Industrial Tribology 1983

INDUSTRIALTRIBOLOGY

TRIBOLOGY SERIES

Advisory Editor: DOUGLAS SCOTT Editorial Board W.J. Bartz (Germany) R. Bassani (Italy) C.A. Brockley (Canada) H. Czichos (Germany) D. Dowson (UK) N.Gane (Australia) W.A. Glaeser (USA)

M. Goder (France) H.E. Hinterman (Switzerland) I.V. Kragelskii(USSR) K.C. Ludema (USA) G.W. Rowe (UK) T. Sakurai (Japan) J.P. Sharma (India)

Vol. 1 Tribology - A Systems Approach to the Science and Technology of Friction, Lubricationand Wear (Czichos) Vol. 2 Impact Wear of Materials (Engel) Vol. 3 Tribology of Natural and Artificial Joints (Dumbleton) Vol. 4 Tribology of Thin Layers (Iliuc) Vol. 5 Surface Effects in Adhesion, Friction, Wear and Lubrication (Buckley) Vol. 6 Frictionand Wear of Polymers (Bartenev and Lavrentev) Vol. 7 Microscopic Aspects of Adhesion and Lubrication (Georges, Editor) Vol. 8 IndustrialTribology- The PracticalAspects of Friction, Lubrication and Wear (Jones and Scott, Editors) Vol. 9 Mechanics and Chemistry in Lubrication (Dorinson and Ludema) Vol. 10 Microstructure and Wear of Materials (Zum Gahr) Vol. 11 Fluid Film Lubrication- Osborne Reynolds Centenary (Dowson et al., Editors) Vol. 12 Interface Dynamics (Dowson et al., Editors) Vol. 13 Tribology of Miniature Systems (Rymuza) Vol. 14 Tribological Design of Machine Elements (Dowson et al., Editors) Vol. 15 Encyclopediaof Tribology (Kajdas et al.) Vol. 16 Tribology of Plastic Materials (Yamaguchi) Vol. 17 Mechanics of Coatings (Dowson et al., Editors)

TRIBOLOGY SERIES, 8

INDUSTRIAL TRIBOLOGY The PracticalAspects of Friction, Lubrication and Wear

edited by

MERVIN H. JONES Department of Mechanical Engineering, University College of Swansea, Swansea, U.K.

and

DOUGLAS SCOTT Consultant, Editor of "Wear", Secretary of The Institution of Engineers and Shipbuilders in Scotland, Glasgow, U.K.

ELSEVIE R SCl ENTl F IC PUBLISHING COMPANY AMSTERDAM -OXFORD -NEW YORK

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat25 P.O. Box 21 1,1000 AE Amsterdam, The Netherlands Distributorsfor the USA and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas New York, NY 10010, USA

Library of Congress Cataloging in Publication Data

Main e n t r y under t i t l e : Industrial tribology.

(Tribology s e r i e s ; 8 ) Includes b i b l i o g r a p h i c a l r e f e r e n c e s and indexes. 1. Tribology. I. J o n e s , Mervin H., 193911. S c o t t , Douglas, 1916111. S e r i e s . TJ1075.148 1983 621.8’9 82-24248 ISBN 0-444-42161-0 (U. S. )

.

ISBN 0444421614 (Vol. 8 ) ISBN 044441677-3 (Series)

First edition 1983 Second impression 1991

0 Elsevier Scientific Publishing Company, 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 21 1,1000 AE Amsterdam, The Netherlands.

Special regulations for readers in the USA. This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, MA 01970, USA. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products. instructions or ideas contained in the material herein. Printed in The Netherlands

V

CONTENTS

ACKNOWLEDGEMENT

XXI

FOREWORD

XXIII

CHAPTER 1.

TRIBOLDGY IN PERSPECTIVE, D.

SCOTT, CONSULTANT,

EDITOR OF WEAR

1

1.1

Introduction

1

1.2

Impact o f T r i b o l o g y

1

1.3

Economic a s p e c t s o f t r i b o l o g y

2

1.4

Mechanisms o f wear

1.5

Surface stu d ie s

3 4

1.6

Lubrication

5

1.7

Lubricants

1.8

Materials

5 6

1.9

Surface treatments

1.10 Computer a i d e d d e s i g n

7 8

1.11 Machinery c o n d i t i o n m o n i t o r i n g

8

3

1.12 C o n c l u s i o n s

10

References

CHAPTER 2.

WEAR,

D.

SCOTT, CONSULTANT, EDITOR OF WEAR

12 12

2.1

Introduction

2.2

The wear p r o c e s s

14

2.3

Scuffing

14

2.4

A b r a s i v e wear

2.5

Fretting

18

2.6

F l u i d and c a v i t a t i o n e r o s i o n

19

2.7

R o l l i n g contact f a t i g u e

20

2.8

Wear d e t e c t i o n and assessment

24

2.9

Conclusions

29

References

I

17

VI CHAPTER 3.

SELECTION OF BEARINGS, M.J.

NEALE, MICHAEL NEALE

AND ASSOCIATES LTD.

3.1

Introduction

31 31

3.2

Bearing types

31

3.3

Performance of v a r i o u s t y p e s o f b e a r i n g

33

3.3.1

Rubbing b e a r i n g s

33

3.3.2

R o l l e r bearings

35

3.3.3

F l u i d f i l m bearings

35

3.3.4

F l e x i b l e members

36

Selection o f a s u i t a b l e bearing

37

3.4

3.4.1

Applications w i t h u n i d i r e c t i o n a l load

37

and c o n t i n u o u s movement

3.4.2

Applications with o s c i l l a t i n g

40

movernent

3.4.3

Applications with multidirectional

42

l o a d and c o n t i n u o u s movement

CHAPTER 4.

DESIGN OF PLAIN BEARINGS,

D E S I G N CHARTS, F.A. GLACIER METAL CO.

4.1 4.2

LTD.

45 45

4.1.1

Journal b e a rin g design l i m i t s

45 46

4.2.1

Notation

Limits o f operation

46

4.2.1.1

Thin o i l f i l m l i m i t

4.2.1.2

High b e a rin g temperature

47 48

4.2.1.3

O i l oxidation l i m i t

50

4.2.1.4

O i l f i l m instabil

50 51

Region o f s a f e o p e r a t i o n

C a l c u l a t i o n and d e s i g n p r o c e d u r e s

-

4.3.1

Dimensionless d a t a

4.3.2

Design p r o c e d u r e s w i t h hea

4.3.3 4.4

GARNER, THE

Introduction

4.2.2 4.3

USE OF BEARING DATA

MARTIN AND D.R.

52

Stage 1

52 ba 1 ance

Stage 1 1

Improvement i n d e s i g n a i d s

57

-

Stage 111

57

Design p r o c e d u r e f o r c y l i n d r i c a l b o r e b e a r i n g s

58

4.4.1

Method o f approach

4.4.2

Guidance f o r s a f e o p e r a t i o n

58 60

VII

4.4.2.1

O i l f i l m t h i c k n e s s and

4.4.2.2 4.4.3

4.5

4.6

temperatures

60

O i l f i l m instability

64

Performance p r e d i c t i o n

65

4.4.3.1

Minimum o i l f i l m t h i c k n e s s

67

4.4.3.2

Misalignment

67

4.4.3.3

Power l o s s and o i l f l o w

67

4.4.3.4

Temperatures

69

High speed a p p l i c a t i o n s

72

4.5.1

Non l a m i n a r o p e r a t i o n

72

4.5.2

P r o f i l e bore bearings

72

Example o f use o f design a i d s

75

4.6.1

75

Problem

4.6.2

76

Procedure

4.6.2.1

Diametral c l e a r a n c e

76

4.6.2.2

Maximum s p e c i f i c l o a d r a t i n g

76

4.6.2.3

Region o f s a f e o p e r a t i o n

76

4.6.2.4

P r e d i c t i o n o f minimum o i l

77

f i l m thickness

4.6.2.5

Check for l a m i n a r o p e r a t i o n

77

4.6.2.6

P r e d i c t i o n o f power loss

77

4.6.2.7

Prediction o f o i l flow

77

4.6.2.8

P r e d i c t i o n o f maximum b e a r i n g

78

temperature

4.6.2.9

Prediction o f o i l o u t l e t

78

temperature

78

Re f e r ence s

CHAPTER 5 .

THE D I A G N O S I S OF PLAIN BEARING FAILURES, R.W. AND E.B.

SHONE,

SHELL RESEARCH CENTRE.

WILSON

80

5.1

Introduction

80

5.2

Properties required o f bearing m a t e r i a l s

80

5.2.1

Fatigue resistance

81

5.2.2

Compressive s t r e n g t h

81

5.2.3

Conformability

81

5.2.4

Embeddability

81

5.2.5

S t r e n g t h a t e l e v a t e d temperatures

81

5.2.6

Compatibility

81

5.2.7

Corrosion resistance

81

5.2.8

Cost

82

VIII 5.3

Type, c o n s t r u c t i o n and c h a r a c t e r i s t i c s o f p l a i n bearing materials

5.4

82

5.3.1

White m e t a l s ( B a b b i t t s )

82

5.3.2

Copper l e a d a l l o y s

86

5.3.3

Lead bronzes

86 87

5.3.4

Aluminium a l l o y s

5.3.5

Phosphor and s i l i c o n bronzes

88

5.3.6

Silver

88

5.3.7

Porous and s e l f l u b r i c a t i n g b e a r i n g s

88

5.3.8

Discussion o f m e t a l l i c b e a r i n g m a t e r i a l s

88

Bearing f a i l u r e s

89

5.4.1

89

M e t a l l u r g i c a l d e f e c t s i n new b e a r i n g s 5.4.1.1

Bad bonding

89

5.4.1.2

Gas c a v i t i e s

81

5.4.1.3

Oversize cuboids

91

5.4.1.4

Excessive l e a d c o n t e n t i n t i n based w h i t e m e t a l s

5.4.1.5

copper- l e a d

5.4.2

91

Uneven l e a d d i s t r i b u t i o n i n and lead-bronze a I I o y s

o r o p e r a t i n g environment

96

5.4.2.1

96

Bad f i t t i n g

5.4.2.2

Extraneous p a r t i c l e s

5.4.2.3

Corrosion

102 117

97

5.4.2.4

C a v i t a t i o n and e r o s i o n

5.4.2.5

E l e c t r i c a l d i scha r g e damage

119

5.4.2.6

Wire wool f a i l u r e s

122 123

5.4.2.7

Fatigue

6.4.2.8

Thermal c y c l i n g damage

125

5.4.2.9

A l l o y i n g i n service

125

5.4.2.10

Inadequate v i s c o s i t y and l a c k of lubricant

5.5

126

Conclusions

127

Ref e rence s

CHAPTER 6.

ROLLING ELEMENT BEARINGS, SKF (UK)

91

F a i l u r e s a s s o c i a t e d w i t h design, f i t t i n g

130

D.G.

HJERTZEN AND R.A.

JARVIS,

LTD.

132

6.1

Introduction

132

6.2

Bearing s e l e c t i o n

132

IX 6.3

Bearing types

133

6.3.1

S i n g l e row deep groove b a l l b e a r i n g

133

6.3.2

Self-a1 i g n i n g b a l l b e a r i n g s

133

6.3.3 6.3.4 6.3.5

Angular c o n t a c t b a l l b e a r i n g

134 134

6.3.6 6.3.7

Double row c y l i n d r i c a l r o l l e r b e a r i n g s

135 136

Needle r o l l e r b e a r i n g s

136

6.3.8

Taper r o l l e r b e a r i n g s

137

6.3.9 6.3.10

Double row s p h e r i c a l r o l l e r b e a r i n g s Spherical r o l l e r t h r u s t bearings

137 138

6.3.11

B a l l t h r u s t bearings

138

6.3.12

Bearings w i t h s p h e r i c a l o u t s i d e surface

Double row a n g u l a r c o n t a c t b a l l b e a r i n g C y l i n d r i c a l r o l l e r bearing

and extended i n n e r r i n g w i d t h

6.4

6.5 6.6 6.7 6.8 6.9

F a t i g u e l i f e and l o a d c a r r y i n g c a p a c i t y

6.4.1 6.4.2

Hertzian contact

6.4.3

F u r t h e r development o f t h e l i f e e q u a t i o n

R e l a t i o n s h i p between l o a d and l i f e

139 139 140 141

Usage

143 144 144

Speed l i m i t s

145

F r ic t i o n

150

Lubrication

151 152

Boundary dimensions

6.9.1

Greases

6.9.1.1 6.9.2

Temperature range

O i l lubrication

6.10 Seals 6.11 Noise 6.12 A n t i c i p a t i n g b e a r i n g damage 6.13 D e t e c t i o n o f b e a r i n g damage by shock p u l s e mea su rement

6.14 F i t s ( s h a f t and housing) 6.15 C o n d i t i o n s o f r o t a t i o n 6.16 I n f l u e n c e o f l o a d and temperature

152 155 157 159 162 163 163 165

6.17 Bearing a p p l i c a t i o n s

165 166

6.18 Bearing c a r e

167

6.19 Bearing mounting 6.20 D im o u n t i n g b e a r i n g s

167 168

6.21 Mounting and dismounting by o i l i n j e c t i o n

169 171

6.22 Cleaning o f b e a r i n g s

X 6.23 R e c o g n i t i o n o f b e a r i n g f a i l u r e s 6.23.1 6.23.2

Wear

172

I n c o r r e c t mounting

6.23.3 6.23.4

Cage f a i l u r e s

174 175 177

6.23.5

Vibrations

6.23.6

Rust and o t h e r t y p e s o f c o r r o s i o n

6.23.7

Smearing

181

PRACTICAL GEAR TRIBOLOGY, T . I .

FOWLE, CONSULTANT

184 184 184

7.1

Introduction

7.2

Alignment

7.3

Tooth a c t i o n

187

Tooth s u r f a c e d i s t r e s s

189

7.4.1

Pitting

189

7.4.2 7.4.3 7.4.4

Scuffing A b r a s i v e wear

191 194

Other forms o f gear wear

195

7.4

7.5

Problems i n l u b r i c a t i o n systems

References Gear problems : causes and remedies

CHAPTER 8.

178 181

Passage o f e l e c t r i c c u r r e n t through bearings

CHAPTER 7.

172

195 196 197

MATERIALS FOR TRIBOLOGICAL APPLICATIONS, 0. SCOTT,

8.1 8.2

Introduction

205 205

Types o f m a t e r i a l s

205

8.3

Materials f o r specific applications

CONSULTANT, EDITOR OF WEAR

8.3.1 8.3.2

P l a i n bearings

207 7-07

Gears

21 0

8.3.3 8.3.4

R o l l i n g bearings

21 1

Wear r e s i s t a n t m a t e r i a l s

21 3

8.3.5

Tools

214

8.3.6 8.3.7

Cutting tools

8.3.8

Cams and t a p p e t s

215 21 5 216

8.3.9

F r i c t i o n materials

217

8.3.10

P l a s t i c bearings

21 7

Piston rings

XI 8.4

8.5

Surface t r e a t m e n t s and c o a t i n g s

218

Conclusions

22 1 221

References

CHAPTER 9.

SELECTION OF LUBRICANTS, A.R.

LANSDOWN, SWANSEA 223

TRIBOLOGY CENTRE 9.1

Introduction

223

9.2

Selecting the l u b r i c a n t type

224

9.3

Properties o f mineral o i l s

226

9.3.1

Viscosity

226

9.3.2

Viscosity

9.3.3

Viscosity

9.3.4

A n t i - w e a r , extreme p r e s s u r e and a n t i -

-

temperature r e l a t i o n s h i p

229

pressure r e l a t i o n s h i p

23 1

f r i c t i o n properties

9.4

CHAPTER 10.

9.3.5

Stability

9.3.6

Con tami nan t s

Choice o f base o i l 9.4.1

L i m i t a t i o n s o f mineral o i l s

9.4.2

High temperatures

9.4.3

Flammability

9.4.4

Compatibil i t y

9.5

Greases

9.6

Sol i d l u b r i c a n t s

9.7

Gas l u b r i c a t i o n

LUBRICANT ADDITIVES, THEIR APPLICATION, PERFORMANCE AN0 LIMITATIONS, D.M.

SOUL, LUBRIZOL INTERNATIONAL 242

LABORATORIES. Introduction

242

10.2

Basic p r o p e r t i e s of l u b r i c a n t s

242

10.3

Lubricant a d d i t i v e s

243

10.1

10.3.1

Detergent and d i s p e r s a n t a d d i t i v e s

243

10.3.1.1

Detergent t y p e s

243

.1 Sulphonates

245

.2 Phosphonates and/or thiophosphonates

245

.3 Phenates

248

.4 A l k y l s u b s t i t u t e d sal i c y l a t e s

249

XI1 10.3.1.2

Mode o f a c t i o n o f d e t e r g e n t add i t i v e s

10.3.1.3

10.3.1.4

249

Di spe rsant a d d i t i v e s

249

. 1 Cop0 1 yme r s

250

.2 S u b s t i t u t e d Succinimides

250

.3 Amides

250

.4 Other chemicals

251

Mode o f a c t i o n o f d i s p e r s a n t additives

10.3.2

251

A n t i o x i d a n t s and b e a r i n g c o r r o s i o n inhibitors

10.3.3

251

10.3.2.1

Di t h i op ho sphates

252

10.3.2.2

Hindered phenol

252

10.3.2.3

N i t r o g e n bases

252

10.3.2.4

S u l ph uri sed p o l y o l e f i n s

252

10.3.2.5

Mode o f a c t i o n o f a n t i o x i d a n t s

252

Corro si on i n h i b i t o r s

253

10.3.3.1

Di t h i op ho sphates

253

10.3.3.2

Di t h i carb amates

253

10.3.3.3

Sulphur p r o d u c t s

254

10.3.3.4

Phosphorus

10.3.3.5

T r i a z o l e s and c h e l a t i n g agents

10.3.3.6

-

Sulphur p r o d u c t s

2 54

Mode o f a c t i o n o f c o r r o s i o n inhibitors

10.3.4

Rust i n h i b i t o r s

10.3.5

V i s c o s i t y improvers 10.3.5.1

10.3.6

254 254

255

Mode o f a c t i o n o f V . I .

improvers

Pour p o i n t depressants 10.3.6.1

2 56 257

Mode o f a c t i o n o f pour p o i n t d ep ressa nts

10.3.7

254

Dime rcap t o t h iad i o z o l e derivatives

10.3.3.7

254

257

258

Extreme p ressu re a d d i t i v e s 10.3.7.1

Comrnerical E.P.

t h e i r application

258

10.3.7.2

Automotive E.P.

259

10.3.7.3

An t i wea r a d d i t i v e s

260

10.3.7.4

E.P.

a d d i t i v e s f o r t u r b i n e o Is

262

10.3.7.5

E.P.

a d d i t i v e s f o r c u t t i n g o Is

262

10.3.7.6

Mode o f a c t i o n o f E.P.

a d d i t i v e s and

gear o i l s

a d d i t ve s

263

XIII

10.4

10.3.8

Emulsifiers

264

10.3.9

Friction modifiers

264 265

Conclusions

266

References

CHAPTER 11. CONSUMPTION AND CONSERVATION OF LUBRICANTS,

A.R.

LANSDOWN, SWANSEA TRIBOLOGY

CENTRE

26 7

11.1

Consumption

26 7

11.2

Reducing consumption

270

11.3

Reclamation and r e - r e f i n i n g

271

11.4

Economics

273

CHAPTER 12. HEALTH AND SAFETY ASPECTS OF LUBRICANTS, A.R.

EYRES, MOBIL EUROPE INC.,

275

12.1

Introduction

275

12.2

Composition o f l u b r i c a n t s

275

12.3

M i n e r a l base o i l f a c t o r s

2 76

12.3.1

Acute t o x i c i t y

276

12.3.2

Dermatitis

277

12.3.3

O i l mist

277

12.3.4

O i l vapours

278

12.3.5

Skin cancer

278

12.3.6

Eye i r r i t a t i o n

280

12.4

Additive factors

280

12.4.1

Lead compounds

280

12.4.2

Orthophosphates

281

12.4.3

C h l o r i n a t e d Naphthalenes

12.4.4

Sodium N i t r i t e and Amines

12.4.5

Sodium Mercaptobenzothiazole

282

12.4.6

Trichloroethylene

282

281 28 1

282

12.5

B a c t e r i a and b i o c i d e s

12.6

Synthetic l u b r i c a n t s

283

12.7

Used and r e c l a i m e d o r r e - r e f i n e d o i l s

283

12.8

H e a l t h and s a f e t y p r e c a u t i o n s

284 284

12.8.1

Suppliers r e s p o n s i b i l i t i e s

12.8.2

Skin p r o t e c t i o n

285

12.8.3

O i l m i s t and vapour

287

12.8.4

S k i n cancer

288

XIV 12.8.5 12.9

B a c t e r i a and b i o c i d e s

Conclusions

289 289

References

289

CHAPTER 13. EFFECTIVE CONTAMINATION CONTROL I N FLUID POWER

SYSTEMS, J.B.

SPENCER,

13.1

Introduction

13.2

D i r t Input

13.1.1

291

A syst e mat i c approach t o f i l t r a t i o n

-

291

t yp es and sources o f c o n t a m i n a t i o n

292 294

13.2. I

New o i 1

294

13.2.2

B u i l t i n contamination

294

13.2.3

Environmental Co nt amination

294

13.2.4

E n t r y p o i n t s f o r e nvironmental

13.2.5 13.3

SPERRY VICKERS

contamination

295

13.2.4.1

A i r breathers

295

13.2.4.2

Power u n i t access p l a t e s

295

13.2.4.3

Cylinder seals

295

Generated c o n t a m i n a t i o n

296

E f f e c t s o f t ype s and s i z e s o f p a r t i c l e s

296

13.3.1

Catastrophic f a i l u r e

296

13.3.2

Intermittent f a i l u r e

296

13.3.3

Deg rad at i on f a i l u r e

297

13.3.4

Pumps

297

13.3.5

Motors

300

13.3.6

D i r e c t i o n a l valves

30 1

13.3.7

Pressure c o n t r o l s

302

13.3.8

Flow c o n t r o l s

302

13.3.9

Summary

303

13.4

S p e c i f y i n g con t a mi na t i o n l e v e l s

305

13.5

Selecting the f i l t e r

305

13.5.1

Nominal r a t i n g

305

13.5.2

A b sol u t e r a t i n g

306

13.5.3

The b ub bl e t e s t

306

13.5.4

Mean f i l t r a t i n g r a t i n g

306

13.5.5

Multipass f i l t e r test

307

13.5.6

Beta r a t i o

307

13.5.7

Practical c l a s s i f i c a t i o n of f i l t e r performance

307

13.5.7.1

Pulsating flow

308

13.5.7.2

Bypass i n g

308

xv 13.5.8 13.6

Filter sizing

Locating the f i l t e r

13.6.1

318

Pump i n l e t f i l t r a t i o n

318

Pressure 1 i n e f i l t r a t i o n

320

13.6.3

Return 1 i n e f i 1 t r a t i o n

322

13.6.4

O f f line filtration

324

13.6.2

13.7

312

327

Summary

CHAPTER 14. SEALS FOR FLUID POWER EQUIPMENT, PART ONE,

B.D.

HALLIGAN, JAMES WALKER 6 CO. LTD.

330

14.1

Introduction

330

14.2

Materials

331

14.3

Seal designs

338

14.4

Tribological considerations

339

14.4.1

Film conditions

339

14.4.2

Surface f i n i s h

343

14.4.3

Seal f r i c t i o n

344

14.4.4

Type o f f l u i d

346

14.4.5

Filtration

347

A i r entrainment

347

14.4.6 14.5 14.6

Selection

348

Storage

350

14.6.1

14.7

Recommendations

350

14.6.1.1

Temperature

350

14.6.1.2

Humidity

351

14.6.1.3

Light

351

14.6.1.4

Oxygen and ozone

351

14.6.1.5

Deformation

351 351

Assembly

CHAPTER 15. SEALS FOR FLUID POWER EQUIPMENT, PART TWO, ROTARY SHAFT L I P SEALS, B.D.

HALLIGAN, JAMES WALKER E CO. LTD.

353

15.1

Introduction

353

15.2

Design

354

15.3

Material

355

15.4

Seal l u b r i c a t i o n

356

15.5

Shaft surfaces

357

15.6

Friction

358

15.7

Speeds

359

XVI 15.9

Pressure

359 360

15.10 Care and h a n d l i n g

360

15.10.1

Storage

15.10.2

Handling

361

15.10.3

Fitting

36 1 362

15.11 S e r v i c e problems and t h e i r s o l u t i o n s 15.11.1

362

Unacceptable leakage

CHAPTER 16. SEALS FOR FLUID POWER EQUIPMENT, PART THREE, COMPRESSION PACKINGS, 16.1

Appendix 1

B.D. HALLIGAN, JAMES WALKER t CO. LTD.

370

The packed g l a n d

370

16.1.1

Pumps

371

16.1.2

Valves

371

16.2

Operating p r i n c i p l e s

373

16.3

Gland design

374

16.4

Packing c o n s t r u c t i o n and m a t e r i a l s

376

16.4.1

Fibre material

3 76

16.4.2

Lubricants

376

16.4.3

Construct ion

3 76

16.4.4

Additional material

379

16.4.5

Selection

383

16.4.6

Fitting

388

16.4.7

Fault finding

390

16.4.8

Standardization

391

16.4.9

The f u t u r e

392 393

Safety o f asbestos g l a n d packings and gaskets

CHAPTER 17. CENTRALISED LUBRICATION SYSTEMS DESIGN, J.G.

MERRETT,

ENGINEERING AN0 GENERAL EQUIPMENT LTD.

395

17.1

Introduction

395

17.2

Points of l u b r i c a t i o n

396

17.3

Selecting the l u b r i c a n t 17.3.1

-

o i l or grease

the other 17.3.2

397

L u b r i c a t i n g grease

-

t h e t y p e s and how

t o s e l e c t them 17.3.3

396

O i l o r grease7 when t o use one and when

Lubricating o i l s s e l e c t them

398

-

types and how t o 400

XVII

17.3.3.1 17.3.3.2

The p r o p e r t i e s o f t h e o i l Specialist lubricants

40 1 402

17.4

Pipe diameter vs f l o w c h a r a c t e r i s t i c s

402 402

17.5

L u b r i c a t i o n requirements f o r p l a i n b e a r i n g s

402

17.6

S e l e c t i n g t h e r i g h t t y p e o f l u b r i c a t i o n system Grease l u b r i c a t i o n systems

403 403

17.6.1.1

D i r e c t f e e d i n g systems

403

17.6.1.2

I n d i r e c t o r l i n e systems

405

17.6.1.3

Comparison o f p a r a l l e l

17.3.4

17.6.1

Summarising l u b r i c a n t s e l e c t i o n

-

systems

s i n g l e l i n e and

dual l i n e

17.6.1.4

412

systems

17.6.1 - 5

Lubrication o f p l a i n o r sleeve bearings

17.6.1.6

O i l l u b r i c a t i o n systems

17.6.2.1

T o t a l l o s s systems

17.6.2.2

Systems designed w i t h small amount o f h e a t removal

17.6.2.3

17.6.4 17.7

412 413 413 416

Systems designed f o r l u b r i c a t i o n

418

c o o l ing

17.6.3

41 2

Lubrication o f a n t i f r i c t i o n ( b a l l and r o l l e r ) b e a r i n g s

17.6.2

412

S e l e c t i n g grease l u b r i c a t i o n

M i c r o - f o g l u b r i c a t i o n systems

41 9

17.6.3.1 17.6.3.2

Working p r i n c i p l e s O i l quality

419 421

17.6.3.3

Compressed a i r

421

17.6.3.4

Systems d e s i g n c o n s i d e r a t i o n s

421

17.6.3.5

Some t y p i c a l a p p l i c a t i o n s

421 423 424

Check l i s t

Summary

425

References

CHAPTER 18. ON CONDITION MAINTENANCE, R.A.

COLLACOTT, FAULT

DIAGNOSIS CENTRE

427

18.1

Introduction

18.2

Background

18.3 18.4

Management o f cond i t i o n mon i t o r ing

427 427 428 428

F a i l u r e modes and e f f e c t s a n a l y s i s

XVIII 18.5

Monitoring technique s e l e c t i o n

432

18.5.1

V i b r a t i o n monitoring

433

18.5.1.1

Proximity probes

433

18.5.1.2

Seismic v e l o c i t y transducers, accelerometers,

vibration

meters, spectrum analysers

18.5.1.3

18.6

Waveform a n a l y s i s

434

4 36

18.5.2

Inspection, i n t e g r i t y s u r v e i l l a n c e

436

18.5.3

Contaminant a n a l y s i s

4 38

18.5.4

Trends a n a l y s i s

444 444

Deterioration l i m i t s

445

References

CHAPTER 19. THE TRIBOLOGY OF METAL CUTTING, E.M.

TRENT, U N I V E R S I T Y

OF BIRMINGHAM

446

19.1

Introduction

446

19.2

Metal c u t t i n g phenomena

19.3

Conditions a t the tool-work

19.3.1

19.4

19.5

446 interface

Tool forces and stresses

448

19.3.2

C u t t i n g speed

451

19.3.3

Heat i n metal c u t t i n g

45 1

19.3.4

Seizure o f the tool-work i n t e r f a c e

452

19.3.5

C u t t i n g t o o l temperatures

456

19.3.6

SI i d i n g a t the tool-work i n t e r f a c e

4 58

C u t t i n g t o o l wear

459

19.4.1

Abrasion

46 1

19.4.2

Surface shearing

46 1

19.4.3

D i f f u s i o n and i n t e r a c t i o n

463

19.4.4

Attrition

465 467

Coolants and l u b r i c a n t s

470

References

CHAPTER 20. ROLE OF LABORATORY TEST MACHINES, F.T.

M.H.

448

JONES, UNIVERSITY COLLEGE

BARWELL AND

O F SWANSEA

471

20.1

Experimental method

471

20.2

Lubricant type approval t e s t i n g machines

4 72

20.2.1

Engine t e s t s

472

20.2.2

Gear t e s t s

474

20.2.3

Simulation o f i n d u s t r i a l s i t u a t i o n s

479

XIX 20.3

20.4

A n a l y s i s i n t o system elements o f t h e b a s i s f o r selection o f laboratory tests

480

Equipment f o r b a s i c re sea rch

488

Ref e r enc e s

492

GLOSSARY

493

AUTHOR INDEX

502

SUBJECT INDEX

506

This Page Intentionally Left Blank

XXI

ACKNOWLEDGEMENT

The e d i t o r s g r a t e f u l l y acknowledge t h e a s s i s t a n c e o f M r . G. W i l l i a m s and Mr.

J . Thomas o f t h e l u b r i c a t i o n department o f B r i t i s h Steel C orporation,

Po r t T a l b o t and D r . G. Thomas o f t he E x t r a Mural Department, U n i v e r s i t y Colle g e , Swansea, who have been a c t i v e l y a s s o c i a t e d w i t h t h e annual Seminars on i n d u s t r i a l T r i b o l o g y s i n c e t h e i r i n c e p t i o n .

Acknowledgement i s a l s o due t o v i s i t i n g l e c t u r e r s who generously gave t h e i r t i m e i n d e v e lo p in g t h e Seminar t o i t s p r e s e n t successful f o r m a t . i n c l u d e R. Gronbech P.

Gadd

-

-

Davey Un i t e d, J. Bathgate

NAML Gosport, N.W.

International, Dr.

E.T.

Morris

Jagger

-

-

-

F arva l ube Ltd.,

Angus Seals Ltd.,

These

David Brown Gears Ltd.,

D. H a t t o n

and D r .

D.J.

-

Shell

Haines

-

B r i t i s h A i r c r a f t Corp ora t i o n.

The e d i t o r s e x te nd t h e i r g r a t i t u d e t o t h e t y p i n g e x p e r t i s e o f Mrs. M.A. W i l l i a m s and Mrs. P.T. ready copy.

Hancock who have so e x p e r t l y produced t h i s camera

This Page Intentionally Left Blank

XXIII

FOREWORD Some e le v e n ye ars ago M r .

Ronald Dale, B r i t i s h Steel C orporation,

P o r t Talbot,

v i s i t e d t h e C o lleg e t o propose t h e i n t r o d u c t i o n o f a course i n T r i b o l o g y f o r Steelworks Design S t a f f .

T h i s p rop osa l was r e a d i l y accepted because prepar-

a t i o n s f o r t h e e s t a b l i s h m e n t o f t h e T r i b o l o g y Centre were a l r e a d y w e l l advanced. The f i r s t c o urse was r a t h e r a m b i t i o u s i n s o f a r as i t was o f two weeks duration,

t h e f i r s t week b e i n g devoted t o f a i r l y heavy t h e o r y w h i l s t more

p r a c t i c a l m a t e r i a l was i n t r o d u c e d d u r i n g t h e second week.

A particular feature

was t h e i n c l u s i o n o f p r o j e c t s based on contemporary problems a t t h e steelw orks. These p r o j e c t s were a l l o c a t e d t o small teams and, on t h e f i n a l day,

the leader

o f each team had t o r e p o r t t o t h e whole co urse on t h e s o l u t i o n a r r i v e d a t by h i s team. The demand f o r re pe at courses was s t r o n g and they have been r u n a t annual i n t e r v a l s e v e r s i nce .

However, t h e p a t t e r n o f i n s t r u c t i o n has been m o d i f i e d from

y e a r t o y e a r as t h e r e s u l t o f q u e s t i o n n a i r e s completed by course members and i n response t o r e p r e s e n t a t i o n s by employers.

Thus t h e p r e s e n t course i s o f s h o r t e r

d u r a t i o n and i s o r i e n t a t e d towards p r a c t i c e from t h e o u t s e t , t h e o r y o n l y b e i n g i n t r o d u c e d when r e q u i r e d t o p o i n t t h e way towards t h e s o l u t i o n s o f p r a c t i c a l problems. One o f t h e o b j e c t s o f t h e f i r s t course,

t o c o n t r i v e t h e maximum degree o f

i n te r c h a n g e o f i n f o r m a t i o n between p a r t i c i p a n t s , h a s

been r e t a i n e d and developed,

p a r t i c u l a r l y s i n c e t h e course i s now r e c r u i t e d from a number o f i n d u s t r i e s be s id e s t h e s t e e l i n d u s t r y and indeed f rom several c o u n t r i e s . The t e r m ' T r i b o l o g y ' , as f o l l o w s : -

d e f i n e d i n t h e Chambers T w e n t i e t h Century D i c t i o n a r y

a sci e nce and t e chn ol o gy embracing a l l s u b j e c t s i n v o l v e d when s u r -

fa c e s i n c o n t a c t move i n r e l a t i o n t o each o t h e r (Greek t r i b e i n , t o rub, and logos, speech, a d i s c o u r s e ) had then been o n l y r e c e n t l y i n t r o d u c e d t o emphasize the m u l t i - d i s c i p l i n a r y

n a t u r e o f t h e study o f b e a r i n g system.

The term 'Lub-

r i c a t i o n ' which had been used p r e v i o u s l y was considered t o be inadequate because i t focussed a t t e n t i o n on one element o n l y o f t h e b e a r i n g system

-

n o t a b l y the

l u b r i c a n t t o t h e e x c l u s i o n o f o t h e r f a c t o r s such as t h e m a t e r i a l o f c o n s t r u c t i o n o f t h e i n t e r a c t i n g elements. The need f o r t h e i n t r o d u c t i o n o f t h e new term was n o t n e c e s s i t a t e d by any f a i l u r e t o d e v e l o p t h e s u b j e c t f rom t h e re search and development p o i n t o f view b u t r a t h e r t o draw a t t e n t i o n t o a body o f knowledge w hich was n o t thought t o be s u f f i c i e n t l y a p p l i e d w i t h i n i n d u s t r y a t t h a t p a r t i c u l a r time.

The development

o f T r i b o l o g y has always been r e l a t e d t o advances i n t h e S t a t e o f t h e A r t o f

XXIV Engineering.

Thus, d u r i n g t h e n i n e t e e n t h c e n t u r y , progress was dominated by

the r a i l w a y and t h e steamship.

The hydrodynamic t h e o r y o f l u b r i c a t i o n was

developed t o e x p l a i n Beachamp Towers experiments on b e a r i n g s used on t h e Metrop o l i t a n RaiIway,and M i c h e l l ' s t i l t i n g pad t h r u s t b e a r i n g s were considered t o be very s u i t a b l e for s h i p s ' p r o p e l l e r s h a f t s .

The t h i r t i e s t h i s c e n t u r y were

devoted t o t h e a i r c r a f t and a ut omo bi l e ;

t h e f o r t i e s t o the gas t u r b i n e .

o f th e e a r l y work o f N.E.L.

Cl e veland was d i r e c t e d t o the problems

o f high-speed b e ari ng s. institution,

and N.A.S.A.

Then came at omi c energy.

Much

The o r i g i n o f o u r s i s t e r

The N a t i o n a l T r i b o l o g y L a b o r a t o r y a t R i s l e y , can be a t t r i b u t e d t o

t h i s need and, more re cen t l y, spa ce

technology has i n t r o d u c e d a whole range o f

new problems p a r t i c u l a r l y r e l a t e d t o o p e r a t i o n i n r a r i f i e d atmospheres. I f one has t o d eci d e on t o d a y ' s s p e c i a l c h a r a c t e r i t i s t h e urge f o r g r e a t e r p r o d u c t i v i t y m a i n l y achieved by au t o mat i c and o t h e r w i s e c a p i t a l - i n t e n s i v e equipment.

T h i s has focussed a t t e n t i o n on t h e importance o f r e l i a b i l i t y and o f

reduced maintenance,which c a l l f o r t h e utmost refinement o f t r i b o l o g i c a l design. Means f o r m o n i t o r i n g t h e c o n d i t i o n o f machines so as t o a n t i c i p a t e f a i l u r e and t o r a t i o n a l i s e maintenance have now assumed t h e utmost importance. The t r i b o l o g y o f t h e m a n u fa c t u ri ng i n d u s t r y w i l l p r o b a b l y be t h e most important grow th area o f o u r s u b j e c t d u r i n g t h e n e x t decade. N o t w i t h s t a n d i n g t h e o u t p u t o f p r e s e n t day research schools i n T r i b o l o g y , o f which t h e r e a r e a growing number, t h e body o f knowledge which has been b u i l t up d u r i n g t h e p a s t c e n t u r y p r o v i d e s p owerf ul t o o l s f o r any engineer who wishes t o improve a p r o d u c t o r p r a c t i c e and i t i s hoped t h a t t h e f o l l o w i n g volume w i l l p o i n t t h e way t o t h e a p p l i c a t i o n o f sound t r i b o l o g y i n many i n d u s t r i e s .

F.T. Barw ell U n i v e r s i t y C ollege o f Swansea

1

1 TRIBOLOGY

IN PERSPECTIVE

D. SCOTT, C o n s u l t a n t , E d i t o r o f Wear 1.1

INTRODUCTION As o u r t e c h n o l o g i c a l c i v i l i z a t i o n expands, m a t e r i a l and energy c o n s e r v a t i o n

i s becoming i n c r e a s i n g l y i mp ort an t .

Wear i s a major cause o f m a t e r i a l wastage,

so any r e d u c t i o n o f wear can e f f e c t c o n s i d e r a b l e savings.

Friction is a

p r i n c i p a l cause o f energy d i s s i p a t i o n and c o n s i d e r a b l e savinas a r e p o s s i b l e by improved f r i c t i o n c o n t r o l .

L u b r i c a t i o n i s t h e most e f f e c t i v e means o f c o n t r o l -

l i n g wear and r e du ci ng f r i c t i o n .

Thus t r i b o l o g y , which i s t h e science and tech-

nolo g y o f f r i c t i o n l u b r i c a t i o n and wear,

i s o f c o n s i d e r a b l e importance i n

The h i s t o r y o f t h i s r e l a t i v e l y new science

m a t e r i a l and energy c o n s e r v a t i o n .

which i s concerned w i t h problems t h a t have always presented man w i t h a c h a l l e n g e has been r e c o r d e d 111, and t h e fundamentals reviewed [ 2 ] .

1.2

IMPACT OF TRIBOLOGY Since t h e p u b l i c a t i o n o f t h e L u b r i c a t i o n Report [ 3 ] t h e r e has been an i n -

c r e a s i n g awareness t hro ug ho ut i n d u s t r y o f t h e s u b j e c t o f t r i b o l o g y .

I n t h e UK

t h e N a t i o n a l Centre f o r T r i b o l o g y and I n d u s t r i a l U n i t s o f T r i b o l o g y have been s e t up t o p r o v i d e a d v i c e t o i n d u s t r y on t he u t i l i s a t i o n o f e x i s t i n g knowledge. These a r e now v i a b l e e s t a b l i s h m e n t s o p e r a t i n g as c o n t r a c t research o r g a n i s a t i o n s s e l l i n g t h e i r s e r v i c e s a t commercial r a t e s .

Over t h i r t y u n i v e r s i t i e s , p o l y -

t e c h n i c s and t e c h n i c a l c o l l e g e s have i n c o r p o r a t e d courses on v a r i o u s aspects o f t r i b o l o g y i n t o t h e i r syllabuses.

A b a s i c t r i b o l o g y module

mechanical e n g i n e e r i n g courses has been drawn up. j e c t f o r t h e h i g h e r n a t i o n a l c e r t i f i c a t e (H.N.C.)

[41 f o r undergraduate

T r i b o l o g y i s an e l e c t i v e subi n e n g i n e e r i n g i n the U n i t e d

Kingdom and a t r i b o l o g y c o n t e n t i s i n c l u d e d i n some committee f o r n a t i o n a l academic awa ds (C.N.N.A.)

courses.

Post-graduate research i n t r i b o l o g y ,

l e a d i n g t o h gher degrees i s c a r r i e d o u t a t several u n i v e r s i t i e s ; c h a i r s i n t r b o log y.

t h r e e have

Various courses and t r a i n i n g programmes a r e a l s o a v a i l a b e

to i n d u s t r y .

T r i b o l o g y i s now reco gn i zed u n i v e r s a l l y and P r e s i d e n t C a r t e r o f U.S.A.

151

d e c l a r e d i t t o be a g e n e r i c t ech no l og y u n d e r l y i n g many i n d u s t r i a l s e c t i o n s and t h e p r o s p e c tu s f o r an I n d u s t r i a l T r i b o l o g y I n s t i t u t e a t Rensselaer Technolooy Center has been p rese nt ed [ 6 ] . Numerous papers on t r i b o l o g y a r e p u b l i s h e d a n n u a l l y and many r e p o r t research d i r e c t e d towards a b e t t e r under.standing o f t h e fundamental p r i n c i p l e s governinq

2 i n t e r a c t i n g surfaces.

U n f o r t u n a t e l y , most o f t h e i n f o r m a t i o n p r o v i d e d i s n o t

s u i t a b l e f o r d i r e c t use by d esi g ne rs and en gi neers as research workers g e n e r a l l y f i n d i t more c o n ve ni e nt t o express r e s u l t s i n terms o f non-dimensional parameters r a t h e r than as t h e s p e c i f i c da t a r e q u i r e d f o r design purposes. bology handbook

A tri-

[7] has been produced w i t h t h e o b j e c t o f p r o v i d i n g i n f o r m a t i o n

t o i n d u s t r y i n a f o rm t h a t i s r e a d i l y a c c e s s i b l e and understood by e n g i n e e r i n g de s ig n e r s , draughtsmen and works e ng i ne ers.

A synoptic j o u r n a l

[8] has been

i ntr o d u c e d t o reduce t i me spent i n l i t e r a t u r e p e r u s a l . There i s a steady g rowt h i n t h e f o r m a t i o n o f T r i b o l o g y S o c i e t i e s on an i n t e r n a t i o n a l s c a l e.

1.3

ECONOMIC ASPECTS OF TRIBOLOGY

The L u b r i c a t i o n Report [ 3 ] est i mat ed , w i t h i n an e r r o r o f t w e n t y - f i v e p e r c e n t , t h a t an amount exceeding f i v e hundred m i l l i o n pounds per annum can be saved i n t h e c i v i l i a n s e c t o r o f t h e UK economy by improvements i n e d u c a t i o n and res e a r c h i n t r i b o l o g y .

Such improvements a r e s i g n i f i c a n t , n o t merely i n c o s t

s a v in g s , b u t a r e c r u c i a l t o t e c h n o l o g i c a l pro gress and have doubly s i g n i f i c a n t i m p l i c a t i o n s f o r t he economic w e l l - b e i n g o f t he n a t i o n and t h e r e p u t a t i o n o f i t s e n g i n e e r i n g p r o d uct s. The ASME Research Committee on L u b r i c a t i o n i n t h e i r " Strategy Conservation t h r ou gh T r i b o l o g y "

f o r Energy

[ 9 ] r e p o r t e d t h e magnitudes o f energy conserv-

a t i o n t h a t can p o t e n t i a l l y be o b t a i n e d i n t h e f o u r major areas o f road t r a n s p o r t a t i o n , power g e n e r a t i o n , t u r b o machinery and i n d u s t r i a l processes through progres s i n t r i b o l o g y .

The e s t i m a t e d 1 1 p er c e n t t o t a l savings i n annual US energy

consumption i s e q u i v a l e n t t o some s i x t e e n b i l l i o n US d o l l a r s by an e x p e n d i t u r e i n r e s e a r c h and development o f an e s t i m a t e d t w e n t y - f o u r m i l l i o n d o l l a r s . A techno-economic

st ud y [ l o ] concluded t h a t t h e a p p l i c a t i o n o f t r i b o l o g i c a l

p r i n c i p l e s and p r a c t i c e s can e f f e c t n a t i o n a l energy savings o f c o n s i d e r a b l e magnitude i n t h e U n i t e d Kingdom,

i n t he areas covered which comprise t h e major

p a r t s o f 87% o f energy consumption.

These savings a r e e s t i m a t e d a t €468 t o

f 7 0 0 m i l l io n p e r annum. E r o s io n can be expensive and i t has been r e p o r t e d [ l l ] t h a t t h e i n g e s t i o n o f du s t c l o u d s can reduce t h e l i v e s o f h e l i c o p t e r engines by as much as 90 per c en t;

l o c a l s t a l l can be caused by removal o f as l i t t l e as 0 . 0 5 mm o f m a t e r i a l

from t h e l e a d i n g edges o f compressor blades.

I n pneumatic t r a n s p o r t a t i o n o f

m a t e r i a l t h r o u g h p i pe s, t h e e r o s i v e wear a t bends can be up t o f i f t y times more than t h a t i n s t r a i g h t s e c t i o n s .

Even wood c h i p s can cause such wear [ 1 2 ] .

Analyses o f t h e f a i l u r e o f b o i l e r tubes i n d i c a t e t h a t about one t h i r d o f a l l occ u r r e n c e s were due t o e r o s i o n [131.

3 Although a b r a s i v e wear i s u s e f u l t o shape and p o l i s h e n g i n e e r i n g components, i t s unwanted occurrence i s p r o b a b l y t h e most s e r i o u s i n d u s t r i a l wear problem. In the a g r i c u l t u r a l

i n d u s t r y as many as f o r t y p e r c e n t o f t h e components

r e p l a c e d on equipment have f a i l e d by a b r a s i v e wear [ 1 4 ] . The wear o f t o o l s used f o r c u t t i n g m e t a l s i s o f c o n s i d e r a b l e importance t o t h e economics o f t h e e n g i n e e r i n g i n d u s t r y ,

I t was e s t i m a t e d i n 1971 [ 1 5 ] t h a t

f o r t y b i l l i o n d o l l a r s was spent i n t h e USA on t h e machining o f metal p a r t s .

In

t h e UK about twenty m i l l i o n c a r b i d e c u t t i n g t o o l s a r e used p e r year a t a c o s t o f f i f t y m i l l i o n pounds. Several e s t i m a t e s have been made on t h e c o s t o f f r i c t i o n and wear.

Jost [16]

s t a t e d t h a t f r i c t i o n and wear i n t h e USA accounted f o r an e x p e n d i t u r e o f one hundred b i l l i o n d o l l a r s p e r annum. Technology of F.R.G.

A Committee o f t h e M i n i s t r y o f Research and

[ 1 7 ] e s t i m a t e d t h a t f r i c t i o n and wear caused a n a t i o n a l

economic waste o f t e n b i l l i o n OM p e r annum o f which about f i f t y p e r c e n t i s due t o a b r a s i v e wear.

Rabinowicz [18] has e s t i m a t e d t h a t about t e n p e r c e n t o f a l l

energy generated by man i s d i s s i p a t e d i n f r i c t i o n processes. T r i b o l o g i c a l f a i l u r e s a r e i n v a r i a b l y a s s o c i a t e d w i t h b e a r i n g s and t o i l l u s t r a t e t h e c o s t s which can be i n v o l v e d i t has been r e p o r t e d [19] t h a t a simple b e a r i n g f a i l u r e i n a f u l l y i n t e g r a t e d s t e e l m i l l can l e a d t o a t o t a l shut down which a t f u l l o u t p u t r a t e may c o s t one hundred and f i f t y t o t h r e e hundred pounds p e r minute.

A s i m i l a r b e a r i n g f a i l u r e on a modern g e n e r a t o r s e t c o u l d i n v o l v e

t h e C e n t r a l E l e c t r i c i t y Generating Board i n a l o s s o f one t o twenty pounds s t e r l i n g p e r m i n u t e t i l l t h e s e t was a g a i n o p e r a t i o n a l .

A s i m i l a r bearing

f a i l u r e i n t h e USA has been quoted t o c o s t t w e n t y - f i v e thousand d o l l a r s p e r day [ZO].

I t has been r e p o r t e d [ Z l ]

t h a t t h e t o t a l c o s t o f wear f o r a US naval

a i r c r a f t amounted t o two hundred and f o r t y t h r e e d o l l a r s p e r f l i g h t hour.

1.4

MECHANISMS OF WEAR Progress i n wear c o n t r o l can be a i d e d by a b e t t e r u n d e r s t a n d i n g o f t h e

mechanisms by which i t occurs.

Research workers have tended t o i s o l a t e and

study s p e c i f i c wear mechanisms such as adhesion, a b r a s i o n , e r o s i o n and f a t i g u e . Such r e s e a r c h has g e n e r a l l y been d i r e c t e d towards t h e study o f s u r f a c e s i n r e l a t i v e m o t i o n , t h e changes brought about by t h e i r i n t e r a c t i o n and t h e e f f e c t s o f t h e l u b r i c a n t and t h e environment p r e s e n t .

L i t t l e a t t e n t i o n has been g i v e n

t o t h e p r o d u c t s o f wear, t h a t i s t o t h e d e b r i s generated.

Recently, p a r t i c l e

t r i b o l o g y [ 2 2 ] has a l l o w e d p o s t u l a t i o n o f t h e mechanisms o f t h e i r f o r m a t i o n which t o g e t h e r w i t h r e f i n e d techniques o f s u r f a c e i n v e s t i g a t i o n and t h e study of sub-surface changes a i d s t h e e l u c i d a t i o n of t h e wear process. Advances i n u n d e r s t a n d i n g emerge o n l y f r o m a w i l l i n g n e s s t o q u e s t i o n accepted t h e o r i e s .

Q u e s t i o n i n g o f t h e t h e o r i e s o f wear a i d e d by r e f i n e d

4 i n v e s t i g a t i o n t e ch ni q ue s a r e now s t i m u l a t i n g the p i o n e e r i n g s p i r i t . d e l a m i n a t i o n t h e o r y o f wear

[23]

Suh's

i s t h e t y p i c a l example o f r e c e n t progress.

Surface e x a m in at i on and wear p a r t i c l e a n a l y s i s has l e d t o t h e h y p o t h e s i s [24] t h a t i n t e r a c t i o n p o l i s h e s t h e surf ace s and c r e a t e s a shear mix l a y e r o f s h o r t c r y s t a l l i n e o r d e r o f a l most s u p e r d u c t i l e m a t e r i a l which spreads over t h e s u r f a c e as f i r s t proposed by B e i l b y

[25].

Repeated r u b b i n g c o n t a c t causes t h e shear

mix l a y e r t o f a t i g u e and c h a r a c t e r i s t i c p a r t i c l e s f l a k e o f f . F u r t h e r work i s r e q u i r e d t o p r o v i d e a more complete d e s c r i p t i o n o f t h e s u r fac e b e h a v io u r o f m a t e r i a l s and t h e wi d er a p p l i c a t i o n o f new t h e o r i e s must a w a i t th e a d d i t i o n a l evidence.

I t may thus be p o s s i b l e t o p r e d i c t t h e wear r a t e s o f

m a t e r i a l s based on f i r s t p r i n c i p l e s and fundamental p r o p e r t i e s . The a p p l i c a t i o n o f a system a n a l y s i s t o wear problems i s r e c e i v i n g c o n s i d e r a b l e a t t e n t i o n [26]. The complex n a t u r e o f wear has delayed i t s i n v e s t i g a t i o n b u t i t now appears t h a t t he e r a when wear was considered a branch o f s t u d i e s i n f r i c t i o n and l u b r i c a t i o n i s coming t o an end.

The success o f t h e f i r s t I n t -

e r n a t i o n a l Conference on Wear o f M a t e r i a l s 1271 e s t a b l i s h e d wear as a s u b j e c t o f i n t e r n a t i o n a l importance i n i t s own r i g h t . i o n a l Conference

[29]

A second [ 2 8 ] and a t h i r d I n t e r n a t -

have been h e l d and a f o u r t h i s planned.

Microscopic

[30] and c a l c u l a t i o n methods f o r [31]. A s t a t e o f the a r t r e v i e w o f

asp e c ts o f wear a r e r e c e i v i n g a t t e n t i o n f r i c t i o n and wear have been reviewed available

1.5

wear i s

[32].

SURFACE STUDIES The f r i c t i o n a l and wear b eh avi o ur o f m a t e r i a l s i s g r e a t l y dependent upon t h e

s u r f a c e m a t e r i a l and i t s topography.

Surface i n t e r a c t i o n causes changes i n

these p r o p e r t i e s b u t d e t a i l e d knowledge o f happenings i n t h e i n t e r f a c e when wear i s occurring i s d i f f i c u l t t o acquire.

I t has been usual t o study surfaces a t

v a r i o u s stages o f wear t o p o s t u l a t e t h e sequence o f events.

Besides t h i s p r o -

cedure g r e a t e r a t t e n t i o n i s now b e i n g g i v e n t o the s i z e , morphology and s t r u c t u r e o f wear p a r t i c l e s as w e l l as t o t h e l o c a l i s e d n a t u r e o f damage t o surface,

i n t e r f a c e and su bsu rf a ce m a t e r i a l .

Several new t o o l s a r e a v a i l a b l e f o r

t h e s tu d y o f s u r fa ces a t a t o mi c l e v e l , n o t a b l y Auger e l e c t r o n spectroscopy, x - r a y photon e l e c t r o n spectroscopy,

scanning ion spectroscopy and i o n s c a t t e r i n g

spectroscopy which w i t h complementary i n f o r m a t i o n from x - r a y energy a n a l y s i s i n t h e scanning e l e c t r o n microscope and micro-probe a n a l y s i s a i d t h e t r i b o l o g i c a l e l u c i d a t i o n o f s u r f a c e phenomena

[33,34].

Advances have been made i n t h e a p p l i c a t i o n o f s t a t i s t i c a l techniques t o the c h a r a c t e r i s a t i o n o f rough s u r f a c e s

[35,36].

The e n t i r e s t a t i s t i c a l m i c r o -

geometry o f c e r t a i n rough s u r f a c e s can now be c o m p l e t e l y described i n terms o f t h e number o f peaks and mean l i n e c r o s s i n g s counted on a s i n g l e p r o f i l e .

These

5 te c h n iq u e s a r e now b e i n g a p p l i e d i n t r i b o l o g y and i t appears t h a t i n instrume n t a t i o n t h r e e - d imen si on al mapping i s now w e l l e s t a b l i s h e d [ 3 7 ] .

For t h e

measurement, assessment and c h a r a c t e r i s a t i o n o f v e r y f i n e surfaces, a l a s e r beam te c h n iq u e o f l i g h t s c a t t e r i n g appears p o t e n t i a l l y a t t r a c t i v e f o r q u a l i t y c o n t r o l purposes [ 3 8 ] .

1.6

LUBRICATION Since Reynolds [ 3 9] produced h i s e q u a t i o n f o l l o w i n g t h e p i o n e e r i n g work o f

Tower,

(see [ 4 0 ] )

t h e mathematical e x p r e s s i o n o f t h e process o f f i l m f o r m a t i o n

between r e l a t i v e l y moving s u r f a c e s has been fundamental t o a l l l u b r i c a t i o n theory.

Equations have been d e r i v e d and a p p l i e d t o t h e s t u d y o f the v a r i o u s

s u r f a c e c o n f i g u r a t i o n s used i n p r a c t i c e , and t h e i n t r o d u c t i o n o f t h e high-speed d i g i t a l computer a l l o w e d t h e si mu l t a ne ou s s o l u t i o n o f Reynolds e q u a t i o n t o g e t h e r w i t h e q u a t i o n s f o r t h e e l a s t i c d ef orma t i o n o f t h e surfaces. el a s to h y d r o d y n a mi c l u b r i c a t i o n (EHL) f i l m s ,

Optical studies o f

i n f r a - r e d temperature measurements

and t h e e l u c i d a t i o n o f t h e response o f vi sco us l i q u i d s t o h i g h frequency shear have g r e a t l y improved t h e u nd erst an di n g o f e l astohydrodynamic c o n t a c t s .

It i s

perhaps b e t t e r t o d e s c r i b e t h e l u b r i c a n t i n a h i g h l y loaded EHL c o n t a c t as an e l a s t o - p l a s t i c s o l i d r a t h e r t ha n a s a vi scou s f l u i d .

Based on t h e new under-

s ta n d in g , a t h e o r y o f EHL t r a c t i o n has been advanced [ 4 1 ] which may be a p p l i e d t o e n g i n e e r i n g components such as r o l l i n g b e a r i n g s and v a r i a b l e - s p e e d d r i v e s .

The e la s t o h y d r o d yn ami cs o f e l l i p t i c a l c o n t a c t s has been a p p l i e d t o b a l l and r o l l e r bearing l u b r i c a t i o n [42]. Progress i n hydrodynamic l u b r i c a t i o n appears t o be c e n t r e d on d e t a i l e d developments r a t h e r t h an improved fundamental understanding.

Work on boundary

l u b r i c a t i o n seems t o be o r i e n t e d towards s p e c i f i c problem areas such as e l e v a t e d temperatures and h o s t i l e environments.

Two c e n t u r i e s o f study have

f a i l e d t o u n r a v e l c o m p l e t e l y t he m y s t e r i e s o f l u b r i c a t i o n problems most i m p o r t a n t t o mankind, t he mechanism o f human j o i n t s .

Following the t e n t a t i v e

pr o p o s a l o f squeeze f i l m s [43] and t h e emphasis on t h e p r o t e c t i v e motion o f t h e s im p le squeeze f i l m [44]

i t i s co nsi d ere d t h a t t h e prospect o f EHL i s good

b u t t h a t t h e p r o m i s i n g mode i s squeeze f i l m and n o t r o l l i n g s l i d i n g [451. C o l l e c t i o n s o f i n f o r m a t i o n a r e a v a i l a b l e on t h e t r i b o l o g y o f n a t u r a l and a r t i f i c i a l j o i n t s [46] and t h e mechanical p r o p e r t i e s o f b i o m a t e r i a l s [ 4 7 ] .

1.7

LUBRICANTS When f a i l u r e i n s e r v i c e o ccurs i t i s co mmercially more a c c e p t a b l e t o change

t h e l u b r i c a n t r a t h e r t ha n t h e d esi g n.

Thus research and development work i s

c o n t i n u o u s l y d i r e c t e d towards improved l u b r i c a n t s , a d d i t i v e s t o impart o r r e i n f o r c e d e s i r a b l e p r o p e r t i e s and s y n t h e t i c l u b r i c a n t s w i t h unique p r o p e r t i e s . The more r e c e n t ma j or developments i n l u b r i c a n t f o r m u l a t i o n appear t o have

6 been on c u t t i n g f l u i d s , f i r e - r e s i s t a n t carbon f l u i d s .

h y d r a u l i c f l u i d s and s y n t h e s i s e d h y d r o -

Although t h e l a t t e r may c o s t more than m i n e r a l - o i l based

p r o d u c t s , e x p e r i e n c e i n d i c a t e s t h a t t h e y may g i v e an o v e r a l l c o s t s a v i n g . Complications caused by l u b r i c a n t s l e a d t o c o n s i d e r a t i o n o f w e a r - r e s i s t a n t m a t e r i a l s w i t h good f r i c t i o n a l p r o p e r t i e s which can o p e r a t e w i t h o u t l u b r i c a t i o n . A n t i - p o l l u t i o n and c o n s e r v a t i o n i s p l a c i n g emphasis on sealed, l u b r i c a t e d - f o r l i f e machinery u s i n g s o l i d l u b r i c a n t s and s u r f a c e t r e a t m e n t s which l u b r i c a t e . Under such c o n d i t i o n s i n t e r f a c i a l c o n d i t i o n s become i m p o r t a n t .

Plastics

m a t e r i a l s a r e r e c e i v i n g increased a t t e n t i o n e s p e c i a l l y where chemical and therma 1 i n e r t n e s s a r e requ i red.

.

Pol y t e t r a f 1 u o r o e t h y 1 ene ( P .T. F. E ) has become

the standard s o l i d l u b r i c a n t i n cryogenic applications. f l o w has been c o n t r o l l e d by s u i t a b l e r e i n f o r c e m e n t .

I t s tendency t o c o l d

Newer polymers a r e b e i n g

i n c r e a s i n g l y used where h i g h thermal s t a b i l i t y i s r e q u i r e d .

Metal f i l m l u b r i c -

a n t s a r e now f i n d i n g use and p o t e n t i a l developments i n s o l i d l u b r i c a n t technol o g y may a r i s e f r o m composite s o l i d - l i q u i d l u b r i c a n t s t o use t h e s p e c i f i c p r o p e r t i e s o f each.

1.8

MATERIALS The emergence o f new design concepts i s a m a j o r i n c e n t i v e f o r t h e development

o f w e a r - r e s i s t a n t m a t e r i a l s and t h e a c q u i s i t i o n o f m a t e r i a l s data. and s t r e s s problems a s s o c i a t e d w i t h advanced t r i b o - e n g i n e e r i n g strength,

light-weight materials.

The thermal

require high-

Conventional m a t e r i a l s have been improved by

o r t h o d o x methods almost t o t h e l i m i t o f t h e i r p o t e n t i a l mechanical p r o p e r t i e s so t h a t new t y p e s o f m a t e r i a l s such as composites,

s y n t h e t i c diamond and s a p p h i r e ,

new g r a p h i t e 5 and c a r b i d e s , metal b o r i d e s and n i t r i d e s which approach t h e hardness o f n a t u r a l diamond a r e b e i n g developed.

To u t i l i z e t h e i r s p e c i f i c p r o -

p e r t i e s new d e s i g n concepts a r e r e q u i r e d as t h e s u b s t i t u t i o n o f such m a t e r i a l s i n e x i s t i n g designs can l e a d t o problems and f a i l u r e s i n s e r v i c e .

Besides

r e p l a c i n g m e t a l s , ceramics may be used as c o a t i n g s t o complement d e s i r a b l e metal characteristics with refractory properties,

i n s u l a t i n g and e r o s i o n , wear,

o x i d a t i o n and c o r r o s i o n r e s i s t a n c e . I n t h e f i e l d o f p l a i n b e a r i n g s n o major development o f s o f t metal b e a r i n g s appears l i k e l y i n t h e immediate f u t u r e as t h e p o s s i b l e a l l o y s o f a l l commercially f e a s i b l e s o f t e r metals have been f u l l y e x p l o i t e d ,

A v a i l a b l e m a t e r i a l s come

c l o s e to u t i l i s i n g f u l l y t h e p o t e n t i a l i t y o f p l a i n b e a r i n g s o f c u r r e n t designs and l u b r i c a t i o n systems.

The development o f p l a s t i c s b e a r i n g m a t e r i a l s capable

o f b e i n g manufactured t o and m a i n t a i n i n g t h e c l o s e t o l e r a n c e s o f m e t a l s c o u l d cause something o f a r e v o l u t i o n i n t h e b e a r i n g f i e l d . P l a s t i c s and t h e i r composites dominate t h e d r y b e a r i n g scene m a i n l y due t o t h e a v a i l a b i l i t y o f design and performance d a t a [48,49].

A s i g n i f i c a n t advance

i n fundamental understandinn o f t h e wear o f p l a s t i c s composites has been t h e

7 r e c o g n i t i o n o f t h e dominant r o l e o f t h e c o u n t e r f a c e metal [50]. d e p o s i t i o n techniques such as s p u t t e r i n g , chemical vapour d e p o s i t i o n (C.V.D)

ion-implantation,

Vacuum

i o n - p l a t i n g and

appear p o t e n t i a l l y a t t r a c t i v e f o r s o l i d f i l m

l u b r i c a n t s o l u t i o n s t o a wide range o f d r y b e a r i n g problems. I n the f i e l d o f r o l l i n g b e a r i n g m a t e r i a l s , methods such as r e d u c t i o n o f gas c o n t e n t [ 5 1 ] and d e l e t e r i o u s c a r b i d e s e g r e g a t i o n [ 5 2 ] a r e b e i n g developed t o improve r o l l i n g c o n t a c t f a t i g u e r e s i s t a n c e .

Developments i n h i g h speed t o o l

s t e e l b e a r i n g s have c e n t r e d around a f i n e r d i s p e r s i o n o f c a r b i d e s [ 5 3 ] and weight s a v i n g [ 5 4 ] .

The use o f h i g h e r than normal a d d i t i o n s o f a l l o y i n g

elements t o p r o v i d e m a r g i n a l l y improved p r o p e r t i e s may n o t j u s t i f y t h e increased cost.

There appears t o be a steady b u t u n s p e c t a c u l a r development o f ceramic

m a t e r i a l s such as s i l i c o n n i t r i d e [ 5 5 ] . Brakes and c l u t c h e s r e q u i r e t o d i s s i p a t e c o n t i n u a l l y g r e a t e r e n e r g i e s due t o loads and speeds g e n e r a l l y i n c r e a s i n g and improved m a t e r i a l s a r e c o n s t a n t l y demanded t o contend w i t h more arduous d u t i e s and h i g h e r temperatures.

Owing t o

t h e p o s s i b l e h e a l t h hazard t h e r e i s c o n s i d e r a b l e p r e s s u r e t o r e p l a c e asbestos, t h e most e f f e c t i v e f i l l e r m a t e r i a l f o r p h e n o l i c r e s i n s due t o i t s f i b r o u s n a t u r e and h e a t r e s i s t a n c e .

S i n t e r e d metal m a t r i c e s a r e now used f o r severe d u t y

a p p l i c a t i o n s b u t a t t e m p t s t o i n t r o d u c e o t h e r o r g a n i c and i n o r g a n i c m a t e r i a l s have n o t y e t succeeded i n d i s p l a c i n g c o n v e n t i o n a l m a t e r i a l s except i n h i g h l y specialised f i e l d s .

Concorde uses carbon composites a g a i n s t themselves and

these m a t e r i a l s w i t h cheaper f i b r e s and f i l l e r s may be a p r o m i s i n g method o f approach t o t h e replacement o f a s b e s t o s - f i l l e d p h e n o l i c r e s i n s .

1.9

SURFACE TREATMENTS The m a t e r i a l o f e n g i n e e r i n g components must have s t r u c t u r a l c h a r a c t e r i s t i c s

t o s a t i s f y t h e design requirements and s u r f a c e c h a r a c t e r i s t i c s t o contend w i t h wear, f a t i g u e and environmental e f f e c t s . promise t o these requirements.

Surface c o a t i n g s o f f e r t h e b e s t com-

Surface t r e a t m e n t s a r e a l s o a t t r a c t i v e as an

a l t e r n a t i v e t o design o r l u b r i c a n t changes t o combat wear i n s e r v i c e [56,571. Besides t h e c o n v e n t i o n a l t r e a t m e n t s r e c e n t l y developed, s u r f a c e t r e a t m e n t s i n v o l v i n g t h i n s u r f a c e f i l m s w i t h s p e c i f i c p r o p e r t i e s a r e now f i n d i n g i n c r e a s i n g use and p r o v i n g t o be advantageous as w e a r - r e s i s t a n t

coatings.

i n c l u d e p h y s i c a l and chemical vapour d e p o s i t i o n processes.

The t r e a t m e n t s

The u s e o f T i c

c o a t i n g s on s i n t e r e d c a r b i d e c u t t i n g t o o l s i s a t y p i c a l example o f r e d u c i n g t o o l wear and c u t t i n g c o s t s [58].

Low temperature CVD processes and c o n t r o l l e d

n u c l e a r thermo-chemical d e p o s i t i o n a r e b e i n g developed t o produce equiaxed g r a i n m a t e r i a l o f e x c e p t i o n a l hardness.

Ion p l a t i n g and i o n i m p l a n t a t i o n a l s o

appear t o be f i n d i n g use f o r s p e c i f i c a p p l i c a t i o n s .

8

1.10

COMPUTER-AIDED D E S I G N

I t i s o n l y by t he co mbi n at i on o f improved s c i e n t i f i c undetstanding and i t s speedy i n d u s t r i a l u t i l i s a t i o n t h a t r a p i d t e c h n o l o g i c a l progress can be achieved. I n the p a s t, a major d i f f i c u l t y has been t h e d e l a y o r l a c k o f feedback from i n d u s t r y and t h u s t h e de l ay i n t h e t i m e t ake n f r o m t h e i n c e p t i o n o f a good idea t o i t s f r u i t f u l application.

To e l i m i n a t e such delays i n c r e a s i n g use i s b e i n g

made o f t h e computer i n de si gn t o en ab l e al most instantaneous feedback.

In

the f i e l d o f p l a i n b e a r i n g s , u s i n g a s u i t a b l e programme t h e designer need o n l y tra n s m it i n f o r m a t i o n on b e a r i n g d esi g n t o be informed o f performance c h a r a c t e r istics.

I n t h i s way he can have i n s t a n t feedback and make use o f t h e l a t e s t

research r e s u l t s w i t h o u t b e i n g an e x p e r t i n t h e f i e l d s o f t r i b o l o g y , o r programming.

In effect,

computation

he has a t hand what amounts t o a u n i v e r s a l t e s t i n g

machine i n which he can p l a n h i s design,

t e s t i t s c h a r a c t e r i s t i c s , modify the

design and a g a i n measure i t s c h a r a c t e r i s t i c s , c o n t i n u i n g t h e process u n t i l s a t i s f i e d t h a t he has t h e optimum de si gn b e f o r e c o m m i t t i n g h i m s e l f t o f u l l - s c a l e t e s t , production o r service.

M a t e r i a l s s e l e c t i o n f o r optimum performance by

computer i s now approaching r a p i d l y .

1.11

MACHINERY CONDITION MONITORING

Economic p r e s sure s a r e ca usi n g t h e p r a c t i c e o f w i t h d r a w i n g equipment from s e r v i c e a t p e r i o d i c i n t e r v a l s f o r i n s p e c t i o n and maintenance t o be r e p l a c e d by f a i l u r e p r e v e n t i o n maintenance.

Thus, means have been developed t o determine

th e c o n d i t i o n o f machinery w h i l s t i n s e r v i c e and t o d e t e c t any d e t e r i o r a t i o n o f performance so t h a t remedial a c t i o n can be t a ken b e f o r e t h e breakdown p o i n t i s reached.

The m o n i t o r i n g t ech ni q ue chosen depends upon the s p e c i f i c i n f o r m a t i o n

r e q u i r e d and t h e c o s t o f a c q u i r i n g t h e i n f o r m a t i o n compared w i t h t h e savings such i n f o r m a t i o n can e f f e c t . There has been a g rad ua l acceptance o f v i b r a t i o n a n a l y s i s a l t h o u g h t h i s has proved t o be n e i t h e r t h e s i m p l e s t n o r t h e most e f f e c t i v e method t o use.

The

problem o f d a t a i n t e r p r e t a t i o n u s u a l l y c r e a t e s t h e need f o r expensive t r e n d a n a l y s i s f r o m a massive b u i l d - u p o f da t a . The h i s t o r y o f a wear process i s reco rde d i n t h e wear d e b r i s produced and magnetic p l u g s and s p e c t r o g r a p h i c o i l a n a l y s i s (SOAP) a r e now e x t e n s i v e l y used t o d e t e c t abnormal wear.

The US Defense Department spends f o r t y m i l l i o n d o l l a r s

pe r y e a r on o i l a n a l y s i s [ZO] t o p r e d i c t o n l y c e r t a i n types o f f a i l u r e i n one power system, th e a i r c r a f t gas t u r b i n e , d i r e c t r e p a i r costs.

t o save t w i c e t h i s f i g u r e i n terms o f

A l t ho ug h t h ese t ech ni q ues have proved e f f e c t i v e i n p r o -

v i d i n g wa r n in g o f changes i n a system, t he y have some disadvantages.

SOAP

p r o v i d e s a knowledge o n l y o f t h e q u a n t i t y o f metal i n t h e l u b r i c a n t b u t no i n f o r m a t i o n on t he s i z e o r shape o f t he wear p a r t i c l e s .

Some damage has u s u a l l y

9

o c c u r r e d when t h e magnetic p l u g p i c k s up d e b r i s l a r g e enough f o r o b s e r v a t i o n . Ferrography [24,59]

a convenient method f o r t h e i s o l a t i o n and a n a l y s i s o f

wear p a r t i c l e s has opened up a new dimension i n wear d e t e c t i o n and assessment i n the form o f p a r t i c l e t r i b o l o g y ,

[22].

N o n - m e t a l l i c p a r t i c l e s can a l s o be

i s o l a t e d f r o m l u b r i c a n t s so t h a t l u b r i c a n t degeneration p r o d u c t s can be i d e n t i f i e d t o assess c o n d i t i o n and performance.

Recent developments [ 6 0 ] have

enabled t h e a d o p t i o n o f f e r r o g r a p h y t o b i o - e n g i n e e r i n g f o r t h e study o f p r o s t h e s i s j o i n t s , which should a s s i s t t h e development o f improved m a t e r i a l s and design o f a r t i f i c i a l i m p l a n t s .

As a r t h r i t i c j o i n t s a r e s u b j e c t e d t o wear t h e

a n a l y s i s o f a s p i r a t e d s y n o v i a l f l u i d appears p o t e n t i a l l y a t t r a c t i v e f o r t h e s t u d y o f wear r a t e s , mechanisms and b i o l o g i c a l responses t o wear i n human j o i n t s . F e r r o g r a p h i c s y n o v i a l f l u i d a n a l y s i s should augment u n d e r s t a n d i n g o f t h e e t i o l o g y and pathogenesis o f d e g e n e r a t i v e a r t h r i t i s and p r o v i d e a method f o r t h e d i a g n o s i s , documentation,

p r o g n o s t i c a t i o n and t r e a t m e n t o f t h e disease.

f i f t e e n m i l l i o n USA c i t i z e n s a r e a f f l i c t e d w i t h o s t e o a r t h r i t i s [ 2 0 ] .

Some

The f i r s t

I n t e r n a t i o n a l Conference on Ferrography has been planned [611.

1 .12

CONCLUS I ONS

I n t h e s h o r t t i m e s i n c e t r i b o l o g y was launched as a concept on i t s own,

it

has been d e s c r i b e d as t h e w o r l d ' s f a s t e s t growing a p p l i e d science, as s t i l l i n i t s i n f a n c y [ 6 2 ] and as a means o f n a t i o n a l w e a l t h c r e a t i o n w i t h o u t commensurate c a p i t a l investment [ 6 3 ] .

So t h a t t r i b o l o g y may q u i c k l y achieve m a t u r i t y s t i l l

g r e a t e r use must be made o f e x i s t i n g knowledge.

Most i n d u s t r i a l t r i b o l o g i c a l

problems can be s o l v e d s a t i s f a c t o r i l y by a l o g i c a l s y s t e m a t i c i n v e s t i g a t i o n o f t h e problem and t h e a p p l i c a t i o n o f e x i s t i n g knowledge.

I t t h u s appears t h a t

increased e f f o r t i s r e q u i r e d t o d i s s e m i n a t e knowledge i n a r e a d i l y understood f o r m t o e f f e c t g r e a t e r energy, m a t e r i a l s and manpower savings a t a minimum c o s t . I t i s hoped t h a t t h e subsequent c h a p t e r s go some way towards doing t h i s .

F u t u r e t r e n d s may be t o experiment l e s s b u t t o measure and i n t e r p r e t more. Research may be j u s t i f i e d o n l y i f i t can p r o v i d e i n f o r m a t i o n t o a l l o w i n d u s t r y t o s o l v e i t s immediate problems o r can produce s i g n i f i c a n t advances i n technol o g i c a l progress.

The e n v i r o n m e n t a l i s t s may i n f l u e n c e t r i b o l o g i s t s by

demanding reduced n o i s e l e v e l s o f mechanisms and t h e e l i m i n a t i o n o f p o l l u t i o n and t o x i c i t y f r o m l u b r i c a n t s .

L u b r i c a n t s g e n e r a l l y do n o t wear o u t b u t become

contaminated and s o , f r o m t h e p o i n t o f view o f t h e environment and c o n s e r v a t i o n , r e c l a m a t i o n , which i s p r e s e n t l y o n l y p r a c t i s e d i f economical, w i l l tend t o become o f major importance as we approach an e r a i n which q u a l i t y o f l i f e , s a f e t y and a c l e a n environment may w e l l be t h e m o t i v a t i n g f o r c e behind t e c h n o l o g i c a l innovation.

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1 2

32,

44,

5,

11

177,

157. Reynolds,O., P h i l . Trans. Roy. Soc., 1886, Cameron,A. Proc. I n s t . Mech. Engrs., 1979, 193, P r e p r i n t No.25, London. Proc. Roy. SOC. A., 1977, 356, 215. 41 Johnson,K.L. and Tevazwark,J.L., 42 Hancock,B.T., and Dowson,D., B a l l Bearing L u b r i c a t i o n - The E l a s t o hydrodynamics o f E l l i p t i c a l Contacts, 1981, W i l e y & Sons, N.Y. 43 Fein,R.S., Proc. I n s t . Mech. Engrs., London, 1967, 181, ( 3 J ) , 125. ( 3 J ) , 45. 44 Dowson,D., Proc. I n s t . Mech. Engrs., London, 1967, P r e p r i n t 33/77. 45 Higginson,G., Proc. I n s t . Mech. Engrs., London, 1977, 46 Dumbleton,J.H., (Ed.), The T r i b o l o g y o f N a t u r a l and A r t i f i c a l J o i n t s , 1980, E l s e v i e r , T r i b o l o g y S e r i e s , 3, Amsterdam. Mechanical P r o p e r t i e s o f B i o m a t e r i a l s , 47 Hastings,G.W., and Williams,D.F. 1980, W i l e y & Sons, N.Y. 48 A Guide t o t h e Design and S e l e c t i o n o f Dry Rubbing Bearings, 1976, E.S.D.U. Data Item, 76029. T r i b o l o g y I n t . , 1973, 6, 219. 49 Lancaster,J.K., and Lavrentev,V.V., F r i c t i o n and Wear o f Polymers, 50 Bartenev,G.M., 1981, E l s e v i e r , T r i b o l o g y S e r i e s , 6 , Amsterdam. 339. 51 Scott,D. and McCullagh,P.J., Wear, 1973, 52. Scott,D. and B l a c k w e l l ,J., Wear, 1975, 46, 273. 149. 53 Scott,D. and B l a c k w e l l , J . , Wear, 1978, 54 Scott,D., T r i b o l o g y I n t . , 1976, 2, 261. 55 Scott,D., Wear, 1977, 43, 71. Proc. 1 s t T u r o . T r i b o l o g y Congress, 1975, p.165, I . Mech. 56 Wilson,R.W., Engrs., London. 283. 57 Scott,D., Wear, 1978, Wear, 1978, 407. 58 Hintermann,H.E., Tribology Int., 59 Bowen,R., Scott,D., Seifert,W.W. and Westcott,V.C., 1976, 2, 261. Proc. I n s t . Mech. Engrs., London, 1978, 60 Scott,D. and Westcott,V.C., P r e p r i n t C42/78, 123. 61 F i r s t I n t e r n a t i o n a l Conf. on Ferrography, 1982 (Sept.), Univ. Swansea ( I n Press). 62 E u r o t r i b , ‘7 7, 1977, Bundesrepublik, Deutschland, D u s s e l d o r f . T r i b o l o g y , 1978, l J , 34. 63 Jost,H.P.,

39

40

T L

191,

3,

3,

5,

5,

12

9 D.

2.1

WEAR

SCOTT, Consultant, E d i t o r of Wear

INTRODUCTION Wear may be d e f i n e d as the undesired displacement o r removal o f surface mat-

e r i a l , although under some circumstances,

the i n i t i a l stages o f wear o r m i l d

wear which tends t o smooth surfaces, may be b e n e f i c i a l f o r the running-in o f mechanisms.

The economic i m p l i c a t i o n s o f wear cause concern i n industry, as a

reasonable l i f e i s r e q u i r e d o f mechanical equipment t o cover c a p i t a l and maintenance costs.

Whilst,

i n many instances, wear may n o t place an absolute l i m i t

on the l i f e o f an investment,

i t c e r t a i n l y causes a great deal o f expenditure

on maintenance t h a t must take place;

such maintenance i s c o s t l y i n i t s e l f , b u t

a l s o c o s t l y i n l o s t p r o d u c t i v i t y w h i l s t i t i s being c a r r i e d o u t . Although wear has f o r long been a subject o f p r a c t i c a l i n t e r e s t , fundamental knowledge o f wear i s sparse.

This i s due probably t o the i n t e r d i s c i p l i n a r y

nature o f wear making i t d i f f i c u l t t o e l u c i d a t e and the f a c t t h a t wear has been accepted as i n e v i t a b l e and unavoidable and so mechanical p a r t replacement technology has dominated wear c o n t r o l technology. Progress i n wear c o n t r o l and prevention can be made o n l y a f t e r a b e t t e r understanding o f the mechanisms by which i t occurs and o f the c o n t r o l l i n g f a c t o r s has been acquired.

2.2

THE WEAR PROCESS Wear may take many forms depending upon surface topography, c o n t a c t c o n d i t i o n s

and environment b u t , g e n e r a l l y , t h e r e are two main types, mechanical and chemical. Mechanical wear involves processes which may be associated w i t h f r i c t i o n , abrasion, erosion and f a t i g u e .

Chemical wear a r i s e s from surface a t t a c k by

r e a c t i v e compounds and the subsequent rubbing o r breaking away o f the r e a c t i o n products by mechanical a c t i o n .

The d i f f e r e n t types o f wear may occur s i n g l y ,

s e q u e n t i a l l y o r simultaneously, b u t a l l wear phenomena centre on a common chara c e r t i s t i c , an o v e r s t r e s s i n g o f the surface 111. When two surfaces a r e i n contact,

the r e a l area o f contact i s considerably

less than the apparent area o f c o n t a c t being confined t o a number o f small areas where opposing h i g h spots touch.

Pressure i n these areas w i l l be h i g h and the

surface m a t e r i a l deformed by the a p p l i e d load u n t i l the c o n t a c t area becomes s u f f i c i e n t l y l a r g e t o support the load.

According t o Bowden and Tabor 121, i n

13 the absence o f an e f f e c t i v e s e p a r a t i n g f i l m a j u n c t i o n may be formed between th e s u r fa c e s , and r e l a t i v e mo t i o n w i l l cause t h e j u n c t i o n t o be broken, r e s u l t i n g i n t h e removal o f m a t e r i a l f ro m one o r t h e o t h e r o f t h e surfaces. Ming Feng [ 3 ] c o n s i d e r s t h a t p l a s t i c f l o w o c c u r s a t c o n t a c t i n g a s p e r i t i e s so t h a t m a t i n g s u r f a c e s conform i n a p a t t e r n o f r i d g e s and grooves and g i v i n g r i s e t o s t r a i n hardening.

Mechanical i n t e r l o c k i n g p r e v e n t s s l i p a t t h e i n t e r f a c e

d u r i n g r e l a t i v e mo t i o n and shear o ccurs i n t h e s o f t e r subsurface m a t e r i a l una f f e c t e d by s t r a i n hardening.

The sheared o f f m a t e r i a l may be i n t h e form o f

d e b r i s o r i f t h e she ari n g process produces a s u f f i c i e n t r i s e i n temperature, t h e wear p r o d u c t s may become a t t a c h e d t o one o f t h e surfaces. The a d h e s iv e wear t h e o r y as d e s c r i b e d by Archard [4,5]

postulates the

f o r m a t i o n o f wear p a r t i c l e s a t c o n t a c t i n g a s p e r i t i e s which a r e hemispherical i n shape.

Rabinowicz [ 6 ] c o n s i d e r s t h a t t he r a t i o o f s u r f a c e energy t o m a t e r i a l

hardness i s an i m p o r t a n t f a c t o r i n wear and may have some e f f e c t on wear p a r t i c l e size.

Al t ho ug h wear eq ua t i o ns [4,6]

c o n s i s t e n t w i t h e xpe ri men t a l r e s u l t s ,

d e r i v e d f r o m these t h e o r i e s a r e

t he y do n o t account f o r t h e b a s i c

m e t a l l u r g y o f t h e m a t e r i a l s and a r e based on s e v e r a l a r b i t r a r y assumptions. S e i f e r t and We s tco t t [7,8]

have demonstrated t h a t r u b b i n g wear p a r t i c l e s take

t h e f o r m o f t h i n f l a k e s o f metal w i t h h i g h l y p o l i s h e d s u r f a c e s and a r e n o t hemis p h e r i c a l fragments g e n e r a l l y proposed by adhesion t h e o r y . [9,10]

Suh and o t h e r s

have proposed a d e l a m i n a t i o n t h e o r y o f wear based on the behaviour o f

d i s l o c a t i o n s a t t h e r u b b i n g su rf a ce,

su bsu rf a ce v o i d and c r a c k f o r m a t i o n and

t h e subsequent j o i n i n g o f c r a c k s by shear d e f o r m a t i o n o f t h e surface.

The de-

l a m i n a t i o n t h e o r y p r e d i c t s t h a t t h e wear p a r t i c l e shape i s t h i n f l a k e - l i k e sheets as opposed t o t h e he mi sph eri ca l shape proposed by t h e adhesion t h e o r y and t h a t t h e s u r f a c e l a y e r can undergo l a r g e p l a s t i c deformation.

Experimental

r e s u l t s showing t h e process o f wear sheet f o r m a t i o n by d e l a m i n a t i o n a r e claimed t o s u b s t a n t i a t e t h e t h e o r y [ll]. Suh has r e p o r t e d t h a t b u l k m a t e r i a l hardness i t s e l f i s n o t t h e c o n t r o l l i n g f a c t o r on wear and t h a t t h e d e l a m i n a t i o n t h e o r y s a t i s f i e s t h e thermodynamic req ui re men t s o f t h e f r i c t i o n a l and wear behaviour o f m e t a l s [ 1 2 ].

W e st co t t and o t h e r s [8,13]

suggest t h a t s u r f a c e i n t e r a c t i o n

p o l i s h e s t h e s u r f a c e s and c r e a t e s a shear mi x l a y e r of s h o r t c r y s t a l l i n e o r d e r o f a lm o s t s u p e r d u c t i l e m a t e r i a l which spreads o v e r t h e s u r f a c e t o e f f e c t the smooth n a t u r e o f r u n - i n s u r f a c e s as f i r s t proposed by B e i l b y [14].

Repeated

r u b b i n g causes t h e shear mi x l a y e r t o become f a t i g u e d and r u b b i n g wear p a r t i c l e s f l a k e o f f [ 1 3 ].

Such a s t a t e o f e q u i l i b r i u m m a i n t a i n s benign wear, b u t d i s -

r u p t i o n o f t h i s s t a t e can cause i n i t i a t i o n o f a more severe mode o f wear [13]. A r h e o l o g i c a l mechanism of p e n e t r a t i v e wear [ 1 5 ] has been proposed f o r the

formation o f p l a t e - l i k e debris.

The st ud y o f subsurface e f f e c t s d u r i n g the

s l i d i n g o f m e t a l s su pp ort s t h e assumption o f d e l a m i n a t i o n 1161.

Study o f the

14 wear behaviour o f u l t r a h i g h m o l e c u l a r we i gh t (UHMW) p o l y t h e n e r e v e a l e d a p l a s t i c a l l y f lo wed smooth s u r f a c e and sub surf ace c r a c k i n g which separated sheets o f polymer f rom t h e wear t r a c k i n t h e form o f smooth s u r f a c e d p l a t e l e t s [17].

H i r s t [18],

K r a g e l s k i i [19] and S c o t t [20] have reviewed t h e s u b j e c t o f

Bickerman 1211 has c r i t i c a l l y reviewed t h e t h e o r i e s o f adhesion and

wear.

f r i c t i o n a l phenomena i n s l i d i n g c o n t a c t and Barw ell [22] has reviewed t h e t h e o r i e s o f wear and t h e i r s i g n i f i c a n c e f o r e n g i n e e r i n g p r a c t i c e .

2.3

SCUFFING Under s l i d i n g c o n d i t i o n s ,

the c h i e f ta sk o f a l u b r i c a n t i s t o a l l o w r e l a t i v e

m o t io n between s urf ace s, w i t h low f r i c t i o n and no damage.

T h i s can be achieved

i f th e l u b r i c a n t f i l m i s t h i c k enough t o keep t h e surfaces a p a r t and hydrodynamic c o n d i t i o n s p r e v a i l .

I f however, i d e a l c o n d i t i o n s cannot be maintained,

t h e s u r fa c e s w i l l come i n t o c o n t a c t and wear o r damage i n the form o f s c u f f i n g

w i l l o c c u r , Fig.1.

The m e t a l l o g r a p h i c a l l y changed, s c u f f e d m a t e r i a l , Fig.2,

i s c o n s i d e r a b l y h ard er ( v a r y i n g f r o m 300 t o 850HV) than the o r i g i n a l carbon

Fig.2. (H= x110, V= ~ 1 1 0 0 ) Taper s e c t i o n through a s c u f f e d surface.

Fig.1. ( ~ 7 5 )S c uf f e d s t e e l surface

.

s t e e l (180HV). [23,241

Electron microscopical

i n v e s t i g a t i o n o f the s c u f f e d m a t e r i a l

r e v e a l s t h a t i t i s m a r t e n s i t e and tempered m a r t e n s i t e o r t r o o t s t i t e ,

i n d i c a t i n g t h a t t h e m a t e r i a l has been heated t o above t h e a u s t e n i t i s i n g tempe r a t u r e and r a p i d l y coo l ed .

Hydrocarbon l u b r i c a n t breakdown i n t h e c o n t a c t

15 zone [ 1 5 ] can l e a d t o i n crea se o f carbon c o n t e n t and hardness due t o d i f f u s i o n o f carbon and gases i n t o t h e heated deformed m a t e r i a l . The complex mechanism o f s c u f f i n g i s d i f f i c u l t t o e l u c i d a t e as t h e process, by c u m u la t iv e a c t i o n , d e s t r o y s evidence o f i t s i n i t i a l stages.

To study

s c u f f i n g and t o f o l l o w t h e development o f s u r f a c e f a i l u r e a cros,sed c y l i n d e r machine has been used [26] i n whi ch one c y l i n d e r i s r o t a t e d and a mating c y l i n d e r , a t r i g h t an gl e s t o i t , i s so t r a v e r s e d t h a t t h e area o f c o n t a c t moves Examination o f t h e h e l i c a l t r a c k round a

al o n g t h e s u r f a c e o f b o t h c y l i n d e r s .

t e s t e d c y l i n d e r r e v e a l s how s u r f a c e damage b u i l d s up w i t h t e s t d u r a t i o n and l o a d 1271.

The b e a r i n g t r a c k s on l u b r i c a t e d s t e e l c y l i n d e r s a r e d i s t i n g u i s h e d

by t h e in c r e a s e d o p t i c a l r e f l e c t i v i t y . i n i t i a t e d by s h o r t f i n e marks,

As t h e l o a d increases f a i l u r e i s

f o l l o w e d by i n c i p i e n t s c u f f i n g and continuous

f a i 1 ure. Study o f t a p e r s e c t i o n s has h el p ed c o n s i d e r a b l y i n t h e e l u c i d a t i o n o f t h e e f f e c t s o f rubbing a c t i o n ;

t h e i n i t i a l p o l i s h i n g seems t o be achieved by

smoothing o f t h e g r i n d i n g a s p e r i t i e s by p l a s t i c deformation as suggested by Westcott Fig.3.

[7,81 and Suh 191, Fig.3.

Subsurface metal l o g r a p h i c changes occur,

Heat produced by ru bb i ng a c t i o n appears t o temper t h e hard s t e e l .

Surface hardness may be pre serve d by r a p i d quenching from above the austeni t i s i n g t e m p e r a t ure by l u b r i c a n t o r b u l k m a t e r i a l . surface m a t e r i a l u s u a l l y contains cracks.

The w h i t e - e t c h i n g ,

hard

Thermal s o f t e n i n g may occur by con-

d u c t i o n o f h e a t i n t he subsurface are a away from l u b r i c a n t quenching a c t i o n . Local h e a t i n g , and subsequent quenching by l u b r i c a n t o r c o l d b u l k metal may be s u f f i c i e n t t o m e t a l l o g r a p h i c a l l y change and harden t h e s u r f a c e o f s o f t s t e e l , so t h a t b o t h h a r d and s o f t s t e e l when s c u f f e d ,

and t h e s c u f f e d m a t e r i a l ,

develop a s i m i l a r m e t a l l o g r a p h i c s t r u c t u r e and hardness.

The e l e c t r o n micro-

scope has r e v e a l e d t h e f i n e m e t a l l o g r a p h i c s t r u c t u r e o f rubbed m a t e r i a l , the n a t u r e o f t h e o r i g i n a l ground s u r f a c e and t he smooth r u n - i n s u r f a c e and has rev e a le d t h a t th e i n i t i a t i o n o f wear on a sub-microscopic s c a l e develops i n a s i m i l a r manner t o t h a t observed on a l a r g e r scale.

With l i g h t loads, t h e sur-

fa c e s a r e smoothed by p l a s t i c d ef orma t i o n o f t h e a s p e r i t i e s , m e t a l l u r g i c a l t r a n s f o r m a t i o n s occu r and s c u f f i n g appears t o i n i t i a t e from small scores. W i t h i n t h e scores on h a r d s t e e l , f e a t u r e s a r e v i s i b l e s u p p o r t i n g t h e Bowden and Tabor ( 2 ) mechanism o f f a i l u r e ,

Fig.4a.

P l a s t i c deformation and roughening o f

s o f t s t e e l i n d i c a t e o v e r - s t r e s s i n g o f t h e s u r f a c e m a t e r i a l which i s n o t incons i s t e n t w i t h t h e ideas o f B l o k [ l ] and Ming-Feng pre s s u r e (e.p.1

131, Fig.4b.

Extreme

a d d i t i v e s a r e used t o p r e v e n t metal t o metal c o n t a c t between

h e a v i l y loaded moving surf ace s.

D e t a i l e d e xamination o f surfaces by t h e

e l e c t r o n microscope has shown t h e b u i l d up o f p r o t e c t i v e f i l m s and p r o v i d e s exp e r im e n ta l e v ide nce f o r t h e g e n e r a l l y agreed a c t i o n o f these a d d i t i v e s ; r e a c t i o n w i t h s u r f a c e metal t o form adherent s u r f a c e f i l m s w i t h good boundary p r o p e r t i e s [27].

16

Fig.3a.

(x80)

F i n e l y qround

surface adjacent to run-in surface.

F i g . 3 ~ . ( ~ 7 5 0 0 ) Run-in s u r f a c e .

( ~ 7 5 0 0 ) F i n e l y ground

Fig.3b. surface.

Fig.3d.

(H= x 1 0 0 , V= ~ 1 1 0 0 )

Taper s e c t i o n t h r o u g h a s c u f f e d surface.

17

Fig.4b.

( ~ 7 5 0 0 ) I n i t i a t i o n of

Fig.4a.

(~7500) Initiation of

s l i d i n g wear on a ha rd s t e e l

s l i d i n g wear on a s o f t s t e e l

s u r fa c e .

surface,

2.4

ABRASIVE WEAR From an economic p o i n t o f view, a b r a s i v e wear caused by p l o u g h i n g o r gouging

o f a h a r d s u r f a c e , h ard p a r t i c l e s o r d e b r i s , a g a i n s t a r e l a t i v e l y s o f t e r mating s u r f a c e i s p r o b a b l y t he most s e r i o u s s i n g l e cause o f wear i n e n g i n e e r i n g practice.

There a r e i n d i c a t i o n s t h a t a b r a s i o n , p r i m a r i l y a crude machining

process,

i s r e l a t e d t o i n d e n t a t i o n hardness and hence t o s t a t i c y i e l d s t r e s s

[28,31].

Evidence o f e x t e n s i v e damage o r i g i n a t i n g from a small p a r t i c l e o f

d e b r i s has been r e p o r t e d [32,34],

Fig.5.

A p a r t i c l e o f hard b r i t t l e m a t e r i a l

may cause damage i n a s i n g l e pass t hro ug h t he area o f minimum f i l m t h i c k n e s s o f a b e a r in g .

However,

i n so passing,

i t may be rendered i n e f f e c t i v e due e i t h e r

t o breakdown i n t o s m a l l e r p a r t i c l e s o f dimensions s m a l l e r than t h e minimum o i l f i l m t h i c k n e s s o r by b ei n g comp l et el y embedded i n the s o f t e r o f t h e mating materials.

On t h e o t h e r hand a s o f t m a t e r i a l p a r t i c l e may work harden on

passage between r e l a t i v e l y moving su rf a ces.

I n gouging t h e s o f t e r b e a r i n g

m a t e r i a l s u r f a c e , t h e p a r t i c l e may, i f t h e b e a r i n g m a t e r i a l a l s o work hardens, be o n l y p a r t i a l l y embedded i n an e q u a l l y ha rd s u r f a c e area and become a source o f f u r t h e r damage t o t h e mat i ng su rf a ce.

18

a.

Large b e a r i n g machined

b.

by hard p a r t i c l e Fig.5.

Wire-wool

( ~ 1 5 0 ) S e c t i o n through t h e p a r t i a l l y embedded h a r d p a r t i c l e

type b e a r i n g f a i l u r e .

I f a b r a s i v e p a r t i c l e s a r e conveyed by a f l u i d stream t h e impact o f t h e

abr a s iv e p a r t i c l e s u r fa c e [35].

laden f l u i d w i l l g i v e r i s e t o e r o s i v e wear o f any i n t e r p o s e d

The e x t e n t and t ype o f wear depends upon t h e impinging angle of

t h e p a r t i c l e s and t h e d u c t i l i t y o f t h e su rf a ce.

2.5

FRETTING F r e t t i n g i s a s p e c i f i c f o rm o f wear wh i ch occurs when t h e r e i s s l i g h t

v i b r a t o r y movement between loaded su rf a ces i n c o n t a c t and which m a n i f e s t s i t s e l f by p i t t i n g o f t h e su rf a ces and t h e accu mul a t i on o f o x i d i s e d d e b r i s ,

Fig.6.

An

e l e c t r o n m ic r o s c o pi ca l st ud y of t h e i n i t i a t i o n o f f r e t t i n g [23] has p r o v i d e d s u p p o r t f o r t h e suggestion t h a t a t t h e o u t s e t i t i s no d i f f e r e n t from o t h e r forms o f wear, b u t t h a t t h e f i n e d e b r i s produced by t h e i n i t i a l damage due t o metal t o metal c o n t a c t and r e l a t i v e mo t i o n p r o v i d e s t h e s t a r t i n g p o i n t f o r a c u m u l a t i v e a b r a s i v e a c t i o n [36,38].

The d e b r i s , b e i n g l a r g e l y t h e o x i d e o f t h e

m e t a l s in v o lv e d , occupies a g r e a t e r volume t h an t h a t o f t h e metal destroyed and i n a l i m i t e d space,

t h i s can l e a d t o a p ressu re b u i l d up and seizure.

The form

and e x t e n t o f f r e t t i n g damage depends on t h e chemical n a t u r e o f t h e environment and on whether or n o t t he d e b r i s can escape or i s b u i l t up between t h e surfaces.

19

I t has been suggested t h a t t h e i n i t i a l d e b r i s has a p l a t e i k e form produced by a process of d e l a m i n a t i o n

[39].

The a c t u a l r a t e o f wear may slow down i f t h e d e b r i s ac s as a b u f f e r between t h e two s u r fa c e s .

Thus a p roce ss which i n i t i a t e s as adhesive wear may change

t o a b r a s i o n and t he n t h e wear r a t e may sl o w down due t o d e b r i s keeping t h e s u r f a c e s a p a r t . The f i n a l f a i l u r e may t he n be by f a t i g u e f r a c t u r e , c r a c k i n i t i a t i o n b e i n g e f f e c t e d by t h e s t r e s s r a i s i n g r o l e o f f r e t t i n g p i t s .

(~7500) I n i t i a t i o n

Fig.6a.

o f f r e t t i n g on h a r d s t e e l .

2.6

Fig.6b.

(x10,ODO)

Fretting

damage on a t i t a n i u m implant.

FLUID AND CAVITATION EROSION These wear mechanisms a r i s e f rom t h e impact o f f l u i d s a t h i g h v e l o c i t i e s .

F l u i d e r o s i o n damage caused by smal l drops o f l i q u i d can occur i n steam t u r b i n e s and f a s t f l y i n g a i r c r a f t t hro ug h t h e impact o f w a t e r d r o p l e t s causing p l a s t i c d e p r e s s ions i n t h e su rf a ce.

As t h e f l u i d f l o w s f r o m t h e deformed zone

i t can cause shear d e f o r m a t i o n i n p e r i p h e r a l areas and repeated deformation

causes a f a t i g u e t y p e o f damage by p i t t i n g and roughening o f t h e surface. C a v i t a t i o n e r o s i o n damage is caused by impact from t h e c o l l a p s e o f vapour or gas bubbles formed i n c o n t a c t w i t h a r a p i d l y moving o r v i b r a t i n g surface.

The

p h y s i c a l damage t o m e t a l s i s c h a r a c t e r i s e d by p i t t i n g s u g g e s t i v e o f a f a t i g u e origin.

The u l t i m a t e r e s i l i e n c e o f m a t e r i a l , measured as t h e energy t h a t can be

d i s s i p a t e d b e f o r e a p p r e c i a b l e de f o rmat i on and c r a c k i n g occurs, appears t o be an

20 i mp o r ta n t p r o p e r t y o f met al s i n c a v i t a t i o n r e s i s t a n c e .

2.7

ROLLING CONTACT FATIGUE The u s e f u l l i f e o f r o l l i n g elements i s l i m i t e d by s u r f a c e d i s i n t e g r a t i o n

p i t s o r f r a c t u r e b ei n g caused by a f a t i g u e process dependent upon t h e p r o perties o f the material,

[40,42].

t h e n a t u r e o f t h e l u b r i c a n t and t h e environment

The phenomenon i s c h a r a c t e r i s e d by t h e sudden removal o f s u r f a c e

m a t e r i a l o r f r a c t u r e due t o repeated a l t e r n a t i n g stresses.

The process has

t h r e e phases, p r e c o n d i t i o n i n g o f t he m a t e r i a l p r i o r t o c r a c k i n i t i a t i o n , c r a c k i n i t i a t i o n and c r a c k pro pa ga t i o n. R o l l i n g c o n t a c t f a t i g u e c r a c k s i n i t i a t e e i t h e r a t t h e s u r f a c e and propagate i n t o t h e m a t e r i a l , o r s t a r t below t h e s u r f a c e i n t h e area o f c a l c u l a t e d maximum H e r t z i a n s t r e s s and propagate towards t h e s u r f a c e depending upon p r e v a i l i n g circumstances.

The p r o p a g a t i o n o f s u r f a c e c r a c k s seems t o be c o n t r o l l e d by t h e

n a t u r e o f t h e l u b r i c a n t and t h e environment th e r o l l i n g d i r e c t i o n , Fig.7,

Fig.7.

[43].

The cracks, t r a n s v e r s e t o

propagate s t e a d i l y i n t o t h e m a t e r i a l a t an a c u t e

( ~ 1 5 , 0 0 0 ) R o l l ing

Fig.8.

Single f a i l u r e p t i n

contact f a t i g u e cracks i n

a mineral o i l l u b r i c a t e d

En31 b a l l b e a r i n g s t e e l .

bearing b a l l .

an g le t o t h e r o l l i n g d i r e c t i o n ,

t he n i n f l u e n c e d by t h e maximum sheai ng

s t r e s s e s , propagate p a r a l l e l t o t h e s u r f a c e t o detach s u r f a c e m a t e r i a l and form a p i t , Fig.8.

I f t h e environment i s d e l e t e r i o u s ,

hydrogen e m b r i t t l e m e n t m a t e r i a l , Fig.9,

f o r example,

i f i t leads t o

[ 4 4 , 4 5 ] t h e c r a c k s may propagate r a p i d l y , deep i n t o t h e

so t h a t f r a c t u r e ensues,

Fig.10.

21

Fig.9.

( ~ 7 5 ) Rapid crack

Fig.10.

Fractured non-

propagation i n a non-flammable

flammable f l u i d l u b r i c a t e d

f l u i d l u b r i c a t e d bearing b a l l .

bearing b a l l .

Subsurface cracks i n i t i a t e a t depths associated w i t h the region o f c a l c u l ated maximum H e r t r i a n s t r e s s and propagate p a r a l l e l t o the surface t o remove surface m a t e r i a l , Fig.11.

Crack i n i t i a t i o n may be f a c i l i t a t e d by b r i t t l e , non-

m e t a l l i c i n c l u s i o n s i n the stressed region which crack, break the m e t a l l i c c o n t i n u i t y and a c t as s t r e s s r a i s e r s . Owing t o r o l l i n g and s l i d i n g a c t i o n , mechanical and metallographic changes occur i n the stressed surface and immediate subsurface m a t e r i a l o f r o l l i n g elements [41,42,46].

The s t r u c t u r e o f conventional En 31 b a l l bearing s t e e l

c o n s i s t s o f f i n e l y dispersed carbide spheroids i n martensite, Fig.12.

The

m e t a l l o g r a p h i c a l l y changed m a t e r i a l o f the surface l a y e r i s devoid o f carbides as a r e s u l t o f h i g h c o n t a c t stresses and l o c a l h i g h temperature f l a s h e s causing s o l u t i o n o f the carbides f o l l o w e d by r a p i d quenching under pressure,

Fig.13.

Absorption o f gases from l u b r i c a n t breakdown may c o n t r i b u t e t o surface hardening and crack i n i t i a t i o n .

S t r i n g e r type carbides may form i n the subsurface area

o f c o n t a c t due t o a n n i h i l a t i o n o f the coarse carbides by p l a s t i c deformation, Fig.14.

Sections transverse t o the r o l l i n g d i r e c t i o n may reveal the presence

o f l o c a l i s e d areas o f tempered martensite; Fig.15.

cracks develop i n such areas,

There i s a t h r e s h o l d s t r e s s l e v e l above which metallographic change

22

Fig.11.

( ~ 3 7 5 ) Subsurface c r a c k

i n an En31 s t e e l b e a r i n g b a l l .

Fig.13.

(~12,000)

Deformed m e t a l -

Fig.12.

(~8000) S t r u c t u r e o f

En31 b a l l b e a r i n g s t e e l .

Fig.14.

(~15,000)

S t r i n g e r type

l o g r a p h i c a l l y changed, s p h e r i c a l

c a r b i d e s i n subsurface m e t a l l o g r a p h i c -

carbide free, surface material o f a

a l l y changed b a l l b e a r i n g s t e e l .

used b e a r i n g b a l l .

23 oc c u r s , s u g g e s t ing t h a t t h e changes may be due t o a y i e l d i n g o r p l a s t i c f l o w phenomenon r a t h e r than a tempering e f f e c t o r be i n d i c a t i v e o f o v e r l o a d o r a l o n g duration o f stressing. S i m i l a r s u b s u rf a ce changes and a s s o c i a t e d c r a c k s a r e found i n s e c t i o n s o f r o l l i n g elements p a r a l l e l t o t h e r o l l i n g d i r e c t i o n t o g e t h e r w i t h a s s o c i a t e d e l o n g a t e d w h i t e e t c h i n g a rea s o f i n crea sed hardness,

Fig.16.

The extreme hard-

ness o f t h e w h i t e e t c h i n g m a t e r i a l may be due t o a f i n e c e l l s i z e o r the almost c o l l o i d a l d i s p e r s i o n o f v e r y f i n e c a r b i d e s formed p o s s i b l y by s t r a i n induced p r e c i p i t a t i o n f o l l o w i n g s o l u t i o n o f coa rse c a r b i d e s , Fig.17.

F i g 15.

( ~ 5 7 5 ) Tempered m a r t e n s i t e i n

t h e s u b s u r f a c e a re a o f r o l l i n g c o n t a c t .

Fig.16

( ~ 1 1 0 0 ) Subsurface c r a c k s and

associated white etching m a t e r i a l .

t appears t h a t seve ral d i f f e r e n t modes o f r o l l i n g c o n t a c t f a t i g u e can cause

c r a ks t o n u c l e a t e and p rop ag at e i n de pe nd en t ly a t v a r i o u s r a t e s ;

thus the

phenomenon i s g r e a t l y i n f l u e n c e d by h i g h l y l o c a l i s e d c o n d i t i o n s .

Whilst the

ge n e r a l p r o p e r t i e s o f t h e b u l k m a t e r i a l a r e i mportant, s p e c i f i c aspects such as t h e s t e e lm a k in g process, gas c o n t e n t and c l e a n l i n e s s a r e a l s o e q u a l l y important [48,52].

The n a t u r e o f t h e l u b r i c a n t and t h e environment can have a dominant

e f f e c t on f a i l u r e . M a t e r i a l c o m bi n at i on [53] and m a t e r i a l l u b r i c a n t combination r e q u i r e c a r e f u l c o n s i d e r a t i o n t o ensure s a t i s f a c t o r y performance [54,55]. I n r o l l i n g c o n t a c t i n t h e absence o f a l u b r i c a n t , f a i l u r e occurs, n o t by the usual f a i l u r e mechanism b u t by e xcessi ve wear l i m i t i n g t h e u s e f u l l i f e due t o v i b r a t i o n and n o i s e tS6,571.

24

Fig.17.

( ~ 7 0 0 0 ) F i ne s t r u c t u r e i n

subsurface w h i t e e t c h i n g m a t e r i a l o f En31 s t e e l .

2.8

WEAR DETECTION AND ASSESSMENT One o f t h e most d i f f i c u l t problems i n e n g i n e e r i n g design i s t h e p r e d i c t i o n

and assessment o f p o s s i b l e wear.

An e q u a l l y i m p o r t a n t and d i f f i c u l t t a s k i s t h e

d e t e c t i o n o f wear d u r i n g t h e o p e r a t i o n o f machines.

Simple methods o f wear

measurement such as t h e d e t e r m i n a t i o n o f changes i n s u r f a c e topography by s t y l u s measurement and t h e d e t e r m i n a t i o n of w e i g h t loss have disadvantages. Machinery must be d i s m a n t l e d f o r t h e measurements t o be made and i n a c c u r a c i e s i n weig h t may a r i s e due t o o x i d a t i o n and a b s o r p t i o n o f l u b r i c a n t . As t h e h i s t o r y o f t h e wear process i s re corded i n t h e wear d e b r i s produced

[581 an a t t r a c t i v e method o f wear d e t e c t i o n and assessment i s contaminant a n a l y s i s o f t h e l u b r i c a n t used.

L u b r i c a n t s i n o p e r a t i n g mechanisms may be con-

v e n i e n t l y checked f o r t h e d e t e c t i o n o f wear by s p e c t r o g r a p h i c a n a l y s i s a l t h o u g h t h e method has some disadvantages [59] b e i n g i n some i n s t a n c e s r e l a t i v e l y b l i n d t o large particles.

A si mpl e d e v i c e such as a magnetic p l u g i n an o i l sump can

c o l l e c t f e r r o u s d e b r i s and i n d i c a t e wear o f a moving p a r t 1601 b u t some s e r i o u s damage may have o c c u r r e d b e f o r e d e b r i s l a r g e enough t o be d e t e c t e d has been collected. Wear p a r t i c l e s a r e unique, h a v i n g i n d i v i d u a l c h a r a c t e r i s t i c s which bear evidence o f t h e c o n d i t i o n s under which t he y were formed [8,58,61].

Careful

25 ex a m in a tio n o f t he morphology and d e t e r m i n a t i o n o f the composition o f wear p a r t i c l e s can t h u s y i e l d s p e c i f i c i n f o r m a t i o n concerning t h e surfaces from which they were produced, t h e mechanism o f t h e i r f o r m a t i o n and t h e o p e r a t i v e wear mode i n t h e system f r o m whi ch t he y were e x t r a c t e d . Ferrography [7,8,62,63]

i s a t e chn i qu e developed t o separate wear d e b r i s and

contaminant p a r t i c l e s c o n v e n i e n t l y f ro m a l u b r i c a n t f o r examination and a n a l y s i s . The duplex Fe r r o gra ph a n a l y s e r c o n s i s t s o f two p a r t i c l e separators, a standard a n a l y s e r and a d i r e c t r e a d i n g (DR) Ferrograph.

The DR Ferrograph i s a simple

i ns tr u m e n t used t o d et ermi n e t he amount and s i z e d i s t r i b u t i o n o f wear p a r t i c l e s i n a l u b r i c a n t sample f ro m which s i g n i f i c a n t numerical d a t a can be d e r i v e d [ 6 2 , 641.

When s u c c e ssi ve l u b r i c a n t samples y i e l d c o n s t a n t d e n s i t y readings i t may

be concluded t h a t t h e machine i s o p e r a t i n g n o r m a l l y and producing benign wear p a r t i c l e s a t a steady r a t e .

A r a p i d i ncre ase i n t h e q u a n t i t y o f p a r t i c l e s and

i n p a r t i c u l a r i n t h e r a t i o o f l a r g e t o small p a r t i c l e s i n d i c a t e s t h e i n i t i a t i o n o f a more severe wear process.

The use o f a simple e q u a t i o n p r o v i d e s a s i n g l e

f i g u r e f o r a c o mpa rat i ve s e v e r i t y o f wear index. u s i n g t h e b i c h r o m a t i c microscope [62,63],

F u l l Ferrographic a n a l y s i s

e l e c t r o n microscopy [62,651 and

h e a t i n g te c h n iq u es [66] may be used t o supplement t h e i n f o r m a t i o n from p a r t i c l e s p r e c i p i t a t e d a c c o r d i n g t o s i z e on a p rep are d s u b s t r a t e by the a n a l y t i c a l Ferrograph. P a r t i c l e s generated by d i f f e r e n t wear mechanisms have c h a r a c t e r i s t i c s which may be i d e n t i f i e d w i t h t h e s p e c i f i c wear mechanism [8,61,62,67]. t i c l e a t l a s has been p rep are d [68].

A wear par-

Rubbing wear p a r t i c l e s found i n t h e lub-

r i c a n t o f most machines have t h e f o r m o f p l a t e l e t s and i n d i c a t e normal permissi b l e ' w e a r , Fig.18.

C u t t i n g wear p a r t i c l e s t a k e t h e form o f m i n i a t u r e s p i r a l s ,

l oo p s and b e n t w i r e s s i m i l a r t o c u t t i n g s f r o m a machining o p e r a t i o n ,

Fig.19.

A c o n c e n t r a t i o n o f such p a r t i c l e s i s i n d i c a t i v e o f a severe a b r a s i v e wear process. The o p e r a t i v e regimes o f s l i d i n g wear can be c l a s s i f i e d by t h e p a r t i c l e s produced.

S i x regimes wh i ch ge ne rat e c h a r a c t e r i s t i c p a r t i c l e s have been

i d e n t i f i e d [13].

Free metal p a r t i c l e s a r e produced i n regimes 1,2 and 3 and

thes e regimes may be re cog ni se d by p a r t i c l e s i z e ,

r a n g i n g f r o m t h e small p a r -

t i c l e s o f regime 2 a s s o c i a t e d w i t h hydrodynamic l u b r i c a t i o n , m e t a l l i c p a r t i c l e s o f regime

3,

Fig.20,

Fig.18,to

large

which may v a r y i n s i z e up t o 2 5 0 ~ 1

when t h e shear mixed s u r f a c e l a y e r becomes u n s t a b l e and l o c a l i s e d adhesion oc cu r s . Three d i s t i n c t p a r t i c l e types, w i t h r o l l i n g bearing fatigue.

l ami n ar, s p h e r i c a l and chunks a r e a s s o c i a t e d

Laminar p a r t i c l e s a r e t h i n metal p a r t i c l e s up t o

50um i n major dimension c o n t a i n i n g h o l e s formed i n passage through the r o l l i n g contact.

Such p a r t i c l e s a r e generated t hro ug hout t h e l i f e o f t h e bearing.

T h e i r c o n c e n t r a t i o n increases w i t h t h e onset o f s p a l l i n g .

Spherical p a r t i c l e s ,

26

Fig.18.

( ~ 7 5 0 ) Rubbing wear particles and friction polymer.

( ~ 7 5 0 ) Optical micrograph Fig.19.

( ~ 4 5 0 ) Scanning electron micrograph

Cutting wear particles.

21

Fig.20.

a.

( ~ 7 5 0 )O p t i c a l

( ~ 4 0 0 ) Large m e t a l l i c wear p a r t i c l e .

micrograph Fig.21.

Fig.21,

b.

(x3,000) Scanning e l e c t r o n m ic r o g raph

Steel spherical p a r t i c l e s .

a r e generated w i t h i n a p r o p a g a t i n g f a t i g u e c r a c k and t h e i r d e t e c t i o n

g i v e s warning o f impending f a t i g u e f a i l u r e [ 6 9 ] .

X-ray energy a n a l y s i s i n t h e

scanning e l e c t r o n microscope can e s t a b l i s h t h a t t h e y a r e composed o f r o l l i n g bearing material.

F a t i g u e chunks c o n s t i t u t e t h e m a t e r i a l removed by s u r f a c e

d i s i n t e g r a t i o n and p i t f o r m a t i o n .

28 Fatigue p a r t i c l e s from a gear t o o t h although o f s i m i l a r dimensions d i f f e r from those from r o l l i n g bearings, a r e g e n e r a l l y o f i r r e g u l a r shape, f r e e from holes and have a smooth surface. f a t i g u e process progresses.

The number o f p a r t i c l e s increases as the

Larger f a t i g u e chunks o f gear m a t e r i a l a r e ind-

i c a t i v e o f surface d e t e r i o r a t i o n by p i t t i n g . S c u f f i n g o f gears causes an increase i n the number o f wear p a r t i c l e s which tend t o have a rough surface and an i r r e g u l a r shape.

As the s e v e r i t y of

s c u f f i n g increases, the l a r g e r p a r t i c l e s produced have surface s t r i a t i o n s indicative o f s l i d i n g action.

Owing t o the thermal e f f e c t s o f s c u f f i n g , p a r t i c l e s

may be p a r t i a l l y o x i d i s e d w i t h a range o f temper colours. As both a r t h r i t i c and a r t i f i c i a l j o i n t s a r e subjected t o wear, Ferrographic a n a l y s i s of synovial f l u i d i s a p o t e n t i a l l y a t t r a c t i v e method o f studying the mechanisms and b i o l o g i c a l responses t o wear i n human j o i n t s

2.9

1701.

CONCLUSIONS I n the present and foreseeable f u t u r e world economic s i t u a t i o n , m a t e r i a l and

energy conservation i s becoming i n c r e a s i n g l y important.

As wear i s a major

cause o f m a t e r i a l wastage, any reduction i n wear can e f f e c t considerable savings i n m a t e r i a l and the energy necessary f o r t h e i r production.

Thus increasing

emphasis w i l l be given t o methods o f wear c o n t r o l and prevention. The complex mechanisms o f wear, however, a r e n o t e a s i l y e l u c i d a t e d as the process by cumulative a c t i o n o b l i t e r a t e s evidence o f the important i n i t i a l stages o f damage.

Being an i n t e r d i s c i p l i n a r y subject, a m u l t i d i s c i p l i n a r y app-

roach i s r e q u i r e d f o r the i n v e s t i g a t i o n o f wear and the c o n t r o l l i n g f a c t o r s t o enable the m s t s u i t a b l e design t o be chosen embodying the best m a t e r i a l and the c o r r e c t l u b r i c a t i o n t o ensure minimum wear and s a t i s f a c t o r y s e r v i c e performance from moving mechanisms.

REFERENCES

1 Blok,H.,

Engineering, London,

1952, 173(4502)594.

and Tabor,D., "The F r i c t i o n and L u b r i c a t i o n o f Sol ds", Clarendon, Oxford. Ming-Feng,l., J.Appl.Phys., 1952, 23(9)1011-1019. Archard,J.F., J.Appl.Phys., 1953, 24(8)981-988. Archard,J.F., Research, 1952, 5(8)395-396. London. Rabinowicz,E., " F r i c t i o n and Wear o f Materials", 1966, J.Wiley Seifert,W.W. and Westcott,V.C., Wear, 1972, 21,2742. Scott,D., Seifert,W.W. and Westcott,V.C., Scient.Amer., 1974,

2 Bowden,F.P.

1950,

3 4 5 6 7 8

230(5)88-97. . .. 9

Suh,N.P.

and co-workers,

Wear,

1977, 44,l-162.

10 Suh,N.P., Wear, 1973, 25,111-124. 1 1 Jahanmir,S., Suh,N.P. and Abrahamson,E.P.,

Wear, 1974, 28, 235-249. 12 Suh,N.P. and Sridharan,P., Wear, 1975, 34,291-299. 13 Reda,A.A., Bowen,E.R. and Westcott,V.C., Wear, 1975, 34,261-273. 14 Beilby,G., "Aggregation and Flow o f Solids", Macmillan, London, 1921. Ludema,K.C. and Brainard,W.A., Wear, 1974, 30,365-375. 15 Bates,T.R.,

29

16 17 18 19 20

21 22

23

24 25

26 27 28 29 30

31 32

33

34 35 36 37 38 39 40 41 42

Kirk,J.A. and Swanson,T.D., Wear, 1975, 35, 63-67. Dumbleton,J.H. and Shen,C., Wear, 1976, 37, 279-289. Hirst,W., M e t a l l . Rev., 1965, 10, 145-172, I . M e t a l s , London. Kragelskii,I.V., " F r i c t i o n and Wear", 1955, B u t t e r w o r t h s , London. Scott,D. (Ed.), " T r e a t i s e on M a t e r i a l s Science and Technology, 13, "Wear", 1979, Academic Press, N.Y. Bickerman,J.J., Wear, 1976, 39, 1-14. B a r w e l l ,F.T., i n " T r e a t i s e on M a t e r i a l s Science and Technology, 13, "Wear", ( S c o t t , D.Ed.) 1979, 1-83, Academic Press, N.Y. Scott,D. and Scott,H.M., Proc. Conf. L u b r i c a t i o n and Wear, 1957, 609-612, I n s t . Mech. Engrs., London. Scott,D., Proc. I n s t . k c h . Engrs., 1967, 181 ( 3 L ) 39-51. Welsh,N.C., J . I n s t . M e t a l s , 1959, 88, 103-111. Barwel1,F.T. and Milne,A.A., B r . Pat. No. 732, 447, 1955. Milne,A.A., Scott,D. and Macdonald,D., Proc. Conf. L u b r i c a t i o n and Wear, 1957, 735-741, I n s t . Mech. Engrs., London. Wright,K.H.R., E n g i n e e r i n g , London, 1961, 191 (4956) 546-547. Kruschov,M.M. and Babichev, , Akad. Nauk. SSR, 1960, 66-76. NEL T r a n s l a t i o n 893, N a t i o n a l E n g i n e e r i n g L a b o r a t o r y , East K i l b r i d e , Glasgow. Kruschov,M.M., Wear, 1974, 28, 69-88. Moore,M.A., Wear, 1974, 28, 59-68. Barwel1,F.T. and Scott,D., Proc. 4 t h L u b r i c a t i o n and Wear Convention, 1966, 277-297, I n s t . Mech. Engrs., London. Hother-Lushington,S., Proc. 4 t h L u b r i c a t i o n and Wear Convention, 1966, 243-252, I n s t . Mech. Engrs., London. Dawson,P.H. and F i d l e r , F . , Proc. I n s t . Mech. Engrs., 1965/66, 180, 513-530. Engel ,P.A., "Impact Wear o f M a t e r i a l s " , 1976, E l s e v i e r , Amsterdam. Wright,K.H.R., Proc. I n s t . Mech. Engrs., 1952/53, lB(l1) 556-574. Godfrey,D. and Bailey,J.M., L u b r i c . Engng., 1954, 10, 155. Waterhouse,R.B., " F r e t t i n g Corrosion", 1972, Pergamon, Oxford. Waterhouse,R.B,, i n "Wear o f Metals", 1977, 55, ASME, N.Y. Scott,D., i n " F a t i g u e i n R o l l i n g Contact", 1963, 103-115, I n s t . Mech. Engrs., London. Scott,D., i n "Low A l l o y Steels", 1965, 203-209, I.S. I., London. Scott,D., R o l l i n g Contact F a t i g u e i n Wear, i n (Sc0tt.D. Ed.) " T r e a t i s e on M a t e r i a l s Science and Technology", 1979, 13, 321-361, Academic Press,

N.Y.

43

Scott,D.,

Proc.

I n s t . Mech. Engrs. Conf. L u b r i c a t i o n and Wear,

1957,

463-468. 44 45 46

Grunberg,L. and Scott,D., J. I n s t . P e t r o l . , 1958, 44 (419), 406-410. Grunberg,L., Scott,D. and Jamieson,D.T., P h i l . Mag. 1963, 8(93) 1553-1568. Scott,D., Loy,B. and MilIs,G.H., Proc. l n s t . Mech. Engrs., 1967,

47 48

Scott,D. Scott,D.

181 (31) 94-106. and McCullagh,P.J. Wear, 1973, 24, 235-242. and B l a c k w e l l , J . , Proc. I n s t . Mech. Engrs.,

1964, 178, (3N)

81-89. 49 50

Scott,D. Scott,D.

and B l a c k w e l l ,J., Wear, and B l a c k w e l l , J . , Proc.

1978, 46, 273-279. I n s t . Mech. Engrs.,

1968, 182(3N)

239-242. 51 52 53 54 55 56 57 58 59

Scott,D. and McCullagh,P.J., Wear, 1975, 34, 222-237. Scott,D. and McCullagh,P.J., Wear, 1973, 25, 339-344. Scott,D., i n " R o l l i n g Contact F a t i g u e and Performance o f L u b r i c a n t s " , (Tourret,R. and Wright,E.P. Eds.), 1977, 3-17, Heydon and Sons, London. Scott,D. and Blackwell,J., Proc. I n s t . Mech. Engrs., 1966, 180, (3K) 32-37. Scott,D., Proc. I n s t . Mech. Engrs., 1968, 182(3J) 116-123. Scott,D., Proc. I n s t . Mech. Engrs., 1967, 182(3A) 325-341. Scott,D. and Blackwell,J., NEL Report 278, 1967, N a t i o n a l Engineering L a b o r a t o r y , East K i 1 b r ide. Scott,D., Proc. I n s t . Mech. Engrs., 1976, 189/75, 623-633. Seifert,W.W. and Westcott,V.C., Wear, 1973, 23, 239-249.

30

60 61 62

Col l a c o t t , R . A . , "Mechanical F a u l t Diagnos s and C o n d i t i o n M o n i t o r i n g " , 1977, Chapman and H a l l , London. Scott,D., Wear, 1975, 34, 15-22. Tribology I n t . , Bowen,E.R., Scott,D., Seifert,W.W. and Ve t c o t t , V . C . ,

1976, 9 ( 3 ) 109-115. 63 64 65 66 67 68 69 70

Scott,O. and Westcott,V.C., Proc. E u r o t r i b 77, 1977, Band 1, paper 70, 1-6 Westcott,V.C., Naval Research Reviews, 1977 (March) 1-8, O f f i c e o f Naval Research, Wash i n g t o n . Scott,D. and Mills,G.H., i n "Scanning E l e c t r o n Microscopy", 1974, P a r t I V , 838-888, I.I.T., Chicago. B a r w e l l , F . T . , Bowen,E.R. and Westcott,V.C., Wear, 1977, 44, 163-171. Ruff,W. Wear, 1977, 42, 49-62. Bowen,E.R. and Westcott,V.C., "Wear P a r t i c l e A t l a s " , 1976, Foxboro/Transo n i c s I n c . , Mass, U . S . A . Scott,D. and Mills,G.H., Wear, 1973, 24, 235-242. Mears,D.C., Wear, 1978, 50, 115-126.

31

'{

SELECTION OF BEARINGS

M.J.

NEALE

Michael Neale and A s s o c i a t e s L t d .

3.1

INTRODUCTION The s e l e c t i o n o f an a p p r o p r i a t e t y p e o f b e a r i n g , f o r use i n a p a r t i c u l a r

a p p l i c a t i o n , i s a d e c i s i o n t h a t i s u s u a l l y made v e r y e a r l y i n a design process. A t t h a t stage, v e r y d e t a i l e d i n f o r m a t i o n on b e a r i n g performance i s n o t u s u a l l y necessary, and what i s r e a l l y r e q u i r e d i s broad guidance on t h e i m p o r t a n t _ L . .

a c t e r i s t i c s o f t h e v a r i o u s types.

The i n f o r m a t i o n presented i n t h i s s e c t i o n

ntended t o meet t h i s requirement.

3.2

B E A R I N G TYPES

The b a s i c f u n c t i o n o f b e a r i n g s i s t o a l l o w a l o a d t o be t r a n s m i t t e d between two s u r f a c e s which a r e i n r e l a t i v e motion.

There a r e t h r e e main t y p e s o f

b e a r i n g as shown i n F i g u r e 1, and these a r e p l a i n b e a r i n g s , r o l l i n g b e a r i n g s and f l e x u r e s .

In p l a i n bearings the load i s transmitted over a considerable

area, w h i l e i n r o l l i n g b e a r i n g s t h e area a c t u a l l y i n c o n t a c t , and t r a n s m i t t i n g t h e load,

i s very small.

The t h i r d t y p e depends on t h e use o f f l e x i b l e com-

ponents and i s o n l y s u i t a b l e f o r o s c i l l a t o r y movement. I t can be seen f r o m F i g u r e 1 t h a t t h e r e a r e f i v e b a s i c p r i n c i p l e s b e h i n d t h e o p e r a t i o n o f t h e v a r i o u s types and these a r e : ( i ) To p e r m i t t h e two s u r f a c e s t o r u b t o g e t h e r and t o a r r a n g e t h e s u r f a c e p r o p e r t i e s so t h a t s e i z u r e o r excess f r i c t i o n does n o t o c c u r and so t h a t an a c c e p t a b l e r a t e o f wear i s o b t a i n e d .

In practice t h i s i s usually

achieved by t h e c h o i c e o f m a t e r i a l s w i t h s u i t a b l e b u l k p r o p e r t i e s o r by t h e use of some f o r m o f s u r f a c e c o a t i n g , which may e i t h e r be a p p l i e d i n advance o r a l l o w e d t o form i n s i t u . ( i i ) To keep t h e s u r f a c e s separated by a f i l m o f f l u i d , so t h a t t h e r e l a t i v e movement can o c c u r w i t h i n t h e f i l m .

To do t h i s t h e f l u i d must be

maintained a t a s u f f i c i e n t pressure t o h o l d the surfaces apart against

32

L J Z h

1

I

The basic problem

To transmit load between two surfaces in relative motior!

Plain rubbing bearings

The surfaces are allowed to rub together

' J(

( (((),

, ,j ]

s-4

tn 0

.-C G

Plain fluid film bearings

Pressure In the film keeps the surfaces apart

Magnetic or electrostatic bearings

Surfaces held apart by mutual repulsion

Rolling element bearings

Surfaces separated by rolling elements of circular cross section

.-r

Rocker pods

The pivot point moves as the bearing rocks

8 a

Knife edges

The sharp edge gives the minimum pivot point movement

Rubber bearings

Movement by elastic deflection of the rubber

Crossed strip

Rotation about the crossover point by strip deflection

.-Q

L

E

tn

m L

m

Q

.--E

I

I

tn

!3 !! X

Q)

h Cables or torsion rods

Fig.1

Movement permitted by elastic deflection

Various types o f b e a rin g

the a p p l i e d load.

T h i s p r e s s u r e may be o b t a i n e d by f e e d i n g i n f r o m an

e x t e r n a l h i g h p r e s s u r e source, o r may be g e n e r a t e d w i t h i n t h e f i l m by r e l a t i v e movement o f s u i t a b l y shaped s u r f a c e s f l o o d e d w i t h a v i s c o u s fluid.

33 ( i i i ) One s u r f a c e may be a l l o w e d t o r o l l on t h e o t h e r .

T h i s , however,

produces an i n t e r a c t i o n between t h e a n g u l a r and t r a n s l a t i o n a l movements o f t h e a d j a c e n t components, which i s d e t e r m i n e d by t h e shape o f t h e r o l l i n g s u r f a c e s , and i n p r a c t i c e may n o t always a l l o w t h e r e q u i r e d degrees o f freedom.

To overcome t h i s p r o b l e m t h e r o l l i n g s u r f a c e s a r e

o f t e n p e r m i t t e d t o s l i d e a s w e l l as r o l l , such as i n g e a r t e e t h , o r t h e p r o b l e m may be overcome by i n s e r t i n g a t h i r d element between t h e s u r f a c e s o f t h e two o r i g i n a l components as i n a r o l l i n g element b e a r i n g . ( i v ) To produce a r e p u l s i v e f o r c e between t h e s u r f a c e s by magnetic o r e l e c t r o s t a t i c means. ( v ) To p o s i t i o n a f l e x i b l e member between t h e two components which can d e f l e c t t o a l l o w a r e l a t i v e o s c i l l a t i n g movement t o o c c u r between them. S u i t a b l e members can be formed by f i l l i n g most o f t h e space between t h e s u r f a c e s w i t h an adequate t h i c k n e s s o f e l a s t o m e r i c m a t e r i a l o r by u s i n g t h i n connecting ligaments o f higher strength materials.

3.3

PERFORMANCE OF V A R I O U S TYPES OF BEARING Since t h e a b i l i t y t o t r a n s m i t a l o a d w i t h r e l a t i v e movement i s t h e b a s i c

f u n c t i o n o f a b e a r i n g , a s t u d y o f t h e r e l a t i o n s h i p between t h e a l l o w a b l e l o a d and speed f o r b e a r i n g s o f v a r i o u s s i z e s and t y p e s s h o u l d p r o v i d e a c o n v e n i e n t s t a n d a r d f o r t h e comparison o f t h e i r performance. For t h e d e t e r m i n a t i o n o f l o a d and speed c h a r a c t e r i s t i c s o f t h e v a r i o u s b e a r i n g t y p e s , t h e f i r s t i n f o r m a t i o n t h a t r e q u i r e s t o be e s t a b l i s h e d i s t h e , a p p r o x i m a t e shape o f t h e r e l a t i o n s h i p , so t h a t a g e n e r a l comparison can be made between t h e v a r i o u s t y p e s .

The b a s i c forms o f t h e s e r e l a t i o n s h i p s can be

d e r i v e d f r o m t h e p h y s i c a l p r i n c i p l e s w h i c h govern t h e o p e r a t i o n o f t h e v a r i o u s b e aring types.

T h i s i s d i s c u s s e d below and t h e r e s u l t i n g performance c u r v e s

a p p l i e d t o j o u r n a l b e a r i n g s compared i n F i g . 2 .

3.3.1

Rubbing B e a r i n g s

I n a b e a r i n g w h i c h o p e r a t e s by p e r m i t t i n g t h e two s u r f a c e s t o r u b t o g e t h e r , t h e p h y s i c a l l i m i t a t i o n s on performance a r e t h e r i s k o f o v e r h e a t i n g and s e i z u r e and t h e p o s s i b i l i t y o f e x c e s s i v e wear. The g e n e r a t i o n o f h e a t a t t h e r u b b i n g s u r f a c e s a r i s e s f r o m t h e movement a g a i n s t t h e f r i c t i o n a l r e s i s t a n c e o f t h e c o n t a c t , and t h i s h e a t has t o be cond u c t e d away a l o n g h e a t - f l o w p a t h s , o f which t h e a r e a w i l l be a p p r o x i m a t e l y p r o p o r t i o n a l t o t h e p r o j e c t e d b e a r i n g a r e a A. The r i s k o f o v e r h e a t i n g a t a s l i d i n g speed V i s t h e r e f o r e a p p r o x i m a t e l y proportional t o

'r

-o r

'PV

34

Pressure limit

PV limit

RPM

Fatigue limit

N

RPM

(al Rubbing bearings

/-Available

N

(bl Rolling bearings

pressure limits Fall due ta effect of

3

Fall d k t o effect

of speed on vixosity

V

8

_1

Film thickness limit RPM

RPM

N

fcl Externally pressurized film

I

N

( d l Pressure fed film

r

Pressure limit

i0)i

Film thickness limit

(rPV limit

8 limit RPM

N

(el Self- contained film

FIGURE 2

RPM

N

( f l Porous metal

Performance of various types of journal bearing flog W plotted against log N )

35 When two s u r f a c e s a r e i n r u b b i n g c o n t a c t , t h e volume o f m a t e r i a l worn f r o m the rubbing surfaces a f t e r s l i d i n g a distance x w i t h a load W i s approximately p r o p o r t i o n a l t o Wx. The wear volume i s , however, o f l i t t l e s i g n i f i c a n c e i n t h e performance o f a b e a r i n g , and t h e depth wear r a t e i s much more r e l e v a n t as a design f a c t o r , s i n c e t h i s i s a measure o f t h e r a t e a t which slackness i n t h e assembly i s l i k e l y t o be produced. The d e p t h wear r a t e w i l l be a p p r o x i m a t e l y p r o p o r t i o n a l t o wx

o r PV At Thus b o t h t h e p h y s i c a l l i m i t a t i o n s t o t h e performance o f r u b b i n g b e a r i n g s i n d i c a t e t h a t ' t h e s e v e r i t y o f t h e b e a r i n g o p e r a t i o n i s r e l a t e d t o PV ( b e a r i n g p r e s s u r e x s l i d i n g speed).

Fig.2a

a rubbing type o f journal bearing.

shows t h e shape o f t h e load-speed c u r v e f o r T h i s i s a composite c h a r a c t e r i s t i c i n c l u d i n g

a PV l i m i t and a l s o a maximum a l l o w a b l e p r e s s u r e l i m i t a s s o c i a t e d w i t h t h e r i s k o f f a t i g u e o r e x t r u s i o n o f t h e b e a r i n g m a t e r i a l , which i s f r e q u e n t l y nonmetal 1 i c .

3.3.2

R o l l e r Bearings

Owing t o t h e c o n c e n t r a t e d c o n t a c t s t r e s s e s i n t h i s t y p e o f b e a r i n g t h e u l t i m a t e l i m i t on performance a r i s e s from t h e p o s s i b l e f a t i g u e o f t h e r o l '"9 elements o r races, a l t h o u g h i n p r a c t i c e r o l l i n g b e a r i n g s o f t e n f a i l as we 1 from l a c k o f p r o p e r l u b r i c a t i o n , d i r t c o n t a m i n a t i o n and u n s u i t a b l e l o a d i n g .

f

f a t i g u e i s taken as t h e performance l i m i t i t i s reasonable t o assume t h a t the l i f e o f a r o l l i n g c o n t a c t b e a r i n g measured i n r e v o l u t i o n s i s p r o p o r t i o n a l t o (l/W)3.

Consequently f o r a g i v e n l i f e W

shown i n Fig.2b.

a

N-1'3

and t h i s c h a r a c t e r i s t i c i s

As b e f o r e , t h e a c t u a l maximum l o a d i s a l s o l i m i t e d by t h e cap-

a c i t y o f t h e b e a r i n g t o c a r r y s t a t i c l o a d which i n t h i s case i s l i m i t e d by t h e r e s i s t a n c e o f t h e races t o b r i n e l l i n g by t h e r o l l i n g elements.

3.3.3

F l u i d F i l m Bearings

I f these a r e o f t h e e x t e r n a l l y p r e s s u r i z e d o r h y d r o s t a t i c t y p e , t h e l o a d c a p a c i t y i s p r i m a r i l y dependent on t h e a v a i l a b l e supply p r e s s u r e , and r u n n i n g speed has v e r y l i t t l e e f f e c t , a l t h o u g h w i t h l i q u i d l u b r i c a t i o n a s l i g h t f a l l i n l o a d c a p a c i t y may o c c u r a t h i g h e r speeds owing t o t h e r e d u c t i o n i n l u b r i c a n t v i s c o s i t y caused by h i g h e r temperatures.

The r e s u l t i n g load-speed c h a r a c t e r i s t i c

i s t h e r e f o r e as shown i n Fig.2c. With t h e hydrodynamic t y p e o f f l u i d f i l m b e a r i n g , t h e l o a d c a p a c i t y increases w i t h speed p r o v i d e d t h a t t h e f i l m i s k e p t adequately s u p p l i e d w i t h l u b r i c a n t , a l t h o u g h t h e r e i s n o r m a l l y a tendency w i t h l i q u i d l u b r i c a n t s f o r t h e l o a d capa c i t y t o f a l l away a t t h e h i g h e s t speeds owing t o t h e h e a t i n g o f t h e l u b r i c a n t .

36 T h i s g i v e s a c h a r a c t e r i s t i c o f t h e g en era l shape shown i n Fig.2d. I n o r d e r t o keep t h e f i l m f u l l y s u p p l i e d w i t h l u b r i c a n t ,

t h i s has t o be f e d

under p r e s s u r e , which i n t u r n r e q u i r e s some f o r m o f l u b r i c a n t supply system. I t i s n o t always co nve ni e nt o r economical t o h a v e t o i n c l u d e such a supply system and c o n s eq ue nt l y many hydrodynamic f l u i d f i l m b e a r i n g s a r e used w i t h self-contained l u b r i c a n t supplies.

I n the l a r g e r sizes, the o i l i s g e n e r a l l y

c o n t a in e d i n a sump below t h e s h a f t and l i f t e d by a r i n g o r d i s c , w h i l e i n t h e s m a l l e r s i z e s i t may be f e d f r o m an o i l - s o a k e d pad o r r e t a i n e d i n t h e b e a r i n g s t r u c t u r e by making i t porous.

These methods o f o i l f e e d i n g have a lower

e f f i c i e n c y than a p ressu re f e d arrangement and r e s u l t i n t h e b e a r i n g o p e r a t i n g w i t h a l o a d - c a r r y i n g f i l m o f reduced c i r c u m f e r e n t i a l l e n g t h and t h e r e f o r e o f reduced l o a d c a p a c i t y .

I t i s a l s o u s u a l l y found w i t h these systems t h a t t h e

volume o f o i l d e l i v e r e d p e r r e v o l u t i o n of t h e s h a f t decreases w i t h speed and consequently t h e l o a d - c a r r y i n g c a p a c i t y a l s o decreases w i t h speed. I n a d d i t i o n t o t h i s e f f e c t , t h e absence o f a l u b r i c a t i o n system means t h a t a l l t h e h e a t g e ne rat ed has t o be d i s s i p a t e d d i r e c t l y t o t h e surroundings, and w i t h a maximum a l l o w a b l e t e mpe rat ure f o r a l ong l i f e o f m i n e r a l o i l l u b r i c a n t s , p l a c e s a l i m i t on t h e maximum a l l o w a b l e speed. T,hese v a r i o u s e f f e c t s r e s u l t i n load-speed c h a r a c t e r i s t i c s f o r b e a r i n g s o f t h i s t y p e which a r e o f t h e ge ne ral shape shown i n Figs.2e and 2 f .

3.3.4

F l e x i b l e Members

B e a r in g s o f t h i s t ype , wh i ch use e i t h e r el astomers i n shear o r h i g h t e n s i l e l i ga m e n t s i n bending t o a l l o w an o s c i l l a t i n g motion t o occur, a r e i n many cases p h y s i c a l l y l i m i t e d i n performance by t h e f a t i g u e s t r e n g t h o f t h e m a t e r i a l o r i t s bonding on t o th e a d j a c e n t components.

The s t r e s s e s a r i s e b o t h from t h e

d e f l e c t i o n and f r o m t h e a p p l i e d l o ad and a r e u s u a l l y a d d i t i v e a t some c r i t i c a l r e g i o n o f t h e assembly, w i t h t h e r e s u l t t h a t t h e a l l o w a b l e loads tend t o decrease w i t h t h e p e r m i t t e d a n g l e o f movement.

Blocks o f e l a s t o m e r i c m a t e r i a l used

i n compression do n o t have t h i s a d d i t i v e s t r e s s c o n d i t i o n , b u t i n t h i s case t h e a l l o w a b l e d e f l e c t i o n i s l i m i t e d by t h e b l o c k t h i c k n e s s , which i n t u r n l i m i t s t h e maximum l o a d i f t h e assembly i s t o be s t a b l e a g a i n s t excessive b u l g i n g o r buckling.

T h i s can be overcome t o some e x t e n t by t h e use o f s t i f f e n i n g p l a t e s

i n t h e e la s t o m e r a rran ge d t o be p a r a l l e l t o t h e r e q u i r e d d i r e c t i o n o f motion, b u t even when these a r e i n c o r p o r a t e d t h e maximum d e f l e c t i o n i s s t i l l l i m i t e d by t h e amount t h a t t h e l o a d can be p e r m i t t e d t o be o f f s e t . I t can t h e r e f o r e be g e n e r a l l y assumed t h a t t h e l o a d - d e f l e c t i o n c h a r a c t e r i s t i c

o f f l e x i b l e b e a r in gs w i l l g e n e r a l l y show a decrease i n a l l o w a b l e l o a d as t h e d e f l e c t i o n i s increased.

31

3.4

SELECTION OF A SUITABLE BEARING The c h a r a c t e r i s t i c r e l a t i o n s h i p s between l o a d and'movement f o r t h e v a r i o u s

typ e s o f b e a r i n g s can be compared w i t h t h e b e a r i n g performance r e q u i r e d i n v a r i o u s a p p l i c a t i o n s and used as a g ui d e t o t h e s e l e c t i o n o f a s u i t a b l e t y p e o f bea r in g . Any p o s s i b l e a p p l i c a t i o n f o r a b e a r i n g i n a machine o r s t r u c t u r e w i l l have some f o r m o f c h a r a c t e r i s t i c r e l a t i o n s h i p between t h e t y p e o f l o a d t o be c a r r i e d and t h e movement t o be a l l owed .

I n p r a c t i c e an important f e a t u r e i s whether

t h e l o a d and movement a r e n o m i n a l l y steady o r whether they v a r y i n some c y c l i c manner.

T h i s can g i v e r i s e t o f o u r p o s s i b l e combinations o f l o a d and movement

as i n d i c a t e d w i t h examples i n Table 3.1. P r o b a b ly t h e g r e a t e s t number o f b e a r i n g a p p l i c a t i o n s a r e o f t h e u n i d i r e c t i o n a l l o a d and c o n t i n u o u s movement t y p e and i n t h i s c a t e g o r y t h e way i n which t h e b e a r i n g l o a d v a r i e s w i t h r o t a t i o n a l speed, d u r i n g t h e o p e r a t i o n o f t h e machine, i s an i m p o r t a n t f a c t o r i n b e a r i n g s e l e c t i o n and design. u n i d i r e c t i o n a l load s have been su b-d i vi de d

For t h i s reason t h e

i n Table 3.1 t o draw a t t e n t i o n t o

t h i s situation. Ta b le 3.1

Examples o f v a r i o u s t ype s o f l o a d and movement p a t t e r n s

Type o f lo a d Unidirectional Cons t a n t

Type o f movement

Examples

Continuous

T urbine j o u r n a l b e a r i n g s

Rising w i t h speed

Continuous

Marine gearbox p i n i o n b e a r i n g s

Falling with speed

Continuous

H y d r a u l i c motor b e a r i n g s

Un i d ir e c t iona 1

Osc i 1 l a t o r y

B r i d g e support b e a r i n g s G r i n d i n g machine t a b l e s

M u l t i d i r e c t i o n a l Continuous

Pi st on-engine c r a n k s h a f t b e a r i n g s

Multidirectional Oscillatory

Linkage b e a r i n g s

3.4.1

A p p l i c a t i o n s w i t h U n i d i r e c t i o n a l Load and Continuous Movement

T h i s i s t h e f i r s t c a t e g o r y o f b e a r i n g a p p l i c a t i o n s l i s t e d i n Table 3.1 and t h e v a r i o u s t y p e s o f b e a r i n g which c o u l d be used a r e t h e rubbing, r o l l i n g element and f l u i d - f i l m types.

F i g . 2 shows t h a t these v a r i o u s types o f b e a r i n g

have q u i t e d i f f e r e n t load-speed c h a r a c t e r i s t i c s . I n f a c t , examples a r e g i v e n i n Table

3.1 o f v a r i o u s a p p l i c a t i o n s w hich a l s o

have s e v e r a l forms o f r e l a t i o n s h i p between l o a d and speed, and i d e a l l y a b e a r i n g s h o u ld be s e l e c t e d w i t h a matching c h a r a c t e r i s t i c .

T h i s k i n d o f app-

roach t o t h e problem i n d i c a t e s t h a t t u r b i n e j o u r n a l b e a r i n g s should i d e a l l y be o f

38 the e x t e r n a l l y p r e s s u r i z e d o r h y d r o s t a t i c t ype, w h i l e an a p p l i c a t i o n such as marine gearbox p i n i o n b e a r i n g i s p a r t i c u l a r l y w e l l s u i t e d t o t h e simple type o f hydrodynamic b e a ri ng . practice,

W h i l e t h i s l a t t e r example i s i n l i n e w i t h c u r r e n t

t u r b i n e s a r e n o t a t p r e s e n t u s i n g t he h y d r o s t a t i c type o f b e a r i n g

except i n those used t o d r i v e high-speed d e n t a l d r i l l s .

T h i s may be an example

o f a design s i t u a t i o n where a p a r t i c u l a r t yp e o f machine i s developed on the b a s i s o f a workable b u t n o t i d e a l f orm o f b e a r i n g design and i t then r e q u i r e s c o n s i d e r a b l e commercial courage t o make t h e necessary change on a v i t a l component i n a machine o f such h i g h c a p i t a l val u e.

From t h e economic p o i n t o f

view, however, h y d r o s t a t i c b e a r i n g s a r e p a r t i c u l a r l y a p p l i c a b l e i n s i t u a t i o n s where a source of h i g h p r e s s u r e f l u i d i s a l r e a d y a v a i l a b l e , and t h i s i s i n f a c t t h e case w i t h a steam t u r b i n e .

Problems o f e r o s i o n may occur i f t h e steam i s

al lo we d t o condense i n t h e b e a r i n g c l e a r a n c e b u t t h i s problem should n o t be

in supe r a b 1 e

.

H y d r a u l i c motors were suggested as an example o f an a p p l i c a t i o n i n w hich t h e l oa d decreased w i t h i n c r e a s i n g speed, a l t h o u g h t h i s c h a r a c t e r i s t i c tends t o be common t o any machine d r i v e n by a source o f a p p r o x i m a t e l y c o n s t a n t power.

For

such an a p p l i c a t i o n , a r o l l i n g c o n t a c t b e a r i n g would appear t o be i d e a l i n t h a t i t has a matching c h a r a c t e r i s t i c ,

as w e l l as a low s t a r t i n g f r i c t i o n which i s

u s u a l l y a l s o r e q u i r e d i n t h i s p a r t i c u l a r t yp e o f a p p l i c a t i o n .

A rubbing type o f

b e a r i n g would also appear t o be p o s s i b l e , b u t i n f a c t t h e loads and speeds a l l o w a b l e w i t h t h i s t y p e o f b e a r i n g a r e c o n s i d e r a b l y below t h e corresponding v a lu e s f o r t h e r o l l i n g - c o n t a c t

t ype .

A lt h o u g h t h i s t ech ni q ue o f matching t h e f o r m o f t h e load-speed r e l a t i o n s h i p o f t h e a p p l i c a t i o n w i t h t h a t o f v a r i o u s b e a r i n g s i s a u s e f u l guide i n b e a r i n g selection,

i t i s s t i l l necessary t o c o n s i d e r t h e a c t u a l values o f l o a d and

speed which can be c a r r i e d by d i f f e r e n t s i z e s o f b e a r i n g s o f t h e v a r i o u s types, i n o r d e r t o be c e r t a i n t h a t t h ey a r e s u i t a b l e .

A c o n v e n ie n t way o f d oi n g t h i s i s t o p l o t t h e performance o f t h e v a r i o u s ty p e s o f b e a r i n g on one diagram so t h a t comparisons can be made, and i f t h i s i s done on l o g a r i t h m i c axes, t h e whole span o f e n g i n e e r i n g loads and speeds can be covered. T h i s t e c h n iq u e can be used t o show t h e performance o f s t e a d i l y loaded j o u r n a l b e a r i n g s w i t h c o n t i n u o u s l y r o t a t i n g s h a f t s , and Fig.3 shows t h e t y p e o f diagram which r e s u l t s . Fig.3

i s o n l y i nt en de d t o g i v e b roa d guidance b u t i t does show t h e general

trends q u i t e c l e a r l y .

I t indicates,

f o r example, t h a t f l u i d - f i l m p l a i n b e a r i n g s

a r e the b e s t ty p e t o use a t h i g h speeds and t h a t t h i s i s p a r t i c u l a r l y t r u e i n th e case o f a l l s h a f t s g r e a t e r t h an about 100 mm diameter.

This f i g u r e a l s o

i n d i c a t e s t h a t t he r u b b i n g t ype o f b e a r i n g i s o n l y r e a l l y s u i t a b l e f o r low

0.01

0.1

1

10

100

1000

10000

100000

Shaft speed ,n ( rev/s)

FIGURE 3

Indication of the performance of common types of journal bearings for shafts of various diameters

40 r o t a t i o n a l speeds, and a t a n y t h i n g more than one o r two hundred r e v o l u t i o n s p e r minute a r o l l ing bearing

3.4.2

s usually a better solution.

A p p l i c a t i o n s w i t h O s c i l l a t i n g Movement

The r u b b i n g t y p e o f b e a r i n g i s , however, p a r t i c u l a r l y good f o r a p p l i c a t i o n s w i t h o s c i l l a t i n g movement, s i n c e i t does n o t need t o b u i l d up an o i l f i l m each time i t s t a r t s t o move, and w i t h small a n g l e s o f o s c i l l a t i o n i t cannot b r i n e l l i n t h e same way as a r o l l i n g b e a r i n g can do under these circumstances. I t i s t h e r e f o r e u s e f u l t o c o n s i d e r t h e p r o b a b l e performance o f t h e v a r i o u s

types o f r u b b i n g b e a r i n g under c o n d i t i o n s o f o s c i l l a t i n g movement.

As i n t h e

case o f c o n t i n u o u s r o t a t i o n , t h e performance w i l l be l i m i t e d by t h e maximum a l l o w a b l e l o a d p r e s s u r e and by t h e wear which w i l l occur as a r e s u l t o f t h e rubbing movement.

A convenient method o f showing a comparison between t h e

v a r i o u s types i s t o p l o t t h e i r a l l o w a b l e b e a r i n g p r e s s u r e and s l i d i n g speed and t h i s been done i n Fig.4.

The values g i v e n h e r e a r e o n l y approximate b u t should

h e l p t o g i v e an i n d i c a t i o n o f t h e v a r i o u s types o f m a t e r i a l which can be used i n any p a r t i c u l a r a p p l i c a t i o n . The f l e x i b l e member type o f b e a r i n g i s a l s o s u i t a b l e f o r a p p l i c a t i o n w i t h o s c i l l a t i n g motion, and Fig.5 g i v e s an i n d i c a t i o n o f t h e a l l o w a b l e maximum l o a d i n g on f l e x i b l e b e a r i n g s which can be f i t t e d w i t h i n a g i v e n space.

These

b e a r i n g s a r e commonly used i n v a r i o u s t y p e s o f l i n k a g e and t h e c r o s s - s e c t i o n a l area a v a i l a b l e a t t h e end o f a l e v e r which r e q u i r e s t o be connected t o a n o t h e r component i s g e n e r a l l y t h e l i m i t i n g f a c t o r on s i z e r a t h e r than t h e diameter of any c o n n e c t i n g p i n which c o u l d be regarded as t h e e q u i v a l e n t o f t h e s h a f t i n t h e a p p l i c a t i o n s examined p r e v i o u s l y .

For t h i s reason t h e l o a d i n F i g . 5 i s

p l o t t e d i n terms o f t h e a l l o w a b l e b e a r i n g p r e s s u r e on t h e o u t s i d e diameter o f rubber bushes and e q u i v a l e n t shaped crossed f l e x u r e p i v o t s .

T h i s diagram i n -

d i c a t e s t h a t f l e x i b l e member b e a r i n g s a r e o n l y usable up t o an a b s o l u t e maximum e q u i v a l e n t p r e s s u r e o f 14 MN/m*

2000 I b / i n 2 w i t h v e r y low a n g u l a r movements,

and w i t h much lower loads up t o a maximum p o s s i b l e a n g u l a r movement o f 30 degrees. Outside these c o n d i t i o n s , pin-and-bush

i t i s necessary t o r e v e r t t o t h e more c o n v e n t i o n a l

t y p e o f b e a r i n g design, f o r which t h e m a t e r i a l can then be s e l e c t e d

as i n d i c a t e d by Fig.4.

I t may appear from Fig.5 t h a t rubber bushes a r e always l i k e l y t o be s u p e r i o r t o f l e x u r a l ligament b e a r i n g s .

While t h i s i s t r u e i n terms o f compressive l o a d

and a l l o w a b l e d e f l e c t i o n , i t must be remembered t h a t f l e x u r a l l i g a m e n t s can be designed t o have a much lower s t i f f n e s s f o r a g i v e n l o a d c a p a c i t y compared t o a rubber bush, and they w i l l a l s o accept a much w i d e r range o f environmental conditions.

The d a t a presented f o r f l e x u r a l l i g a m e n t s a r e based on crossed

f l e x u r e p i v o t s and i t i s i m p o r t a n t t o remember t h a t t h e design can be s i m p l i f i e d t o a much s i m p l e r s i n g l e l i g a m e n t , u s u a l l y i n t e n s i o n ,

i f the r o t a t i o n a l centre

41

I

I

Hard steel on ,hard steel

Q, c

E!

.-

X

0.01

0 01

a

0.1

1

10

100

Approximate maximum rubbing speed (m/sl

Fig.4

Indication of the performance of dry rubbing bearing with oscillating motion

42

.+ .C 3 . I -

0

al

0 m

+

3 0

C 0

Maximum deflection each way (degrees)

Fig.5

I n d i c a t i o n o f t h e performance o f f l e x i b l e member b e a r i n g s w i t h o s c i l l a t i n g motion

o f m o t i o n does n o t have t o be k e p t under c l o s e c o n t r o l .

3.4.3

A p p l i c a t i o n s w i t h M u l t i d i r e c t i o n a l Load and Continuous Movement

I n a p p l i c a t i o n s o f t h i s k i n d , o f which t y p i c a l examples a r e t h e c r a n k s h a f t b e a r i n g s o f p i s t o n engines and r e c i p r o c a t i n g compressors, t h e b e a r i n g p r e s s u r e s used i n p r a c t i c e a r e c o n s i d e r a b l y g r e a t e r than those a p p l i e d t o u n i d i r e c t i o n a l l y loaded b e a r i n g s .

T h i s i s p o s s i b l e because

( i ) t h e l o a d frequency changes i n d i r e c t i o n as w e l l as i n magnitude, and t h e s h a f t does n o t have t i m e t o squeeze t h e o i l f r o m t h e f i l m t o a s u f f i c i e n t e x t e n t t o make metal-to-metal

c o n t a c t b e f o r e t h e l o a d r e v e r s e s and l i f t s

t h e s h a f t away again. ( i i ) s i n c e v a r i o u s p o s i t i o n s o f t h e b e a r i n g and s h a f t s u r f a c e c a r r y t h e minimum f i l m r e g i o n i n t u r n ,

t h e l o c a l thermal c o n d i t i o n s a r e n o t as

severe as i n a s t e a d i l y loaded b e a r i n g where one r e g i o n o f t h e b e a r i n ?

43 metal i s c o n t i n u o u s l y s u b j e c t e d t o l o c a l h e a t i n p u t .

As a r e s u l t lower

v a l u e s o f f i l m t h i c k n e s s may be p e r m i t t e d f o r a g i v e n r e v o l u t i o n speed and s h a f t s i z e , p r o v i d e d t h a t s h a f t s u r f a c e f i n i s h i s smooth enough. The methods o f c a l c u l a t i n g t h e performance o f d y n a m i c a l l y loaded b e a r i n g s a r e e s s e n t i a l l y r a t h e r complex and t h e most comprehensive method o f assessing t h e i r p r o b a b l e performance i s t o c a l c u l a t e t h e p r o b a b l e l o c u s o f t h e s h a f t w i t h a computer

.

From t h e p o i n t of view o f v e r y g e n e r a l design guidance, however,

i t can

p r o b a b l y be a p p r e c i a t e d t h a t s i n c e much g r e a t e r b e a r i n g p r e s s u r e s a r e a l l o w a b l e , t h e s t r e n g t h o f t h e b e a r i n g m e t a l s becomes an i m p o r t a n t f a c t o r i n b e a r i n g s e l e c t i o n , t o g e t h e r w i t h t h e method o f f e e d i n g o i l t o t h e b e a r i n g s i n o r d e r t o ensure t h a t t h e b e a r i n g c l e a r a n c e s a r e k e p t as f u l l o f o i l as p o s s i b l e i n readiness f o r c a r r y i n g t h e dynamic loads. I t i s d i f f i c u l t t o quote p r e c i s e data f o r t h e a l l o w a b l e loads on v a r i o u s b e a r i n g m a t e r i a l s s i n c e f a c t o r s such as s h a f t d e f l e c t i o n , e f f i c i e n c y o f l u b r i c a t i o n and t h e t y p e o f l o a d i n g p a t t e r n have a c o n s i d e r a b l e e f f e c t .

Table 3.2

however, g i v e s some broad guidance on t h e t y p e o f m a t e r i a l s t h a t a r e l i k e l y t o be s u i t a b l e . Table 3.2

P o s s i b l e m a t e r i a l s f o r p l a i n b e a r i n g s i n r e c i p r o c a t i n g machinery

Maximum b e a r i n g p r e s s u r e lb/in2 MN/m2

Possible bearing material

Below 2500

Whitemetal

17

2500- 4000

17

-

28

Copper leads

2500- 5000

17

-

34

Aluminium t i n s

4000- 6000

28

-

41

Low t i n l e a d bronzes

6000-10000

41

High t i n l e a d bronzes

70

-

70

10000-15000

100

Phosphor bronzes

The s t r o n g e r m a t e r i a l s a r e h a r d e r t h a n t h e weaker ones and i t i s t h e r e f o r e d e s i r a b l e t o p i c k a m a t e r i a l which i s o n l y j u s t adequately s t r o n g enough i n o r d e r t o have t h e maximum p o s s i b l e c o n f o r m a b i l i t y w i t h t h e s h a f t and embedda b i l i t y for d i r t . h a r d shaft,

With h a r d e r b e a r i n g m a t e r i a l s i t i s a l s o necessary t o u s e a

p a r t l y t o m a i n t a i n an adequate hardness d i f f e r e n t i a l w i t h t h e

b e a r i n g m a t e r i a l and p a r t l y because t h e h i g h e r b e a r i n g pressures which have c a l l e d f o r t h e h a r d e r m a t e r i a l a r e a l s o l i k e l y t o r e s u l t i n lower f i l m t h i c k nesses and t h e r e f o r e g r e a t e r d i r t s e n s i t i v i t y . As a v e r y g e n e r a l guide,

loads below 24 MN/m2

i t i s n o t u s u a l l y necessary t o harden t h e s h a f t a t

(3500 l b / i n Z ) b u t g e n e r a l l y d e s i r a b l e t o do so a t loads

44 above 31 MN/m2

(4.500 l b / i n 2 ) .

R o l l i n g element b e a r i n g s can a l s o be used f o r p i s t o n engine c r a n k s h a f t be a r in g s , a l t h o u g h i n most cases they o f f e r no b e t t e r performance than a f l u i d -

f , i l m b e a r i n g , a t t h e expense i n l a r g e r en gi n es o f a r a t h e r more c o m p l i c a t e d design.

R o l l i n g b e a r i n g s do have t h e p a r t i c u l a r m e r i t o f r e q u i r i n g much l e s s

o i l supply than f l u i d - f i l m b e a r i n g s and t h i s makes them a f i r s t c h o i c e f o r twos t r o k e engines o f t h e p e t r o i l type, and f o r o t h e r small cheap engines where t h e o p p o r t u n i t y t he y o f f e r o f e l i m i n a t i n g t h e o i l supply system r e s u l t s i n a more commercial design.

F i g u r e s were r e p r i n t e d by p e r m i s s i o n o f t h e C ouncil o f t h e I n s t i t u t i o n o f Mechanical Engineers f r o m t h e Proceedings o f 1967 I n t e r n a t i o n a l Conference on L u b r i c a t i o n and \!ear.

45

4

DESIGN OF PLAIN BEARINGS Use of Bearing Data Design Charts

MARTIN and D.R.

F.A.

GARNER

Research 8 Development O r g a n i s a t i o n The G l a c i e r Metal Company L i m i t e d .

4.1

INTRODUCTION The d e s ig n o f hydrodynamic b e a r i n g s can be a d a u n t i n g t a s k f o r t h e average

machinery d e s ig ner.

W i t h r o l l i n g element b e a r i n g s t h e design process i s very

much b i a s e d towards b e a r i n g s e l e c t i o n d at a, where m a n u f a c t u r e r s ' catalogues g i v e t a b u l a t e d i n f o r m a t i o n on s i z e ,

load, speed and l i f e .

i t i s n o t so easy t o f i n d such ' p o t t e d '

information,

For p l a i n bearings

and i n t h e p a s t t h e

de s ig n e r had e i t h e r t o become i n v o l v e d i n t h e mathematical c o m p l e x i t i e s o f the hydrodynamics, o r r e l y on t h e v a g a r i e s o f ru l e-of-thumb guides.

With the con-

t i n u a l u p r a t i n g o f machinery, t h i s l a t t e r approach i s seldom adequate, but i t can be d i f f i c u l t t o o b t a i n more d e t a i l e d i n f o r m a t i o n i n a r e a d i l y d i g e s t i b l e form. V a r io u s t e c h ni qu es f o r a n a l y s i n g and ch ecking t h e performance o f p l a i n j o u r n a l b e a r i n g s a r e p rese nt ed here, s o t h a t t h e designer has t h e means t o appr e c i a t e t h e e f f e c t s o f t h e v a r i o u s parameters on b e a r i n g performance and design, w i t h o u t t he need f o r e x t e n s i v e mathematical m a n i p u l a t i o n .

4.1.1 b 'd

Notat ion = a x i a l b e a r i n g l e n g t h (m) = b e a r i n g d i a m e t r a l c l e a r a n c e (m)

C

= b e a r i n g r a d i a l cl e ara nce (m)

d

= b e a r i n g d i ame t e r (m)

e

= d i s t a n c e between b e a r i n g and s h a f t c e n t r e s (m)

hmin

= minimum o i l f i l m t h i c k n e s s (m)

H

= power loss (\I)

M

= e f f e c t i v e r o t o r mass a t b e a r i n g (kg)

N

= s h a f t r o t a t i o n a l speed ( r e v / s )

46 = r o t a t i o n a l speed o f l o a d v e c t o r ( r e v / s )

NL P

= b e a r i n g s p e c i f i c l o a d W/bd (Pa o r N/m2)

Q

= o i l f l o w req ui re men t (m3/s)

W

= a p p l i e d load

6

= j o u r n a l mi sa l i gn men t o ver b e a r i n g l e n g t h (m)

I-

= e f f e c t i v e f i l m v i s c o s i t y (Ns/m2)

ee

(N)

= e f f e c t i v e t emp era t u re ("C)

emax

= maximum t emp era t u re ( " C )

p

= o i l d e n s i t y (kg/m3)

w

= a n g u la r v e l o c t y ( r a d / s )

Dimensionless Terms ( i n any c o n s i s t e n t s e t o f u n i t s eg. as g i v e n above)

W'

load v a r i a b l e

& [?I2

=

P

=

e/cr

d2N2p

turbulence v a r i a b l e

eccentricity r a t i o

E

di m e n s io n le s s c r i t i c a l mass c

4.2

=

[t]

= 1

- hmin/cr

w 2 M/W

JOURNAL BEARING D E S I G N LIMITS The t o t a l d e si gn p roce ss f o r most mechanical components,

be a r in g s ,

including p l a i n

i n v o l v e s many stages, w i t h f u n c t i o n a l , economic and perhaps a e s t h e t i c

asp e c ts a l l needing t o be considered.

I t i s t h e f i r s t o f these w hich n o r m a l l y

i n v o l v e s t h e d e si g ne r i n most e f f o r t and whi ch i s t o be considered i n d e t a i l here.

For convenience we w i l l s p l i t t h i s f u n c t i o n a l design stage i n t o two:( i ) e n s u r i n g t h a t t h e b e a r i n g i s cap ab l e o f o p e r a t i n g s a t i s f a c t o r i l y under

th e imposed c o n d i t i o n s , and t h a t i t i s n e i t h e r t o o c l o s e t o i t s l i m i t s o f o p e r a t i o n t o endanger r e l i a b i l i t y n o r so f a r away t h a t i t has p e n a l t i e s i n over-design, ( i i ) p r e d i c t i n g t h e performance o f t h e component as i t a f f e c t s the design o f i t s a s s o c i a t e d p a r t s o r t he o v e r a l l system. i n i t i a l l y , therefore,

4.2.1

t h e l i m i t s o f o p e r a t i o n o f b e a r i n g s must be d e f i n e d .

L i m i t s o f Ope rat i on

Consider a b e a r i n g o f some g i v e n s i z e and geometry, w i t h a d e f i n e d l u b r i c a n t grade and f e e d i n g c o n d i t i o n s .

The l i m i t i n g c o n d i t i o n s o f load and speed w hich

t h i s b e a r i n g can s u c c e s s f u l l y a ccep t a r e shown i n Fig.1; be c o n s id e r e d i n more d e t a i l .

these l i m i t s w i l l now

47 Thin O i l F i l m L i m i t

4.2.1.1

The danqer h e r e i s o f met al -t o-me t a l

c o n t a c t o f t h e surfaces, w i t h con-

sequent severe wear (and perhaps o v e r h e a t i n g ) l e a d i n g t o a breakdown i n b e a r i n g op e r a t io n .

There i s e vi de nce t o show t h a t t h i s c o n t a c t o c c u r s a t a p r e d i c t e d

f i l m t h i c k n e s s which i s a f u n c t i o n o f t h e s u r f a c e roughness [l],w hich i n t u r n T h i s has been considered

i s dependent upon machining process and b e a r i n g s i z e .

i n some d e t a i l i n r e f e r e n c e [ 2 ] , and t h e c o n c l u s i o n reached t h a t a r e a l i s t i c ' f a i l u r e ' v a l u e o f t h e f i l m t h i c k n e s s i s g i v e n by t h e p e a k - t o - v a l l e y

surface

f i n i s h (Rmax) o f t h e j o u r n a l , assuming t h a t t h e s u r f a c e f i n i s h o f t h e b e a r i n g i s o f t h e same o r d e r . An a p p r o x im a te c o r r e l a t i o n between Ra and Rmax values i s a l s o g i v e n i n r e f e r e n c e [ 2 ] , e n a b l i n g t h e i n f o r m a t i o n g i v e n i n Fig.2

t o be presented.

The

s u r f a c e f i n i s h e s shown h ere a r e r e p r e s e n t a t i v e o f those w hich can be o b t a i n e d by normal m a n u f a c t u r i n g methods. f a i l u r e values,

Since t h e Rmax values have been thought o f as

some a d d i t i o n a l f a c t o r must be a p p l i e d i f we a r e t o s p e c i f y

s a f e v a lu e s o f o i l f i l m t h i ckne ss.

T h i s f a c t o r has t o a l l o w f o r s l i g h t u n i n -

t e n t i o n a l m is a lignme nt which may t ake p l a c e between s h a f t and bearing, and f o r d i r t c o n t a m i n a t i o n i n t h e o i l supply. i n Fig.2

The a l l o w a b l e f i l m t h i c k n e s s values shown

a r e a f a c t o r o f t h r e e above t h e f a i l u r e values, and have been found by

exp e r ie n c e t o be acce pt ab l e.

For v e r y h i g h standards o f b u i l d and o p e r a t i o n

a f a c t o r o f two can be s a t i s f a c t o r y ,

BEARIN( LOAD

THIN OIL FILM LIMIT danger of metal -to - metal contact if load lies above this line

O :/ F

b u t i t i s considered t h a t t h e values shown

HIGH BEARING TEMPERATURE LIMIT danger of lining material wiping if load lies above this h e

/..

SAFE OPERATION

)

OIL OXIDATION LIMIT danger of excessive oil oxidation if speed lies beyond OILthis FILM lineWHIRL LIMIT

I

danger of unacceptable vibration if speed lies beyond this line

JOURNAL SPEED Fig.1

L i m i t s o f s a f e o p e r a t i o n f o r hydrodynamic j o u r n a l b e a r i n g s

48 i n Fig.2

should be used as a general g u i d e t o a v o i d w o r k i n g t o o c l o s e t o

failure limits.

I t must be emphasised t h a t t h e r e l a t i o n s h i p between Ra and

Rmax i s o n l y approximate,

as evidenced by t h e "spread o f r e s u l t s " band on t h e

f i g u r e , b u t i t i s adequate t o show t h e o r d e r o f r e d u c t i o n i n t h e a l l o w a b l e f i l m t h i c k n e s s t h a t can be p e r m i t t e d by improving s u r f a c e f i n i s h .

To increase t h e

o p e r a t i n g o i l f i l m t h i c k n e s s , f o r g i v e n c o n d i t i o n s o f l o a d and speed, t h e b e a r i n g s i z e must be increased, o r a t h i c k e r l u b r i c a n t and/or a reduced i n l e t temperature used (change i n o i l supply p r e s s u r e w i l l u s u a l l y have l i t t l e e f f e c t ) . An increase i n c l e a r a n c e may e i t h e r increase o r reduce t h e f i l m t h i c k n e s s , depending upon t h e p r e c i s e o p e r a t i n g c o n d i t i o n s .

1

2

I

6 810

20

LO

1inI

JOURNAL DIAMETER

Fig.2

4.2.1.2

Guidance on s h a f t s u r f a c e f i n i s h and a l l o w a b l e o i l f i l m t h i c k n e s s

High Bearing Temperature

The shearing which t a k e s p l a c e i n t h e b e a r i n g c l e a r a n c e space i s l s e e n ' e x t e r n a l l y as power d i s s i p a t i o n ,

and a l s o as an i n c r e a s e i n t h e temperature o f

s h a f t and b e a r i n g s u r f a c e s and o f t h e l u b r i c a n t . The l i m i t i n g a c c e p t a b l e temperature o b v i o u s l y depends upon t h e l i n i n g m a t e r i a l used, b u t whatever t h e m a t e r i a l t h e temperature must always be k e p t w e l l below i t s theoretical melting point.

For example,

i n t i n based w h i t e metal t h e

m e l t i n g temperature o f 232OC cannot even be c l o s e l y approached due t o t h e s o f t e n i n g and subsequent p l a s t i c f l o w o f t h e m a t e r i a l which o c c u r s a t temper-

49 atures w e l l below 200°C under the i n f l u e n c e o f hydrodynamic pressure. et al.

[3]

Booser

observed l i m i t i n g whitemetal temperatures i n j o u r n a l bearings as low

as 130°C;

however, since o n l y c a l c u l a t e d temperatures are a v a i l a b l e a t the

design stage, and c u r r e n t methods o f e s t i m a t i o n are known t o be f a r from i t i s s a f e r t o lower the l i m i t t o about 120'C.

accurate,

I n machines i n which

some dynamic loading can occur on top o f the normal steady load, and where bearing f a t i g u e i s t h e r e f o r e a p o s s i b l e problem, keep the temperature below 100°C.

i t i s customary t o t r y and

I f higher temperatures have t o be accomm-

odated then the use o f an aluminium t i n o r a copper lead m a t e r i a l must be considered.

The former o f these, when c o n t a i n i n g about 40% t i n , can be used a t

temperatures up t o 15O-16O0C, and i s almost equal t o whitemetal t o withstand seizure c o n d i t i o n s and d i r t contamination.

less f o r g i v i n g i n t h i s respect, and i d e a l l y should have a s o f t , p l a t e t o a i d i n bedding-in,

in i t s ability

Copper lead i s much t h i n overlay

but i t can be used a t temperatures o f 200°C plus.

I t r e q u i r e s a hardened s h a f t (about 300 HV) and very good l u b r i c a n t f i l t r a t i o n .

I t i s worth n o t i n g t h a t i f a h i g h temperature c o n d i t i o n i s present i n a bearing t h e r e i s u s u a l l y n o t h i n g t h a t can be done t o the o i l feeding c o n d i t i o n s t o improve the s i t u a t i o n , a p a r t from changing the o i l grade.

The oft-used

' p a l l i a t i v e s ' o f increasing o i l feed pressure o r grooving area i n order t o f o r c e more o i l through the bearing a r e u s u a l l y n o t successful since they o n l y reduce the b u l k temperature o f the o i l passing through the bearing (see section 4.2.1.3).The

bearing m a t e r i a l temperature i s c o n t r o l l e d by the amount o f lub-

r i c a n t passing through the a c t i v e p a r t o f the o i l f i l m , s e n s i t i v e t o feeding conditions, starved.

and t h i s i s very i n -

provided t h a t the bearing i s n o t grossly

As a rough approximation,

the maximum bearing temperature w i l l be

dropped by about a h a l f o f any decrease i n o i l i n l e t temperature,

i e a 10°C

reduction i n i n l e t temperature may be expected t o decrease the maximum bearing temperature by about 5°C.

The bearing temperature may a l s o g e n e r a l l y be de-

creased by increasing the bearing s i z e o r clearance, o r by using a t h i n n e r grade o f o i l . One f u r t h e r p o i n t on bearing temperatures which i s sometimes ignored i s the n e c e s s i t y o f considering the influence o f machine temperatures on the bearings.

I f t h e r e can be appreciable heat soak along the s h a f t , o r ( l e s s o f t e n ) through the bearing housing, then the design must a l l o w f o r adequate o i l f l o w t o deal w i t h it.

Additionally,

i n some machinery, the worst temperature c o n d i t i o n s a t

t h e bearing can occur a f t e r shut down when heat soak r a i s e s temperatures w e l l above peak running values, and the choice o f m a t e r i a l should then be d i c t a t e d by c o n d i t i o n s which a r e o f t e n n o t drawn t o the a t t e n t i o n o f the bearing designer.

50 4.2.1.3

O i l Oxidation L i m i t

S t r a i g h t mineral o i l s i n a normal (oxygen c o n t a i n i n g ) atmosphere can be r a p i d l y o x i d i s e d a t the order o f temperatures t h a t we have been discussing above.

There i s no p r e c i s e 'go/no-go'

l i m i t f o r t h i s process, r a t h e r the r a t e

o f degradation i s a f u n c t i o n o f temperature [4].

I n d u s t r i a l mineral o i l s

u s u a l l y c o n t a i n a n t i - o x i d a n t s which r e t a r d t h i s process, b u t f o r comnonly used t u r b i n e o i l s , f o r l i v e s i n the order o f thousands o f hours,

i t i s necessary t o

r e s t r i c t b u l k temperatures o f o i l i n tanks,

t o about 75-80°C.

r e s e r v o i r s etc.

Thus a b u l k d r a i n temperature from a bearing a t a higher value than t h i s i s u n l i k e l y t o be acceptable.

As mentioned above, t h e d r a i n temperature can u s u a l l y be reduced by i n creasing o i l supply pressure o r p r o v i d i n g bleed grooves i n the bore, and thereby h e l p i n g t o avoid the o x i d a t i o n l i m i t .

I t i s u n f o r t u n a t e t h a t the b u l k o u t l e t

temperature i s o f t e n used t o judge bearing performance, presumably because i t i s the e a s i e s t temperature t o measure, since t h i s j u g g l i n g w i t h supply cond i t i o n s can r a d i c a l l y a l t e r d r a i n temperatures w i t h o u t appreciably a f f e c t i n g m a t e r i a l temperatures.

The o u t l e t temperature can be used as a long-term

monitoring device ( i e a temperature which has been steady a t 70-75'C should n o t suddenly r i s e t o 80-85°C) misleading,

4.2.1.4

f o r months

b u t i t i s n o t s u i t a b l e , and may indeed be

i f used for ' s e t t i n g up' o i l feed c o n d i t i o n s on a new machine.

O i l F i l m Instability

Under c e r t a i n c o n d i t i o n s , normally a t low load and/or h i g h s h a f t speeds, a s e l f e x c i t e d and s e l f s u s t a i n i n g motion can occur i n which the s h a f t c e n t r e precesses around the bearing c e n t r e a t something s l i g h t l y l e s s than h a l f s h a f t speed, t y p i c a l l y 0.42-0.47

o f s h a f t speed.

Under these c o n d i t i o n s the hydro-

dynamic a c t i o n o f the bearing i s a l l but l o s t , and metal-to-metal contact can occur;

i n practice,

i f l e f t f o r long periods, a f a t i g u e type damage i s pro-

duced due t o t h e h i g h temperatures generated.

There can a l s o be a large, and

perhaps unacceptable, v i b r a t i o n t r a n s m i t t e d through the machine.

Guidance on

the l i k e l i h o o d o f i n s t a b i l i t y i n c y l i n d r i c a l bore bearings i s given i n Fig.3. This c h a r t i s s t r i c t l y o n l y a p p l i c a b l e t o r i g i d , simply supported s h a f t s [ 5 ] , but experience has shown i t t o g i v e a f a i r guide f o r o t h e r systems, f o r example overhung rotors.

I t can be seen t h a t i f the o p e r a t i n g e c c e n t r i c i t y r a t i o i s

g r e a t e r than about 0.8

then the bearing i s stable.

This i s the reason why

- adding e x t r a grooving i n the loaded region, using a grade o f o i l etc. - can sometimes cure h a l f speed w h i r l problems.

d e r a t i n g a bearing thinner

A d d i t i o n a l l y , whatever t h e o p e r a t i n g e c c e n t r i c i t y r a t i o there i s some value o f the dimensionless c r i t i c a l mass below which the bearing w i l l always be stable. I n h e a v i l y loaded a p p l i c a t i o n s ,

such as gearboxes,

the a p p l i e d load i s normally

51 o f a much higher magnitude than the s h a f t mass and i n s t a b i l i t y , a t any r e a l i s t i c speed,

i s n o t a problem.

However there may well. be some p a r t load

c o n d i t i o n , f o r example a s p i n t e s t , a t which s t a b i l i t y needs t o be c a r e f u l l y checked.

1

1

Cr radial clearance shaft angular velocity M effective rotor mass at bearing

0

W bearing load

- 100 -80

-

With consistent units e.g. m, radls, kg. N.

60

4 0VL3 1 ~

-20

-I

-10 - 8

UNSTABLE 0.1 0.5

. . 1.0

t

~ 0

-6

4: g

3 2

w 4

-4

1.5

I

2 4

I

length diamotrr

v)

z W

STABLE

E - 2

'

" 0.2

"

'

0-4

" 0.6

' 0-8

-

1.0

ECCENTRICITY RATIO Fig.3

O i l f i l m w h i r l i n s t a b i l i t y o f c y l i n d r i c a l bore j o u r n a l bearings L i m i t i n g dimensionless mass

4.2.2

-

Region o f Safe Operation

The l i m i t s j u s t defined serve t o bound a region o f safe operation.

The

simplest approach t o design i s merely t o c a l c u l a t e such o p e r a t i n g parameters as f i l m thickness and temperatures and t o compare them against the defined limits.

If b o t h the l i m i t s and the design p o i n t a r e p l o t t e d , i n a form s i m i l a r

t o Fig.1,

then judgements on the degree o f s a f e t y can e a s i l y be made.

There a r e various methods o f t a c k l i n g t h i s , and the most comnon w i l l now be described;

the f i n a l design c h a r t s presented enable very r a p i d and accurate

estimates o f bearing o p e r a t i o n t o be made w i t h minimal e f f o r t .

52 4.3

CALCULATION AND D E S I G N PROCEDURES The a n a l y s i s o f o i l f i l m c o n d i t i o n s w i t h i n a b e a r i n g , desc ibed by t h e

Reynolds e q u a tio n,

i s a v e r y i n v o l v e d mathematical e x e r c i s e .

The e q u a t i o n i s

d e c e p t i v e l y s im p l e, b u t an a n a l y t i c a l s o l u t i o n cannot be o b t a ned f o r any b u t th e s i m p l e s t o f arrangements.

I nst ea d re cou rse must be made t o numerical

techniques, u s u a l l y i n v o l v i n g computer s o l u t i o n s .

A f u r t h e r c o m p l i c a t i o n i n o i l l u b r i c a t e d b e a r i n g s i s t h e need t o make a r e a l i s t i c estimation o f the o i l v i s c o s i t y w i t h i n the f i l m . normal m i n e r a l o i l s is v e r y dependent on temperature,

The v i s c o s i t y of

and a r e l a t i v e l y a c c u r a t e

assessment o f t h e l a t t e r must be made f o r r e a l i s t i c design.

One method o f

do in g t h i s i s t o s o l v e t h e Reynolds and energy equations simultaneously.

This

c o n s i d e r a b l y in c rea ses t h e comp ut at i on al e f f o r t and t h e r e s u l t i n g s o l u t i o n s a r e somewhat r e s t r i c t e d i n t h e i r g e n e r a l i t y . I n s te a d i t i s u s u a l l y more co nve ni e nt t o s o l v e t h e Reynolds e q u a t i o n assuming a s i n g l e , g l o b a l , e f f e c t i v e be determined a t a l a t e r stage.

v i s c o s i t y value,

which then can, and must,

T h i s i s done by e q u a t i n g t h e work done i n

s h e a r in g t h e o i l w i t h i n t h e b e a r i n g c l e a r a n c e space t o t h e amount o f h e a t c a r r i e d away, b o t h by t h e o i l ( t h e predominant mechanism) o r through t h e b e a r i n g and housing. It i s , unfortunately,

balance i s o m i t t e d ,

n o t uncommon t o see design methods i n w hich t h i s h e a t

t h ere by assuming t h a t t r e n d s i n dimensionless groups acc-

u r a t e l y p o r t r a y t r e n d s i n r e a l performance. clusions;

T h i s can l e a d t o i n c o r r e c t con-

f o r example an i n crea sed c l e a r a n c e may a p p a r e n t l y reduce f i l m

thickness i f the v i s c o s i t y i s a r t i f i c i a l l y h e l d constant, b u t i n p r a c t i c e the o p e r a t i n g v i s c o s i t y may i ncre ase and p o s s i b l y r e v e r s e t h e trend. Whatever c a l c u l a t i o n method i s chosen, t h e r e s u l t s must then be compared a g a i n s t t h e e s t i m a t e d o r known l i m i t s o f s a t i s f a c t o r y o p e r a t i o n . Fig.4 Stage

i l l u s t r a t e s t h e v a r i o u s stages i n p r oducing design data:-

1

i s t h e s o l u t i o n of t h e Reynolds e q u a t i o n t o g i v e dimensionless data.

Stage I I

ta k e s t h i s d a t a and d et ermi n es a r e a l i s t i c o i l v i s c o s i t y t o g i v e s p e c i f i c answers.

Stage I l l

uses t h e r e s u l t s f r o m t h e p r e v i o u s two stages t o produce 'easy-to-use'

4.3.1

d esi g n c h a r t s .

Dimensionless Data

- Stage

I

Dimensionless d esi g n d a t a have been p u b l i s h e d f o r t h e most commonly used b e a r i n g c o n f i g u r a t i o n s [6,7,8,9]

and Fig.5

shows a t y p i c a l c h a r t r e l a t i n g a

l o a d v a r i a b l e t o a f i l m t h i c k n e s s r a t i o f o r a c y l i n d r i c a l bore j o u r n a l b e a r i n g w i t h a steady ( o r p ure r o t a t i n g ) load.

T h i s may be used f o r b e a r i n g s w i t h two

a x i a l o i l f e e d grooves (l oa de d midway between them), w i t h a s i n g l e f e e d groove

53

1

I

I Fundamental theory

program e.g. solution to

StageI- Dimensionless groups StagelI-Specific answer for selected input Stagem- Infinite number of answers (certain restrictions usually necessary)

Fig.4

Stages in design

I

4

I

54 i n th e unloaded h a l f o r w i t h a c e n t r a l c i r c u m f e r e n t i a l groove;

w i t h the l a t t e r

the l e n g t h i s t h a t of one l an d and t h e l o a d i s h a l f t h e t o t a l a p p l i e d load. The c e n t r a l c i r c u m f e r e n t i a l l y grooved b e a r i n g i s g e n e r a l l y used i n a p p l i c a t i o n s where th e l o a d i s r o t a t i n g o r where an unknown l o a d d i r e c t i o n has t o be c a t e r e d for.

When a b e a r i n g has t o c a r r y a r o t a t i n g l o a d t h e l o a d c a p a c i t y i s p r o -

p o r t i o n a l t o ( 1 - 2 N ~ / N l , as shown i n Fig.5 where NL i s t h e a n g u l a r r o t a t i o n a l speed o f t h e l o a d measured i n t h e same d i r e c t i o n as s h a f t r o t a t i o n (N). i s f u r t h e r i l l u s t r a t e d i n Fig.6,

which shows d i a g r a m n a t i c a l l y t h e o i l f i l m

forma t ion.

FILM THICKNESS RATIO

Fig.5

This

hmin/cr

B e a r in g l o a d c a p a c i t y for steady and p u r e r o t a t i n g l o a d cases

55

An examination of dimensionless data can also be instructive where gross effects on performance are apparent.

For instance the position of oil feed

grooves in a steadily loaded bearing is very important, since they can seriously derate the load-carrying capability of the bearing.

Such a derating effect is

clearly demonstrated in Fig.7. Case (a) shows a n uninterrupted coverging oil film which generates a substantial hydrodynamic pressure to support the load ( W ) . Case (b) shows the same converging oil film a s (a) but interrupted by an oil groove.

This will only support a very reduced load because of the

smaller integrated pressure. Case (c) shows an uninterrupted film with the same load a s (a) which needs a smaller oil film thickness to generate sufficient pressure.

+

ongulor velocity of load ongulor velocity of shaft

hmin position oheod of pressure region

I

I

N

mL

hmin position logs behind pressure region

Analogous

-

Fig.6

DRAGGING OIL

SQUEEZING

PUSHING OIL

Film formation and relative load capacity under rotating load conditions.

56

E;

I

c

Cr

~

groove

me min

min smaller load

t

I

(a)

I

W

Fig.7

(b)

The derating effect of grooving positioned in a converging oil film

- 0.1

Fig.8

Variation o f load capacity with direction of load for a bearing with two axial grooves

67 The d im e n s io n le s s l o a d and f i l m t h i c k n e s s c h a r t , Fig.8,

shows more p r e c i s e l y

th e v a r i a t i o n i n l o a d c a p a c i t y w i t h any l o a d d i r e c t i o n r e l a t i v e t o t h e grooves. In t h i s f i g u r e a b e a r i n g w i t h a l e n g t h equal t o h a l f the diameter i s cons id e r e d , h a v i n g two a x i a l grooves d i s p l a c e d 180" a p a r t , each groove e x t e n d i n g

30"

around t h e b e a r i n g .

These a r e t y p i c a l groove arrangements.

Wit h t h e l o a d d i r e c t i o n e x a c t l y between two grooves t h e l o a d c a p a c i t y shown i n Fig.8

i s t h e same as t h a t i n F i g . 5 and v a r i a t i o n s i n t h e angle w i t h i n

f 20" cause l i t t l e d i f f e r e n c e .

As t h e l oa d l i n e g e t s near t o a groove p o s i t i o n

th e lo a d c a p a c i t y d r a s t i c a l l y reduces as seen by the r e d u c t i o n i n t h e m o d i f i e d load v a r i a b l e ( W ' / T )

f o r t h e same f i l m t h i c k n e s s .

I t can be seen t h a t t h e load

v a r i a b l e i s h a l v e d when g o i n g f rom p o i n t (a) t o p o i n t (b) i n Fig.8, p o i n t s (a),

(b) and ( c ) can be i d e n t i f i e d w i t h cases (a),

where

(b) and ( c ) i n Fig.7.

S i m i l a r l y f o r t h e same v a l u e o f l o a d parameter t h e f i l m t h i c k n e s s f o r case (c) i s much s m a l l e r t ha n case ( a ) . I t s h o u ld be n ot ed t h a t t h e c o n d i t i o n o f l o a d i n g d i r e c t l y i n t o a groove does n o t n o r m a l l y produce t h e l owest o i l f i l m t h i c k n e s s ; t h e l o a d i s d i r e c t e d j u s t b e f o r e t h e groove.

i n s t e a d t h i s occurs when

T h i s i s because t h e peak o i l f i l m

pre s s u r e o c c u r s j u s t beyond t h e l o a d l i n e ( f o r a steady l o a d c o n d i t i o n ) , and a groove i n t h i s p o s i t i o n t h e r e f o r e more s e v e r e l y d e r a t e s t h e b e a r i n g .

4.3.2

Design Procedures w i t h Heat Balance

-

Stage I I

The h e a t b a lan ce procedure can be c a r r i e d o u t by hand c a l c u l a t i o n i n a r e l a t i v e l y s t r a i g h t f o r w a r d manner, b u t i s i d e a l l y s u i t e d t o simple computer techniques.

A p a r t i c u l a r l y co nve ni e nt and we1 1 documented procedure f o r

[9] which [9] enables t h e

c y l i n d r i c a l bore b e a r i n g s i s g i v e n i n r e f e r e n c e

can form t h e b a s i s o f

a computer programme [ l o ] .

influence o f a very

The procedure

wide range o f v a r i a b l e s t o be co nsi d ere d and i s an extremely v a l u a b l e design t o o l , e s p e c i a l l y when i n t h e f orma t o f a c o n v e r s a t i o n a l type computer programme.

4 . 3 . 3 Improvement i n Design A i d s

-

Stage I l l

W h i l s t t h e above arrangement i s s u i t a b l e f o r p r e d i c t i n g b e a r i n g performance w i t h ' s i n g l e s h o t ' answers,

i t does n o t p r o v i d e a ' f e e l '

for how near a

p a r t i c u l a r d e s ig n i s t o t h e v a r i o u s s a f e o p e r a t i n g l i m i t s .

Therefore, t h e r e i s

a need f o r s t a g e I l l where t h e 'computed answers' f o r b e a r i n g cases a r e repres e n t e d i n d e s ign c h a r t form.

These e na bl e t h e designer t o answer such

q u e s t i o n s as:-

- W i l l t h i s bearing operate s a f e l y ? - How n e a r i s i t t o t h e l i m i t s of o p e r a t i o n ? -

What changes can be made t o improve t h e design?

58 When assessing safe l i m i t s o f o p e r a t i o n many f a c t o r s a r e involved, f o r example the bearing data considered must include diameter, length, o i l grade, load and speed.

The various permutati.ons,for a range o f these f a c t o r s r e s u l t s

i n many thousands o f cases t o be considered.

I f such data were represented i n

conventional graphical form, a t h i c k volume o f unmanageable design a i d s would result.

The ' s l i d e c h a r t ' obviates the need f o r t h i s and gives a d i r e c t and

i n s t a n t f e e l f o r the e f f e c t o f changing any o f t h e v a r i a b l e s , thus b r i n g i n g the power o f the computer t o the designers' desk top.

4.4

DESIGN PROCEDURE

4.4.1

FOR CYLINDRICAL BORE BEARINGS

Method o f Approach

The procedure d e t a i l e d here presents design a i d s which a l l o w f o r l i m i t i n g c o n d i t i o n s o f o p e r a t i o n and t h e p r e d i c t i o n o f bearing performance i n c y l i n d r i c a l bore j o u r n a l bearings which have two a x i a l grooves spaced 180" a p a r t . type o f bearing, w i t h a steady l o a d midway between the grooves, most common i n use.

This

i s one o f the

A l l the tedious heat balance c a l c u l a t i o n s have already

been c a r r i e d o u t by computer, based on the design procedure given i n ESDU Data Item

66023 [ g ] ,

and a r e consequently inherent i n the design aids.

Comparison of experimental work w i t h r e s u l t s from t h i s theory i n d i c a t e d t h a t w h i l s t c o r r e l a t i o n was good . f o r most v a r i a b l e s , the p r e d i c t e d maximum tempe r a t u r e could,

i n many circumstances, be considerably i n e r r o r .

A new method

was evolved based on considerable experimental evidence which r e l a t e s the maximum bearing temperature t o the c a l c u l a t e d e f f e c t i v e temperature [ l l ] . The r e s u l t i n g procedure was used t o c a l c u l a t e bearing performance over a wide range o f o p e r a t i n g c o n d i t i o n s .

However, t o keep the problem t o a manage-

able s i z e i t was decided t h a t c e r t a i n v a r i a b l e s should be f i x e d throughout, values commonly found i n c u r r e n t p r a c t i c e .

These were:-

(i) o i l groove dimensions: a x i a l l e n g t h = 0.8 o f bearing l e n g t h c i r c u m f e r e n t i a l width = 0.25 o f bearing diameter. ( i e 150" between the edges o f grooves, i n top and bottom o f bearing). Grooves w i t h 'square' ends

(ii) o i l feed c o n d i t i o n s a t the bearing: pressure 0.1 MPa (approx 15 I b f / i n Z ) temperature 50°C

to

59 ( i i i ) o i l grades: i n o r d e r t h a t t h e o i l grade c o u l d be d e f i n e d s i m p l y , a t r e n d i n viscosity-temperature characteristics typical o f industrial m i n e r a l o i l s was assumed t o a p p l y throughout.

The d e f i n i t i o n o f

an o i l grade can t h en be made by q u o t i n g one v i s c o s i t y a t a certain (arbitrary)

temperature;

40°C was used ,here t o d e f i n e

t h e o i l grade, t h i s b e i n g t h e same b a s i c r e f e r e n c e as f o r IS0 v i s c o s i t y grades.

Fig.9 shows t h e v i s c o s i t y t r e n d s considered.

I n a d e s ig n problem t h e b e a r i n g d i ame t e r and o p e r a t i n g c o n d i t i o n s o f speed and l o a d a r e g e n e r a l l y determined p r i o r t o t h e b e a r i n g design stage, s i n c e they u s u a l l y f o r m p a r t o f an o v e r a l l system.

For s i m i l a r reasons t h e l u b r i c a t i n g

o i l grade t o be used i s o f t e n a l s o imposed l e a v i n g c l e a r a n c e and b e a r i n g l e n g t h as t h e two dimensions s t i l l t o be determined.

Fig.9

Viscosity

%

Temperature c h a r a c t e r i s t i c s f o r t h e range o f o i l s co nsi d ere d i n t h e design c h a r t s

60

Several authors have considered optimising on clearance [12,13] to give the largest possible oil film thickness in the bearing or the lowest power loss. Unfortunately these two optima do not coincide and a compromise solution is usually required.

Indeed when the problem is treated purely as a mathematical

exercise there is a danger of having excessively slack clearances for optimum power loss and, in some instances, very tight clearances for optimum film thickness, especially if the effect of change in clearance on viscosity is ignored.

4.4.2 Guidance for Safe Operation 4.4.2.1 Oil Film Thickness and Temperatures It is appropriate to consider the tight clearance end of the tolerance range

when examining conditions at h i g h temperature.

These tight clearances may also

be used when making judgements on safe operation relating to small oil films since the variation of clearance throughout m s t practical tolerance ranges generally has little effect on oil film thickness.

The design aids, Figs.lO(a),

(b) and (c), associated with both temperature and film thickness limits, have been developed using the minimum clearance values shown in Fig.11.

These

values relate to an extreme of the tolerance range; average manufactured clearance will be larger.

These clearances are based on many years' expe.rience

with much 'feed-back' relating to the performance of practical designs. With clearances known it is possible, for any particular values of diameter, length and lubricant grade, to plot limiting lines of load against speed, representing film thickness limit and temperature limits (as in Fig.1).

The design

aids, in 'slide chart' form, enable the designer to 'plot' these limiting lines in a matter of seconds, not just for a discrete number of variable combinations, b u t for an infinite number of cases.

In fact the designer merely has to move

an already drawn curve to its correct position relative to the axes, the bearing conditions dictating exactly where this position is. Fig.lO(a) is used to 'plot' the film thickness limit lines, and Fig.lO(b) is used to 'plot' the temperature limit lines, both on the grid of load and speed given in Fig.lO(c).

A transparent version of the chart in question,

Fig.lO(a) or (b), should be placed on the backing sheet, Fig.lO(c) using the guide lines on the top (transparent) sheet to keep the two sheets square.

The

transparency should then be moved to a position where a point in its grid, defined by the diameter and oil grade values, is coincident with the cross on the backing sheet.

The relevant length/diameter line then shows the limit of

load against speed for the conditions considered. By using the two transparencies the relative position of a design point

(defined by its speed and load values) to the limiting lines can be seen.

If

this point is within the limits then the bearing, under reasonable environmental

61

Fig.lO(a)

Load c a p a c i t y s l i d e c h a r t :

Thin o i l f i l m l i m i t (transparency)

62

This chart may not be applicable at high speeds if non laminar conditions prevail.

-

U C

4

LC

z

5 eE 2 C

a

Y

I

a

.-C a

K .C

\

lest at LO'C I

Ensure that the design point of speed and load lies to the left of the appropriate guidance curve, otherwise full lines -there is a danger of EXCESSIVE OIL OXIDATION broken lines - the BEARING MATERIAL may be TOO HOT

03'

) 0%

guide line -keep square with backing sheet

Fig.lO(b)

Load c a p a c i t y s l i d e c h a r t :

High temperature 1 i m i t s (transparency)

63

such that appropriate DIAMETER and OIL GRADE are coincident with this point Bearing load

length diameter

Ices

Journal weed Irev/minl

Fig.lO(c)

Load c a p a c i t y s l i d e c h a r t :

Load-speed g r i d (backing s h e e t )

64 c o n d i t i o n s , should o pe rat e s a t i s f a c t o r i l y .

I f i t i s c l o s e t o o r outside the

l i m i t s t h e n t h e s l i d e c h a r t s p r o v i d e a q u i c k method o f d e t e r m i n i n g which v a r i a b l e o r v a r i a b l e s can be changed t o improve t h e design.

For i n s t a n c e

increasing the bearing length w i l l r a i s e the l i m i t l i n e f o r f i l m thickness, b u t may worsen a h i g h temperature s i t u a t i o n . e x p e r i m e n t a l l y by Brown and Newman [ 1 4 ] .

The l a t t e r e f f e c t was n o t e d

A t these h i g h speeds the c h a r t s dem-

o n s t r a t e and q u a n t i f y t he advantage o f t h i n n e r o i l s .

I f t h e use o f t h i c k o i l s

i s u n a v o id a b le t he n an i n crea sed cl e ara nce , above t h e v a l u e s g i v e n by Fig.11, may h e l p a t h i g h speeds.

The s l i d e c h a r t s a r e then n o t a p p l i c a b l e , b u t a

s u i t a b l e d e s ig n method i s d e t a i l e d i n s e c t i o n 4.4.3. Finally,

i t i s advisable,

l i m i t i n g lines;

wherever p o s s i b l e , t o work away from t h e a c t u a l

t h i s a l l o w s an e x t r a s a f e t y margin f o r u n i n t e n t i o n a l adverse

c o n d i t i o n s such as small mi sa l i gn men t (see s e c t i o n

4 . 4 . 3 1 , contaminated o i l , etc.

I t i s a l s o a d v i s a b l e t o work w e l l w i t h i n t h e l i m i t i n g l i n e s where t h e r e i s a

chance o f h a v in g two modes o f f a i l u r e a t t he same t i m e ( i e . a t t h e d o t t e d apex i n Fig.1). C u r r e n t p r a c t i c e i s t o q u a n t i f y t h i s a d d i t i o n a l l i m i t i n terms o f s p e c i f i c l oa d ( l o a d d i v i d e d by p r o j e c t e d area, W/bd).

For hydrodynamic b e a r i n g s i t i s

usual t o keep t h e s p e c i f i c l o a d below about 4 MPa, s t i l l checking t h a t f i l m t h i c k n e s s and t e mp era t u res are-acceptable.

However,

t h e l o a d on a b e a r i n g a t

s t a r t - u p o r r u n down must be c o n s i d e r a b l y lower, no more than 1 t o 1.3 MPa o t h e r w i s e h i g h p re ssure j a c k i n g o i l must be s u p p l i e d . Fig.12 has been developed t o g i v e t he maximum s p e c i f i c l o a d r a t i n g based on the t h i n o i l f i l m l i m i t ;

t h i s assumes t h e same geometry and o i l feed c o n d i t i o n s

as c o n s id e r e d i n t h e s l i d e c h a r t .

4.4.2.2

O i l Film I n s t a b i l i t y

T h i s i s o n l y l i k e l y t o be a problem i n l i g h t l y loaded b e a r i n g s a t h i g h speeds. As a f i r s t check c a l c u l a t e t h e v a l u e o f t h e dimensionless c r i t i c a l mass parameter shown i n Fig.3.

I f t h i s i s l e s s t ha n the minimum v a l u e on t h e p a r t i c u l a r

b/d c u r v e , t h e n t h e b e a r i n g sh ou l d be s t a b l e .

I f n o t , then s t a b l e o p e r a t i o n

may s t i l l be p r e d i c t e d p r o v i d e d t h a t t h e e c c e n t r i c i t y r a t i o (l-hmin/cr) l a r g e enough;

s e c t i o n 4.4.3

g i v e s a method for o b t a i n i n g t h i s value.

is The

,

r e s i s t a n c e t o i n s t a b i l i t y i s r a i s e d by any e x t e r n a l damping w i t h i n the system, f o r example gear meshing, so t h a t t h e l i m i t s can be unduly p e s s i m i s t i c .

I f i n s t a b i l i t y i s a problem t h e b ore p r o f i l i n g can o f t e n p r o v i d e t h e s o l u t i o n ; s e c t i o n 5. c o n s i d e r s t h i s i n some d e t a i l .

65

MINIMUM DIAMETRAL CLEARANCE

MINIMUM

(in.]

0.02

0.016

0.012

0.008

0

-

SPEED I rcv/min 1

Fig. 11

4.4.3

Recommended minimum c l e a r a n c e

Performance P r e d i c t i o n

Having determined t h a t a g i v e n b e a r i n g i s l i k e l y t o o p e r a t e s a t i s f a c t o r i l y , t h e d e s ig n e r t h e n o f t e n needs t o know t h e power loss and o i l f l o w requirement o f t h e b e a r in g .

As w e l l as b e i n g o f i n t e r e s t as f a r as t h e o v e r a l l e f f i c i e n c y

o f a machine i s concerned, these q u a n t i t i e s have a d i r e c t i n f l u e n c e on the l u b r i c a n t s u p p ly system coolers etc.

-

t h e s i z e o f o i l pump and supply l i n e s , t h e need f o r

T h i s system must be ca pa bl e o f adequately s u p p l y i n g s u f f i c i e n t

c o o l e d o i l (heat i s t h e t a n g i b l e f o rm o f b e a r i n g power l o s s ) f o r any b e a r i n g w i t h i n t h e spread o f t h e man uf act uri n g t o l e r a n c e on clearance.

When d e t e r m i n i n g

t h e o i l pump c a p a c i t y , t h e b e a r i n g f l o w a t maximum p o s s i b l e c l e a r a n c e ( w i t h i n t h e t o l e r a n c e range) sho ul d be co nsi d ere d s i n c e t h i s has the maximum f l o w req u ir e m e n t .

Maximum power l o s s on t h e o t h e r hand,

i m p o r t a n t when c o n s i d e r i n g

t h e h e a t d i s s i p a t i o n from t h e o v e r a l l system. can occur anywhere w i t h i n t h e t o l e r a n c e range o f cl e ara nce .

The cl e ara nce has t h e r e f o r e been l e f t as a

v a r i a b l e i n t h e p r e d i c t i o n c h a r t s f o r power l o s s , o i l f l o w and temperature. However, t h e c l e a r a n c e range sh ou l d p r e f e r a b l y s t i l l have a minimum v a l u e c o r r e s p o n d in g w i t h Fig.11,

s i n c e t h e s l i d e c h a r t s (which i n h e r e n t l y c o n t a i n

these c l e a r a n c e s ) can t h en be used t o check f o r safe o p e r a t i o n p r i o r t o p r e d i c t i n g b e a r i n g performance.

66

Fig.12

Maximum specific load rating for a two axial groove bearing based on thin oil film limit

67 Under some c r i t i c a l c o n d i t i o n s t h e need t o c o n t r o l t h e v a r i a t i o n s i n f i l m t h i c k n e s s , power loss o r o i l f l o w m i g h t r e g u l a t e t h e range o f t h e c l e a r a n c e t o l e r a n c e s , b u t u s u a l l y p r e d e f i n e d m a n u f a c t u r i n g d e t a i l s w i l l impose a g i v e n c l e a r a n c e range which t h e d e s i g n e r must c a t e r f o r adequately.

The s i z e o f t h i s

t o l e r a n c e range i s m a i n l y a f u n c t i o n o f economic c o n s i d e r a t i o n s , and may t h e r e f o r e vary considerably.

However, a p r a c t i c a l g u i d e i s t h a t t h e t o l e r a n c e (mm)

on d i a m e t r a l c l e a r a n c e l i e s w i t h i n t h e range (2.10 -6 x d i a ) l l 3 t o (5.10 -6 x d i a ) 1/3 depending on t h e m a n u f a c t u r i n g process;

' d i a ' i s t h e nominal s h a f t diameter i n

mn. The minimum f i l m t h i c k n e s s r a t i o (hmin/cr)

i s a s i g n i f i c a n t term when p r e -

d i c t i n g t h e power loss, o i l f l o w and maximum b e a r i n g temperature.

Using t h i s

r a t i o , p r e d i c t i o n c h a r t s i n nomograph f o r m have been d e v i s e d which p e r m i t q u i c k and a c c u r a t e d e t e r m i n a t i o n o f t h e v a r i o u s parameters w i t h v e r y l i t t l e calculation.

4.4.3.1

Minimum O i l F i l m Thickness

The f i l m t h i c k n e s s r a t i o may be o b t a i n e d f r o m Fig.13 f o r known o p e r a t i n g conditions.

B a s i c a l l y t h r e e g r i d s a r e used t o d e f i n e t h e problem, and by

l i n k i n g these a l o n g t h e a p p r o p r i a t e g u i d e l i n e s , as i n d i c a t e d by t h e arrows on the chart, a p o i n t i n the fourth,

'answer' g r i d i s o b t a i n e d .

T h i s c h a r t i s a l s o u s e f u l f o r p r e d i c t i n g t h e minimum f i l m t h i c k n e s s i n a b e a r i n g which has a d i f f e r e n t c l e a r a n c e t o t h e minimum v a l u e shown i n Fig.11, i e a b e a r i n g which cannot be c o n s i d e r e d on t h e s l i d e c h a r t s .

The a c c e p t a b i l i t y

o f any f i l m t h i c k n e s s can be checked on Fig.2.

4.4.3.2

Misalignment

Good a l i g n m e n t between s h a f t and b e a r i n g can be c r i t i c a l l y i m p o r t a n t because o f t h e d r a m a t i c r e d u c t i o n i n o i l f i l m t h i c k n e s s which misalignment causes. There a r e many ways i n which misalignment can o c c u r , f r o m poor b u i l d o f t h e machine t o mechanical d i s t o r t i o n s due t o l o a d o r temperature, and each o f them produces d i f f e r e n t c o n d i t i o n s w i t h i n t h e o i l f i l m .

However, a g e n e r a l guide t o

t h e d e r a t i n g e f f e c t o f misalignment i s g i v e n i n Fig.14.

T h i s shows t h e r e -

d u c t i o n i n o i l f i l m t h i c k n e s s from t h e p e r f e c t l y a l i g n e d case (Fig.13) g i v e n misalignment a c r o s s t h e l e n g t h o f t h e b e a r i n g (6).

for a

The r e s u l t i n g minimum

f i l m t h i c k n e s s , which o c c u r s a t one a x i a l end o f t h e b e a r i n g , s h o u l d be checked

on Fig.2 f o r a c c e p t a b i l i t y .

4.4.3.3

Power Loss and O i l Flow

Power l o s s and o i l f l o w may be determined f r o m Figs.15 and 16.

The method

o f use i s shown on each c h a r t and i s f u r t h e r i l l u s t r a t e d by t h e example i n s e c t i o n 4.6.The

f l o w g i v e n i s t h e b e a r i n g requirement f l o w , and should be

68

Fig.13

P r e d i c t i o n o f minimum o i l f i l m thickness

69 9’0

so E 7’0

5 L.

E‘O 2’0 L‘O

The d e r a t i n g e f f e c t o f mi sa l i gn men t on f i l m t h i c k n e s s

Fia.14

increased when d e t e r m i n i n g pump c a p a c i t y t o a l l o w f o r b e a r i n g wear, t o m u l t i - s u p p l i e d bearings etc.

unequal f l o w

T y p i c a l l y t h e f i g u r e s sho u ld be incre a se d by

20-25%, and a pump chosen which can a de qu at el y su ppl y t h i s amount.

4.4.3.4

Temperatures

The v a l u e o f t h e o i l o u t l e t temp era t u re may be e s t i m a t e d f r o m t h e power l o s s and o i l f l o w v al ues, as read f r o m t h e c h a r t s , u s i n g t h e e q u a t i o n : O u t l e t tem perature = i n l e t temperat ure

-

50

+

t e mpera t ure r i s e

+

A H/Q ( " C )

where A = 0 . 0 0 0 5 f o r H kW and Q m3/s

A = 5

f o r H hp and Q gal / mi n.

(A = 6 f o r hp and US gal /mi n) The maximum b e a r i n g temperature may be determined f r o m Fig.17;

ag a in an

example o f use i s shown on t h e c h a r t and i s f u r t h e r a m p l i f i e d i n s e c t i o n 4.6. I t s houl d be noted t h a t t h i s temp era t u re o c c u r s a t t h e s u r f a c e o f t h e l i n i n g m a t e r i a l and a t some c i r c u m f e r e n t i a l p o s i t i o n w hi ch i s n o t w e l l d e f i n e d ; r o u g h l y c o i n c i d e s w i t h t h e p o s i t i o n o f t h e minimum o i l f i l m t h i c k n e s s .

it

I0

POWER LOSS

UUD IN1 1 U 1

Fig.15

P r e d i c t i o n of b e a r i n g power loss

L FLOW REQUIREMENTS 70 :

1w: 200. OM (mn 500 400 3w

2w /

7000 10000 20000 30000

Fig.16

P r e d i c t i o n o f b e a r i n g o i l f l o w requirement

71

I

Fig.17

here

-

I

P r e d i c t i o n of maximum b e a r i n g temperature

72 Unless a great deal o f thought and care i s taken i n determining the p o s i t i o n o f temperature instrumentation then the recorded temperature w i l l be less than the a c t u a l maximum value.

4.5

H I G H SPEED APPLICATIONS The general t r e n d towards l a r g e r sizes and f a s t e r o p e r a t i n g speeds i n equip-

ment such as compressors and t u r b i n e s has caused d i f f i c u l t i e s w i t h both the design and the o p e r a t i o n o f conventional c y l i n d r i c a l bore bearings.

The design

d i f f i c u l t y occurs when the o i l f l o w w i t h i n the bearing clearance space becomes non-laminar,

and the information presented so f a r ceases t o be v a l i d .

The

operational d i f f i c u l t i e s concern p o s s i b l e i n s t a b i l i t y o f e i t h e r the o i l f i l m (see Sect ion 4.2.1.4)

4.5.1

or of the complete r o t o r system.

Non-Laminar Operation

C o l l o q u i a l l y i t i s common t o r e f e r t o e i t h e r laminar o r t u r b u l e n t o p e r a t i o n , but i n p r a c t i c e there i s a wide o p e r a t i n g band separating the two regimes i n which o t h e r forms o f f l u i d motion, eg Taylor v o r t i c e s , occur,

Since the design

methods d e t a i l e d i n the previous sections a r e f o r laminar operation o n l y i t i s necessary t o have some check on whether laminar o r non-laminar c o n d i t i o n s apply. Fig.18 i s a quick way o f performing t h i s check;

note t h a t i t uses the value o f

f i l m thickness r a t i o (hmin/cr) obtained from the laminar c h a r t Fig.13.

I f non-

laminar operation i s p r e d i c t e d then recourse must be made t o a l t e r n a t i v e design methods and procedures [ 1 5 ] . I t should be emphasised t h a t turbulence per se i s n o t a problem as f a r as safe bearing operation i s concerned, merely a d i f f i c u l t y ( a l b e i t a r e l a t i v e l y major one) w i t h the design process. l a r g e clearances o r t h i n o i l s .

Turbulence i s increased by h i g h speeds,

Bearings o p e r a t i n g i n the t u r b u l e n t regime can

have s i g n i f i c a n t l y higher power losses and temperatures than p r e d i c t e d by

I ami nar theory.

4.5.2

P r o f i l e Bore Bearings

A c y l i n d r i c a l bore bearing has a s i n g l e converging clearance space (which may i n some circumstances be i n t e r r u p t e d by a groove) i n which o i l f i l m pressure i s generated t o support the e x t e r n a l load.

I n c o n t r a s t the bore o f a p r o f i l e

bearing i s arranged so t h a t two o r more separate converging regions a r e present under normal operation, each developing f i l m pressures which a c t i n various d i r e c t i o n s around the bearing.

The s t a b i l i t y o f such a bearing i s b e t t e r than a

c y l i n d r i c a l bore, b u t t h e r e a r e u s u a l l y p e n a l t i e s t o be p a i d i n terms o f reduced load c a p a c i t y o r increased power loss and l u b r i c a n t f l o w requirement.

73

Fig.18

Guidance on t h e 1 i k e l ihood o f l a m i n a r o r non-laminar o p e r a t i o n

The range o f poss b l e p r o f i l e s a r e shown i n Fig.19,

i s almost i n f i n i t e b u t t h e more common forms

t o g e t h e r w i t h an i n d i c a t i o n o f t h e i r r e l a t i v e m e r i t s i n

terms o f v a r i o u s o p e r a t i n g parameters.

O f n e c e s s i t y these comparisons a r e

approximate o n l y , s i n c e minor changes t o e i t h e r o f t h e two c l e a r a n c e s i n v o l v e d i n p r o f i l e b o r e b e a r i n g s ( t h e so c a l l e d 'shake c l e a r a n c e ' which i s a measure o f p o s s i b l e s h a f t movement and t h e hydrodynamic c l e a r a n c e which i s t h e d i f f e r e n c e between r a d i i o f formance.

i n d i v i d u a l lobes and t h e s h a f t ) can s i g n i f i c a n t l y a l t e r p e r -

Experience has shown t h a t as a general r u l e t h e 'shake c l e a r a n c e ' ,

as a r a t i o ( i e c d / d ) ,

should be no t i g h t e r than 0.001 mm/mrn.

T h i s then a v o i d s

p o s s i b l e problems w i t h l o s s o f c l e a r a n c e on s t a r t - u p due t o d i f f e r e n t i a l

74 thermal expansion o f t h e s h a f t r e l a t i v e I n f o r m a t i o n on s t i f f n e s s and damping

o t h e b e a r i n g and housing.

s i m p o r t a n t when c o n s i d e r i n g t h e

dynamics o f t h e machine as a whole, w i t h s t i f f n e s s i n f l u e n c i n g t h e c r i t i c a l speeds o f t h e r o t o r , and damping c o n t r o l i n g t h e v i b r a t i o n a m p l i t u d e s when running through t h e c r i t i c a l s . In o r d e r t o g i v e a f e e l f o r i n s t a b i l i t y t h r e s h o l d speeds f o r r e a l i s t i c

clearances,

Fig.20,

has been prepared.

T h i s shows t h e minimum speed a t which

o i l f i l m i n s t a b i l i t y can o c c u r f o r a h o r i z o n t a l , w e i g h t loaded, r o t o r system ( i e where t h e b e a r i n g l o a d i s p u r e l y due t o t h e mass).

Fig.19

Comparison o f s t a t i c and dynamic c h a r a c t e r i s t i c s o f commonly used journal bearing types

15

Comprehensive and d e t a i l e d p r o c e d u r e s f o r t h e d e s i g n o f p r o f i l e b o r e b e a r i n g s have n o t been p u b l i s h e d , and c u r r e n t l y t h e d e s i g n e r must e i t h e r r e s o r t t o a f u n d a m e n t a l t h e o r e t i c a l s t u d y o r seek a d v i c e , u s u a l l y on a s p e c i f i c c a s e basis,

f r o m s p e c i a l i s t s [16].

IS0 VG L6 @, 50' INLET 1 MEDIUM TURBINE GRADE 1 b/d : 0.5

OIL:

LOO.

-

CLEARANCE VARIED TO MAINTAIN :

300-

DRAIN TEMPERATURE < 75'C MATERIAL TEMPERATURE < 100°C

W

LL

0

m

4LL 9m

200-

100-

LEMON BORE

0

F 9.20

4.6

10000 20000 30000 JOURNAL SPEED I revlmin 1

O i l f i l m whirl

-

LOO00

i n s t a b i l i t y of various types of journal bearings L i m i t i n g speeds f o r h o r i z o n t a l r o t o r s

EXAMPLE OF USE OF DESIGN AIDS

4.6.

Problem

A g e a r b o x b e a r i n g 200 mm d i a m e t e r and 50 mm l o n g h a s t o c a r r y a l o a d o f 20 kN a t 3600 r e v / m i n .

bearing length,

The b e a r i n g has t w o a x i a l g r o o v e s e x t e n d i n g 0.8 o f t h e

s i t u a t e d a t ? 90" t o t h e l o a d l i n e .

The o i l

i s w i t h i n t h e IS0

VG 32 s p e c i f i c a t i o n ( a c t u a l t y p i c a l v i s c o s i t y v a l u e s a r e g i v e n a s 30 c S t a t 40°C and 5.2 c S t a t l O O " C , temperature

t h e o i l d e n s i t y b e i n g 880 kg/m3).

The o i l f e e d

i s 50°C and t h e o i l f e e d p r e s s u r e 1 b a r (0.1 MPa).

Check t h a t t h i s b e a r i n g w i l l o p e r a t e s a f e l y and e s t i m a t e o i l f i l m t h i c k n e s s , power l o s s , o i l f l o w r e q u i r e m e n t , maximum b e a r i n g t e m p e r a t u r e and o i l o u t l e t temperature.

76 4.6.2

Procedure

T h i s p a r t i c u l a r e x a m p l e h a s been i n c o r p o r a t e d o n t h e d e s i g n a i d s a s dashed

1 ines.

4.6.2.1

Diametral Clearance t h e minimum d i a m e t r a l c l e a r a n c e , f o r a 200 mm d i a m e t e r b e a r i n g

From F i g . 1 1 ,

o p e r a t i n g a t 3600 r e v / m i n ,

4.6.2.2

Maximum S p e c i f i c Load R a t i n g t h e maximum s p e c i f i c l o a d r a t i n g ( b a s e d o n t h i n o i l f i l m

From F i g . 1 2 , limit),

i s 0.26 mm.

i s o b t a i n e d by f o l l o w i n g t h e g u i d e l i n e s , through t h e r e l e v a n t o i l grade,

l e n g t h , speed and d i a m e t e r .

The a l l o w a b l e l i m i t i s seen t o be 4.3

MPa, w e l l

above t h e a c t u a l s p e c i f i c l o a d o f 2 MPa.

4.6.2.3

Region o f Safe O p e r a t i o n

Use F i g . l O ( a ) , on g r i d ,

transparent copy, w i t h Fig.lO(b),

(200 mm).

Place transparency on backing sheet,

l i n i n g u p marked p o i n t w i t h

c a n t h e n e a s i l y be i n t e r p o l a t e d .

parency, F i g . l O ( b ) ,

Mark p o i n t

and d i a m e t e r

The l i m i t i n g l i n e f o r s a f e o p e r a t i o n , w i t h b / d

datum p o i n t ( s e e F i g . 2 l ( a ) ) . equal t o 0.25,

backing sheet.

c o r r e s p o n d i n g t o o i l g r a d e (30 c S t a t 40°C)

Fig.lO(a),

The t e m p e r a t u r e l i m i t t r a n s -

i s p o s i t i o n e d i n a n i d e n t i c a l manner ( s e e F i g . 2 1 ( b ) ) .

BACKING SHEET TRANSPARE,NT OVERLAY r - - - - - - --1

I

I I

I

I

I

I

I

I I

I I< 1;

b" /

o'/\ /'

'

THIN FILM LIMIT

i __----SPEED

I I

I I

I I J

I $1

HIGH

A\

I I I

SI

J

( a1

Fig.21

I

TEMPERATURE LIMITS

SPEED

Example use o f s l i d e c h a r t s

DESIGN POINT IS WITHIN LIMITS \

I

I

0

a

1 0

3600 rei/rnin

I SPEED

(C)

These t w o c o m b i n e d g i v e g u i d a n c e t o a r e g i o n o f s a f e o p e r a t i o n o n a l o a d , speed f r a m e w o r k a s shown i n F i g . Z l ( c ) . well within t h i s l i m i t i n g region,

4.6.2.4

The a c t u a l o p e r a t i n g c o n d i t i o n s a r e

i n d i c a t i n g a r e l i a b l e design.

P r e d i c t i o n o f Minimum O i l F i l m T h i c k n e s s

F i g . 1 3 p r e d i c t s minimum o i l f i l m t h i c k n e s s a n d i s v e r y e a s y t o u s e , a l t h o u g h looking f a i r l y formidable.

F i r s t mark p o i n t s on t h e f o l l o w i n g t h r e e g r i d s .

1) C l e a r a n c e r a t i o and speed (0.0013 and 3600 r e v / m i n )

2 ) l e n g t h t o d i a m e t e r r a t i o and o i l ( 0 . 2 5 a n d 30 c S t a t 40°C)

( 2 MPa a n d 30 c S t a t 40°C)

3) s p e c i f i c l o a d a n d o i l

J o i n t h e s e p o i n t s a s shown, f o l l o w i n g t h e g u i d e l i n e s , and c o n t i n u e t o t h e answer g r i d i n t h e t o p r i g h t hand c o r n e r ; ness r a t i o ( h m i n / c r )

Minimum o i l f i l m t h i c k n e s s hmin

4.2.6.5

a t b/d equal t o 0.25 t h e f i l m t h i c k -

i s r e a d o f f d i r e c t l y a s 0.16. =

0.16

x 0 . 1 3 = 0.021

mm

Check f o r L a m i n a r O p e r a t i o n

Fig.18 g i v e s guidance on whether l a m i n a r , as p r e s e n t e d h e r e , o r non-laminar d e s i g n methods [ l 5 ] s h o u l d be used.

In consistent units P = 2 . 1 0 6 N/m2

d

=

0.2rn,

N

I n F i g . 1 8 mark p o i n t where Y

=

-

and c i d = G . 0 0 1 3 d =

60 rev/s,

ti

=

880 kg/m3,

f r o m w h i c h Y = 48.74.

g=

1.76 and b / d

= 0.25

(on lower g r i d ) .

P r o j e c t v e r t i c a l l y upwards a s i n d i c a t e d b y dashed l i n e t o f i l m t h i c k n e s s r a t i o

o f 0.16.

The r e s u l t i n g p o i n t a p p e a r s i n t h e l a m i n a r r e g i o n ,

therefore the

design a i d s presented here a r e s u i t a b l e .

4.6.2.6

P r e d i c t i o n o f Power L o s s

On F i g . 1 5 ,

using the value of hmin/cr a l r e a d y obtained (0.16),

j o i n up t h e

a p p r o p r i a t e p o i n t s on t h e s c a l e s a s shown. Power l o s s = 5.3

4.6.2.7

kW

P r e d i c t i o n o f O i l Flow

On F i g . 1 6 ,

s t a r t i n g a t t h e s p e e d - c l e a r a n c e r a t i o g r i d , j o i n up t h e a p p r o p -

r i a t e p o i n t s a s shown.

O i l f l o w r e q u i r e m e n t = 0.00021 m 3 / s

(0.21

1/51

78 4.6.2.8

P r e d i c t i o n o f Maximum Bearing Temperature

From Fig.17,

t h e maximum b e a r i n g temperature i s o b t a i n e d by f o l l o w i n g t h e

rd

numbered steps s h o w n ~ i n t h e sketch i n t h e lower l e f t hand c o r n e r . Ca 1 c u 1a t e

t

lo3

]

(=o.oao47)

M u l t i p l y t h i s by t h e m o d i f i e d s p e c i f i c l o a d P

(=0.136)

S t a r t i n g a t hmin/cr equal t o 0.16 f o l l o w t h e g u i d e l i n e s as shown, through values o f b/d r a t i o , o i l grade, and t h e two terms c a l c u l a t e d above. e f f e c t i v e temperature ( O e ) , value o f

emax/ee

(=1.55)

A v a l u e of

can be o b t a i n e d a t t h e end o f s t e p 3 (=65"C), and a

f r o m s t e p 5 (which completes t h e pentagon).

Maximum temperature = l 0 l ' C

4.6.2.9

P r e d i c t i o n o f O i l O u t l e t Temperature

From e q u a t i o n (1)

-

Section

4 4.1.4.

O i l o u t l e t temperature = 50 + 0.0005

= 50

-HQ

+ 0.0005 x 5.3 0.00021

= 63°C

REFERENCES 1 2

3 4 5 6

7 8

9

10

11

Elwel1,R.C. and Booser,E.R., 'Low Speed L i m i t o f L u b r i c a t i o n P a r t 1, What i s a "Too Slow" B e a r i n g ? ' , Machine Design, 15 June, 1972. Martin,F.A., 'Minimum A l l o w a b l e O i l F i l m Thickness i n S t e a d i l y Loaded Journal B e a r i n g s ' , Proc. L u b r i c a t i o n and Wear Convention 1964 ( I n s t . Mech. Engrs. London), V01.178, P t . 3 N pp. 161-167. Booser,E.R., Ryman,F.D. and Linkinhoker,C.L., 'Maximum Temperature f o r Hydrodynamic Bearings under Steady L o a d ' , ASLE Trans. 1970, Vol.26, No.7. T r i b o l o g y Handbook, 1973, S e c t i o n B1, B u t t e r w o r t h s . Lund,J.W., ' S e l f - e x c i t e d S t a t i o n a r y W h i r l O r b i t s o f a Journal i n a Sleeve B e a r i n g ' , Thesis, 1966, Rensselaer P o l y t e c h n i c I n s t i t u t e , N Y . Pinkus,O. and S t e r n l i c h t , B . , 'Theory o f Hydrodynamic L u b r i c a t i o n ' , 1961, Chapter 4, McGraw-Hi 11. Cameron ,A. ' P r i n c i p l e s o f L u b r i c a t i o n I , 1966, Chapter 13, Longmans. Lund,J.W. and Thomsen,K.K., 'A C a l c u l a t i o n Method and Data f o r t h e Dynamic C o e f f i c i e n t s o f O i l - L u b r i c a t e d J o u r n a l B e a r i n a s ' f r o m 'TODiCS i n F l u i d F i l m Bearings and Rotor B e a r i n g System Designland O p t i m i z a t i o n ' , 1978, ASME. ' C a l c u l a t i o n Methods f o r S t e a d i l y Loaded Pressure Fed Hydrodynamic Journal B e a r i n g s ' , Engineering Sciences Data U n i t Item 66023, Sept. 1966 ( I n s t . Mech. Engrs., London). 'Computer S e r v i c e f o r P r e d i c t i o n o f Performance o f S t e a d i l y Loaded Pressure Fed Hydrodynamic Journal B e a r i n g s ' , Engineering Sciences Data U n i t , Item 69002, Sept. 1972, Amendment B. Martin,F.A. and Garner,D.R., ' P l a i n Journal Bearings under Steady Loads Design Guidance f o r Safe O p e r a t i o n ' , F i r s t European T r i b o l o g y Congress, 1973 ( I n s t . Mech. Engrs., London) paper C313/73.

79 12

13 14 15

16

Blok,H., D i s c u s s i o n t o Conference. Proc. Conf. L u b r i c a t i o n and \.!ear, 1957 ( I n s t . Mech. Engrs., London), pp. 745-747. Moes,H. and Bosma,R., ' D e s i g n C h a r t s f o r Optimum B e a r i n g C o n f i g u r a t i o n 1 . The F u l l J o u r n a l B e a r i n g ' A p r i l 1971, ASME Trans. Vol.93, S e r i e s F, No.2, pp. 302-306. Brown,T.W.F. and Newman,A.O., 'High-speed H i g h l y Loaded B e a r i n g s and T h e i r Development', Proc. Conference on L u b r i c a t i o n and Wear, 1957 ( I n s t . Mech. E n g r s ) , pp. 20-27. Garner,D.R., Jone5,G.J. and Martin,F.A. ' T u r b u l e n t J o u r n a l B e a r i n g s Design C h a r t s f o r Performance P r e d i c t i o n ' , J u l y 1977, ASLE Trans. V01.20, No.3, pp. 221-232. Garner,O.R., Lee,C.S, and Martin,F.A., ' S t a b i l i t y o f P r o f i l e Bore B e a r i n g s : I n f l u e n c e o f B e a r i n g Type S e l e c t i o n ' , October 1980, T r i b o l o g y I n t e r n a t i o n a l , pp. 204-210.

80

5 R.W.

THE DIAGNOSIS

WILSON and E . B .

S h e l l Research L t d . ,

5.1

OF PLAIN BEARING FAILURES

SHONE Thornton Research Centre, P.O.

Box 1, C h e s t e r CH1 3SH

I NTRODUCT I ON

A f e w y e a r s ago t h e c a s u a l o b s e r v e r m i a h t have c o n c l u d e d t h a t b o t h t h e m e t a l l u r g y o f b e a r i n g s a n d t h e u n d e r s t a n d i n g o f t h e i r f a i l u r e mechanisms h a d r e a c h e d a s e t t l e d s t a g e i n w h i c h b e a r i n g m a t e r i a l s were a v a i l a b l e t o meet a l m o s t every requirement and t h a t c h a r a c t e r i s t i c f a i l u r e s were well-documented.

Thus,

most b e a r i n g f a i l u r e s c o u l d be d e s c r i b e d q u i t e a d e q u a t e l y f r o m a s t r i c t l y metallurgical o r s t r i c t l y engineerins viewpoint. only i n passing;

The l u b r i c a n t was m e n t i o n e d

a s l o n g a s i t had t h e r i q h t v i s c o s i t y , p r o v i d e d a d e q u a t e

c o o l i n g and d i d n o t c o r r o d e t h e b e a r i n o a l l o y ,

i t gave l i t t l e c a u s e f o r c o n c e r n .

O p e r a t o r s f r e q u e n t l y blamed l u b r i c a n t s f o r b e a r i n q f a i l u r e s , b u t b e a r i n g manu f a c t u r e r s a c k n o w l e d g e d t h a t v e r y f e w f a i l u r e s c o u l d be a t t r i b u t e d t o i n f e r i o r lubricants.

T h e r e was n o s p e c i a l p r e s s u r e o n l u b r i c a t i o n t e c h n o l o q i s t s t o

become e x p e r t s o n b e a r i n g f a i l u r e s ;

i n any case, b e a r i n g manufacturers g e n e r a l l y

provided e x c e l l e n t t e c h n i c a l s e r v i c e t o t h e i r customers.

However, e n g i n e e r i n g

d e v e l o p m e n t s i n r e c e n t y e a r s have made i n c r e a s i n g l y s e v e r e demands o n p l a i n b e a r i n g s and have g i v e n r i s e t o r a t h e r more c o m p l i c a t e d f a i l u r e mechanisms. Many o f t h e s e f a i l u r e s c a n be d e s c r i b e d a s t r i b o l o g i c a l f a i l u r e s ;

they cannot

be a s c r i b e d t o a p a r t i c u l a r d e f e c t i n t h e d e s i g n o r i n t h e m e t a l l u r g y o f t h e bearing or t o a shortcoming i n t h e l u b r i c a n t

-

t h e y a r e t h e consequence of

the

i n t e r a c t i o n o f a number o f f a c t o r s . Before i t i s p o s s i b l e t o t a l k about p l a i n bearing f a i l u r e s

it i s essential t o

have an u n d e r s t a n d i n g o f t h e m a t e r i a l s u s e d , how t h e y a r e made a n d t h e r e q u i r e ments t h a t must be met.

T h e r e a r e a number o f v e r y good p u b l i c a t i o n s d e s c r i b i n g

t h e c o n s t r u c t i o n and p r o p e r t i e s of p l a i n b e a r i n g s and we i n t e n d t o b r i e f l y summarise t h e s e .

5.2

PROPERTIES R E Q U I R E D OF BEARING MATERIALS

I t i s e v i d e n t t h a t a b e a r i n g must w i t h s t a n d a v a r i e t y o f imposed c o n d i t i o n s . No b e a r i n g i s e q u a l l y good w i t h r e s p e c t t o a l l r e q u i r e m e n t s ,

and t h e s e l e c t i o n

o f t h e most s u i t a b l e b e a r i n g m a t e r i a l f o r a p a r t i c u l a r s e t o f circumstances r e q u i r e s a c a r e f u l e v a l u a t i o n o f t h e most c r i t i c a l o p e r a t i n g f a c t o r s . t h e f a c t o r s t h a t h a v e t o be c o n s i d e r e d a r e o u t l i n e d i n S e c t i o n s 5.2.1

Some o f

-

5.2.8.

81 5.2.1

Fatigue Resistance

This is the most important property in those applications where the load varies.

However, fatigue failure in bearings is frequently associated with some

other factor, such a s corrosion, which reduces the strength o f the bearing, or wear, which reduces the load-carrying area.

The fatigue strength of a fund-

amentally weak bearing alloy can be increased by making the bearing alloy thin and bonding i t firmly to a bronze or steel backing. 5.2.2

Compressive Strength

This is the steady load that the bearing alloy can support without extruding. There is little correlation between compressive strength and fatigue resistance when a single class o f bearing material is considered.

5.2.3

Conformability

This is the ability t o compensate for misalignment that occurs a s a consequence o f bad design or manufacture or that may develop in service.

5.2.4

Embeddabil ity

This i s the ability to tolerate and absorb foreign particles, thereby avoiding scoring or wear.

5.2.5

Strength at Elevated Temperatures

High sliding speeds and heavy loads can generate considerable heat, even when a bearing is operating hydrodynamically.

One of the main functions of the lub-

ricant and the bearing alloy i s to conduct heat away from the sliding surfaces; even so, bearings are often required to operate at elevated temperatures, and a lack o f high-temperature strength may result in extrusion of the bearing alloy and/or fatigue failure.

5.2.6 Compa t i b i 1 i ty All bearing assemblies experience some metal-to-metal contact at some stage i n their lives;

the resistance of the bearing metal/journal combination to

seizure is therefore important.

However some of the harder bearing alloys do

not function satisfactorily against unhardened steel journals.

5.2.7

Corrosion Resistance

Bearings may be exposed to weak organic acids formed a s a result of the oxidation of lubricating oils in service.

They may also be subject to corrosion

by weak organic acids and strong mineral acids derived from fuel combustion products.

Sometimes the operating environment is corrosive

chemical plants and marine installations.

-

for example. on many

a2

cost

5.2.8

I t i s e s s e n t i a l t h a t b e a r i n g a l l o y s a r e cheap and easy t o manufacture.

TYPE, CONSTRUCTION AN0 CHARACTERISTICS OF PLAIN BEARING MATERIALS

5.3

Some o f t h e more w i d e l y used b e a r i n g a l l o y s a r e l i s t e d i n Table 5.1;

the

more commonly used o v e r l a y s a r e d e s c r i b e d i n Table 5.2.

-

Sections 5.3.1

5.3.7

o u t l i n e t h e methods by which b e a r i n g s a r e u s u a l l y

manufactured f r o m these a l l o y s and d e s c r i b e t h e c h a r a c t e r i s t i c s o f t h e v a r i o u s bearing materials.

The c h a r a c t e r i s t i c s o f t h e v a r i o u s b e a r i n g m a t e r i a l s a r e

summarised i n Table 5.3 and t h e g e n e r a l p r o p e r t i e s b r i e f l y discussed i n Section 5.3.8.

5.3.1

White M e t a l s ( B a b b i t s )

These a r e t i n - b a s e d o r lead-based a l l o y s , t h e o r i g i n a l t i n - b a s e d w h i t e metal being invented by Isaac B a b b i t t i n 1839. t h e manufacture o f b o t h t h i c k ,

large,

T h i s a l l o y i s w i d e l y used today f o r

i n d i v i d u a l l y manufactured b e a r i n g s and

t h i n - w a l l e d b e a r i n g s made f r o m c o n t i n u o u s s t r i p . The coated s t r i p i s c u t i n t o s e c t i o n s o f a s u i t a b l e s i z e which a r e p r e s s Newer m a n u f a c t u r i n g techniques p e r m i t t h e use of b e a r i n g

formed t o shape.

metal combinations and c o n t r o l l e d m i c r o s t r u c t u r e s t h a t c o u l d n o t o t h e r w i s e be produced. Another i n n o v a t i o n i s t h e three-component

(or tri-metal)

bearing,

i n which

one o f t h e harder b e a r i n g m a t e r i a l s such as bronze, a l r e a d y bonded t o a s t e e l backing,

i s covered w i t h a t h i n (20-200 urn), e l e c t r o d e p o s i t e d o r c a s t o v e r l a y

o f white metal. The t i n - b a s e d a l l o y c o n s i s t s of a t i n - r i c h m a t r i x , w i t h some antimony and copper i n s o l i d s o l u t i o n ; needles o f Cu6Sn5.

d i s p e r s e d i n t h e m a t r i x a r e cuboids o f SbSn and

T h i s m i c r o s t r u c t u r e l e d t o t h e erroneous b e l i e f t h a t

b e a r i n g a l l o y s must c o n s i s t o f h a r d c r y s t a l s d i s p e r s e d i n a s o f t m a t r i x .

In

f a c t , t h e i n t e r m e t a l l i c compounds i n w h i t e m e t a l s appear t o serve no purpose o t h e r than t o s t r e n g t h e n t h e a l l o y s . White m e t a l s a r e o u t s t a n d i n g l y good b e a r i n g a l l o y s i n many r e s p e c t s , t h e i r main d e f e c t b e i n g l a c k o f l o a d - c a r r y i n g c a p a c i t y , p a r t i c u l a r l y a t e l e v a t e d temperatures.

The c o r r o s i o n r e s i s t a n c e o f lead-based w h i t e m e t a l s i s i n f e r i o r

t o t h a t o f t i n - b a s e d w h i t e metals;

nevertheless,

w i d e l y used, p a r t i c u l a r l y i n t h e U.S.A.

lead-based a l l o y s a r e q u i t e

TABLE 5.1 Some l u b r i c a t e d b e a r i n g m a t e r i a l s Bearing m a t e r i a l

Major a l l o y i n a elements

1. Lead-based w h i t e metal ( lead-Babb i t t )

8-16% antimony 5-11% t i n

Lead

2. Tin-based w h i t e metal ( t i n Babbitt)

7-14% a n t imony 3- 9% copper

Tin

3. S i n t e r e d c o p p e r - l e a d 4. Cast c o p p e r - l e a d

20-50% l e a d

Copper

20-30% l e a d

Copper

5. Lead-bronze

20-30% l e a d 3- 5% t i n

Copper

6. Aluminium-low t i n

7. Aluminium-high t i n 8. A l uminium-si 1 i c o n

6% t i n , 1.5% s i l i c o n 20% t i n ,

1.0% copper

11% s i l i c o n ,

1 % copper

A l umi n i um

A l umi n i um

A l umin i um

(overlay plated) 9. A l umin i um-Babb i t t

10% lead, 2% t i n

A l umi n ium

8-12% t i n , 0.2-1.0% phosphorous

Copper

11. S i l icon-bronze

1.5-4% s i l i c o n

Copper

12. Lead-bronzes ( g e n e r a l l y s t e e l -backed)

5-10% t i n 8-12% l e a d

Copper

10. Phosphor-bronze ( c a s t )

13. S i l v e r

14. Porous bronze

8-12% t i n

15. B r o n z e - g r a p h i t e composites

5-40% g r a p h i t e

16. Laminated r e s i n s

F i b r e - r e i n f o r c e d phenol i c s o r epoxies

17. PTFE and f i 1 l e r i n

PTFE and l e a d

Metal

Bronze and g r a p h i t e

PTFE

fillers 19. Metal-backed thermoplastics

Nylon o r p o l y a c e t a l

G e n e r a l l y used w i t h 90% lead-10% t i n o r 96% lead-4% i n d i u m e l e c t r o d e p o s i t e d o v e r l a y , 25 urn t h i c k ; l e a d bronzes can a l s o have t h i n - B a b b i t t o v e r l a y s

1 1

Hardened j o u r n a l s p r e f e r r e d

S t i l l under development

P r i ma r i 1 y bush i ng mate r ia 1

E l e c t r o p l a t e d , 2 5 0 - 5 0 0 ~ 1t h i c k , o f t e n used w i t h l e a d - i n d i u m o v e r l a y Self-lubricating,

T i n , bronze

impregnated w i t h o i l

Can be used d r y Usually water-lubricated

metal m a t r i x

18. PTFE m a t r i x and metal

Spec i a 1 f e a t u r e s

Remainder

Good under c o n d i t i o n s o f m a r g i n a l lubrication

W

TABLE 5.2 Commonly used overlays Overlay type

Special features

Major alloying elements

~

~~

A

lead-tin, electroplated

8-12% tin

Simultaneously precision-plated, about inch thick (25 urn)

0.001

B

C

D

Lead-tin-copper, electroplated Lead-indium, electroplated

Lead-Babbitt, cast Tin-Babbitt, cast

E Satco alloy, cast

8 - 1 2 % tin 1% copper

4-8% indium

Antimony-tin Antimony-copper Lead, with 2% tin and 0.5% calcium

Simultaneously precision-plated, about inch thick (25 urn)

0.001

Precision-plated, first lead then indium, then alloyed by diffusion; 0.001 inch thick (25 urn) 0.005-0.015

inch thick (125-375

about thick)

TABLE 5 . 3 C h a r a c t e r i s t i c s o f b e a r i n g rnater i a l s M a t e r i a l (see Table 1)

Relative load-carrying capacity MN/m2 Ib/in2

Embeddabil i t y S e i z u r e resistance (tolerance for dirt)

Maximum operating temp. "C

Tolerance f o r Corrosion r e s i s t a n c e m i s a l i g n m e n t Organic a c i d s M i n e r a l a c i d s

1

2000

14

Excellent

Very good

130

Very good

Moderate

Fair

2

2000

14

Excellent

Very good

130

Very good

Excel l e n t

Very good

3

3000-5000

20-35

4

4000-6000

28-41

5 6

5000-8000

35-55

7 8

Moderate with-Moderate w i t h out overlay; out overlay; good w i t h good w i t h o v e r Iay o v e r I ay

150

Good

Fair

160

Moderate

Poor

170

Fair

Fa i r

Poor

Fair

180

Poor

Good

Poor Fa i r

8000

55

5000

35

Poor

Fa i r

170

Fa i r

Good

Fa i r

8000

55

Poor

Moderate

180

Fa i r

Good

Moderate

9

5000

35

Poor ?

Moderate

160

Fa i r

Poor ?

Fair ?

10

8000+

55+

Poor

Moderate

220

Fa i r

Fa i r

Fa i r

11

8000+

55+

Poor

Fa i r

220

Poor

Moderate

Fa i r

12

6000

41

Moderate

Moderate

180

Moderate

Fa i r

Moderate

13

8000+

55+

Poor

Poor

180

Poor

Good e x c e p t f o r sulphur

Moderate

130

Fa i r

Poor

Fa i r

Withstands Fair occasional very h i g h temp.

Fa i r

Fa i r

14

2000-4000

15

Very v a r i a b l e

16

High

14-28

Moderate

Good Good

Moderate

Good

100-300

Moderate

Good

Good Good

Good Good

Good Good Good

Good

Good

Very Good

depending on resin

17 18 19

Comparat i v e l y poor; depends on o p e r a t i n g conditions

Fair

Very Good

Fa i r

Very Good

Fair

Good

250 250 100-350 depending on

Spec i a 1 features

1

Resistance t o organic acids g r e a t 1 y improved by o v e r l a y

Excellent for high loads a t low speeds

I

Some p l a s t i c s a r e damaged by t h e s o l v e n t a c t i o n of 1 ubricants

86 5.3.2

Copper-lead A l l o y s

These can be regarded as t h e f i r s t o f t h e modern a l l o y s .

They a r e blanked

from c o n t i n u o u s l y c o a t e d s t e e l s t r i p , t h e b e a r i n g a l l o y b e i n g a p p l i e d e i t h e r by c a s t i n g or by s i n t e r i n g . the l i q u i d s t a t e , and,

Lead and copper a r e i m m i s c i b l e i n b o t h t h e s o l i d and

i n o r d e r t o a v o i d complete s e g r e g a t i o n when c a s t i n g , t h e

moving s t e e l s t r i p i s c h i l l e d immediately a f t e r c a s t i n g by a w a t e r spray on t h e underside.

T h i s r e s u l t s i n a pronounced d e n d r i t i c s t r u c t u r e , w i t h c r y s t a l s o f

copper and l e a d normal t o t h e s t e e l backing and t h e b e a r i n g s u r f a c e . I n t h e s i n t e r i n g process t h e copper-lead powder i s compacted a f t e r a p p l i c a t i o n and then s i n t e r e d .

T h i s method o f manufacture g i v e s good c o n t r o l o f l e a d d i s t -

r i b u t i o n and r e s u l t s i n a homogeneous and e q u i a x i a l s t r u c t u r e . Both types o f copper-lead b e a r i n g a r e s a i d t o r e l y on a t h i n , e x t r u d e d s u r f a c e f i l m o f l e a d f o r s a t i s f a c t o r y performance.

I n many ways they a r e

s i m i l a r , b u t t h e d i f f e r e n t methods o f manufacture g i v e r i s e t o i m p o r t a n t d i f f erences i n performance i n c e r t a i n r e s p e c t s .

I n t h e c a s t a l l o y t h e l o n g copper

d e n d r i t e s enhance t h e l o a d - c a r r y i n g c a p a c i t y and f a c i l i t a t e h e a t f l o w away from t h e b e a r i n g surface. distribution.

I t i s , however, d i f f i c u l t t o o b t a i n a s a t i s f a c t o r y l e a d

On t h e o t h e r hand,

i t i s much e a s i e r t o ensure a u n i f o r m l e a d

d i s t r i b u t i o n by s i n t e r i n g techniques, and t h e s i n t e r e d a l l o y i s t h e r e f o r e less susceptible t o corrosion. Copper-lead b e a r i n g s have g r e a t e r s t r e n g t h and b e t t e r h i g h - t e m p e r a t u r e performance than w h i t e m e t a l s , b u t i n most o t h e r r e s p e c t s t h e y a r e i n f e r i o r . I n p a r t i c u l a r , they a r e s u s c e p t i b l e t o c o r r o s i o n , cannot t o l e r a t e as much d i r t and g e n e r a l l y r e q u i r e hardened j o u r n a l s .

I n o r d e r t o overcome these d i s a d -

vantages, copper-lead b e a r i n g s a r e g e n e r a l l y o v e r l a y - p l a t e d , e l e c t r o d e p o s i t o f a 90% l e a d

-

applied t o the bearing surface. plated;

1 0 % t i n o r a 95% l e a d

-

25 urn)

t h e a l l o y i s co-

t h e l e a d i s p l a t e d f i r s t and then t h e

indium, which i s f i n a l l y d i f f u s e d i n t o t h e lead. o v e r l a y i s merely a r u n n i n g - i n a i d .

(%

5% indium a l l o y b e i n g

I n t h e case o f l e a d - t i n ,

i n t h e case o f lead-indium,

a thin

It i s often stated that t h i s

T h i s i s n o t t h e case;

expected t o l a s t t h e l i f e o f t h e b e a r i n g .

the overlay i s

It provides a seizure-resistant

surface, a l l o w s s o f t s h a f t s t o be used, increases t h e a b i l i t y o f t h e b e a r i n g t o absorb d i r t and combats c o r r o s i o n o f t h e pure l e a d i n t h e u n d e r l y i n g copperlead.

Since i t i s so t h i n i t d e r i v e s c o n s i d e r a b l e s u p p o r t f r o m t h e u n d e r l y i n g

b e a r i n g a l l o y , and t h e f a t i g u e s t r e n g t h o f t h e composite i s h a r d l y impaired.

5.3.3

Lead Bronzes

The h i g h - t e m p e r a t u r e performance and l o a d - c a r r y i n g c a p a c i t y o f copper-lead a l l o y s can be improved by t i n a d d i t i o n s . copper, t h e r e b y s t r e n g t h e n i n g t h e b e a r i n g ; subject t o corrosion.

The t i n d i s s o l v e s c o m p l e t e l y i n t h e t h e l e a d remains u n a l l o y e d and

L i k e copper-lead b e a r i n g s , lead-bronze b e a r i n g s can be

c a s t o r s i n t e r e d , and show t h e same disadvantages.

L a r g e r - s i z e b e a r i n g s may have a t h i n l a y e r (100-200 pin)

usually overlay-plated.

o f tin-based w h i t e metal c a s t on them; micro-Babbitt o r tri-metal This m i c r o - B a b b i t t

For t h i s reason they a r e

such b e a r i n g s a r e sometimes c a l l e d

bearings.

l a y e r should always be separated from t h e lead-bronze by

a d i f f u s i o n b a r r i e r , o t h e r w i s e t h e bronze w i l l a l l o y w i t h t h e t i n - r i c h B a b b i t t t o form a hard,

i n t e r m e t a l l i c phase which can damage t h e s h a f t .

Lead-bronzes a r e o f t e n used f o r l i t t l e - e n d bushes i n p i s t o n s f o r t u r b o charger bearings.

5.3.4

Aluminium A l l o y s

Two a l l o y s a r e i n widespread use, one c o n t a i n i n g about 6% t i n and t h e o t h e r 20% t i n .

The 6% t i n a l l o y may be used i n massive f o r m o r bonded t o s t e e l , b u t ,

unless i t i s o v e r l a y - p l a t e d ,

i t r e q u i r e s hardened j o u r n a l s i f wear i s t o be

kept w i t h i n a c c e p t a b l e l i m i t s .

The 20% t i n a l l o y i s a more r e c e n t development

and i t s method o f manufacture p r o v i d e s a good example o f t h e advanced t e c h n o l ogy a p p l i e d t o b e a r i n g manufacture. H i g h - t i n aluminium a l l o y s , as c a s t , have v e r y poor mechanical p r o p e r t i e s , because t h e t i n forms a c o n t i n u o u s network e n c l o s i n g t h e p r i m a r y aluminium crystals.

By c o l d - w o r k i n g and low-temperature h e a t t r e a t m e n t t h i s continuous

phase can be broken up t o produce an i n t e r l o c k i n g network s t r u c t u r e ,

t h e so-

c a l l e d r e t i c u l a r s t r u c t u r e which has g r e a t l y improved mechanical p r o p e r t i e s . However, t h e a l l o y i s s t i l l i n s t r i p form and must be bonded t o a t h i n , s t e e l s t r i p backing.

T h i s i s achieved by a c o n t i n u o u s p r e s s u r e - w e l d i n g o p e r a t i o n

c a r r i e d o u t between r o l l e r s , t h e bonding between t h e two s t r i p s b e i n g promoted by a v e r y t h i n sheet o f p u r e aluminium. T h i s t y p e o f b e a r i n g has a h i g h e r l o a d - c a r r y i n g c a p a c i t y than copper-lead a l l o y s and y e t can be used i n c o n j u n c t i o n w i t h s o f t j o u r n a l s .

I t s tolerance

f o r d i r t i s n o t good and i t i s o c c a s i o n a l l y o v e r l a y - p l a t e d . The l a t e s t development i n aluminium b e a r i n g s i s t h e a l u m i n i u m - B a b b i t t a l l o y . A t l e a s t t h r e e s u p p l i e r s a r e known t o be a c t i v e i n t h i s f i e l d , each u s i n g a

d i s t i n c t l y d i f f e r e n t m a n u f a c t u r i n g technique.

I n one case t h e method o f manuf-

a c t u r e i s n o t u n l i k e t h a t used f o r t h e 20% t i n - 8 0 % aluminium b e a r i n g s . another, a c o n t i n u o u s s i n t e r i n g process i s used.

In

The B a b b i t t a l l o y i s about

90% lead-10% t i n , and s i n c e t i n i s o n l y s p a r i n g l y s o l u b l e i n aluminium, t h e t i n remains i n s o l u t i o n i n t h e lead.

T h i s means t h a t t h e l e a d - r i c h phase ( t h e

B a b b i t t ) should remain c o r r o s i o n - r e s i s t a n t copper-lead and lead-bronze a l l o y s .

-

i n contrast t o the s i t u a t i o n w i t h

88

5.3.5

Phosphor and S i l i c o n Bronzes

These a l l o y s a r e c a s t as e i t h e r i n d i v i d u a l b e a r i n g s o r tubes from which be a r in g s can be machined.

I n r e c e n t yea rs, c e n t r i f u g a l and semi-continuous

c a s t i n g procedures have been used.

M a t e r i a l s o f t h i s t y p e a r e used m a i n l y f o r

bushes, p a r t i c u l a r l y l i t t l e - e n d bushes, where heavy loads and h i g h temperatures a r e encountered.

The presence o f phosphorus a t about t h e 0.5% l e v e l has a

marked e f f e c t on r e s i s t a n c e t o pounding,

i.e.

the a l l o y has o u t s t a n d i n g r e s -

i s t a n c e t o wear when s u b j e c t t o heavy l oa ds a t low s l i d i n g speeds.

5.3.6

Silver

S i l v e r b e a r in gs, sometimes p l a i n , sometimes w i t h a t h i n o v e r l a y o f leadindium, have been adopted by t h e a i r c r a f t i n d u s t r y and a r e used on one w e l l known make o f d i e s e l engine. and i s about 0.5 mm t h i c k .

The s i l v e r i s e l e c t r o d e p o s i t e d on a s t e e l backing E l e c t r o d e p o s i t e d s i l v e r i s much harder than c a s t

s i l v e r , and s i l v e r b e a r i n g s a r e un eq ua l l ed w i t h respect t o l o a d - c a r r y i n g capa c i t y and f a t i g u e r e s i s t a n c e .

U n f o r t u n a t e l y , they a r e prone t o s e i z u r e and a r e

v e r y s e n s i t i v e t o t h e n a t u r e o f t h e l u b r i c a n t and t o c e r t a i n l u b r i c a t i n g - o i l add i t ives .

5.3.7

Porous and S e l f - L u b r i c a t i n g Bearings

Most porous b e a r i n g s c o n s i s t o f s i n t e r e d bronze o r i r o n powders w i t h i n t e r c on n e c t in g pores. operation,

These p ore s may t ake up 10-30% o f t h e t o t a l volume and, i n

l u b r i c a t i n g o i l i s s t o r e d i n them and i s subsequently f e d t o t h e

bearing surface. i l l a r y action.

Any o i l e scap i ng f ro m t h e loaded zone i s reabsorbed by cap-

A t y p i c a l bronze c o n t a i n s 90% copper and 10% t i n ;

may be added t o t h e mi x t o enhance t h e s e l f - l u b r i c a t i n g p r o p e r t i e s . p o r o s i t y and h i g h l u b r i c a t i n g - o i l

1-4% g r a p h i t e H igh

c o n t e n t a r e r e q u i r e d f o r high-speed,

light-

l oa d a p p l i c a t i o n s , whereas a l o w - p o r o s i t y m a t e r i a l w i t h a h i g h g r a p h i t e c o n t e n t i s b e t t e r f o r o s c i l l a t o r y and r e c i p r o c a t i n g movement, where i t i s d i f f i c u l t t o e s t a b l i s h an o i l f i l m .

5.3.8

Dis c u s s io n on M e t a l l i c Be ari n g M a t e r i a l s

No b e a r i n g m a t e r i a l combines a l l t he d e s i r a b l e p r o p e r t i e s .

However, t h e

i m p o r ta n t p r o p e r t i e s o f b e a r i n g s can be grouped i n two main c a t e g o r i e s . includes surface c h a r a c t e r i s t i c s ,

One

such as wear r e s i s t a n c e , j o u r n a l c o m p a t a b i l i t y ,

c o n f o r m a b i l i t y and e m b e d d a b i l i t y , t h e o t h e r i n c l u d e s mechanical p r o p e r t i e s , such a f a t i g u e s t r e n g t h and l o a d - c a r r y i n g c a p a c i t y . are divergent,

Since these two c a t e g o r i e s

t o have optimum s u r f a c e c h a r a c t e r i s t i c s s t r e n g t h must be sac-

r i f i c e d and v i c e versa.

However, by u s i n g modern manufacturing methods,

it i s

g e n e r a l l y p o s s i b l e t o a r r i v e a t a good compromise s o l u t i o n as, f o r example, the case o f t r i - m e t a l

b ea ri ng s.

in

89 There i s n o c l e a r u nd erst an di n g o f t h e way i n which b e a r i n g a l l o y s f u n c t i o n , and general t h e o r i e s r e g a r d i n g t h e i r b eh avi o ur can almost always be d i s c r e d i t e d by r e f e r e n c e t o some p a r t i c u l a r b ea ri ng . p r a c t i c e has always been ahead o f t he ory.

In the formulation o f bearing a l l o y s , T h i s i s n o t t o say t h a t b e a r i n g

design and manufacture i s a backward i n d u s t r y ;

i n f a c t , t h e manufacturing

techniques d e s c r i b e d p rove t h e o p p o s i t e , and b e a r i n g manufacturers can p r o v i d e s a t i s f a c t o r y s o l u t i o n s t o a l most any b e a r i n g problem.

N evertheless, many

be a r in g s s t i l l f a i l pre mat ure l y i n s e r v i c e and t h e r e i s a g r e a t need t o t e l l designers about t h e b e a r i n g m a t e r i a l s a l r e a d y a v a i l a b l e o r under development.

5.4

BEARING FAILURES

-

S e c tio n s 5.1

5.3 have p r o v i d e d t h e reader w i t h a b r i e f i n t r o d u c t i o n t o the

s u b je c t, and i t i s now p o s s i b l e t o d e s c r i b e and d i s c u s s some o f t h e l i k e l y causes o f b e a r i n g f a i l u r e s , g i v i n g p a r t i c u l a r emphasis t o t h e i d e n t i f i c a t i o n o f factors that give r i s e t o failures.

5.4.1

M e t a l l u r g i c a l Def ect s i n New Be ari n gs

Modern p l a i n b e a r i n g s can be v e r y complex c o n s t r u c t i o n s and much may go wrong d u r i n g t h e i r manufacture.

Neve rt h el e ss, v e r y few d e f e c t i v e b e a r i n g s e n t e r

s e r v i c e , s i n c e t h e ma j or b e a r i n g man uf act ure r s m a i n t a i n a h i g h standard o f quality control. bea r in g s , e.g.

Def ect s a r e more l i k e l y t o occur on i n d i v i d u a l l y manufactured

l a r g e , whi t e-me t a l b e a r i n g s , t han on mass-produced bearings o f

the c o p p e r - le a d o r a l u m i n i u m - t i n described i n S e ct i on s 5.4.1.1

5.4.1.1

-

varieties.

D efects t h a t may be encountered a r e

5.4.1.5.

Bad Bonding

Bonding i s s t i l l a ma j or problem on l a r g e , w hite-metal bearings, a l t h o u g h poor bonds a r e e xt re mel y r a r e on o t h e r t ype s o f bearing. white-metal

Bearing s h e l l s f o r

b e a r i n g s must be t i n n e d b e f o r e t h e w h i t e metal i s c a s t i n place,

and t h e te m p e r a tu re o f b o t h t h e s h e l l and t h e w h i t e metal must be c a r e f u l l y controlled.

A m e t a l l u r g i c a l bond between t h e b e a r i n g and t h e s h e l l strengthens

the w h i t e metal and f a c i l i t a t e s t he f l o w o f heat away from the b e a r i n g surface. In t h e p a s t , some "wh i t e m e t a l l e r s " have argued t h a t i f they machined d o v e t a i l grooves i n t h e s h e l l , these h e l d t h e w h i t e metal i n p l a c e and t h e r e was no need t o i n s i s t on good bonding. sighted p o l i c y ;

Experience p rove s t h a t t h i s i s an extremely s h o r t -

d o v e t a i l grooves a r e no s u b s t i t u t e f o r good bonding and,

in

a d d i t i o n , t h e sharp edges a t t h e sh ou l de rs o f t h e grooves a c t as l o c a l i s e d s t r e s s r a i s e r s and can i n i t i a t e c r a c k s ( F i g . 1 ) .

A general v ie w o f a cop pe r-l ea d b e a r i n g which f a i l e d due t o bad bonding, t h e copper-lead b e a r i n g a l l o y h a v i n g separated c l e a n l y from t h e s t e e l s h e l l , shown i n Fig.2.

is

90

Fig.1

Dovetail in white-metal thrust pad bearing.

Fig.2

Bad bonding:

copper-lead bearing.

91 Bad bonding on new b e a r i n g s can be d e t e c t e d r e a d i l y u s i n g u l t r a s o n i c o r e l e c t r i c a l r e s i s t a n c e measuring techniques,

a l l y a v a i l a b l e t e s t i nst rume nt s.

and t h e r e a r e a number o f commerci-

On a f a i l e d bearing, bad bonding can be

d i s t i n g u i s h e d f r om f a t i g u e f a i l u r e because w i t h t h e former, t h e b e a r i n g metal detaches c l e a n l y f ro m t h e s h e l l (Fig.3).

5.4.1.2

Gas C a v i t i e s

I n l a r g e , wh ite -met al b e a r i n g s , t o o l ow a c a s t i n g temperature o r i n s u f f i c i e n t p r e h e a t i n g o f t h e s h e l l can g i v e r i s e t o gas c a v i t i e s near t h e s h e l l s u r f a c e . Such c a v i t i e s weaken t h e b e a r i n g metal and a d v e r s e l y a f f e c t heat f l o w ;

some-

times, t h e h y d r o s t a t i c pre ssure above t h e c a v i t i e s may cause t h e i r c o l l a p s e (F ig s . 4 and 5). Cast c o p p e r - le ad a l l o y s sometimes e x h i b i t c a s t i n g p o r o s i t y , w hich i n t h i s i n s ta n c e i s g e n e r a l l y assumed t o be due t o hydrogen e v o l u t i o n from t h e copper. This p o r o s i t y due t o o u t - g a s s i n g must be d i s t i n g u i s h e d from c o r r o s i o n o f t h e lead phase by o i l - o x i d a t i o n pro du ct s.

Cast i ng p o r o s i t y i s sub-surface,

whereas

w i t h corrosion, the lead i n the surface l a y e r s i s p r e f e r e n t i a l l y dissolved (F ig .6 ) .

5.4.1.3

Ov e r s ize Cuboids

T h i s p r o b le m i s s p e c i f i c t o wh i t e -met al b earings.

I n l a r g e r b e a r i n g s , slow

c o o l i n g t h r o u g h t h e s o l i d i f i c a t i o n range can g i v e r i s e t o o v e r s i z e (> 0 . 1 mm) t i n - a n t i m o n y c u b o i ds (F i g. 7).

T h i s i n t e r m e t a l l i c compound i s b r i t t l e and the

l a r g e c u b o id s can c r a c k and break-up i n s e r v i c e , causing s c o r i n g o f t h e j o u r n a l s and damage elsewhere i n t h e o i l system ( F i g . 8 ) .

I n general,

i f cuboids

a r e r e a d i l y v i s i b l e t o t h e naked eye t h e y a r e t o o b i g .

5.4.1.4

Excessive Lead Content \ i n Tin-Based White Metals

The le a d c o n t e n t o f t i n-b ase d w h i t e me t a l s should be below 0.5% t o prevent th e f o r m a t i o n o f a l o w - m e l t i n g ,

l e a d - t i n e u t e c t i c i n t h e g r a i n boundaries.

The e u t e c t i c weakens t h e a l l o y and makes i t more s u s c e p t i b l e t o w i p i s g . s i m i l a r reasons,

For

i t i s bad p r a c t i c e t o mix lead-based and t i n - b a s e d w hite-metal

h a l f - b e a r i n g s , as i s sometimes done t o save i n i t i a l c o s t s , t i n b e i n g used on t h e loaded h a l f and l e a d on t h e unloaded h a l f .

Carry-over o f l e a d from one

h a l f - b e a r i n g t o t h e o t h e r can r e s u l t i n t h e f o r m a t i o n of t h e l o w - m e l t i n g - p o i n t eutect ic.

5.4.1.5

Uneven Lead D i s t r i b u t i o n i n Copper-Lead and Lead-Bronze A l l o y s

Copper-lead and lead-bronze b e a r i n g a l l o y s can be manufactured by m e l t i n g and c a s t i n g o r by s i n t e r i n g techniques.

Lead d i s t r i b u t i o n i s more r e a d i l y

c o n t r o l l e d by p o wde r-met al l u rgy t e chn i qu es, so uneven d i s t r i b u t i o n i s m a i n l y

92

Fig.3

Micro-sections.

93

Fiq.4

Cross-section o f White-metals Showing Gas Cavities

Fig.5

Collapsed surface o f white-metal thrust pad bearing.

94

Fig.6

C a s t i n g p o r o s i t y i n c a s t copper-lead big-end b e a r i n g .

Fiq.7

O v e r s i z e t i n - a n t i m o n y cuboids i n w h i t e - m e t a l steam t u r b i n e

t h r u s t pad from

95

Fig.8

S i n g l e cuboids showing c r a c k s .

Fig.9

Bad l e a d d i s t r i b u t i o n .

96 encountered on c a s t b e a r i n g s .

Copper and l e a d a r e i m m i s c i b l e and good l e a d

d i s t r i b u t i o n i n c a s t a l l o y s depends on s p e c i a l m a n u f a c t u r i n g techniques, p a r t i c u l a r on r a p i d c o o l i n g o f t h e a l l o y a f t e r c a s t i n g .

in

This r a p i d cooling

r e s u l t s i n a h i g h l y o r i e n t e d s t r u c t u r e w i t h l o n g copper and l e a d d e n d r i t e s normal t o t h e s u r f a c e o f t h e b e a r i n g s h e l l . laxed, t h e l e a d can be unevenly d i s t r i b u t e d ;

If manufacturing c o n t r o l s are ref o r example,

i t may be p r e s e n t i n

l a r g e s l u g s , normal o r p a r a l l e l t o t h e s u r f a c e ( F i g . 9 ) .

5.4.2

F a i l u r e s A s s o c i a t e d w i t h Design, F i t t i n g o r O p e r a t i n g Environment

The f i n a l m a n i f e s t a t i o n o f f a i l u r e r e s u l t i n g f r o m adverse o p e r a t i n g c o n d i t i o n s o f t h i s type i s generally fatigue.

The l o a d - c a r r y i n g c a p a c i t i e s and tem-

p e r a t u r e l i m i t a t i o n s o f t y p i c a l b e a r i n g a l l o y s a r e i n d i c a t e d i n Table 5 . 2 . However, these depend on a number o f f a c t o r s , such as t h e t h i c k n e s s o f t h e b e a r i n g a l l o y and t h e support i t g e t s f r o m i t s s u b s t r a t e o r s h e l l .

Also, the

s t r e n g t h o f b e a r i n g a l l o y s decreases w i t h i n c r e a s i n g temperature, and b e a r i n g s u r f a c e temperatures a r e g e n e r a l l y a t l e a s t 2OoC i n excess o f measured o i l temperatures. The design o f b e a r i n g s , p a r t i c u l a r l y t h e l o c a t i o n o f o i l feeds and o i l grooves,

i s o u t s i d e t h e scope o f t h i s paper.

N e v e r t h e l e s s , i t should be em-

phasised t h a t l o a d - c a r r y i n g c a p a c i t y i s r a r e l y enhanced by a p r o l i f e r a t i o n of o i l grooves.

5.4.2.1

Bad F i t t i n g

Modern b e a r i n g s a r e manufactured t o v e r y c l o s e t o l e r a n c e s and s h o u l d be assembled w i t h g r e a t care.

I f t h e y a r e n o t a c c u r a t e l y a l i g n e d , t h e l o a d i n g w i l l be

uneven and premature f a i l u r e due t o f a t i g u e may o c c u r .

I f t h e b e a r i n g does n o t

f i t p r o p e r l y i n i t s housing, s l i g h t movement may o c c u r , which can r e s u l t i n

severe f r e t t i n g on t h e back o f t h e b e a r i n g .

Loose f i t t i n g and/or f r e t t i n g can

a d v e r s e l y a f f e c t h e a t t r a n s f e r and can g i v e r i s e t o e x c e s s i v e b e a r i n g - s u r f a c e temperature (Fig.10). D i r t p a r t i c l e s trapped between a b e a r i n g and i t s housing c o n s t i t u t e a n o t h e r

cause o f poor heat t r a n s f e r .

Such p a r t i c l e s can a l s o cause h i g h spots on t h e

b e a r i n g s u r f a c e , which may g i v e r i s e t o l o c a l i s e d w i p i n g and f a t i g u e (Fig.11). I t w i l l be c l e a r f r o m t h e above t h a t e x a m i n a t i o n o f t h e back o f a f a i l e d

b e a r i n g can p r o v i d e i m p o r t a n t i n f o r m a t i o n r e g a r d i n g i t s mode o f f a i l u r e . comment has been made t h a t t h e b e s t way t o e v a l u a t e "experts"

The

on b e a r i n g

f a t i g u e s i s t o n o t e whether t h e y examine t h e backs o f b e a r i n g s w i t h as much c a r e as t h e sl i d i n g surface.

91

Fig.10

5.4.2.2

F r e t t i n g and d i s t o r t i o n due t o bad f i t t i n g .

Extraneous P a r t i c l e s

T h i s i s p r o b a b l y t h e m s t common cause o f b e a r i n g f a i l u r e .

The more e x p e r t

we become a t d e t e c t i n g and i d e n t i f y i n g f o r e i g n m a t e r i a l , t h e more obvious i t i s t h a t c l e a n l i n e s s on assembly and good l u b r i c a n t f i l t r a t i o n i n s e r v i c e a r e e s s e n t i a l f o r lo ng, t r o u b l e - f r e e s e r v i c e l i v e s .

A l l too frequently, engineering

components a r e assembled w i t h o u t adequate c l e a n i n g i n d i r t y environments, w i t h the r e s u l t t h a t machining swarf , moulding sand and o t h e r k i n d s o f d e b r i c c i r c u l a t e w i t h the l u b r i c a t i n g o i l .

Large p a r t i c l e s a r e g e n e r a l l y removed by t h e

o i l f i l t e r s , b u t medium-sized p a r t i c l e s may embed i n t h e b e a r i n g s and v e r y small p a r t i c l e s c o n t i n u e t o c i r c u l a t e w i t h t he o i l .

F o r e i g n p a r t i c l e s and wear

d e b r i s can a l s o accumulate d u r i n g s e r v i c e , and v e r y l o n g o i l - c h a n g e p e r i o d s , now much i n f a v o u r , ag gra vat e t h e s i t u a t i o n .

98

Fig.12

Shaft wear due t o embedded p a r t i c l e s i n bearinq.

99 B e a r in g a l l o y s , p a r t i c u l a r l y w h i t e met al s, have a c o n s i d e r a b l e c a p a c i t y f o r abrasive materials.

When t h i s c a p a c i t y i s exceeded, t h e b e a r i n g s u r f a c e can a c t

as a l a p , a b r a d i n g away t h e j o u r n a l s u r f a c e , t h e r e b y i n c r e a s i n g clearances and leading t o f a t i g u e f a i l u r e .

T h i s l a p p i n g a c t i o n by embedded p a r t i c l e s i s t h e

prime cause o f j o u r n a l wear (F i g. 12 ).

In most in s t a nces, t h e b e a r i n g s u r f a c e w i l l be i n r e l a t i v e l y good c o n d i t i o n (unless complete f a i l u r e has o c c u r r e d ) , a l t h o u g h t h e o v e r a l l t h i c k n e s s o f t h e be a r in g can a c t u a l l y i n crea se owing t o t h e amount o f embedded m a t e r i a l . P a r t i c l e s embedded i n b e a r i n g s u r f a c e s d i s p l a c e metal and t h e r e f o r e a r e o f t e n surrounded by a r a i s e d , b urn i she d h a l o (F i g. 13).

I f t h e c o m p o si t i o n o f embedded p a r t i c l e s can be determined, t h e i r o r i g i n may be e s t a b l i s h e d and t h e a p p r o p r i a t e s t e p s taken t o e l i m i n a t e them.

A v a r i e t y o f i d e n t i f i c a t i o n t ech ni q ue s a r e now a v a i l a b l e .

For example, t h e

p a r t i c l e s may be e x t r a c t e d c h e m i c a l l y o r m e c h a n i c a l l y f o r i d e n t i f i c a t i o n by X-ray d i f f r a c t i o n o r X-ray sp ect rome t ry.

They can be examined i n s i t u u s i n g an

el e c tr o n - p r o b e .

Alternatively,

chemical e x t r a c t i o n techniques have much t o

recommend them;

t h e amount o f d e b r i s g e n e r a l l y o b t a i n e d i s enough t o h o r r i f y

most p l a n t o p e r a t o r s (F i g . 1 4). P a r t i c l e s i n e l e c t r o d e p o s i t e d l e a d o v e r l a y s can be e x t r a c t e d w i t h an a c e t i c acid/hydrogen p e r o x i d e m i x t u r e , which d i s s o l v e s t h e o v e r l a y ; - t i n b e a r i n g s can be e x t r a c t e d w i t h c a u s t i c soda.

those i n aluminium

Both m i x t u r e s can loosen

p a r t i c l e s i n t i n - b a s e d w h i t e met al s. Embedded f e r r o u s p a r t i c l e s ( t h e l a r g e s t s i n g l e group) can be q u i c k l y i d e n t i f i e d by " i r o n p r i n t i n g " (Fig.15). T h i s i n v o l v e s so aki n g an unglazed paper i n a 5 % s~o l u t i o n o f potassium f e r r i cyanide t o wh ic h a few drops of h y d r o c h l o r i c a c i d and w e t t i n g agent have been added.

The s u r p l u s l i q u i d i s d r a i n e d o f f t h e paper, w hich i s then p l a c e d i n con-

t a c t w i t h t h e degreased surf ace o f t h e b e a r i n g f o r about 30 seconds.

The paper

i s then removed, and b l u e sp ot s i n d i c a t e t h e presence o f f e r r o u s p a r t i c l e s . p a r t i c l e s t h a t remain i n t h e b e a r i n g s u r f a c e a r e a l s o s t a i n e d b l u e .

The

Similar

techniques a r e a v a i l a b l e f o r t h e i d e n t i f i c a t i o n o f p a r t i c l e s o f most common nonf e r r o u s m e t a ls . Since f e r r o u s p a r t i c l e s c o n s t i t u t e t h e most common t y p e o f a b r a s i v e wear p a r t i c l e , t h e use o f magnetic f i l t e r s ,

i n a d d i t i o n t o o r d i n a r y f i l t e r s , has much t o

recommend i t. Fin e p a r t i c l e s , s m a l l e r t ha n normal b e a r i n g clearances, can c i r c u l a t e w i t h the l u b r i c a n t and erode t h e b e a r i n g s u i f a c e .

Hard p a r t i c l e s erode deep, w e l l -

d e f i n e d channels, w h i l e s o f t p a r t i c l e s g i v e r i s e t o more general e r o s i o n (Fig.16), p a r t i c u l a r l y on s o f t e l e c t r o d e p o s i t e d o v e r l a y s . E r o s io n by f i n e p a r t i c l e s i s most p r e v a l e n t on high-speed b e a r i n g s and may be associated w i t h c a v i t a t i o n erosion.

100

Fig.13

P a r t i c l e s embedded i n b e a r i n g s u r f a c e .

Fig.14

Steel p a r t i c l e s e x t r a c t e d from b e a r i n g .

101

Fig.16

Erosion damage.

102

5.4.2.3

Cor r o s i o n

L u b r i c a n t s d e t e r i o r a t e i n s e r v i c e i n two ways

-

they become contaminated and

th e y undergo p h y s i c a l and chemical changes due t o o x i d a t i o n .

I n engines,

the

common contaminants a r e a i r b o r n e du st and wear products, unburnt f u e l , f u e l combustion p r o d u c t s and wat er. e r i a l s and a s p h a l t e ne s. sludges and lacquers.

The o x i d a t i o n p r o d u c t s a r e m a i n l y a c i d i c mat-

Asphaltenes i n a s s o c i a t i o n w i t h f u e l contaminants form The a c i d i c m a t e r i a l s - r e s u l t i n g f r o m t h e o x i d a t i o n o f

l u b r i c a n t s a r e g e n e r a l l y weak o r g a n i c a c i d s , a l t h o u g h i n extreme cases s t r o n g m i n e r a l a c i d s may be produced.

However, a l most a l l s t r o n g m i n e r a l a c i d contami

n a t i o n i n l u b r i c a n t s i s d e r i v e d f rom f u e l combustion p r o d u c t s , s u l p h u r i c a c i d b e i n g a major contaminant i n d i e s e l e ng i ne s and halogen a c i d s i n g a s o l i n e eng ines. Since b e a r i n g s a r e o f complex c o n s t r u c t i o n ,

the way i n w hich t h e i r s t r u c t u r e

and c o m p o s i t i o n a f f e c t s t h e i r c o r r o s i o n r e s i s t a n c e w i l l be discussed w i t h r e f erence t o v a r i o u s c o r r o s i o n mechanisms. I t s h o u ld be emphasised t h a t b e a r i n g f a i l u r e s t h a t a r e p r i m a r i l y due t o any form o f c o r r o s i o n a r e c o m p a r a t i v e l y r a r e and t h a t b e a r i n g f a i l u r e s caused by i n h e r e n t d e f i c i e n c i e s on t h e p a r t o f t h e l u b r i c a n t a r e extremely r a r e .

Never-

t h e l e s s , c e r t a i n s p e c i f i c forms o f c o r r o s i o n , such as t i n o x i d e f o r m a t i o n on ti n- b a s e d white-metal

b e a r i n g s and sul p hu r c o r r o s i o n o f phosphor-bronze a l l o y s ,

have a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n .

5.4.2.3

A l s o , as w i l l be apparent i n S e c t i o n

( c o r r o s i o n by weak o r g a n i c a c i d s ) and 5 . 4 . 2 . 4

i n c r e a s i n g l y severe o p e r a t i n g c o n d i t i o n s ,

( c a v i t a t i o n and e r o s i o n ) ,

such as longer-oil-change

periods,

v e r y h i g h o p e r a t i n g temperatures and i n crea sed speeds, can g i v e r i s e t o p a r t i c u l a r problems.

These a r e problems i n which c o r r o s i o n p l a y s a p a r t , b u t i n

which o t h e r f a c t o r s a r e also o p e r a t i v e . ( i ) Co r r o s io n by weak o r g a n i c a c i d s Weak o r g a n i c a c i d s a r i s e e i t h e r f r o m prolonged exposure o f t h e l u b r i c a n t a t e l e v a t e d temperatures o r by c o n t a m i n a t i o n o f t h e l u b r i c a n t w i t h p a r t i a l l y b u r n t combustion p rod uct s.

These a c i d s a t t a c k l e a d f a r more

r e a d i l y t h an o t h e r met al s, and can d i s s o l v e t h e lead phase i n copper-lead and lead-bronze b e a r i n g s (F i g . 1 7). Some e n g in e l u b r i c a t i n g o i l s p e c i f i c a t i o n s i n c l u d e t e s t s t o determine t h e c o r r o s i v i t y o f l u b r i c a t i n g o i l s towards copper-lead bearings.

The loss

o f le a d i s assessed b y we i gh i ng t h e b e a r i n g s h e l l s b e f o r e and a f t e r t e s t . I n s e r v i c e f a i l u r e s , where t h e wei g ht s o f new b e a r i n g s a r e n o t a v a i l a b l e , le a d c o r r o s i o n can be d e t e c t e d by m e t a l l u r g i c a l s e c t i o n i n g (Fig.18). Copper-lead and lead-bronze b e a r i n g s may be manufactured by s i n t e r i n g o r casting.

I n s i n t e r e d a l l o y s i t i s p o s s i b l e t o ensure t h a t t h e lead

103

Fig.17

Fig.18

Corroded copper-lead b e a r i n g

Corroded s i n t e r e d copper-lead b e a r i n g

104 phase i s w e l l d i s t r i b u t e d and d i s c o n t i n u o u s , s o t h e c o r r o s i o n w i l l be r e s t r i c t e d t o the surface layers.

On t h e o t h e r hand, w i t h c a s t a l l o y s ,

s a t i s f a c t o r y l e a d d i s t r i b u t i o n depends on h i g h l y s p e c i a l i s e d manufactur i n g techniques and r a p i d c o o l i n g o f t h e a l l o y a f t e r c a s t i n g .

This

rapid cooling gives r i s e t o a h i g h l y o r i e n t e d structure, w i t h long copper and l e a d d e n d r i t e s normal t o t h e s u r f a c e o f t h e b e a r i n g s h e l l . The l o n g l e a d d e n d r i t e s p r o v i d e an easy p a t h f o r t h e p e n e t r a t i o n o f a corrosive lubricant.

I n these circumstances i t i s p o s s i b l e f o r almost

a l l o f t h e l e a d phase t o be leached o u t o f a c a s t copper-lead a l l o y (Fig.19).

Fig.19

Corroded c a s t copper-lead b e a r i n g .

Most copper-lead b e a r i n g s now have a p r e c i s i o n e l e c t r o d e p o s i t e d o v e r l a y o f a l e a d - t i n o r lead-indium a l l o y . the c o r r o s i o n r e s i s t a n c e o f t h e lead.

The indium o r t i n a d d i t i o n s improve We have e s t a b l i s h e d t h a t about

3%

t i n o r 4 . 8 % indium i s r e q u i r e d t o render t h e o v e r l a y c o m p l e t e l y r e s i s t a n t t o c o r r o s i o n (Fig.20). Bearing manufacturers g e n e r a l l y p r o v i d e o v e r l a y s w i t h about 5% indium o r 10% t i n .

However, a t t h e h i g h temperatures t h a t b e a r i n g s can e x p e r i e n c e

i n s e r v i c e , t h e indium o r t i n d i f f u s e s q u i t e r a p i d l y and m i g r a t e s i n t o

105

Average w t loss m 9.

200Indium alloys 150-

100

Tin alloys

-

A Lead-tin alloys X Lead- indium alloys

50

1.0

0

Fig.20

2.0

3.0 4.0 Percentage alloying element

- 6.0

5.0

Corrosion of l e a d - t i n and lead-indium a l l o y s .

the u n d e r l y i n g copper-lead.

This d i f f u s i o n can r e a d i l y be observed both

on bearings removed from s e r v i c e and on bearings t e s t e d i n the laboratory. I n the case o f t i n , the t i n a l l o y s w i t h the copper t o form c o p p e r - t i n i n t e r m e t a l l i c compounds.

I n the case of indium, enrichment occurs ad-

j a c e n t t o t h e copper, but we have no evidence o f compound formation. S i m i l a r e f f e c t s can be observed on s i l v e r bearings w i t h l e a d - r i c h e l e c t r o deposited overlays. This m i g r a t i o n o f indium and t i n from the surface l a y e r s o f the o v e r l a y leaves them s u s c e p t i b l e t o c o r r o s i o n (Figs.21-23).

I t i s s a i d t h a t one

way t o prevent t h i s d i f f u s i o n i s t o interpose a dam between the overlay and the u n d e r l y i n g copper-lead Very t h i n n i c k e l ,

(Fig.24).

i r o n o r brass l a y e r s have been used as dams.

However,

many bearings users a r e u n w i l l i n g t o face the increased cost associated w i t h the use o f dams and,

i n any case, t h e i r e f f e c t i v e n e s s i s being

questioned. There i s another way i n which the indium i n lead-indium overlays may be depleted.

I f the indium content f a l l s t o below about 3%, i n t e r n a l

o x i d a t i o n o f the indium can occur i n the g r a i n boundaries (Fig.25).

The

indium oxide formed i n t h i s way e m b r i t t l e s the a l l o y and renders i t very susceptible t o f a t i g u e f a i l u r e as w e l l as t o c o r r o s i v e a t t a c k (Fig.26).

106

Fig.21

Corrosion o f b e a r i n g o v e r l a y .

Fig.22

E l e c t r o n probe micrographs showing indium d i s t r i b u t i o n .

107

A f t e r 13,000 h r s . Fig.23

Fig.24

A f t e r 200 h r s . a t f u l l l o a d .

l n t e r m e t a l 1 i c compound f o r m a t i o n i n o v e r l a y b e a r i n g s ( c o p p e r - l e a d ) .

O v e r l a y b e a r i n g w i t h dam.

108

Fig.25

Internal oxidation o f lead indium overlay.

Fig.26

Corrosion o f lead indium overlay.

109 Another consequence o f t h e d i f f u s i o n o f t i n f r o m l e a d - r i c h o v e r l a y s i s t h e roughening o f t h e o v e r l a y s u r f a c e (Fig.27).

Although copper

has a much h i g h e r a f f i n i t y f o r t i n t h a n has lead, t i n d i f f u s e s much more r a p i d l y i n l e a d than i n copper.

T h i s means t h a t where t h e l e a d

phase i n copper-lead a l l o y i s continuous w i t h t h e o v e r l a y ,

the t i n

can p e n e t r a t e deeply i n t o t h e a l l o y i n t h e l e a d phase (Fig.28) cause s i n k i n g o f t h e o v e r l a y .

and

T h i s movement o f t i n away f r o m t h e

o v e r l a y can g i v e t h e o v e r l a y a roughened appearance, which may be mistaken f o r c o r r o s i o n . ( i i ) C o r r o s i o n by s t r o n g m i n e r a l a c i d s The main source o f s t r o n g m i n e r a l a c i d s i n l u b r i c a n t s i s c o n t a m i n a t i o n by f u e l combustion p r o d u c t s .

Diesel fuels,

p a r t i c u l a r l y marine

diesel f u e l s , contain s i g n i f i c a n t q u a n t i t i e s o f sulphur;

t h i s sulphur

i s t h e source o f s u l p h u r i c a c i d , which may f i n d i t s way i n t o t h e lubricant.

Gasolines, on t h e o t h e r hand, c o n t a i n v e r y l i t t l e s u l p h u r ,

b u t do c o n t a i n c h l o r i n e and bromide compounds which a r e added t o scavenge t h e l e a d a n t i k n o c k compounds.

Thus, l e a d h a l i d e compounds

may accumulate i n t h e l u b r i c a t i n g o i l i n g a s o l i n e engines and,

in

c e r t a i n circumstances, halogen a c i d s can form. The s t r o n g a c i d s g e n e r a l l y a t t a c k bare s t e e l s u r f a c e s r a t h e r than bearing a l l o y s .

T h i s u s u a l l y r e s u l t s i n j o u r n a l s b e i n g roughened by

c o r r o s i o n , and b e a r i n g s t h e n f a i l e i t h e r because o f t h i s o r as a r e s u l t o f damage by c o r r o s i o n p r o d u c t s ( r u s t ) . I n t h e presence o f aluminium and m o i s t u r e , l e a d h a l i d e s can d e l i q u e s c e , g i v i n g r i s e t o halogen a c i d s , which c o r r o d e aluminium.

This corrosion

r e a c t i o n was f i r s t observed on a l u m i n i u m - a l l o y p i s t o n s and can r e a d i l y be d u p l i c a t e d i n t h e l a b o r a t o r y .

F o r t u n a t e l y , examples o f t h i s t y p e

o f a t t a c k on a l u m i n i u m - t i n b e a r i n g s i n engines a r e e x t r e m e l y r a r e , a l t h o u g h b e a r i n g s a r e s u s c e p t i b l e t o h a l i d e c o r r o s i o n a f t e r removal f r o m engines. Halogen a t t a c k has a l s o been observed on l e a d o v e r l a y b e a r i n g s o p e r a t e d a t v e r y h i g h engine temperatures, when mixed b r o m i d e / c h l o r i d e l a y e r s can f o r m on t h e o v e r l a y s u r f a c e and cause b l a c k e n i n g ( F i g s .

29 and 30).

E l e c t r o n - p r o b e s t u d i e s o f t h e r e a c t i o n l a y e r show t h a t

i t c o n t a i n s indium a t t h e same c o n c e n t r a t i o n as i n t h e b e a r i n g , i n d i c a t i n g t h a t t h e l e a d h a l i d e s have been formed i n s i t u as a r e s u l t o f c o r r o s i o n and have n o t been d e r i v e d d i r e c t l y f r o m t h e f u e l . ( i i i ) Sulphur c o r r o s i o n T h i s i s a general d e s c r i p t i o n which i s a p p l i e d t o most forms of c o r r o s i o n encountered on s i l v e r - r i c h o r c o p p e r - r i c h b e a r i n g a l l o y s .

110

Fig.27

Roughening o f overlay due t o d i f f u s i o n .

Fig.28

' S i n k i n g ' o f overlay.

111

Fig.29

Blackened b i g - e n d b e a r i n g s .

Fig.30

Lead h a l i d e l a y e r .

112 There i s no doubt t h a t sulphur compounds i n l u b r i c a t i n g o i l s can promote the corrosion of these p a r t i c u l a r a l l o y s .

On the o t h e r hand, b o t h

n a t u r a l l y o c c u r r i n g sulphur compounds i n l u b r i c a n t s and sulphur-containing a d d i t i v e s (e. g. z i n c d i a l k y l d it h iophosphates) confer b e n e f i c i a l p r o p e r t i e s on l u b r i c a n t s .

Thus, the dithiophosphates show considerable a,ntioxidant

and anti-wear e f f e c t s .

With regard t o n a t u r a l l y o c c u r r i n g sulphur com-

pounds, modern r e f i n i n g techniques can remove them completely, b u t experience shows t h a t t h i s i s most unwise, since some o f these compounds p l a y a l a r g e p a r t i n i n h i b i t i n g the c o r r o s i o n o f many metals, p a r t i c u l a r l y lead. On s i l v e r bearings the problem,

i n theory,

i s f a i r l y simple.

S i l v e r has

a h i g h a f f i n i t y f o r sulphur, and dithiophosphate-type a d d i t i v e s cannot be used.

However, a s m a l l amount o f e.p.

(extreme-pressure) a c t i v i t y on the

p a r t o f the l u b r i c a n t i s necessary, and the n a t u r a l sulphur can sometimes meet t h i s requirement. be used.

A l t e r n a t i v e l y , a c h l o r i n e - c o n t a i n i n g a d d i t i v e may

A s i l v e r bearing a f t e r s e r v i c e would be expected t o show a

c e r t a i n amount o f d i s c o l o r a t i o n due t o the formation o f s i l v e r sulphide o r s i l v e r c h l o r i d e , which have a b e n e f i c i a l e f f e c t on the f r i c t i o n a l characteristics. corrosion.

However, too much r e a c t i v e sulphur can cause severe

I t can be concluded t h a t the f o r m u l a t i o n o f o i l s t o l u b r i c a t e

s i l v e r bearings r e q u i r e s t h e e x p e r t i s e o f an o i l chemist. The c o r r o s i o n problems o f copper a l l o y s a r e more complex because the a l l o y s themselves a r e complex. phase MY

On simple copper-lead a l l o y s the copper

be attacked by sulphur, but t h i s i s a comparatively r a r e

occurrence, the problem o f sulphur c o r r o s i o n being much more acute on phosphor-bronze a1 loys (Fig.31).

This i s because phosphor-bronze a1 loys,

which a r e very popular f o r l i t t l e - e n d bushes i n d i e s e l engines, a r e expected t o operate a t considerably higher temperatures than copper-lead bearings.

I n any case, t h e r e a r e very few bare copper-lead bearings i n

use today. There i s no general agreement about the c o r r o s i o n mechanism.

Some

engine manufacturers and users h o l d dithiophosphate a d d i t i v e s e n t i r e l y responsible, but t h i s o p i n i o n cannot be substantiated, f o r severe c o r r o s i o n can occur when o i l s c o n t a i n i n g o n l y n a t u r a l sulphur compounds a r e used. TWO important f a c t o r s i n f l u e n c i n g the s e v e r i t y o f c o r r o s i o n a r e the

amount o f a l l o y i n g element i n s o l u t i o n i n the copper-rich phase and the p o r o s i t y o f the a l l o y .

I n phosphor bronzes,

i f the amount o f t i n i n

s o l u t i o n can be increased by special c a s t i n g techniques, such as continuous c a s t i n g , o r by s o l u t i o n treatment a f t e r manufacture, the resistance t o c o r r o s i o n i s g r e a t l y increased.

The presence o f z i n c

113 and/or s i l i c o n as a l l o y i n g elements i n copper a l l o y s a l s o increases t h e r e s i s t a n c e o f these a l l o y s t o su l ph ur c o r r o s i o n .

However,

it is d i f f i -

c u l t t o make sound s i l i c o n - b r o n z e c a s t i n g s .

I f t h e a l l o y i s porous, t h e l u b r i c a n t i s drawn i n t o the pores, where i t s ta g n a t e s and, i f o p e r a t i n g temperatures a r e h i g h , can become very c o r r o s i v e (F i g. 32 ).

The p a r t i c u l a r temperature a t which c o r r o s i o n be-

comes severe depends on t h e t ype o f d i t h i o p h o s p h a t e used;

very a c t i v e

v a r i e t i e s can s t a r t t o co rrod e a t about 1 3 O o C , whereas o t h e r v a r i e t i e s may be c o mp ara t i ve l y s t a b l e up t o 180°C. l i t t l e t r o u b l e below 1 7 O O C .

N a t u r a l sulphur compounds g i v e

Most phosphor-bronze c a s t i n g s a r e micro-

porous, and t h e g r e a t e r t h e p o r o s i t y t he g r e a t e r the r i s k o f c o r r o s i o n . Cases a r e known where o n l y c e r t a i n bushes corroded i n a p a r t i c u l a r engine, and m e t a l l u r g i c a l e xami n at i on showed t h a t the bushes which c o r roded were porous.

Co nt i nu ou s-cast i ng techniques g i v e sounder a l l o y s

t h a n o t h e r c a s t i n g t ech ni q ue s and,

i n addition, a greater proportion o f

t i n s t a y s i n s o l u t i o n , t h ere by improving t h e i n t r i n s i c c o r r o s i o n resistance.

However,

i f t h e problem i s t o be c o n t r o l l e d , t h e o n l y s a t -

i s f a c t o r y s o l u t i o n i s t o use an a l l o y t h a t i s r e s i s t a n t t o sulphur corrosion.

L ab ora t o ry t e s t s f o l l o w e d by e x t e n s i v e f i e l d experience ex-

t e n d i n g o v e r t e n yea rs have shown t h a t a l l o y s o f t h e gun-metal

type,

i.e.

c o p p e r - t i n a l l o y s w i t h 2-4% z i n c , a r e completely immune f r m sulphur corrosion.

These gun-metal a l l o y s a r e r a t h e r s o f t e r than t h e t r a d i t i o n a l

phosphor bronzes, and n i c k e l may a l s o be added t o compensate f o r the reduced hardness. The importance o f m i c r o s t r u c t u r e i n t h i s type o f c o r r o s i o n was r e c e n t l y demonstrated by some phosphor-bronze bushes which corroded i n r e g u l a r l y spaced bands, d e s p i t e t h e f a c t t h a t t h ey were f r e e from p o r o s i t y and had been o p e r at ed on a l u b r i c a n t c o n t a i n i n g o n l y a small amount o f a very s t a b l e d it hi o ph osp ha t e .

M e t a l l u r g i c a l examination showed t h a t they had

been manufactured by a semi-continuous c a s t i n g process, which gave r i s e t o marked s e g r e g a t i o n i n t h e a l l o y , making i t v e r y s u s c e p t i b l e t o c o r r o s i o n i n c e r t a i n are as (F i g. 33 ). Re c e n t ly some cases o f what i s cl a i med t o be sulphur c o r r o s i o n have been encountered on t i n - b a s e d whi t e-me t a l b e a r i n g a l l o y s .

I t appears t h a t very

a c t i v e s u l p hu r compounds can s e l e c t i v e l y a t t a c k t h e c o p p e r - r i c h ,

copper-

t i n i n t e r m e t a l l i c compounds i n t h e w h i t e metal, and t h a t t h e r e s u l t i n g c o r r o s i o n p r o d u c t s , r i c h i n Cu2S, can spread over t h e b e a r i n g surface. The d a r k e ni n g caused by these c o r r o s i o n products should n o t be confused w i t h t h e d arke ni n g due t o t i n o x i d e f o r m a t i o n .

114

Fig.31

Corrosion o f phosphor bronze.

Fig.32

Subsurface a t t a c k on phosphor-bronze.

115 ( i v ) C o r r o s io n o f t i n - b a s e d w h i t e m e t a l s :

t i n oxide formation

I n r e c e n t ye ars t h e f o r m a t i o n o f hard, b l a c k l a y e r s on t h e s u r f a c e o f t i n based wh ite -met al b e a r i n g s i n s h i p s ' t u r b i n e s has a t t r a c t e d a l o t o f attention.

Yet t h i s i s by no means a new problem;

i t has been encoun-

t e r e d i n t h e main d i e s e l e ng i ne s o f s h i p s f o r many years.

Some t u r b i n e

b u i l d e r s and s h i p owners have blamed t h e c o r r o s i o n on t h e i n c r e a s i n g use o f a d d i t i v e s i n l u b r i c a t i n g o i l s , b u t t h e f a c t t h a t d i e s e l engine bearings o p e r a t i n g on base o i l s have s u f f e r e d f r o m t h i s c o r r o s i o n f o r many years throws some doubt on t h e i r assumptions. The c h a r a c t e r i s t i c f e a t u r e s o f t h e c o r r o s i o n a r e as f o l l o w s .

A hard,

b l a c k l a y e r forms on t h e s u r f a c e o f t h e bearings, sometimes o n l y on the wo r k in g s u r f a c e s b u t g e n e r a l l y on a l l t h e exposed s u r f a c e s (Fig.34). The presence o f t h i s l a y e r i s e xt re mel y harmful f o r two reasons: i t i s v e r y h a r d and may damage t h e a d j a c e n t s t e e l surfaces,

i t s f o r m a t i o n decreases normal b e a r i n g clearances.

firstly,

and secondly,

M e t a l l u r g i c a l sections

f r o m f a i l e d b e a r i n g s show t h a t t h e h a r d l a y e r i s formed from t h e t i n - r i c h m a t r i x o f t h e b e a r i n g a l l o y (F i g . 3 5).

The c o p p e r - t i n and a n t i m o n y - t i n

i n t e r m e t a l l i c compounds a r e c o m p l e t e l y unattacked.

Microhardness t e s t s

show t h a t t h e hardness o f t h e b l a c k m a t r i x i s between 200 and 600 DPN, and u s u a l l y between 400 and 500 DPN.

The hardness o f t h e untransformed

m a t r i x i s about 25 OPN. X-ray d i f f r a c t i o n e xami n at i on shows t h a t t h e b l a c k l a y e r c o n s i s t s m a i n l y o f c a s s i t e r i t e , Sn02, u s u a l l y w i t h a l i t t l e stannous o x i d e , SnO, associated with it.

Dn t h r u s t pads, where b e a r i n g clearances a r e l a r g e , the

b l a c k l a y e r can grow t o a c o n s i d e r a b l e t h i c k n e s s , b u t e v e n t u a l l y i t d i s i n t e g r a t e s and t h e h a r d d e b r i s c i r c u l a t e s w i t h t h e l u b r i c a n t . Some y e a r s ago, Bryce and Roehner d i scussed t h i s problem i n d e t a i l , b u t f a i l e d t o a r r i v e a t a s a t i s f a c t o r y explanation.

However,

i t has been

e s t a b l i s h e d t h a t t h e c o r r o s i o n o ccurs o n l y when aqueous e l e c t r o l y t e s a r e p r e s e n t i n t h e l u b r i c a n t , which suggests t h a t t h e t i n d i o x i d e i s formed by an e l e c t r o c h e m i c a l mechanism.

Some i n v e s t i g a t o r s c l a i m t o have r e -

produced t h e c o r r o s i o n i n t h e l a b o r a t o r y by making a p i e c e o f w h i t e metal t h e anode i n sea-water b u t , a l t h o u g h t h i s produces a b l a c k l a y e r , the l a y e r i s s o f t and amorphous.

We have found t h a t t h e n a t u r a l c u r r e n t

between p i e c e s o f w h i t e metal ( t i n - b a s e d t o B.S.S.

3332/2) and copper,

p a r t i a l l y immersed i n f u l l - s t r e n g t h o r d i l u t e d s y n t h e t i c sea-water under o i l a t 60"C, w i l l cause t h e w h i t e me t a l t o corrode, f o r m i n g t h i c k , hard, adherent sta nn ic oxide. The b l a c k l a y e r s formed i n t h i s way have the c r y s t a l l o g r a p h i c and metall o g r a p h i c c h a r a c t e r i s t i c s o f t h e l a y e r s found i n engines. s i m p l e set-up

With t h i s

i t has been p o s s i b l e t o i n v e s t i g a t e t h e c o r r o s i o n mechanism

116

Fig.33

Effect o f micro-structure on corrosion.

fig.34

T i n oxide on t h r u s t bearing

117 and t o s t u d y t h e i n f l u e n c e o f v a r i o u s o i l a d d i t i v e s on t h e c o r r o s i o n r a t e . Some a d d i t i v e s reduced o r p r e v e n t e d t i n d i o x i d e f o r m a t i o n i n t h e t e s t cells;

u n f o r t u n a t e l y , almost a l l t h e a d d i t i v e s t h a t behaved i n t h i s way

would have an adverse e f f e c t on o t h e r components i n engines.

For example,

one a d d i t i v e t h a t c o m p l e t e l y prevented c o r r o s i o n o f t h e t i n was c o r r o s i v e t o copper a l l o y s . The mechanism o f t i n o x i d e f o r m a t i o n on b e a r i n g s i s s t i l l n o t f u l l y understood.

However, i t has been d e f i n i t e l y e s t a b l i s h e d t h a t c o r r o s i o n

occurs o n l y when e l e c t r o l y t e s a r e p r e s e n t i n t h e l u b r i c a n t and when t h e r e i s a r e s t r i c t e d supply o f oxygen.

The f a c t t h a t b e a r i n g s on bronze

supports appear t o e x p e r i e n c e more t r o u b l e than b e a r i n g s on s t e e l supports suggests t h a t t h i s i s p r i m a r i l y g a l v a n i c c o r r o s i o n .

T h i s i s n o t a problem

t h a t can be e a s i l y overcome by changes i n l u b r i c a t i n g o i l c o m p o s i t i o n o r changes i n a l l o y composition.

The b e s t remedy i s t o keep e l e c t r o l y t e s o u t

o f t h e system. ( v ) C o r r o s i o n o f c o p p e r - l e a d a l l o y s by water p r e s e n t i n t h e o i l T h i s f o r m o f c o r r o s i o n i n uncommon and i n d i c a t e s t h a t c o n s i d e r a b l e q u a n t i t i e s o f water a r e p r e s e n t i n t h e l u b r i c a t i n g o i l system.

Analysis

may i n d i c a t e t h a t t h e o i l i s i n e x c e l l e n t c o n d i t i o n and t h a t t h e water c o n t e n t i s n o t abnormal.

However, t h e water c o n t e n t o f a sample taken f o r

a n a l y s i s can depend v e r y much on when and where i t was taken.

I f sub-

s t a n t i a l amounts o f water a r e p r e s e n t i t c o u l d be e x e m p l i f i e d by t h e need t o change f i l t e r s f r e q u e n t l y

-

as t h e y s w e l l and b l o c k when i n c o n t a c t

w i t h water. The damage a s s o c i a t e d w i t h t h e presence o f water i n t h e o i l takes t h e f o r m o f removal o f t h e lead-based o v e r l a y m a t e r i a l and severe l o c a l i s e d c o r r o s i o n o f t h e l e a d phase i n t h e c o p p e r - l e a d b e a r i n g a l l o y (Figs. and 3 7 ) .

36

The o v e r l a y i s even removed f r o m t h e unloaded shoulders o f t h e

b e a r i n g , and t h e i n t e n s i v e l o c a l i s e d n a t u r e o f t h e a t t a c k i s i n d i c a t i v e o f galvanic corrosion, attacked

5.4.2.4

t h e l e s s - n o b l e ( a n o d i c ) l e a d being p r e f e r e n t i a l l y

.

C a v i t a t i o n and E r o s i o n

C a v i t a t i o n and e r o s i o n a r e i n c r e a s i n g l y a cause o f f a i l u r e on b e a r i n g s , owing t o i n c r e a s i n g l y h i g h and v a r i a b l e loads and speeds. these terms.

I t i s important t o define

Two t y p e s o f c a v i t i e s can form i n l u b r i c a n t s , vaporous c a v i t i e s

and gaseous c a v i t i e s .

Gaseous c a v i t i e s , which a r e formed by o u t g a s s i n g o f t h e

l u b r i c a n t , can f o r m and c o l l a p s e o n l y s l o w l y and cannot cause any mechanical damage d i r e c t l y .

However, t h e i r presence reduces t h e l o a d - c a r r y i n g c a p a c i t y o f

the l u b r i c a n t f i l m ,

t h e r e b y p r o m o t i n g o t h e r k i n d s o f f a i l u r e , such as f a t i g u e .

118

Tig.35

Fig.36

General o x i d a t i o n o f white-metal.

Damage associated w i t h the presence o f water i n o i l .

119 Vaporous c a v i t i e s , which c o n t a i n vapour o f t h e

i q u i d concerned, and l i t t l e o r

no gas, can f o r m and c o l l a p s e e x t r e m e l y r a p i d l y

and t h e v e r y h i g h p r e s s u r e

a s s o c i a t e d w i t h t h e i r c o l l a p s e can cause mechan c a l ( i m p a c t ) damage on metal surfaces.

Both t y p e s o f c a v i t y can be generate

by t h e p r e s s u r e f l u c t u a t i o n s

a s s o c i a t e d w i t h t h e f l o w o f l u b r i c a n t through a b e a r i n g and t h e f l u c t u a t i n g loads imposed on i t .

Vaporous c a v i t a t i o n can a l s o be caused by t h e v i b r a t i o n

o f a metal s u r f a c e i n c o n t a c t w i t h a l u b r i c a n t , as i n an u l t r a s o n i c c l e a n e r . The damage a s s o c i a t e d w i t h b o t h types o f c a v i t a t i o n may be aggravated by t h e presence o f f i n e p a r t i c l e s i n t h e l u b r i c a n t ;

i t i s i n such circumstances t h a t

i t may be j u s t i f i e d t o t a l k o f e r o s i o n damage and c a v i t a t i o n e r o s i o n .

However,

i t should be emphasised t h a t vaporous c a v i t a t i o n can cause severe damage even i n

the absence o f sol i d p a r t i c l e s . U n l i k e o t h e r types o f damage, vaporous c a v i t a t i o n damage i s g e n e r a l l y encount e r e d on t h e unloaded areas o f b e a r i n g s , where o i l - f i l m pressures a r e low, and t h i s p r o v i d e s a u s e f u l means o f i d e n t i f i c a t i o n ( F i g . 3 8 ) .

Microsections o f

damaged areas show s i g n s o f l o c a l work-hardening and f a t i g u e c r a c k i n g .

When t h e

damage i s due s o l e l y t o c a v i t a t i o n , t h e t e x t u r e i n t h e damaged areas i s rough (Fig.39);

when p a r t i c l e s a r e p r e s e n t ( c a v i t a t i o n e r o s i o n ) , t h e damaged surfaces

a r e smooth (Fig.40). Vaporous c a v i t a t i o n can remove p r o t e c t i v e f i l m s , and i n i t i a t e c o r r o s i o n .

such as o x i d e s , from m e t a l s

I n a d d i t i o n , t h e v e r y h i g h l o c a l pressures and temp-

e r a t u r e s a s s o c i a t e d w i t h t h e f i n a l stage o f c a v i t y c o l l a p s e can induce chemical r e a c t i o n s which would n o t n o r m a l l y t a k e p l a c e .

There i s some evidence t h a t

c e r t a i n o i l a d d i t i v e s a r e u n s t a b l e under c a v i t a t i n g c o n d i t i o n s and t h a t t h e decomposition p r o d u c t s can be c o r r o s i v e .

I n such circumstances, b e a r i n g s u r -

faces can be s u b j e c t e d t o t h e combined e f f e c t s o f c a v i t a t i o n and c o r r o s i o n . Work c a r r i e d o u t a t Thornton Research Centre i n d i c a t e s t h a t t h e r e i s v e r y l i t t l e one can do t o a commercial l u b r i c a n t t o e l i m i n a t e t h e e f f e c t s o f c a v i t a t i o n , and c a v i t a t i o n must be regarded p r i m a r i l y as a d e s i g n problem.

5.4.2.5

E l e c t r i c a l Discharge Damage

On e l e c t r i c a l machinery, and o c c a s i o n a l l y on o t h e r types o f machinery, p o t e n t i a l d i f f e r e n c e s can be b u i l t up and e l e c t r i c a l discharges may o c c u r across the b e a r i n g s u r f a c e s . Each d i s c h a r g e g i v e s r i s e t o a small p i t , and a l a r g e number o f discharges can e v e n t u a l l y cause damage o f t h e t y p e i l l u s t r a t e d i n Fig.41.

This type o f

damage i s c h a r a c t e r i s e d by t h e f a c t t h a t t h e p i t t i n g o c c u r s on b o t h b e a r i n g and j o u r n a l surfaces.

120

Fig.37

Section showing the i n t e n s i v e l o c a l i s e d n a t u r e o f the a t t a c k .

Fig.38

C a v i t a t i o n damage.

121

Fig.39

C a v i t a t i o n of w h i t e m e t a l .

Fig.40

Cavitation erosion o f white metal.

122

Fig.41

5.4.2.6

E l e c t r i c a l p i t t i n g i n l e a d base b a b b i t t b e a r i n g .

Wire-wool

Failures

T h i s i s th e name g i v e n t o a c a t a s t r o p h i c t y p e o f f a i l u r e t h a t has been enc ou n t e r e d d u r i n g t h e l a s t 10 ye ars on t i n-b ased j o u r n a l and t h r u s t b e a r i n g s on b o t h land-based and mari n e t u r b i n e i n s t a l l a t i o n s .

A hard, b l a c k scab (Fig.421,

which i n some cases has been shown t o be an amorphous m i x t u r e o f i r o n and t i n c o n t a i n i n g d i s p e r s e d i r o n c a r b i d e s and p o s s i b l y n i t r i d e s , forms on t h e surfaces o f t h e t i n - b a s e d whi t e-me t a l b e a r i n g s and proceeds t o machine-away t h e a d j a c e n t s t e e l s u r fa c e s .

S t e e l s l i v e r s a r e u s u a l l y found i n and around t h e b e a r i n g ,

sometimes i n s u f f i c i e n t q u a n t i t y t o g i v e t h e appearance o f w ire-w ool. f a i l u r e s a lm o s t always occu r w i t h i n a few hours o f s t a r t - u p ,

The

e i t h e r f r o m new o r

a f t e r an o v e r h a ul , and a r e c h a r a c t e r i s e d by a v e r y h i g h wear r a t e (sometimes c e n t i m e t r e s o f s t e e l may be l o s t ) w i t h o u t h i g h c o e f f i c i e n t s o f f r i c t i o n o r much temperature r i s e . ium-steel

The f a i l u r e s have been a t t r i b u t e d b o t h t o t h e use o f chrom-

r o t o r s h a f t s and t o e . ~ . o i l s .

There i s no c l e a r understanding o f t h e

sequence o f e v e n t s l e a d i n g t o f a i l u r e , and i n p a r t i c u l a r o f t h e r o l e s p l a y e d by metal s u r fa c e s , t h e base o i l and t h e a d d i t i v e s p r e s e n t i n t h e base o i l . can be s a i d i s t t i a t t h i s i s indeed a t r i b o l o g i c a l f a i l u r e .

A l l that

However, v a r i o u s

i n v e s t i g a t o r s have e s t a b l i s h e d t h a t t h e t r i g g e r i n g agent r e s p o n s i b l e f o r w i r e -

wool f a i l u r e s i s a d i r t p a r t i c l e , p a r t i a l l y embedded i n a w hite-metal b e a r i n g . I t i s n o t c e r t a i n what causes p a r t i c u l a r p a r t i c l e s t o t r i g g e r t h e sequence o f events l e a d i n g t o f a i l u r e , b u t whether o r n o t a b l a c k scab "machine t o o l " develops depends v e r y much on t h e n a t u r e o f t h e l u b r i c a n t and the composition o f

123 the r o t o r s t e e l .

O i l a d d i t i v e s (e.g.

sulphur-containing

e.p.

additives) that

can p r e v e n t b l a c k scab f o r m a t i o n w i t h ch romi um-containing-steel

r o t o r s may

a c t u a l l y promote scab f o r m a t i o n when used w i t h o t h e r r o t o r s t e e l s such as m i l d s t e e l and

4% molybdenum

that chlorine-containing

With these l a t t e r s t e e l s i t has been claimed

steel. e.p.

a d d i t i v e s a l s o increase t h e s u s c e p t i b i l i t y t o

failure,

b u t t h i s i s n o t supported by o u r i n v e s t i g a t i o n s a t Thornton Research

Centre.

What i s more pro ba bl e i s t h a t some l u b r i c a n t s a r e more l i k e l y t o a l l o w

f a i l u r e when t h e i r s p e c i f i c a n t i - r u s t / a n t i - w e a r The b e s t way t o a v o i d wire-wool

agent becomes depleted.

f a i l u r e s i s t o avoid d i r t .

t h e o u t s e t must be as c l e a n as p o s s i b l e .

The system from

The o i l should be c o n t i n u o u s l y f i l t e r e d

and t h e oil p r e ssu re i n t he t h r u s t ho usi n g under r u n n i n g c o n d i t i o n s should always be a t l e a s t 7 I b f / i n 2 (48 kN/m2). Now t h a t b l a c k scab f a i l u r e s have been w i d e l y p u b l i c i s e d , may have been encountered p r e v i o u s l y b u t passed unrecognised.

i t appears t h a t they

For example,

severe wear o f n i t r i d e d s t a i n l e s s - s t e e l s h a f t j o u r n a l s on power-recovery t u r b i n e s o f a i r c r a f t p i s t o n engines has been observed from t i m e t o t i m e and these appear t o have s u f f e r e d severe ma chi n i ng -t yp e wear.

It i s p a r t i c u l a r l y s i g n i f -

i c a n t t h a t t h e b e a r i n g s i n c o n t a c t w i t h these f a i l e d j o u r n a l s a r e i n g e n e r a l l y q u i t e good c o n d i t i o n , showing much l e s s damage than would be expected from the s t a t e o f the j o u r n a l s .

T h i s i s a l s o a c h a r a c t e r i s t i c o f b l a c k scab f a i l u r e s .

However, t h e a i r c r a f t b e a r i n g s were s i l v e r w i t h lead-indium o v e r l a y s .

I f these

f a i l u r e s a r e o f t h e b l a c k scab t yp e, then b l a c k scab i s n o t p e c u l i a r t o w h i t e metal b e a r in g s .

5.4.2.7

Fatigue

A p l a i n b e a r i n g may f a i l by f a t i g u e when i t has achieved i t s designed l i f e expectancy;

however,

i f f a i l u r e occu rs p rematurely, t h i s w i l l be because e i t h e r

an i n c o r r e c t b e a r i n g m a t e r i a l has been used o r t h e b e a r i n g has been i n c o r r e c t l y fitted. I n f a t i g u e f a i l u r e s t h e c r a c k s s t a r t a t t h e b e a r i n g s u r f a c e , propagate n o r mal t o t h e s u r f a c e u n t i l t he y approach t h e s h e l l , then t u r n through 90" and exte n d p a r a l l e l t o t h e bond between t h e b e a r i n g metal and the s h e l l l e a v i n g a t h i n l a y e r o f b e a r i n g metal a t t a c h e d t o t h e s h e l l (Fig.43). Bad bonding and f a t i g u e a r e s u p e r f i c i a l l y

similar.

However, w i t h bad bonding

th e b e a r i n g m e ta l separates c l e a n l y f r o m t h e s h e l l , whereas w i t h f a t i g u e the c r a c k s s t a r t a t t h e b e a r i n g s u r f a c e , propagate normal t o t h e b e a r i n g s u r f a c e u n t i l t h e y approach t h e s h e l l and t h en ext en d p a r a l l e l t o the bond between t h e b e a r i n g m e ta l and t h e s h e l l , always l e a v i n g a l a y e r o f b e a r i n g metal a t t a c h e d t o the s h e l l .

For a comparison o f t h e two t ypes o f f a i l u r e see Fig.44.

124

Fig.42

Black scab formation on a thrust bearing.

Fi9.43

F a t i g u e of lead b r o n z e bearing.

125

Fig.44

5.4.2.8

White met al b e a r i n g s

Thermal C y c l i n g Damage

T h i s i s o n l y a problem on t i n - r i c h b e a r i n g a l l o y s . o f t i n crystals i s anisotropic,

i.e.

The thermal expansion

t h e l i n e a r c o e f f i c i e n t o f thermal expansion

i s d i f f e r e n t a l o n g t h e t h r e e p r i n c i p a l c r y s t a l l o g r a p h i c axes. ti n - b a s e d w h ite - met al

Consequently,

b e a r i n g s exposed t o c o n s i d e r a b l e thermal c y c l i n g can ex-

p e r i e n c e g r a in - b o un da ry d i s t o r t i o n and c r a c k i n g (Fig.45).

Bad bonding f a c i l -

i t a t e s t h i s type o f f a i l u r e .

5.4.2.9

A l l o y i n g i n Service

T h i s can be a problem on e l e c t r o p l a t e d l e a d - a l l o y o v e r l a y b e a r i n g s and on micro-Babbitt

b eari n gs.

I n b o t h cases t h e t i n i n t h e s u r f a c e l a y e r m i g r a t e s

towards and a l l o y s w i t h t h e c o p p e r - r i c h phase o r s i l v e r phase i n t h e underlay. Hard,

i n t e r m e t a l l i c compounds a r e formed which,

the j o u r n a l surfaces.

i f they a r e exposed, can score

T h i s p rob l em i s most a c u t e on m i c r o - B a b b i t t bearings,

where complete a l l o y i n g o f t h e 200-pm t h i c k , t i n - r i c h s u r f a c e has been observed i n s e r v i c e (Fig.46).

Ob vi ou sl y, t h e a l l o y i n g r e a c t i o n i s dependent on b o t h

ti me and temperature;

e xpe ri men t s show t h a t i t can proceed v e r y r a p i d l y a t

150°C ( Fig .4 7 ) .

126

Fig.45

5.4.2.10

D i s t o r t i o n s i n t i n due t o thermal c y c l i n g .

Inadequate V i s c o s i t y and Lack o f L u b r i c a n t

Should t h e s up pl y o f l u b r i c a n t t o a b e a r i n g be i n t e r r u p t e d , even f o r a m a t t e r o f seconds, c a t a s t r o p h i c f a i l u r e can occu r.

The damage u s u a l l y takes t h e form

o f complete s e i z u r e and e x t e n s i v e m e l t i n g o f t h e b e a r i n g a l l o y .

I f the f l o w o f

a l u b r i c a n t i s l i a b l e t o i n t e r r u p t i o n , some degree o f temporary p r o t e c t i o n can

be p r o v i d e d by t h e use o f s p e c i a l s u r f a c e t r e a t m e n t s on t h e s t e e l surfaces, by using p l a s t i c o r plastic-impregnated

b e a r i n g s and by t h e use o f g r a p h i t e o r

molybdenum d i s u l p h i d e i f t h e l a t t e r i s c o r r e c t l y a p p l i e d . Some f a i l u r e s due t o o i l s t a r v a t i o n a r i s e because o f l a c k o f l u b r i c a n t on start-up,

and on many items o f heavy machinery i t i s e s s e n t i a l t o have a means

of c i r c u l a t i n g t he o i l p r i o r t o s t a r t - u p t o p r o v i d e l u b r i c a t i o n and c o o l i n g .

127

I n t e r m e t a l l i c compound f o r m a t i o n on m i c r o - B a b b i t t b e a r i n g .

Fig.46

D i l u t i o n o f t h e l u b r i c a n t by f u e l can o c c u r i n b o t h g a s o l i n e and d i e s e l engines.

I t i s most f r e q u e n t l y encountered on l i g h t l y loaded, c o o l - r u n n i n g

engines, e s p e c i a l l y those w i t h poor combustion c h a r a c t e r i s t i c s . fuel

i n t h e l u b r i c a n t lowers i t s v i s c o s i t y ,

capacity.

The presence o f

thereby r e d u c i n g i t s l o a d - c a r r y i n g

I f t h e r e d u c t i o n i n v i s c o s i t y i s m a r g i n a l , premature f a i l u r e by

f a t i g u e may o c c u r ;

however, i f t h e r e d u c t i o n i s severe,

then sudden, c a t a s t -

r o p h i c f a i l u r e may t a k e p l a c e .

5.5

CONCLUS IONS I n t h e p r e c e d i n g pages we have attempted t o d e s c r i b e t h e m e t a l l u r g i c a l

f e a t u r e s o f some o f t h e b e a r i n g f a i l u r e s t h a t we have i n v e s t i g a t e d i n r e c e n t years.

Bearing f a i l u r e mechanisms a r e becoming i n c r e a s i n g l y complex and i t i s

no l o n g e r always p o s s i b l e t o g i v e r e l i a b l e , on-the-spot

diagnosis.

Metallo-

graphic examination o f sections from f a i l e d bearings w i l l o f t e n provide the necessary i n f o r m a t i o n t o e s t a b l i s h t h e cause o f t h e t r o u b l e .

For t h e more

d i f f i c u l t and complex cases t h e r e i s a wide v a r i e t y o f i n v e s t i g a t i o n a l t e c h niques t h a t can be a p p l i e d t o t h e problem.

To a s s i s t o t h e r s i n t h e d i a g n o s i s

o f p l a i n b e a r i n g f a i l u r e s we have compiled Table 5.4, v a r i o u s f a i l u r e s a r e described.

i n which t h e f e a t u r e s o f

W h i l s t t h i s i s n o t a s u b s t i t u t e f o r experience,

we b e l i e v e t h a t , c o u p l e d w i t h t h e accompanying b i b l i o g r a p h y , i t may be an a i d t o e s t a b l i s h i n g t h e cause o f f a i l u r e s .

128

Electro deposited t i n lead overlay

on

A f t e r 2 hrs. a t 170°C

Fig.47

25pm

After 2 h r s at 130'C

After 2 hrs.at

Electron-probe micrograph showing rate of diffusion o f tin.

150°C

129 TABLE 5.4 Cause o f f a i 1 ure

Typical f e a t u r e s

Extraneous matter c i r c u l a t i n g i n 1 ubr icant

S o f t p a r t i c l e s , e.g. carbonaceous matter, can erode w h i t e metals and overlays. Hard p a r t i c l e s , e.g. m e t a l l i c wear d e b r i s c u t s w e l l - d e f i n e d channels.

Extraneous matter embedded i n bearing

Burnished spots round embedded p a r t i c l e s . Wear o f j o u r n a l - can lead t o f a t igue.

Fatigue

Cracks, i n i t i a t e d a t bearing surface, propagate normal t o surface; then, near backing, t u r n and run p a r a l l e l w i t h bearing surface.

Bad bonding

Bearing a l l o y l i f t s c l e a r l y away from no evidence o f a l l o y i n g . backing Comnon on l a r g e , white-metal bearings.

The rma 1 cyc 1 ing

Can cause roughening o f tin-based whitemetal bearing surfaces

Surface p i t t i n g o f tin-based w h i t e metals

Cracking o f o v e r s i z e tin-antimony cuboids

Hard, black surface l a y e r s a l l over tin-based white-metal bearings

T i n - r i c h m a t r i x o f a l l o y transformed t o hard t i n oxides. Antimony-tin and c o p p e r - t i n phases unchanged.

Sudden c a t a s t r o p h i c wear o f a s i n g l e t u r b i n e j o u r n a l o r t h r u s t bearing shortly after start-up

Black scab formation i n white-metal bearing, which "machines" s h a f t , producing wire-wool.

Electrical p i t t i n g

Fine p i t s on both bearing surface and journal.

Cavitation i n lubricant f i l m

Local ised metal removal, g e n e r a l l y i n unloaded areas o f bearing.

Corrosion by weak o r g a n i c a c i d s

Surface roughening and f i l l i n g . I n copper-lead and lead-bronze, pure lead phase i s leached o u t and surface may d i s i n t e g r a t e . Lead overlays d e f i c i e n t i n t i n o r indium may be corroded.

Corrosion by s t r o n g a c i d s

Takes a v a r i e t y o f forms; bearing metal o r j o u r n a l may be attached. See t e x t .

"Sulphur" a t t a c k

D i s c o l o r a t i o n and c o r r o s i o n o f copper and s i l v e r - r i c h a l l o y s . Can be very severe i n phosphor bronzes.

Corrosion by water i n the o i l

Takes t h e form o f removal o f the o v e r l a y and l o c a l i s e d i n t e n s i v e a t t a c k o f the lead phase o f copper-lead bearing a l l o y s .

Excessive o p e r a t i n g temperatures

\ t i p i n g o f surface layers. Fatigue f a i l u r e .

Inadequate v i s c o s i t y o f l u b r i c a n t

If marginal, may g i v e r i s e t o premature f a i l u r e by f a t i g u e . I f severe, f o r example due t o excessive f u e l d i l u t i o n , can g i v e sudden c a t a s t r o p h i c f a i l u r e .

Lack o f l u b r i c a n t

Complete seizure. Extensive m e l t i n g o f bearing a l l o y .

Bad f i t t i n g

F r e t t i n g o f backs o f bearing. Lack o f thermal c o n t a c t w i t h housing may cause bearing t o bow.

Misalignment

Uneven c o n t a c t and wear across bearing surface.

Manufacturing defects, e.g. poor lead d i s t r i b u t i o n i n copper-lead alloys

Requires expert. m e t a l l u r g i c a l examinat ion.

Surface hardening o f t i n B a b b i t t overlay

Cracking o f o v e r l a y , wear of j o u r n a l .

I n t e r n a l o x i dat i o n o f lead- i n d i um over I ays

Corrosion and f a t i g u e o f o v e r l a y .

-

130 REFERENCES

1

2 3

Sleeve Bearing M a t e r i a l s , 1949, ASM Cleveland, Ohio. Metals Handbook, 1961, 8 t h e d i t i o n , Vol.1, p.843-863, ASM Cleveland, Ohio. ( 3 ) , p.29, "The Modern Bus and Truck - Fuels and L u b r i c a t i o n , 1953, Lubricants". ( 5 ) , p.57, "Some Problems A s s o c i a t e d w i t h L u b r i c a t i o n L u b r i c a t i o n , 1953, o f Large Engines". L u b r i c a t i o n , 1953, 8, I ( l l ) , p.45, "Automotive Engine Bearings". L u b r i c a t i o n , 1955, 10, ( 4 ) , p.37, " I n d u s t r i a l B e a r i n g L u b r i c a t i o n " . L u b r i c a t i o n , 1957, ( 8 ) , p.85, "Abrasives and Wear". L u b r i c a t i o n , 1958, 13, ( 9 ) , p.245, "Petroleum L a b o r a t o r y I n v e s t i g a t i o n s " . ( 6 ) , p.81, "Diesel Power Plants". L u b r i c a t i o n , 1963, ( 7 ) , p.77, " P l a i n Bearing F a i l u r e s " . L u b r i c a t i o n , 1964, F o r r e s t e r , P.G., Modern M a t e r i a l s , 1964, 4, p.173, Academic Press, New York and London, " M a t e r i a l s f o r P l a i n BeaTings". " E l e c t r o - g r a p h i c Methods o f Hunter,M.S., C h u r c h i l l , J.R. and Mear, R.B., Surface A n a l y s i s " , Metal Progress 1942, 42, p.1070. Crooks, C.S. and Eastham, D.R. " P l a t i n g z r Bearing A p p l i c a t i o n s " . Trans. I n s t . o f Metal F i n i s h i n g 1982 v01.60. Rafique, S.O., I n s t . Mech. Eng. L u b r i c a t i o n and Wear, Second Convention " F a i l u r e s o f P l a i n Bearings and t h e i r Causes". 1964, p.180. Love,P.P., Forrester,P.G. and Burke,A.E., I n s t . Mech. Eng. Auto. D i v . p.29. "Function o f M a t e r i a l s i n Bearing Operation". Proc., 1953-54, L u b r i c a n t s and L u b r i c a t i o n ( e d i t o r B r a i t h w a t e , E.R.) E l s e v i e r , London,

8, 8,

E,

4 5 6

7 8 9

5, z,

2,

1967. M o r r i s , J.A., Ch.7. p.310, " M e t a l l i c Bearing M a t e r i a l s " . P r a t t , G.C., Ch.8, p.377, " P l a s t i c - b a s e d Bearings". 10

Pratt,G.C. and Perkins,C.A., " S i l i c o n Aluminium Bearings f o r High-speed Diesels", D i e s e l and Gas T u r b i n e Worldwide Vol. X I I I , No.10, p.76-78. 1 1 Davis,T.A., " P l a i n b e a r i n g wear i n IC Engines", Automotive Engineer, Aug./Sept. 1981. 12 P r i n c i p l e s of L u b r i c a t i o n ( e d i t o r A. Cameron) Longmans Green and Co., London, 1966. Holligan,P.T., Ch.25, p.511. " P l a i n Bearings - Bearing M a t e r i a l s and Diagnosis o f Bearing Fa i 1 ures". 1 3 Engineering, 1967, 20, p.260. "Bond S t r e n g t h o f White M e t a l l i n g " . 14 Rose, A. Trans. l n s t . Mar. Eng. 1967, 79, p.233, "Marine Bearings". J o i n t Course on T r i b o l o g y , I n s t i t u t i o n o f 15 Wilson,R.W. and Shone,E.B., M e t a l l u r g i s t s , London, 1968, Paper 4, " M e t a l l u r g i c a l S t u d i e s o f Bearing Failures". 16 Wilson,R.W. and Shone,E.B., A n t i - C o r r o s i o n Methods and M a t e r i a l s , 1970, 17, p.9. "The C o r r o s i o n o f Lead Overlay Bearings". "The Science o f T r i b o l o g y 17 Quayle, J.P., Copper, 1969, 3, ( 5 ) , p.12. P a r t 3Il. 18 Bryce,J.B. and Roehner,T.G., Trans. I n s t . Mar. Engs., 1961, 73, p.377, "The C o r r o s i o n o f Tin-Base B a b b i t t Bearings i n Marine Steam Turbines". I n s t . Mech. Engs., T r i b o l o g y Convention 1969, 19 Lloyd,K.A. and Wilson,R.W., Paper 10, p.76. "Formation o f T i n Oxides on White Metal Bearinqs". "Corrosion o f T i n Base B a b b i t t Bearings t o f o r m T i n Oxides", 20 H i l e y , R.W., Trans l n s t . Mar. Eng., 1979, 91, (2) p. 52-66. 21 Dawson, P.H. and F i d l e r , F., I n s t . Mech. Engs. ( L u b r i c a t i o n and Wear F i f t h Convention), 1967, 1811, p. 207, "Wire-wool Type F a i l u r e s ; The e f f e c t o f S t e e l Composition, S t r u c t u r e and Hardness". 22 Dowson, D., Godet, M. and T a y l o r , C.M., " C a v i t a t i o n and R e l a t e d Phenomena i n L u b r i c a t i o n " , U n i v e r s i t y o f Leeds, Yorks, England. Leeds-Lyon Symposium on T r i b o l o g y , l s t , Prog. Pap and Discuss., U n i v e r s i t y o f Leeds, Yorks, England, Sept. 1974, Publ. by Mech. Eng. Publ. f o r I n s t i t u t e o f T r i b o l o g y , Publ. Ltd., New York, NY, 1974, 248. Leeds U n i v e r s i t y , Yorks, England.

131

23 24 25 26 27 28 29

James, R.D., " C a v i t a t i o n Damage i n P l a i n Bearings". T r i b o l o g y I n s t . Feb. 1978, 1 1 , (1) p. 22-23. Garner, D.R., James, R.D. and W a r r i n e r , J.F., " C a v i t a t i o n E r o s i o n i n Engine B e a r i n g s - Theory and P r a c t i c e " , 1 3 t h CIMAC Conf. Vienna 1979. F o r r e s t e r , P.G., " B e a r i n g M a t e r i a l s " , M e t a l l u r g i c a l Reviews, 1960, 5, p. 507. F o r r e s t e r , P.G., " E l e c t r o d e p o s i t i o n i n P l a i n B e a r i n g Manufacture", Trans. I n s t . Met. F i n i s h i n g , 1961, 38, p.52. Booser, E.R., " P l a i n B e a r i n g M a t e r i a l s " , Machine Design, 1970, 42, p. 14. Standard Handbook o f L u b r i c a t i o n E n g i n e e r i n g , ASLE, M c G r a w - H i l l , 1968. Chapter 18, "S1 i d i n g Bearings". T r i b o l o g y Handbook ( e d i t o r M.J. N e a l e ) , B u t t e r w o r t h s , London 1973.

132

(;

ROLLING ELEMENT BEARINGS

D.G.

HJERTZEN and

6.1

INTRODUCTION

R.A.

JARVIS,

SKF (U.K.)LTD.

The p r e s e n t development o f r o l l i n g b e a r i n g s i s c h a r a c t e r i s e d by numerous a p p a r e n t l y s m a l l i n t e r n a l improvements i n t h e b e a r i n g s and t o m a t e r i a l q u a l i t y r a t h e r than t h e i n t r o d u c t i o n o f r a d i c a l new designs.

There i s c o n s i d e r a b l e

t e c h n i c a l r e s e a r c h i n t o r o l l i n g b e a r i n g technology and i m p o r t a n t progress i s b e i n g made i n m a n u f a c t u r i n g and i n s p e c t i o n methods, improved l u b r i c a n t s and l u b r i c a t i o n equipment, e t c . I t i s possibly s u r p r i s i n g , but true,

t h a t although r o l l i n g bearings are

e x t e n s i v e l y used, t h e r e a r e many misconceptions and t h e methods o f s e l e c t i n g t h e most s u i t a b l e b e a r i n g s a r e n o t always f u l l y understood o r a p p l i e d .

It

has been known f o r some u n f o r t u n a t e e x p e r i e n c e w i t h a c e r t a i n b e a r i n g t y p e t o r e s u l t i n l o s s of confidence; cular

6.2

consequently,

the c h a r a c t e r i s t i c s o f t h a t p a r t i -

bearing are o f t e n n o t f u l l y u t i l i s e d .

BEARING SELECTION The major f u n c t i o n o f b e a r i n g s i s t o t r a n s m i t loads between machine p a r t s i n

r e l a t i v e motion, b u t a d d i t i o n a l l y t h e r e may be s p e c i a l performance o r e n v i r o n ment requirements a f f e c t i n g c h o i c e o f b e a r i n g s . I t i s e s s e n t i a l f o r t h e d e s i g n e r t o s e l e c t from t h e wide range o f b e a r i n g s a v a i l a b l e t h e b e a r i n g design s u i t a b l e t o c a r r y t h e loads i n v o l v e d under t h e v a r i o u s o p e r a t i n g c o n d i t i o n s and t o s a t i s f y t h e requirements o f r o t a t i o n a l speed, temperature v a r i a t i o n s , b e a r i n g housing misalignment, and r i g i d i t y e t c . I t i s a l s o i m p o r t a n t t o ensure t h a t c o r r e c t f i t s a r e used between t h e b e a r i n g i n n e r r i n g t o s h a f t and o u t e r r i n g t o housing.

The c h o i c e o f f i t depends

e s s e n t i a l l y on t h e w o r k i n g c o n d i t i o n s , b u t t h e r e a r e o t h e r c o n s i d e r a t i o n s such as b e a r i n g t y p e and s i z e , b e a r i n g i n t e r n a l c l e a r a n c e and method o f assembly, etc.

133 6.3

BEARING TYPES

6.3.1

S i n g l e Row Deep Groove B a l l B e ari n g ( Fig.1)

The depth o f t h e b a l l t r a c k s cou pl e d w i t h a r e l a t i v e l y l a r g e b a l l s i z e and h i g h degree of c o n f o r m i t y ( r a t i o o f t r a c k r a d i u s t o b a l l r a d i u s ) g i v e s t h i s bearing considerable a x i a l - c a r r y i n g capacity i n a d d i t i o n t o r a d i a l capacity, even a t h i g h speeds.

These b e a r i n g s n o r m a l l y have a c y l i n d r i c a l bore and a r e

mounted d i r e c t o n t o t h e s h a f t .

For l o c a t i o n purposes, a groove can be p r o v i d e d

i n t h e o u t e r r i n g f o r a snap r i n g wh i ch can be used f o r a x i a l l o c a t i o n . These b e a r i n g s can be f i t t e d w i t h s h i e l d s o r s e a l s .

The s h i e l d s a r e intended

as grease r e t a i n e r s and t o keep o u t a c e r t a i n amount o f f o r e i g n m a t t e r , b u t t h e s e a l s a r e a c t u a l l y r u b b i n g s e a l s and designed f o r dusty environments,

etc.

Be a r in g s w i t h two s h i e l d s o r s e a l s a r e i n i t i a l l y charged w i t h t h e c o r r e c t quant i t y o f grease and, co nse qu en t l y, do n o t r e q u i r e r e l u b r i c a t i o n .

These bearings

a r e f r e q u e n t l y c a l l e d l u b r i c a t e d - f o r - l i f e bearings.

Fig.1

6.3.2

S i n g l e Row Deep Groove B a l l B e a r ing .

F ig.2

Self-A igning

B a l l Bearing.

S e l f - a l i g n i n g B a l l B e ari n gs (F i g. 2)

These have two rows o f b a l l s each i n i t s own groove on the i n n e r r i n g , b u t i n a common sphered t r a c k i n t h e o u t e r r i n g .

The i n n e r r i n g and b a l l s form a

u n i t wh ic h can a l i g n f r e e l y a bo ut t h e b e a r i n g c e n t r e .

T h i s f e a t u r e i s an advan-

tage i n cases where i t i s d i f f i c u l t t o o b t a i n a c c u r a t e p a r a l l e l i s m between t h e s h a f t and h o u s in g bo re, o r where t h e r e i s a d e f l e c t i o n o f t h e s h a f t .

Due t o

the sphered o u t e r r i n g t r a c k , t h e b e a r i n g does n o t have h i g h l o a d - c a r r y i n g

134 c a p a c i t y and t h e a x i a l - c a r r y i n g c a p a c i t y i s l i m i t e d . The degree o f mis-alignment o f t h e s h a f t o r housing i s l i m i t e d by t h e b a l l s c o n t a c t i n g t h e edges o f t h e o u t e r r i n g and p e r m i s s i b l e m i s - a l i g n m e n t n o r m a l l y v a r i e s from 2'

-

3'.

These b e a r i n g s a r e manufactured w i t h c y l i n d r i c a l o r t a p e r b o r e and t h e l a t t e r i s u s u a l l y mounted on a s p l i t s l e e v e .

6.3.3

Angular Contact B a l l B e a r i n g [ F i g . 3 )

The d i r e c t i o n o f l o a d through t h e b a l l s i s a t an a n g l e t o t h e b e a r i n g a x i s which makes these b e a r i n g s p a r t i c u l a r l y s u i t a b l e f o r c a r r y i n g combined r a d i a l and a x i a l loads.

A r a d i a l l o a d imposed on a s i n g l e row a n g u l a r c o n t a c t b a l l

b e a r i n g g i v e s r i s e t o an induced a x i a l l o a d which must be c o u n t e r a c t e d ; t h e r e f o r e these b e a r i n g s need t o be arranged so t h a t t h e y can be a d j u s t e d a g a i n s t a second b e a r i n g .

Fig.3

Angular Contact B a l l B e a r i n g .

Fig.4

Double Row Angular Contact B a l l Bearing.

These b e a r i n g s a r e f r e q u e n t l y mounted i n p a i r s i n f a c e - t o - f a c e ,

back-to-back

o r tandem f o r m a t i o n , and i n o r d e r t o achieve t h i s t h e s i d e faces o f t h e b e a r i n g r i n g s a r e ground t o achieve t h e c o r r e c t r e s i d u a l i n t e r n a l c l e a r a n c e . The c o n t a c t a n g l e can v a r y between 15' 6.3.4

- 25'

-

30'

-

40'.

Double Row Angular Contact B a l l Bearing ( F i g . 4 )

T h i s b e a r i n g has s i m i l a r c h a r a c t e r i s t i c s t o two s i n g l e row a n g u l a r c o n t a c t b a l l b e a r i n g s mounted back-to-back;

consequently, t h e l i n e s o f p r e s s u r e o r

135 c o n t a c t through t h e b a l l s a r e d i r e c t e d outwards, rigidity.

thereby g i v i n g increased

These b e a r i n g s have v e r y l i t t l e a x i a l c l e a r a n c e , g i v i n g c l o s e a x i a l

l o c a t i o n o f t h e s h a f t and e l i m i n a t i n g t h e n e c e s s i t y

f o r a x i a l adjustment.

These b e a r i n g s can be s u p p l i e d t o g i v e a p r e - l o a d c o n d i t i o n when mounted so t h a t even when s u b j e c t e d t o a x i a l load, t h e a x i a l displacement o f t h e s h a f t i s very small.

6.3.5

C y l i n d r i c a l R o l l e r Bearings ( F i g . 5 )

The r o l l e r s i n these b e a r i n g s a r e g u i d e d between i n t e g r a l f l a n g e s on the o u t e r o r i n n e r r i n g , thereby a l l o w i n g t h e r i n g s t o move a x i a l l y r e l a t i v e t o each o t h e r , which i s an advantage when t h e s h a f t expansion i s g r e a t e r than t h e housing expansion i n an a x i a l plane.

The f l a n g e d r i n g and r o l l e r s a r e h e l d

t o g e t h e r by a cage t o form an assembly which can be removed from t h e o t h e r r i n g . T h i s s e p a r a b l e f e a t u r e i s o f t e n u t i l i s e d t o ease assembly problems.

The bear-

i n g has h i g h r a d i a l l o a d - c a r r y i n g c a p a c i t y w i t h a c c u r a t e g u i d i n g o f t h e r o l l e r s , r e s u l t i n g i n a c l o s e approach t o t r u e r o l l i n g .

Consequently,

t h e low f r i c t i o n

p e r m i t s h i g h speeds.

Fig.5

C y l i n d r i c a l R o l l e r Bearing.

Fig.6

Double Row C y l i n d r i c a l R o l l e r Bearing.

V a r i o u s designs a r e made w i t h d i f f e r e n t f l a n g e arrangements and a d d i t i o n a l

loose f l a n g e s can be i n c o r p o r a t e d t o a x i a l l y l o c a t e t h e r o l l e r s i n b o t h d i r e c tions.

The r o l l e r ends and r i n g f l a n g e s a r e capable o f s u s t a i n i n g r e l a t i v e l y

h i g h a x i a l loads.

136 6.3.6

Double Row C y l i n d r i c a l R o l l e r B e ari n gs ( F i g . 6 )

These b e a r in g s a r e o f sep ara bl e d esi g n which a l l o w s a f l a n g e d r i n g w i t h r o l l e r and cage assembly and f l a n g e l e s s r i n g t o be f i t t e d independently, thus f a c i l i t a t i n g mounting and d i smou nt i ng . Low c r o s s - s e c t i o n a l

h e i g h t and h i g h l o a d - c a r r y i n g c a p a c i t y g i v e r i g i d bear-

i n g arrangements f o r t h e accommodation o f heavy r a d i a l loads.

They a r e , there-

f o r e , m a i n l y used f o r machine t o o l and r o l l i n g m i l l a p p l i c a t i o n s . r i n g n o r m a l l y has a t ap ere d

The i n n e r

b ore t o e na bl e t h e r i n g t o be d r i v e n up a tapered

j o u r n a l t o a c h ie ve a g i v e n r a d i a l cl e ara nce o r even p r e - l o a d .

They can be

supplied i n special o r u l t r a - p r e c i s i o n execution.

6.3.7

Needle R o l l e r Be ari n gs (F i g. 7)

These a r e s i m i l a r t o c y l i n d r i c a l r o l l e r b e a r i n g s and have h i g h l o a d - c a r r y i n g capacity.

They a r e made i n v a r i o u s d esi g ns and a r e p a r t i c u l a r l y s u i t a b l e f o r

a p p l i c a t i o n s i n c o n f i n e d spaces.

These b e a r i n g s a r e used i n gudgeon p i n s and

u n i v e r s a l j o i n t s , b u t they a r e n o t recomnended where t h e r e i s l i k e l y t o be misal ig n m e n t o r s h a f t bending o r any c o n d i t i o n where t h e r o l l e r s can be s u b j e c t e d t o t i l t i n g f o r c e s whi ch can cause a c l u t c h a c t i o n .

Fig.7

Needle R o l l e r Bearing.

Fig.8

Taper R o l l e r Bearing.

The d ia m e t e r s o f ne ed l e r o l l e r s a r e s m a l l , u s u a l l y 1 . 5 t o 5 mm and t h e r o l l e r l e n g t h i s a p p r o xi ma t e l y 2 . 5 t i mes t h e d i ame t er.

137

6.3.8

Taper R o l l e r Bearings ( F i g . 8 )

The r o l l i n g elements i n these b e a r i n g s a r e t r u n c a t e d cones and t h e t r a c k o f b o t h t h e i n n e r r i n g (cone) and t h e o u t e r r i n g (cup) a r e tapered.

The t a p e r s of

th e r o l l e r s and t h e i n n e r r i n g have a common apex on t h e b e a r i n g a x i s and t h e greater the i n c l i n a t i o n o f the r o l l e r s , the greater the a x i a l carrying capacity. As w i t h s i n g l e row a n g u l a r c o n t a c t b a l l be ari ngs, t a p e r r o l l e r bearings must always be mounted i n p a i r s o r a d j u s t e d towards another b e a r i n g capable o f dealing w i t h a x i a l forces a c t i n g i n the opposite d i r e c t i o n .

The t a p e r b e a r i n g can

a l s o be s u p p l i e d as a p a i r e d u n i t w i t h a common i n n e r o r o u t e r r i n g .

This

b e a r i n g i s used e x t e n s i v e l y i n t h e Automobile I n d u s t r y .

6.3.9

Double Row S p h e r i c a l R o l l e r B e ari n gs ( F i g . 9 )

Due t o t h e s i z e , shape, and number o f r o l l e r s , t o g e t h e r w i t h the accuracy w i t h wh ic h t h e r o l l e r s a r e guided, t h i s b e a r i n g has e x c e l l e n t l o a d - c a r r y i n g c a p a c i t y i n b o t h a r a d i a l and a x i a l d i r e c t i o n .

Since t h e b e a r i n g i s s e l f -

a l i g n i n g , a n g u l a r mi sa l i gn men t between s h a f t and housing has no d e t r i m e n t a l e f f e c t and f u l l c a p a c i t y i s always a v a i l a b l e .

Accurate r o l l e r guidance i n the

normal ' C ' d e s ig n i s by means of a l oo se r e l a t i v e l y narrow guide r i n g of t h i c k r a d i a l s e c t i o n between t h e rows o f r o l l e r s and by t h e cage.

Fig.9

Double Row S p h e r i c a l R o l l e r B e a r in g

Fig.10

Spherical R o l l e r Thrust Bearing

S p h e r i c a l r o l l e r b e a r i n g s a r e manufactured w i t h c y l i n d r i c a l o r tapered bores. They can be mounted on t ap ere d sleeves and a r e used i n Railway and Heavy Engineering.

These b e a r i n g s a r e regarded as t h e heavy-weight o f t h e standard b e a r i n g

138 types and a r e used e x t e n s i v e l y i n plummer b l o c k s .

6.3.10

S p h e r i c a l R o l l e r T h r u s t Bearings (Fig.10)

U n l i k e most o t h e r t ype s o f t h r u s t be ari n g,

t h e l i n e o f a c t i o n o f t h e load

a t t h e c o n t a c t s between t h e t r a c k s and t h e r o l l e r s forms an a n g l e w i t h t h e b e a r i n g a x i s wh ich makes these be ari n gs s u i t a b l e f o r c a r r y i n g a r a d i a l load up t o 55% o f t h e s i m u l t a n e o u s l y a c t i n g a x i a l l o ad.

The sphered t r a c k o f t h e hous-

i n g washer p r o v i d e s s e l f - a l i g n i n g p r o p e r t i e s w hich p e r m i t a c e r t a i n angular displacement o f t h e s h a f t r e l a t i v e t o t h e housing, due t o mounting e r r o r s o r shaft deflection. I n o r d e r t o p r e v e n t u na ccep t a bl e s l i d i n g a t t h e r o l l e r - t o - t r a c k c o n t a c t s under t h e i n f l u e n c e o f c e n t r i f u g a l f o r c e and g y r a t o r y moments,

i t i s necessary

t o a p p l y a c e r t a i n minimum a x i a l l o a d t o t h e bearing. The s h a f t washer,

r o l l e r s and cage f orm an assembly w hich i s separable from

the housing washer, a l l o w i n g t h e washers t o be mounted independently.

These

be a r in g s a r e s u i t a b l e f o r heavy l o ad s and f o r r e l a t i v e l y h i g h speeds.

6.3.11

B a l l T h r u s t Bearings (Fig.11)

These c o n s i s t o f a row o f b a l l s , r e t a i n e d i n p o s i t i o n by a cage and two washers known as t h e s h a f t and h ou si ng washer, each w i t h a s h a l l o w b a l l t r a c k groove.

The s h a f t washer has a s m a l l e r bo re than t h e o t h e r washer and

l o c a t e d by th e s h a f t .

5

The ho usi n g washer has a l a r g e r o u t s i d e diameter than

the s h a f t washer f o r l o c a t i o n i n a housing. These b e a r in g s a r e o n l y capable o f c a r r y i n g a x i a l l o a d i n one d i r e c t on.

Fig.11

B a l l T hru st B e ari n g

Fig.12

B e a r i n g w i t h S p h e r i c a l Outside Surf a c e and Extended I n n e r R ing Width.

139 6.3.12

Bearings w i t h S p h e r i c a l Ou t si d e Su rf ace and Extended I n n e r Ri n g Width (Fig.12)

These b e a r i n g s a r e e s s e n t i a l l y a v a r i a t i o n o f a deep groove b a l l b e a r i n g and are n o r m a l l y used i n c o n j u n c t i o n w i t h a range o f c a s t i r o n o r pressed s t e e l housings.

They

have a sphered o u t e r s u r f a c e and t h i s , when f i t t e d i n t o a

sphered s e a t i n g i n t h e housing, a l l o w s t h e u n i t t o accommodate any i n i t i a l misalignment o f t h e s h a f t whi ch may o c c u r d u r i n g mounting.

These bearings normally

have an extended i n n e r r i n g w i t h some f o r m o f l o c k i n g d e v i c e w hich i s used t o l o c k t h e b e a r i n g and, hence, t h e u n i t t o t h e s h a f t . made t o t h e same degree o f accuracy as

Although the b e a r i n g i s

deep groove b a l l b e a r i n g s , t h e method

o f l o c k i n g t h e b e a r i n g t o t h e s h a f t does n o t g i v e t h e same c e n t e r i n g accuracy as when b e a r i n g s a r e mounted w i t h an i n t e r f e r e n c e f i t on t h e s h a f t .

These

be a r in g s a r e used e x t e n s i v e l y i n machinery where t h e r o t a t i o n a l accuracy r e q u i r e ments a r e n o t t o o s t r i n g e n t , such as a g r i c u l t u r a l machinery and conveyors, e t c . where t h e y o f f e r a s i m p l e and economical s o l u t i o n .

6.4.

FATIGUE LIFE AND LOAD-CARRYING CAPACITY The concept o f a r o l l i n g b e a r i n g i s t h a t l o a d between t h e s t a t i o n a r y and

r o t a t i n g machine components i s t r a n s f e r r e d t hrough t h e b e a r i n g by means of i nte r p o s e d r o l l i n g elements.

Pure r o l l i n g seldom occurs and t h e n e t e f f e c t

u s u a l l y a c o m b in at i o n o f r o l l i n g and s l i d i n g . f u n c t i o n o f t h e b e a r i n g t ype , b u t

is

The exact p r o p o r t i o n s a r e a

90-99% o f t h e l o a d i s r e l a t e d t o r o l l i n g motion

and 1-10% t o s l i d i n g .

I f a b e a r i n g i s p r o p e r l y handled, c o r r e c t l y mounted, l u b r i c a t e d , and p r o t e c ted, a l l causes o f f a i l u r e a r e e l i m i n a t e d except one

-

fatigue o f the material.

The l i f e o f a r o l l i n g b e a r i n g as d e f i n e d by IS0 ( I n t e r n a t i o n a l Standards Organi s a t i o n ) i s t h e number o f r e v o l u t i o n s ( o r number o f o p e r a t i n g hours a t a g i v e n c o n s t a n t speed) whi ch t h e b e a r i n g i s cap ab l e o f e n d u r i n g b e f o r e f a t i g u e occurs on one o f i t s r i n g s o r r o l l i n g elements.

Repeated t e s t s have v e r i f i e d t h a t

when a group o f a p p a r e n t l y i d e n t i c a l be ari n gs a r e run under t h e same c o n d i t i o n s o f l o a d and speed t h e y have d i f f e r e n t l i v e s . "life"

A c l e a r d e f i n i t i o n o f t h e term

i s , therefore, essential f o r the c a l c u l a t i o n o f bearing s i z e .

Dynamic

l oa d r a t i n g s g i v e n i n b e a r i n g ma nu f a ct u rers catalogues a r e based on t h e l i f e t h a t 90% o f a s u f f i c i e n t l y l a r g e group o f a p p a r e n t l y i d e n t i c a l bearings can be expected t o a t t a i n o r exceed.

T h i s i s c a l l e d t h e b a s i c r a t i n g l i f e ( o r the

nominal l i f e ) and agrees w i t h t h e IS0 d e f i n i t i o n .

A t y p i c a l l i f e d i s p e r s i o n cu rve

is shown i n Fig.13, and i t can be seen t h a t

h a i f t h e b e a r in g s a c h i e v e an average l i f e f i v e times g r e a t e r than the nominal l i f e on which t h e c a l c u l a t i o n s a r e based.

140

F%

The classic curve of hearing life shows the life distribution for identical bearings run under idcntical conditions of load and speed. S is the perccntagc of still serviceable brarings, F the percentagc showing signs of fatigue, and L, the life of an individual bearing, given in t e r n of I.,, as explained in the text

S,%

Fig.13

6.4.1

A T y p i c a l B e a r i n g L i f e D i s p e r s i o n Curve.

The H e r t z i a n Contact

W h i l s t t h e H e r t z t h e o r y r e l a t i n g t o t h e c o n t a c t between s o l i d bodies i s s t i l l of i n t e r e s t i n c o n n e c t i o n w i t h c a l c u l a t i n g c o n t a c t pressures and deforma t i o n s i n r o l l i n g b e a r i n g s , t h e g r e a t e s t i n t e r e s t i s i n t h e shear s t r e s s e s beneath th e c o n t a c t s u r f a c e as t he se a r e con sidered t o be t h e cause o f b o t h p l a s t i c d e f o m a t i o n s and f a t i g u e c r a c k s i n t h e c o n t a c t zone. F i g u r e 14 shows t he s t r e s s d i s t r i b u t i o n i n a H e r t z i a n c o n t a c t and from a f a t i g u e aspect t h e shear s t r e s s e s a t t h e edge o f t h e c o n t a c t zone a r e t h e most dangerous ones.

They a r e p a r a l l e l t o t h e c o n t a c t s u r f a c e and a r e s i t u a t e d

beneath th e s u r f a c e . age o f t h e r o l l i n g

As these shear s t r e s s e s change d i r e c t i o n w i t h t h e pass-

body, m a t e r i a l f a t i g u e w i l l o c c u r i f any weak p o i n t such

as a s l a g i n c l u s i o n comes w i t h i n t h e su bsu rf ace zone where these shear s t r e s s e s a r e c o n s id e r a b le . s t i t u t e weak

I t has been proved t h a t even t i n y s l a g i n c l u s i o n s can con-

p o i n t s o f t h i s k i n d and t h a t under t h e i n f l u e n c e o f the a l t e r n a -

t i n g shear s t r e s s e s these i n c l u s i o n s r e s u l t i n m i c r o c r a c k s w hich subsequently i nc r e a s e i n s i z e , f i n d t h e i r way up t o t h e s u r f a c e , and l e a d t o s u r f a c e f a t i g u e f a i l u r e (see F ig. 15 ).

141

Fig.14 T

max

Shear Stresses i n t h e Contact Zone.

z g r e a t e s t shear s t r e s s i n t h e c o n t a c t zone and occurs a t a depth z

below t h e c o n t a c t s u r f a c e f o r m i n g an a n g l e o f the contact surface. and zmax z Tzy

max

0.786

max w i t h the plane o f

45'

For l i n e c o n t a c t o f i n f i n i t e l e n g t h

b where ' b '

T~~~~

0.30Uo

i s h a l f t h e w i d t h o f t h e c o n t a c t zone.

g r e a t e s t shear s t r e s s p a r a l l e l t o t h e p l a n e o f t h e c o n t a c t s u r f a c e and o c c u r s at a depth"zo" t a c t o f i n f i n i t e length

beneath T~~

max

=

the contact surface.

0.25Uo and zo

=

For l i n e con-

0.50 b.

R e l a t i o n s h i p between Load and L i f e

6.4.2

The r e l a t i o n s h i p between b e a r i n g l o a d and l i f e shown i n F i g u r e 16 has been o b t a i n e d by t e s t i n g b e a r i n g s under loads o f d i f f e r e n t magnitude. s e r i e s were r u n by t h e ings

under loads P 1 , P 2 and P3.

d o t s on t h e h o r i z o n t a l l i n e s . i n each t e s t s e r i e s ,

by t h e numerals 1, 2 and

3.

i.e.

The l i v e s o b t a i n e d by 90% o f t h e bear-

t h e nominal b e a r i n g l i f e , has been i n d i c a t e d

I t w i l l be seen i n t h e graph t h a t these t h r e e

p o i n t s l i e on an a p p r o x i m a t e l y s t r a i g h t l i n e .

T h i s means t h a t i n v i e w o f t h e

f a c t t h a t t h e s c a l e s o f b o t h axes a r e l o g a r i t h m i c ,

t h e l i f e can be expressed

as a power o f t h e load, hence:

where

L = nominal b e a r i n g l i f e i n m i l l i o n s o f r e v o l u t i o n s ; P

C p

=

Three t e s t

The l i v e s o b t a i n e d a r e i n d i c a t e d

e q u i v a l e n t b e a r i n g l o a d i n Newtons b a s i c dynamic load r a t i n g o f t h e b e a r i n g i n Newtons 3 f o r b a l l b e a r i n g s and 10/3 f o r r o l l e r b e a r i n g s .

142

Fig.15

Normal F a t i g u e F a i l u r e

The b a s i c dynamic l o a d r a t i n g o f t h e b e a r i n g i s d e f i n e d as t h e load t h a t gives a

nominal b e a r i n g l i f e o f one m i l l i o n r e v o l u t i o n s .

On t h e graph,

the

l o a d a t t h e p o i n t a t t h e i n t e r s e c t i o n o f t h e o r d i n a t e f o r t h e l i f e one m i l l i o n r e v o l u t i o n s w i t h t h e l i n e r e p r e s e n t i n g t h e r e l a t i o n s h i p between l o a d and l i f e i s t h e b a s i c dynamic l o a d r a t i n g " C " .

T h i s i s t h e l o a d r a t i n g g i v e n i n SKF

catalogues and i s used f o r t h e c a l c u l a t i o n o f t h e b e a r i n g l i f e .

143

Fig.16 6.4.3

R e l a t i o n s h i p between Load and L i f e .

F u r t h e r Development o f t h e L i f e E q ua tion

With t h e advancement o f technology,

g r e a t e r r e l i a b i l i t y i s r e q u i r e d from

r o l l i n g b e a r i n g s and o f t he c a l c u l a t i o n methods used.

I t i s necessary t o con-

s i d e r f a c t o r s n o t i n c l u d e d i n t h e b a s i c l i f e e q u a t i o n even though t h e r e s u l t s g i v e n by t h e b a s i c l i f e e q u a t i o n a r e s a t i s f a c t o r y i n the m a j o r i t y o f cases.

IS0 has suggested t h a t t h e l i f e e q u a t i o n should be as f o l l o w s : -

al

=

r e l i a b i l i t y f a c t o r , which enables t h e b e a r i n g l i f e t o be c a l c u l a t e d f o r any g i v e n p r o b a b i l i t y o f f a t i g u e (al

a

=

m a t e r i a l f a c t o r (a

::

1 f o r 90% p r o b a b i l i t y ) .

1 f o r a r o l l i n g b e a r i n g o f good q u a l i t y s t e e l w i t h

normal hardness and s t r u c t u r e s . a

l u b r i c a t i o n f a c t o r (a, = 1 f o r normal l u b r i c a t i o n ) . A lt h o u g h t h e new l i f e e q u a t i o n i s a development o f t h e o l d e q u a t i o n , a m u l t i -

p l i c a t i v e c o m b inat i on o f the t h r e e m o d i f i e r s r e s u l t i n an improvement. i ns ta n c e , material.

is

a 1 , az, and a

does n o t always

The n e g a t i v e e f f e c t o f inadequate l u b r i c a t i o n , f o r

n o t compensated f o r by u s i n g say a vacuum remelted s t e e l b e a r i n g

144 6.5

BOUNDARY D I HENS I ONS AN0 I NTERNAL CONTROLS

Every s t a n d a r d b e a r i n g w i t h m e t r i c boundary dimensions belongs t o a "dimens i o n a l s e r i e s " which forms p a r t o f g e n e r a l p l a n s p r e p a r e d by t h e I n t e r n a t i o n a l Organisation f o r Standardisation. d i f f e r e n t outside

For any g i v e n b o r e t h e r e a r e a s e r i e s o f

d i a m e t e r s and w i t h i n each d i a m e t e r s e r i e s t h e r e a r e b e a r i n g s

o f d i f f e r e n t w i d t h s , as shown i n F i g . 1 7 .

Each s t a n d a r d b e a r i n g b e l o n g i n g t o

a dimension s e r i e s i s d e s i g n a t e d by a t w o - d i g i t number.

The l a s t two d i g i t s i n

a f o u r o r f i v e d i g i t number a r e 1/5 o f t h e b o r e when t h i s i s i n m i l l i m e t r e s ; i . e . b e a r i n g 6004 has a b o r e o f 20 mm.

Supplementary symbols f a l l i n t o v a r i o u s groups r e l a t i n g t o i n t e r n a l d e s i g n f e a t u r e s such as cage, m a t e r i a l , s e a l i n g ,

i n t e r n a l r a d i a l c l e a r a n c e and p r e -

cision etc. The r a d i a l c l e a r a n c e f o r s i n g l e row deep groove b a l l b e a r i n g s has been agreed b o t h i n t e r n a t i o n a l l y and by B r i t i s h Standards O r g a n i s a t i o n s .

A general

summary i s a s f o l l o w s : -

SKF

I.S.O.

Designation c2 Norma I

c3 c4

& B.S.

A l s o r e f e r r e d t o as

Group 2

0

Norma 1

00

3 4

000 OD00

SKF a l s o manufacture c e r t a i n b e a r i n g s w i t h s m a l l e r r a d i a l c l e a r a n c e s ( C I ) and l a r g e r r a d i a l c l e a r a n c e s (C5).

6.6

USAGE C1

-

used on machine t o o l s p i n d l e s where minimum movement and maximum r i g i d i t y are required.

145 C2

-

s u i t a b l e f o r f r a c t i o n a l h orse power motors f o r domestic appliances p a r t i c u l a r l y where s i l e n t run ni n g i s r e q u i r e d .

Normal c le a r a nce ture,

-

C3

-

used f o r normal a p p l i c a t i o n s where t h e r e a r e no tempera-

speed, o r i n t e r f e r e n c e f i t problems.

o f t e n used on l a r g e e l e c t r i c motors, p a r t i c u l a r l y where the i n n e r r i n g temperature exceeds t h a t o f t h e o u t e r r i n g , o r when t h e r i n g s a r e mounted w i t h heavy i n t e r f e r e n c e f i t s .

C4 -

used f r e q u e n t l y on

a p p l i c a t i o n s such as l a r g e t r a c t i o n motors f o r

d i e s e l e l e c t r i c and e l e c t r i c l o como t ives where t h e r e a r e temperature d i f f e r e n t i a l s between the i n n e r and o u t e r r i n g and t h e i n t e r f e r e n c e f i t s a r e much g r e a t e r than those used f o r C3 bearings.

-

C5

t h i s c o n t r o l i s o f t e n used f o r b ea ri ngs i n furnace t r u c k s where t h e r e a r e l a r g e t emp era t u re d i f f e r e n t i a l s between t h e i n n e r and o u t e r r i n g s c o u p le d w i t h heavy i n t e r f e r e n c e f i t s .

6.7

SPEED LIMITS

Due t o t h e many f a c t o r s combining t o de t e rmine t h e maximum speed l i m i t s f o r r o l l i n g b e a r i n g s such as b e a r i n g t yp e, s i z e , t i o n s , method o f l u b r i c a t i o n ,

r a d i a l clearance, c o o l i n g condi-

l oa d, degree of p r e c i s i o n and environment e t c . ,

it

i s d i f f i c u l t t o g i v e p r e c i s e l i m i t s and a l l attempts t o g i v e l i m i t i n g speeds must be t o p r o v i d e an approximate general g ui de. For comparison purposes between speed and b e a r i n g s i z e , r o l l i n g b e a r i n g speeds a r e u s u a l l y expressed i n terms o f "ndm"

and

and n

JbFi,

ndm = A x f l x f 2 f o r r a d i a l b e a r i n g s

where n

/% = n = dm

d

= =

A x f

x f f o r t h r u s t bearings 1 2 speed, rpm

mean d i ame t e r o f b e a r i n g , 0.5(d+D)mm b e a r i n g bo re, rnm

D = b e a r i n g o u t s i d e d i ame t e r, mm H

=

f

h e i g h t o f s i n g l e t h r u s t be ari n g, mm c o r r e c t i o n f a c t o r f o r b e a r i n g s i z e (Fig.18)

1 f2

=

c o r r e c t i o n f a c t o r f o r b e a r i n g l oad (Fig.19)

A

=

c o n s t a n t , whi ch i s a f u n c t i o n o f t h e l u b r i c a n t and l u b r c a t i o n method (Fig.20).

A lt h o u g h t h e speed l i m i t f ormu l a i s based on p r a c t i c a l experience re s e a r c h i n t o more a c c u r a t e d e t e r m i n a t i o n o f t h e f a c t o r s

A,

fl

extensive

and f 2 has

r e s u l t e d i n l i m i t i n g speeds i n c l o s e ap pro xi mation t o t h e c o n d i t i o n s found i n practice. Approximate speed l i m i t s f o r a l o ad g i v i n g t h e b e a r i n g a minimum l i f e Lh

=

100,000 h o u rs ( f 2 = 1) a r e shown on F i gs.21 and 22.

I f t h e load i s g r e a t e r

146

1

0.5

50

100

200

300

500

1000

dm Correction factor for bcaring size (d,,,=mean diameter o bearing, mm)

(1

Fig.18

Correction Factor f o r Bearing S i z e .

fi 1

0.5

I ,

50

, , , , f

100

I

Mo

I

300

I

500

J d?n

lo00

Correction factor f* for bearing load, expressed as the resulting life L b in hours (d,=mcm diameter o f bearing, mm)

Fig.19

Correction Factor f o r Bearing Load

147

Bearing type Radial bearing^: dn = . f , f a A Deep groove ball bearings

"

Factor A

Remarks

Normally Maximum

500'000 1 000 000

Pressed-steel cages Solid brass cages

Self-aligning ball bearings

Vormally Maximum

500 000 800 000

Pressed-steel cages Solid brass cages

Single row angular contact ball bearings

Normally Maximum

500OOO

Pressed-steel cages Solid brass cages

Double row angular contact hall bearings

Normally Maximum

200000 400000

Cylindrical roller bearings

Normally Maximum

400000 Presstd-steel cages 600 000 Solid brass cages

Spherical roller radial bearings

Normally Maximum

200000 400OOO

Taper roller bearings

Normally Maximum

200 OOO In the case of a predominant 400000 thrust load 20-40% lowei

%wt

600000

limit values apply, depending o n the working conditions

bearitigJ:

" I/DH=f,f, A Ball thrust bearings Spherical roller thrust bearings

Normally Maximum Normally Maximum

1OOOOO

200 000 200000 Good, natural cooling gene rally sufficient. 300000 Effective cooling necessary.

At high speeds the bearing slackness must usually be greater than normal Thrust bearings operating at high speeds must carry a certain minimum load Fa n i r , as shown in the graph in Fig.

F i g . 20

148 th e l i m i t values a r e reduced by m o d i f y i n g t h e f a c t o r f 2 .

F i g u r e s 21 and 22

show two speed l i m i t s (normal and maximum) f o r each b e a r i n g type.

Bearings

o p e r a t i n g up tomorma1 l i m i t s can be f i t t e d w i t h standard cages and, as a general r u l e , grease l u b r i c a t i o n may be used.

I t should be n o t e d t h a t B a l l

Th r u s t and S p h e r i c a l R o l l e r T h r u s t b e a r i n g s shown i n Fig.22 should be mounted f r e e f r o m s la c k n ess and c a r r y a c e r t a i n minimum a x i a l load, o t h e r w i s e t h e t r a c k s may be damaged (due t o smearing) a s a r e s u l t o f t h e g y r a t o r y f o r c e s a c t i n g on th e r o l l i n g elements.

--__-I_

UBAlllNGS

M-

d-trr (1 ) far d u m r r mrs: -I_

u

2

-_ 23

3

4

I8

zu

24

25

2934

26 31

21 34

3240 36 46 44 53

43 41 54

46 54

51 58 65

1 I

zn

M

22

n

60

n

25

60 68 75

60 65

65 70

73

13

18

0

7B

85

83 93 9 0 %

95 113 10

I05 I13 I25

1 5 1 1113 0 3 I 110 U8 I 118

18 25 33

140 141

I5

I25

2U 25

133 14U

40 48 58

1% 168 175

13 54

148 155 168

65 75

183 195

65 75 n8

180 195 210

05

235

35

265

I5

225 240 250

65 ml

4u 55 Ml

265

95 I0 40

325 3 u

Y O 370 511 3 w

70 OU

410

all

60

w5

95

535

411

In Jo

uy) 3IU

UI W

03

m : 90

%

I33

9O 215

20 1 25U

w r)5 m 310

)80

4w

YI I 4 7 5

w

420 4 4w P

50

505

55

7u

51) 5uJ

u 10 u615 o

1x1

UI

570

UI5

h&d kariuEs. Approximate s p e d limits for a load (predorninurtly rcrdi.1 in the o f roller bcarinp) givin the bearings a minimum life L, of IOOOM) houn. Lower limits apply to axially loadtd d i a l roller bearings See adjoining table for thc values d. case

m 5w

845 87U

tMJ

1111

615 645 675 718

$55 755 IM 135 i75

795 M! 8%

us

94,

-

Fig.21

Approximate speed l i m i t s .

Rad i al Bearings.

149

n

.C.

m

BALL

Ill

-

113

- 114

15

31

Y

u

36

m

(0

U 51

45

51

(P

63 M 16

11 24

a

61

81

64 11

m

P 30

95

3a

n n

101

34

U 45 46

26

f

86

107 It4

P

111

m

n im

IW IU

44

U

I43

62 11 15

121 1%

im

52 56

m

141 151 115

n

141 1U

I(0

m

U

IN

6(

I14

IU

12

Ed

IW

M

16

no

51

s

79

16

P

M

96

00

I39

1%

IS3

191

111

in

40

m

M

!I4

I35

M

up

16

92

w

!43 !59

36

QI

62

m

I3

I24

59

id0

m

m

b9

Fig.22

11 21 XI

54 61

41

m -

15 I6 I1

31 35

UI

-

14

16 19 2a

P

LI

'8.

130

rn

ef

Tkvrr bariag~. Approximate 8 Umiu for a Land giving the bearings a pinimum lice l&MO houn. Scc adjoining uble for the values P H . It rhould be noted a y y t ofb m + d that the value 0.001 C, C p s u t i c carrying c it often grater than tte numerial value o F. obtam from the graph. The former value i8 then t a k a . It should .Is0 h noted chat the F . i lina have diffuent n l u a for ball mnd mUu baring$

I1 I3

16

25

E N. -

-

>

L78

w

112 I35

-

Approximate speed 1imits.Thrust bearings.

.I.

150 I n the top p a r t o f Fig.21 the speed curves w i t h A = 1500000 and A = 3000000 have been drawn i n and the broken l i n e shows the extreme values which have been achieved w i t h bearing types i n a few known cases.

Such speeds, however, do

r e q u i r e experience i n mounting and the gr a t e s t care must be exercised. FRICTION

6.8

The extensive use o f r o l l i n g bearings than s l i d i n g f r i c t i o n .

s due t o r o l l i n g f r i c t i o n being less

F r i c t i o n a l losses i n r o l l ing bearings are u s u a l l y very

low, hence the term a n t i f r i c t i o n bearings. The f r i c t i o n a l resistance o f a r o l l i n g bearing i s dependent on several f a c t o r s such as bearing load, speed o f r o t a t i o n , and the p r o p e r t i e s o f the l u b r i cant.

Under c e r t a i n c o n d i t i o n s (bearing load P

0.1C.

adequate l u b r i c a t i o n

and normal o p e r a t i n g c o n d i t i o n s ) i t i s p o s s i b l e t o c a l c u l a t e the f r i c t i o n a l resistance w i t h s u f f i c i e n t accuracy using t h e c o e f f i c i e n t s o f f r i c t i o n given i n Table 6.1.

Where rubbing seals are used t h e i r f r i c t i o n a l resistance, which

can be g r e a t e r than t h a t i n the bearing, must a l s o be taken i n t o account. f r i c t i o n torque "M"

M =

x F x

d 2

o f a bearing i s obtained from the equation:

(Nmn)

where IJ = c o e f f i c i e n t o f f r i c t i o n f o r the bearing (see Table 6.1)

F = bearing load,

N

d = bearing bore, mm.

TABLE 6.1

Coefficient o f f r i c t i o n

Bearing type

IJ

S e l f a1 i g n i n g B a l l Bearings

.001

C y l i n d r i c a l R o l l e r Bearings

.0011

Thrust B a l l Bearings

.0013

Deep Groove B a l l Bearings

.0015

Spherical R o l l e r Bearings

.0018

Taper R o l l e r Bearings

.0018

Spherical R o l l e r Thrust Bearings

.0018

Angular Contact B a l l Bearings Single Row

.002

Double Row

.0024

Needle R o l l e r Bearings

.0025

C y l i n d r i c a l R o l l e r Thrust Bearings

.004

Needle R o l l e r Thrust Bearings

.004

The

151 Higher c o e f f i c i e n t o f f r i c t i o n values than those given i n Table 6.1 a r e obtained w i t h new bearings and t h i s a p p l i e s p a r t i c u l a r l y t o r o l l e r bearings which have n o t been run i n . and using

Higher values w i l l a l s o be achieved when s t a r t i n g

excessive q u a n t i t i e s o f l u b r i c a n t .

The f r i c t i o n loss i s :

wF =

9.8 x

M x n (Watts)

-103 where

M

=

n

6.9

f r i c t i o n moment, Nmm speed, rpm.

LUBRICATION R o l l i n g bearings must be l u b r i c a t e d t o prevent m e t a l l i c contact between the

r o l l i n g elements, t r a c k s and cage and a l s o t o p r o t e c t the bearing from corrosion and wear.

The most favourable running temperature f o r a r o l l i n g bearing i s

achieved when the minimum o f l u b r i c a n t necessary t o ensure r e l i a b l e l u b r i c a t i o n i s used. L u b r i c a t i n g p r o p e r t i e s d e t e r i o r a t e due t o ageing and mechanical working and a l l l u b r i c a n t s become contaminated i n s e r v i c e and must, t h e r e f o r e be replenished or

changed

periodically.

R o l l i n g bearings may be l u b r i c a t e d w i t h grease o r o i l , o r i n special cases with a s o l i d lubricant.

When considering l u b r i c a t i o n f o r bearings the choice

is between o i l and grease and various aspects need t o be considered.

Grease

l u b r i c a t i o n has c e r t a i n advantages which are:-

1. Costs involved i n mounting a r e lower than w i t h o i l . 2 . Less maintenance i s required and i t i s n o t necessary t o

ncorpora t e

p i p i n g o r pumping equipment.

3.

Constant o i l l e v e l devices n o t required.

4. Easier t o c o n t a i n grease i n housing than o i l . 5. Cleaner t o use grease as there i s no splashing as w i t h o 1 .

6. Cheaper t o seal f o r grease than f o r o i l . 7. Grease a s s i s t s i n s e a l i n g a g a i n s t the e n t r y o f moisture and o t h e r impur i t ies Tests

.

have shown t h a t o n l y small amounts o f grease adhere t o the surfaces

o f the bearing.

Reservoirs o f grease form on the cage and a g a i n s t the side

faces o f the bearing.

The b u l k o f t h e grease c o l l e c t s o u t s i d e the bearing and

i n the grease c a v i t i e s of the housing and, as a r e s u l t , t h i s i s u s u a l l y i n a c t i v e . I t can be argued

t h a t t h i s reserve o f grease helps t o m a i n t a i n an o i l bleed t o

the bearings, b u t experience suggests t h a t although t h i s reserve may be i n reasonable q u a n t i t i e s i n the c a v i t i e s ,

i t i s s t i l l p o s s i b l e f o r the bearing to

152 f a i l due t o i n s u f f i c i e n t l u b r i c a n t .

The b a s i c r u l e f o r a normal b e a r i n g arrange-

ment i s t h a t th e b e a r i n g s h o u l d be w e l l packed w i t h grease w i t h t h e housing no I f t h e space round t h e b e a r i n g i s e x c e s s i v e l y f i l l e d w i t h

more than h a l f f u l l .

grease then c h u r n i n g o f t h e grease i n t h e b e a r i n g can o c c u r w hich c o u l d l e a d t o a r a p i d breakdown o f t h e grease s t r u c t u r e due t o o v e r h e a t i n g .

I n such cases

the grease s o f t e n s and t h e o i l i n t h e grease tends t o b l e e d from t h e soaps. The s t i f f n e s s o r hardness o f a grease i s c a l l e d c o n s i s t e n c y and i s u s u a l l y quoted i n terms o f t h e N a t i o n a l L u b r i c a t i n g Grease I n s t i t u t e (NLGI) s c a l e and Consistencies

2 o r 3 a r e c o m p l e t e l y s a t i s f a c t o r y i n normal a p p l i c a t i o n s f o r b a l l and r o l l e r bear i ng 1 u b r i c a t i o n .

A ' 3 ' c o n s is te n cy grease would be used i n an a p p l i c a t i o n such as an axlebox o r t r a c t i o n motor where t h e r e i s c o n s i d e r a b l e v i b r a t i o n and a r i s k o f t h e grease slumping.

6.9.1

Greases

L u b r i c a t i n g greases a r e t h i c k e n e d m i n e r a l o i l s o r s y n t h e t i c f l u i d s . soaps a r e m a i n l y used

as t he t h i c k e n i n g agent.

depends m o s t l y on t h e t ype and q u a n t i t y o f t h e t h i c k e n i n g agent used. s e l e c t i n g a grease,

i t s con si st e ncy,

Metal

The c o n s i s t e n c y o f t h e grease When

t emp era t ure range and r u s t - i n h i b i t i n g p r o -

p e r t i e s a r e th e most i m p o r t a n t f a c t o r s t o be considered. 6.9.1.1

Temperature Range Sodium Base Greases.

(i)

-30 t o +8OoC,

These greases may be used a t temperatures between

a l t h o u g h some s p e c i a l v e r s i o n s may be used up t o +120°C

base greases a r e w a t e r - s o l u b l e ,

Sodium

i . e . they a b s o r b w a t e r t o a c e r t a i n e x t e n t and

form a r u s t - i n h i b i t i n g emulsion wi ch ou t t h e i r l u b r i c a t i n g p r o p e r t i e s b e i n g impaired.

These greases w i l l p r o t e c t t h e be arings s u f f i c i e n t l y a g a i n s t r u s t

p r o v i d i n g t h a t w a t e r cannot e n t e r t h e b e a r i n g arrangement.

Where w ater c2n

e n t e r , such greases a r e e a s i l y washed o u t o f t h e b e a r i n g housing. Calcium Base Greases.

(ii)

1 t o 3% w a t e r .

Most c a l c i u m based greases a r e s t a b i l i s e d w i t h

W i t h i ncre ase d t emp era t u re t h e w ater evaporates and s e p a r a t i o n

o f t h e grease i n t o m i n e r a l o i l and soap occurs.

The upper temperature l i m i t

f o r t h e s e greases i s t h e r e f o r e ap pro xi mat el y +6OoC. base

Some h e a t - s t a b l e c a l c i u m

greases a r e a v a i l a b l e wh i ch p e r m i t o p e r a t i n g temperatures up t o +120°C.

( i ii )

L i t h i u m Base Greases.

temperatures

These greases a r e g e n e r a l l y s u i t a b l e f o r use a t

between -30 t o +llO°C,

b u t a few greases o f t h i s t y p e a r e s u i t a b l e

f o r wo r k in g temperatures up t o +150°C. L i t h i u m and c a l c i u m base greases a r e v i r t u a l l y i n s o l u b l e i n w a t e r and do n o t therefore give p r o t e c t i o n against corrosion.

Such greases should t h e r e f o r e

never be used u n l e s s t he y c o n t a i n a r u s t - i n h i b i t o r .

153 For heavily-loaded r o l l i n g bearings, e.g.

r o l l i n g - m i l l bearings, greases

c o n t a i n i n g EP a d d i t i v e s are used since these increase the load-carrying a b i l i t y o f the l u b r i c a n t f i l m .

Such greases are a l s o g e n e r a l l y recommended f o r the

l u b r i c a t i o n o f medium and l a r g e sized r o l l e r bearings.

The r u s t i n h i b i t i n g

p r o p e r t i e s o f calcium and I i thium base greases c o n t a i n i n g EP a d d i t i v e s (mainly lead compounds) a r e good.

These greases adhere w e l l t o the bearing surfaces as

w e l l as being i n s o l u b l e i n water.

They are,

therefore, p a r t i c u l a r l y s u i t a b l e

f o r a p p l i c a t i o n s where water can penetrate the bearing arrangement, e.g.

paper-

making machines o r r o l l i n g - m i l l s . Creases c o n t a i n i n g inorganic thickeners instead o f metal soaps, e.g.

clay or

s i l i c a , may be used f o r s h o r t periods a t higher temperatures than l i t h i u m base greases.

Synthetic greases, e.g.

those made from d i e s t e r or s i l i c o n e f l u i d s ,

may be used a t both higher and lower temperatures than greases made from mineral

oils. Grease r e - l u b r i c a t i o n i n t e r v a l s as recomnended by SKF a r e given i n graphs i n these i n t e r v a l s are conservative and are known t o g i v e a wide s a f e t y

Fig.23; margin

.

Relubrication interval hours 01 operation

t b

20000

300 200 160 100

a Radial wl bearings b Cylindrical roller bearings,needc roller bearings c Sphericd mler bearings. taper roller bearings, tnrust ball bearings

Fig.23

Re-lubrication Intervals

-

n r/min

164 To p r e v e n t t h e p o s s i b i l i t y o f m i x i n g i n c o m p a t i b l e greases w i t h i t s i n h e r e n t problems, i t i s a d v i s a b l e t o ensure t h a t o n l y greases h a v i n g t h e same t h i c k e n e r and w i t h a s i m i l a r

base o i l a r e used when r e - l u b r i c a t i n g .

There i s o f t e n doubt as t o t h e q u a n t i t y o f grease t o be used i n a b e a r i n g and w h i l s t i t i s d i f f i c u l t t o be e x a c t , t h e f o l l o w i n g g u i d e can be used:

0.005 x D x B

G

where

G

=

grease q u a n t i t y , grams

D

=

b e a r i n g o u t s i d e di a met er, m

B

b e a r i n g w i d t h , mm.

For high-speed b e a r i n g s n e c e s s i t a t i n g f r e q u e n t r e - l u b r i c a t i o n i n t e r v a l s i t i s e s s e n t i a l t o a v o i d o v e r - f i l l i n g t h e housings, s i n c e t o o much grease causes the grease t o churn,

r e s u l t i n g i n an e x c e s s i v e r i s e i n temperature.

Churning can

l ea d t o a breakdown i n t h e l u b r i c a t i n g p r o p e r t i e s o f t h e grease w i t h a f u r t h e r r i s e i n t e m p e r a t ure and t he b e a r i n g s o p e r a t i n g i n a p r e - l o a d c o n d i t i o n .

This

problem can be a voi d ed by u t i l i s i n g a grease escape v a l v e arrangement, as shown i n Fig.24.

The v a l v e c o n s i s t s o f a d i s c which r o t a t e s w i t h t h e s h a f t and,

c o n j u n c t i o n w i t h a ho usi n g end cove r, forms a narrow r a d i a l gap.

in

Excess grease

Cnum d v m for mnd-fnmm typ. m l e r t c motor

Fig.24

Grease Escape Valve Arrangement

i s thrown o u t by t he d i s c i n t o an a n g u l a r c a v i t y and e j e c t e d through an opening on t h e u n d e r s id e o f t h e end cover.

T h i s system o f escape v a l v e i s extremely

u s e f u l f r o m a maintenance a spe ct as e xami n at i on o f t h e e j e c t e d grease can g i v e v a l u a b l e e v id e n c e r e g a r d i n g t h e c o n d i t i o n o f t h e bearing,

i.e.

i f the bearing

i s f i t t e d w i t h a b rass cage and t h i s i s b e g i n n i n g t o wear t h e grease w i l l become

165 F i g u r e 25 shows t h e

d i s c o l o u r e d , and t h i s can be e a s i l y seen and recognized.

temperature e f f e c t o f o p e r a t i n g b e a r i n g s a t h i g h speeds w i t h and w i t h o u t grease v a lv e s .

Experiments i n t h e machine t o o l i n d u s t r y where b e a r i n g s a r e r u n w i t h

o n l y a l i g h t smear o f grease on t h e t r a c k s i n d i c a t e d t h a t i n i t i a l l u b r i c a t i o n c o u l d be as l i t t l e as 1 cm3 p e r 50 mm mean b e a r i n g diameter.

The use o f such

small q u a n t i t i e s i n 9Omm bo re c y l i n d r i c a l r o l l e r b e a r i n g s i n j i g b o r e r s p r e loaded 0.0025 mm

and o p e r a t i n g a t a speed o f 2500 rpm has reduced t h e temper-

a t u r e r i s e t o a slow as

Al t ho ug h l o n g - r u n n i n g p e r i o d s have a l s o been

8OC.

a c h ie v e d u s i n g t h i s technique,

t h e method i s extremely d e l i c a t e and n e c e s s i t a t e s

special t r a i n i n g f o r the f i t t e r s .

-u Y

b 60 10

L

a

" 0

2

0

24

48

72

L

0

96 h

120

144

1

188 l.54

2

1

1

1

1

1

1

24

48

72

96

120

144

I

1

168 192

h

Y

a

E

a

a

a

A

h

A1 n

C

Y I-

I

0

46

& ,

96

h

6.9.2

t

'

k

L

I

-

d

I

I

h

Typical comparison between lubication with a grease valve and ordinary grease lubication for a high-speed bearing in a cloaed housing. The steady high temperature in diagram (b) indicates that the grease has deteriorated during the first high temperature running period. Grease has seeped out through the seals and less grease remains than in the housing with grease valve. The temperature peaks in diagram (a) normally occur at each lubrication and a r e permissible with soda-base greases.

Fig.25

50-

P-

(a) Roller bearing 22328M running at 1100rev/min in conjunction with grease valve (b) Same type of roller bearing without grease valve (c) Ball bearing 6310 MA running at 8000rev/min in conjunction with grease valve (d) Same type of bearing running at 12400rev/min in conjunction with grease valve x = relubrication point

E f f e c t on Temperature o f I n c o r p o r a t i n g Grease Valve.

O i l Lubrication

O i l has s e v e r a l

advantages compared t o grease such as ease o f d r a i n i n g and

r e p l e n i s h i n g when necessary and p a r t i c u l a r l y when t h e r e l u b r i c a t i o n i n t e r v a l f o r grease i s v e r y s h o r t . o r operating

O i l l u b r i c a t i o n i s g e n e r a l l y used when h i g h speeds

temperatures p r o h i b i t t h e use o f grease and i s u s e f u l when i t i s

necessary t o d i s s i p a t e f r i c t i o n a l o r . a p p l i e d h e a t f r o m t h e b e a r i n g . t i o n o f a l u b r i c a t i n g o i l i s e a s i e r t ha n t h e c h o i c e o f a grease.

The selec-

O i l s a r e more

u n i f o r m i n t h e i r c h a r a c t e r i s t i c s and i f r e s i s t a n t t o o x i d a t i o n , gumming, and e v a p o r a t i o n can be s e l e c t e d on t h e b a s i s o f a s u i t a b l e v i s c o s i t y (Fig.26).

156

Graph for selection of oil. d = bore of bearing. mlp., spccd, I. m. Example : spheric11 roller thxust b r i n g 29469 : d = &O mm, I 500 r.p.m. UK an oil which has a minimwm viscosity of 13.5 cSt at working temperature. When the working I C I n p c n N r C is known the ap mximarc riscosit of the oil required at 50' C. can b;obuinef; with the aid of t i e thin. oblique lines. At a working tcmpcnturc of 70" C. the oil in the cnmplc should hare a viscosit of appmx. 27 cSr at 50" C. Select an oil from those avaiible who= this d u e . for cmmplc. Ckwylc viscosity is thc ~ r a ( i / DTE Oil Medium (27.3 cSt at 50" C.) n

-

P

Figure 26.

Selection o f Oil.

Due t o t h e f a c t t h a t he at -ge ne rat ed i n r o l l i n g b e a r i n g s increases w i t h v i s c osity,

i t i s necessary to s e l e c t a t h i n o i l f o r high-speed o p e r a t i o n , o t h e r w i s e

t h e b e a r i n g t e m pera t u re would be t o o h i g h . such

For v e r y slow speeds, a p p l i c a t i o n s

as s p h e r i c a l r o l l e r t h r u s t b e a r i n g s i n cranes, an extremely t h i c k o i l

(minimum v i s c o s i t y 400 c S t a t 5D°C) oil film.

For normal ambient temperatures and w o r k i n g c o n d i t i o n s , an o i l w i t h

a v i s c o s i t y o f 12-22c S t a t 5OoC oil is

i s used t o ensure a s u f f i c i e n t l y s t r o n g

i s suitable.

A t moderate speeds, no s p e c i a l

r e q u i r e d for b e a r i n g s i n gear-boxes s i n c e they can be l u b r i c a t e d by t h e

gear o i l p r o v i d e d t h e b e a r i n g s a r e a de qu at el y p r o t e c t e d a g a i n s t wear p a r t i c l e s from th e gears e n t e r i n g t h e be ari n gs. bea r in g s

I f t h i s cannot be prevented,

must be s e p a r a t e l y l u b r i c a t e d , u s u a l l y w i t h

oil.

then t h e

Grease can be used,

b u t t h e s e a l s must have t h e a b i l i t y t o p r e v e n t t h e gear o i l f l u s h i n g t h e grease o u t o f the bearing.

I t must be mentioned t h a t ,

as w i t h grease, an e x c e s s i v e

q u a n t i t y o f o i l can cause c h u r n i n g and c o n s i d e r a b l e heat;

therefore,

for normal

r e s e r v o i r systems i t i s e s s e n t i a l f o r t h e maximum o i l l e v e l t o be no h i g h e r th a n t h e c e n t r e o f t h e l owest r o l l i n g element.

I f t h e o i l l e v e l exceeds t h i s

t h e r e c o u l d be a t e mpe rat ure r i s e due t o c h u r n i n g .

F i g u r e 27 shows t h e e f f e c t

on t e m p e r a t u r e r i s e and f r i c t i o n t o r q u e o f i n c r e a s i n g t h e q u a n t i t y o f o i l . When the q u a n t i t y

o f o i l reaches a minimum l e v e l ,

i.e.

t h e d o t t e d zone, m e t a l - t o -

metal c o n t a c t o ccu rs, r e s u l t i n g i n r a p i d t emperature r i s e and p o s s i b l e b e a r i n g seizure.

B e a r i n g f r i c t i o n t o r q u e i s a l s o a f u n c t i o n o f o i l q u a n t i t y and i t can

be seen t h a t t h e t o r q u e i ncre ase s w i t h t h e q u a n t i t y o f o i l . rep r e s e n t

As t h i s c o u l d

c o n s i d e r a b l e power loss i t i s e s s e n t i a l t o ensure t h a t the o i l l e v e l

does nmt exceed a l e v e l c o m p a t i b l e w i t h adequate l u b r i c a t i o n .

O i l lubrication

167 can be by c i r c u l a t i o n , d r i p feed, w i c k feed, o r o i l m i s t .

In a circulatory

system t h e o u t l e t must be g r e a t e r t ha n t h e i n l e t t o p r e v e n t t h e p o s s i b i l i t y o f exc e s s iv e o i l i n t h e b ea ri ng .

For high-speed a p p l i c a t i o n s such as g r i n d i n g

s p i n d l e s , o i l m i s t l u b r i c a t i o n i s o f t e n used.

I n t h i s system a m i s t o f o i l and

a i r i s t r a n s p o r t e d t h rou gh p i p e s t o t h e b ea rings.

Condensing n i p p l e s immediately

b e f o r e each b e a r i n g pos t i o n cause t h e o i l t o be s u p p l i e d t o the b e a r i n g i n d r o p l e t form.

The smal

q u a n t i t i e s o f o i l can be a c c u r a t e l y r e g u l a t e d , conse-

quently the l u b r i c a t i o n f r i c t i o n i s n e g l i g b l e .

F i g u r e 28 shows a t y p i c a l o i l

mist u n i t .

The dependence of burlng friction on quintlcy of 011 (in It+/hr) Lower curve: frlction torque, Ib. In. Upper curve: outer rln; tempenwre. 'C

Fig.27

6.10

E f f e c t on B e ari n q F r i c t i o n o f V a ryi ng a u a n t i t y o f O i l

SEALS

Selecting the c o r r e c t bearings f o r a p a r t i c u l a r a p p l i c a t i o n necessitates more t h a n a s c e r t a i n i n g c o r r e c t t y p e and s i z e . satisfactorily,

If bearings are t o f u n c t i o n

t h e y must be p r o p e r l y l u b r i c a t e d and p r o t e c t e d f r o m t h e oper-

a t i n g environment by means o f c o r r e c t l y designed seals. Seals a r e n o r m a l l y i n t e nd ed t o p r e v e n t f o r e i g n m a t e r i a l e n t e r i n g t h e b e a r i n g and i n c e r t a i n cases t o p r e v e n t t h e i n g r e s s o f m o i s t u r e and c o r r o s i v e media. The f r i c t i o n developed by a r u b b i n g se al must be considered r e l a t i v e t o t h e power i n p u t .

The seal o r s i d e p l a t e must a l s o r e t a i n t h e l u b r i c a n t i n t h e

b e a r i n g o r housing.

R o t a t i o n a l speed o r r u b b i n g s e a l s must a l s o be considered

t o ensure e x c e s si ve h e a t i s n o t developed.

158

Pipeline layout for condensed oil mist lubricating system

1 I 3 4

5 Mstribution box 6-8 Condenstng nipples 9 Branch pipelines

Compressed air cut-off valve OU mint lubricator

Pressure control valve

10 Spray nomle

Mein pipeline

Fig.28

O i l M i s t L u b r i c a t i o n System

Simple gap seals a r e e f f i c i e n t using grease l u b r i c a t i o n , the purpose o f the gap seals being t o keep o u t r e l a t i v e l y small a m u n t s o f f o r e i g n m a t e r i a l and t o r e t a i n the grease w i t h o u t excessive temperature problems. long a x i a l l y and as small as p o s s i b l e r a d i a l l y . gap should be 0.1

t o 0 . 3 mm.

A gap seal should be

For normal a p p l i c a t i o n s the

Gap seals can be used w i t h o i l l u b r i c a t i o n , but

i t i s advisable t o incorporate a groove i n the s h a f t o r , a l t e r n a t i v e l y ,

fit a

r i n g , both arrangements causing the o i l t o f l o w back i n t o the housing.

A fur-

t h e r a l t e r n a t i v e would be t o machine a small groove i n the s h a f t adjacent t o the gap seal (which has a d r a i n hole) thereby encouraging the o i l t o f l o w back i n t o the housing. L a b y r i n t h seals a r e e x t e n s i v e l y used w i t h grease l u b r i c a t i o n i n d i r t y and wet c o n d i t i o n s .

The l a b y r i n t h c o n s i s t s o f a number o f r a d i a l l y separated

tongues w i t h a small r a d i a l clearance and can be considered as e l a b o r a t e grooved gap seals.

The grooves a r e o f t e n f i l l e d w i t h grease t o prevent the ingress o f

169 d i r t and a t y p i c a l l a b y r i n t h system f o r a t r a c t i o n motor bearing i s shown i n Fig.29.

lkaring srbcmc /or rrartion motor armatwe

Fig.29

6.11

Bearing Scheme f o r T r a c t i o n Hotor Armature

NOISE

Noise o r i g i n a t e s from an o s c i l l a t i n g body which produces a moving l o n g i t u d i n a l wave c o n s i s t i n g o f v a r i a t i o n i n pressure.

I n order t o reduce noise i r r i t a t i o n t o

a minimum t h e r e i s a constant demand f o r s i l e n t running bearings p a r t i c u l a r l y i n domestic and o f f i c e equipment such as vacuum cleaners, f l o o r p o l i s h e r s , c i r c u l a t o r y pump motors f o r c e n t r a l heating and fans e t c .

A clean h i g h q u a l i t y r o l l i n g bearing runs w i t h an even p u r r i n g sound and depending on the bearing arrangement, the n o i s e may be so low as t o be i n a u d i b l e o r i t can be unpleasant and d i s t u r b i n g .

Bearing noise i s a f u n c t i o n o f the l e v e l

o f v i b r a t i o n i n the bearing and the q u i e t e r the bearing the lower the l e v e l o f vibration.

V i b r a t i o n s i n a bearing depend on many f a c t o r s such as surface

f i n i s h , speed, load and accuracy o f geometric form e t c . Clearance i n a bearing i s a f u r t h e r c o n t r i b u t o r y f a c t o r t o n o i s e and i t i s necessary t o s e l e c t bearings and f i t s t o achieve zero clearance i n the bearing under o p e r a t i n g c o n d i t i o n s . bearing clearances.

Fig.30

shows the e f f e c t on noise of d i f f e r e n t

Freedom from clearance i n mounted b a l l bearings can a l s o be

160 achieved by a d j u s t i n g t h e b e a r i n g a g a i n s t each o t h e r by means o f d i s c s p r i n g s as shown i n Fig.31.

T h i s method ensures p r a c t i c a l l y z e r o c l e a r a n c e under a l l

working c o n d i t i o n s .

The t o t a l s p r i n g p r e s s u r e should be a p p r o x i m a t e l y 5

Newtons per mm o f s h a f t diameter i . e .

100 Newton p r e s s u r e f o r a 20 mm s h a f t .

Clearance in mounted bearing The dependence of motor noise on bearing clearance. Measurement of vibration of electric motor over the frequency range 400--800 c/s with progressive variation of bearing clearance A=clearance range obtained with C2 bearings and normal fits B=clearance range obtained with normal bearings and normal fits

Fig.30

E f f e c t o f Noise on D i f f e r e n t Bearing C 1 earances

For b a l l and c y l i n d r i c a l r o l l e r b e a r i n g s o p e r a t i n g a t h i g h speeds (ndm 500000 and 400000 r e s p e c t i v e l y ) t h e temperature d i f f e r e n t i a l between t h e i n n e r and o u t e r r i n g s i s g r e a t e r than a t normal speeds and t h e e f f e c t o f t h i s must be taken i n t o account.

Any r e d u c t i o n i n c l e a r a n c e due t o temperature d i f f e r e n t i a l

must be compensated f o r by u s i n g b e a r i n g s w i t h g r e a t e r i n i t i a l r a d i a l clearance. W h i l s t v i b r a t i o n l e v e l s i n b e a r i n g s can be reduced, i t i s e q u a l l y i m p o r t a n t t o ensure t h a t t h e o t h e r components i n t h e machine a r e a l s o manufactured t o s i m i l a r accuracy t o ensure t h e y a r e n o t t h e cause o f v i b r a t i o n , o t h e r w i s e t h e q u i e t running p r o p e r t i e s o f t h e b e a r i n g w i l l n o t be u t i l i s e d .

From a n o i s e

aspect t h e o v a l i t y and t a p e r o f t h e s h a f t and housing s e a t i n g s should be accura t e and l i e w i t h i n h a l f t h e t o l e r a n c e range f o r grades I T 5 and IT6 f o r s h a f t and housing r e s p e c t i v e l y .

B e a r i n g a l i g n m e n t must a l s o be c o n s i d e r e d , a s mis-

a l i g n m e n t can a l s o be a source o f n o i s e .

F i g u r e 32 shows t h e h i g h and c o r r e s -

ponding d i a m e t r i c a l l y opposed low s p o t s on a lobed t y p e o f out-of-roundness, and Fig.33

shows t h e e f f e c t o f a l i g n m e n t e r r o r s on n o i s e i n t e n s i t y .

There a r e

161

Fig.31

Spring preloading of deep groove ball bearings. 1 with spring washers, 2 with helical springs.

Fig.32

Lobing. For every high spot there is a corresponding low spot diametrically opposite. There are always an odd number of lobes.

162

Noise intensity

dR

I

Fig.33

Errors of alignment

-

E f f e c t o f Alignment E r r o r s on Noise

many a p p l i c a t i o n s where n o i s e must be t o l e r a t e d i n o r d e r t o a c h i e v e a s a t i s f a c t o r y bearing l i f e . o f railway vehicles,

I n e l e c t r i c t r a c t i o n motors geared t o t h e d r i v i n g a x l e s t h e o p e r a t i n g c o n d i t i o n s d i c t a t e heavy f i t s w hich i n t u r n

n e c e s s i t a t e b e a r i n g s w i t h l a r g e i n i t i a l r a d i a l clearance.

The increased r a d i a l

c l ea r a n c e r e s u l t s i n combined r o l l e r and cage d r o p as they come o u t o f t h e load zone which can cause increased b e a r i n g no i se, a l t h o u g h i t has no e f f e c t on t h e l i f e o f t h e b e a r i n g and cannot be heard when t h e motor is f i t t e d under t h e l oc o m o tiv e .

A h i g h p i t c h e d whine can o c c a s i o n a l l y o c c u r a t c e r t a i n speed ranges

w i t h c y l i n d r i c a l r o l l e r be ari n gs, d e s p i t e adequate l u b r i c a t i o n .

T h i s has been

a t t r i b u t e d t o resonance caused by c e r t a i n f r e q u e n c i e s o f the r o l l i n g elements c o i n c i d i n g w i t h t h e n a t u r a l frequency of t h e b e a r i n g end frame and, i n c e r t a i n cases, whine has been s i g n i f i c a n t l y reduced by m o d i f y i n g t h e mass d i s t r i b u t i o n o f t h e housing.

6.12

ANTICIPATING BEARING DAMAGE

The r u n n in g performance o f r o l l i n g b e a r i n g s from a n o i s e aspect can be checked w i t h reasonable accuracy u s i n g a wooden l i s t e n i n g s t i c k and l i s t e n i n g t h r o u g h t h e h a n d le t o t h e t r a n s m i t t e d n oi se .

A wooden s t i c k i s p a r t i c u l a r l y u s e f u l i n

t h a t i t t r a n s m i t s n o i s e r e l a t i n g t o t h e c o n d i t i o n o f t h e b e a r i n g and c u t s o u t most o f t h e extraneous n o i s e from o t h e r machine components w hich can cause p r o blems f o r t h e more s e n s i t i v e stethoscope.

With experience, an o p e r a t o r becomes

tuned t o c e r t a i n n o i s e s and can a s c e r t a i n t h a t b e a r i n g examination should be c a r r i e d o u t a t t h e n e x t shutdown p e r i o d .

Naturally,

if say a c o n s i s t e n t knocking

develops w i t h t h e normal n o i s e l e v e l , t h en an immediate i n s p e c t i o n o f t h e bearing

163 should be made.

Damaged t r a c k s caused by c a r e l e s s mounting produce pronounced

low p i t c h n o i s e and inadequate r a d i a l c l e a r a n c e produces m e t a l l i c n o i s e .

6.13

DETECTION OF BEARING DAMAGE BY SHOCK PULSE MEASUREMENT

When a r o l l i n g b e a r i n g s u f f e r s f a t i g u e f a i l u r e , element s u r f a c e s o r i n t h e t r a c k s .

f l a k i n g occurs i n t h e r o l l i n g

A b e a r i n g component coming i n t o c o n t a c t

w i t h t h e damaged zone causes a mechanical shock, causing t r a n s i e n t v i b r a t i o n s which a r e t r a n s m i t t e d t o t h e b e a r i n g housing.

These v i b r a t i o n s g e n e r a t e e l e c -

t r i c v o l t a g e s i n an acce l ero met er and t h e i r a m p l i t u d e i s determined by a shock p u l s e meter.

The c o n d i t i o n o f t h e b e a r i n g i s monitored, t a k i n g readings a t

suitable intervals.

I n c i p i e n t damage can, t h e r e f o r e be d e t e c t e d a t an e a r l y

s ta g e and b e a r i n g replacement planned a c c o r d i n g l y . Earphones can be used t o l i s t e n t o t h e rhythm o f t h e shocks and, p r o v i d e d t h e damage i s n o t t o o complex,

t h e rhythm w i l l suggest w hich component o f t h e

b e a r i n g i s damaged. Temperature i s a f u r t h e r method f o r gauging b e a r i n g c o n d i t i o n , and b e a r i n g f a i l u r e s a r e sometimes preceded by a d r o p i n temperature f o l l o w e d by a r a p i d i nc r e a s e i n temperature,

u s u a l l y caused by metal-to-metal

quent f a t i g u e f a i l u r e o r even s e i z u r e .

c o n t a c t w i t h subse-

The o l d method o f checkina t h e b e a r i n g

te m p e r a t u r e by f e e l i n g t h e h ou si ng i s n o t s a t i s f a c t o r y ,

and i n a p p l i c a t i o n s

where breakdowns cause maintenance problems i t i s usual t o use thermal-couples p o s i t i o n e d as c l o s e t o t h e b e a r i n g o u t e r r i n g as p o s s i b l e .

When u s i n g t h e

te m p e r a t u r e method o f ch ecki n g b e a r i n g c o n d i t i o n i t must be noted t h a t t h e r e

w i l l always be a t emp era t u re r i s e w i t h new o r freshly-greased

bearings.

This

o n l y a p p l i e s d u r i n g i n i t i a l r u n n i n g and once t h e new l u b r i c a n t has d i s t r i b u t e d itself,

6.14

t h e te m p era t u re w i l l s t a r t t o f a l l .

FITS (SHAFT AND HOUSING)

To le r a n c e s f o r t h e bo re and o u t s i d e d i ame ter o f m e t r i c r o l l i n g b e a r i n g s a r e i n t e r n a t i o n a l l y s t a n d a r d i s e d and t h e r e q u i r e d f i t s a r e achieved by s e l e c t i n g s h a f t and h o u s in g t o l e r a n c e s u s i n g t h e IS0 t o l e r a n c e system ( i n c o r p o r a t e d i n BS 4 5 0 0 : P a r t 1:1969).

Only a sma l l s e l e c t i o n o f t h e IS0 t o l e r a n c e zones need

t o be c o n s id e r e d f o r r o l l i n g b e a r i n g , and Fig.34 shows these r e l a t i v e t o t h e b e a r i n g b o r e ( a ) and b e a r i n g o u t s i d e d i ame t e r ( b ) . The s h a f t t o l e r a n c e i s i n d i c a t e d by a sma ll l e t t e r and a number and the

A t y p i c a l s h a f t and housing

housing b o r e by a c a p i t a l l e t t e r and a number. bore t o l e r a n c e c omb i na t i o n would be w r i t t e n j 6 obtained from t ole ran ce tab le s.

-

J7, t h e values f o r each being

164

b

Fig.34

Shaft and Housing F i t s .

The various symbol gradings a r e as f o l l o w s : Shafts Clearance

Bearings always have clearance f i t i f the s h a f t t o l e r a n c e

Transition

Bearings can be clearance o r i n t e r f e r e n c e f i t depending on

i sl l f l l

the a c t u a l dimensions, i f t h e shaft t o l e r a n c e is w i t h i n t h e range 'lg Interference

- j".

Bearings w i l l always have an i n t e r f e r e n c e f i t i f t h e s h a f t t o l e r a n c e range i s w i t h i n "k

-

r'l.

Housings C I earance

Bearings w i l l always have clearance i f t h e housing tolerance

Transition

Bearings w i l l have a clearance or an i n t e r f e r e n c e f i t depend-

range i s I'G

- H".

i n g on the a c t u a l dimensions i f the housing tolerance i s w i t h i n the range "J Interference

-

N".

Bearings w i l l have i n t e r f e r e n c e f i t i f the housing tolerance i sl l p l l

The most important f a c t o r s t o consider when s e l e c t i n g f i t s are;-

1 . Conditions o f r o t a t i o n 2 . Magnitude o f the load

3 . Temperature c o n d i t i o n s

165 6.15

C O N D I T I O N S OF ROTATION

When a bearing r i n g i s t o r o t a t e r e l a t i v e t o t h e d i r e c t i o n of the r a d i a l load,

i t must be mounted w i t h s u f f i c i e n t i n t e r f e r e n c e on the s h a f t o r i n the

housing t o prevent " r o l l "

o r "creept'.

I f creep occurs between heavily-loaded

d r y surfaces, t h e surfaces undergo r a p i d wear. the f i t used i s t i g h t enough t o

It i s therefore essential that

ensure t h a t no clearance e x i s t s and none can

I f the load i s always d i r e c t e d towards

develop due t o the a c t i o n o f the load.

the same p o i n t on the r i n g , however, no creep o'ccurs and a clearance f i t i s g e n e r a l l y permissible. Various loading c o n d i t i o n s can be c l a s s i f i e d as f o l l o w s ; (i)

R o t a t i n g inner r i n g load.

The s h a f t r o t a t e s r e l a t i v e t o the d i r e c t i o n

o f the load so t h a t a l l p o i n t s on the inner r i n g t r a c k a r e subjected t o load d u r i n g one r e v o l u t i o n . Example

-

Shaft w i t h b e l t d r i v e .

( i i ) S t a t i o n a r y inner r i n g load.

The s h a f t i s s t a t i o n a r y r e l a t i v e t o the

d i r e c t i o n o f loading so t h a t the load i s always towards the same sector o f the inner r i n g . Example

- Automobile

f r o n t hub.

( i i i ) S t a t i o n a r y o u t e r r i n g load.

The bearing housing remains s t a t i o n a r y

r e l a t i v e t o t h e d i r e c t i o n o f loading so t h a t

t h e load i s always d i r e c t e d

towards t h e same sector o f the o u t e r r i n g . Example

-

Shaft w i t h b e l t d r i v e .

( i v ) Rotating o u t e r r i n g load.

The bearing housing r o t a t e s r e l a t i v e t o the

d i r e c t i o n o f loading so t h a t a l l p o i n t s on t h e o u t e r r i n g t r a c k come under load d u r i n g one r e v o l u t i o n . Example

- Automobile

f r o n t hub.

I n many a p p l i c a t i o n s o p e r a t i n g c o n d i t i o n s cannot be r e l a t e d t o any o f these simple loading cases and v a r i a b l e e x t e r n a l loads o r out-of-balance

forces

i n f l u e n c e t h e d i r e c t i o n o f loading, an a p p r o p r i a t e f i t being c l a s s i f i e d as " d i r e c t i o n o f loading indeterminate".

I n t h i s instance, bearing r i n g "creep"

can o n l y be prevented by using an i n t e r f e r e n c e f i t f o r both r i n g s and i n such cases bearings w i t h increased r a d i a l clearance a r e u s u a l l y necessary.

6.16

INFLUENCE OF LOAD AND TEMPERATURE

The load compresses t h e inner r i n g i n a r a d i a l d i r e c t i o n which s t r e t c h e s the r i n g i n a c i r c u m f e r e n t i a l d i r e c t i o n and compresses t h e s h a f t , thereby loosening the f i t .

S i m i l a r l y , as the bearing inner r i n g t r a c k is warmer d u r i n g o p e r a t i o n

than the bearing bore, t h i s a l s o has the e f f e c t o f loosening the f i t o f the

166 b e a r i n g i n n e r r i n g on i t s j o u r n a l . I n t h e m a j o r i t y o f a p p l i c a t i o n s i t i s unnecessary t o c a l c u l a t e t h e r e q u i r e d

i n t e r f e r e n c e s i n c e e xpe ri en ce i s a good g u i d e i n s e l e c t i n g s u i t a b l e f i t s f o r d i f f e r e n t o p e r a t i n g c o n d i t i o n s and o n l y i n p a r t i c u l a r l y d i f f i c u l t or unusual c o n d i t i o n s i s i t necessary t o r e s o r t t o s p e c i a l c a l c u l a t i o n s .

6.17

BEARING A P P L I C A T I O N S

When d e s i g n i n g b e a r i n g arrangements t h e r e a r e c e r t a i n b a s i c r u l e s w hich should be f o l l o w e d . l o c a t i o n purposes.

Wherever p o s s i b l e o n l y one b e a r i n g should be used f o r T h i s means t h a t t h e b ea ring o u t e r r i n g should be h e l d

a x i a l l y i n i t s hou si ng w i t h t h e i n n e r r i n g l o c a t e d on t h e s h a f t i n a s i m i l a r manner.

All o t h e r be ari n gs on t h e same s h a f t should be a x i a l l y f r e e , e i t h e r

on t h e s h a f t o r i n t h e housing, as shown on Fig.35.

I f t h i s basic r u l e i s

ignored and two a x i a l l y l o c a t e d b e a r i n g s a r e used, any s h a f t expansion o c c u r r i n g due t o generated o r e x t e r n a l h e a t c o u l d cause severed l o c k i n g ( p r e l o a d i n g ) acr o s s t h e b e a r in gs, r e s u l t i n g i n premature b e a r i n g f a i l u r e .

In certain appli-

c a t i o n s u s i n g a ng ul a r c o n t a c t b a l l o r t a pe r r o l l e r bearings i t i s n o t p o s s i b l e t o use only one b ea ri ng f o r l o c a t i o n and t h e bearings must be a d j u s t e d by end covers.

FIG.

35

FTG.

36

W it h these b e a r ing s g r e a t c a r e must be t ake n t o ensure t h a t t h ? bearings can accommodate a x i a l v a r i a t i o n s and i n sane a p p l i c a t i o n s a small gap i s l e f t between t h e b e a r i n g o u t e r r i n g and t h e abutment cover, as shown i n Fig.36. The a x i a l expansion problem can be r e s o l v e d by i n c o r p o r a t i n g a c y l i n d r i c a l r o l l e r b e a r in g a t t h e no n-l o cat ed p o s i t i o n , t h i s arrangement being used extens i v e l y on e l e c t r i c motors where a b a l l b ea ri ng i s used a t t h e commutator end and a r o l l e r b e a ri ng a t t h e d r i v e end.

I n a d d i t i o n t o r e s o l v i n g t h e thermal

expansion problem such an arrangement has an added advantage i n t h a t a c y l i n d r i c a l r o l l e r b e ari n g has s i g n i f i c a n t l y h i g h e r I o a d - c a r r y i n g c a p a c i t y (compared t o a d i m e n s i o n a l l y e q u i v a l e n t b a l l be ari n g) which makes i t p a r t i c u l a r l y s u i t a b l e f o r r e a c t i n g heavy d r i v e f o r c e s .

167 6.18

BEARING CARE

Cleanliness i s o f paramount importance when handling and mounting bearings, but i n s p i t e o f the f a c t t h a t bearings a r e p r e c i s i o n made, one has o n l y t o look around the average workshop t o see open bearings l e f t on benches o r i n dusty environments.

I t should be noted t h a t i f f o r e i g n matter i s allowed t o enter a

bearing the r o l l i n g elements i n passing over i t d u r i n g s e r v i c e can cause indenta t i o n s leading t o f a t i g u e f a i l u r e i n t h e r o l l i n g elements o r t r a c k s which can shorten the l i f e o f the bearing considerably.

As a general r u l e , i n order t o

ensure the bearings remain f r e e from i m p u r i t i e s , they should n o t be removed from t h e i r o r i g i n a l packing u n t i l they a r e required f o r i n s t a l l a t i o n .

Before

packing bearings a r e u l t r a - s o n i c a l l y washed and coated w i t h a bearing preservat i v e which mixes r e a d i l y w i t h most l u b r i c a n t s , a p a r t from c e r t a i n c l a y o r synthetic-based greases,

i n which case i t i s advisable t o contact the bearing manu-

facturers. Although i n i t i a l washing o f bearings by t h e user i s n o t recommended due t o the p o s s i b i l i t y o f the washing f l u i d n o t completely evaporating,

i f washing i s

necessary then the bearings should be washed w i t h c l e a n w h i t e s p i r i t a f t e r which the bearings must be thoroughly d r i e d before adding l u b r i c a n t .

6.19

BEARING MOUNTING

I f the bearing inner r i n g i s t o be a t i g h t f i t on t h e s h a f t , i t can be d r i v e n onto the s h a f t j o u r n a l by means o f a t u b u l a r d r i f t which should bear evenly

I f the bearing outer

a g a i n s t t h e f a c e o f t h e inner r i n g , as shown i n Fig.37.

r i n g i s t o be a t i g h t f i t i n i t s housing then the r i n g should be d r i v e n i n t o the housing by a p p l y i n g f o r c e t o the outer r i n g . Hollow drift for bearings with an interference fit on the shaft

I

FIG-

37

Hollow drift for bearings with an interference fit both in the housing and on the shaft

I FIG.

38

Pressure must n e v e r be a p p l i e d t o the inner o r outer bearing r i n g i n order t o mount the o t h e r r i n g which has a t i g h t f i t , otherwise t h e r e i s a danger o f the t r a c k s being indented and the bearing would probably f a i l i n s e r v i c e a f t e r

168 a s h o r t t im e .

I f b o t h t he i n n e r and o u t e r r i n g s a r e a t i g h t f i t , then a t o o l

o f t h e t y p e shown i n F i g . 3 8 must be used whi ch c o n t a c t s b o t h s i d e f a c e s o f t h e be a r in g r i n g s . Care must be t ake n t o e nsu re t h a t t h e b e a r i n g r i n g b e i n g pressed on i s c o r r e c t l y a l i g n e d , p a r t i c u l a r l y i n t h e case o f small r i n g s .

Excessive m i s a l i g n -

ment p u t s severe s t r e s s on t h e cage, wh i ch can r e s u l t i n premature b e a r i n g failure. C y l i n d r i c a l r o l l e r b ea ri ng s sometimes g i v e assembly problems due t o t h e r o l l e r s being scored as t h e y a r e f e d o v e r t h e t r a c k s .

T h i s problem can be r e -

s o lv e d by u s i n g a s p e c i a l mounting s l e e v e as shown on Fig.39.

Mounting sleeves simplify the mounting of cylindrical roller bearings and prevent damage to the tracks

FTG.

39

For l a r g e r o l l i n g b e a r i n g s w i t h c y l i n d r i c a l bores where t h e i n n e r r i n g i s a

t i g h t f i t , mounting can be s i m p l i f i e d by h e a t i n g t h e complete b e a r i n g i n an o i l ba th a t a t e m p e ra t u re o f 70°

-

8OoC above ambient.

I f t h e b e a r i n g needs t o be

t r a n s p o r t e d some d i s t a n c e i t can be heated t o a h i g h e r temperature t o compens a t e f o r c o o l i n g down d u r i n g t r a n s i t .

Bearings can be heated up t o 12OoC i n

t h i s way, b u t t h i s t e mpe rat ure sh ou l d n o t be exceeded as, above 120°C,

there i s

a danger o f r e d uci n g t h e b e a r i n g m a t e r i a l hardness.

6.20

DISMOUNTING BEARINGS

I f an i n t e r f e r e n c e f i t has been used t o l o c a t e a b e a r i n g on a s h a f t i t i s e s s e n t i a l t o use a s u i t a b l y designed wi t h dra wal t o o l t o remove t h e b e a r i n g . The b a s i c p r i n c i p l e s for mounting a b e a r i n g a p p l y a l s o t o dismounting.

The d i s -

mounting f o r c e must be a p p l i e d t o t h e b e a r i n g r i n g having t h e i n t e r f e r e n c e f i t and n o t t o t h e loose r i n g and t hro ug h t o t h e o t h e r r i n g by means o f t h e r o l l i n g

169 elements.

Claw-type p u l l e t s a r e o f t e n used t o remove b a l l bearings,

but design-

e r s must t a k e t h i s i n t o c o d s i d e r a t i o n d u r i n g t h e d e s i g n s t a g e by i n c o r p o r a t i n g s u i t a b l e grooves i n t h e abutment shoulder a d j a c e n t t o t h e b e a r i n g f a c e t o enable the b e a r i n g i n n e r r i n g t o be g r i p p e d by t h e p u l l e r c l a w s .

N a t u r a l l y , the p u l l e r

f o r c e must be a p p l i e d t o t h e i n n e r r i n g and n o t t r a n s m i t t e d through t h e r o l l i n g elements.

I f t h e i n n e r r i n g i s i n a c c e s s i b l e i t i s necessary t o p u l l on t h e

o u t e r r i n g , b u t b e a r i n g damage can be avoided by l o c k i n g t h e p u l l e r c e n t r e screw w i t h a spanner and r o t a t i n g t h e claws.

The o u t e r r i n g w i l l then r o t a t e

d u r i n g t h e w i t h d r a w l process, d i s t r i b u t i n g t h e load over t h e t r a c k s and thereby preventing the p o s s i b i l i t y o f indentations.

The b e a r i n g i s t h e n p u l l e d o f f

s u f f i c i e n t l y t o enable t h e i n n e r r i n g t o be g r i p p e d w i t h t h e p u l l e r ( F i g . 4 0 ) .

Extractor

U FIG. 40 6.21

MOUNTING AND DISMOUNTING B Y O I L INJECTION

I n o r d e r t o overcome many o f t h e mounting and dismounting problems SKF, i n 1940, i n t r o d u c e d an o i l - i n j e c t i o n t e c h n i q u e which i s now w i d e l y used i n many branches o f e n g i n e e r i n g .

O i l under h i g h p r e s s u r e i s i n j e c t e d between t h e bear-

ing i n n e r r i n g and s h a f t j o u r n a l d u r i n g mounting o r dismounting.

An o i l f i l m

i s formed which b o t h separates and l u b r i c a t e s t h e c o n t a c t s u r f a c e s .

The o i l

separates t h e c o n t a c t s u r f a c e s except f o r a narrow zone a t each end o f t h e r i n g . The s u r f a c e p r e s s u r e i s g r e a t e r i n these zones due t o t h e i n f l u e n c e o f t h e s h a f t m a t e r i a l beyond t h e end o f t h e r i n g and t h e zones a c t a s an o i l l o c k which r e t a i n s t h e o i l between t h e c o n t a c t s u r f a c e s .

When t h e o i l p r e s s u r e i s released

t h e o i l i s f o r c e d a u t o m a t i c a l l y back through t h e s u p p l y d u c t s , thereby r e s t o r i n g the o r i g i n a l f r i c t i o n . The advantage o f u s i n g o i l - i n j e c t i o n i s t h a t t h e f o r c e r e q u i r e d t o move t h e component i s s i g n i f i c a n t l y reduced and t h a t by e l i m i n a t i n g d i r e c t c o n t a c t and t h e r e s u l t i n g f r i c t i o n between t h e c o n t a c t s u r f a c e s , t h e p o s s i b i l i t y o f damage t o t h e s u r f a c e s o c c u r r i n g d u r i n g t h e mounting o r dismounting process i s minimised.

170 A f u r t h e r advantage i s t h a t

components can be dismounted o r a d j u s t e d w i t h o u t

t h e r i s k o f the f i t d e t e r i o r a t i n g . I f t h e b e a r i n g r i n g i s mounted on a t a p e r e d j o u r n a l w i t h a s e l f - r e l e a s i n g taper, the

r i n g w i l l be e j e c t e d w i t h some f o r c e when t h e o i l i s i n t r o d u c e d and

some f o r o f a x i a l r e s t r a i n t such as a n u t w i l l be necessary.

I f , however, a

l o c k i n g t a p e r i s used then an e x t e r n a l f o r c e a d d i t i o n a l t o t h e o i l i n j e c t i o n f o r c e w i l l be r e q u i r e d .

F i g u r e s 41, 42 and 43 show t y p i c a l arrangements.

Position of distribution groove in a tapered and a cylindrical seating for a rolling bearing

u

Cylindrical seating

FIG.

41

Tapered seating

FIG.

42

The f i r s t two show a b e a r i n g on a p a r a l l e l s h a f t and a b e a r i n g on a tapered shaft.

The t h i r d shows o i l i n j e c t i o n b e i n g a p p l i e d t o a gear wheel mounted on

a parallel shaft. bearing r i n g i s

I t should be n o t e d t h a t , w i t h a p a r a l l e l s h a f t , once t h e

p a s t t h e o i l e n t r y h o l e t h e r e i s no l o n g e r any o i l p r e s s u r e

and t h e r i n g c o u l d

lock.

I t has been found, however, t h a t i f t h e r i n g i s w i t h -

drawn r a p i d l y then i t w i l l t r a v e l o v e r t h e remaining area w i t h o u t t o o much d i f f iculty

Large r o l l i n g b e a r i n g s w i t h a t a p e r b o r e and mounted on a d a p t o r o r w i t h -

d rawa 1 sleeves can be e a s i l y mounted o r dismounted u s i n g h y d r a u l i c n u t s designed by SKF

The h y d r a u l i c n u t c o n s i s t s o f an i n t e r n a l l y threaded s t e e l r i n g w i t h a

groove i n one f a c e i n t o which i s f i t t e d an a n n u l a r p i s t o n sealed w i t h O - r i n g s . When o 1 i s pumped i n t o t h e a n n u l a r space behind t h e p i s t o n i t i s f o r c e d o u t wards,

thereby f o r c i n g t h e b e a r i n g on o r p u l l i n g t h e b e a r i n g o f f t h e s l e e v e o r

shaft.

F i g u r e 4 4 shows a h y d r a u l i c n u t b e i n g used t o mount a s p h e r i c a l r o l l e r

b e a r i n g on an adaptor s l e e v e and Fig.45 shows an arrangement f o r d i s m o u n t i n g a b e a r i n g on an adaptor s l e e v e .

An arrangement f o r dismounting a b e a r i n g from a

withdrawal sleeve i s shown on Fig.46.

171

H M V nut for driving up a bearing on an adapter sleeve

Cylindrical mating surface having two distribution grooves

FIG.

43

H M V nut used to free a withdrawal sleeve

H M V nut and stop rings in position to press loose an adapter sleeve

FIG.

6.22

46

C L E A N I N G OF B E A R I N G S

A s p r e v i o u s l y mentioned,

i t i s n o t necessary t o c l e a n b e a r i n g s taken d i r e c t

from t h e i r packages and any a t t e m p t t o do so c o u l d be d e t r i m e n t a l from b o t h a c l e a n l i n e s s and f u t u r e l u b r i c a t i o n aspect. bearings need t o

There a r e occasions, however, when

be dismounted, cleaned, and inspected i n s p i t e o f the f a c t

t h a t unnecessary dismounting may cause d e t e r i o r a t i o n o f t h e s h a f t and housing f i t s as w e l l as p o s s i b l e damage t o t h e t r a c k s .

172 A f t e r d is m o u n t i n g f o r c l e a n i n g purposes a l l used grease should f i r s t be caref u l l y removed. ing f l u i d

Small b e a r i n g s sh ou l d be immersed i n w h i t e s p i r i t o r o t h e r clean-

and s w i r l e d round, t he r e s i d u a l grease and d i r t b e i n g removed u s i n g a

good q u a l i t y b r ush . tr a p p e d between

Care must be taken t o ensure t h a t none o f the b r i s t l e s a r e

t h e cage and r o l l i n g elements.

A f t e r t h i s p r e l i m i n a r y washing t he b e a r i n g s should be washed i n one o r two additional b a t h s o f white s p i r i t .

A warm (lOO°C maximum) l i g h t m i n e r a l o i l

s h o u ld then be f l u s h e d t h rou gh t h e b e a r i n g as i t i s s l o w l y r o t a t e d .

Under no

circumstances s ho ul d t h e b e a r i n g be r o t a t e d u n t i l t h e o i l passes through i t , o t h e r w i s e any f o r e i g n p a r t i c l e s i n t h e b e a r i n g w i l l be r o l l e d i n t o t h e t r a c k s c a u s in g i n d e n t a t i o n s . they s h o u ld

I f t h e b e a r i n g s a r e n o t intended f o r immediate mounting

be l u b r i c a t e d w i t h a good q u a l i t y o i l c o n t a i n i n g a w a t e r - r e p e l l a n t

additive.

6.23

RECOGNITION OF B E A R I N G FAILURES

Ob v io u s ly premature b e a r i n g f a i l u r e s o c c u r , b u t i n t h e m a j o r i t y o f cases i f the symptoms a r e reco gn i sed t h e cause o f t h e f a i l u r e s can be e l i m i n a t e d .

If

r o l l i n g b e a r in g s a r e p r o p e r l y mounted, sea l ed, l u b r i c a t e d and maintained, they w i l l run u n t i l f a t i g u e f a i l u r e ( f l a k i n g ) occurs on t h e b e a r i n g i n n e r o r o u t e r r i n g o r r o l l i n g elements. f a i l u r e can

As a l r e a d y mentioned, b e a r i n g l i f e based on f a t i g u e

be p r e d i c t e d as a f u n c t i o n o f t h e l i f e d i s p e r s i o n curve, and i f a

b e a r i n g f a i l s w e l l s h o r t o f i t s normal L,o

l i f e i t i s important to a s c e r t a i n

th e cause o f the f a i l u r e b e f o r e f i t t i n g new b e a r i n g s .

6.23.1

Wear

I f a b e a r i n g housing i s i na de qu at el y se al ed, a b r a s i v e p a r t i c l e s can e n t e r the bearing

c a u s in g wear i n t h e t r a c k s ,

be a r in g s wear i n t h e g u i d e f l a n g e s .

r o l l i n g elements and cage, and i n c e r t a i n The worn s u r f a c e s a r e d u l l i n appearance

and an example o f worn t r a c k s i s shown i n Fig.47.

Wear can a l s o o c c u r between

th e i n n e r r i n g bo re and i t s j o u r n a l o r between t h e o u t e r r i n g o u t s i d e diameter and h o u s in g

bore due t o creep.

T h i s i s u s u a l l y the r e s u l t

o f excessive c l e a r -

ance and i s p a r t i c u l a r l y s e r i o u s because n o t o n l y the b e a r i n g b u t a l s o t h e s h a f t o r h o u s in g c o u l d be damaged and r e q u i r e r e p l a c i n g . faces

Creep between mating s u r -

causes f r e t t i n g c o r r o s i o n and wear, and p a r t i c l e s o f r u s t from t h e c o r r o -

ded areas may a l s o e n t e r t h e b e a r i n g cau si ng wear i n t h e t r a c k s and r o l l i n g e l e ments.

Wear can a l s o

be caused

having a h i g h l y planished f i n i s h . failure.

by inadequate l u b r i c a t i o n ,

t h e worn surfaces

T h i s t y p e o f wear develops i n t o f a t i g u e

173

Fig.47

Worn T r a c k s due t o Abrasive P a r t i c l e s .

174 Incorrect

6.23.2

Mounting

F a u l t y mounting o f t e n r e s u l t s i n t h e b e a r i n g b e i n g s u b j e c t e d t o h i g h loads which can

cause m a t e r i a l f a t i g u e i n t h e b e a r i n g r i n g s o r r o l l i n g elements w e l l

b e f o r e t h e nominal c a l c u l a t e d l i f e o f t h e b e a r i n g has been reached.

Indenta-

t i o n s w i t h t h e same p i t c h as t h e r o l l i n g elements can be caused by t h e mounting f o r c e b e i n g a p p l i e d through t h e r o l l i n g elements.

D u r i n g s e r v i c e an o v e r l o a d

occurs each t i m e a r o l l i n g element passes o v e r an i n d e n t a t i o n , and a f t e r a r e l a t i v e l y s h o r t t i m e small fragments o f b e a r i n g m a t e r i a l break away, t h e c o n d i t i o n b e i n g g e n e r a l l y r e f e r r e d t o as " f l a k i n g " .

F i g u r e 48 shows t h e i n n e r r i n g o f a

deep groove b a l l b e a r i n g w i t h advanced areas o f f l a k i n g on t h e r i g h t - h a n d s i d e o f the track.

Fig.48

F l a k i n g caused by F a u l t y Mounting

F i g u r e 49 shows t h e i n n e r and o u t e r r i n g o f a b a l l b e a r i n g w i t h a f i l l i n g s l o t . The f l a k e d wear on t h e r i g h t - h a n d s i d e o f t h e i n n e r r i n g and on t h e l e f t - h a n d side

o f t h e o u t e r r i n g show t h a t f a i l u r e has been caused by a heavy t h r u s t l o a d

due t o c r o s s - l o c a t i o n .

F a t i g u e f a i l u r e o f t h i s t y p e begins a t a p o i n t below t h e

s u r f a c e o f t h e t r a c k o r r o l l i n g elements. work up t o t h e s u r f a c e where,under

Small c r a c k s develop, which g r a d u a l l y

repeated o v e r l o a d a t such weak p o i n t s , even-

t u a l l y cause fragments t o break away, t h e s e fragments b e i n g r o l l e d i n t o o t h e r p a r t s o f t h e t r a c k thereby i n i t i a t i n g f u r t h e r areas o f weakness.

175

Fig.49

F a i l u r e caused by a Heavy T h r u s t Load due t o Cross L o c a t i o n .

6.23.3

Cage F a i l u r e s

With an adequately l u b r i c a t e d b e a r i n g o p e r a t i n g i n normal c o n d i t i o n s , t h e

I f t h e l u b r i c a t i o n i s inadequate,

cage i s t h e most l i g h t l y - l o a d e d component.

wear w i l l o c c u r where t h e cage makes c o n t a c t w i t h t h e r i n g s and r o l l i n g elements and e v e n t u a l l y t h e cage may f r a c t u r e .

Highly planished surfaces are u s u a l l y

a s s o c i a t e d w i t h inadequate cage l u b r i c a t i o n .

Excessive misalignment o f t h e

i n n e r and o u t e r b e a r i n g r i n g r e l a t i v e t o each o t h e r i s a n o t h e r cause o f cage f a i l u r e and severe misalignment can r e s u l t i n t h e cage, o r even t h e b e a r i n g rings, cracking.

Wear can a l s o be caused by f o r e i g n p a r t i c l e s e n t e r i n g t h e

bearing, t h e p a r t i c l e s jamming o r wedging between t h e cage and r o l l i n g elements. F i g u r e 50 shows a s p h e r i c a l r o l l e r b e a r i n g case w i t h worn r o l l e r prongs and wear i n t h e bottom o f t h e r o l l e r pockets. where t h e prongs have a c t u a l l y sheared.

F i g u r e 51 shows a more advanced case

176

Fig.50

Spherical R o l l e r Bearing Cage w i t h Worn R o l l e r Prongs and Wear i n the Bottom of the R o l l e r Pockets.

Fig.51

More Advanced Case than F i q . 5 0 , w i t h Prongs Sheared.

6.23.4

Smearing

Smearing

i s a t r a n s f e r e n c e o f m a t e r i a l from one body t o another when two

inadequately l u b r i c a t e d

surfaces s l i d e a g a i n s t each o t h e r .

Local s t r e s s con-

c e n t r a t i o n s a r e induced by smearing, and these produce cracks i n t h e s u r f a c e l a y e r s and subsequent f l a k i n g . which

F i g u r e 52 shows a smeared c y l i n d r i c a l r o l l e r

has been caused by r a p i d a c c e l e r a t i o n o f t h e r o l l e r i n t h e loaded zone

coupled w i t h inadequate o r i n c o r r e c t l u b r i c a t i o n . subjected

I f c y l i n d r i c a l r o l l e r s are

t o severe t h r u s t f o r c e s combined w i t h inadequate o r u n s u i t a b l e l u b r i -

c a t i o n , smearing develops a t t h e ends o f t h e r o l l e r and on t h e g u i d e f l a n g e s

Fig.52

Smeared C y l i n d r i c a l R o l l e r caused by Rapid A c c e l e r a t i o n o f t h e R o l l e r i n t h e Loaded Zone coupled w i t h Inadequate o r Incorrect Lubrication.

o f b e a r i n g r i n g s , and Fig.53 shows smearing on t h e end o f a r o l l e r .

Smearing

can a l s o o c c u r on l i g h t l y - l o a d e d high-speed b a l l t h r u s t b e a r i n g s where t h e g y r a t o r y moment may f o r c e t h e b a l l s t o s l i d e t a n g e n t i a l l y t o t h e d i r e c t i o n o f

178

Fig.53

Smearing Caused by Severe Thrust Forces combined with Inadequate o r Unsuitable Lubrication.

rolling, causing spiral-shaped smearing.

Lightly-loaded high-speed ball thrust

bearing must therefore always have a small pre-load or spring load when mounted as given on Fig.22.

6.23.5

Vibrations

A phenomenon known as 'False Brinelling' can occur in rolling bearings fitted to stationary machines, due to vibrations from adjacent machinery.

Bearings in

ships ancillary equipment subject t o vibrations from the ship's machinery are particularly prone to this damage and this can apply to bearings in machines being transported by sea.

179

Fi9.54

Fig.55

Vibration Damage.

Vibration Oamage.

180

Rotor clamp for preventing damage due t o vibration during transport

FIG.

Fig.57

56

Corrosion a t the Contact Surfaces due t o Water.

181 A c h a r a c t e r i s t i c f e a t u r e o f v i b r a t i o n damage i s t h a t t h e damaged areas a r e spaced a t t h e same p i t c h as t h e r o l l i n g elements.

The v i b r a t i o n s induce small

movements a t t h e c o n t a c t surf ace s between t h e r o l l i n g elements and t h e r i n g s , and t h e r e s u l t i n g wear causes mi cro scop i c p a r t i c l e s o f m a t e r i a l t o break away. These p a r t i c l e s o x i d i s e and, on m i x i n g w i t h t h e l u b r i c a n t , cause a l a p p i n g a c t i o n , and hence an i n crea se i n t h e o x i d a t i o n r a t e .

The h i g h e r t h e frequency

o f t h e v i b r a t i o n a l movement a t t h e c o n t a c t s u r f a c e s , t h e more r a p i d t h e damage, each r o l l i n g element g r a d u a l l y f o r m i n g a cavity’. C y l i n d r i c a l r o l l e r and n ee dl e r o l l e r b e a r i n g s a r e more prone t o t h i s t y p e o f damage t h a n b a l l b e a r i n g s and s p h e r i c a l r o l l e r b e a r i n g s , and one method o f overcoming t h e problem i s t o use spri n g-l o ad ed b a l l bearings.

The p r e - l o a d

s h o u ld be a p p r o x i m a t e l y equal t o 20 Newtons per m i l l i m e t e r o f s h a f t diameter. Be fo r e p r e - l o a d i n g ,

however,

i t i s necessary t o check t h a t t h e combination o f

e x t e r n a l f o r c e s and a d d i t i o n a l p r e - l o a d does n o t o v e r l o a d t h e bearing. T y p i c a l examples o f v i b r a t i o n damage t o b e a r i n g s d u r i n g t r a n s p o r t a t i o n due t o r e l a t i v e movement between t h e s h a f t and housing, whether by sea, r a i l , o r road, can be r e s o l v e d by d r i v i n g a wooden wedge between t h e s h a f t o r c o u p l i n g and a r o b u s t p a r t o f t h e c r a t e .

Alternatively,

t h e s h a f t can be locked r e -

l a t i v e t o t h e h o usi n g by means o f a clamp, as shown i n Fig.56.

6.23.6

Rust and Other Types o f C o r r o s i o n

R o l l i n g b e a r i n g s w i l l r u s t i f wat er o r m o i s t u r e i s p r e s e n t and t h e bearings a r e n o t p r o t e c t e d by a l u b r i c a n t c o n t a i n i n g a r u s t - i n h i b i t o r .

P i t t i n g develops

i n t h e c o r r o d e d are as, r e l e a s i n g small p a r t i c l e s o f r u s t , and i f t h e r e a r e such areas i n t h e t r a c k s t h e r u s t mixes w i t h grease and a c t s as a l a p p i n g agent. Fig.57 shows c o r r o s i o n on a s p h e r i c a l r o l l e r b e a r i n g and i n t h i s case t h e grease was u n a b l e t o p r o t e c t t h e b e a r i n g f rom w ater.

E l e c t r o l y t i c a c t i o n due

t o t h e wa t e r has r e s u l t e d i n c o r r o s i o n a t t h e c o n t a c t s u r f a c e s between t h e r o l l e r s and r i n g s .

6.23.7

Passage o f E l e c t r i c Cu rren t Through Bearings

E l e c t r i c c u r r e n t p assi n g t hro ug h a r o l l i n g b e a r i n g causes damage t o t h e t r a c k s o r r o l l i n g elements, which can r e s u l t i n premature f a i l u r e o f t h e bearing. A l t e r n a t i n g and d i r e c t c u r r e n t have a s i m i l a r e f f e c t and where t h e r e i s a r i s k o f e l e c t r i c a l leakage, o f current.

r o l l i n g b e a r i n g s must be p r o t e c t e d a g a i n s t t h e passage

I f a c o n t i n u o u s c u r r e n t passes through a r o t a t i n g b e a r i n g a dark

c o l o u r e d f i l m i s produced on t h e t r a c k s and r o l l i n g elements which g r a d u a l l y develop i n t o a washboard f o r m a t i o n as shown i n Fig.58. The b a l l s i n b a l l b e a r i n g s s u b j e c t e d t o e l e c t r i c c u r r e n t do n o t u s u a l l y develop t h e washboard surf ace , b u t become u n i f o r m l y dark-coloured over the whole surface.

T h i s i s due t o t he b a l l s s p i n n i n g when t h e b e a r i n g r o t a t e s .

The

182

Fig.58

\,/ashboardEffect Caused by Continuous Passage of Electric Current.

rings have fluting similar to that on rollers, but the bands are much narrower. On applications such a s traction motors, craters and burns occur instead of the washboard formation and a typical burn is shown on Fig.59. Generally pitting from electrical leakage does not mean rapid failure and on railway applications, it is known that pitted bearings have run for very long periods before requiring replacing. The main problem is removing the debris released during the formation of the craters, and provided this is carried out and fresh lubricant added, preferably by means of a grease escape valve, the bearings should function successfully for a further period. There are many problems relating to ball and roller bearings, but providing the basic principles governing bearing selection and a knowledge of lubrication is developed, then the calculated nominal bearing life can be achieved.

183

Fig.59

Typical Burn Caused by E l e c t r i c Current Leakage i n A Traction Motor.

184

I-

4

PRACTICAL GEAR TRIBOLOGY

T.I.

FOWLE, C o n su l t a nt , Tenterden

7.1

INTRODUCTION The i d e n t i f i c a t i o n o f t h e causes o f t h e v a r i o u s forms o f d i s t r e s s appearing

on gear t e e t h i s seldom an easy m a t t e r because o f t h e i r g r e a t v a r i e t y and because few e n g i ne ers have t h e o p p o r t u n i t y t o see even a m i n o r i t y o f them a t f i r s t hand.

The t rou bd esh oo t i n g c h a r t s g i v e n a t t h e end o f & i s

chapter a r e

i n te n d e d t o s i m p l i f y t h e i d e n t i f i c a t i o n o f p o s s i b l e causes and t h e s e l e c t i o n o f a p p r o p r i a t e remedies f r o m t h e observed symptoms.

Systematic c o n s i d e r a t i o n o f

the v a r i o u s p o s s i b i l i t i e s should a t l e a s t narrow down t h e number and suggest t e s t s which m ig h t be a p p l i e d t o c o n f i r m t h e f i n a l i d e n t i f i c a t i o n .

The f o l l o w i n g

not e s a r e g i v e n t o a m p l i f y and e x p l a i n t h e reasons f o r t h e most i m p o r t a n t effects.

7.2

ALIGNMENT There a r e two a spe ct s o f gear a l i g n m e n t : e x t e r n a l and i n t e r n a l .

e x t e r n a l a lig n m e nt ,

t h a t i s a l i gn men t w i t h t h e connected

loads on t h e b e ari n gs and c o u p l i n g s ,

Errors i n

machines, p l a c e over-

r i s k i n g f a i l u r e o r a t l e a s t n o i s e which

c o u ld be e r r o n e o u s l y a t t r i b u t e d t o t h e gears themselves.

Errors i n internal

alig n m e n t cause uneven d i s t r i b u t i o n o f t h e l oad a l o n g t h e gear t e e t w i t h consequent r i s k o f damage and n o i s y r u n n i n g and a r e o f p a r t i c u l a r concern i n t h i s c h a p te r . With p a r a l l e l - s h a f t gears t h e r e a r e b a s i c a l l y two e r r o r s o f i n t e r n a l a l i g n ment:

t h e s h a f t s may n o t be p a r a l l e l , and they may n o t be i n t h e same p l a n e .

Bo t h these e r r o r s may, o f course, be p r e s e n t t o g e t h e r .

The v a r i o u s combinations

produce p a t t e r n s of c o n t a c t , o r o f damage such as p i t t i n g o r s c u f f i n g , as shown i n F i g u r e 1.

Only i n a few cases a r e gears p r o v i d e d w i t h means for a d j u s t i n g

t h e p a r a l l e l i s m o f t h e two s h a f t s , and indeed, w i t h modern machining i t i s u n l i k e l y t h a t t h e s h a f t s w i l l n o t be p a r a l l e l . eve n ly supported on i t s f e e t by t h e f ou nd at i ons,

However,

i f t h e gearcase i s n o t

t h e s h a f t s w i l l n o t be i n t h e

same p la n e . Checking t h a t t h e two s h a f t s a r e i n t h e same p l a n e i s b e s t c a r r i e d o u t by removing t h e t o p o f t h e gear c a s i n g and, o r b e a r i n g keeps.

i f necessary, t h e top halves o f bearings

A s t r a i g h t edge w i t h a spacer b l o c k t o a l l o w f o r any d i f f e r -

ence i n diameter s u p p o r t i n g a p r e c i s i o n s p i r i t l e v e l can then be placed across

185 b o t h ends o f t h e s h a f t s i n t u r n t o check t h a t t h e ends a r e i n t h e same plane. An accuracy o f 1 p e r 60 000 i s s a t i s f a c t o r y .

The t h i c k n e s s o f the shims r e q u i r e d

can then be e a s i l y determined.

Single helical and spur

€2

Double helical

Gear axes parailei but not

co-planar

Single helical and spur

€3

Q

Gear axes co-pianar but not parailei Single helical and spur

iii ozzl

Gear axes neither co-planar nor parallel

Fig.1

I n t e r n a l misalignment p a t t e r n s on p a r a l l e l s h a f t gears

I n t e r n a l a l i g n m e n t can a l s o be checked by d e t e r m i n i n g t h e e x t e n t o f t o o t h c o n t a c t w i t h marking b l u e .

A f t e r e n s u r i n g t h a t t h e j o u r n a l s cannot l i f t o u t o f

t h e i r b e a r i n g s and t h a t t h e j o u r n a l s a r e l u b r i c a t e d , a t h i n c o a t i n g o f marking b l u e i s a p p l i e d i n a t h i n a x i a l band on one o f the gears.

The gears a r e then

t u r n e d so t h a t t h e marking i s t r a n s f e r r e d o n t o t h e o t h e r gear. then be taken by means of a S e l l o t a p e impression. depends on t h e c o n d i t i o n s

A r e c o r d may

The degree o f c o n t a c t required

o f s e r v i c e as i n d i c a t e d i n F i g u r e 2 ( a ) t o (d) which

i s based on BS 1807 f o r t u r b i n e gears and s i m i l a r d r i v e s .

' S p l i t Marking' as

186 shown i n F i g u r e 2 ( e ) , which i s due t o t h e hob n o t h a v i n g been c o n c e n t r i c w i t h i t s mandrel,

i s u n d e s i r a b l e because i t p r e v e n t s t h e most f a v o u r a b l e p a r t o f t h e

tooth p r o f i l e ,

i.e.

t h a t w i t h t h e l e a s t s l i d i n g , from s u p p o r t i n g t h e l o a d and

transfers i t instead t o less favourable p a r t s .

25%

25%

50%

la)

T

I

I

I

I

(6)

I

I

I

I

Fig.2

Contact area

( a ) , (b) and ( c ) r e p r e s e n t t h e minimum c o n t a c t areas r e q u i r e d by BS 1 8 0 7 f o r t u r b i n e gears and s i m i l a r d r i v e s , c l a s s e s B, A 2 and A1 r e s p e c t i v e l y . The s p e c i f i c a t i o n does n o t s t i p u l a t e t h e d i s t r i b u t i o n o f t h e c o n t a c t areas, o n l y t h e i r depth and t o t a l l e n g t h . Classes A 2 and A1 a r e f o r gears w i t h p i t c h l i n e speeds o v e r 50 m / s , A 2 b e i n g suggested f o r 50-100 m/s and A1 f o r 50-150 m / s . ( d ) r e p r e s e n t s t h e requirements o f some a u t h o r i t i e s f o r p r e c i s i o n gears. ( e l S p l i t marking.

187 W i t h n o n - p a r a l l e l s h a f t gears,

such as b e v e l g e a r s and worm gears, a l l o w a n c e

has t o be made f o r t h e i n e v i t a b l e d i s t o r t i o n under l o a d , as shown i n F i g u r e s and 4.

3

I n a l l cases c o n t a c t s h o u l d n o t e x t e n d t o t h e ends o f t h e t e e t h as l o a d s

No load Fig.3

'

Full load C o n t a c t marks on b e v e l

gears

.+

No load

Full load

Fig.4

C o n t a c t marks on worm gears

t h e r e would be h i g h l y c o n c e n t r a t e d .

I n t h e case o f worm g e a r s t h e r e must a l s o

be c l e a r a n c e where t h e worm e n t e r s t h e c o n t a c t , even a t t h e h i g h e s t l o a d s , so t h a t t h e o i l on i t s s u r f a c e i s n o t scraped o f f by t h e edge o f t h e t o o t h .

I f

t h i s c l e a r a n c e i s n o t a v a i l a b l e t h e f r i c t i o n and wear a r e g r e a t l y i n c r e a s e d and t h e t r a n s m i t t e d power 1 i m i t e d .

7.3

TOOTH ACTION An u n d e r s t a n d i n g o f t o o t h a c t i o n h e l p s t o e x p l a i n many a s p e c t s o f g e a r t r i b o -

logy.

I n s p u r , h e l i c a l and b e v e l g e a r t o o t h a c t i o n i s as r e p r e s e n t e d i n F i g . 5 .

The p o i n t o f c o n t a c t moves c o n t i n u o u s l y o v e r b o t h t e e t h and i n t h i s sense t h e y r o l l o v e r one a n o t h e r . p r e s s u r e s t o be c a r r i e d .

The t r a n s i e n t n a t u r e o f t h e c o n t a c t e n a b l e s v e r y heavy

A t t h e p i t c h p o i n t b o t h t o o t h s u r f a c e s a r e moving i n

t h e same d i r e c t i o n a t t h e same speed without sliding.

and so m o m e n t a r i l y r o l l o v e r one a n o t h e r

A t a l l o t h e r p o i n t s t h e s u r f a c e s a r e moving a t d i f f e r e n t

speeds so t h a t t h e r e i s s l i d i n g as w e l l as r o l l i n g .

188

Opposing surface slides in these directions Line of action Dr

Fig.5

Contact c o n d i t i o n i n Spur, H e l i c a l and Bevel t e e t h . The h a t c h i n g i n d i c a t e s t h e d i r e c t i o n o f d i s t o r t i o n o f the surface layers.

On t h e s u r f a c e o f t h e d r i v i n g gear t h e s l i d i n g i s always away from t h e p i t c h l i n e , w h i l e on t h e d r i v e n t o o t h i t i s always towards t h e p i t c h l i n e . lubrication i s

When

inadequate t h e s l i d i n g shear f o r c e s thus tend t o draw m a t e r i a l

away from the p i t c h l i n e o f t h e d r i v e r and t o p i l e i t up a t t h e p i t c h l i n e o f the d r i v e n tooth.

T h i s produces t h e c o n d i t i o n known as g r o o v i n g and r i d g i n g

( r e f e r symptom 1 1 o f t h e T r o u b l e - s h o o t i n g c h a r t ) .

Another e f f e c t o f t h i s system

of s l i d i n g i s t h a t on t h e dedendum s u r f a c e o f b o t h gears any cracks a r e p u l l e d open i n advance o f t h e c o n t a c t . favourable d i r e c t i o n ,

The cra cks b e i n g a l s o i n c l i n e d i n t h e most

t h e o i l r e a d i l y e n t e r s t h e crack.

F u r t h e r movement o f

t h e c o n t a c t t h e n s e a l s t h e mouth o f t h e c r a c k and compresses t h e o i l i n i t t o exte n d t h e c r a c k s t i l l f u r t h e r .

I n c o n t r a s t , any cracks on t h e addendum surfaces

s l o p e away from t h e on-coming c o n t a c t which,

furthermore,

tends t o push t h e

s i d e s o f t h e c r a c k t o g e t h e r i n advance o f c o n t a c t so t h a t o i l i s n o t encouraged t o e n t e r t h e c r a ck.

The r e s u l t i s t h a t f a t i g u e p i t t i n g tends t o occur almost

e x c l u s i v e l y on t h e dedendum s u r f a c e o f gear t e e t h ,

b o t h d r i v e r and d r i v e n .

An i m p o r t a n t f e a t u r e o f spur and h e l i c a l gears i s t h a t t h e r e i s no s l i d i n g al o n g t h e l i n e o f c o n t a c t , whi ch i s a l s o v i r t u a l l y t h e case w i t h bevel gears. I n h y p o id gears and worm gears, however, t h e r e i s a c o n s i d e r a b l e component o f s l i d i n g a l o n g t h e l i n e o f c o n t a c t whi ch makes f o r g r e a t e r d i f f i c u l t y i n

189 lubrication,

because any a s p e r i t y on one s u r f a c e i s i n c o n t a c t w i t h t h e o t h e r

f o r a g r e a t e r t ime and d i s t a n c e .

I n worm gears p a r t i c u l a r l y ,

s i d e r a b l y in c r e a se s f r i c t i o n and t e mpe rat ure r i s e .

t h i s s l i d i n g con-

Much o f t h e f r i c t i o n , more-

over, m e r e ly serves t o d i s t o r t t h e wheel towards c r i t i c a l c o n d i t i o n s o f c o n t a c t (see symptom 20).

7.4

TOOTH SURFACE DISTRESS

O f t h e t h r e e main forms o f s u r f a c e d i s t r e s s : p i t t i n g , a b r a s i v e wear and s c u f f i n g o r a d h e si ve wear, t h e f i r s t tends t o o c c u r a t t h e lower end o f t h e speed range w h i l e s c u f f i n g tends t o o ccur a t t h e h i g h e r end as i n d i c a t e d i n Figure 6 .

S c u f f i n g may f o low p i t t i n g , b u t p i t t i n g does n o t occur where t h e r e

i s s c u f f i n g o r where t h e r e i s a b r a s i v e wear.

The i d e a l c o n d i t i o n o f f u l l - f i l m

o r elastohydrodynamic l u b r c a t i o n (EHL) occu rs a t low loads and h i g h speeds.

Bending fatigue failure, impact f radure

I

'tr

U

0

4

Partial E.H.L.

Speed F i g. 6

-

Zones o f gear t o o t h d i s t r e s s .

The d i f f e r e n t slopes o f l i m i t s f o r p i t t i n g i n h a r d and i n s o f t gears i l l u s t r a t e s e p a r a t e l y t h e two e f f e c t s o f speed on f i l m t h i c k n e s s and t o o t h e r r o r s .

7.4.1

Pitting

About t h e most common f orm o f s u r f a c e d i s t r e s s i n s o f t o r through-hardened gears i s p i t t i n g , wh i ch i s a t y p e o f f a t i g u e f a i l u r e . p i t t i n g i s r a t h e r unusual i n surface-hardened g e a r i n g .

As i n d i c a t e d i n F i g u r e Various sub-types and

6,

190 causes a r e l i s t e d a g a i n s t sympton 2, and an example i s shown i n F i g u r e 7. t h i s Figure the t y p i c a l oyster-shell

In

shaped c r a t e r s produced by t h e o i l p r i s i n g

t h e fragments l o o s e can be seen.

Fig.7

Surface f a t i g u e p i t t i n g .

Note c h a r a c t e r i s t i c o y s t e r - s h e l l shape on t h e l e f t . Note a l s o t h e e f f e c t s o f ' s p l i t m a r k i n g ' . (Photo c o u r t e s y o f S h e l l I n t e r n a t i o n a l Co.Ltd.) A s well as t h e s t r e n g t h o f t h e m a t e r i a l , t h e o i l f i l m t h i c k n e s s , o r r a t h e r t h e r a t i o o f t h e o r e t i c a l o i l f i l m t h i c k n e s s t o t h e s u r f a c e roughness o f t h e harder t o o t h , i s a most i m p o r t a n t f a c t o r , and t h i c k f i l m s o r f i n e f i n i s h e s o r b o t h a r e d e s i r a b l e t o reduce t h e i n c i d e n c e o f p i t t i n g . t o be avoided as much stresses.

Shock-loading i s a l s o

as p o s s i b l e as i t c o n s i d e r a b l y increases t h e s u r f a c e

T h i s i s i n d i c a t e d by t h e n e g a t i v e s l o p e o f t h e p i t t i n g l i m i t f o r s o f t

gears i n F i g u r e 6. P i t t i n g may be e i t h e r o f t h e i n i t i a l o r t h e p r o g r e s s i v e t y p e .

With the

former t h e removal o f a s p e r i t i e s and prominent areas by p i t t i n g increases t h e a c t u a l area o f c o n t a c t and reduces t h e s t r e s s e s s o t h a t i t a r r e s t s i t s e l f . W i t h overloaded s u r f a c e s , however, p i t t i n g c o n t i n u a l l y reduces t h e a r e a o f

191 a c t u a l c o n t a c t and promotes f u r t h e r p i t t i n g .

But i f one gear i s a b l e t o r e s i s t

p i t t i n g and can m a i n t a i n i t s shape, p i t t i n g w i l l progress less r a p i d l y on t h e o t h e r (see sympton 2 ( f ) ) . The s i z e o f t h e p i t s i s g e n e r a l l y r e l a t e d t o t h e s i z e

o f the gears, b u t there

may be wid e v a r i a t i o n s due, perhaps, t o s u r f a c e a s p e r i t i e s and o t h e r d e t a i l s . I n p a r t i c u l a r , where t h e o i l f i l m t h i c k n e s s i s r e l a t i v e l y v e r y t h i n t h e s u r f a c e t r a c t i o n can be v e r y h i g h and produce v e r y l a r g e s h a l l o w p i t s (see 2 ( j ) ) . The e f f e c t o f extreme p r e s s u r e (EP) a d d i t i v e o i l s i s v a r i e d .

I n laboratory

t e s t s u s i n g v e r y a c c u r a t e gears o r d i s c s , EP o i l s a r e g e n e r a l l y found t o a c c e l e r a t e p i t t i n g , b u t where p r a c t i c a l gears h a v i n g s u r f a c e u n d u l a t i o n s o r s i m i l a r i n a c c u r a c i e s a r e concerned, EP o i l s a r e o f t e n a b l e t o d e l a y o r suppress t h e problem.

A p p a r e nt l y,

a t l e a s t some EP ag en t s can prevent t h e c r e s t s o f such

u n d u l a t i o n s from work-hardening so t h e c o n t a c t zones can deform t o increase t h e i r area and thus reduce s t r e s s e s and temperatures.

7.4.2

Scuffing

Severe a d h e s ive wear t ake s seve ral d i f f e r e n t forms i n gear t e e t h according t o c o n d i t i o n s and i s g i v e n even more d i f f e r e n t names. s c o r i n g , g a l l i n g and p l u c k i n g .

For example, s c u f f i n g ,

The nomenclature i s even f u r t h e r confused by t h e

f a c t t h a t what i s c a l l e d s c u f f i n g i n t h e UK i s known as s c o r i n g i n t h e USA.

In

l a b o r a t o r y t e s t r i g s t h e v a r i o u s forms a r e n o t t o o d i f f i c u l t t o recognise and a r e reasonably w e l l d e f i n e d , as f o r example s c u f f i n g and s c o r i n g i n I P 166 f o r the I A E Gear Rig, b u t i n n on -st a nd ard i sed t e s t s d i s t i n c t i o n s a r e sometimes made between high-speed and low-speed

s c u f f i n g ( l e s s than about 4 m/s p i t c h l i n e

speed a c c o r d i n g t o some a u t h o r i t i e s ) and between s e l f - p r o p a g a t i n g and s e l f h e a l i n g forms i n b o t h cases. I n p r a c t i c a l gears appearances may v a r y t o an even g r e a t e r e x t e n t , b u t a r e d i f f e r e n c e s i n degree, n o t o f k i n d .

I n comparison w i t h l a b o r a t o r y t e s t gears i t

should be borne i n mind t h a t i n t h e l a t t e r a l a r g e p a r t o f t h e power i s used t o overcome t o o t h f r i c t i o n i n t h e t e s t gears so t h a t when l u b r i c a t i o n f a i l s and s c u f f i n g o c c u r s , t h e r e i s a c o n s i d e r a b l e i n c r e a s e i n t h e power r e q u i r e d .

There

i s thus an immediate f a l l i n speed wh i ch f u r t h e r increases f r i c t i o n and noise, as w e l l as p r o d u ci n g

smoke and sometimes sp arks.

Under these c o n d i t i o n s

s c u f f i n g o f t e n does n o t l a s t f o r more than a f r a c t i o n o f a minute and, FZG r i g , f o r example,

never more than 15 mi n utes.

i n the

I n p r a c t i c e , however, the

e x t r a power absorbed when gear t e e t h s c u f f i s o n l y a v e r y small f r a c t i o n o f t h e power b e in g t r a n s m i t t e d so t h a t t h e o nse t o f t h e damage i s u s u a l l y unnoticed and may c o n t i n u e f o r many hours.

Under such c o n d i t i o n s t h e v e r y severe type known

as ' g a l l i n g ' a r i s e s . Except p o s s i b l y a t low speeds,

s c u f f i n g appears t o be an e s s e n t i a l l y thermal

phenomenon due t o t o o t h s l i d i n g f r i c t i o n .

The speed of s l i d i n g depends mainly on

192

S c u f f e d p i n i o n t o o t h f r o m FZG t e s t . (Photo c o u r t e s y o f S h e l l I n t e r n a t i o n a l Co. L t d . )

Fig.8

t h e p e r i p h e r a l speed o f t h e gears and t h e s i z e o f t h e t e e t h .

And s i n c e l a r g e

t e e t h can be more h e a v i l y loaded than small t e e t h , t o o t h s i z e tends tohave a preponderant i n f l u e n c e on t h e i n c i d e n c e o f s c u f f i n g , as i n d i c a t e d i n Table 7.1 below, which summarises general e x p e r i e n c e , a l s o i n s e c t i o n 3 ( f ) . Table 7.1

Danger o f s c u f f i n g i n spur, h e l i c a l and bevel gears.

Module p i t c h (mm) 1.25

Danger o f s c u f f i n g None

2.5

Only a t v e r y h i g h speeds w i t h t h i n o i l .

5

A t moderate speeds even w i t h medium o i l .

10

A t low speeds even w i t h heavy o i l .

193 A s i n d i c a t e d i n t h e above T a b l e , and a l s o i n symptom 3 ( d ) , t h e h i g h e r t h e

v i s c o s i t y grade o f t h e o i l t h e g r e a t e r t h e p r o t e c t i o n i t a f f o r d s a g a i n s t s c u f f ing.

I f t h e grade i s t o o v i s c o u s ,

however, t h e r e c o u l d be d i f f i c u l t i e s i n

s t a r t i n g up f r o m c o l d and t h e power loss and t e m p e r a t u r e r i s e i n high-speed b e a r i n g s m i g h t be e x c e s s i v e .

Compromises have t o be made, t h e r e f o r e , and l i g h t e r

grades have t o be used f o r high-speed gears which,

i n any case, a r e more a c c u r -

a t e l y made and have s m a l l e r t e e t h and c o r r e s p o n d i n g l y s m a l l e r l o a d s .

Viscosity

grade recommendations f o r s p u r , h e l i c a l and bevel gears a r e , t h e r e f o r e , o f t e n g i v e n i n terms o f speed o n l y , as i n T a b l e 7.2 below, which i s f o r gears operat i n g a t ambient temperatures between about 10°C and 25OC. Table 7.2

O i l v i s c o s i t y grades f o r spur, h e l i c a l and bevel gears P i t c h l i n e speed m/s

Range o f I S 0 V i s c o s i t y Grades c S t a t 4OoC

ft/min

0.5

100

460

1.3

250

320

2.5

500

220

5

1000

150

12.5

2500

100

25

5000

68

10000

46

50

-

1000

680 460 320

220 150 100

H i g h e r v i s c o s i t y grades may be needed where t h e ambient t e m p e r a t u r e exceeds 2S0C, where t h e gears a r e s u b j e c t t o shock loads, o r where b o t h gears a r e made o f through-hardened n i c k e l - c h r o m e s t e e l s .

Lower v i s c o s i t y grades may be used

where t h e ambient t e m p e r a t u r e i s below 10°C,

o r where t h e t e e t h have been g i v e n

a scuff-resistant coating t o a s s i s t running-in. For d o u b l e - r e d u c t i o n g e a r i n g t h e low v i s c o s i t y end of t h e range f o r t h e low speed t r a i n s h o u l d be taken, and f o r m u l t i p l e t r a i n s t h e mean f o r t h e two slowest trains. Worm gears need t o be t r e a t e d r a t h e r more g e n e r o u s l y because t o o t h f r i c t i o n i s o f much g r e a t e r importance.

To m i n i m i s e t o o t h f r i c t i o n H V I o i l s a r e p r e -

f e r r e d t o o t h e r t y p e s and v i s c o s i t y grades a r e h i g h e r than f o r o t h e r types o f g e a r i n g , a s i n d i c a t e d i n T a b l e 7 . 3 below. For d e s i g n e r s , t h e r i s k o f s c u f f i n g can be assessed by t h e use o f B l o k ' s C r i t i c a l Contact Temperature t h e o r y o r t h e Niemann and S e i t z i n g e r b u l k t o o t h temperature c r i t e r i o n o f s t r a i g h t o r EP o i l s . a t i o n o f T a b l e 7.1

[l]and a d e c i s i o n made on whether t o recommend t h e use

For f i e l d use s i m p l e r guidance i s needed and c o n s i d e r -

i s recommended.

EP o i l s should,

f u r t h e r m o r e , be c o n s i d e r e d

f o r p a r a l l e l - s h a f t gears where t h e t o o t h c o n t a c t markings do n o t reach t h e e x t e n t

194 r e q u i r e d by t h e speed as i n d i c a t e d i n F i g u r e 2, where t h e gears a r e s u b j e c t t o dynamic o v e r lo a d s, and where t h e gears s t e p up t h e speed. T a bl e 7.3 IS0 o i l v i s c o s i t y grades r e q u i r e d f o r enclosed worm gears Output o r wormwheel r.p.m.

Centre d i s t a n c e ( in c h e s ) (mm) 2.5

800

50 and under

64

HVI 1000

HVI 460

150 and o v e r

HVI 320

4

100

460

320

320

10

250

3 20

220

220

20

500

320

2 20

220

EP o i l s s h o u ld g e n e r a l l y be used f o r h y p o i d gears because o f t h e i r h i g h com-

ponent o f s l i d i n g a l o n g t h e l i n e o f c o n t a c t , t h e i r l a r g e t e e t h and t h e i r l i a b i l i t y t o c o n s i d e r a b l e dynamic o verl o ad s

itl

a ut omotive s e r v i c e .

They should n o t ,

however, be used f o r worm gears u n l e s s t h e o i l temperature i s c o n s i s t e n t l y below about 6OoC because o f t h e r i s k o f e xcessi ve c o r r o s i v e wear o f t h e bronze. Since s c u f f i n g i s a thermal phenomenon, o v e r h e a t i n g can be a cause, and i t s courses and t h e a p p r o p r i a t e counter-measures d e t a i l e d under symptom 23 should be c o n s i dered.

7.4.3

A b r a s i v e Wear

There a r e t w o k i n d s of a b r a s i v e wear.

One, as i n symptom 4, where a rough,

hard s u r f a c e rubs a g a i n s t a s o f t e r one, which i s known as 'two-body'

abrasion;

t h e o t h e r , as i n symptom 5, where a b r a s i v e d i r t a c t s between two r u b b i n g surfaces, which i s known as ' t h r e e - b o d y '

a bra si on .

The f i r s t k i n d o ccurs where a rough surface-hardened p i n i o n runs a g a i n s t a s o f t s t e e l o r p l a s t i c wheel, and a l s o where a rough surface-hardened worm runs a g a i n s t a bronze wheel.

I n t h e l a t t e r case t h e a s s o c i a t e d h i g h f r i c t i o n may so

d i s t o r t t h e gears t h a t c o n t a c t i s b rou gh t o n t o t h e i n l e t edge o f t h e wheel t e e t h and f r i c t i o n f u r t h e r i n crea ses t o t h e e x t e n t t h a t power t r a n s m i s s i o n may be l i m i t e d (see symptom 20). smoother f i n i s h , e.g.

The remedy i s t o stone o r l a p t h e harder member t o a

0 . 5 t o 0.2 microns Ra.

T y p i c a l contaminants cau si ng three-body a b r a s i o n a r e sand and m i l l s c a l e . F i l t r a t i o n down t o 20 mi cro ns (nominal)

i s n o r m a l l y t h e best p r a c t i c a b l e s o l u t i o n ,

though s m a l l e r p a r t i c l e s can s t i l l cause a b r a s i o n .

The maximum amount o f conta-

minant t o l e r a b l e i n t h e o i l depends on i t s hardness r e l a t i v e t o t h a t o f t h e gears concerned.

For example,

0.3% w by D I N 51 592

,

i n steel m i l l practice a typical l i m i t f o r millscale i s

i . e.

r e t a i n e d on a 0.45 micron m i l l i p o r e f i l t e r ) .

c o r r e s p o n d in g l i m i t f o r sand would be 0.1% w.

The

Greases a r e p a r t i c u l a r l y l i a b l e

196 t o p e r m i t a b r a s i o n s i n c e t h e y t e n d t o keep t h e a b r a s i v e c o n t a m i n a n t s and t h e wear p r o d u c t s i n t h e v i c i n i t y o f t h e mesh.

Fig.9

Three-body a b r a s i o n o f a g e a r t o o t h by sand i n t h e o i l Note t h e sh o rt l e n g t h o f t h e scars. (Photo c o u r t e s y o f S h e l l I n t e r n a t i o n a l Co.Ltd.1

7.4.4

O t h e r forms o f gear wear

The o t h e r forms o f g e a r wear l i s t e d under symptoms 6 t o 19 a r e r e l a t i v e l y r a r e and space does n o t p e r m i t s p e c i a l d i s c u s s i o n h e r e .

7.5

PROBLEMS I N LUBRICATION SYSTEMS The main problems a r i s i n g i n s u p p l y i n g and c o n t r o l l i n g t h e f l o w o f o i l t o

and f r o m t h e t o o t h mesh a r e :

o v e r h e a t i n g , windage ( i n v e r y h i g h speed g e a r s ) ,

196 e x c e s s i v e foaming ( i n b a t h l u b r i c a t e d systems) and e x c e s s i v e a e r a t i o n ( i n l a r g e c i r c u l a t i o n systems). One f r e q u e n t source o f o v e r h e a t i n g i n b a t h systems i s when one gear d i p s t o o deeply i n t o t h e b a t h (see symptom 2 3 ( a ) ,

(b) and ( c ) ) .

T h i s can sometimes be

avoided by s u r r o u n d i n g t h e lower p a r t o f t h e d i p p i n g gear by a s p e c i a l t r o u g h . When t h e gear i s a t r e s t t h e t r o u g h f i l l s w i t h o i l , b u t d u r i n g o p e r a t i o n t h e excess i s thrown o u t and t h e meshing t e e t h r e c e i v e s u f f i c i e n t o i l f o r l u b r i c a t i o n and c o o l i n g from h o l e s i n t h e bottom o f t h e trough. Above about 15 m/s p i t c h l i n e speed even t h i s s p e c i a l f o r m o f b a t h l u b r i c a t i o n tends t o cause e x c e s s i v e power loss and temperature r i s e so t h a t spray l u b r i c a t i o n f r o m c i r c u l a t i o n systems i s n o r m a l l y adopted. i n t h e range 0.3 t o 1.5 b a r .

Pump p r e s s u r e s a r e u s u a l l y

Pressures below 0.3 b a r may be s a t i s f a c t o r y , b u t

whenever t h e gauges read below 1 b a r a v i s u a l check should be made t o ensure t h a t the o i l i s reaching t h e p a r t s required.

(Upper l i m i t s t o o i l p r e s s u r e may

be d i c t a t e d by t h e s a f e t y l i m i t s f o r pumps, c o o l e r s , f i l t e r s , e t c . ) .

A t speeds above about 50 m/s windage e f f e c t s become n o t i c e a b l e and a t around 100 m/s and o v e r t h e y need t o be s p e c i a l l y c a t e r e d for i n t h e d e s i g n by arrangi n g f o r p l e n t y o f space between t h e gears and t h e housing, by a r r a n g i n g comple t e l y s e p a r a t e o r even dual d r a i n l i n e s w i t h s h i e l d s , o t h e r w i s e t h e c a s i n g may become choked w i t h o i l . The l o c a t i o n o f t h e o i l s p r a y e r s needs s p e c i a l c o n s i d e r a t i o n where p i t c h l i n e speeds a r e above 50 m/s.

Above t h a t speed t h e sprayers should n o t be d i r e c t e d

s t r a i g h t i n t o t h e mesh, b u t a l i t t l e i n advance.

In t h i s way a l l t h e o i l

serves t o c o o l t h e gears, t h e excess above t h a t r e q u i r e d f o r l u b r i c a t i o n i s thrown o f f b e f o r e t h e gears mesh. 'down-going'

gears.

Above

T h i s arrangement i s e s p e c i a l l y r e q u i r e d f o r

a p p r o x i m a t e l y 75 m/s,

about 80-90% o f t h e f l o w

should be d i r e c t e d o n t o t h e o u t g o i n g s i d e f o r c o o l i n g , w i t h t h e remainder b e i n g sprayed s l i g h t l y i n advance o f mesh f o r l u b r i c a t i o n . Excessive foaming and a e r a t i o n o f t h e o i l i s due e i t h e r t o a d e t e r i o r a t i o n o f the properties o f t h e o i l from contamination o r t o excessive ingress o f a i r i n t o the o i l .

The former may be r e a d i l y checked by c a r r y i n g o u t t h e a p p r o p r i a t e

t e s t s on t h e o i l , e.g. 24(a),

I P 146 and I P 313, t h e l a t t e r by r e f e r e n c e t o symptom

(b) and ( c ) .

REFERENCES

1 Fowle, T. I . , Lubn. Engg.,

1976, 32 N O . 1,

17.

GEAR PROBLEMS : CAUSES AND REMEDIES Symptom

1. Broken t e e t h

P o s s i b l e Causes (a) F a t i g u e ( i ) Load unevenly d i s t r i b u t e d . ( i i ) Sharp c o r n e r i n t o o t h r o o t . ( i i i ) Notches i n t o o t h r o o t caused by improper f i l i n g o r g r i n d i n g . ( i v ) Overload. (v) Torsional v i b r a t i o n s . ( v i ) Bruises on t e e t h , e.g. caused by dropping. ( v i i ) Coarse m a r t e n s i t i c s t r u c t u r e i n hardened s t e e l . (viii)Blow-holes i n roots o f cast teeth. ( i x ) Surface f a t i g u e p i t s a d j a c e n t t o root o f tooth.

Remedies Temporary c o r r e c t i o n s may be made by c u t t i n g o u t broken t e e t h and cracked s e c t i o n s u n t i l o n l y sound t e e t h a r e l e f t , and r u n n i n g a t p r o p o r t i o n a l l y reduced load; by i n s e r t i n g pegs and b u i l d i n g up new t e e t h on them by w e l d i n g , f o l l o w e d by reshaping

.

Apart from e l i m i n a t i n g overloads t h e o n l y permanent c o r r e c t i o n i s , however, t o procure gears w i t h o u t t h e f a u l t s l i s t e d , t o have them c o r r e c t l y l i n e d - u p and run w i t h any shock loads l i m i t e d by shock absorbing c o u p l i n g s .

(b) F r a c t u r e ( i ) Repeated heavy loads. ( i i ) Hard f o r e i g n o b j e c t s jammed i n gear mesh. 2. Pitting (a) Small w i d e l y s c a t t e r e d p i t s on worki n g s u r f a c e s .

(a ) F a t i g u e a t s u r f a c e a s p e r i t i e s d u r i n g i n i t i a l running.

(a) None: t h e p i t t i n g w i l l cease t o spread and w i l l be o f no consequence.

(b) P i t s c o n ce nt rat ed a t one o f tooth o r helix.

(b) C o n c e n t r a t i o n o f load due t o s l i g h t mi sa l i gnment.

(b) Check alignment o f s h a f t s and c a s i n g f o r d i s t o r t i o n . The p i t t i n g may cease t o spread.

(c) Concentration o f p i t s i n r e g u l a r ba nd s a l o n g tooth width.

( c ) C o n c e n t r a t i o n o f load on s u r f a c e undulations.

( c ) The p i t t i n g may cease t o spread, b u t lapping o r stoning advisable i n severe cases.

Remed ies

P o s s i b l e Causes

SvmDtom 2. P i t t i n g ( c o n t d . ) ( d ) P i t s c o n ce nt rat ed a l on g pitch line.

(d ) ( i ) F a u l t y p r o f i l e . ( i i ) Excessive wear o r s c u f f i n g has produced a r i d g e a l o n g t h e p i t c h l i n e w hich becomes overloaded.

(d) ( i ) As above. ( i i ) Use o i l o f h i g h e r v i s c o s i t y o r o f h i g h e r EP a c t i v i t y .

( e ) P i t s c o n ce nt rat ed on dedendum su rf a ces o f one gear.

(e) ( i ) I n s u f f i c i e n t hardness o f s o f t e r gear o r excessive f r e e - f e r r i t e i n rnicros t r u c t u r e r e l a t i v e t o loading. ( i i ) E xcessive amounts o f hard nonm e t a l l i c inclusions i n the metal. ( i i i ) I n i t i a l s u r f a c e f i n i s h t o o rough and o i l v i s c o s i t y t o o low. ( i v ) Overload, e s p e c i a l l y by shock, t o r s i o n a l o s c i l l a t i o n s and h i g h s t a r t i n g torque. ( f ) I n s u f f i c i e n t hardness o f b o t h gears r e l a t i v e t o the loading.

(e) The p i t t i n g w i l l cease t o spread i f opposing gear u n a f f e c t e d . An i n c r e a s e i n o i l v i s c o s i t y c o u l d be b e n e f i c i a l

(g) P i t t i n g i n casehardened g ea rs.

(g) Teeth t o o s o f t e i t h e r from a c c i d e n t a l d e c a r b u r i s a t i o n o r inadequate quenching.

(9) Reduce load.

(h) M i c r o - p i t t i n g o r ' f r o s t i n g ' i n s u r f a c e hardened gears ( c l o s e l y spaced p i t s s m a l l e r t ha n 0.1mm across).

(h) ( i )

(h) ( i )

( f ) P i t s on dedendum s u r faces o f b o t h gears, often with d i s t i n c t step a t the p i t c h l i n e .

!ii)

N i t r i d e d gears: t h i n s u r f a c e l a y e r o f b r i t t l e super-rich n i t r i d e s (white l a y e r ) . Inadequate o i l f i l m .

F i t shock absorbing c o u p l i n g .

( f ) Reduce l o a d i n g i n s e r t shock absorbing t y p e o f coupling,between source o f shock and gear. I f possible, increase operating v i s c o s i t y o f o i l . Change t o EP o i l , m a i n l y t o p r e v e n t subsequent s c u f f i n g o f remaining c o n t a c t areas. I f p o s s i b l e , l a p o r stone t e e t h t o improve s u r f a c e and r u b i n d r y MoS2 powder.

A 25 micron l a y e r o r l e s s i s gene r a l l y harmless b u t removal o f t h i c k e r l a y e r s by l a p p i n g o r g r i n d i n g should be considered. ( i i ) Increase o p e r a t i n g v i s c o s i t y o f the o i l .

Symptom

P o s s i b l e Causes

Remed i es

2 . P i t t i n g (contd.) ( i ) Very l a r g e oyster-shaped p i t s e x te nd i ng ove r p r a c t i c a l l y t h e whole o f t h e a c t i v e f l a n k o f casehardened t e e t h .

3. S c u f f i n g

( i ) Hi gh t o o t h f r i c t i o n due t o d i r e c t c o n t a c t between t h e t e e t h ; t h e o r e t i c a l o i l f i l m thickness l e s s t han 0.5 micron.

( i ) Increase o p e r a t i n g v i s c o s i t y o f t h e o i l . NB. P i t t i n g o f s o f t and through-hardened s t e e l gears i s so common t h a t i t cannot be counted as a f a i l u r e . But, i f p o s s i b l e , i t i s prudent t o c o r r e c t as d e t a i l e d above. In some cases, a f r e s h s t a r t can be made by t u r n i n g round one o r b o t h gears and l a p p i n g them t o g e t h e r as a p p r o p r i a t e . I n severe cases where n o i s e i s e x c e s s i v e o r reduced areas o f c o n t a c t cannot be prevented from s c u f f i n g , o r t h e r e i s a danger o f s u r f a c e p i t t i n g i n i t i a t i n g t o o t h breakage, i t may be necessary t o r e p l a c e gears.

(a) Tooth l o a d i n g t o o h i g h .

(a) Reduce l o a d i n g

(b) I n s u f f ic ien t 1 ub r i c a t i o n ( i ) O i l - b a t h l e v e l t o o low. ( i i ) O i l sprayer nozzles choked.

(b) ( i )

( i i i ) Pump s u c t i o n s t r a i n e r choked. ( i v ) Bearings r o b o i l from sp rayer n o z z l e s . ( v ) Windage i n h i g h speed gears d e f l e c t s o i l spray. ( v i ) L u b r i c a n t channels a t low s t a r t i n g temperatures. ( v i i ) Clearance between gears and c a s i n g t o o s m a l l : o i l cannot f l o w back t o bath.

Raise o i l l e v e l so t h a t gear d i p s 1-3 t o o t h h e i g h t s when running. ( i i ) Check o i l nozzles, c l e a r as necessary, f i l t e r o r change o i l . ( i i i ) C h e c k s u c t i o n s t r a i n e r , c l e a n as necessary, f i l t e r o r change o i l . ( i v ) R e s t r i c t o i l supply t o bearings. Increase o i 1 pressure. Reposition sprayer nozzles. ( v i ) Use l u b r i c a n t w i t h lower channel p o i n t .

(v)

( v i i ) I f p o s s i b l e use lower v i s c o s i t y o i l . Increase clearance.

( c ) Ope rat ing temperatures excessive.

( c ) Reduce temperatures (see Symptom 23)

(d ) O i l v i s c o s i t y t o o low ( p a r t i c u l a r l y s o f t o r through-hardened gears).

(d) Use h i g h e r v i s c o s i t y grade;

improve c o o l i n g .

N

Symptom

3. S c u f f i n g ( c o n t d . )

0

(e) L u b r i c a n t has i n s u f f i c i e n t EP a c t i v i t y ( p a r t i c u l a r l y hardened g e a r s ) .

(e) Use more a c t i v e EP o i l .

( f ) T e e t h have e x c e s s i v e addendum h e i g h t f o r t h e speed.

( f ) Check d e s i g n . Use more a c t i v e EP o i l .

(9) Teeth do n o t have s u f f i c i e n t c o n t a c t because o f u n d u l a t i o n s , s p l i t markings o r m i s a l i g n m e n t .

(g) Reduce u n d u l a t i o n s by s t o n i n g , l a p p i n g o r shaving. Check a l i g n m e n t . Check t h a t c a s i n g has n o t d i s t o r t e d f r o m uneven s e t t l i n g o f f o u n d a t i o n s . Check t h a t b e a r i n g s a r e riot worn.

(h) T e e t h a r e n o t a d e q u a t e l y r e l i e v e d .

(h) A p p l y t i p - r e l i e f

( i ) F u l l - l o a d a p p l i e d b e f o r e gears have been a d e q u a t e l y r u n - i n .

( i ) Use a c t i v e EP o i l t o p r e v e n t f u r t h e r s c u f f i n g w h i l e c o n t i n u e d r u n n i n g makes s u r f a c e s smoother.

( j ) B o t h gears have h i g h n i c k e l - c h r o m i u m c o n t e n t and a r e n o t case-hardened.

4. Wear o f s o f t e r member

0

Remed i es

P o s s i b l e Cause

by s h a v i n g o r s t o n i n g .

(j) Use a c t i v e EP o i l o r h i g h e r v i s c o s i t y grade. NB. P r o v i d e d s c u f f i n g has n o t roughened t h e surfaces unduly, e s p e c i a l l y w i t h s p i r a l b e v e l and h e l i c a l g e a r s , once t h e b a s i c cause has been e l i m i n a t e d f u r t h e r r u n n i n g , p r e f e r a b l y w i t h an EP o i l , w i l l c o r r e c t t h e problem.

S u r f a c e f i n i s h o f h a r d e r member t o o coarse.

Stone o r l a p t e e t h t o a f i n e r f i n i s h .

A b r a s i v e d i r t i n l u b r i c a n t becoming embedded i n s o f t e r inember.

Change o i l o r pass i t t h r o u g h a f i n e f i l t e r . F i t f i l t e r s on a i r v e n t s .

6. Wear a t low speed.

Lubricant f i l m too t h i n .

Use h i g h e r v i s c o s i t y l u b r i c a n t .

7. Wear a t h i g h speed.

E x c e s s i v e f r i c t i o n caused by o v e r l o a d , overspeed, loss o f b a c k l a s h , o r f a u l t y lubrication.

Reduce o v e r l o a d s o r overspeed, and d i s t r i b u t i o n .

o n l y o f gear p a i r .

5. Wear o f h a r d e r member i s greater.

improve o i l f l o w

Symptom

Remed ies

P o s s i b l e Causes

8. Wear o f worm wheel t e e t h .

M a t e r i a l combination may be u n s u i t a b l e . (See a l s o No.20).

I f p o s s i b l e use case-hardened s t e e l worm and c e n t r i f u g a l l y cast phosphor bronze.

9 . T e e th t i p s rounded and

I n t e r f e r e n c e : gears n o t p r o p e r l y matched o r c e n t r e d i s t a n c e t o o s m a l l .

Check design. distance.

(a ) Combination o f s o f t m a t e r i a l and re pe at ed shock loads l e a d i n g t o separation o f t e e t h w i t h re-contact i n s u f f i c i e n t l y damped by o i l f i l m (peening)

(a) Reduce shock loads, use h i g h e r v i s c o s i t y o i l , reduce backlash, change t o harder gear m a t e r i a l .

(b) Very heavy steady loads and s o f t materials ( r o l l i n g ) .

(b) Reduce loads, i n c r e a s e gear s u r f a c e hardness.

11. Groove a l o n g p i t c h l i n e o f d r i v i n g t e e t h and r i d g e a lo n g p i t c h l i n e o f d r i v e n teeth.

When a s s o c i a t e d w i t h s c u f f i n g o f t h e r e s t o f t h e t e e t h i t may be due t o complete f a i l u r e o f t h e l u b r i c a n t su pp l y. When n o t , o i l f i l m may be t o o thin.

Check o i l supply, e.g. t h a t o i l b a t h l e v e l i s c o r r e c t when gears a r e running, t h a t o i l supply p i p e s , f i l t e r s and sprayer nozzles a r e n o t choked.

12. Grooves a l o n g p i t c h l i n e o f both d r i v i n g and d r i v e n teeth.

E r o s i o n by spark discharge.

E s t a b l i s h source o f s t r a y e l e c t r i c c u r r e n t s and lead t o e a r t h by (a) e a r t h i n g brushes o f generous s i z e , and (b) s t o p p i n g o t h e r p o s s i b l e paths through t h e gear mesh by i n s u l a t i n g pads under p e d e s t a l s and i n s u l a t i n g bushes f o r holding-down b o l t s .

13. Rippling.

E xce ssi ve s u r f a c e f r i c t i o n a t low speeds.

Use hypoid t y p e f u l l EP o i l .

14. B u lk p l a s t i c d e f o r m a t i o n o f

Very severe o v e r h e a t i n g due t o f a i l u r e o f o i l su pply.

Repair t e e t h o r renew gears. E l i m i n a t e cause o f o i l supply f a i l u r e . I n s t a l l alarms so t h a t u n i t can be stopped q u i c k l y i n event o f o i l supply f a i l u r e .

dedendum s u r f aces gouged.

10. P l a s t i c f l o w o f t o o t h s u r faces w i t h pronounced f i n at tips.

I f p o s s i b l e extend c e n t r e

.

t e e t h e s p e c i a l l y a t middle o f tooth width.

N

0

System

Remed i es

P o s s i b l e Causes

15. I n d e n t a t ions.

Hard p a r t i c l e s i n system, o f t e n s w a r f , o c c a s i o n a l l y from EP o i l carbonised on h i g h l y r a t e d h e a t e r s i n system.

Thoroughly c l e a n system, check h e a t e r surfaces, c l e a n and reduce s u r f a c e temperature as necessary .

16. Cracks i n s u r f a c e of

Overheating d u r i n g g r i n d i n g , h e a t t r e a t m e n t s , o r both.

incorrect

Check w i t h m a n u f a c t u r e r .

17. L o n g i t u d i n a l c r a c k i n g and f l a k i n g i n casehardened gears .

Case i s t o o t h i n and c o r e t o o s o f t so t h a t s u r f a c e has c o l l a p s e d under l o a d .

Check w i t h m a n u f a c t u r e r .

18. Red-brown spots on s u r -

A t t a c k by c o r r o s i v e substances such as s a l t s from hardening process.

Clean spots w i t h emery s t i c k . Clean, f l u s h and r e f i l l l u b r i c a t i o n system.

F r e t t i n g due t o v i b r a t i o n w h i l e under s t a t i o n a r y load.

Arrange f o r t h e gears t o be f l u s h e d w i t h o i l and s l o w l y r o t a t e d .

(a) I n s u f f i c i e n t allowance f o r d i s t o r t i o n under load has brought c o n t a c t o n t o e n t r y s i d e o f wheel t e e t h .

(a) A d j u s t p o s i t i o n o f wheel so t h a t even under f u l l l o a d c o n t a c t i s n o t on e n t r y s i d e o f wheel t e e t h and o i l can be drawn i n t o contact.

(b) Excessive t o o t h f r i c t i o n , due e i t h e r t o rough worm s u r f a c e o r t o u n s u i t a b l e combination o f gear m a t e r i a l s , causing e x c e s s i v e d i s t o r t i o n .

(b) Improve s u r f a c e f i n i s h o f worm, reduce o i l temperature o r use h i g h e r v i s c o s i t y grade. Best m a t e r i a l combination i s casehardened s t e e l worm and c e n t r i f u g a l l y c a s t phosphor bronze wheel.

hardened gears, o f t e n i n net-like pattern.

faces o f case-hardened gears.

19. Red-brown c o n t a c t marks on t e e t h and r e d - t - o w n powder i n n o m i n a l l y s t a t i o n a r y gears. 20. Worm gear f a i l s t o transmi t f u l l t o r q u e ,

N

Symptom

21.

22.

Vibration

Unusual n oi se .

P o s s i b l e Causes

Remed i es

(a) D e f e c t i v e b e a r i n g s o r coup1 i n g s .

(a) Check bearings and c o u p l i n g and r e p l a c e as necessary.

(b) S h a f t s m i s a l i g n e d .

(b) Check t h a t c a s i n g i s not d i s t o r t e d , r e a l i g n shafts.

( c ) Check and rebalance as necessary.

( c ) Check and rebalance as necessary.

(a) D e f e c t i v e r o l l i n g element bearing. (b) D e f e c t i v e o i l pump.

Check these p a r t s and r e p l a c e as necessary.

(c) Defective coupling.

23.

Overheating.

(d) Tooth s u r f a c e s e x c e s s i v e l y p i t t e d o r roughened.

(d) Stone o r l a p t e e t h and rub i n MoS2 powder.

(e) Continuous t o o t h double h e l i c a l gears r u n n i n g 'down-going' w i t h apex t r a i l i n g , s q u i r t i n g o i l o u t o f mesh.

(e) Reduce excessive o i l supply t o mesh by reducing immersion i n b a t h o r reducing f l o w t o sprayers and d i r e c t i n g spray i n advance o f mesh.

(a ) O i l l e v e l t o o h i g h i n bath.

(a) A d j u s t l e v e l when gear r u n n i n g t o d i p 1-3 t o o t h heights.

(b) O i l v i s c o s i t y too high.

(b) Change t o lower v i s c o s i t y grade.

( c ) Speed t o o h i g h f o r b a t h l u b r i c a t i o n .

( c ) Change t o spray l u b r i c a t i o n system.

(d) TOO much o i l sprayed t o o c l o s e t o i n g o i n g mesh o f high-speed gears.

(d) D i r e c t o i l spray f u r t h e r i n advance o f mesh; r e s t r i c t amount o f o i l t o sprayers.

(e)

(e) Improve drainage o r r e s t r i c t amount o f o i l t o sprayers.

Inadequate drainage from housing.

( f ) Clogged c o o l e r .

( f ) Check o i l and w ater s i d e s and c l e a n as necessary

(9) Cooler inadequate.

(9) Change c o o l e r f o r l a r g e r s i z e o r s w i t c h t o c o o l e r w ater supply.

(h) Heat r a d i a t e d from surroundings.

(h) I n t e r p o s e r a d i a t i o n s h i e l d s . PJ

0

w

tu Symptom

2 3 . Overheating (con t d . )

24. Excessive foaming and aeration.

Remed ies

P o s s i b l e Causes

( i ) Inadequate a i r f l o w o v e r gearbox.

( i ) Increase v e n t i l a t i o n o f surrounding a i r space.

( j ) D i r t accu mulations on casing.

( j ) Clean d i r t away.

( k ) (Worm g e a r s ) , u n s u i t a b l e l u b r i c a n t .

( k ) Change t o HVI m i n e r a l o i l o r , p r e f e r a b l y , p o l y g l y c o l type s y n t h e t i c o i l .

(1) E xce ssi ve power l o s s i n p l a i n bearings.

(1) Check b e a r i n g design.

(a ) Gear d i p s t o o deeply i n t o o i l bath.

(a) A d j u s t l e v e l when gear r u n n i n g t o d i p 1 - 3 tooth heights.

(b) A i r l e a k s on s u c t i o n s i d e o f c i r c u l a t i n g system.

(b) Remake s u c t i o n - s i d e j o i n t s i n c l u d i n g pump gland.

(c) O i l cascades down v e r t i c a l r e t u r n pipes i n t o r e s e r v o i r .

( c ) Rearrange r e t u r n 1 ines t o a1 low smooth f l o w i n t o tank below o i l l e v e l .

(d) O i l contaminated by grease, j o i n t i n g compound, another, and i n c o m p a t i b l e o i l etc.

(d) Renew o i l charge.

0 P

205

MATERIALS FOR TRIBOLOGICAL APPLICATIONS

D. SCOTT, Consultant,

8.1

E d i t o r o f Wear

I NTRODUCT I O N Engineering design i s t h e c r e a t i o n o f i n s t r u c t i o n s f o r making an a r t i c l e t o

s a t i s f y a s p e c i f i c requirement.

From a t r i b o l o g i c a l p o i n t o f view, t h e mat-

e r i a l s o f c o n s t r u c t i o n and the

u b r i c a n t a r e i m p o r t a n t f a c t o r s i n such s p e c i f -

ications.

For t r i b o l o g i c a l app i c a t i o n s t h e i m p o r t a n t p r o p e r t i e s of m a t e r i a l s

a r e those p r o p e r t i e s which must be taken i n t o account i n d e s i g n i n g a component t o w i t h s t a n d t h e mechanical and thermal s t r e s s e s t o which i t w i l l be exposed and the e f f e c t s o f t h e environment

n which i t has t o f u n c t i o n [ 1 , 2 ] .

There i s a

continuous demand f o r m a t e r i a l s o f improved p r o p e r t i e s and w i t h b e t t e r s t r e n g t h t o weight r a t i o s .

Mechanisms operaLing under arduous c o n d i t i o n s o f h i g h speed,

heavy l o a d o r extremes o f environment r e q u i r e m a t e r i a l s o f h i g h s t r e n g t h .

If

s u b j e c t e d t o r e l a t i v e m o t i o n they may r e q u i r e m a t e r i a l s o f g r e a t hardness, wear and c o r r o s i o n r e s i s t a n c e and s t r u c t u r a l and dimensional s t a b i l i t y

[31.

Newer

m a t e r i a l s [ 4 ] may meet s t r i n g e n t design requirements beyond t h e c a p a b i l i t i e s o f the more commonly used m a t e r i a l s b u t a v a i l a b i l i t y and c o s t make c o n v e n t i o n a l m a t e r i a l s more a t t r a c t i v e commercially and encourage i n n o v a t i o n . The d e s i g n e r has a v a s t range o f m a t e r i a l s f r o m which t o s e l e c t . erence book

[5] gives

35000 p r o p r i e t a r y m a t e r i a l c o m p o s i t i o n s .

however, besides s e a r c h i n g f o r

One r e f -

The d e s i g n e r

improved m a t e r i a l s must o f t e n seek t h e cheapest

m a t e r i a l t o s a t i s f y h i s requirements and sometimes t h e more r e a d i l y a v a i l a b l e indiginous materials.

A d e t e r m i n i n g f a c t o r i n t h e e x t e n s i v e use o f many mat-

e r i a l s i s t h e a m e n a b i l i t y o f t h e m a t e r i a l to m a n i p u l a t i o n and t h e e x t e n t t o which t h e d e s i g n e r can c o n t r o l and v a r y p r o p e r t i e s such as s t r e n g t h , hardness and d u c t i l i t y w i t h i n t h e range o f s p e c i f i c e n g i n e e r i n g requirements. o f m a t e r i a l s i s o f t e n r e s t r i c t e d by t h e m a n u f a c t u r i n g f a c i l i t i e s

8.2

dt

The c h o i c e h i s disposal.

TYPES OF MATERIALS M a t e r i a l s may be c o n v e n i e n t l y d i v i d e d i n t o f o u r p r i n c i p a l t y p e s : -

n o n - f e r r o u s , n o n - m e t a l l i c and composite m a t e r i a l s .

ferrous,

The abundance o f i r o n and

i t s a l l o y s c o m p r i s i n g t h e b u l k o f m e t a l s made, t h e i r f a v o u r a b l e economics and d i v e r s e p r o p e r t i e s make f e r r o u s m a t e r i a l s t h e d e s i r a b l e c h o i c e f o r t r i b o l o g i c a l applications.

Modern c a s t i r o n s and s t e e l s f i n d e x t e n s i v e use i n t r i b o -

206 e n g i n e e r i n g as a l l o y i n g and h e a t - t r e a t m e n t e nables them t o be t a i l o r e d t o specific applications.

The p r i n c i p a l methods o f s t r e n g t h e n i n g s t e e l i n c l u d e

work-hardening,

d ecre asi n g t h e g r a i n s i z e , s o l i d s o l u t i o n and d i s p e r s i o n

s t r e n g th e n in g .

I n c u r r e n t l y used s t e e l s ,

the martensite transformation pro-

duces th e b e s t c o mbi n at i on o f s t r e n g t h and d u c t i l i t y b u t as hardness increases, d u c t i l i t y decreases and a t t h e h i g h e s t s t r e n g t h l e v e l s produced by c o n v e n t i o n a l heat t r e a t m e n t procedures, d u c t i l i t y i s d i m i n i s h e d t o l e v e l s considered a t pre s e n t unacceptable f o r most e n g i n e e r i n g a p p l i c a t i o n s [ 6 ] .

I f a thermo-

mechanical t r e a t men t i s used whereby a u s t e n i t e i s s t r a i n hardened b e f o r e t r a n s f o r m a t i o n t o m a r t e n s i t e unusual d u c t i l i t y , f a t i g u e and impact p r o p e r t i e s a r e ob ta in e d .

Ausforming may t hu s a l l o w increased s t r e n g t h s above t h e p r e s e n t

usa b le l i m i t s w i t h o u t s a c r i f i c e of d u c t i l i t y

[7].

With m a t e r i a l s g e n e r a l l y , h i g h hardness i s u s u a l l y a s s o c i a t e d w i t h a h i g h melting point.

Powder m e t a l l u r g y has widened t h e f i e l d o f a v a i l a b l e hard

m e t a l l i c m a t e r i a l s by making p o s s i b l e metal combinations u n o b t a i n a b l e by conv e n t i o n a l m e l t i n g and c a s t i n g t ech ni q ue s.

Hard s i n t e r e d c a r b i d e s may be used t o

advantage i n many a p p l i c a t i o n s r e q u i r i n g a h i g h degree o f wear r e s i s t a n c e . However, such m a t e r i a l s a r e u s u a l l y expensive t o manufacture and d i f f i c u l t t o form and s u r f a c e t r e a t m e n t s and c o a t i n g s on o r d i n a r y m a t e r i a l s may be used t o i nc r e a s e s t r e n g t h and improve wear r e s i s t a n c e . For use a t e l e v a t e d temperatures m e t a l s must form a dense, tough,

impervious

o x i d e l a y e r which r e s i s t s c r a c k i n g under l o ad and p r e v e n t s a t t a c k o f t h e metal by h o s t i l e environments.

The e s t a b l i s h e d n o n - f e r r o u s metal a l l o y systems i n

c u r r e n t use a r e based on n i c k e l r i c h and c o b a l t r i c h a l l o y s .

I n b o t h cases t h e

necessary r e s i s t a n c e t o o x i d a t i o n and c o r r o s i o n i s c o n f e r r e d by the i n t r o d u c t i o n o f chromium.

The n i c k e l based a l l o y s a r e s t i f f e n e d p r i n c i p a l l y by the a d d i t i o n

o f t i t a n i u m and aluminium.

I n c o b a l t a l l o y s s t i f f e n i n g i s e f f e c t e d by complex

c a r b i d e s o f molybdenum, n i o b i u m and t an t a l um. High speed t o o l s t e e l s and s i m i l a r s p e c i a l s t e e l s a r e a l s o used f o r e l e v a t e d temperature s e r v i c e .

Corrosion resistance requires c a r e f u l m a t e r i a l s e l e c t i o n

and t h e use o f s t a i n l e s s s t e e l s , chromium,

n o n - f e r r o u s m e t a l s such as aluminium, n i c k e l ,

t i t a n i u m and t h e i r a l l o y s o r n o n - m e t a l l i c m a t e r i a l s such as p l a s t i c s

o r e la s to m e r s .

To r e s i s t severe a b r a s i v e wear, cemented c a r b i d e s , cermets or

even diamond may be r e q u i r e d . As c o n v e n t i o n a l m a t e r i a l s have been improved by o r t h o d o x methods almost t o t h e l i m i t o f t h e i r p o t e n t i a l mechanical p r o p e r t i e s , new types o f m a t e r i a l a r e bein g developed.

Composites whi ch combine m a t e r i a l s o f d i s s i m i l a r mechanical

and p h y s i c a l p r o p e r t i e s , can have p r o p e r t i e s s u p e r i o r t o one o r b o t h o f t h e i r constituents.

There a r e two p r i n c i p a l t ype s.

In one, a m a t r i x may be r e i n -

fo r c e d w i t h f i b r e s o r p a r t i c l e s t o improve i t s p r o p e r t i e s .

I n the o t h e r the

207 the m a t r i x i s e s s e n t i a l l y a g l u e t o h o l d t o g e t h e r f i b r e s o r p a r t i c l e s which have d e s i r a b l e p r o p e r t i e s b u t which by themselves cannot be used as e n g i n e e r i n g materials.

C e r t a i n d i f f i c u l t i e s r e q u i r e t o be surmounted b e f o r e composites

achieve t h e i r f u l l p o t e n t i a l .

Whiskers and f i b r e s a r e expensive and have prob-

lems w i t h s t r e s s c o n c e n t r a t i o n a t t h e i r ends which can i n f l u e n c e c r a c k initiation.

Conventional methods a r e n o t s u i t a b l e f o r t h e manufacture o f com-

ponents from composites n o r f o r t h e f o r m a t i o n and j o i n i n g o f f i b r e r e i n f o r c i n g materials.

By u s i n g r e i n f o r c e m e n t s o f o x i d e and n o n - m e t a l l i c w h i s k e r s which

approach t h e t h e o r e t i c a l s t r e n g t h , v e r y h i g h u l t i m a t e s t r e n g t h s i n composites are p o s s i b l e . metals.

Glass, carbon,

s i l i c o n n i t r i d e and alumina a r e a t t r a c t i v e non-

Besides r e p l a c i n g m e t a l s , ceramics may be used as c o a t i n g s t o comple-

ment d e s i r a b l e metal c h a r a c t e r i s t i c s by a d d i n g r e f r a c t o r y p r o p e r t i e s ,

insulation,

and e r o s i o n , wear, o x i d a t i o n and c o r r o s i o n r e s i s t a n c e . The s t r e n g t h e n i n g o f m e t a l s f o r use a t h i g h temperatures can be achieved by d i s p e r s i n g n o n - m e t a l l i c p a r t i c l e s i n them t o m a i n t a i n u s e f u l p r o p e r t i e s t o w i t h i n 5O-10O0C o f t h e m e l t i n g p o i n t o f t h e m a t r i x m e t a l .

Only small amounts o f

the d i s p e r s o i d a r e r e q u i r e d and n i c k e l a l l o y s w i t h t h o r i u m , TD n i c k e l , a r e commercially a v a i l a b l e .

Other newer m a t e r i a l s f o r arduous k o n d i t i o n s i n c l u d e

s y n t h e t i c diamond and sapphire, new g r a p h i t e s and m a t e r i a l s such as t h e carbides, borides and n i t r i d e s o f c e r t a i n m e t a l s which approach t h e hardness o f diamond.

8.3

MATERIALS FOR SPECIFIC APPLICATIONS

Adequate m a t e r i a l p r o p e r t i e s f o r design a r e u s u a l l y ensured by i n d i r e c t means mainly by t h e d e s i g n e r s p e c i f y i n g chemical a n a l y s i s , h e a t - t r e a t m e n t and mecha n i c a l p r o p e r t i e s a l t h o u g h such s p e c i f i e d p r o p e r t i e s may n o t be d i r e c t l y r e p r e s entative i n service.

For instance,

t h e most i m p o r t a n t m a t e r i a l p r o p e r t y may be

r e s i s t a n c e t o a b r a s i o n o r r e s i s t a n c e t o s c u f f i n g and s e i z u r e , o r t o r o l l i n g contact f a t i g u e o r t o l u b r i c a n t a t t a c k o r corrosion.

A p r o p e r t y such as

dimensional s t a b i l i t y may c o m p l e t e l y determine t h e s e r v i c e l i f e .

As t h e u l t -

imate assessment o f a m a t e r i a l i s performance i n p r a c t i c e , f u l l s c a l e t e s t i n g and s e r v i c e s i m u l a t i o n t e s t i n g a r e u s u a l l y r e s o r t e d t o as a means o f m a t e r i a l selection. T r i b o - e n g i n e e r i n g depends i n many i n s t a n c e s upon b e a r i n g s , components which a l l o w r e l a t i v e m o t i o n between members o f a mechanism w h i l s t t r a n s f e r r i n g load. Bearings may t a k e many forms b u t t h e most w i d e l y used types a r e p l a i n b e a r i n g s , gears and r o l l i n g b e a r i n g s .

8.3.1

P l a i n Bearings

I n p l a i n b e a r i n g s t h e l o a d i s t r a n s m i t t e d between moving p a r t s by s l i d i n g c o n t a c t and t h e c r i t e r i o n o f s a t i s f a c t o r y b e a r i n g performance i s minimum wear o f the components t o g e t h e r w i t h freedom from s e i z u r e and freedom f r o m mechanical

f a i l u r e by deformation o r f a t i g u e .

To c a r r y a h a r d s t e e l s h a f t , u s u a l l y spec-

i f i e d f o r i t s mechanical p r o p e r t i e s , a b e a r i n g m a t e r i a l must be c o m p a r a t i v e l y s o f t t o a v o i d wear o f t h e harder m a t e r i a l y e t s t r o n g enough t o w i t h s t a n d heavy loads w i t h o u t d i s t o r t i o n and w i t h o u t s u f f e r i n g f a t i g u e .

Soft bearing m a t e r i a l s

a l s o a l l o w a b r a s i v e p a r t i c l e s t o become embedded and thus reduce a b r a s i v e wear. As a low hardness i s u s u a l l y a s s o c i a t e d w i t h a low m e l t i n g p o i n t , h i g h s p o t s o f s o f t b e a r i n g s a r e removed by s l i d i n g c o n t a c t w i t h o u t damage t o t h e m a t i n g s u r face and w i t h o u t t h e r i s k o f s e i z u r e .

However, low hardness i s u s u a l l y assoc-

i a t e d w i t h low f a t i g u e s t r e n g t h and, as s t r e s s l e v e l s a r e r a i s e d , t h e demand i s f o r harder b e a r i n g m a t e r i a l s t o improve t h e l o a d c a r r y i n g c a p a c i t y b u t w i t h t h e minimum loss o f f r i c t i o n and wear p r o p e r t i e s .

As a general r u l e i t i s a d v i s a b l e

t o use t h e s o f t e s t b e a r i n g m a t e r i a l p o s s i b l e . White m e t a l , a w i d e l y used p l a i n b e a r i n g m a t e r i a l i s based on t i n o r l e a d o r t h e i r intermediate a l l o y s .

A t y p i c a l t i n based a l l o y c o n t a i n s 7-10% Sb and

3-5% Cu, t h e p r i n c i p a l c o n s t i t u e n t s b e i n g SbSn, CueSns and a t e r n a r y p e r i t e c t i c complex, Fig.1. composition [ 8 ] .

Hardness and mechanical p r o p e r t i e s a r e l i t t l e a f f e c t e d by A t lOO"C,

t h e hardness ranges f r o m 11-16 HV and t h e f a t i g u e

s t r e n g t h f o r l o 7 c y c l e s from 1.6-1.9

MN/m2.

Lead based a l l o y s c o n t a i n i n g Sb and Sn and Cu i n t h e f o r m o f i n t e r m e t a l l i c SbSn and CusSns may be cheaper than s i m i l a r t i n based a l l o y s b u t a r e s l i g h t l y i n f e r i o r r e g a r d i n g wear and f a t i g u e p r o p e r t i e s .

Intermediate a l l o y s o f h i g h

lead and t i n c o n t e n t a r e w i d e l y used b u t appear t o have no advantages o v e r t h e o t h e r w h i t e metals.

The success o f w h i t e m e t a l s i s g e n e r a l l y regarded as b e i n g

due t o t h e c o r r e c t compromise between s o f t n e s s t o a v o i d wear and s t r e n g t h t o resist fatigue. Copper-based a l l o y s , s t r o n g e r b e a r i n g m a t e r i a l s than t h e w h i t e m e t a l s a t o p e r a t i n g temperatures,

range from t h e phosphor bronzes (10% Sn, 0.5% P) through

the leaded bronzes t o t h e copper l e a d a l l o y s o f up t o 50% Pb, Fig.2. p r o p e r t i e s o f leaded bronze a r e b e t t e r than those o f w h i t e m e t a l . o f copper-lead v a r i e s a c c o r d i n g t o composition from 30-70 H.V.

The wear

The hardness

J o u r n a l wear

increases w i t h increase i n hardness b u t f a t i g u e s t r e n g t h increases r o u g h l y i n t h e same p r o p o r t i o n as j o u r n a l wear. minimize wear.

Increased j o u r n a l hardness can h e l p t o

A d i s a i v a n t a g e o f copper-lead a l l o y s i s t h e i r s u s c e p t i b i l i t y t o

l u b r i c a n t c o r r o s i o n of t h e l e a d phase. A compromise between a l l o y s s o f t enough t o a v o i d wear, those h a r d enough t o

r e s i s t f a t i g u e and those a b l e t o r e s i s t c o r r o s i o n has e v o l v e d by t h e use o f o v e r l a y b e a r i n g s i n which a s t r o n g m e t a l , such as copper-based m e t a l , has a s o f t metal o v e r l a y .

For economic reasons, t h e c o p p e r - l e a d may be used as an

i n t e r l a y between a s t e e l base and t h e o v e r l a y ,

Fig.3.

To a v o i d f a t i g u e under

the a p p l i e d loads, t h e o v e r l a y i s u s u a l l y t h i n n e r than 5 urn.

The o v e r l a y p l a t e d

209

Fig.1

(x75) S t r u c t u r e o f g r a v i t y c a s t t i n based w h i t e m e t a l .

Fig.2

(x75) S t r u c t u r e o f copper-lead on s t e e l base.

copper-lead b e a r i n g i s w i d e l y used f o r h i g h d u t y e n g i n e b e a r i n g s b u t t h e continuous search i s f o r s u p e r i o r r e a d i l y a v a i l a b l e m a t e r i a l s .

Fig.3

(x75) S o f t o v e r l a y on copper lead bearing.

Fig.4

(x120) S t r u c t u r e o f aluminium 20% t i n b e a r i n g .

210 Aluminium, a c o m p a r a t i v e l y cheap m a t e r i a l i n abundant supply, has met w i t h some success by t h e c o n v e n t i o n a l approach o f u s i n g hard metal compounds i n the aluminium m a t r i x t o produce a s t r u c t u r e analogous t o t h a t of w h i t e m e t a l .

The

use o f a n o t h e r s o f t met al , t i n , w i t h a l umi n i um has produced good r e s u l t s .

By

s u i t a b l e c o l d wo rki ng f o l l o w e d by h e a t - t r e a t m e n t and r e c r y s t a l l i z a t i o n the d i s t r i b u t i o n o f about 20% t i n produces a r e t i c u l a r s t r u c t u r e , Fig.4, adequate b e a r i n g p r o p e r t i e s .

with

The a d d i t i o n o f a small amount o f hardener such as

copper i s b e n e f i c i a l and seems t o o f f e r t h e b e s t combination of l o a d c a r r y i n g c a p a c i t y , wear and c o r r o s i o n r e s i s t i n g p r o p e r t i e s c u r r e n t l y a v a i l a b l e . o v e r l a y s f o r aluminium b e a r i n g s a i d r u n n i n g - i n , r e d u c t i o n o f t i n c o n t e n t t o about 9%.

Suitable

minimise j o u r n a l wear and a l l o w

S i m i l a r aluminium-lead b e a r i n g s e f f e c t

f u r t h e r economies. For j o u r n a l s , t h e cheapest p o s s i b l e m a t e r i a l i s u s u a l l y chosen. i s used f o r l e s s arduous a p p l i c a t i o n s ,

Mild steel

and p l a i n carbon s t e e l s can be heat-

.

t r e a t e d t o meet most c o n v e n t i o n a l a p p l i c a t i o n s , t h e p r o p e r t i e s improving w i t h i n c r e a s e o f carbon c o n t e n t .

Medium-carbon s t e e l s used f o r s m a l l e r s i z e s o f

j o u r n a l s and e n gi n e c r a n k s h a f t s have low h a r d e n a b i l i t y .

For more massive p a r t s ,

which a r e r e q u i r e d i n t h e hardened and tempered c o n d i t i o n , l o w - a l l o y s t e e l s a r e needed t o e n a b le s a t i s f a c t o r y p r o p e r t i e s t o be obtained.

Depending upon t h e

s p e c i f i c p r o p e r t i e s r e q u i r e d , manganese, n i c k e l , chromium, molybdenum and vanadium,

used s e p a r a t e l y o r i n v a r i o u s co mbinations, p r o v i d e a wide range o f

m a t e r i a l s f o r h i g h l y s t r e s s e d t r a n s m i s s i o n components and t h e more s t r i n g e n t applications.

Surface-hardening

t ech ni q ue s, such as c a r b u r i z i n g and n i t r i d i n g ,

a r e b e n e f i c i a l f o r p r o v i d i n g an e x t r e m e l y hard, w e a r - r e s i s t a n t s u r f a c e u s u a l l y on s p e c i a l l y manufactured low-carbon manganese o r tough a l l o y s t e e l s c o n t a i n i n g small amounts o f manganese, chromium, n i c k e l and molybdenum.

Nickel steels are

p a r t i c u l a r l y s u i t a b l e f o r case h ard en i ng a s such s t e e l s p r o v i d e a strong, wear-resistant

tough,

case w i t h a d u c t i l e c o r e w h i l s t t h e h a r d e s t n i t r i d e d cases a r e

o b t a i n e d w i t h a lu mi ni u m-con t a i ni n g

steels.

C o m p a t a b i l i t y o f m e t a l s i n s l i d i n g c o n t a c t i s a dominant f a c t o r i n b e a r i n g performance and b o t h t h e b e a r i n g m a t e r i a l and t h e h a r d e r s h a f t r e q u i r e c a r e f u l selection.

M a t e r i a l s f o r h i g h t emp era t u re b e a r i n g s and s l i d i n g a p p l i c a t i o n s

have been surveyed [ 9 ] .

8.3.2

Gears

Gears i n s e r v i c e a r e s u b j e c t e d t o r o l l i n g , s l i d i n g , a b r a s i v e , chemical, v i b r a t o r y and s h ock-l o ad i ng a c t i o n . scuffing,

pitting, fretting,

T h e i r u s e f u l l i f e may be t e r m i n a t e d by

a b r a s i o n , c o r r o s i o n and f r a c t u r e .

must be chosen t o r e s i s t t he se phenomena. a r e t h e carbon-manganese s t e e l s ;

Gear m a t e r i a l s

The most e x t e n s i v e l y used gear s t e e l s

manganese c o n t r i b u t e s markedly t o s t r e n g t h

211 and hardness b u t i t s e f f e c t depends upon t h e carbon c o n t e n t . hardenability, strength.

I t a l s o enhances

and f i n e - g r a i n e d manganese s t e e l s a t t a i n unusual toughness and

For more s t r i n g e n t gear a p p l i c a t i o n s , a l l o y s t e e l s , h e a t - t r e a t e d t o

p r o v i d e t h e optimum p r o p e r t i e s , a r e used.

Nickel provides solid-solution

s t r e n g t h e n i n g and i ncre ase s toughness and r e s i s t a n c e t o impact, p a r t i c u l a r l y a t low temperatures,

lessens d i s t o r t i o n i n quenching,

ance and a l l o w s more l a t i t u d e i n h ea t t rea t ment.

improves c o r r o s i o n r e s i s t Chromium increases harden-

a b i l i t y and has a s t r o n g tendency t o f orm s t a b l e c a r b i d e s which hamper g r a i n growth and p r o v i d e f i n e - g r a i n e d ,

tough s t e e l s .

Vanadium forms s t a b l e c a r b i d e s

t h a t do n o t r e a d i l y go i n t o s o l u t i o n and which a r e n o t prone t o agglomeration by tempering.

I t i n h i b i t s g r a i n growth, t h u s i m p a r t i n g s t r e n g t h and toughness Molybdenum and vanadium a r e g e n e r a l l y used i n combin-

t o heat-treated steels.

a t i o n w i t h o t h e r a l l o y i n g elements. f a s t e r machining r a t e s ,

Lead may be added t o gear s t e e l s t o a t t a i n

i n crea sed p r o d u c t i o n and l o n g e r t o o l l i f e .

S u r fa c e h a r d e ni n g t o reduce wear i s e x t e n s i v e l y a p p l i e d t o gear s t e e l s without s a c r i f i c i n g desirable core properties. l i q u i d , gas o r pack c a r b u r i z e d . inium-containing steels.

Carbon and a l l o y s t e e l s can be

N i t r i d i n g i s u s u a l l y a p p l i e d t o s p e c i a l alum-

Flame and i n d u c t i o n hardening methods a r e a l s o used.

Other s u r f a c e t r e a t m e n t s such as S u l ph i nu z, phosphating and s o f t n i t r i d i n g which reduce f r i c t i o n and a i d l u b r i c a t i o n , can be b e n e f i c i a l .

8.3.3

R o l l i n g Be ari n gs

A lth o u g h b a l l and r o l l e r b e a r i n g s a r e b a s i c a l l y r o l l i n g elements,

i n oper-

a t i n g mechanisms t h e y a r e a l s o s u b j e c t e d t o some wear by s l i d i n g and t o chemical a t t a c k by l u b r i c a n t and environment.

Their useful l i f e i s usually

l i m i t e d by s u r f a c e d i s i n t e g r a t i o n , p i t s b e i n g formed by a f a t i g u e process dependent upon t h e p r o p e r t i e s o f t h e m a t e r i a l , t h e n a t u r e o f t h e l u b r i c a n t and the environment [10,11,12].

The p r i n c i p a l q u a l i t i e s o f b a l l - b e a r i n g m a t e r i a l s

a r e dimensional s t a b i l i t y , h i g h hardness t o r e s i s t wear,

high e l a s t i c l i m i t

t o a v o i d p l a s t i c d e f o r m a t i o n under load, and good f a t i g u e r e s i s t a n c e t o contend w i t h h i g h a l t e r n a t i n g stresses.

A h i gh -carb on s t e e l s a t i s f i e s these r e q u i r e -

ments i f a c a r b i d e - f o r m i n g element i s i n c o r p o r a t e d t o increase hardness, g i v e h a r d e n a b i l i t y and a l l o w o i l quenching t o mi n imize d i s t o r t i o n d u r i n g heattre a t m e n t.

EN 31, 535A99 o r SEA 52100 (1.0% C, 1.5% Cr) through hardening

s t e e l i s used f o r c o n v e n t i o n a l b e a r i n g s , Fig.5.

Vacuum degassed, vacuum r e -

melted a i d e l e c t r o - s l a g r e f i n e d m a t e r i a l o f improved mechanical p r o p e r t i e s may be used f o r in c r e ase d r o l l i n g c o n t a c t f a t i g u e r e s i s t a n c e .

For convenience,

i n t h e manufacture o f t h e l a r g e r s i z e s o f r o l l e r b e a r i n g s case-hardening s t e e l s c o n t a i n i n g chromium, n i c k e l and molybdenum a c c o r d i n g t o t h e degree o f hardena b i l i t y , shock r e s i s t a n c e and c o r e hardness r e q u i r e d a r e used.

For use i n a

212 c o r r o s i v e environment m a r t e n s i t i c s t a i n l e s s s t e e l s a r e used w i t h some loss of f a t i g u e resistance. For use a t e l e v a t e d temperatures,

conventional r o l l i n g bearing s t e e l s a r e not

s a t i s f a c t o r y owing t o loss o f hardness and f a t i g u e r e s i s t a n c e and h i g h speed t o o l s t e e l s w i t h h i g h tempering temperatures a r e used, Fig.6.

H igh speed t o o l

s t e e l s c o n t a i n i n g p r i n c i p a l l y t un gst en , molybdenum and vanadium a r e a l s o l e s s prone t o d e l e t e r i o u s l u b r i c a n t e f f e c t s t h an EN 31 s t e e l [ l l ] .

M a t e r i a l com-

b i n a t i o n and m a t e r i a l l u b r i c a n t co mbi n at i on a r e i m p o r t a n t t o ensure adequate r o l l i n g c o n t a c t f a t i g u e l i f e [13,14,15].

Fi g. 5

( ~ 4 0 0 0 ) S t r u c t u r e o f EN 31 b a l l bearing s t e e l .

F i g. 6 ( ~ 6 , 5 0 0 ) S t r u c t u r e o f 18%W h i g h speed t o o l s t e e l .

Under c o n d i t i o n s o f u n l u b r i c a t e d r o l l i n g c o n t a c t , f a i l u r e occurs n o t by t h e usu a l f a t i g u e mechanism b u t by exce ssi ve wear l i m i t i n g u s e f u l l i f e , owing t o v i b r a t i o n and rough, n o i s y run ni n g.

Va ri ou s s u p e r a l l o y s w i t h base composition

o f chromium, molybdenum and c o b a l t and c o n t a i n i n g s i g n i f i c a n t amounts o f n i c k e l , tu n g s t e n , vanadium and o t h e r a l l o y i n g elements, a l s o cermets and ceramics, a r e p o t e n t i a l l y s u i t a b l e and have been used under arduous t e s t conditions.

Under c e r t a i n t e s t c o n d i t i o n s

1131,

tungsten c a r b i d e was t h e b e s t o f

t h e m a t e r i a l s t r i e d , g i v i n g t h e l owest wear r a t e and being r e l a t i v e l y u n a f f e c t e d by temperature.

The b e s t r e s u l t s were o b t a i n e d w i t h t h e s m a l l e s t c a r b i d e s i z e

and t h e lo w e s t percentage o f m a t r i x m a t e r i a l , Fig.7. r i d e was a l s o s u i t a b l e .

Hot pressed s i l i c o n n i t -

213

F i n e grained Fig.7

8.3.4

Coarse grained

( ~ 5 0 0 )S t r u c t u r e o f 6% Co tungsten c a r b i d e .

Wear R e s i s t a n t M a t e r i a l s

Wear r e s i s t a n t m a t e r i a l s r e q u i r e t h e c o r r e c t combination o f hardness t o r e s i s t a b r a s i o n and d u c t i l i t y t o contend w i t h shock loads and c y c l i c s t r e s s i n g .

AS these a r e c o n f l i c t i n g requirements, s u i t a b l e m a t e r i a l s e l e c t i o n i n v o l v e s compromise.

Other f a c t o r s such as t h e types o f wear,

t h e n a t u r e o f any

abrasive, t h e o p e r a t i n g temperature and t h e environment a f f e c t t h e c h o i c e o f material. For c o n d i t i o n s o f h i g h s t r e s s and impact, t h e toughness and work hardening

p r o p e r t i e s o f a u s t e n i t i c manganese s t e e l a r e u s u a l l y r e q u i r e d .

For lower s t r e s s

s l i d i n g c o n d i t i o n s where toughness i s l e s s i m p o r t a n t , depending upon t h e c o s t f a c t o r , hardened s t e e l s , a l l o y c a s t i r o n s , h a r d f a c i n g m a t e r i a l s , c o n c r e t e o r ceramics may be used.

Where maximum wear r e s i s t a n c e i s r e q u i r e d and c o s t i s

immaterial, cemented c a r b i d e s may be used.

Corrosion resistance requires the

use of s t a i n l e s s m e t a l s , s u i t a b l e rubbers o r p l a s t i c s .

Suitably reinforced

polymer m a t e r i a l may be used where a low c o e f f i c i e n t o f f r i c t i o n as w e l l as wear resistance i s required.

High chromium a l l o y s o f i r o n and s t e e l o f f e r t h e b e s t

wear r e s i s t a n c e t o e l e v a t e d temperature problems o f c r a c k i n g , s p a l l i n g and thermal shock. The wear r e s i s t a n c e o f a metal v a r i e s w i t h d i f f e r e n t a b r a s i v e s and t h e e f f e c t i v e hardness o f an a b r a s i v e has been d e f i n e d as t h e maximum v a l u e o f hardness o f a metal t h a t can be abraded by i t .

T h i s i s o f importance i n m a t e r i a l

s e l e c t i o n where a known a b r a s i v e i s t o be encountered i n p r a c t i c e . o f importance i n a b r a s i v e s e l e c t i o n f o r m a t e r i a l removal.

It i s also

Next t o diamond, t h e

hardest a b r a s i v e a v a i l a b l e , s i l i c o n c a r b i d e i s t h e most i m p o r t a n t a b r a s i v e f o r lapping and g r i n d i n g and f o r g r i n d i n g wheels.

Diamond can be s y n t h e s i z e d from

214 carbon and i n a s i m i l a r manner, c u b i c c r y s t a l l i n e boron n i t r i d e o f s i m i l a r hardness can be produced.

I t s extreme hardness and d u r a b i l i t y make t h i s mat-

e r i a l o f economic s i g n i f i c a n c e even though t h e p r e s e n t i n i t i a l c o s t i s h i g h .

8.3.5

To o ls

The e v o l u t i o n o f modern p r o d u c t i o n methods has been l a r g e l y dependent on the development o f tool s t e e l s cap ab l e of o p e r a t i n g e f f i c i e n t l y under i n c r e a s i n g High hardness i s a r e q u i s i t e o f almost every t o o l s t e e l

arduous c o n d i t i o n s .

b u t t h e optimum hardness depends upon t h e a p p l i c a t i o n and, as hardness and toughness v a r y i n v e r s e l y , maximum hardness can o n l y be used i n t h e absence of shock l o a d i n g .

Hardness a t e l e v a t e d t e mpe rature i s sometimes e s s e n t i a l as w e l l

as r e s i s t a n c e t o a b r a s i o n and d i s t o r t i o n .

The c h o i c e o f s t e e l i s u s u a l l y based

on a c o n s i d e r a t i o n o f t h e r e l a t i v e importance of t h e p r o p e r t i e s r e q u i r e d .

As

no s i n g l e s t e e l possesses a l l d e s i r a b l e p r o p e r t i e s , a compromise i s g e n e r a l l y necessary.

Economic a spe ct s can r a r e l y be ignored.

The s i m p l e s t t o o l s t e e l s a r e p l a i n carbon (0.6 t o 1.4% C) types, which by wa t e r quenching, d eve l op a h a r d case.

Low a l l o y v a r i e t i e s c o n t a i n up t o 0.25%

V f o r c a r b i d e s t a b i l i z a t i o n and t o r e t a r d g r a i n growth, and up t o 0.5% C r t o i n c r e a s e h a r d e n a b i l i t y and case depth.

O i l - h a r d e n i n g manganese s t e e l s (1.0 t o

2.0% Mn) p r o v i d e h i g h hardness w i t h minimum d i s t o r t i o n .

Chromium may be added

t o h e l p h a r d e n a b i l i t y w h i l s t a d d i t i o n s of t u ngsten and vanadium improve wear r e s i s t a n c e and c o n t r o l g r a i n growth. high-carbon

(2.0% C),

For t he more arduous a p p l i c a t i o n s t h e

high-chromium (14% C r )

s t e e l s a r e used t o r e s i s t wear and

c o r r o s i o n and t o m a i n t a i n h i g h d i men si on al s t a b i l i t y .

They m a i n t a i n a good

c u t t i n g edge f o r c o l d b l a n k i n g t o o l s .

I f shock r e s i s t a n c e i s r e q u i r e d , s t e e l s o f lower carbon c o n t e n t (0.4

to

0.5% C) a r e u s u a l l y r e q u i r e d and a l l o y i n g elements such as s i l i c o n , tungsten, vanadium and chromium may be added t o improve a b r a s i o n r e s i s t a n c e .

For h o t -

w o r k in g , high-speed t o o l o r h o t d i e s t e e l s which r e t a i n h i g h hardness and a good c u t t i n g edge a t e l e v a t e d temperatures a r e needed. t y p e i s t h e 18-4-1

( t u n g s t e n , chromium, vanadium)

The most w i d e l y used

type and h o t hardness may be

increased by r a i s i n g t h e vanadium c o n t e n t o r by a d d i t i o n o f c o b a l t .

It i s

p o s s i b l e t o p a r t l y r e p l a c e t u ng st e n w i t h molybdenum as i s t h e case w i t h , f o r example, M.2,

6.5.4.2

(W,

Mo, C r ,

V) s t e e l .

Tungsten, chromium and molybdenum

chromium h o t d i e s t e e l s may be regarded as l e a n type high-speed t o o l s t e e l s i n which improved toughness can be developed i f t h e carbon c o n t e n t i s reduced t o below 0.4% C.

N i c k e l , chromium, molybdenum s t e e l s g e n e r a l l y used as s t r u c t u r a l

s t e e l s , may by s u i t a b l e heat t rea t me nt be used as t o o l s t e e l s .

They do n o t

possess t h e h i g h hardness o r a b r a s i o n r e s i s t a n c e o f high-speed t o o l s t e e l s b u t advantage can be taken o f t h e i r s u p e r i o r toughness.

215

8.3.6

C u t t i n g Tools

From an economic p o i n t o f view, one o f t h e most i m p o r t a n t f a c t o r s i n machining i s t h e r a t e a t which c u t t i n g t o o l s wear 1161. most w e a r - r e s i s t a n t ,

Generally the hardest,

t o o l m a t e r i a l which r e s i s t s b r i t t l e f r a c t u r e should be

used. High-speed s t e e l s have good shock r e s i s t a n c e and can be r e a d i l y shaped by f o r g i n g and machining, so a r e t h e p r e f e r r e d c u t t i n g t o o l m a t e r i a l s f o r a l a r g e range o f a p p l i c a t i o n s .

Surface t r e a t m e n t s which increase hardness and minimize

adhesion t o t h e workpiece a r e b e n e f i c i a l .

Cast cobalt-chromium-tungsten a l l o y s ,

the S t e l l i t e s a r e s u i t a b l e f o r a p p l i c a t i o n s i n v o l v i n g h i g h temperatures, where c o o l i n g i s i m p r a c t i c a b l e b u t where impact i s absent.

Sintered carbide t o o l s

may be s u c c e s s f u l l y used f o r most m e t a l - c u t t i n g o p e r a t i o n s and t h e i r e x c e p t ional performance r e s u l t s from h i g h hardness and compressive s t r e n g t h s .

The

s t r a i g h t t u n g s t e n c a r b i d e s have t h e h i g h e s t hardness and a r e used f o r general applications,

b u t grades c o n t a i n i n g t i t a n i u m and t a n t a l u m c a r b i d e a r e more

r e s i s t a n t t o c r a t e r i n g and used g e n e r a l l y f o r machining s t e e l .

The use o f t h i n

CVD c o a t i n g s o f T i c improves t o o l l i f e c o n s i d e r a b l y . The i n t r o d u c t i o n o f new m a t e r i a l s which a r e d i f f i c u l t t o machine and t h e focus on h i g h p r o d u c t i v i t y , has accentuated t h e need f o r harder c u t t i n g t o o l s . Ceramic and cermet t o o l s a r e now i n use.

The h e a t - r e s i s t i n g p r o p e r t i e s o f

ceramic t o o l s enable them t o be used a t speeds u n a t t a i n a b l e by c a r b i d e t o o l s w i t h consequently improved s t o c k removal r a t e s .

Their high r i g i d i t y prevents

c h a t t e r i n g and a l l o w s smoother c u t s and s u p e r i o r work f i n i s h .

T h e i r h i g h hard-

ness and w e a r - r e s i s t a n t p r o p e r t i e s a l l o w c l o s e maintenance o f dimensions and enable t h e machining o f h i g h hardness m a t e r i a l s .

Being n o n - m e t a l l i c ,

ceramics

do n o t weld t o t h e m a t e r i a l b e i n g c u t and t h e i r extreme r e f r a c t o r i n e s s obv i a t e s t h e need f o r c o o l a n t s .

Diamond t o o l s a r e used f o r s p e c i a l purpose

c u t t i n g where t h e h i g h c o s t can be j u s t i f i e d ;

t o o l l i f e , however,

i s superior

t o t h a t o f carbide.

8.3.7

P i s t o n Rings

Apart f r o m speed, temperature and load, t h e p r i n c i p a l f a c t o r s which i n f l u e n c e wear o f p i s t o n r i n g s a r e c o r r o s i o n , frequency o f use and d i r t .

Whilst high

temperatures may cause wear by a d v e r s e l y a f f e c t i n g l u b r i c a t i o n , l o w tempera t u r e s cause c y l i n d e r and r i n g wear due t o c o r r o s i o n by t h e condensate o f combustion p r o d u c t s , t h i s i s p a r t i c u l a r l y so w i t h i n f r e q u e n t l y used engines. A i r b o r n e d u s t can be a s e r i o u s problem. Grey c a s t i r o n i s p r o b a b l y t h e most w i d e l y used m a t e r i a l f o r compression and

o i l r i n g s b u t h a r d e r m a t e r i a l s , such as carbon s t e e l o r even En31 b a l l b e a r i n g s t e e l , f i n d use.

For g r e a t e r hardness and wear r e s i s t a n c e , chromium p l a t i n g

i n s u f f i c i e n t t h i c k n e s s (0.005

inch) can be used on r i n g s and g i v e s reduced

216 wear r a t e s f o r b o t h c y l i n d e r and r i n g s .

Chromium p l a t i n g does n o t r u n s a t i s -

f a c t o r i l y a g a i n s t i t s e l f , so o n l y one mat i ng s u r f a c e i s p l a t e d .

Although occ-

a s i o n a l l y used f o r l i g h t e r s e r v i c e on passenger c a r engines, chromium p l a t i n g i s g e n e r a l l y used on compression r i n g s on heavy duty engines; d i e s e l and n a t u r a l gas engines.

i t i s w i d e l y used i n

As chromium p l a t i n g reduces f a t i g u e s t r e n g t h

i t i s usual t o p l a t e m a t e r i a l s o f h i g h f a t i g u e s t r e n g t h such as h i g h carbon

Molybdenum c o a t i n g s o f hardness o ver 1000 HV and a p o r o s i t y o f 15-25%

steels.

may now be f a v o ure d i n p r e f e r e n c e t o chromium p l a t i n g . r i n g s a r e f i n d i n g i n c r e a s i n g use.

Carbon-graphite,

p i s t o n r i n g m a t e r i a l f o r u n l u b r i c a t e d compressors, b u t i t has l i t t l e i n h e r e n t f l e x i b i l i t y ,

Sintered iron piston

used f o r many years as a i s a satisfactory material

needs support and depends f o r i t s low

wear p r o p e r t i e s on t he presence o f condensed w ater [ 1 7 ] .

Non-metallic p i s t o n

r i n g s , p a r t i c u l a r l y f a b r i c r e i n f o r c e d PF r e s i n s , have been used where t h e r e a r e doubts about l u b r i c a t i o n so t h a t ,

i n t h e e vent o f l u b r i c a t i o n f a i l u r e , t h e s o f t

r i n g s w i l l n o t damage t h e c y l i n d e r w a l l . Dry r u n n in g PTFE p i s t o n r i n g s a r e su ccessful

i n medium p r e s s u r e oxygen com-

p r e s s o r s as t h e y possess low f r i c t i o n a l c h a r a c t e r i s t i c s , dimensional s t a b i l i t y a t o p e r a t i n g temperatures,

good wear r e s i s t a n c e ,

c a r r y i n g a b i l i t y and chemical i n e r t n e s s . pr e s s u r e a p p l i c a t i o n s , i s reinforced.

h i g h s t r e n g t h and load-

However, f o r n o n - l u b r i c a t e d h i g h -

dimensional s t a b i l i t y can o n l y be r e t a i n e d i f t h e PTFE

G l a s s - f i b r e r e i n f o r c e m e n t appears t o be p o t e n t i a l l y more s u i t -

able than s t e e l reinforcement.

Other f i l l e r s commonly used a r e carbon, powd-

e r e d m e t a ls , MoS2, ce rami cs and carbon f i b r e s .

The concept o f avoidance o f

damage t o t h e c y l i n d e r w a l l and i n c r e a s i n g e xperience w i t h PTFE r i n g s on unlubr i c a t e d compressors have l e d t o t h e idea o f r e p l a c i n g m e t a l l i c r i n g s on l u b r i c a t e d compressors w i t h PTFE r i n g s t o a l l o w reduced l u b r i c a n t feed r a t e s . o f t h e a r t r e v i e w o f t h e i r use has been pre sented [ I S ] .

A state

A l i t e r a t u r e survey o f

m a t e r i a l and m e t a l l u r g i c a l asp ect s o f p i s t o n r i n g s c u f f i n g has been c a r r i e d out [lg].

8.3.8

Cams and Tappets

I n modern high-speed,

high-output,

small automtoive u n i t s d i s t r e s s o f cams

and t a p p e ts can be a major problem due t o a c c e l e r a t e d normal wear, p i t t i n g , s c u f f i n g and b u r n i s h i n g . camshaft m a t e r i a l .

Hardenable g rey c a s t i r o n i s t h e most w i d e l y used

Water-quenched hi g h-ca rbo n o r oil-quenched a l l o y s t e e l s o f

h i g h carbon c o n t e n t a r e used i n t h e through-hardened c o n d i t i o n .

Carburizing

s t e e l s may a l s o be used w i t h s e l e c t i v e f l a me o r i n d u c t i o n hardening o f s u r f a c e areas.

For a u t o mot i ve en gi n e cams, chromium and molybdenum c o n t a i n i n g i r o n s a r e

g e n e r a l l y used w i t h i n d i v i d u a l cams s u r f a c e hardened.

Tappet m a t e r i a l s a r e

u s u a l l y through-hardened h i g h carbon, chromium o r molybdenum types o f c a r b u r i z e d low-alloy steels.

The most commn t a p p e t m a t e r i a l s i n automotive a p p l i c a t i o n s

217 are grey hardenable c a s t i r o n c o n t a i n i n g chromium, molybdenum and n i c k e l o r c h i l l e d cast iron.

Oxide c o a t i n g s on hardened s t e e l tappet faces improve

f r i c t i o n a l q u a l i t i e s and a c c e l e r a t e w e a r i n g - i n . t o ensure adequate l i f e ;

Surface f i n i s h e s a r e important

s t r e s s - r e l i e v i n g can be b e n e f i c i a l .

Salt-bath

n i t r i d i n g t r e a t m ent can be b e n e f i c i a l t o cam f o l l o w e r s .

8.3.9

F r i c t i o n Materials

Te c h n o lo g ic a l p rog ress i n t h e aeronaut i c a ? and automobile i n d u s t r i e s makes demands on f r i c t i o n m a t e r i a l s i n c r e a s i n g l y more severe. m a t e r i a l s must have h i g h c o e f f i c i e n t s o f f r i c t i o n , temperatures,

High q u a l i t y brake

s t a b i l i t y a t a l l operating

good wear r e s i s t a n c e and s t r e n g t h , h i g h thermal c o n d u c t i v i t y and

corrosion s t a b i l i t y .

For arduous a p p l i c a t i o n s where s u r f a c e temperatures a r e

hi gh , o r g a n i c f r i c t i o n m a t e r i a l

i s no l o ng er s u i t a b l e .

A t o p e r a t i n g temper-

a t u r e s above 350"C, wear o f o r g a n i c m a t e r i a l becomes e x t r e m e l y severe.

Copper-

base f a c i n g s can o p e r a t e up t o lO0O'C b u t new m a t e r i a l s a r e under development f o r a e r o n a u t i c a l a p p l i c a t i o n s where r u b b i n g s u r f a c e s may w e l l reach temperatures w e l l above t h i s .

S i n t e r e d m a t e r i a l s o f f e r a w ide range o f f r i c t i o n a l c h a r a c t e r -

i s t i c s by t h e d i s p o s a l o f c a r e f u l l y graded n o n - m e t a l l i c p a r t i c l e s i n a m e t a l l i c matrix; level.

t h e c o m p o s i t i o n can be a d j u s t e d t o minimise wear, s e i z u r e and n o i s e Cermet and carbon f r i c t i o n m a t e r i a l s a r e b e i n g s u c c e s s f u l l y used i n

many a p p l i c a t i o n s .

8.3.10

P l a s t i c Be ari n gs

Since t h e f i r s t appearance on t h e market i n t h e 1 9 3 0 ' s o f b e a r i n g s based on t h e r m o s e t t i n g r e s i n s , t h e r e has been a con t i n uous increase i n t h e u t i l i z a t i o n o f polymers and polymer based composites i n t h e b e a r i n g f i e l d ,

[20,21].

Nylon

was t h e f i r s t o f t h e t h e r m o p l a s t i c m a t e r i a l s used f o l l o w e d by PTFE and more recently the polyacetals.

P l a s t i c s o f f e r a number o f advantages over metals.

T h e i r p h y s i c a l and mechanical p r o p e r t i e s can be v a r i e d over a wide range by s u i t a b l e c h o i c e o f polymer t ype , f i l l e r and r e i n f o r c e m e n t ; easy t o shape.

some a r e cheap and

Many polymers a r e r e s i s t a n t t o chemical a t t a c k and e x h i b i t low

c o e f f i c i e n t s o f f r i c t i o n during unlubricated sliding.

T h e i r wear r a t e s s l i d i n g

a g a i n s t smooth metal c o u n t e r f a c e s a r e low and they do n o t n o r m a l l y e x h i b i t s c u f f i n g o r seizure.

L u b r i c a t i o n by f l u i d s can o f t e n be dispensed w i t h , b u t

when l u b r i c a t e d , polymers a l l o w elastohydrodynamic metals.

They have disadvantages compared w i t h metals, e s p e c i a l l y r e g a r d i n g

u l t i m a t e strengths, pansion.

l u b r i c a t i o n more r e a d i l y than

e l a s t i c mo du l i , cre ep r e s i s t a n c e and c o e f f i c i e n t s o f ex-

Polymers may a l s o r e a d i l y absorb f l u i d s r e s u l t i n g i n dimensional

instability.

T h e i r low thermal c o n d u c t i v i t i e s can cause problems w i t h d i s s -

i p a t i o n o f f r i c t i o n a l h ea t .

They a l s o have t emperature l i m i t a t i o n s r e g a r d i n g

218 softening,

m e l t i n g and thermal d eg rad at i on .

The p r im e v i r t u e s o f t h e r m o - s e t t i n g reinforced,

r e s i n based bearings, c l o t h o r f i b r e

i s t h e i r h i g h s t r e n g t h and . e x c e l l e n t performance under c o n d i t i o n s

o f wa t e r l u b r i c a t i o n .

A t y p i c a l use i s r o l l neck b e a r i n g s o f s t e e l r o l l i n g

m i l l s where heat removal by wat er i s e s s e n t i a l .

On a s m a l l e r scale,

reinforced

r e s i n b e a r in g s a r e used i n pumps f o r wat er c i r c u l a t i o n and w i t h o i l and grease l u b r i c a t i o n i n a ut o mot i ve a p p l i c a t i o n s . Nylon and p o l y a c e t a l bushes p r o v i d e good d r y b e a r i n g l i v e s i f t h e PV f a c t o r i s k e p t low.

These m a t e r i a l s s i n t e r e d t o p r o v i d e c o n t r o l l e d p o r o s i t y a r e used

f o r o i l impregnated b e a r i n g s a t much g r e a t e r PV values w i t h low wear.

Thermo-

p l a s t i c b e a r i n g s f i n d e x t e n s i v e use i n f r a c t i o n a l HP e l e c t r i c motors, automotive a p p l i c a t i o n s and washing machines.

P o l y e t h y l e n e and h i g h d e n s i t y p o l y e t h y l e n e

a r e used i n p r o s t h e s i s human j o i n t s . PTFE based b e a r i n g s a r e now used a l most anywhere where c l e a n l i n e s s , resistance,

heat

freedom f ro m s e i z u r e and l u b r i c a n t e l i m i n a t i o n i s r e q u i r e d .

Thin

f i l m s o f PTFE f i n d e f f e c t i v e use i n many a p p l i c a t i o n s , p a r t i c u l a r l y t o a v o i d fretting.

Depending upon t h e a p p l i c a t i o n and the p r o p e r t i e s r e q u i r e d , PTFE

may be f i l l e d w i t h g l a s s , asbestos, carbon f i b r e s , MoS2, g r a p h i t e , bronze, n i c k e l and i r o n o x i d e .

A p r i n c i p a l a p p l i c a t i o n o f PTFE i s f o r gas l u b r i c a t e d

b e a r i n g s t o contend w i t h s t o p p i n g and s t a r t i n g .

8.4

SURFACE TREATMENTS AND COATINGS M a t e r i a l s f o r t r i b o l o g i c a l a p p l i c a t i o n s must f u l f i l two i m p o r t a n t f u n c t i o n s .

They must have s t r u c t u r a l p r o p e r t i e s f o r l o a d c a r r y i n g and s u r f a c e c h a r a c t e r i s t i c s t o a l l o w r e l a t i v e mot i on w i t h low f r i c t i o n and t h e minimum amount o f wear damage. demands.

Surface t re at men t s and c o a t i n g s a l l o w design scope t o meet these A ls o , w i t h wear problems i t i s o f t e n d i f f i c u l t t o make w o r t h w h i l e

i n n o v a t i o n s i n d e si gn , l u b r i c a t i o n o r m a t e r i a l s s e l e c t i o n ,

b u t many s u r f a c e

t r e a t m e n t s and c o a t i n g s a r e a v a i l a b l e which can e f f e c t wear r e s i s t a n c e and improved f r i c t i o n a l p r o p e r t i e s r Z Z , Z 3 ] .

Hard wear r e s i s t a n t m a t e r i a l s a r e

u s u a l l y expensive t o manufacture and d i f f i c u l t t o shape, and f o r economic reasons, w e a r - r e s i s t a n t

s u r f a c e t r e a t m e n t s may be a p p l i e d t o more common,

cheaper t o produce and more e a s i l y shaped m a t e r i a l s . Many s u r f a c e t r e a t m e n t s a r e a v a i l a b l e t o combat wear, and t h e c h o i c e depends l a r g e l y upon t h e t y p e and s e v e r i t y o f wear i n v o l v e d .

Some treatments

a r e s h o r t l i v e d b u t t h e more common s u r f a c e t r e a t m e n t s a r e those which a r e expected t o l a s t t h e l i f e o f t h e component.

Both types o f treatment may be com-

b i n e d t o e f f e c t p r o t e c t i o n d u r i n g arduous c o n d i t i o n s o f r u n n i n g i n and l o n g service effectiveness.

Treatments expected t o l a s t t h e l i f e o f a component

i n v o l v e s u r f a c e ha rde ni n g w i t h o u t changing t h e composition, s u r f a c e hardening by d i f f u s i o n t r e a t m ent s and by t h e a p p l i c a t i o n o f s u r f a c e c o a t i n g s .

Carbon and

219 alloy steels of a hardenable composition and cast iron may be surface hardened by flame or induction methods.

Shot peening work hardens the surface o f metals.

An extensively used method o f surface hardening to improve wear resistance depends upon diffusing specific elements into the surfaces of metal by such treatments a s carburizing, carbo-nitriding, nitriding, chromosing, boronising and siliconising.

Care must be taken to ensure a sufficient depth of case of

the correct structure and hardness and a satisfactory transition zone, Fig.8. Electro-deposition o f hard metal such a s chromiumprovides hard surface coatings and alloy coatings may be used to improve wear resistance.

Composite

coatings may be produced by the co-deposition o f hard particles and electrodeposited metal, Fig.9.

Coatings o f silicon carbide in nickel, tungsten carbide

in nickel and cobalt and silicon nitride in nickel have been used effectively. Diamond containing coatings have been developed for specific applications 1241. Hard surfacing o r facing finds wide use in many applications to provide specific wear resistant alloy o r ceramic coatings.

Almost any metal o r alloy

which can be cast may be used as a welding rod to apply a coating.

With plasma

spraying even the most refractory materials can be deposited with good surface bonding

Fig.8

.

(x75) Micro hardness survey o f a case hardened gear tooth with superimposed HV.

Fig.9

( ~ 1 5 0 ) Co-deposited Fe-b%Ni and chromium carbide.

220 Some s u r f a c e t r e a t m e n t s , besides e f f e c t i n g wear r e s i s t a n c e , may a l s o a i d lubrication.

Thus Sulphinuz [25] t r e a t m e n t can n i t r o g e n harden a s u r f a c e w i t h

t h e a s s o c i a t e d compressive s t r e s s e s b e n e f i c i a l t o f a t i g u e r e s i s t a n c e , as w e l l as produce a s u l p h u r - r i c h s u r f a c e l a y e r w i t h good l u b r i c a n t p r o p e r t i e s under boundary c o n d i t i o n s . an oxygen-ion r i c h ,

S o f t n i t r i d i n g c a r r i e d o u t i n an o x i d i s i n g b a t h produces low f r i c t i o n s u r f a c e b e n e f i c i a l i n r e d u c i n g wear and a i d i n g

l u b r i c a t i o n under arduous c o n d i t i o n s . may s o f t e n h a r d m a t e r i a l s .

Sulphinuz and s o f t n i t r i d i n g t r e a t m e n t s

T h i s can be avoided w i t h t h e Noskuff process [25]

which i n c o r p o r a t e s a quenching t r e a t m e n t o f t h e S u f t BT process [25] which p r o duces a s u l p h u r r i c h l a y e r , by an e l e c t r o l y t i c t r e a t m e n t i n a low temperature s a l t bath.

Various t y p e s o f phosphating t r e a t m e n t s p r o v i d e a t h i n , porous

c r y s t a l l i n e l u b r i c a t i n g s u r f a c e f i l m o f i n s o l u b l e phosphate, Fig.10,

which can

a l s o r e t a i n l u b r i c a n t o r p r o v i d e an e f f e c t i v e base f o r a s o l i d l u b r i c a n t . S o l i d l u b r i c a n t f i l m s such as PTFE and MoS2 can a l s o c o n t r o l wear,

reduce

f r i c t i o n and a i d l u b r i c a t i o n .

Fig.10

Phosphate c o a t i n g .

A v e r y hard, s t r o n g l y adherent, homogeneous t h i n f i l m u s u a l l y o f h a r d c a r b i d e such as TIC may be a p p l i e d t o metal by chemical vapour d e p o s i t i o n (CVD) [26,27].

R e f r a c t o r y c o a t i n g s such as alumina and s i l i c o n n i t r i d e may a l s o be

chemical vapour deposited. P h y s i c a l vapour d e p o s i t i o n techniques a r e a v i a b l e approach t o t h e p r o d u c t i o n o f s u r f a c e f i l m s o f extreme v e r s a t i l i t y i n d e p o s i t c o m p o s i t i o n .

V i r t u a l l y any

m e t a l , a l l o y , r e f r a c t o r y o r i n t e r m e t a l l i c compound, some p o l y m e r i c t y p e m a t e r i a l s and t h e i r m i x t u r e s can be d e p o s i t e d [ 2 8 ] .

Metal f i l m s d e p o s i t e d by

221 i o n - p l a t i n g a r e s t r o n g l y bonded t o t h e s u r f a c e as t h e f i l m i s d e p o s i t e d on a surface cleaned by s p l u t t e r e t c h i n g

[29,30].

S o f t metal l u b r i c a n t f i l m s may

a l s o be bonded t o a metal s u r f a c e by i o n - p l a t i n g

[31].

I o n - n i t r i d i n g speeds up

the n i t r i d i n g o p e r a t i o n .

8.5

CONCLUSIONS Mechanisms can o n l y p e r f o r m s a t i s f a c t o r i l y i f t h e d e s i g n and t h e m a t e r i a l s

o f c o n s t r u c t i o n a r e c o r r e c t l y chosen t o contend w i t h t h e o p e r a t i n g c o n d i t i o n s and t h e environment i n which t h e y a r e r e q u i r e d .

I t i s o n l y by e f f i c i e n t s e l -

e c t i o n and c a r e f u l s p e c i f i c a t i o n t h a t t h e most e f f e c t i v e use o f m a t e r i a l s can be accomplished t o e f f e c t m a t e r i a l c p n s e r v a t i o n and energy s a v i n g i n t h e i r p r o duct ion. Economics i n m a t e r i a l s and m a n u f a c t u r i n g c o s t s can o f t e n be made by j u d i c i o u s use o f s u r f a c e c o a t i n g s and s u r f a c e t r e a t m e n t s .

REFERENCES

1 2

3 4

Scott,O., T r i b o l o g y , 1968, 1, 14. Scott,D, I n T r i b o l o g y - Proc. J. R e s i d e n t i a l Course, 1968, Paper 1 , I n s t . M e t a l l u r g i s t s , London. Scott,D., Proc. I n s t . Mech. Engrs., London I n t . Conf. on L u b r i c a t i o n and Wear, 1967, ( 3 A ) , 325. New E n g i n e e r i n g M a t e r i a l s , Proc. I n s t . Mech. Engrs, London, 1965/66,

182,

180, ( 3 D ) .

5 6 7 8 9 10 11 12

13 14 15

16 17 18 19 20

21 22

mdman,N.E., E n g i n e e r i n g A l l o y s , 1962, Chapman & H a l l , London. Special Zakay,V.F. and Justusson,W.M., I n High S t r e n g t h S t e e l s - I . S . I . Report 76, 1962, 14. I r o n & S t e e l l n s t . , London. Ouckworth,W.E., Leak,D.A. and P h i l l i p s , R . , I n High S t r e n g t h S t e e l s , I.S.I. Special Report 76, 1962, 22, I r o n & S t e e l l n s t . , London. Forrester,P.G., Metal1 Rev., 1960, (ZO), 507. Amateau,M.F., Nicholson,D.W. and Glaeser,W.A., 1961, O.T.S. PB 171625, O f f i c e o f T e c h n i c a l Services, Washington D.C. Scott,O., I n F a t i g u e i n R o l l i n g Contact, 1963, 103, I n s t . Mech. Engrs. London. Scott,D., I n Low A l l o y S t e e l s , 1968, 203, I r o n and S t e e l I n s t . London. Scott,D., I n R o l l i n g Contact F a t i g u e , (Ed. Tourret,R., and Wright,E.P.), 1977, 3, Heyden, London. Scott,D., Wear, 1977, 71. Scott,D., Proc. I n s t . Mech. Engrs., 1969, 183, (3L), 9. Scott,D., R o l l i n g Contact F a t i g u e , I n , Wear, (Scott,D., Ed.), T r e a t i s e on M a t e r i a l s Science and Technology, 1978, 13, 321, Academic Press, NY. Trent,E.M. Metal C u t t i n g , 1977, B u t t e r w o x h s , London. Summers-Smith,D., Wear, 1966, 425. Summers-Smith,D., Proc. T r i b o l o g y Conf., 1971, Paper C93/71, I n s t . Mech. Engrs. Scott,D., Smith,A.I., T a i t , J . and Tremain,G.R., Wear, 1975, 2, 293. Pratt,G.C., P l a s t i c Based Bearings i n " L u b r i c a t i o n and L u b r i c a n t s " , (E.R. B r a i t h w a i t e - Ed.), 1967, 377, E l s e v i e r , Amsterdam. Evan5,F.C. and Lancaster,J.K., The Wear o f Polymers, i n Wear, (Scott,D.), T r e a t i s e on M a t e r i a l s Science and Technology, 1978, 13, 85, Academic Press, NY. Scott,D., Wear, 1978, 283.

5,

9,

9,

48,

222

23 Wilson,R.W., Proc. 1st Euro, Tribology Congr., 1973, 165, Inst. Mech. Engrs., London. 24 Sharp,W.F., Wear, 1975, 32, 315. 25 Gregory,J.C., Tribology, 1970, 3, 73. 26 Gass,H. and Hintermann,H.E., SwTss Patent 452.205, 1968. 27 Hintermann,H.E. and Aubert,F., Proc. 1 s t Euro Tribology Congr., 1973, 207, Inst. Mech. Engrs., London. 28 Bunshah,R.F. and Juntz,R.S., J. Vac. Sci. Technol., 1972, 9, 1389. 29 Spalvins,T., Przbyszewski,J.S. and Buckley,D.H., NASA Tech. Note TN D - 3707, 1966. 30 Teer,D.G., Tribology, 1975, 1, 245. 211. 31 Sherbiney,M.A. and Halling,J., Wear, 1977,

3,

223

!)

SELECTION OF LUBRICANTS

A.R.

LANSDOWN, D i r e c t o r , Swansea T r i b o l o g y Centre, U.K.

9.1

I NTRODUCT I ON The v a r i e t y o f a v a i l a b l e l u b r i c a n t s i s enormous.

I f we simply c o n s i d e r

broad, b a s i c t y p es t h e r e a r e p r o b a b l y many hundreds:

i f we take i n t o account

minor d i f f e r e n c e s i n comp osi t i on and t h e v a r i o u s commercial brands, t h e r e a r e probably t e n s o f thousands.

To t h e n o n - s p e c i a l i s t

t h e problem o f proper l u b r i -

cant s e l e c t i o n can t h e r e f o r e seem v e r y c o n f u s i n g . For many a p p l i c a t i o n s t h e s e l e c t i o n o f a l u b r i c a n t i s i n f a c t n o t c r i t i c a l , and a wide v a r i e t y o f l u b r i c a n t s c o u l d work q u i t e s a t i s f a c t o r i l y .

For such

a p p l i c a t i o n s t h e o b j e c t o f l u b r i c a n t s e l e c t i o n should p r o b a b l y be t o ensure t h e lowest o v e r a l l l i f e c o s t f o r t h e system as a whole. For some a p p l i c a t i o n s , however, t h e s e l e c t i o n o f l u b r i c a n t may be v e r y c r i t i cal indeed, and t h e r e may be v e r y few o r even no l u b r i c a n t s capable o f e n s u r i n g satisfactory operation. Because t h e r e a r e thousands o f d i f f e r e n t l u b r i c a n t s a v a i l a b l e , and because many a p p l i c a t i o n s a r e n o t c r i t i c a l , t h e r e i s o f t e n a tendency t o leave l u b r i c a n t s e l e c t i o n t o a v e r y l a t e st ag e i n t h e de si gn process.

The r e s u l t may be t h a t a

design i s completed and a machine c o n s t r u c t e d f o r which no s u i t a b l e l u b r i c a n t e x i s t s , and t h e r e have been cases where v e r y expensive m o d i f i c a t i o n has been necessary t o r e s o l v e t h e problem o f l u b r i c a t i o n . One im p o r t a n t p r i n c i p l e i s t h e r e f o r e t h a t l u b r i c a n t requirements should always be c o n s id ere d a t an e a r l y st a ge i n d esign.

In o r d e r t o approach l u b r i c a n t s e l e c t i o n r e a I i s t i c a l l y , we should be c l e a r as t o what a r e t h e o b j e c t s o f l u b r i c a t i o n . The p r i m a r y o b j e c t o f l u b r i c a t i o n i s t o reduce f r i c t i o n o r wear, o r u s u a l l y both f r i c t i o n and wear. cant

.

There a r e i n (i)

T h i s i s t h e f a c t o r w hich d e f i n e s a m a t e r i a l as a l u b r i -

a d d i t i o n t h r e e secondary f u n c t i o n s o f a l u b r i c a n t : -

To a c t as a c o o l a n t .

I n some systems t h i s w i l l be a v i t a l f u n c t i o n o f

t h e l u b r i c a n t because f r i c t i o n a l o r process heat must be removed and no a l t e r n a t i v e c o o l i n g f l u i d can be used. (ii)

To remove wear d e b r i s o r o t h e r contaminants, o r t o p r e v e n t o t h e r contamin a n ts from e n t e r i n g t h e system.

( i i i ) To p r o t e c t m e t a l s a g a i n s t c o r r o s i o n .

There i s no good reason why a

224 l u b r i c a n t sho ul d be expected t o p r o v i d e such p r o t e c t i o n , b u t because mine r a l o i l s are very e f f e c t i v e corrosion preventives,

many designers have

come t o expect t he same o f a l l l u b r i c a n t s . The l u b r i c a n t w i l l o f t e n s a t i s f a c t o r i l y f u l f i l a l l these tasks, b u t where the a v a i l a b l e choice o f l u b r i c a n t s i s l i m i t e d ,

i t may be necessary t o choose t h e

l u b r i c a n t o n l y t o meet t h e f r i c t i o n and wear requirements, and t o use o t h e r techniques t o s o l v e t h e c o o l i n g , c o n t a m i n a t i o n and c o r r o s i o n problems.

9.2

SELECTING THE LUBRICANT TYPE L u b r i c a t i o n systems as such a r e o u t s i d e t h e scope of t h i s chapter, b u t the

problem o f l u b r i c a n t s e l e c t i o n cannot e n t i r e l y be separated from t h a t o f select i n g t h e l u b r i c a t i o n system. The most s t r a i g h t f o r w a r d way t o s e l e c t b o t h l u b r i c a n t and l u b r i c a t i o n system i s p r o b a b ly t o s t a r t w i t h t h e s i m p l e s t t e chn i que and t o progress from t h a t o n l y

a s f a r as i s necessary t o ensure s a t i s f a c t o r y o p e r a t i o n .

The s i m p l e s t technique

w i l l u s u a l l y have t h e l owest i n i t i a l c o s t , and w i l l o f t e n also be t h e most re1 i a b l e . The s i m p l e s t system c o n s i s t s o f a sma l l q u a n t i t y o f p l a i n m i n e r a l o i l i n p l a c e i n t h e l u b r i c a t e d component, w i t h o u t any f a c i l i t y f o r r e - l u b r i c a t i o n . Such a system w i l l cope w i t h a s u r p r i s i n g l y wide v a r i e t y o f mechanisms, i n c l u d i n g watches and c l o c k s and many o t h e r p r e c i s i o n instruments, d o o r - l o c k s and h in g e s , sewing-machines,

t y p e w r i t e r s , b i c y c l e s , r o l l e r skates, skate-boards,

and so on. I t ceases t o cope when t h e r e i s t o o much l oad o r speed o r heat o r d e b r i s , o r when t h e l i f e r e q u i r e d i s so l on g t h a t t h e o i l o x i d i s e s . evaporates, o r creeps away f r o m th e b e a r i n g surf ace s.

I t i s then necessary t o use a more s o p h i s t i c a t e d

o i l , a grease, a s o l i d l u b r i c a n t , o r sometimes even a gas l u b r i c a n t , o r t o use a more complex l u b r i c a t i o n system. T a b le 9.1 shows some o f t h e p o s s i b l e cho i ces o f a l t e r n a t i v e systems when t h e s i m p l e s t system i s no l on ge r adequate.

Some o f these choices a r e concerned w i t h

t h e l u b r i c a t i o n system r a t h e r than t h e c h o i c e o f l u b r i c a n t , b u t whatever l u b r i c a t i o n system i s used, a c h o i c e o f l u b r i c a n t i s needed. As t h e demands on a b e a r i n g i ncre ase , a p o i n t i s e v e n t u a l l y reached where a p l a i n m i n e r a l o i l i s no l o ng er adequate, and i t i s a t t h i s p o i n t t h a t t h e p r o blem o f l u b r i c a n t s e l e c t i o n b eg i ns.

A c o n v e n ie n t approach t o l u b r i c a n t s e l e c t i o n w i l l be t o c o n s i d e r f i r s t t h e p r o p e r t i e s which i n f l u e n c e t h e s e l e c t i o n o f d i f f e r e n t m i n e r a l o i l s , then t h e v a r i o u s o t h e r o i l s which can be used, and f i n a l l y t h e a l t e r n a t i v e s t o l u b r i c a t i n g o i l s , namely greases, s o l i d l u b r i c a n t s and gases. I t may be u s e f u l t o r e f e r t o F i g u r e 1, whi ch i n d i c a t e s broad l i m i t s o f speed

and l o a d w i t h i n w hi ch d i f f e r e n t c l a s s e s o f l u b r i c a n t can be used.

SPEED ( f t / m i n ]

1,ooo.000

2

10

boo0

100

10.000

100.000

100.000

2.

t

3.

- looeooo

For 1 iqiiid Iiihricnnts t h c rmlnmcntnl pronerty is v i s c o s i t y , :ind t h i s f i m i r c L I S S I U ~ C St h a t t h e corrert v i s c o s i t y h;is hecn chosen. 1:xtcrnalIy nressiiriscd hcnrinps can he uscd ovcr a wide rnnp,c of snecd :ind lo:1d.

10.000

>

.c _

E

a

t

-

N

N

\

10.000 1,000

Y

-ea

1,000 100

100

10

100

1.000

10.000

rw,ooo

14.5

r,ooo,ooo

SPEED AT BEARING CONTACT lrnm/s) --C

Fig.1

Speed/load limitations f o r d i f f e r e n t types o f lubricant. N

tu cn

226 Table 9.1 P o s s i b l e ch oi ce s when a small q u a n t i t y o f p l a i n m i n e r a l o i l i n p l a c e i s no lonqer adequate. Too much l oa d

More viscous o i l Grease Extreme p r e s s u r e o i l Extreme p r e s s u r e grease Externally pressurised l u b r i c a t i o n Sol i d l u b r i c a n t

Too much speed (which may l ea d t o t o o much he at )

Greater q u a n t i t y o f o i l Less viscous o i l O i l c i r c u l a t i o n system Gas l u b r i c a t i o n

Too much heat

O i l w i t h anti-oxidant More viscous o i l Greater q u a n t i t y o f o i l O i 1 c i r c u l a t i o n system Synthetic o i l Sol i d l u b r i c a n t

Too much d e b r i s

Greater q u a n t i t y o f o i l C i r c u l a t i o n system w i t h f i l t r a t i o n

Contamination

O i l c i r c u l a t i o n system Grease Solid lubricant

Long 1 i f e r e q u i r e d

O i l with anti-oxidant More viscous o i l Great q u a n t i t y o f o i l R e l u b r i c a t i o n system Synthetic o i l Grease Sol i d l u b r i c a n t

PROPERTIES OF MINERAL OILS

9.3

9.3.1

Viscosity

V i s c o s i t y i s t h e most i m p o r t a n t s i n g l e p r o p e r t y o f a l u b r i c a t i n g o i l , s i n c e i t i s th e s o l e p r o p e r t y which determines whether t h e o i l i n a b e a r i n g a t a

s p e c i f i c lo a d and speed w i l l g i v e f u l l f l u i d f i l m s e p a r a t i o n o f t h e b e a r i n g s u r fa c e s . V i s c o s i t y may be expressed i n any one o f a number o f d i f f e r e n t u n i t s .

Pro-

ba b ly t h e most commonly used i n E n gl i sh -spe aking c o u n t r i e s i s the c e n t i s t o k e ( c S t ) which p r o p e r l y d e s c r i b e s K i n ema t i c V i s c o s i t y , and i s d i r e c t l y measured by t h e v a r i o u s s t a n dard I n s t i t u t e o f Petroleum, ASTM, B r i t i s h Standard and IS0 t e s t methods.

The A b sol u t e V i s c o s i t y i s commonly expressed i n terms o f t h e

c e n t i p o i s e (cP) and i s t h e v i s c o s i t y used i n t h e v a r i o u s e n g i n e e r i n g c a l c u l a t i o n s such as e qu at i on s ( 1 ) t o ( 3 ) .

227 Each o f these u n i t s has i t s S t e q u i v a l e n t , b u t t h e i n t r o d u c t i o n o f t h e S I U n i t s has been t he s u b j e c t of c o n s i d e r a b l e d i s p u t e . c e n t i s t o k e i s t h e m i l l i m e t r e z pe r second (mm2 s - ' )

The SI e q u i v a l e n t o f t h e and t h e S I e q u i v a l e n t o f t h e

c e n t i p o i s e i s t h e m i l l inewton second p e r metre2 o r m i l 1 i p a s c a l second ( m N s / m Z ) . There a r e a l s o t h r e e o t h e r u n i t s wh i ch a r e s t i l l w i d e l y used, a l t h o u g h c u r r e n t p o l i c y i s t o d i s c o n t i n u e them i n f a v o u r o f k i n e m a t i c v i s c o s i t y i n c e n t i stokes.

They a r e Redwood Seconds, used i n B r i t a i n , Saybolt Seconds (SUS)

i n the

United S ta t e s , and degrees E n gl e r, used i n Germany and o t h e r p a r t s o f c o n t i n e n t a l Europe. There a r e s e v era l e q u a t i o n s which r e l a t e t h e o i l f i T m t h i c k n e s s t o t h e o i l For g en era l hydrodynamic l u b r i c a t i o n t h e Reynolds Equation [ l ]

viscosity. applies, ax

*

[

h3

ax1

+

aY

I

h3

3

aY1

f o r elastohydrodynamic

=

6pU

ah

+ 6ph

+

12pV

(1)

l u b r i c a t i o n w i t h c y l i n d r i c a l elements, as i n r o l l e r

bearings and many gear c o n f i g u r a t i o n s , an e q u a t i o n o f t h e Dowson-Higginson type,

121 a p p l i e s ,

"0.7 hmin/R

=

2.65

G0.54

w0.03

w h i l e f o r e la s t o hyd rod yna mi c l u b r i c a t i o n w i t h s p h e r i c a l elements, as i n b a l l bearings, t h e Archard-Cowking Equation

[3]

applies,

where U i s p r o p o r t i o n a l t o v i s c o s i t y .

10-1

z P

I-

uLL

u.

lL 0

Iz W -

2

U W

8

10-2

10.~

10-8 PV/ p

Fig.2

T y p i c a l S t r i b e c k curve

DIMENSIONLESS

228 F i g u r e 2 , sometimes c a l l e d a S t r i b e c k curve, shows how t h e v i s c o s i t y o f a l u b r i c a n t a f f e c t s t h e l u b r i c a t i o n mode and t h e c o e f f i c i e n t o f f r i c t i o n i n a bearing . By t h e use of t h e a p p r o p r i a t e e q u a t i o n t h e o i l v i s c o s i t y r e q u i r e d f o r any b e a r i n g can be c a l c u l a t e d , b u t i f t h i s i s done f o r every b e a r i n g i n every machi n e i n a f a c t o r y t h e r e s u l t w i l l p r o b a b l y be a l i s t of several hundred v i s c o s i ties.

The number o f a v a i l a b l e m i n e r a l o i l s i s so g r e a t t h a t i t m i g h t w e l l be

p o s s i b l e t o o b t a i n o i l s w i t h eve ry v i s c o s i t y r e q u i r e d , b u t t h e r e a r e some very good reasons f o r n o t d o i n g so.

I f a l l t h e b e a r i n g s i n a complex machine can use t h e same

(i)

u b r i c a n t , then

a s i n g l e o i l r e s e r v o i r and c i r c u l a t i o n system can be used, and t h i s w i l l g e n e r a l l y be cheaper and more r e l i a b l e . (ii)

A l a r g e number o f d i f f e r e n t l u b r i c a n t s causes s t o r a g e prob ems.

( i i i ) The g r e a t e r t h e number o f l u b r i c a n t s i n use i n one p l a n t , t h e g r e a t e r i s t h e p r o b a b i l i t y o f t h e wrong one b e i n g used. (iv)

Most b e a r i n g s a r e r e q u i r e d t o o p e r a t e over a range o f temperature,

so the

v i s c o s i t y o f t h e o i l w i l l o n l y be a t t h e optimum v a l u e f o r a p a r t o f the o p e r a t i n g time. F o r t u n a t e l y t h e v i s c o s i t y req ui re men t i s o f t e n n o t c r i t i c a l .

A s l o n g as the

v i s c o s i t y i s h i g h enough t o ensure t h e r e q u i r e d o i l f i l m t h i c k n e s s , harm i n g o in g t o a s l i g h t l y h i g h e r v i s c o s i t y . s i t y i s p a r t l y s e l f -co mpe nsa t i n g.

t h e r e i s no

I n any case an increase i n v i s c o -

The e f f e c t of i n c r e a s i n g v i s c o s i t y i s t o

i n c r e a s e v is c o u s f r i c t i o n , whi ch i n crea ses t h e power d i s s i p a t e d i n t h e bearing. The excess power i s con vert ed t o h ea t , t h e h eat r a i s e s t h e o i l temperature, and t h e h i g h e r t e m p erat ure reduces t h e o i l v i s c o s i t y .

So t h e end r e s u l t of u s i n g a

more v is c o u s o i l i s t h a t t h e power consumption and t h e temperature s t a b i l i s e a t a s l i g h t l y higher level. I t i s t h e r e f o r e p o s s i b l e t o use a sma l l number o f v i s c o s i t y grades t o f i l l a wide range o f o i l requirements,

and i n f a c t many n a t i o n a l and i n t e r n a t i o n a l

standards ( i n c l u d i n g B r i t i s h Standard 4231 " V i s c o s i t y t r i a l L i q u i d Lubricants")

t h e v i s c o s i t y o f each o i l i n c e n t i s t o k e s a t 4OoC pre c e d in g grade.

C l a s s i f i c a t i o n f o r Indus-

r e s t r i c t t h e number o f l u b r i c a n t s i n such a way t h a t i s 50% h i g h e r than t h a t o f t h e

T h i s g i v e s o n l y e i g h t e e n grades t o cover t h e whole range from

2 c S t t o 1500 c S t . So t h e f i r s t t h i n g t o do i n m i n e r a l o i l s e l e c t i o n i s t o c a l c u l a t e t h e lowest

v i s c o s i t y which w i l l ensure f u l l f l u i d f i l m l u b r i c a t i o n o f a l l t h e b e a r i n g s i n t h e system, and t he n t o i d e n t i f y t h e n e x t h i g h e r standard v i s c o s i t y grade. The v i s c o s i t y o f an o i l i s always quoted a t a s t a t e d temperature, u s u a l l y O°F,

7OoF, 100°F

o r 130°F,

b u t i n c r e a s i n g l y 4OoC.

These temperatures w i l l

229 r a r e l y be t h o s e a t wh i ch yo ur b e a r i n g s a r e i n tended t o operate, so t h e second i mp o r t a n t p r o p e r t y t o c o n s i d e r i s t h e v a r i a t i o n o f v i s c o s i t y w i t h temperature.

9.3.2

V i scos i ty/Tempe r a t u r e Re1a t ionsh i p

For a l l l i q u i d s t h e v i s c o s i t y decreases as t h e temperature increases, b u t t h e r a t e o f decrease v a r i e s c o n s i d e r a b l y f ro m

one l i q u i d t o a n o t h e r .

I t follows

t h a t i f we p l o t graphs o f o i l v i s c o s i t y a g a i n s t temperature, w hich g i v e s somet h i n g approaching s t r a i g h t l i n e s , t h e s l o p e o f t h e l i n e s i s d i f f e r e n t f o r d i f f erent o i l s .

F i g u r e 3 shows some t y p i c a l v i s c o s i t y / t e m p e r a t u r e

relationships

for different oils. Standard c u r v es a r e r e a d i l y a v a i l a b l e showing the v i s c o s i t y / t e m p e r a

ur e

r e l a t i o n s h i p f o r d i f f e r e n t o i l s , so t h a t we can c o n v e r t o u r r e q u i r e d v scos i t y t o t h e v i s c o s i t y a t a st an da rd r e f e r e n c e t e mperature, p r o v i d e d t h a t we know which l i n e ( i . e .

whi ch sl o pe ) r e l a t e s t o t h e o i l under c o n s i d e r a t i o n .

The p r o p e r t y whi ch i s most w i d e l y used t o d e s c r i b e t h e v i s c o s i t y / t e m p e r a t u r e beh a v io u r o f an o i l i s t h e V i s c o s i t y Index (V.1).

-

T h i s index r e l a t e s t h e

change o f v i s c o s i t y w i t h t emp era t u re t o two a r b i t r a r y o i l s , one based on a Pe n n s y lv a n ia Grude o i l (V . I .

0) and one on a G u l f Coast o i l (V.1.

-

100).

The v i s c o s i t y index o f an unknown o i l can be c a l c u l a t e d from t h e measured v i s c o s i t i e s a t 4OoC and 100°C by means of e q u a t i o n

V.I.

100 ( L - u ) / ( L - H )

where U i s t h e v i s c o s i t y a t 4OoC o f t h e o i l sample i n c e n t i s t o k e s , L i s t h e v i s c o s i t y a t 4OoC o f an o i l o f 0 v i s c o s i t y index having t h e same v i s c o s i t y a t 100°C as t h e o i l sample, and H i s t h e v i s c o s i t y a t 4OoC o f an o i l of 100 v i s c o s i t y in d e x h a v i n g t h e same v i s c o s i t y a t 100°C as t h e o i l sample.

Tables o f

v al u e s f o r L and H a r e c o n t a i n e d i n t h e st an dard I n s t i t u t e o f Petroleum and ASTM t e s t methods. Briefly,

t h e V i s c o s i t y Index w i l l be low i f t h e v i s c o s i t y changes r a p i d l y

w i t h temperature, and h i g h i f v i s c o s i t y i s l e s s a f f e c t e d by temperature. Standard D.S.4231 The h i g h e s t V . I . synthetic o i l s

British

r e f e r s t o m i n e r a l o i l s h avi ng V i s c o s i t y I n d i c e s of 0, 50 o r 95. f o r a s i m p l e m i n e r a l o i l i s s l i g h t l y over 100, w h i l e c e r t a i n

have V . I .

Ge n e r a lly , a V . I .

g r e a t e r t ha n 200.

h i g h e r t h an 100 i s o n l y o b t a i n e d w i t h a m i n e r a l o i l by t h e

use o f a p o l y m e r i c a d d i t i v e c a l l e d a V i s c o s i t y Index Improver.

V.I.Improvers

a r e used i n m u l t i - g r a d e e ng i ne o i l s and i n h y d r a u l i c f l u i d s w hich a r e r e q u i r e d t o o p e r a t e o v e r a wi d e t emp era t u re range. Most b e a r i n g s have t o o p e r a t e o v e r a range of temperatures, w i d t h o f t h i s range which determines what V . I .

i s required.

and i t i s t h e

A V.I.

o f 0 may be

a c c e p t a b l e i f t h e t emp era t u re range i s v e r y narrow, o r i f a l a r g e change i n v i s c o s i t y i s acceptable.

On t h e o t h e r hand, a V . I .

o f 160 o r more i s s p e c i f i e d

230

Fig.3

Viscosity/temperature characteristics of various oils

231 f o r an a i r c r a f t h y d r a u l i c f l u i d which i s r e q u i r e d t o o p e r a t e f r o m -4OOC t o +15OoC. The e f f e c t i v e n e s s o f a V.I.Improver

i s a f f e c t e d by t h e shear r a t e , t h e r a t e

a t which t h e o i l has t o pass through c o n f i n e d spaces, and a t h i g h shear r a t e i n a high-speed b e a r i n g t h e e f f e c t i v e v i s c o s i t y o f a V . I .

improved o i l may be

l i t t l e o r no d i f f e r e n t from t h a t o f t h e base o 1 . Having chosen t h e r e q u i r e d V i s c o s i t y Index, t h e v i s c o s i t y o f t h e s e l e c t e d o i l a t the r e f e r e n c e temperature can be worked o u t by means o f a graph such as Figure 3 o r a c h a r t such as t h e one i n B.S.423

9.3.3

Viscosity/Pressure Relationship

Before l e a v i n g t h e s u b j e c t o f v i s c o s i t y ,

i t i s perhaps d e s i r a b l e f o r complete-

ness t o mention t h e v a r i a t i o n o f v i s c o s i t y w i t h p r e s s u r e . l u b r i c a t i n g o i l s increases as t h e p r e s s u r e i s increased.

The v i s c o s i t y o f a l l I n p r a c t i c e the r a t e

o f increase i s v e r y low, and i n p l a i n hydrodynamic o r e x t e r n a l l y p r e s s u r i s e d bearings i t can s a f e l y be n e g l e c t e d . I n elastohydrodynamic l u b r i c a t i o n , however, t h e p r e s s u r e generated i n t h e l u b r i c a n t can be s u b s t a n t i a l .

The i n c r e a s e i n l u b r i c a n t v i s c o s i t y i s c o r r e s -

pondingly h i g h , and i s a major f a c t o r i n e n s u r i n g s u c c e s s f u l l u b r i c a t i o n o f gears and r o l l i n g b e a r i n g s . I t i s rare t o specify o r quote a pressure c o e f f i c i e n t o f v i s c o s i t y f o r a

l u b r i c a t i n g o i l , and t h i s i s n o r m a l l y o n l y done f o r v e r y c r i t i c a l s i t u a t i o n s such as i n c e r t a i n s p a c e c r a f t a p p l i c a t i o n s .

9.3.4

A n t i -Wear,

Extreme Pressure and A n t i -Fr ic t i o n P r o p e r t i e s

I n t h e o r y , from t h e p o i n t o f view o f p u r e p h y s i c a l l u b r i c a t i o n , we have completely s p e c i f i e d t h e o i l when we have s p e c i f i e d t h e v i s c o s i t y , t h e V . I . perhaps t h e p r e s s u r e c o e f f i c i e n t o f v i s c o s i t y .

and

I n p r a c t i c e t h i s i s n o t always

so, and i t i s o f t e n necessary t o s p e c i f y t h e a n t i - w e a r ,

extreme p r e s s u r e o r

a n t i - f r i c t i o n properties o f the o i l . R e f e r r i n g a g a i n t o F i g u r e 2, t h e boundary and mixed l u b r i c a t i o n r e g i o n s represent s i t u a t i o n s i n which t h e b e a r i n g s u r f a c e s a r e n o t c o m p l e t e l y separated by a f i l m o f l u b r i c a n t , b u t e x p e r i e n c e some degree o f s o l i d - t o - s o l i d Even i n l i g h t l y - l o a d e d b e a r i n g s s o l i d - t o - s o l i d

contact.

c o n t a c t can occur a t low

speed when a b e a r i n g i s s t a r t i n g o r s t o p p i n g , and t h i s i s p a r t i c u l a r l y i m p o r t a n t i f a mechanism i s i n t e n d e d t o o p e r a t e i n t e r m i t t e n t l y .

The r e s i d u a l o i l f i l m

w i l l o f t e n g i v e some p r o t e c t i o n under these c o n d i t i o n s , b u t g r e a t e r p r o t e c t i o n

can be o b t a i n e d by t h e use

o f an a n t i - w e a r a d d i t i v e .

T h i s i s u s u a l l y an organic

a c i d such as s t e a r i c a c i d o r a n a t u r a l o i l such as rapeseed o i l , b u t s y n t h e t i c o r g a n i c phosphorus compounds such as t r i - x y l y l phosphate a r e a l s o e f f e c t i v e . I n h e a v i l y - l o a d e d b e a r i n g s o r i n t e r m i t t e n t r o l l i n g b e a r i n g s o r c e r t a i n types o f gear, a more a c t i v e t y p e o f a d d i t i v e known as an Extreme Pressure (EP)

232 a d d i t i v e may be needed.

EP a d d i t i v e s a r e u s u a l l y r e a c t i v e s y n t h e t i c o r g a n i c

compounds c o n t a i n i n g phosphorus, s u l p h u r o r c h l o r i n e , a l t h o u g h lead naphthenate i s s t i l l used i n c e r t a i n gear o i l s , and molybdenum d i s u l p h i d e i s o c c a s i o n a l l y used. EP p r o p e r t i e s a r e assessed i n s e v e r a l d i f f e r e n t ways, b u t t h e most w i d e l y

used t e s t methods a r e p rob ab l y t h e Timken and Four-Ball machines. measured by means o f t h e F ou r-Ba l l Machine,

One c r i t e r i o n ,

i s t h e Mean H e r t z load, and t h i s can

v a r y from 30 f o r a p l a i n m i n e r a l o i l t o 85 f o r a pow erful EP gear o i l . One extreme r equ i reme nt f o r EP p r o p e r t i e s i s i n c e r t a i n types o f metalw orking o p e r a t i o n , and t h e most po we rf u l c h l o r i n e and s u l p h u r - c o n t a i n i n g used i n t h e l u b r i c a n t s f o r such a p p l i c a t i o n s .

additives are

Pow erful EP a d d i t i v e s can be

c o r r o s i v e , and s ho ul d t h e r e f o r e n o t be used where t h e y a r e n o t necessary. A n a t u r a l o i l may be added t o an o i l t o decrease t h e f r i c t i o n i n boundary

l u b r i c a t i o n , and thus reduce t h e power consumption and t h e heat generated.

This

may be p a r t i c u l a r l y i m p o r t a n t i n a mechanism which i s s e n s i t i v e t o f r i c t i o n , The n a t u r a l o i l i s then sometimes c a l l e d a l u b r i c i t y addi-

such as a worm g ear.

t i v e , b u t t h e t e rm i s i l l - d e f i n e d and g e n e r a l l y n o t recommended. We have now c onsi d ere d a l l t h e p r o p e r t i e s r e q u i r e d t o g i v e adequate i n i t i a l

1 u b r i c a t io n , b u t t h e r e a r e two o t h e r c o n s i d e r a t i o n s i n e n s u r i n g continued s a t i s f a c t o r y l u b r c a t i on.

9.3.5

Stab l i t y

An o i l w i 1 decompose c h e m i c a l l y i n s e r v i c e because o f e i t h e r heat (thermal decomposition) o r a co mbi n at i on o f h ea t and oxygen ( o x i d a t i o n ) .

Thermal s t a b i -

l i t y can be improved i n man uf act uri n g i f t h e more u n s t a b l e components o f t h e o i l can be removed, b u t o t h e r w i s e t h e o n l y s o l u t i o n i s t o I n g e n e r a l, however, t u r e s than o x i d a t i o n ,

thermal decomposition takes p l a c e a t much h i g h e r temperaand i t i s t h e o x i d a t i v e s t a b i l i t y w hich determines t h e

maximum t e m p e r a t ure a t wh i ch t h e o i l can be used. i v e l y reduced by t h e use o f a n t i - o x i d a n t s . presence o f oxygen,

keep t h e temperature down.

O x i d a t i o n can be v e r y e f f e c t -

I f t h e temperature i s h i g h i n t h e

i t may t h e r e f o r e be d e s i r a b l e t o use an o i l c o n t a i n i n g an

anti-oxidant. F i g u r e 4 shows t h e r e l a t i o n s h i p between o i l l i f e and temperature f o r t y p i c a l mineral o i l s . Many o f t h e a d d i t i v e s used i n o i l s w i l l a l s o decompose, and t h e u s e f u l l i f e o f t h e o i l may depend on t h e d e p l e t i o n o f such a d d i t i v e s . used up i n p r e v e n t i n g o x i d a t i o n ,

Anti-oxidants are

so t h a t where h i g h l y o x i d a t i v e c o n d i t i o n s occur

th e a n t i - o x i d a n t s may be r a p i d l y exhausted and leave t h e o i l unprotected.

600

1

500

LOO

I

- 300 W [L

200

2W a

5

100

k

L L O W E R TEMPERATLIRE LIMIT IMPOSED BY 'POUR POINT WHICH SOURCE, VISCOSITY, TREATMENT AND ADDITIVES]

- 100 1

I 2

I

l

l

3

4

5

I 10

20

I l l 30 LO 50

100

LIFE (HOURS) Fig.4

200 ,300LOO 500

VARIES WITH OIL,,+ 1,000

2,wO 3,000 L,OOo5.000 10,000

Temperature/l i f e 1 i m i t s f o r m i n e r a l o i l s

N

w

W

234 9.3.6

Contaminants

The q u a l i t y o f a l u b r i c a n t w i l l a l s o d e t e r i o r a t e i n use because o f contamination.

The contaminants may be s o l i d p a r t i c l e s o f wear d e b r i s o r s o l i d decompo-

s i t i o n p r o d u c t s from a f u e l o r t h e l u b r i c a n t i t s e l f , or they may be d i s s o l v e d a c i d s produced by o x i d a t i o n , o r w a t e r from c o n d e n s a t i o n o r f u e l combustion. They may a l s o e n t e r t h e system f r o m o u t s i d e ,

such as when a t m o s p h e r i c d u s t o r r a i n

e n t e r an o i l f i l l e r o p e n i n g . Some o f t h e s e c o n t a m i n a n t s can be removed by f i l t r a t i o n and o t h e r s w i l l s e t t l e o u t i n t h e r e s e r v o i r , b u t i t i s sometimes necessary t o use a d e t e r g e n t o r d i s p e r s a n t a d d i t i v e t o keep t h e contaminants condition.

i n a r e l a t i v e l y harmless d i s p e r s e d

U l t i m a t e l y t h e o i l change p e r i o d i n some systems may be determined

by t h e e x t e n t o f c o n t a m i n a t i o n .

9.4

CHOICE OF

9.4.1

BASE OIL

L i m i t a t i o n s o f Mineral O i l s

M i n e r a l o i l s a r e t h e most w i d e l y used l u b r i c a t i n g o i l s because they a r e t h e cheapest, and f o r many a p p l i c a t i o n s t h e y a r e a l s o t h e b e s t a v a i l a b l e .

The most

i m p o r t a n t l i m i t a t i o n s on t h e i r use a r i s e f r o m h i g h t e m p e r a t u r e s , f l a m m a b i l i t y and c o m p a t i b i l i t y problems, and t h e r e a r e a number o f o t h e r base o i l s w h i c h can t h e n be used as a l t e r n a t i v e s . T a b l e 9.2 shows some o f t h e i m p o r t a n t f a c t o r s i n t h e s e l e c t i o n o f a l t e r n a t i v e base o i l s .

9.4.2

H i g h Temperatures

More and more i n d u s t r i e s a r e r e q u i r i n g l u b r i c a n t s t o o p e r a t e a t temperatures t o o h i g h f o r m i n e r a l o i l s , and a l t e r n a t i v e f l u i d s a r e t h e r e f o r e becoming more and more w i d e l y used.

E s t e r s were f i r s t developed f o r a i r c r a f t j e t e n g i n e s , and

a r e now p r o b a b l y t h e most common l u b r i c a n t i n a p p l i c a t i o n s w h i c h a r e t o o h o t f o r mineral o i l s .

At h i g h e r t e m p e r a t u r e s v a r i o u s t y p e s o f s i l i c o n e can be used, and

f o r even h i g h e r t e m p e r a t u r e s p o l y p h e n y l e t h e r s a r e a v a i l a b l e , b u t b o t h s i l i c o n e s and p o l y p h e n y l e t h e r s have some s e r i o u s d i s a d v a n t a g e s i n comparison w i t h m i n e r a l o i l s and e s t e r s . Figure oils.

5 shows t h e t e m p e r a t u r e / l i f e l i m i t s f o r s e v e r a l s y n t h e t i c l u b r i c a t i n g

These l i m i t s depend on t h e a c c e p t a b l e v i s c o s i t y ,

t h e degree o f oxygen

access and t h e e x t e n t t o which d e t e r i o r a t i o n i s a c c e p t a b l e .

9.4.3

Flammability

Some i n d u s t r i e s , such as a v i a t i o n and c o a l m i n i n g , have l o n g been concerned w i t h the

f l a m m a b i l i t y o f l u b r i c a n t s and h y d r a u l i c f l u i d s .

With increasing

p l a n t i n t e g r a t i o n and l u b r i c a n t system c a p a c i t i e s , t h i s concern i s now e x t e n d i n g

T a b l e 9.2

Fluid

I m p o r t a n t p r o p e r t i e s i n s e l e c t i n g d i f f e r e n t base o i l s .

property

D i - e s t e r Neopentyl polyol (Conplex) Esters

Maximum temperature

250

300

I20

220

320

305

260

450

200

Maximum temperature i n presence of "..,~_..("C)-, oxyygjen

210

240

120

ICO

250

230

200

320

I5O

Minimum temperature due t o increase j n viscositv P C l

-35

-65

-55

-50

-30

-65

-20

0

pC;bsence

Typical Phosphate Ester

Typical Methyl Silicone

Typical Phenyl Hethyl Silicone

Chlorinated Phenyl Hethyl S i 1icone

Polyglycol (inhibited)

Polyphenyl Ether

Nineral

Remarks

Oil

comparison (for 1 .__ For e s t e r s t h i s temperature w i l l be h i g h e r i n the absence o f n e t a l s

o f oxygen

Density ( g l m l )

0.91

1.01

1.12

0.97

1.06

1.04

1.02

1.19

V i s c o s i t y index

145

140

0

200

175

195

160

-60

F l a s h p o i n t (OC)

230

255

200

310

290

270

180

275

to -50

This l i m i t i s a r b i t r a v y . I t w i l l be h i g h e r i f oxygen c o n c e n t r a t i o n i s low and l i f e i s s h o r t T h i s l i n i t depends on the power a v a i l a b l e t o o v e r c m the e f f e c t o f increased viscosity

0.88 to 140

I 5 O to

-____ A h i g h v i s c o s i t y index i s d e s i r a b l e

-

Above t h i s temperature t h e vapour o f the f l u i d m a y b e i g n i t e d by an open _. f l a w ~

Spontaneous i g n i t i o n temperature

Low

Medium

Very h i g h

High

Boundary l u b r i c a t i o n

Good

Good

Very good

High

Very h i g h

Medium

High

LOW

Fair but

F a i r but

Good

Very good

Fair

Good

poor for

poor f o r s t e e l on

This r e f e r s p r i m a r i l y t o anti-wear p r o p e r t i e s when some metal c o n t a c t i s occurring

Non-toxic

Non-tox?c

Non-toxic

Believed t o be low

Believed t o be low

Slicht

~

(None f o r very h i g h temvera-

Nitrile

s t e e l on steel

Toxicity

Slight

Slight

Sane

steel

toxicity S u i t a b l e rubbers

tiitrile. s? 1icone

Cost ( r e l a t i v e t o

mneral

011)

Silicone

Butyl, EPR

Neoprene, v i ton

tieoprene, viton

Viton, fluorosilicone

Nitrile

Corrosive t o some nanferrous metals when h o t

Enhances covosion

lioncorrosive

tioncorrosive

Corrosive in presence of water t o ferrous mta1s

Noncowosive

~

be ~

~

~

~

~~

E f f e c t on metals

Above t h i s temperature the f l u i d may i g n i t e w i t h o u t any f l a r e b e i n g present

Slightly corrosive to nonferrous metals

5

10

n presecce o f water

10

25

5C

60

5

Noncorrosive

250

Noncorrosive when pure

lhese are rough apprOXIPlatlOnS. and vary d i t h q u a l i t y and supp1:r p o s i t i o n .

N

w

u1

N

w

0)

I 1 -100

'

L-POUR

I

I

I

l

l

I

I

1

1

POINT LIMIT FOR SILICONES AND ESTERS-'

Fig.5

Temperature/life limits for some synthetic oils.

-

I

237 t o most i n d u s t r i e s . The b e s t f i r e - r e s i s t a n t

l i q u i d i s w a t e r , b u t i t i s a poor l u b r i c a n t and i n

i t s r e l a t i v e l y u n m o d i f i e d form i s o n l y used as a h y d r a u l i c f l u i d . i t s l u b r i c a t i n g performance f o r m e t a l - c u t t i n g , used.

To

mprove

d e t e r g e n t s and EP a d d i t ves a r e

For more g e n e r a l l u b r i c a n t use, v a r i o u s t e c h n i q u e s a r e employed t o i n -

crease i t s v i s c o s i t y , i n c l u d i n g t h i c k e n i n g w i t h c l a y s and n a t u r a l o r g a n i c substances, m i x i n g w i t h p o l y g l y c o l s , and e m u l s i f i c a t i o n w i t h m i n e r a l o i l . Other f i r e - r e s i s t a n t

l i q u i d s w i t h good l u b r i c a t i n g p r o p e r t i e s i n c l u d e phos-

phate e s t e r s , c h l o r i n a t e d hydrocarbons and c h l o r i n a t e d s i l i c o n e s , b u t these a l l have c o r r e s p o n d i n g disadvantages.

9.4.4

Compatibility

Problems o f i n c o m p a t i b i l i t y can a r i s e w i t h any l i q u i d .

Mineral o i l s are

i n c o m p a t i b l e w i t h n a t u r a l rubber s e a l s and hoses, e s t e r s a r e i n c o m p a t i b l e w i t h n i t r i l e s , and phosphate e s t e r s a r e i n c o m p a t i b l e w i t h many d i f f e r e n t r u b b e r s . M i n e r a l o i l s and most o t h e r c o m b u s t i b l e l i q u i d s a r e i n c o m p a t i b l e w i t h h i g h p r e s s u r e oxygen, and c e r t a i n s p e c i a l f l u o r i n e - c o n t a i n i n g l u b r i c a n t s a r e p r e f e r r e d f o r b r e a t h i n g oxygen systems.

M i n e r a l o i l s a r e u n s a t i s f a c t o r y i n con-

t a c t w i t h r e d - h o t s t e e l s because t h e y produce c a r b u r i s a t i o n , and rapeseed o i l may be used t o a v o i d t h i s problem. Most c o m p a t i b i l i t y problems can be overcome by c a r e f u l s e l e c t i o n o f s u i t a b l e base o i l s and a d d i t i v e s , b u t i n some i n d u s t r i e s such as f o o d s t u f f s ,

pharmaceu-

t i c a l s and chemicals, even t h e s m a l l e s t l e a k o f any c o n v e n t i o n a l l u b r i c a n t may be unacceptable. lubricant.

I n such a case i t may be p o s s i b l e t o use a process f l u i d as a

For example, i n sugar r e f i n i n g t h e h i g h v i s c o s i t y o f syrups and

molasses enables them t o p r o v i d e e f f e c t i v e l u b r i c a t i o n o f b e a r i n g s .

Where t h e

a v a i l a b l e process f l u i d s have i n s u f f i c i e n t v i s c o s i t y f o r e f f e c t i v e hydrodynamic l u b r i c a t i o n , e x t e r n a l p r e s s u r i s a t i o n can be used.

Such use o f process f l u i d s

may sometimes be p r e f e r r e d t o c o n v e n t i o n a l l u b r i c a t i o n , because i t may e l i m i n a t e t h e need f o r s e a l s and g l a n d s .

9.5

GREASES

A l u b r i c a t i n g grease i s a l i q u i d l u b r i c a n t which has been t h ckened t o a semi-solid consistency.

The base o i l may be m i n e r a l o i l , e s t e r

silicone, or

one o f t h e o t h e r s y n t h e t i c o i l s , and many o f t h e a d d i t i v e s used i n l u b r i c a t i n g o i l s a r e e q u a l l y e f f e c t i v e i n greases.

I n a d d i t i o n , s o l i d l u b r c a n t s such as

g r a p h i t e or molybdenum d i s u l p h i d e can c o n f e r i m p o r t a n t advantages i n greases. I t has been s a i d t h a t more i n d u s t r i a l b e a r i n g s a r e l u b r i c a t e d by grease than

by o i l .

To understand t h e reason f o r t h i s we s h o u l d c o n s i d e r a g a i n t h e s i m p l e s t

f o r m o f l u b r i c a t i o n d e s c r i b e d i n S e c t i o n 9.2,

namely t h e use o f a small quan-

t i t y o f p l a i n mineral o i l i n place i n the bearing.

There a r e two disadvantages

600

500

100

300

2 00

100

WITH UNLIMITED OXYGEN I 0

- 100 ~~

1

2

3

1 5

10

20

30 10 50

100

LIFE (HOURS1

Fig.6

Temperature/life

200 300 100500

1.000

l i m i t s f o r greases.

2.000 3.0001.wO

10,000

239 o f t h a t system wh i c h a r e due o n l y t o t h e f l u i d n a t u r e o f t h e o i l .

The f i r s t i s

t h a t i t i s d i f f i c u l t t o r e t a i n t h e o i l i n t h e b e a r i n g f o r any l e n g t h o f tim e . The second i s t h a t i t i s d i f f i c u l t t o s t o p d i r t o r o t h e r c o n ta m in a n ts g e t t i n g i n t o the bearing. The use o f a g r e a s e may overcome b o t h o f t h e s e problems w h i l e r e t a i n i n g t h e s i m p l i c i t y and economy o f t h e system.

The grease i n a b e a r i n g o f t e n d i s t r i b u t e s

i t s e l f q u i t e q u i c k l y i n such a way t h a t a v e r y s m a ll p r o p o r t i o n i s a c t i v e l y l u b r i c a t i n g the bearing, while the bulk i s outside the faces,

c r i t i c a l bearing sur-

f o r m i n g a s e a l a g a i n s t loss o f l u b r i c a n t o r i n g r e s s o f c o n ta m in a n ts .

The c h i e f l i m i t a t i o n on t h e use o f g r e a s e s i s t h e l a c k o f c o o l a n t f l o w t o remove s u r p l u s h e a t when t h e b e a r i n g i s l a r g e , f a s t o r h e a v i l y loaded, b u t t h e r e a r e greases w h i c h w i l l o p e r a t e temperature/life

9.6

a t t e m p e r a t u r e s as h i g h as 250°C.

F i g u r e 6 shows

l i m i t s f o r some t y p i c a l g r e a s e s .

S O L I D LUBRICANTS S t r i c t l y speak i n g , s o l i d l u b r i c a n t s a r e s o l i d m a t e r i a l s w h ic h can be i n t r o -

duced between b e a r i n g s u r f a c e s t o reduce f r i c t i o n or wear, b u t t h e t i t l e i s n o r m a l l y used i n a more r e s t r i c t e d sense t o c o v e r o n l y r e d u c t i o n i n f r i c t i o n o r p r e v e n t i o n o f adh e s i o n o r s e i z u r e .

I t t h e r e f o r e e x c lu d e s h a r d w e a r - r e s i s t a n t

c o a t i n g s and f r i c t i o n s u r f a c e s such as n o n - s k i d c o a t i n g s and b r a k e m a t e r i a l s . Tabl e 9 . 3 l i s t s a l a r g e number o f d i f f e r e n t s o l i d l u b r i c a n t s , b u t t h e m a j o r i t y o f appl i c a t i o n s use e i t h e r molybdenum d i s u l p h i d e , g r a p h i t e or PTFE ( p o l y t e t r a fluoroethylene).

They can a l l be used i n a v a r i e t y o f ways,

powder, rubbed f i l m ,

bonded f i l m o r s p u t t e r e d f i l m .

including free

G r a p h i t e and PTFE may a l s o

be used i n t h e f o r m o f s o l i d b l o c k s , as may many o t h e r p o ly m e r s , and i n t h i s form t hey a r e commonly known as d r y b e a r i n g m a t e r i a l s . The i m p o r t a n t p r o p e r t i e s o f molybdenum d i s u l p h i d e , g r a p h i t e and PTFE a r e l i s t e d i n T a b l e 9.3. The advantages o f t h e common s o l i d l u b r i c a n t s a r e t h e i r w id e te m p e r a tu r e ranges, chem i cal i n e r t n e s s , low v o l a t i l i t y , and t h e f a c t s t h a t t h e y do n o t need s e a l i n g , do n o t need f e e d systems and

g e n e r a l l y do n o t cause c o n t a m i n a t i o n .

T h e i r di sadvant ag e s compared w i t h o i l or grease a r e r e l a t i v e l y h i g h f r i c t i o n , l a c k o f h e a t removal, f a i l u r e t o p r e v e n t c o r r o s i o n , and s te a d y wear i n s e r v i c e . They a r e t h e r e f o r e o n l y used where t h e i r advantages o v e r o i l s and greases o u t weigh t h e i r

di sa d v a n t a g e s ,

such as a t h i g h o r v e r y l o w te m p e r a tu r e s ,

i n vacuum,

where v e r y l o n g l i f e i s r e q u i r e d w i t h o u t r e l u b r i c a t i o n , o r where c o n t a m i n a t i o n is critical.

9.7

GAS LUBRICATION Gases can be used as l u b r i c a n t s i n t h e same way as o i l s , and t h e p h y s i c a l

laws g o v e r n i n q hydrodynamic o i l l u b r i c a t i o n a o o l v a l s n

tn naqe4

The v p r v

Inw

240

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241 v i s c o s i t y o f gases means t h a t t h e f i l m t h i c k n e s s i s a l s o v e r y low, so t h a t hydrodynamic gas b e a r i n g s ("gas-dynamic

bearings")

are 1 imited t o conditions

o f h i g h speed, low load, small c l e a r a n c e s and v e r y c l o s e l y c o n t r o l l e d t o l e r a n c e s . For t h i s reason e x t e r n a l l y p r e s s u r i s e d gas b e a r i n g s a r e more commonly used which can c a r r y h i g h e r loads, a r e l e s s c r i t i c a l on c l e a r a n c e s and t o l e r a n c e s , and can be used a t lower o r even z e r o speeds. Gas l u b r i c a t i o n can be used a t h i g h e r o r lower temperature than o i l o r grease, but t h e i r main advantages a r e i n g i v i n g h i g h s t i f f n e s s i n h i g h speed p r e c i s i o n bearings, such as i n d e n t a l d r i l l s and p r e c i s i o n g r i n d i n g s p i n d l e s , and i n a v o i d i n g s e a l i n g and c o n t a m i n a t i o n problems where a s u i t a b l e process gas i s a v a i l a b l e t o a c t as t h e gas l u b r i c a n t .

242

I() LUBRICANT ADDITIVES Their Application

, Performance and Limitations

D . M . SOUL, L u b r i z o l I n t e r n a t i o n a l L a b o r a t o r i e s , Hazelwood, Derby, U.K.

10.1

I NTRODUCT I ON

The l u b r i c a t i o n o f i n d u s t r i a l p l a n t and equipment covers t h e whole range o f requirements f o r machinery i n c l u d i n g t h e i n t e r n a l combustion engine and t h e gas turbine.

Dis c u ssi o n o f such a wide s u b j e c t i n one chapter would be impossible

b u t a v e r y wide f i e l d o f a d d i t i v e a p p l i c a t i o n s e x i s t s a p a r t from those f o r en g in e and t r a n s m i s s i o n systems.

These i n c l u d e h y d r a u l i c and gear l u b r i c a n t s ,

metal c u t t i n g and f ormi n g f l u i d s ,

t u r b i n e o i l s , compressor and r e f r i g e r a t o r

o i l s , f i r e - r e s i s t a n t f l u i d s , greases, and a m i s c e l l a n y o f s p e c i a l i s e d products. L u b r i c a n t a d d i t i v e s a r e chemicals i n c o r p o r a t e d i n t o e i t h e r a l i q u i d base (m in e r a l o i l , s y n t h e t i c f l u i d or w a t e r ) ,

a semi-fluid,

o r a grease.

I n general,

th e a d d i t i v e s a r e s o l u b l e o r u n i f o r m l y di sp ersed throughout t h e c a r r i e r medium. I n t h i s r e s p e c t t h i s d i s c u s s i o n does n o t cover s o l i d l u b r i c a n t a d d i t i v e s such as g r a p h i t e ,

molybdenum d l s u l p h i d e and polytetrafluoroethylene.

A d d i t i v e s a r e used t o improve t h e performance o f t h e base c a r r i e r m a t e r i a l t o p r o v i d e a d e s i r e d p h y s i c a l , chemical, o r mechanical p r o p e r t y .

They a r e manufac-

t u r e d t o s t r i c t q u a l i t y c o n t r o l standards t o assure c o n s i s t e n t performance i n s i m i l a r a p p l i c a t i o n s , i n many environments t hroughout t h e w o r l d , base m a t e r i a l and t h e a d d i t i v e system.

f o r both the

Many l u b r i c a n t s p e c i f i c a t i o n s c o v e r i n g

a wide range of i n d u s t r i a l processes can o n l y be s a t i s f i e d by t h e use o f these a d d i t i o n a l chemicals and c e r t a i n t e s t s a r e designed s p e c i f i c a l l y t o assess such factors. 10.2

B A S I C PROPERTIES OF LUBRICANTS

All types o f l u b r i c a t i n g f l u i d s d i s p l a y c e r t a i n b a s i c p r o p e r t i e s .

These may

be used t o advantage i n s e l e c t i n g t h e most a p p r o p r i a t e l u b r i c a n t f o r any

243 application.

I n a p p l i c a t i o n s where hydrodynamic l u b r i c a t i o n f i l m s a r e formed,

an u n t r e a t e d base o i l o f t h e c o r r e c t v i s c o m e t r i c p r o p e r t i e s can p r o v i d e a p e r f e c t l y adequate l u b r i c a t i n g medium.

The p r o p e r t i e s w i t h a p p r o p r i a t e t e s t

references f o r s t an da rd e v a l u a t i o n t e s t s a r e g i v e n i n Table 10.1. M in e r a l o i l p r o p e r t i e s a r e dependent on t h e crude o i l source and t h e r e f i n e r y processes o f s e p a r a t i o n and t rea t me nt .

Lubricating o i l fractions o f varying

By s e l e c t i o n o f t h e processes used i n t h e r e f i n e r y ,

v i s c o s i t y a r e produced.

p r o p e r t i e s such as pour p o i n t , c o l o u r and v i s c o s i t y index may be c o n t r o l l e d o r m o d ifie d so as t o produce b e t t e r f i n a l p r o d u c t s .

10.3

LUBRICANT ADDITIVES

I n Ta b le 10.2 i s a l i s t o f t he a d d i t i v e s n o r m a l l y employed i n l u b r i c a n t s . This t a b l e g i v e s t h e g en era l name o f each t ype t o g e t h e r w i t h a summary o f i t s purpose and a suggested mechanism o f a c t i o n . 10.3.1 10.3.1.1

Detergent and D i s p e r s a n t A d d i t i v e s De t e r g en t Types

A v a r i e t y o f me t a l c o n t a i n i n g compounds c a l l e d detergents' have been described T h e i r f u n c t i o n appears t o be one o f e f f e c t i n g a d i s p e r s i o n

i n the l i t e r a t u r e .

o f p a r t i c u l a t e m a t t e r and n e u t r a l i s i n g

d i r t and d e b r i s .

a c i d s r a t h e r than one o f c l e a n i n g up

Some o f t he se m a t e r i a l s a l s o f u n c t i o n as r u s t i n h i b i t o r s and

emu 1 s i f i e r s . The compounds a r e g e n e r a l l y mo l ecu l es h a v i n g a l a r g e hydrocarbon t a i l and a "polar group" head.

The t a i l s e c t i o n serves as a s o l u b i l i z e r i n t h e base o i l ,

w h i l e t h e p o l a r group serves as t h e f u n c t i o n a l p a r t o f t h e molecule, which a t t r a c t s p a r t i c u l a t e contaminants i n t h e l u b r i c a n t . The most w i d e l y used members o f t h e o r g a n o m e t a l l i c c l a s s a r e t h e sulphonates and phenates.

These i n c l u d e b o t h t h e n e u t r a l barium, c a l c i u m and magnesium

s a l t s , and t h e a l k a l i n e o r h i g h l y b a s i c p r o d u c t s prepared from these o r g a n i c s u b s t r a t e s by i n c o r p o r a t i n g metal i n excess o f t h e s t o i c h i o m e t r i c q u a n t i t i e s i n t o those compounds.

Phosphonates and s a l i c y l a t e s a r e a l s o used commercially

as s u b s t r a t e s f o r met al c o n t a i n i n g a d d i t i v e s . Commercial i n t e r e s t i n barium compounds i s d e c l i n i n g due t o c o s t o f product i o n and e n v ir o n me nt al c o n s i d e r a t i o n s . The meaning o f t h e terms " n e u t r a l " Neutral s a l t

-

and "ba sic" metal s a l t used above:

i s a s a l t o f an a c i d which c o n t a i n s t h e s t o i c h i o m e t r i c amount o f metal r e q u i r e d f o r n e u t r a l i z a t i o n o f t h e a c i d i c group present.

Basic s a l t

-

i s a s a l t o f an a c i d which c o n t a i n s more metal than i s required f o r the n e u t r a l i z a t i o n o f the a c i d i c groups p r e s e n t .

Such excess metal may be present

Lu

P P

Table 10.1 Standard L u b r i c a t i n g O i l Tests

Test

Scope o f T e st

P r a c t i c a l use o f

Data

I . P e t . Method Number and Related Methods

S p e c i f i c Gravi t y

Q u a l i t y o f o i l . Naphthenic h i g h , P a r a f f i n i c medium. A ro matic low. B a t ch c o n t r o l .

Volume t o w e i g h t conversions

160/68,

V i scos i t y

R e l a t i o n o f 'b od y' o i l t o t emp era t u re. Norma l l y e v a l u a t e d a t two temperatures, A l s o v a r i o u s methods f o r low tempera t u r e s and v a r y i n g shear r a t e s .

V i s c o s i t y Index. R e f i n i n g processes. 'Flow ' o f o i l i n machinery.

71/79

Flash P o i n t

Volatility, classification o f fluids, inflammability.

Colour

Depth o f c o l o u r r e l a t e d t o r e f i n e r y processes.

Storage and o p e r a t i n g c o n d i t i o n s o f o i l . 34/75 (Pensky-Martin) T a x a t i o n and s h i p p i n g o f Petroleum products. 170/75 (Abel) 36/67 (Cleveland) Contamination. 15/72

Pour/Cloud P o i n t

R e f i n i n g processes dewaxing. E f f e c t o f low temperatures.

Low temperature v i s c o s i t y and o p e r a t i o n .

Acidity

T ra ce resi d ue s from r e f i n e r y processes.

N ormally low f o r new o i l . D e t e r i o r a t i o n o f used o i l .

1 /74

Carbon Residue

Carbon r e s i d u e a f t e r b u r n i n g o f f oil.

Performance o f h i g h q u a l i t y base high. Paraffinic low. Naphthenic

13/78 (Conradson) 14/65 (Ramsbottom)

-

-

-

59/72,

189179

15/67 (Pour) 219/67 (Cloud)

245 due t o a t r u e b a s i c s a l t s t r u c t u r e , b u t a more l i k e l y e x p l a n a t i o n i s t h a t t h e excess metal i s present i n the f orm o f d i s p e r s e d metal compounds.

Sul phonates

10.3.1.1.1

Normal metal sul p ho na t e s d e r i v e d from "mahogany"

a c i d s ( t h e mahogany-coloured

petroleum s u l p h o n i c a c i d s o b t a i n e d as a b y-product d u r i n g w h i t e o i l manufacture) were f i r s t employed as d e t e r g e n t a d d i t i v e s i n commercial crankcase o i l s d u r i n g World War 1 1 .

Almost w i t h o u t e x c e p t i o n ,

was c a l c i u m o r barium.

a d d i t i v e s w i t h r e s p e c t t o detergency, bea r in g metal

t h e metal p r e s e n t i n such sulphonates

P e t ro l eu m sul p ho na t e s were n o t o n l y s u p e r i o r t o e a r l i e r b u t were much l e s s c o r r o s i v e t o s e n s i t i v e

a l l o y s and responded w e l l t o c o r r o s i o n i n h i b i t o r s .

They can be

rep r e s e n te d by t h e g en era l f o rmul a : RxArS03M where R A r r e p r e s e n t s complex a l k y l a r o m a t i c r a d i c a l s d e r i v e d from petroleum and M i s one e q u i v a l e n t o f a p o l y v a l e n t met al .

Supplementing t h e su pp l y o f

n a t u r a l p et roleum sulphonates a r e t h e s y n t h e t i c

sulphonates d e r i v e d f ro m l on g-ch ai n a l k y l s u b s t i t u t e d benzenes (e.g.

polydodecyl

benzene bottoms) o b t a i n e d as b y-pro du ct s i n t h e manufacture o f household d e t e r gents o r wh ic h a r e manufactured s p e c i f i c a l l y f o r t h i s use. Highly basic

su l ph on at es c o n t a i n from 3 t o 10 o r 15 times as much metal as

the c o r r e s p o n d in g normal su l ph on at es. "hyperbasic"

sulphonates,

C a l l e d "overbased",

"superbasic"

or

t he se p r o d u c t s a r e manufactured by h e a t i n g a m i x t u r e

o f c e r t a i n promoters o r s o l v e n t s w i t h a n e u t r a l sulphonate and a l a r g e excess o f metal o x i d e o r h yd roxi d e and c a r b o n a t i n g w i t h carbon d i o x i d e t o c o n v e r t the metal base t o c o l l o i d a l l y - d i s p e r s e d metal carbonate.

Overbased sulphonates possess

t h e a b i l i t y t o n e u t r a l i z e a c i d i c contaminants formed i n l u b r i c a t i n g o i l s and thus reduce c o r r o s i v e wear o f en gi n e components. 10.3.1.1.2

Phosphonates and/or Thiophosphonates

These d e t e r g e n t a d d i t i v e s can be re pre sen t ed by t h e general formula:

R

-

8 P

(XM12

where R i s a l a r g e a l i p h a t i c r a d i c a l o f a t l e a s t 500 m o l e c u l a r w e i g h t . oxygen and/or s u l p h u r , and metal.

500

-

X is

M i s one e q u i v a l e n t o f a monovalent o r p o l y v a l e n t

They a r e pre pa red by f i r s t h e a t i n g a p o l y o l e f i n such a s polybutenes of

2,000 m o l e c u l a r w e i g h t w i t h a phosphorus reagent, g e n e r a l l y phosphorus

p e n t a s u l p h i d e , t o form a complex o r g a n i c phosphorus-sulphur compound which i s the n n e u t r a l i z e d w i t h a metal base.

By steam treatment o r prolonged h y d r o l y s i s

p r i o r t o o r d u r i n g n e u t r a l i z a t i o n , a p o r t i o n o r s u b s t a n t i a l l y a l l o f the sulphur

TABLE 1 0 . 2 Lubricant A d d i t i v e

Additive

Chemicals

Purpose o f A d d i t i v e

How Additive Works

Antioxidant

Hindered phenols hines Organic sulphides Zinc dithiophosphate

Minimizes the formation o f resins, varnish, acids, sludge and polymers

Reduces organic peroxides terminating the oxidation chain. Reduces formation o f acids by decreasing oxygen taken up i n the o i l . Prevents c a t a l y t i c reactions.

Corrosion I n h i b i t o r

Zinc d i thiophosphates Sulphurized terpenes Phosphosulphurized terpenes Sulphurized o l e f i n s

Protects bearing and other metal surfaces from corrosion

Acts as a n t i c a t a l y s t . Coats metal surfaces which p r o t e c t against a c i d and peroxide attack.

Rust I n h i b i t o r

h i n e phosphates Sodi um.ca1 c i urn and Magnesi um sulphonates A l k y l succinic acids F a t t y acids

Protects ferrous metal surfaces against r u s t

Polar molecules are absorbed p r e f e r e n t i a l l y on the metal surface and serves as a b a r r i e r against water. Neutralizes acids.

Detergent

Normal or basic, calcium, b a r i um, magnesium phosphonates, phenates and sul phonates

Reduces o r prevents deposits i n engines operated a t high temperatures

Controls buildup of varnish and sludge by r e a c t i n g w i t h o x i d a t i o n products t o form o i l soluble material which remains suspended i n the o i l .

Dispersant

Polymers such as n i t r o g e n containing polymethacrylates, a l k y l succinimides, and succinate esters high molecular weight amines and amides

Prevents and r e t a r d s sludge formation and deposition under low temperature operating conditions

Dispersants have a strong a f f i n i t y for d i r t p a r t i c l e s and surround each w i t h o i l soluble molecules which keep the sludge frm agglomerating and depositing i n the engine.

F r i c t i o n Modifier

F a t t y acids F a t t y amines Fats

To increase o i l f i l m strength

Highly p o l a r molecules are absorbed on the metal surface and remain i n place t o cushion and keep metal surfaces apart.

Zinc d i a l k y l d i thiophosphate T r i c r e s y l phosphate

Reduce r a p i d wear i n steel-onsteel applications

Antiwear

t o prevent o i l f i l m rupture

A d d i t i v e reacts w i t h the metal t o form a compound which i s deformed by p l a s t i c f l o w t o allow a new d i s t r i b u t i o n of load.

Extreme Pressure

Sulphurized f a t s , o l e f i n s Chlorinated hydrocarbons Lead s a l t s o f organic acids. Amine phosphates

Prevents seizure and welding between metal surfaces under c o n d i t i o n o f extreme pressure and temperature

EP agent reacts w i t h metal surfaces t o form new compounds having lower shear strength than the base metal and i s sheared p r e f e r e n t i a l l y t o the base metal.

V i s c o s i t y Index Improver

Polyisobutylenes Polymethacrylates Polyacryl ates Ethylene propylene copolymers Styrene maleic e s t e r copolymers Hydrcgenated styrene butadiene copolymers

Reduces the r a t e o f change o f v i s c o s i t y w i t h temperature

Polymer molecule assumes a compact curled form i n a poor solvent ( c o l d o i l ) and an uncurled high surface area i n a b e t t e r solvent ( h o t o i l ) . The uncurled form thickens the o i l .

Pour Depressant

Wax a l k y l a t e naphthalene Wax a k l y l a t e d phenols Polymethacrylates

Lowers the pour p o i n t of t h e o i l

Retards the formation o f f u l l - s i z e wax c r y s t a l s by coating o r c o - c r y s t a l l i z a t i o n w i t h the wax.

Anti foam

S i l i c o n e polymers Polymthacrylates

Prevents the fonnation o f s t a b l e foam

Appears t o attack the o i l f i l m surrounding each bubble reducing i n t e r f a c i a l tension. The small bubbles l i b e r a t e d combine t o form large ones which f l o a t t o the surface.

Emu1s i f ie r

Sodium s a l t s of sulphonic acids, sodium s a l t s o f organic acids, f a t t y mine salts

To nake mineral o i l m i s c i b l e w i t h water

Emulsifier i s absorbed a t the o i l - w a t e r i n t e r f a c e t o reduce i n t e r f a c i a l tension r e s u l t i n g i n an i n t i m a t e dispersion o f one l i q u i d i n the other.

Tackiness

Soaps, polyisobutylene and p o l y s c r y l a t e polymers

To provide the o i l w i t h greater cohension

Increases v i s c o s i t y . and s t r i n g y .

Antiseptic

Phenols, c h l o r i n e compounds, Increases emulsion l i f e and formaldehyde bases prevents odour

Prevents and reduces microorganism growth.

Metal Deactivator

T r i a r y l phosphi tes Sulphur compounds Diamines. Dimercapto t h i a d i a z o l e derivatives.

A p r o t e c t i v e f i l m i s absorbed on metal surfaces which prevents contact between corrosive agents and base metal.

Stop the c a t a l y t i c e f f e c t o f metals on o x i d a t i o n and corrosion

Materials themselves are tacky

N

P 4

248 pr e s e n t i n t h e thiophosphonate group, -P(S) (SMl2, can be s u b s t i t u t e d w i t h oxygen t o y i e l d a phosphonate. The h i s t o r y o f these d e t e r g e n t a d d i t i v e s c l o s e l y p a r a l l e l s t h a t o f sulphonate a d d i t i v e s i n t h a t b a s i c and overbased s a l t s have r e p l a c e d normal s a l t s i n a l l b u t a few commercial a p p l i c a t i o n s .

The manufacture o f such b a s i c and overbased

s a l t s i s c a r r i e d o u t by t h e use o f methods l i k e those d e s c r i b e d i n t h e SULPHONATES sect ion.

10.3.1.1.3

Phenates

Phenates and p he na t e -sul p hi d es

have p l a y e d an i m p o r t a n t r o l e as d e t e r g e n t

a d d i t i v e s e v e r s i n c e t h e i r i n t r o d u c t i o n d u r i n g World War 1 1 .

Among t h e e a r l i e s t

a d d i t i v e s o f t h i s t y p e t o g a i n commercial acceptance were: Calcium and ba ri um phenates o f t e r t i a r y - o c t y l p h e n o l s u l p h i d e and t e r t i a r y - a m y l p h e n o l

sulphide having

t h e ge ne ral f ormu l a:

011

R

R

Calcium phenates o f t e r t i a r y - a m y l p h e n o l -formaldehyde condensation p rod uct s.

OH

OH

R

R

Calcium and b ari u m phenates o f p a r a f f i n wax s u b s t i t u t e d phenol h a v i n g t h e g en era l f ormula:

OM

I n a d d i t i o n t o p e r f o r m i n g a d e t e r g e n t r o l e , phenates sulphides

-

-

and e s p e c i a l l y phenate

e x h i b i t s u b s t a n t i a l a n t i - o x i d a n t p r o p e r t i e s and a r e p a r t i c u l a r l y

u s e f u l f o r h i g h temperature f l u i d s . L i k e t h e o t h e r f a m i l i e s o f d e t e r g e n t a d d i t i v e s discussed, phenates have r e p l ace d normal phenates i n most a p p l i c a t i o n s .

b a s i c and overbased Manufacturing

249 te c h n iq u e s f o r such p r o d u c t s a r e s i m i l a r t o those employed f o r b a s i c sulphonates o r phosphonates. 10.3.1.1.4

Alkyl Substituted Salicylates

These a d d i t i v e s can be rep rese nt ed by t h e general formula:

0

II

-

C

OM

where R i s an o i l - s o l u b i l i z i n g o r g a n i c r a d i c a l and M i s one e q u i v a l e n t o f a polyvalent metal.

One o f t h e f i r s t a d d i t i v e s o f t h i s t ype t o see commercial use was t h e z i n c carboxylate o f di-isopropyl

s a l i c y l i c acid.

More r e c e n t l y , c a l c i u m c a r b o x y l -

at e s of lo n g - c h ai n a l k y l s a l i c y l i c a c i d s have been used.

The manufacture of

such a d d i t i v e s i n v o l v e s c a r b o x y l a t i o n o f a metal phenate w i t h carbon d i o x i d e . As w i t h t h e o t h e r d e t e r g e n t a d d i t i v e s discussed, overbasing techniques have been employed t o p rep are improved p r o d u c t s . 10.3.1.2

Mode of A c t i o n o f Det erg en t A d d i t i v e s

A lt h o u g h t h e mechanism o f d et erg en cy i n non-aqueous media such as m i n e r a l o i l s i s n o t f u l l y understood, o f "soap m i c e l l e s "

re sea rche rs have found evidence f o r t h e e x i s t e n c e

i n non-aqueous s o l v e n t s .

There i s reason, then,

t o believe

t h a t d e t e r g e n t a d d i t i v e s i n m i n e r a l o i l s o l u t i o n can a c t i n a manner s i m i l a r t o aqueous soap s o l u t i o n s .

B a s i c and overbased d e t e r g e n t s a l s o possess t h e a b i l i t y

t o n e u t r a l i z e h armf ul i n o r g a n i c and o r g a n i c a c i d s w hich accumulate i n crankcase lubricants during service.

They can a l s o a c t as h i g h temperature s t a b i l i s e r s as

means o f r e d u c in g thermal d eco mpo si t i o n o f o t h e r a d d i t i v e s by n e u t r a l i s i n g small amounts o f a c i d i c p r o d u c t s which c o u l d cause c a t a l y t i c decomposition r e a c t i o n s t o occur.

10.3.1.3

Dispersant A d d i t i v e s

The te r m " d is pe rsan t "

i s p r e s e n t l y used t o designate a d d i t i v e s w hich a r e

capable o f d i s p e r s i n g t h e " c o l d sl ud ge '' formed i n engines operated f o r t h e most p a r t a t r e l a t i v e l y low b u l k crankcase o i l temperatures.

Unless maintained i n

f i n e suspension i n t h e l u b r i c a t i n g o i l , t h i s sludge d e p o s i t s on o i l f i l t e r s , v a lv e t r a i n components, and o i l c o n t r o l r i n g s where i t i n t e r f e r e s w i t h good engine performance. Since known m e t a l - c o n t a i n i n g

d e t e r g e n t s d i d not appear t o o f f e r a s o l u t i o n

t o t h e c o l d s lu d ge problem, re sea rche rs t u r n e d t h e i r a t t e n t i o n t o m e t a l - f r e e

250 o r g a n i c compounds i n t h e hope t h a t an "ash l ess detergent" would p r o v i d e the answer. Such p r o d u c ts can a l s o be used t o e f f e c t d i s p e r s i o n o f i n s o l u b l e m a t e r i a l in oil,

i.e.

c o l l o i d a l dispersions,

and a l s o t o d i s p e r s e w ater i n o i l t o produce

s t a b l e i n v e r t emulsions c o n t a i n i n g up t o f o r t y percent w ater. The compounds which a r e u s e f u l f o r t h i s purpose a r e a g a i n c h a r a c t e r i z e d by a p o l a r group a t t a c h e d t o a r e l a t i v e l y h i g h m o l e c u l a r w e i g h t hydrocarbon c h a i n . The p o l a r group g e n e r a l l y c o n t a i n s one or more o f t h e elements: n i t r o g e n , oxygen and phosphorus.

The s o l u b i l i z i n g c h a i n s a r e based on p o l y i s o b u t y l e n e .

D i s p e r s a n t s may be d i v i d e d i n t o v a r i o u s chemical f a m i l i e s . 10.3.1.3.1

Copolymers

Copolymers which c o n t a i n a c a r b o x y l i c e s t e r f u n c t i o n and one o r more a d d i t i o n a l p o l a r f u n c t i o n s such as amine,

imide, h y d r o x y l , e t h e r , epoxide, phosph-

o r u s e s t e r , c a r b o x y l , an hyd ri de , o r n i t r i l e g e n e r a l l y have d i s p e r s a n t p r o p e r t i e s . Some o f t h e s e polymers e x h i b i t v i s c o s i t y m o d i f y i n g p r o p e r t i e s and thus f i n d a p p l i c a t i o n as m u l t i - f u n c t i o n a l a d d i t i v e s . a r e i n used today:

Three types o f d i s p e r s a n t V I

improver

p o l y m e t h a c r y l a t e s , s t y r e n e - m a l e i c e s t e r copolymers and

ethy le n e - p r o p y le n e copolymers. 10.3.1.3.2

S u b s t i t u t e d Succinimides

Hydrocarbon polymers may be i n t r o d u c e d i n t o molecules by chemical r e a c t i o n . T y p i c a l p r o d u c t s o f t h i s t y p e a r e o b t a i n e d by t r e a t i n g a p o l y o l e f i n , w i t h a molec u l a r w e i g h t i n t h e range 500-2000,

w i t h phosphorus c h l o r i d e and phosphorus sul-

p h i d e and t h e n w i t h rea ge nt s such as ure a, e t h y l e n e o x i d e and b o r i c a c i d . The N - s u b s t i t u t e d l o n g c h a i n a l k e n y l su cci nimides c o n t a i n t h e c h a r a c t e r i s t i c s u c c i n im id e g r o u p i n g

where R c o n t a i n s upwards o f 50 carbon atoms. extensively i n 10.3.1.3.3

T h i s group o f p r o d u c t s i s used

many t yp es o f crankcase l u b r i c a n t .

Amides

H ig h m o le c u la r w e i g h t amides and polyamides a r e u s u a l l y prepared by t h e r e a c t i o n o f h i g h e r f a t t y a c i d s o r e s t e r s w i t h p o l y a l k y l e n e amines.

Products o f

t h i s t y p e a r e g e n e r a l l y used i n t h e l u b r i c a t i o n o f tw o-cycle engines and l i t t l e i n a u t o m o tiv e crankcase l u b r i c a n t s .

251 10.3.1.3.4

Other Chemicals

E x t e n s iv e r e s ea rch i n t o o t h e r cl asse s o f d e t e r g e n t c h e m i s t r y c o n t i n u e s . Products may be produced f rom t h e f o l l o w i n g : P o ly and benzyl amines H ig h m o l e c u l a r w e i g h t e s t e r s whi ch may be used f o r e m u l s i f i c a t i o n purposes Amine s a l t s o f h i g h m o l e c u l a r w e i g h t a c i d s 10.3.1.4

Mode o f A c t i o n o f Di spe rsan t A d d i t i v e s

Based on microscopy and chromatography s t u d i e s , several groups o f researchers have concluded t h a t a shl e ss d i s p e r s a n t s f u n c t i o n by a d s o r p t i o n on contaminant p a r t i c l e s p r e s e n t i n o i l s , t h us keeping them i n suspension.

Although these r e -

searchers have c o n t r i b u t e d much t o an u nd erst anding o f t h e mechanism o f d i s p e r sancy, t h e r e i s wide agreement t h a t i t i s necessary t o e v a l u a t e t h e p r o d u c t s i n p r a c t i c a l t r i a l s s i n c e t h e r e i s l i t t l e i n t h e way o f l a b o r a t o r y t e s t i n g which can compare p r o d u c t s w i t h t h i s f u n c t i o n . 10.3.2

A n t i o x i d a n t s and B e ari n g C o r r o s i o n I n h i b i t o r s

A n t i o x i d a n t s a r e p r o b a b l y employed i n a w i d e r v a r i e t y o f l u b r i c a n t s than any othe r k i n d o f a d d i t i v e .

I n a d d i t i o n t o t h e i r use i n crankcase o i l s and steam

t u r b i n e o i l s , they w i l l be found i n gas t u r b i n e l u b r i c a n t s , automatic t r a n s m is s io n f l u i d s , gear o i l s , c u t t i n g o i l s , greases and h y d r a u l i c f l u i d s . The f u n c t i o n o f an o x i d a t i o n i n h i b i t o r i s t h e p r e v e n t i o n of d e t e r i o r a t i o n ass o c ia t e d w i t h oxygen a t t a c k on t h e l u b r i c a n t base f l u i d .

Inhibitors function

e i t h e r t o d e s t r o y pe roxi d es o r t h e f r e e r a d i c a l s d e r i v e d from p e r o x i d e s .

The

most w i d e l y used a n t i o x i d a n t s i n t h e l u b r i c a n t f i e l d a r e the p h e n o l i c types, s u l p h u r i s e d p o l y o l e f i n s and t h e z i n c d i t h i o p h o s p h a t e s .

The phenols a r e c o n s i -

dered t o be t h e cha i n-b rea ki ng t y p e whereas t h e l a t t e r two a r e b e l i e v e d t o be p e r o x i de d e s t r o y e r s

.

The c o r r o s i o n o f b e a r i n g m e t a l s i n i n t e r n a l combustion engines i s g e n e r a l l y c on s id e r e d t o be due l a r g e l y t o a c i d o r a c t i v e s u l p h u r a t t a c k on t h e b e a r i n g m e t a ls .

The a c i d s i n v o l v e d i n t h e a t t a c k o r i g i n a t e e i t h e r from p r o d u c t s o f

incomplete combustion o f t h e f u e l which f i n d t h e i r way i n t o t h e l u b r i c a n t as blow-by gases o r from t h e o x i d a t i o n o f t h e l u b r i c a n t .

O x i d a t i o n i n h i b i t o r s can

reduce o r e l i m i n a t e t h e l a t t e r m a t e r i a l s and, hence, reduce b e a r i n g c o r r o s i o n . G e n e r a lly ,

i n most i n d u s t r i a l a p p l i c a t i o n s ,

o x i d a t i o n i s t h e major cause o f a c i d

re 1 ease. I n most environments i n which a l u b r i c a t i n g o i l i s employed,

i t comes i n con-

t a c t w i t h a i r o f t e n a t h i g h temperatures and i n t h e presence o f m e t a l s o r chemic a l compounds wh i ch promote o x i d a t i o n o f t h e o i l .

The o i l undergoes a complex

s e r i e s o f o x i d a t i o n r e a c t i o n s and t h e h armf ul r e s u l t s i n c l u d e , p r i n c i p a l l y , an

252 i nc r e a s e i n v i s c o s i t y o f t h e l u b r i c a n t , t h e f o r m a t i o n o f a c i d i c contaminants such as " p e tr o le u m oxya ci ds" and t h e development o f carbonaceous m a t e r i a l . Among t h e more e f f e c t i v e chemicals employed as commercial a n t i o x i d a n t s today are:10.3.2.1

Dith ioph osp ha t e s

Zinc diorganodithiophosphates ( a l s o e f f e c t i v e i n the r o l e o f corrosion i n h i b i t o r s ) . Other me t a l s may a l s o be used. Ashless d i t h i o p h o s p h a t e s , 10.3.2.2

r e p l a c i n g me t a l by amine can be e q u a l l y e f f e c t i v e .

Hin d e re d Phenol

Hin d e r e d phenols ( i . e . phenols i n whi ch t h e h y d r o x y l group i s s t e r i c a l l y bl o c k e d or "hindered") : 2, 6-di-tertiary-butyl-4-methylphenol

4, 4'-methy 1ene b i s (2,6-d i- t e r t ia r y - b u t y 1phenol ) 4, 4'- t h i o b is (2-methy 1-6-t e r t ia r y - b u t y 1 phenol 10.3.2.3

N i t r o g e n Bases

Amines such as:

N-phenyl-alpha-naphthylamine N-phenyl-beta-naphthylamine

Tetramethyldiaminodiphenylmethane Anthranil i c acid P h e n ot h i azi n e and a l k y l a t e d d e r i v a t i v e s 10.3.2.4

S u l p h u r i s e d p o l y o l e f i n s , where t h e sulphur a c t s i n a s i m i l a r manner

t o n a t u r a l l y o c c u r r i n g s u l p h u r chemicals found i n crude o i l b u t removed d u r i n g refining. O f a l l these a n t i o x i d a n t s , phosphates.

t h e most w i d e l y used a r e t h e z i n c d i o r g a n o d i t h i o -

These have a dual f u n c t i o n o f behaving a l s o as a n t i w e a r agents t o

p r o t e c t cam and t a p p e t s c u f f i n g when used i n crankcase o i l s .

These compounds

a r e a l s o used e x t e n s i v e l y as m i l d l o a d - c a r r y i n g a d d i t i v e s i n gear oils and i n hydraulic o i l s . more t h a n

Performance req ui re men t s today g e n e r a l l y d i c t a t e the use o f

one a n t i o x i d a n t .

Hindered phenols a r e f a vou red f o r h y d r a u l i c and t u r b i n e o i l s because they g i v e e x c e l l e n t o x i d a t i o n l i v e s f o r l o n g f i e l d use. antioxidants,

The phenyl naphthylamine

e s p e c i a l l y t h e b e t a form, or t hose c o n t a i n i n g t r a c e s o f i t , a r e

now regarded as c a r c i n o g e n i c ,

They a r e t h e r e f o r e n o t used t o a g r e a t e x t e n t

and a r e more s p e c i f i c t o some types o f grease. 10.3.2.5

Mode o f A c t i o n o f A n t i o x i d a n t s

There i s g e n e ral agreement among independent i n v e s t i g a t o r s t h a t t h e o x i d a t i o n o f a l u b r i c a t i n g o i l i n v o l v e s a c h a i n o x i d a t i o n r e a c t i o n i n w hich i n i t i a l l y -

253 formed o r g a n i c p ero xi de s a t t a c k u n o x i d i z e d o i l and a r e subsequently regenerated by oxygen i n t h e a i r t o c o n t i n u e such a t t a c k . "peroxide theory",

A c c o r d i n g t o t h i s w i d e l y accepted

an e f f e c t i v e a n t i o x i d a n t i s a chemical compound which reduces

org a n ic p e r o x id e s and co nse qu en t l y causes t h e c h a i n r e a c t i o n t o cease. 10.3.3

Co r r o s ion I n h i b i t o r s

I n the a d d i t i v e industry,

t h e t erm " c o r r o s i o n

i n h i b i t o r ' ' i s app1ied;to

a

m a t e r i a l which p r o t e c t s c o r r o s i o n - s u s c e p t i b l e non-ferrous metal components, p r i n c i p a l l y b e a r i n g s , a g a i n s t a t t a c k by a c i d i c contaminants i n t h e l u b r i c a t i n g oil.

A d i f f e r e n t t erm

-

-

rust inhibitor

i s used t o d e s i g n a t e m a t e r i a l s w hich

p r o t e c t f e r r o u s metal s u r f a c e s a g a i n s t r u s t . Among t h e e a r l i e s t t yp es

o f c o r r o s i o n i n h i b i t o r s t o see commercial a p p l i c a Most o f t h ese were n o t p u r e chemicals b u t r a t h e r

t i o n were o r g a n i c p h o s p h i t e s . m ix tu r e s o f mono-,

di-,

and t r i - o r g a n o p ho sphites o b t a i n e d from t h e r e a c t i o n of

alc o h o ls o r h y d r oxyest ers (e.g.

methyl l a c t a t e ,

t r i m e t h y l c i t r a t e ) w i t h phos-

phous t r i c h l o r i de. The m a jo r c l a s s e s o f c o r r o s i o n i n h i b i t o r s i n commercial use a t t h e present time a r e : 10.3.3.1

D ith iop ho sph at es

Metal d i o r g a n o d i t h i o p h o s p h a t e s , e s p e c i a l l y z i n c diorganodithiophosphates. They possess t h e s t r u c t u r e :

RO

RO

\

P

'

QS

MS'

where R i s an a l i p h a t i c o r a roma t i c r a d i c a l and M i s a p o l y v a l e n t metal such as zinc o r n i c k e l .

T h e i r man uf act ure i n v o l v e s f i r s t h e a t i n g an a l c o h o l o r phenol

w i t h phosphorus p e n t a s u l p h i d e t o form d i o r g a n o d i t h i o p h o s p h o r i c a c i d , then neut r a l i z i n g such a c i d w i t h a metal base.

10.3.3.2

Dith io ca rba mat es

Metal diorganodithiocarbamates, e s p e c i a l l y z i n c diorganodithiocarbamates They a r e d e s c r ib e d by t h e g en era l f ormu l a:

R

\ N - C

I R

HS -

SM

254 where R and M a r e as d e f i n e d i n 10.3.3.1.

T h e i r manufacture i n v o l v e s t h e reac-

t i o n o f an o r g a n i c amine, carbon d i s u l p h i d e , and a metal base. 10.3.3.3

Sulphur P ro du ct s

S u l p h u r i z e d terpenes,

f o r example, s u l p h u r i z e d dipentene.

These p r o d u c t s a r e

manufactured by h e a t i n g elemental s u l p h u r w i t h a terpene hydrocarbon and then o p t i o n a l l y washing t h e cru de p r o d u c t w i t h aqueous a l k a l i o r a l k a l i metal s u l p h i d e t o remove d i s s o l v e d o r " c o r r o s i v e " 10.3.3.4

sulphur.

Phosphorus-Sulphur P ro du ct s

Phosphosulphurized t erp en es, f o r example, phosphorus p e n t a s u l p h i d e - t r e a t e d These a d d i t i v e s a r e manufactured by h e a t i n g phosphorus p e n t a s u l -

turpentine.

phid e w i t h a te r p ene hydrocarbon. O f t h e f o u r m aj or cl asse s o f c o r r o s i o n i n h i b i t o r s l i s t e d above, metal d i t h i o -

phosphates

-

p a r t i c u l a r l y zinc dialkyldithiophosphates

commercial acceptance.

-

have achieved t h e w idest

Many o f t h e compounds l i s t e d above may n o t be t r u e c o r r o -

s i o n i n h i b i t o r s b u t f u n c t i o n by i n h i b i t i n g o x i d a t i o n and so reducing t h e format i o n o f c o r r o s i v e c a r b o x y l i c acids. 10.3.3.5

T r i a z o l e s and C h e l a t i n g Agents

B e n z o t r i a z o l e and i t s d e r i v a t i v e s a r e used t o form a s u r f a c e l a y e r on copper and s i l v e r based a l l o y s by c h e l a t i o n .

T h i s process p a s s i v a t e s t h e s u r f a c e and

reduces i t s a b i l i t y t o a c t as a c a t a l y s t towards f l u i d d e g r a d a t i o n by p r e v e n t i n g s o l u b i l i s a t i o n o f sma l l q u a n t i t i e s o f t h e m e t a l . There a r e v e r s i o n s o f t h e chemical whi ch a r e n o t s o l u b l e

and o t h e r which a r e

s o l u b l e i n b o t h s y n t h e t i c base f l u i d s and aqueous systems. 10.3.3.6 Dirnercapto T h i a d i o z o l e D e r i v a t i v e s These a r e p r e pa red by r e a c t i n g h y d r a z i n e and carbon d i s u l p h i d e f o l l o w e d by v a r i o u s r e a c t i o n s t o make t h e m a t e r i a l o i l s o l u b l e .

They a r e e f f e c t i v e i n red-

ucing corrosion. 10.3.3.7

Mode o f A c t i o n o f C o r r o s i o n I n h i b i t o r s

I t can be s a i d t h a t compounds l i k e 10.3.3.5 inhibitors.

and 10.3.3.6

are t r u e corrosion

They f u n c t i o n by r e a c t i n g c h e m i c a l l y w i t h t h e non-ferrous s u r f a c e

o f t h e metal component:

(e . g .

in

an engine, copper-lead o r lead-bronze b e a r i n g s )

t o form t h e r e o n a c o r r o s i o n - r e s i s t a n t , p r o t e c t i v e f i l m .

T h i s f i l m must adhere

t i g h t l y t o t h e b e a r i n g s u r f a c e l e s t i t be removed by d i s p e r s a n t s o r d e t e r g e n t s and expose th e u n d e r l y i n g metal s u r f a c e t o a t t a c k by a c i d i c components

n the

lubricating o i l . 10.3.4

Rust I n h i b i t o r s

The presence o f wat er i n l u b r i c a n t s v a r i e s between v e r y small l e v e l s t o values as h i g h as f o r t y p erce nt (40) i n i n v e r t e mul sion h y d r a u l i c f l u i d s and n n e t y - f i v e

(95) p e r c e n t i n e mul si o n f l u i d s .

I n h i b i t i n g f e r r o u s surfaces a g a i n s t r u s t i n g i s

t h e r e f o r e a r e q u i reme nt i n a l l types o f

oil.

A range o f r u s t i n h i b i t i n g ch emi ca l s i s necessary t o cope w i t h d i f f e r e n t environments,

and t h e i r chemical o r p h y s i c a l a c t i o n i s i m p o r t a n t .

The s t r o n g s u r -

face a d s o r p t i o n e x h i b i t e d by these chemicals t o r e s t r i c t t h e c o n t a c t w i t h w ater means t h a t c a r e f u l c h o i c e i s necessary t o ensure t h a t o t h e r s u r f a c e a c t i v e chemi c a l s , n o t a b l y t h e a n t i w e a r and extreme p ressure members, a r e a b l e t o perform their function. T y p i c a l chemicals used f o r o i l s o l u b l e systems i n c l u d e t h e f o l l o w i n g : A l k e n y l s u c c i n i c a c i d s and d e r i v a t i v e s

Turbine,

hydrau i c and c i r c u -

A l k y l t h i o a c e t i c a c i d s and d e r i v a t i v e s

) Substituted imidazolines

Gear o i l s

Amine phosphates

Preservative o i

Sulphonates, n e u t r a l o r low base

Engine preserva i v e o i l s . Storage, e t c .

S

A d d i t i o n a l f i l m s t r e n g t h i s a chi e ved by t h e a d d i t i o n o f f a t t y m a t e r i a l s such as l a n o l i n i n t h e case o f t h i c k f i l m s t o r a g e co mposition.

10.3.5

V i s c o s i t y Improvers

V i s c o s i t y improvers a r e m a t e r i a l s which improve t h e v i s c o s i t y temperature relationship o f a lubricant.

They a r e g e n e r a l l y o i l s o l u b l e polymers w i t h mole-

c u l a r w e ig h t r a n g i n g from a pp roxi ma t e l y 50,000 t o 1,000,000.

The polymer mole-

c ul e i n t e r a c t s w i t h t h e o i l t o e f f e c t t h e f i n a l o i l v i s c o s i t y .

The h i g h e r t h e

tempature o f t h e system, t h e l a r g e r t h e polymer volume, t h e g r e a t e r t h e t h i c k e n ing e f f e c t , and hence t h e l e s s t h e " t h i n n i n g "

tendendy o f t h e o i l due t o i n c r e a -

sed temperature. I n a d d i t i o n t o v i s c o s i t y c h a r a c t e r i s t i c s , t h e performance o f these polymers i s dependent on t h e shear s t a b i l i t y o r r e s i s t a n c e t o shear and on t h e i r chemical and thermal s t a b i l i t y .

W i t h a g i v e n polymer, t h e shear s t a b i l i t y decreases w i t h

an in c r e a s e i n m o l e c u l a r w e i g h t .

The loss due t o shear i s r e f l e c t e d i n a de-

crease i n v i s c o s i t y o f t h e l u b r i c a n t i n t h e mechanical system. V i s c o s i t y Index o r " V . I . "

i s an a r b i t r a r y number

-

c a l c u l a t e d from t h e obser-

ved v i s c o s i t i e s o f a l u b r i c a n t a t two w i d e l y separated temperatures

-

which

i n d i c a t e s t h e r e s i s t a n c e o f t h e l u b r i c a n t t o v i s c o s i t y change w i t h temperature. The h i g h e r t h e V . I .

va l ue , t h e g r e a t e r t h e r e s i s t a n c e o f t h e l u b r i c a n t t o thicken

a t low temperatures and t h i n o u t a t h i g h temperatures.

The s i g n i f i c a n c e o f V . I .

i s much l e s s w i t h c u r r e n t o i l s p e c i f i c a t i o n s .

Measured low t e m pe rat ure v i s c o s i t y i s now most i m p o r t a n t .

For motor o i l s such

values a r e r e p o r t e d u s i n g t h e Cold Cranking S i m u l a t o r and f o r automatic t r a n s m i ss io n f l u i d s and gear o i l s u s i n g t h e B r o o k f i e l d Viscometer. The use o f hydrocarbon polymers o f low m o l e c u l a r w e i g h t makes i t p o s s i b l e t o form u la t e m u l t i g r a d e d a x l e o i l s .

Reduction i n o i l drag and consequent f u e l

256 s a v in g s may be achieved w i t h lower v i s c o s i t y index values b u t a h i g h degree o f shear s t a b i l i t y .

A l l im p o r ta n t v i s c o s i t y improvers a r e manufactured by processes o f p o l y m e r i The groups o f polymers, s o l u b l e i n o i l , a r e : -

sation.

(i)

P o l yi sob ut en es

(ii)

A l k y l m e t h a c r y l a t e and a c r y l a t e copolymers

( i i i ) Rubber t ype chemicals such as o l e f i n e co-polymers and b ut ad i en e-st yren e copolymers

10.3.5.1

Mode o f A c t i o n o f V . I .

(OCP)

Improvers

improvers e x e r t a g r e a t e r t h i c k e n i n g e f f e c t on o i l a t h i g h temperatures

V.I.

than they do a t r e l a t i v e l y lower temperatures.

The r e s u l t o f such s e l e c t i v e

t h i c k e n i n g i s t h a t t h e o i l s u f f e r s l e s s v i s c o s i t y change w i t h changing temperature.

I t i s b e l i e v e d t h a t t h e s e l e c t i v e t h i c k e n i n g occurs because t h e polymer

assumes a compact,

c u r l e d form i n a poor s o l v e n t such as c o l d o i l , and an un-

c u r l e d h i g h surFace a rea form i n a b e t t e r s o l v e n t such as h o t o i l . Polymers whose s o l u b i l i t y i n o i l changes v e r y l i t t l e w i t h temperature a c t as thickeners,

b u t a r e n o t as e f f e c t i v e V . I .

improvers as a r e those polymers whose

s o l u b i l i t y i s poor a t low temperatures b u t good a t h i g h e r temperatures.

V.I.

improvers a r e more e f f e c t i v e i n i n c r e a s i n g t h e v i s c o s i t y o r d e r o f low v i s c o s i t y o i l s and become p r o g r e s s i v e l y l e s s e f f e c t i v e as t h e v i s c o s i t y o f t h e base o i l i nc r e a s e s .

V.I.

improvers undergo temporary v i s c o s i t y r e d u c t i o n under shear because o f

th e a lig n m e n t o f t h e polymer mo l ecu l es i n t h e d i r e c t i o n o f f l o w .

T h i s temporary

v i s c o s i t y r e d u c t i o n has t h e e f f e c t o f re du ci ng f r i c t i o n i n h i g h shear zones and g i v e s a s i g n i f i c a n t advantage f o r t h e V . I . o f t h e same v i s c o s i t y . s i t y o f the treated

t r e a t e d o i l remains h i g h , thus m i n i m i z i n g o i l consumption.

Polymer-

o i l s a l s o e x h i b i t lower b e a r i n g wear than t h e i r comparable m i n e r a l o i l

counterparts. engine

i m p r o v e r - t r e a t e d o i l o v e r a base o i l

Another advantage i s t h a t i n low shear zones t h e v i s c o -

These f a c t o r s have been i m p o r t a n t i n t h e acceptance o f m u l t i g r a d e

oils.

Polymers used as v i s c o s i t y - i n d e x improvers must be r e l a t i v e l y s t a b l e t o c h a i n s c i s s i o n under h i g h shear r a t e s . shearing, index.

M o l e c u l a r w e i g h t i s d r a s t i c a l l y reduced by

thus c a usi n g a d e t e r i o r a t i o n of t h e p r o p e r t i e s w hich improve v i s c o s i t y

The shear s t a b i l i t y o f a polymer t y p e i s dependent on m o l e c u l a r w e i g h t

and m o le c u la r w e i g h t d i s t r i b u t i o n and i n crea ses w i t h decreasing m o l e c u l a r w eight. I t can be seen t h a t shear s t a b i l i t y and v i s c o s i t y - i n d e x

d i v e r g e n t m o le c ul ar w e i g h t s . based on a compromise between

improvement r e q u i r e

The m o l e c u l a r w eights o f commercial polymers a r e t he se two p r o p e r t i e s .

I n many cases, t h i s has

r e s u l t e d i n t h e use o f l a r g e r amounts o f l ower-molecular-w eight given a p p l i c a t i o n .

polymers f o r a

257 Chemical and thermal index improver.

s t a b i l i t y a r e i m p o r t a n t p r o p e r t i e s o f a good v i s c o s i t y -

For good s t a b i l i t y ,

i t i s i mportant t h a t c a t a l y s t residues be

completely removed d u r i n g t h e man uf act uri n g process.

Some commercial polymers

are f o r m u la te d w i t h i n h i b i t o r s t o enhance t h e i r s t a b i l i t y . commercial v i s c o s i t y - i n d e x improvers a r e m u l t i - f u n c t i o n a l

Several o f the i n t h a t they may be

e f f e c t i v e p o u r - p o i n t d ep ressa nt s and possess dispersency performance.

10.3.6

Pour P o i n t Depressants

Ever s i n c e l u b r i c a t i n g o i l s were p rep are d from crude o i l s , r e f i n e r s have experienced d i f f i c u l t y w i t h c o n g e l a t i o n o f these p r o d u c t s a t low temperatures. P a r t o f t h e d i f f i c u l t y a r i s e s f rom a n a t u r a l t h i c k e n i n g o f t h e hydrocarbons comprising t h e b u l k o f t h e o i l ;

something which can u s u a l l y be c o r r e c t e d by t h e

use o f a s o l v e n t such as kerosene t o reduce v i s c o s i t y . culty

-

t h e more s e r i o u s

part

-

The r e s t o f t h e d i f f i -

a r i s e s from c r y s t a l l i z a t i o n a t low temperatures

o f t h e p a r a f f i n wax p r e s e n t i n a l most a l l l u b r i c a t i n g o i l f r a c t i o n s .

Upon c r y -

s t a l l i z a t i o n , t h i s wax tends t o form i n t e r l o c k i n g networks w hich adsorb o i l and form

a voluminous g e l - l i k e mass which r e s t r i c t s t h e f l o w o r "pour"

o f the o i l .

Pour p o i n t d e pre ssan t s a r e chemicals whi ch modify t h e wax c r y s t a l l i z a t i o n process i n such a manner t h a t t h e o i l w i l l pour a t low temperatures.

Although

some monomeric compounds such as t e t r a ( l o n g c h a i n a l k y l ) s i l i c a t e s , phenyl t r i s t e a r y l o x y s i l a n e , and p e n t a e r y t h r i t o l t e t r a s t e a r a t e have been shown t o be e f f e c t i v e , a l l c o m m e rci a l l y i m p o r t a n t pour p o i n t depressants a r e polymers: (i)

A l k y l m e t h a c r y l a t e polymers and copolymers

( ii )

V i n y l c a r b o x y l a t e - d i a l k y l fumarate copolymers

( i i i ) A l p h a - o l e f i n polymers and copolymers F r i e d e l - C r a f t s co nd en sat i on p r o d u c t s o f c h l o r i n a t e d wax and

(iv)

a r o m a t i c compounds such as n ap ht halene o r phenol (some i n v e s t i g a t o r s do n o t c l a s s t y p e ( i v ) p r o d u c t s as polymers;

others

maintain that

they a r e r e l a t i v e l y low m o l e c u l a r w e i g h t polymers having a p l u r a l i t y o f a r o m a t i c r i n g s and p a r a f f i n wax r a d i c a l s ) . The m o l e c u l a r w e i g h t range o f polymers e f f e c t i v e as pour p o i n i s g e n e r a l l y below t h a t o f polymers used as V . I .

improvers, and

depressants s usually i n

the a r e a o f 500 t o 100,000.

10.3.6.1

Mode o f A c t i o n o f Pour P o i n t Depressants

Pour p o i n t depressants f u n c t i o n by ad sorb i ng on o r c o - c r y s t a l p r e c i p i t a t i n g wax,

thus i n h i b i t i n g l a t e r a l c r y s t a l growth.

i z i n g w i t h the

T h i s promotes growth

o f s m a l l e r c r y s t a l s t h an t h e p l a t e l e t s formed i n t h e absence o f pour p o i n t depressants.

T h i s change d i m i n i s h e s t h e a b i l i t y o f t h e wax c r y s t a l s t o o v e r l a p

and i n t e r l o c k t o f o rm l a r g e conglomerates o f wax w hich would impede t h e f l o w o f the o i 1 .

258 There i s evidence t h a t a l i p h a t i c polymers f u n c t i o n by a c o - c r y s t a l l i z a t i o n mechanism, and t h a t a l k y l a r o m a t i c t y p e s such as c h l o r i n a t e d p a r a f f i n wax-aromatic compound condensation p r o d u c t s f u n c t i o n by a d s o r p t i o n on t h e nascent wax c r y s t a l . 10.3.7

Extreme Pressure A d d i t i v e s

These a d d i t i v e s ,

commonly c a l l e d "E.P."

agents, a r e chemicals which a r e addec

t o l u b r i c a n t s t o prevent d e s t r u c t i v e metal-to-metal P l a i n mineral

contact during l u b r i c a t i o n .

o i l s p r o v i d e good l u b r i c a t i o n when a f i l m o f o i l i s m a i n t a i n e d be.

tween t h e moving s u r f a c e s (hydrodynamic l u b r i c a t i o n ) b u t f a i l t o

p r o v i d e ade-

quate l u b r i c a t i o n when p r e s s u r e and r u b b i n g speeds a r e such t h a t t h e f i l m o f o i l i s squeezed o r wiped o u t . rication",

The l a t t e r k i n d o f l u b r i c a t i o n , c a l l e d "boundary

lub-

i s governed l a r g e l y by parameters o f t h e c o n t a c t i n g s u r f a c e s such as

s u r f a c e f i n i s h , metal shear s t r e n g t h and t h e c o e f f i c i e n t o f f r i c t i o n between t h e c o n t a c t i n g metal s u r f a c e s . Unless such parameters can be chosen t o meet expected pressures and r u b b i n g needs, d e s t r u c t i v e m e t a l - t o - m e t a l 10.3.7.1

Commercial E.P.

contact w i l l take place.

A d d i t i v e s and t h e i r A p p l i c a t i o n

V i r t u a l l y a l l commercial E.P.

a d d i t i v e s a r e o r g a n i c compounds t h a t c o n t a i n onc

o r more elements o r f u n c t i o n s such as s u l p h u r , halogen, phosphorus, c a r b o x y l , o r c a r b o x y l a t e s a l t w h i c h can r e a c t c h e m i c a l l y w i t h t h e metal s u r f a c e under condit i o n s o f boundary l u b r i c a t i o n . The ease w i t h which an E . P . a d d i t i v e t h e metal s u r f a c e , i . e .

i t s "activity",

reacts w i t h

determines t o a l a r g e e x t e n t whether i t

would be used i n a l u b r i c a n t such as a c u t t i n g o i l , a hypoid gear o i l , h y d r a u l i c o i l , o r a steam t u r b i n e o i l . A n assignment o f l i k e l y f i e l d s o f a p p l i c a t i o n o f commercial E . P . a d d i t i v e s T a b l e 10.3

based on t h e i r r e l a t i v e a c t i v i t y i s shown i n T a b l e 10.3

F i e l d s o f A p p l i c a t i o n o f E.P.

Additives

High A c t i v i t y A d d i t i v e s

"Moderate" o r I n t e r m e d i a t e A c t i v i t v Additives

" M i Id" o r Low A c t i v i t v Additives

Straight cutting o i l s

Hypoid gear o i l s

Worm gear o i l s

Drawing compounds

(e.g. m u l t i - p u r p o s e gear 1 ub r i c a n t s )

S p i r a l bevel gear o i l s

Metal-forming l u b r i c a n t s

I n d u s t r i a l gear o i l s

Manual gear box o i l s

Some hypoid gear o i Is

(e.g.

"open gear" o i Is)

I n d u s t r i a l gear o i l s f o r general a p p l i c a t i o n

Motor o i l s Steam t u r b i n e o i l s Jet aircraft turbine oils Gas t u r b i n e o i l s Automatic t r a n s m i s s i o n fluid Hydraulic o i l s ( f i r e r e s i s t a n t emulsion type) I n d u s t r i a l gear o i l s f o r closed o r c i r c u l a t i n g systems

259

10.3.7.2

A u t o mot i ve E . P .

Gear Oils

These o i l s a r e used t o l u b r i c a t e t h e worm, s p i r a l b e v e l , or hypoid gear d r i v e s o f a u t o m ot i ve v e h i c l e s . o f E.P.

S i n ce hyp oi d gears r e q u i r e t h e g r e a t e s t measure

p r o t e c t i o n o f a l l commercial gear d r i v e s , most a d d i t i v e treatments a r e

designed t o g i v e s a t i s f a c t o r y performance i n t h i s environment. H i s t o r i c a l l y , E.P. general c a t e g o r i e s .

a d d i t i v e s f o r use i n gear s e r v i c e f a l l i n t h e f o l l o w i n g T h i s i n f o r m a t i o n sh ou l d n o t be i n t e r p r e t e d as i n d i c a t i n g

t h a t e a r l y treatments a r e obsolete;

i n f a c t , a l l o f t h e l i s t e d treatments a r e

i n commercial use today, many on i n d u s t r i a l a p p l i c a t i o n s . developed a d d i t i v e s , however,

The more r e c e n t l y

en j oy most o f t h e market.

-

E a r l i e s t s u c cessf ul t rea t me nt

Lead soap (e.g.

lead naphthenate p l u s an

a c t i v e o r "corrosive"

organic sulphur

compound.

-

L a t e 1930's

C h l o r i n e and "Moderately a c t i v e ' ' s u l p h u r p r e s e n t i n t h e same o r d i f f e r e n t o r g a n i c mo l ecu l es

-

World War I I p e r i o d

Sulphur, c h l o r i n e and phosphorus i n s u i t a b l e organic c a r r i e r s

Most r e c e n t typ es (1960-

-

)

Sulphur and phosphorus i n s u i t a b l e o r g a n i c carriers

Typical E.P.

a d d i t i v e s used comme rci al l y i n f o r m u l a t i n g gear l u b r i c a n t s have

i nc lu d e d : C h l o r i n a t e d p a r a f f i n wax (40%

-

60% c h l o r i n e )

Chlornaphtha x a n t h a t e ( r e a c t i o n p r o d u c t o f c h l o r i n a t e d naphtha

and a l k a l i

metal x a n t h a t e ) C h l o r i n a t e d p a r a f f i n wax s u l p h i d e s ( r e a c t i o n p r o d u c t o f c h l o r i n a t e d paraf f i n wax and a l k a l i metal s u l p h i d e s ) S u l p h u r i s e d f a t t y o i l s (e.g.

sulphurised l a r d o i l , sulphurised f i s h o i l ,

s u l p h u r i s e d sperm o i I ) S u l p h u r i s e d hydrocarbons such as po l ybu t e nes S u l p h u r i s e d s y n t h e t i c e s t e r s (e.g.

s u l p h u r i s e d methyl o l e a t e o f f a t t y a c i d s )

S u lp h u r c h l o r i d e - t r e a t e d f a t t y o i l s (e. g. S 2 C I 2

A 1 i p h a t i c and a r o m a t i c p o l y s u l p h i d e s (e.g.

-

treated f i s h o i l )

benzyl d i s u l p h i d e , c h l o r o b e n z y l

d i s u l p h i d e , b u t y l d i s u l p hi d e) Phosphosulphurized f a t t y o i l s (e.g.

l a r d o i l heated w i t h s u l p h u r and phos-

phorus pen t a s u 1 ph i d e ) Organic p h o s p h i t e s ( o b t a i n e d by t r e a t i n g a l c o h o l s w i t h P C I )

3

A l k a r y lp h o s p hat es ( o c t y l p h e n o l t r e a t e d w i t h P205)

260 A l k y l phosphates ( a l c o h o l s t e a t e d w i t h P 0 )

2 5

Lead naphthenate Z i n c and l e a d d i - o r g a n o d i t h i o p h o s p h a t e s Z i n c and le a d d i - a l k y l

dithiocarbamates

An e v a l u a t i o n o f t h e p r a c t i c a l e f f e c t i v e n e s s o f a hypoid gear l u b r i c a n t must be c a r r i e d o u t i n f u l l - s c a l e equipment i n t h e l a b o r a t o r y and i n t h e f i e l d . Bench t e s t r i g s have n o t been a b l e t o p e r f o r m t h i s t a s k , a l t h o u g h they a r e useful

i n s c r e e n in g l i k e l y ca nd i da t e s f o r f u l l - s c a l e e v a l u a t i o n .

A l i s t o f these

t y p i c a l bench t e s t s i s g i v e n i n T a b l e 10.4.

10.3.7.3

A n t iwe ar A d d i t i v e s

A lt h o u g h d i s c u s s i n g p r i m a r i l y gear systems,

t h e advent o f h i g h performance

engines f o r passenger c a r s posed new l u b r i c a t i o n problems.

Engine i n s p e c t i o n

began t o r e v e a l un mi st a kab l e evi d en ce o f e xcessive wear and s c u f f i n g o f v a l v e t r a i n components;

h i g h r o t a t i o n a l speeds and i n c r e a s i n g pressures between cams

and l i f t e r f o o t su rf a ces had a p p a r e n t l y combined t o s h i f t l u b r i c a t i o n r e q u i r e ments i n t o t h e boundary r e g i o n .

The f i r s t and perhaps most e f f e c t i v e E.P.

addi-

t i v e f o r c o n t r o l l i n g o r e l i m i n a t i n g wear and s c u f f i n g i n t h e v a l v e t r a i n area was found t o be t h a t v e r s a t i l e a d d i t i v e

-

zinc dialkyldithiophosphate.

Other a d d i t i v e s found u s e f u l f o r t h e c o n t r o l o f v a l v e t r a i n wear i n c l u d e : T r i c r e s y l phosphate D i l a u r y l phosphate Didodecyl p h o s p h i t e S u l p h u r i z e d terpenes S u l p h u r i z e d sperm o i l C h l o r i n a t e d compounds Zinc d i a l k y l dithiocarbamate I t has been mentioned e a r l i e r t h a t t h e z i n c d i o r g a n o d i t h i o p h o s p h a t e p l a y s a v e r y i m p o r t a n t r o l e i n a n t i w e a r h y d r a u l i c o i l s and i n m i l d EP gear o i l s .

The

c h o i c e o f t h e c o r r e c t t ype o f compound depends upon t h e d e s i r e d performance characteristics. alcohols.

A z i n c d i t h i o p h o s p h a t e may be prepared u s i n g a v a r i e t y of

P r im a ry and secondary a l k y l groups a r e used f o r a p p l i c a t i o n s such a s

g a s o l i n e e n g in e and h y d r a u l i c systems and aro matics ( s u b s t i t u t e d phenols) f o r d i e s e l engines where t h e maj o r r o l e i s as a h i g h temperature a n t i o x i d a n t . It is

e s s e n t i a l t h e r e f o r e t h a t t h e performance o f these m a t e r i a l s i s r e l a t e d

t o the application.

G e n e r a l l y , good a n t i w e a r performance means low thermal

s t a b i l i t y , b u t a good a n t i o x i d a n t .

For h y d r a u l i c system use t h e a d d i t i v e must

be r e s i s t a n t t o h y d r o l y s i s a t t emp era t u res t y p i c a l o f h y d r a u l i c c i r c u i t s . An i n d i c a t i o n o f t h e ma j or performance p r o p e r t i e s o f a range o f z i n c diorganod i t h i o p h o s p h a t e s i s g i v e n i n T ab l e 10.5.

T a b l e 10.4 L u b r i c a n t , F r i c t i o n , Wear and Gear T e s t Machines

T e s t Machine

Type o f Contact

Type o f Measurement

Type o f Load ing

Range o f Load ing (kg)

Almen-Weiland

Conforming area, pin/bearing s h e l l s

F r i c t i o n , wear, load-carrying

Mechanical steps

0-2000

F a l e x I P 241/77T

M u l t i p l e line/area

F r i c t i o n , wear, load-carrying

Mechanical continuous

0-2000

Shell 4-Ball I P 239179 T

Multiple point hardened s t e e l b a l Is

F r i c t i o n , wear, 1 oad- ca r r y ing

Dead w e i g h t steps

0-1800

T i mken I P 240/76 T

B l o c k and r i n g . Line

F r i c t i o n , wear, load-carrying

Dead w e i g h t continuous o r step

0-50

Niemann-FZG I P 33h/77 T

Gear. Two t o o t h Load-carrying forms. P i n i o n t e e t h wear r a t e 16. Wheel t e e t h 24. Spur/Case hardened

Dead w e i g h t step, s t a r t under l o a d

1600 'Normal' load

I .A.E. I P 166/77

Gear-one t o o t h Load-carrying form p i n i o n t e e t h 16. Wheel 16. Spur/ Case hardened

Dead w e i g h t step, s t a r t under l o a d

Gear-one t o o t h form pinion/wheel t e e t h 28

Hydraulic a p p l i e d when running

Ryder FTMS 791a-6508

Load-carrying

Up t o a t l e a s t 70 Lever

Speed

Other c h a r a c t e r i s t i c s

m/s

0

-

0.2

0.1-0.25 1500 rprn

0-400 (0-800rpm)

7.3

A l s o used f o r c o r r o s i o n t e s t on specimens a f t e r r u n n i n g Also f o r corrosion t e s t A l s o w i t h d i f f e r e n t loads speed and specimens f o r wear t e s t s . "Roll ing" u n i t f o r b e a r i n g studies A l s o f o r grease s t u d i e s

A l s o runs a t 2175 r.p.m. O i 1 temperature 9OoC

7.9 4000 rpm

A l s o runs a t 2000 and 6000 rpm O i l temperature 200O-6O0C, 400O-7O0C

10000

oil

rpm

temperature 7 4 ' ~ S t a r t up a t no l o a d N CL Q,

262 TABLE 10.5

R e l a t i v e Performance o f Z i n c Di t hiophosphates Substrate

A1 coho1

Performance

Thermal STabi 1 i t y

AntiOxidancy

Hydrolytic Stabi 1 it y

AntiWear

Bearing Protection

Secondary-1

4

2

2

2

Secondary-2

5

1

1

1

1

P r im a r y - 1

2

4

4

4

4

2

P r ima r y - 2

3

3

3

3

3

Aromatic-1

1

5

5

5

Aromatic-2

1

5

5

5 5

5

These a d d i t i v e s a r e t y p i c a l o f those used f o r l u b r i c a n t s Rating

5

T:

worst c o n d i t i o n

1 = best c o n d i t i o n

10.3.7.4

EP a d d i t i v e s f o r T u r b i n e O i l s

Advances i n t h e de si gn o f steam t u r b i n e , gas t u r b i n e ,

and j e t a i r c r a f t t u r b -

i n e engines and t h e i r a s s o c i a t e d g e a r i n g have i n t r o d u c e d problems o f boundary lubrication.

To s o l v e these problems, t h e a d d i t i v e i n d u s t r y has developed E.P.

a d d i t i v e s o f t h e "low a c t i v i t y " v a r i e t y whi ch p r o v i d e p r o t e c t i o n a g a i n s t excessi v e wear a n d

s c u f f i n g o f t u r b i n e en gi n e components.

Such a d d i t i v e s must w i t h -

s ta n d h i g h temperatures -and i n t h e case o f steam t u r b i n e s , w a t e r c o n t a m i n a t i o n w i t h o u t p r o m o tin g c o r r o s i o n o f t u r b i n e e ng i ne components.

-

For use i n gas t u r -

b i n e and j e t a i r c r a f t t u r b i n e engines f o r m u l a t e d w i t h s y n t h e t i c ester-base f l u i d s d i f f e r e n t ch emi ca l s may be r e q u i r e d . For steam t u r b i n e o i l s a d d i t i v e s based on phosphorus and h y d r o l y t i c a l l y s t a b l e c h l o r i n e c o n t a i n i n g chemicals have been used. t i v e s a r e a l s o known t o have been used.

C e r t a i n sulphur-phosphorus a d d i -

I n t h i s case, t h e s u l p h u r i s i n a c t i v e

towards copper a t normal temperatures and becomes r e l e a s e d o n l y a t h i g h operat i ng temperatures

.

For gas t u r b i n e a p p l i c a t i o n s phosphorus compounds, phates, a r e used.

b o t h phosphites and phos-

I n c e r t a i n cases p o l y g l y c o l f l u i d s may a c t i n t h e same way

-

presumably by s t r o n g a d s o r p t i o n o f t h e hydroxy groups t o metal s u r f a c e s . 10.3.7.5

EP A d d i t i v e s f o r C u t t i n g O i l s

Very h i g h p r e ssu res and temperatures a r e developed l o c a l l y between t h e work and t h e c u t t i n g t o o l i n machining o p e r a t i o n s .

The s o - c a l l e d " s t r a i g h t c u t t i n g

o i l s " w i d e l y employed t o c o o l and l u b r i c a t e t h e work and c u t t i n g t o o l a r e m i n e r a l 0 1 1 s which have been blended w i t h s u i t a b l e E.P.

additives.

Since the

l u b r i c a t i o n o f a m e t a l - c u t t i n g o p e r a t i o n i s almost completely w i t h i n t h e boundary

263 region, E . P .

a d d i t i v e s o f high a c t i v i t y are required f o r best r e s u l t s .

I n the

case o f an o r g a n i c s u l p h u r compound, i t s a c t i v i t y s h o u l d be such t h a t a d i l u t e s o l u t i o n (e.g.

1 o r 2 p e r c e n t ) o f t h e compound i n a m i n e r a l o i l w i l l completely

blacken a copper s t r i p w i t h i n one hour a t 100°C.

Sulphur compounds having t h i s

degree o f a c t i v i t y a r e known i n t h e i n d u s t r y as " c o r r o s i v e s u l p h u r " a d d i t i v e s and a r e used e x t e n s i v e l y i n t h e compounding o f commercial c u t t i n g o i l s . Examples o f E.P.

a d d i t i v e s developed f o r use i n c u t t i n g o i l s i n c l u d e s u l p h u r i z e d m i n e r a l

o i l , sulphurized f a t t y o i l s , sulphur chloride-treated f a t t y o i l s , sulphurized o l e f i n s , s u l p h u r c h l o r i d e - t r e a t e d o l e f i n s , benzyl p o l y s u l p h i d e s , c h l o r i n a t e d p a r a f f i n wax, and c h l o r i n a t e d m i n e r a l o i l s . However, metal w o r k i n g and f o r m i n g o p e r a t i o n s r e q u i r e many types o f chemical a d d i t i v e a p a r t f r o m t h e a c t i v e o r c o r r o s i v e s u l p h u r type. sulphur a d d i t i v e s a r e used i n metal removal o p e r a t i o n s .

Generally, the a c t i v e The l i s t below i n d i c a t e s

the i n c r e a s i n g degree o f s e v e r i t y : (i)

T u r n i n g and M i l l i n g

(ii)

D r i l l i n g and Reaming

( i i i ) Tapping and Threading (iv) As v e r y h i g h

Broaching

emperatures a r e reached, t h e c h o i c e o f s u i a b l e a d d i t i v e dep-

ends a l s o on t h e m a t e r i a l

b e i n g machined, so t h a t t h e b e s t c o o l i n g , s u r f a c e

f i n i s h and t o o l l i f e a r e r e t a i n e d . The chemical a c t i v i t y o f s u l p h u r compounds may be chosen t o g i v e p a r t i c u l a r performance i n c u t t i n g o p e r a t i o n s .

When such m a t e r i a l s a r e i n c o r p o r a t e d i n t o

f l u i d s h a v i n g a dual-purpose n a t u r e , such as machine l u b r i c a t i o n , then t h e choice has t o be made w i t h t h e requirements o f t h e h y d r a u l i c o r c i r c u l a t i n g o i l c i r c u i t m e t a l l u r g y matched a g a i n s t t h e metal w o r k i n g o p e r a t i o n . A d d i t i v e s a r e a l s o used i n metal f o r m i n g o p e r a t i o n s such as r o l l i n g , drawing, stamping and f o r g i n g .

For r o l l i n g o i l s t h e g r e a t e s t requirement i s h i g h s u r -

face f i n i s h , which means t h a t t h e a d d i t i v e s must n o t be c h e m i c a l l y a c t i v e .

Gen-

e r a l l y , f a t t y a l c o h o l s and e s t e r s a r e p r e f e r r e d . Drawing and f o r g i n g o p e r a t i o n s r e q u i r e h i g h l y s p e c i a l i s e d p r o d u c t s .

Fatty-

based systems a r e used c o n t a i n i n g a v a r i e t y o f a d d i t i v e s i n c l u d i n g s o l i d l u b r i cants such as g r a p h i t e and molybdenum d i s u l p h i d e .

10.3.7.6

Mode o f A c t i o n o f E.P.

E.P.additives

Additives

f u n c t i o n by r e a c t i n g w i t h r e l a t i v e l y moving s u r f a c e s under

boundary l u b r i c a t i o n c o n d i t i o n s t o f o r m an adherent f i l m which has lower shear s t r e n g t h than t h a t o f t h e metal s u r f a c e s themselves.

T h i s f i l m a c t s as a s o r t

o f s o l i d l u b r i c a n t , and t a k e s o v e r t h e t a s k o f l u b r i c a t i o n when t h e o i l i s no longer a b l e t o p r o v i d e a s e p a r a t i n g and p r o t e c t i v e l u b r i c a t i n g f i l m .

W i t h an

264 a p p r o p r i a t e E.P.

additive,

t h e r e i s l i t t l e o r no f o r m a t i o n o f such " s o l i d l u b r i -

c a n t " under c o n d i t i o n s o f hydrodynamic l u b r i c a t i o n .

I t forms o n l y a t t h e eleva-

t e d temperatures which develop l o c a l l y between metal surfaces under c o n d i t i o n s o f boundary l u b r i c a t i o n .

Because t e mpe rat ure has been shown t o be t h e most

i n f l u e n t i a l parameter i n t h e f u n c t i o n o f E.P.

l u b r i c a n t s , F.P.Bowden and co-

workers have suggested t h a t t h e t erm "Extreme Temperature" l u b r i c a n t s and a d d i t i v e s m ig h t be more a p p r o p r i a t e . Emulsifiers

10.3.8

G e n e r a l l y , an e m u l s i f i e r i s a chemical f o r d i s p e r s i n g e i t h e r w a t e r i n o i l o r o i l i n wa t e r .

The former,

hydraulic fluids,

t h e more d i f f i c u l t system, produces f i r e - r e s i s t a n t

r o c k d r i l l l u b r i c a n t s , and many types o f w ire-draw ing media.

The e m u l s i f i e r may be o f t h e a l k e n y l s u c c i n i m i d e type, f a t t y e s t e r s , o r o t h e r s c o n t a i n i n g f a t t y resi d ue s.

For me t a l wo rki ng and h y d r a u l i c media o f t h e s o l u b l e

o i l ( o i l i n w a t e r ) type, a l a r g e range o f e m u l s i f i e r s i s a v a i l a b l e .

The c h o i c e

depends upon whether i t i s d e s i r a b l e t o have a chemical w i t h i o n i c c h a r a c t e r i s t i c s , a s a l t , o r one w i t h n o n - i o n i c p r o p e r t i e s such as e s t e r s , phenol e t h e r s and o t h e r oxygenated chemicals.

A s h o r t l i s t i s g i v e n below o f some o f t h e

chemicals: Sodium sul p ho na t es T a l l o i l amides E t ha no l amines Qu at ern ary Ammonium s a l t s P o l y a l k y l e n e phenol e t h e r s and a s s o c i a t e d oxygenated p r o d u c t s Ethoxylated f a t t y acids Salts o f f a t t y acids E m u l s i f i e r s a r e c l a s s i f i e d a c c o r d i n g t o a nu merical value, t h e

HLB

number,

o b t a i n e d by e s t i m a t i n g t h e emu l si on s t a b i l i t y when prepared i n a standard way. For o i l - i n - w a t e r

12-15,

s o l u b l e o i l emulsions,

f o r water-in-oil

t h e HLB range f o r e m u l s i f i e r s i s

i n v e r t emulsions i t i s 4-6 when checked i n a naphthenic

base s t o c k .

10.3.9

F r i c t i o n Modifiers

C e r t a i n d e f i c i e n c i e s a r e sometimes their f r i c t i o n a l characteristics.

observed w i t h l u b r i c a n t s w i t h r e s p e c t t o

Where metal s u r f a c e s a r e designed t o s l i d e ,

two opposing r e q ui re men t s may o ccur: (i)

Smooth s l i d i n g w i t h no v i b r a t i o n and minimum c o e f f i c i e n t o f f r i c t i o n .

( i i ) No s l i d i n g maximum c o e f f i c i e n t o f f r i c t i o n f o r engagement o f c l u t c h s u r fa ces o r f r i c t i o n l o c k i n g d evi ces.

265 Additives f o r ( i ) a r e generally fatty-based, such as f a t t y e s t e r s and amides.

n a t u r a l l y - o c c u r r i n g products

They f i n d use i n s l i d i n g motion systems such

as machine t o o l sl i d eways. I n t h e case o f t y p e ( i i ) , t h e c l u t c h o r f r i c t i o n mechanism should engage o r disengage smoothly w i t h o u t v i b r a t i o n . t o control. (i)

Two t ypes o f a p p l i c a t i o n a r e d i f f i c u l t

A c h o i c e o f d i f f e r e n t p r o d u c t s may be made:-

Anti-chatter

i n l i m i t e d s l i p a x l e s , o r o t h e r metal/metal f r i c t i o n

locking units.

I t i s necessary t o reduce t h e s t i c k - s l i p a c t i o n o f

lubricated steel/steel Amide

-

contacts.

metal d i t h i o p h o s p h a t e combinations

Amine d i t h i o p h o s p h a t e s ( i i ) Anti-squawk a d d i t i v e s reduce v i b r a t i o n w hich g i v e s r i s e t o a u d i b l e noise i n clutches o f d i s s i m i l a r material on s t e e l , e t c . ) .

(bronze on s t e e l , asbestos

Chemicals used f o r t h i s purpose i n c l u d e :

N -acyl sa rcosi n es and d e r i v a t i v e s S u l p h u r i s e d f a t s and e s t e r s Organophosphorus a c i d and f a t t y a c i d m i x t u r e s Esters o f dimerised f a t t y acids Fo r m u la t io n s which meet t h e complex performance d e s i r e d i n automatic t r a n s m is s io n f l u i d s , u n i v e r s a l t r a c t o r e n g i n e - t r a n s m i s s i o n o i l s o r machine t o o l systems a r e t h e

r e s u l t o f c a r e f u l ma t ch i ng o f f r i c t i o n m o d i f i e r s i n t h e whole

a d d i t i v e system.

10.4

CONCLUSIONS

The c h a p te r w i l l se rve t o i n d i c a t e t h e complex n a t u r e o f t h e chemicals used i n lubricant formulations.

M a i n l y , t h e d i s c u s s i o n has been about a d d i t i v e s f o r

m i n e r a l l u b r i c a t i n g o i l s a l t h o u g h t h e r e has been r e f e r e n c e made t o s y n t h e t i c f l u i d s and w a t e r c o n t a i n i n g h y d r a u l i c and metal w orking f l u i d s . I n f o r m u l a t i n g l u b r i c a n t s o f any type,

t h e presence o f these a d d i t i v e s re-

q u i r e s t h a t n o t o n l y sh ou l d each i n d i v i d u a l p r o d u c t c a r r y o u t i t s d e s i r e d funct i o n b u t t h a t i n t e r a c t i o n s between more t ha n one chemical must n o t cause d e t e r i o r a t i o n o f performance. s o l u b l e b y - p r o d u ct s.

N e i t h e r must m i x t u r e s r e a c t t o g e t h e r t o form o i l i n -

The f i n a l o i l f o r m u l a t i o n must be s t a b l e o v e r

t h e range

o f o p e r a t i n g and s t o r a g e temperatures. M i x t u r e s o f a d d i t i v e s may be f o r m u l a t e d and used as a package a t a g i v e n tre a tm e n t l e v e l , r e q u i r i n g o n l y

the c a r r i e r f l u i d .

Again, t h e r e must be no

d e t e r i o r a t i o n a f t e r ma nu f a ct u re o r s t o r a g e and the combination must be s e l e c t e d w i t h g r e a t c a r e so t h a t maximum performance i s achieved a t an economic c o s t . The a u t h o r wishes t o acknowledge t h e a s s i s t a n c e o f h i s colleagues i n t h e L u b r i z o l C o r p o r a t i o n f o r r e v i e w i n g t h i s ch ap ter and suggesting a d d i t i o n a l data;

266 also to the Lubrizol Corporation for permission to compile the information REFERENCES 1

2

Smallheer and Kennedy-Smith, Lubricant Additives, The Lezius-Hiles Co., Cleveland, Ohio. C.V.Smallheer, Lectures on Lubricant Additives, Imperial College, London, March 1970.

267

11 CONSUMPTION AND CONSERVATION OF LUBRICANTS

A.R.

LANSDOWN

D i r e c t o r , Swansea T r i b o l o g y Centre, U.K.

11.1

CONSUMPTION

Apart f r o m t h e r e l a t i v e l y small q u a n t i t i e s o f v e g e t a b l e and animal o i l s , almost a l l modern l u b r i c a n t s a r e d e r i v e d f r o m petroleum, e i t h e r by f r a c t i o n a l d i s t i l l a t i o n o r chemical conversion.

I t i s now g e n e r a l l y recognised t h a t t h e

w o r l d ' s remaining s u p p l i e s o f p e t r o l e u m have o n l y a l i m i t e d l i f e . The v a r i o u s e s t i m a t e s o f t h e remaining l i f e o f p e t r o l e u m reserves depend on many assumptions.

The r a t e o f f u t u r e consumption i s i t s e l f dependent on

several assumptions, fuels,

such as t h e a v a i l a b i l i t y and c o m p e t i t i v e n e s s o f a l t e r n a t i v e

improvements i n e f f i c i e n c y o f u t i l i s a t i o n , and f i s c a l

incentives.

The

estimates o f r e m a i n i n g r e s e r v e s a l s o depend on s e v e r a l assumptions, such as t h e r a t e o f d i s c o v e r i n g new r e s e r v e s and t h e p o t e n t i a l f o r u s i n g low-grade sources. For energy purposes t h i s l a s t f a c t o r reaches i t s l i m i t when t h e energy r e q u i r e d t o e x p l o i t a source i s equal t o t h e energy u l t i m a t e l y o b t a i n e d from t h e source. There i s a general tendency t o d i s c u s s t h e f u t u r e o f p e t r o l e u m p u r e l y from the energy s t a n d p o i n t , and t o r e l a t e i t t o t h e phasing i n o f a l t e r n a t i v e energy sources.

There a r e however many p r o d u c t s f r o m p e t r o l e u m which w i l l be l e s s

r e a d i l y r e p l a c e d f r o m non-petroleum sources, and l u b r i c a n t s a r e i n t h i s category

.

A f u r t h e r l i m i t a t i o n on f u t u r e l u b r i c a n t s u p p l i e s i s t h a t n o t a l l crude o i l s can be used t o produce l u b r i c a n t s w i t h o u t expensive, and energy-expensive, chemical p r o c e s s i n g .

The U n i t e d Kingdom's N o r t h Sea o i l i s i n f a c t g e n e r a l l y

u n s u i t a b l e f o r l u b r i c a n t manufacture. We a r e t h u s f a c e d w i t h t h e s i t u a t i o n t h a t w i t h i n t h e w o r k i n g l i f e o f some o f our younger l u b r i c a n t t e c h n o l o g i s t s petroleum-based l u b r i c a n t s w i l l cease t o be p l e n t i f u l and may become e x t r e m e l y expensive.

Conservation o f l u b r i c a t i n g o i l s

should t h e r e f o r e a l r e a d y be a m a t t e r o f s e r i o u s concern, and w i l l i n e v i t a b l y be recognised as such w i t h i n one, o r a t most two, decades.

268 Table 11.1 shows a breakdown o f t h e t o t a l U n i t e d Kingdom consumption o f pe tr o le u m f o r t he ca l en da r year 1977, and i t can be seen t h a t l u b r i c a t i n g o i l s and greases r e p rese nt j u s t o v e r one m i l l i o n m e t r i c tons, o r 1.17% o f t h e t o t a l . A c c u r a t e f i g u r e s f o r t h e breakdown o f l u b r i c a n t types a r e more d i f f i c u l t t o o b t a i n , b u t Table 11.2 g i v e s e s t i m a t e s o f U.K. th e more im p o r t ant c a t e g o r i e s .

l u b r i c a n t consumption i n some o f

The b i g g e s t s i n g l e c a t e g o r y c o n s i s t s o f a u t o -

m o t i v e e n g in e o i l s , c o m p r i s i n g perhaps one t h i r d o f t h e t o t a l l u b r i c a n t consumpt io n . Table 11.1

U n i t e d Kingdom Consumption o f P e troleum Products 1977 ( F i g u r e s from I n s t i t u t e o f Petroleum "Petroleum S t a t i s t i c s " )

Product c a t e g o r y

Consumption (thousand tonnes)

Motor s p i r i t

17,336

Gas, d i e s e l and f u e

oils

47,920

Aviation fuels

4,218

Other f u e l s

9,639

Naphtha/Light d i s t i l a t e

5,179

L u b r i c a t i n g o i l s and greases

1,029

Other p r o d u c t s

2,835

Tota 1

Table 11.2

88,156

E s tima t e d L u b r i c a n t Consumption by Types End Use

TY Pe

Consumption (%)

Av i a t i o n

0.4

Marine

8.5

Tractor

3.0

Motor

Oils

Motor

Greases

35.0 0.1

Industrial

Hydraul i c o i 1 s

16.0

Industrial

B e ari n g o i l s

22.5

5.5 5.5

lndust r i a l

Me t a l wo rki ng

Industrial

Process o i l s

Industrial

I.C.E.

Industrial

Greases

1 .o

Industrial

Fuel as l u b r i c a n t

1 .o

oils

1.5

269 O v e r a l l l u b r i c a n t s account f o r o n l y a l i t t l e o v e r 1 % o f t h e t o t a l p e t r o l e u m consumption, b u t t h e importance o f t h i s 1 % i s increased by t h e two f a c t o r s p r e v i o u s l y mentioned, namely t h e g r e a t e r d i f f i c u l t y o f f i n d i n g s u b s t i t u t e sources f o r l u b r i c a n t s and t h e l i m i t e d range o f crude o i l s which can be used t o produce l u b r i c a n t s . The u l t i m a t e f a t e o f t h e v a r i o u s l u b r i c a n t s i s a l s o d i f f i c u l t t o assess accurately.

Table 11.3 shows some e s t i m a t e s made i n 1973 o f t h e f a t e o f a u t o -

motive engine o i l s .

From t h e c o n s e r v a t i o n p o i n t o f view t h e r e a r e two i n t e r -

e s t i n g aspects o f these e s t i m a t e s .

The f i r s t i s t h a t some 185 000 t o n s were

a p p a r e n t l y p o t e n t i a l p o l l u t e r s o f l a n d and water.

The second

s t h a t some

258,000 tons c o u l d t h e o r e t i c a l l y be reclaimed. An a r t i c l e p u b l i s h e d i n t h e AA "Drive" t h a t the 58,000

magazine i n January,

1974 suggested

t o n s o f engine o i l s changed each year by motor s t s a t home was

p o s s i b l y disposed o f as shown i n Table 11.4.

The l a r g e r quant t y changed i n

garages was, however, g e n e r a l l y disposed o f more l e g a l l y . Since 1974 t h e r e have been more d e t a i l e d surveys, b u t t h e i n c r e a s i n g awareness o f t h e need f o r c o n s e r v a t i o n , has a l s o l e d t o a tendency t o l o n g e r o i l change p e r i o d s and t o a g r e a t e r c o n t r i b u t i o n o f r e - r e f i n i n g t o t h e d i s p o s a l prob 1 em. Both o f these f a c t o r s ,

r e d u c t i o n i n consumption and r e - r e f i n i n g , w i l l

e v i t a b l y assume g r e a t e r importance i n coming years.

Table 11.3

Table 11.4

Estimates o f t h e Fate o f Motor O i l s Burned (exhaust)

25%

95,000 t o n s

Changed (garages)

50%

190,000 t o n s

Changed (home)

15%

58,000

tons

Leaked

5%

Scrapped w i t h v e h i c l e

1%

4,000

Spi 1 l e d

1%

4,000 t o n s

Railway ( ? )

1%

5,000 tons

19,000 t o n s tons

Estimates o f O i l Disposal by " D o - i t - y o u r s e l f " B u r i e d i n garden Poured down d r a i n s

Car Owners 20% 8%

Burned

18%

Taken t o garage e t c . f o r p r o p e r d i s p o s a l

17%

Otherwise disposed o f

37%

in-

11.2

REDUCING CONSUMPTION

There a r e o f cou rse p u r e l y mechanical ways t o reduce consumption, such as e l i m i n a t i n g leakage and i mpro vi ng s e r v i c e a b i l i t y o f engines.

Improvement may

sometimes a l s o be p o s s i b l e by t h e use o f a more v i s c o u s o i l , b u t t h i s must o b v i o u s l y be done w i t h c a u t i o n .

Not o n l y must c a r e be used t o ensure t h a t a

s u i t a b l e v i s c o s i t y i s used, b u t as a ge ne ral r u l e t h e use o f a h i g h e r v i s c o s i t y o i l w i l l mean h i g h e r power consumption, and the r e s u l t i n g energy wastage w i l l pr o b a b ly more t h an o f f s e t any l u b r i c a n t saving. The b e s t p r o s p e c t f o r red uci n g consumption l i e s i n e n s u r i n g t h a t o i l changes a r e n o t c a r r i e d o u t any more f r e q u e n t l y t ha n they need t o be. Even i n i d e n t i c a l systems t h e r a t e o f o i l degradation can v a r y c o n s i d e r a b l y . The f o l l o w i n g a r e some o f t h e f a c t o r s whi ch l e a d t o r a p i d degradation. Dusty o r d i r t y environments High temperatures Very low t e m pera t u res Temperature f l u c t u a t i o n s ,

l e a d i n g t o con densation

High a l t i t u d e Poor f i l t e r maintenance Low o i l l e v e l s Frequent s t o p - s t a r t o p e r a t i o n Short j o u r n e y l e n g t h s i n a v e h i c l e Contamination by ch emi ca l s Contamination by unburned f u e l Contamination by combustion p r o d u c t s Contamination by wear d e b r i s W i t h i n t h e o i l f o r m u l a t i o n t h e f o l l o w i n g f a c t o r s can a l s o lead t o r a p i d de g r a d a t io n . Poor q u a l i t y base o i l , c o n t a i n i n g u n s t a b l e molecules Inadequate a n t i - o x i d a n t c o n t e n t I n s u f f i c i e n t dispersant o r detergent a d d i t i v e s I n s u f f i c i e n t a nt i -we ar o r EP a d d i t i v e s I n s u f f i c i e n t basic additives w i t h sulphur-containing fuel Excessively r e a c t i v e o r unstable a d d i t i v e s Because o f a l l these v a r i a b l e s ,

t h e r e q u i r e d o i l change p e r i o d i n two

i d e n t i c a l e n g in e s may v a r y by a f a c t o r o f t e n i n d i f f e r e n t o p e r a t i n g c o n d i t i o n s .

I n an e r a o f r e l a t i v e l y cheap l u b r i c a n t s , recommended o i l change p e r i o d s w i l l te n d t o be s h o r t enough t o ensure s a t i s f a c t o r y q u a l i t y i n t h e w o r s t s e t o f

c o n d i t i o n s , because t h e economic p e n a l t y f r o m u s i n g an o i l f i l l t o o l o n g w i l l be f a r g r e a t e r t h a n t he v a l u e o f t h e o i l saved.

271 The s o l u t i o n i s o i l q u a l i t y m o n i t o r i n g , which enables t h e o i l r e m a i n i n g i n an i n d i v i d u a l system t o be assessed p e r i o d i c a l l y , and o n l y changed when i t s cond i t i o n i s approaching an u n s a t i s f a c t o r y l e v e l .

As a bonus, o i l m o n i t o r i n g w i l l

a l s o g i v e v a l u a b l e i n f o r m a t i o n about t h e c o n d i t i o n o f t h e engine o r o t h e r l u b r i c a t e d system. O i l m o n i t o r i n g i s an i m p o r t a n t p a r t o f machinery h e a l t h m o n i t o r i n g , which i s described i n d e t a i l i n Chapter 18.

I t i s t h e r e f o r e unnecessary t o d e s c r i b e t h e

various techniques a t l e n g t h here, b u t t h e r e a r e two a s p e c t s which should be mentioned. The f i r s t o f these i s t h e need t o t a i l o r t h e m o n i t o r i n g techniques t o t h e s i z e and importance o f t h e o i l system b e i n g monitored.

A l a r g e c r i t i c a l system

w i l l j u s t i f y t h e e f f o r t and c o s t i n v o l v e d i n f r e q u e n t s p e c t r o g r a p h i c o i l a n a l y s i s or ferrography.

Even a small system, such as a c a r engine, may w e l l j u s t i f y t h e

use o f a magnetic p l u g t o m o n i t o r wear d e b r i s , examination o f a drop o f o i l from the d i p s t i c k by t h e f i l t e r paper technique, o r v i s c o s i t y by t h e F l o s t i c k method. The second f a c t o r i s t h e need t o e s t a b l i s h f o r any system a c r i t e r i o n f o r d e c i d i n g when t o change t h e o i l .

T h i s may be a l e v e l o f a c i d i t y i n t h e T o t a l

Acid Number, a degree o f v i s c o s i t y change, o r t h e v i s i b l e appearance o f contaminants on a f i l t e r - p a p e r , essential

b u t an o b j e c t i v e c r i t e r i o n o r c r i t e r i a w i l l be

i f o i l change i s t o be determined by o i l q u a l i t y .

I t i s a salutary

thought t h a t t h e i n c i d e n c e o f i n f l i g h t engine f a i l u r e s i n c e r t a i n U n i t e d States

A i r Force a i r c r a f t decreased when n o t o n l y r o u t i n e o i l change, b u t r o u t i n e engine overhaul p e r i o d s were d i s c o n t i n u e d i n f a v o u r o f s c h e d u l i n g i n accordance with spectroscopic o i l analysis.

11.3

RECLAMATION AND RE-REFINING

Even when a l u b r i c a n t has d e t e r i o r a t e d so t h a t i t i s no l o n g e r f i t f o r service,

t h e g r e a t e r p a r t o f i t w i l l s t i l l be unchanged.

Much o f t h e degradation

i s by c o n t a m i n a t i o n , w h i l e a f u r t h e r f a c t o r i s d e p l e t i o n o f a d d i t i v e s .

Only a

very small p r o p o r t i o n o f t h e base o i l w i l l u s u a l l y have been degraded, and t h i s w i l l c o n s i s t o f t h e most u n s t a b l e molecules, u s u a l l y o x i d i s e d t o aldehydes, ketones o r c a r b o x y l i c a c i d s . I t i s t h e o r e t i c a l l y p o s s i b l e t o remove a l l t h e contaminants and t h e degraded a d d i t i v e s and base o i l molecules, t o add f r e s h a d d i t i v e s , and t h u s t o produce a l u b r i c a n t which d i f f e r s l i t t l e i f a t a l l f r o m t h e o r i g i n a l .

The processes

used a r e d e s c r i b e d as r e c l a m a t i o n o r r e - r e f i n i n g , depending on t h e e x t e n t o f treatment i n v o l v e d . I n t h e s i m p l e s t case t h e use o f an i n - l i n e f i l t e r t o remove s o l i d contami n a n t s i s a r e c l a m a t i o n technique.

Another example i s t h a t o f a t r a n s f o r m e r

o i l , which degrades i n s e r v i c e t o generate a small b u t unacceptable l e v e l o f

272 o f e l e c t r i c a l c o n d u c t i v i t y and can be c l e a n e d up i n s i t u t o recover t h e r e -

quired insulation l e v e l .

Such techniques a r e commonly known a s " l a u n d e r i n g "

and have been w i d e l y used f o r many y e a r s .

A well-established

system f o r more i n t e n s i v e r e - r e f i n i n g i s t h e a c i d - c l a y

system, which b a s i c a l l y c o n s i s t s o f t h e f i v e components shown i n F i g . 1 .

The

s t r a i n e r a t t h e i n l e t t o t h e waste o i l s t o r a g e tank removes major s o l i d contaminants, w h i l e water and sludge a r e d r a i n e d o f f f r o m t h e bottom.

Contamin-

a t i n g f u e l o r o t h e r v o l a t i l e m a t e r i a l s a r e removed i n t h e steam s t r i p p e r .

The

c l a r i f i e d o i l from t h e i n t e r m e d i a t e s t o r a g e i s t r e a t e d w i t h h o t s u l p h u r i c a c i d , which r e a c t s w i t h most o f t h e r e a c t i v e compounds p r e s e n t and removes them a s a c i d sludge.

Treatment w i t h heated a c t i v e " e a r t h " or " c l a y "

removes t h e

remaining p o l a r compounds, and t h e s o l i d s a r e f i n a l l y removed i n a f i l t e r - p r e s s . The cleaned o i l o b t a i n e d by t h i s process can be made s u i t a b l e f o r use a s l u b r i c a n t base o i l , b u t i s o f t e n blended i n t o heavy f u e l o i l s .

Strainer

-

sludge treatment

Clay and clean oil

Figure 1

A c i d - c l a y r e - r e f i n i n g process

The main disadvantage o f t h i s process i s t h e problem of d i s p o s i n g o f l a r g e amounts o f a c i d and a c i d c l a y .

An a l t e r n a t i v e process developed by t h e

I n s t i t u t e F r a n c a i s du P e t r o l e uses l i q u i d propane p r e c i p i t a t i o n t o remove t h e degraded m a t e r i a l s and r e s i d u a l a d d i t i v e s .

T h i s c o n s i d e r a b l y reduces t h e

273 ge n e r a tio n of waste b y-pro du ct s, economic

b u t a t pre sent t h e process i s probably n o t

.

A Matthys process c o n s i s t s e s s e n t i a l l y o f two d i s t i l l a t i o n stages.

The

f i r s t i s a t atmospheric p ressu re and 180°C and removes w ater and l i g h t hydroThe second t a kes p l a c e a t 3 4 O o - 3 6 O 0 C and i s designed so t h a t the

carbons.

un d e s ir a b le m a t e r i a l s f o rm a coke which i s c a r r i e d o f f i n t h e l i q u i d p r o d u c t s and removed by c e n t r i f u g i n g . Most r e - r e f i n i n g processes r e q u i r e a f i n i s h i n g stage t o produce f r a c t i o n s f o r re-use as l u b r i c a n t s and t h i s may be c l a y treatment o r a conventional hydrofinishing. The r e c o v e r y o f o i l f rom d i l u t e emulsions o r o i l y p l a n t waste-water difficult,

is

b u t i s i mpo rt a nt n o t o n l y f o r o i l c o n s e r v a t i o n b u t f o r e l i m i n a t i o n

of pollution.

The st an da rd procedure i s t o c r a c k emulsions by chemical t r e a t -

ment and a l l o w t h e p rod uct t o s e t t l e i n h o l d i n g tanks u n t i l t h e o i l can be skimmed o f f .

The o i l f r a c t i o n then tends t o c o n t a i n a h i g h p r o p o r t i o n o f d i r t ,

water and t h e t r e at men t chemical.

I t i s l i k e l y t o be uneconomical f o r r e -

r e f i n i n g and i s o f t e n blended i n t o b urn er f u e l . I n r e c e n t y e a rs, t ech ni q ue s have been developed which use p o l y m e r i c membranes t o f i l t e r o u t p a r t i c l e s and d r o p l e t s f rom t h e w ater, e i t h e r by d i r e c t u l t r a f i l t r a t i o n o r by re verse osmosis.

These t ech niques a r e r e p o r t e d t o g i v e a much

cleaner o i l f r a c t i o n as w e l l as a wat er f r a c t i o n s u i t a b l e f o r discharge t o sewers. One problem a s s o c i a t e d w i t h r e - r e f i n i n g o f engine o i l s i s t h a t t h e r e may be a s l i g h t b u i l d - u p o f p o l y n u c l e a r aro mat i cs, which a r e c a r c i n o g e n i c i n higher c o n c e n t r a t io n s .

I t may t h e r e f o r e be d e s i r a b l e t o t r e a t r e - r e f i n e d o i l s by a

f i n i s h i n g process such as s o l v e n t e x t r a c t i o n w hich w i l l remove p o l y n u c l e a r a roma t ic s

11.4

.

ECONOMICS

The economics o f o p t i m i s i n g o i l - c h a n g e p e r i o d s depends m a i n l y on t a i l o r i n g the m o n i t o r i n g tech ni q ue s t o t h e s i z e and importance o f the system.

Systems

which a r e e i t h e r v e r y l a r g e o r o f c r i t i c a l importance a r e a l r e a d y being moni t o r e d by s o p h i s t i c a t e d techniques.

Smaller and l e s s c r i t i c a l systems may

al re a d y j u s t i f y si mp l e m o n i t o r i n g techniques,

and as l u b r i c a n t a v a i l a b i l i t y

decreases, t h e balance w i l l i n e v i t a b l y s h i f t i n f a v o u r o f increased l e v e l s o f m o n it o r in g . The economics of r e - r e f i n i n g a l s o depends on a v a i l a b i l i t y o f l u b r i c a t i n g o i l . During t h e second World War l a r g e q u a n t i t i e s o f l u b r i c a t i n g o i l s were r e - r e f i n e d i n most c o u n t r i e s , b u t t h e i n d u s t r y d e c l i n e d i n t h e f o l l o w i n g t h i r t y years. The e x t e n t o f t h e d e c l i n e v a r i e d i n d i f f e r e n t c o u n t r i e s , b u t nowhere d i d i t

274 c o m p l e t e l y cease.

The reason f o r t h e d e c l i n e may have been p a r t l y p s y c h o l o g i c a l ,

i n t h a t r e - r e f i n e d o i l s were c o n s i d e r e d t o be i n f e r i o r i n some way, b u t t h e main reason was p r o b a b l y f i n a n c i a l .

The economics o f r e - r e f i n i n g d i d n o t p e r m i t more

than a token r e d u c t i o n i n r e t a i l p r i c e s o f r e - r e f i n e d o i l s . I t has been p o s s i b l e f o r many y e a r s t o produce r e - r e f i n e d o i l s o f s i m i l a r q u a l i t y t o new o i l s , b u t i n a p e r i o d o f r e l a t i v e a f f l u e n c e and p l e n t y most people have been h a p p i e r t o pay t h e m a r g i n a l l y h i g h e r p r i c e f o r "new" o i l . The main economic problem has been t h e c o s t o f c o l l e c t i n g and t r a n s p o r t i n g waste o i l , b u t a second f a c t o r has been t h e d i f f i c u l t y o f persuading o p e r a t o r s t o keep wastes o f d i f f e r e n t q u a l i t i e s c l e a n arld separate. I n t h e Federal Republic o f Germany a d e l i b e r a t e move was made t o reduce l u b r i c a n t consumption and encourage r e - r e f i n i n g by t h e i n t r o d u c t i o n i n 1968 o f t h e "Law on Measures t o Ensure t h e Disposal o f Waste O i l " .

T h i s law enabled

o r g a n i s a t i o n s d i s p o s i n g o f waste o i l s by approved methods t o c l a i m an allowance t o cover c o s t s i n v o l v e d i n d i s p o s a l .

The approved methods i n c l u d e d r e - r e f i n i n g .

The funds were r a i s e d by a s p e c i a l d u t y on imported l u b r i c a n t grade o i l , and t h i s had t h e e f f e c t o f i n t r o d u c i n g a p r i c e margin i n f a v o u r o f r e - r e f i n e d o i l s .

As a r e s u l t , i t was e s t i m a t e d i n 1973 t h a t up t o 30% o f t o t a l l u b r i c a t i n g o i l consumption was b e i n g r e - r e f i n e d i n Germany, compared w i t h o n l y 5% i n B r it a i n .

With d e c l i n i n g a v a i l a b i l i t y o f p e t r o l e u m l u b r i c a n t s , t h e p r o p o r t i o n o f r e r e f i n e d l u b r i c a n t s w i l l p r o b a b l y approach more and more t h e t h e o r e t i c a l l i m i t o f 60-70%.

275

12 HEALTH AND SAFETY ASPECTS OF LUBRICANTS A.R.

EYRES,

M.A.,

M.Sc.

Environmental H e a l t h & Product S a f e t y A d v i s o r , Mobil Europe Inc.

12.1

INTRODUCTION

The m a j o r i t y o f m i n e r a l o i l based l u b r i c a n t s and greases a r e r e l a t i v e l y harml es s t o man.

T h e i r use n o r m a l l y i n v o l v e s n o unusual hazards p r o v i d e d t h a t

reasonable c a r e i s taken t o a v o i d e xcessi ve s k i n c o n t a c t o r i n h a l a t i o n o f m i s t s and vapours.

A small number o f p r o d u c t s may, because o f c o m p o s i t i o n a l r e q u i r e -

ments t o meet p a r t i c u l a r t e c h n i c a l performance needs, p r e s e n t a h i g h e r degree of hazard.

Because l u b r i c a n t s a r e m a i n l y composed o f o r g a n i c chemicals which have

some solvency f o r n a t u r a l components o f t h e s k i n , i t i s u n l i k e l y t h a t p r o v i s i o n o f c o m p le te ly s a f e p r o d u c t s c o u l d be p o s s i b l e .

Synthetic lubricants generally

are s i m i l a r i n hazard p o t e n t i a l t o m i n e r a l o i l products. hazards o f l u b r i c a n t s ,

I n order t o review the

i t i s f i r s t o f a l l necessary t o l o o k b r i e f l y a t t h e i r

composition.

12.2

COMPOSITION OF LUBRICANTS

M in e r a l o i l based l u b r i c a n t s a r e p rep are d from base o i l s manufactured from n a t u r a l l y o c c u r r i n g crud e pe t ro l eu m o i l s .

Crude o i l s occur i n many p a r t s o f t h e

wo r ld and t h e i r comp osi t i on v a r i e s a c c o r d i n g t o source. m ix tu r e s o f p a r a f f i n i c ,

I n a d d i t i o n t o complex

i s o p a r a f f i n i c , n ap ht henic ( c y c l o p a r a f f i n i c ) and aromatic

hydrocarbons, some compounds o f s u l p h u r , oxygen and n i t r o g e n w i l l be present p l u s t r a c e s o f a number o f me t a l s.

D i s t i l l a t i o n o f crude o i l , f o l l o w e d by

v a r io u s o t h e r r e f i n i n g processes such as s o l v e n t e x t r a c t i o n , hydrogenation o r a c i d t r e a t m e n t y i e l d s v a r i o u s f r a c t i o n s i n t h e broad c a t e g o r i e s shown i n Table

12.1. A l u b r i c a t i n g o i l f r a c t i o n t y p i c a l l y c o n t a i n s several thousand i n d i v i d u a l hydrocarbon compounds.

A base s t o c k i s u s u a l l y described as p a r a f f i n i c o r

naphthenic depending on t h e predominant t yp e o f hydrocarbon compound p r e s e n t . This i s a f u n c t i o n o f t h e cru de source.

Crude o i l a l s o n o r m a l l y c o n t a i n s p o l y -

c y c l i c a r o m a t i c hydrocarbons, some o f which (4 t o 6 r i n g compounds) a r e known t o be c a r c i n o g e n i c , eg. benz(a) pyrene.

Some r e f i n i n g processes, such as

s o lv e n t o r severe a c i d t r e a t m e n t , remove most o f these p o l y c y c l i c aromatic compounds so t h a t v e r y few a r e p rese nt i n t h e f i n a l l u b r i c a n t base stock.

However,

276 Table 12.1

Mol ecu 1 a r Size Gases L i g h t Naphtha Gasol i n e Heavy Naphtha/Kerosine Gas O i l s

c1

-

'3

-

'8

'4

-

c12

'9

-

'10-

c4

'16 '26

Lubricating Oils

C,7 upwards

Residuum

CZ6 upwards

Boi 1 i n g Range, "C

so 0 20

100 170

-

100

200 270 400

> 300

i t should be n o t e d t h a t t h e p o l y c y c l i c compounds a r e p r e s e n t i n t h e e x t r a c t e d

Because o f t h e i r b o i l i n g p o i n t s , t h e 4 t o 6 r i n g p o l y c y c l i c compounds

material.

a r e n o t n o r m a l l y p r e s e n t i n d i s t i l l a t e f r a c t i o n s b o i l i n g below about 370°C. To o b t a i n l u b r i c a n t performance c h a r a c t e r i s t i c s which c o u l d n o t be p r o v i d e d by t h e base o i l s themselves, v a r i o u s a d d i t i v e s such as a n t i o x i d a n t s , d e t e r g e n t s / d i s p e r s a n t s , e m u l s i f i e r s , b i o c i d e s , a n t i - c o r r o s i v e s and anti-wear/extreme pressure compounds a r e i n c o r p o r a t e d .

P o t e n t i a l h e a l t h hazards o f such a d d i t i v e s

t h e r e f o r e a l s o need t o be considered. For s p e c i a l a p p l i c a t i o n s , a v a r i e t y o f s y n t h e t i c l u b r i c a n t s have been developed.

These a r e based on s y n t h e t i c hydrocarbons and e s t e r s , p o l y g l y c o l s ,

s i l i c o n e s and phosphate e s t e r s .

A d d i t i v e s o f s i m i l a r t y p e s t o those used i n

m i n e r a l o i l based l u b r i c a n t s a r e g e n e r a l l y i n c o r p o r a t e d .

12.3

MINERAL BASE OIL FACTORS

12.3.1

Acute T o x i c i t y

A l l types o f m i n e r a l o i l base s t o c k s have a low o r d e r o f a c u t e ( s h o r t - t e r m )

toxicity.

Acute o r a l LD5O'S ( t h e dose t o t e s t a n i m a l s r e s u l t i n g i n 50 p e r c e n t

m o r t a l i t y ) f o r t h e r a t a r e w e l l above 109 p e r kg of body w e i g h t .

Extrapolated

t o man t h i s equates t o i n g e s t i o n o f m r e than one l i t r e o f o i l t o cause death. Low v i s c o s i t y p r o d u c t s such as k e r o s i n e p r e s e n t a somewhat g r e a t e r hazard because o f t h e danger o f a s p i r a t i o n i n t o t h e lungs, f o l l o w e d by pneumonitis,

if

v o m i t i n g occurs. M i n e r a l o i l s a r e a l s o e s s e n t i a l l y n o n - t o x i c by a b s o r p t i o n t h r o u g h t h e s k i n . Dermal L O 5 0 ' s f o r t h e r a b b i t a r e w e l l above log p e r kg body w e i g h t , a l e v e l g e n e r a l l y c o n s i d e r e d as harmless.

277 12.3.2

Dermatitis

D e r m a t i t i s i s undoubtedly t h e maj o r p o t e n t i a l h e a l t h problem w i t h m i n e r a l o i l p r o d u c ts , r e s u l t i n g f ro m re pe at ed o r p rolonged s k i n c o n t a c t and inadequate skin care.

Primary i r r i t a t i o n and d e f a t t i n g o f t h e s k i n can occur t o v a r y i n g

e x t e n t s , depending on t h e t y p e o f p r o d u c t and t h e degree o f exposure. l i g h t e r p e t r o le u m o i l s w i t h f i n a l b o i l i n g p o i n t s below about 350°C, k e r o s in e , t e n d t o be d i r e c t s k i n i r r i t a n t s .

The

f o r example,

Since the chemical n a t u r e o f

min e r a l o i l ensures t h a t t h e r e w i l l be some solvency e f f e c t on the n a t u r a l f a t s o f t h e s k i n i t i s u n l i k e l y t h a t p o t e n t i a l problems can be t o t a l l y e l i m i n a t e d by product development.

However, a d o p t i o n o f simple measures t o prevent repeated

and prolonged c o n t a c t , t o g e t h e r w i t h good pe rsonal hygiene p r a c t i c e s and c a r e o f the s k i n , can v i r t u a l l y e l i m i n a t e d e r m a t i t i s problems.

D e r m a t i t i s from l u b -

r i c a n t s i s an a v o i d a b l e o c c u p a t i o n a l di se ase i n t h e v a s t m a j o r i t y o f cases.

12.3.3

O i l Mist

A lt h o u g h i t has o f t e n been suggested t h a t i n h a l a t i o n o f o i l m i s t over extended p e r i o d s may l e a d t o an i n crea sed r i s k o f l u n g cancer, evidence from animal t e s t i n g and exposed human p o p u l a t i o n s does n o t support t h i s . animal exposures t o w h i t e o i l m i s t a t increased l u n g cancer r i s k .

For example,

5 and 100 mg/m3 showed no i n d i c a t i o n s o f

I n f u r t h e r work, no i n j u r y o r i n d i s p o s i t i o n from

i n h a l a t i o n o f o i l m i s t was observed amongst animals exposed f o r 18 months t o m i s t f r o m a s u l p h u r i s e d s o l v e n t - e x t r a c t e d n aphthenic base o i l a t 50 mg per c u b ic meter [1,2,3].

Deco uf l e o f t h e U.S.

N a t i o n a l Cancer I n s t i t u t e [4,5]

has

pu b lis h e d two e p i d e m i o l o g i c a l s t u d i e s o f t h e cancer m o r t a l i t y o f workers exposed t o c u t t i n g o i l mists.

The r e s u l t s i n d i c a t e d t h a t exposure t o o i l m i s t s does

n o t pose a hazard i n terms o f r e s p i r a t o r y cancer and f a t a l non-malignant r e s p i r a t o r y disease, b u t may be a s s o c i a t e d w i t h a s l i g h t increase i n cancer o f t h e g a s t r o i n t e s t i n a l system.

A st ud y o f worke rs i n metal machining p l a n t s i n

Germany by Oraschs [ 6 ] showed no adverse r e s p i r a t o r y e f f e c t s among 443 employees i n 17 f a c t o r i e s .

About 63 p e r c e n t o f these employees were r e p o r t e d t o have

been exposed t o h i g h o i l m i s t l e v e l s i n t h e range 40 t o 80 mg/m3.

Comparison

of smokers and non-smokers i n exposed and non-exposed p o p u l a t i o n s suggested i n f a c t t h a t i n h a l a t i o n o f o i l m i s t may p r o v i d e some p r o t e c t i v e e f f e c t a g a i n s t t h e harmful e f f e c t s o f smoking on t h e b r o n c h i a l system. The c u r r e n t T hresh ol d L i m i t Value ( t h e atmospheric c o n c e n t r a t i o n t o which i t i s b e l i e v e d most workers can be exposed f o r 8 hours d a i l y w i t h o u t adverse e f f e c t s on h e a l t h ) p u b l i s h e d by t h e American Conference o f Gernmental Indu s t r i a l Hygienists i s

5

mg/m3.

T h i s has been s e t on t h e b a s i s o f p r e v e n t i n g

nuisance and unpleasantness t o wo rkers r a t h e r than on h e a l t h e f f e c t aspects and i t i s e s s e n t i a l l y an index o f good i n d u s t r i a l hygiene p r a c t i c e s .

I t i s believed

t h a t t h e 5 mg/m3 l i m i t p r o v i d e s a s a f e t y f a c t o r o f a t l e a s t t e n a g a i n s t even r e l a t i v e l y minor changes i n t h e lungs. As w i t h many chemicals,

i n h a l a t i o n o f very h i g h concentrations o f o i l mists

( o r vapours o f more v o l a t i l e p r o d u c t s such as k e r o s i n e ) may cause i r r i t a t i o n o f t h e lungs and may l e a d t o a chemical pneumonia.

12.3.4

O i l Vapours

A t normal temperatures, t y p i c a l l u b r i c a t i n g o i l s do n o t produce any s i g n i f -

i c a n t l e v e l s o f vapour i n t h e working atmosphere.

S a t u r a t e d vapour concent-

r a t i o n s f o r a t y p i c a l l u b r i c a t i n g o i l have been c a l c u l a t e d by Sanderson [7] t o be 0.016 ppm a t 20°C and

1.4

ppm a t 100°C

adverse h e a l t h e f f e c t s w i l l n o t occur.

-

these a r e so low t h a t any

L i g h t e r p r o d u c t s such as d i e s e l o i l and

k e r o s i n e do have t h e p o t e n t i a l f o r e v a p o r a t i o n o f l i g h t ends t o produce s i g n i f i c a n t vapour c o n c e n t r a t i o n s .

For example,

f o r k e r o s i n e a t 20°C i s about 2000 ppm. i r r i t a t i o n o f mucous membranes.

t h e s a t u r a t e d vapour c o n c e n t r a t i o n

These l i g h t e r p r o d u c t s may produce

I n metal machining o p e r a t i o n s , s i g n i f i c a n t

v a p o r i s a t i o n o f l u b r i c a t i n g o i l s may o c c u r a t t h e h i g h t o o l / w o r k p i e c e tempera t u r e s , b u t on c o o l i n g i n t h e s u r r o u n d i n g atmosphere, t h i s vapour w i l l be condensed t o d r o p l e t s o f o i l m i s t .

12.3.5

Skin Cancer

I t has been known f o r many y e a r s t h a t some t y p e s o f m i n e r a l o i l can cause

s k i n cancer w i t h repeated and p r o l o n g e d exposure o v e r l o n g p e r i o d s o f years.

It

should be n o t e d however t h a t s k i n cancer i s n o r m a l l y l e s s s e r i o u s than o t h e r forms o f cancer and i s u s u a l l y c u r a b l e w i t h e a r l y t r e a t m e n t . I n 1922, L e i t c h [ 8 ] r e p o r t e d t h a t S c o t t i s h s h a l e o i l d i s t i l l a t e s caused cancer o f t h e s k i n when p a i n t e d on t o animals.

I t i s not possible t o review a l l

o f t h e e x t e n s i v e s t u d i e s c a r r i e d o u t s i n c e t h a t t i m e , b u t among t h e most i m p o r t a n t were those r e p o r t e d i n 1966 by Bingham and Horton 191, sponsored by t h e American Petroleum I n s t i t u t e .

I t was shown t h a t base s t o c k s prepared by

s o l v e n t r e f i n i n g , which removes p o l y c y c l i c a r o m a t i c hydrocarbons (PCAH), d i d n o t cause tumours i n mice whereas t y p i c a l a c i d r e f i n e d base s t o c k s d i d .

I n 1968,

t h e UK Medical Research Council p u b l i s h e d a r e p o r t "The Carcinogenic A c t i o n o f Mineral O i l s :

A Chemical and B i o l o g i c a l Study" [lo]. The s a l i e n t c o n c l u s i o n s

from t h i s r e p o r t and o t h e r s t u d i e s can be summarised as f o l l o w s :

-

Some f r a c t i o n s o f c e r t a i n crudes f r o m which l u b r i c a t i n g o i l s a r e r e f i n e d have been shown t o produce tumours on s k i n s o f r a b b i t s o r mice.

*

S o l v e n t e x t r a c t i o n methods o f r e f i n i n g which remove a r o m a t i c compounds markedly reduce t h e c a r c i n o g e n i c a c t i v i t y o f t h e r e f i n e d l u b r i c a n t s .

279

.

C a r c i n o g e n i c i t y o f m i n e r a l o i l s appears t o be r e l a t e d t o t h e presence o f PCAH's, some o f which a r e known t o be carcinogens.

*

I t has n o t been p o s s i b l e t o d e f i n e any simple a n a l y t i c a l parameters which c o r r e l a t e w i t h c a r c i n o g e n i c a c t i v i t y . An Ad-hoc Committee o f t h e UK I n s t i t u t e o f Petroleum s t u d i e d a l l a v a i l a b l e

evidence and a d v i s e d member companies i n 1968 t h a t o i l s which have been s o l v e n t r e f i n e d o r t r e a t e d i n o t h e r ways t o a p p r e c i a b l y reduce t h e c o n t e n t o f p o l y c y c l i c aromatic compounds, were l e s s l i k e l y t o promote s k i n cancer

han o i l s which had

n o t been t h u s r e f i n e d . The d i f f i c u l t y o f d e f i n i n g a n a l y t i c a l parameters t o c o r r e a t e w i t h c a r c i n o genic a c t i v i t y i s i l l u s t r a t e d by r e s u l t s r e p o r t e d by Scala [ 11, shown i n Table 12.2.

Table 12.2

A

OIL

B

C

Pyrene, ppm

3.5

18.3

4.4

Benz ( a ) anthracene, ppm

6.6

7.9

2.7

Benz (a) pyrene, ppm

4.5

1.2

0.2

% CA (carbon i n a r o m a t i c r i n g s ) 15.4

15.2

12.4

+

Cancer a c t i v i t y

+

The reason f o r t h i s i s t h e u n p r e d i c t a b l e e f f e c t s o f cocarcinogens, and a c c e l e r a t o r s .

inhibitors

For example, i t has been shown t h a t s u l p h u r and some organo-

sulphur compounds can increase c a r c i n o g e n i c i t y , as a l s o can c e r t a i n t y p e s o f hydrocarbons. So f a r , d e s p i t e e x t e n s i v e research, no e n t i r e l y s a t i s f a c t o r y method f o r

e v a l u a t i n g t h e p o t e n t i a l c a r c i n o g e n i c i t y o f an o i l has been devised. method t o d a t e i s t h e l o n g term p a i n t i n g o f mice s k i n s .

The b e s t

Such t e s t s t a k e two

years t o complete and r e q u i r e h i g h standards o f experimental techniques; are t h e r e f o r e v e r y c o s t l y .

they

There i s a l s o t h e d i f f i c u l t y o f t r a n s l a t i n g r e s u l t s

o f animal t e s t s t o t h e human exposure s i t u a t i o n . t e s t each i n d i v i d u a l f o r m u l a t i o n .

I t i s t h e r e f o r e impossible t o

However, a l a r g e number o f t e s t s have been

done and these f o r m t h e b a s i s f o r c u r r e n t recommendations t o m i n i m i s e hazards. Cases o f o c c u p a t i o n a l s c r o t a 1 cancer have been r e p o r t e d f r o m many c o u n t r i e s i n c l u d i n g France, Sweden, t h e UK and t h e USA.

The i n c i d e n c e i n t h e UK i s

e q u i v a l e n t t o about 5 cases p e r m i l l i o n males p e r year whereas i n Sweden i t i s lower, a t about 1 case p e r m i l l i o n males p e r year.

Wahlberg [ 1 2 ] has r e p o r t e d

t h a t o n l y 21 p e r c e n t o f Swedish cases had had d e f i n i t e exposure t o m i n e r a l o i l .

280 T h i s c o n t r a s t s markedly w i t h a f i g u r e o f 86 per c e n t r e p o r t e d f o r UK cases. No o b v io u s reason f o r t h i s d i f f e r e n c e has been i d e n t i f i e d , b u t f a c t o r s such as p l a n t and p e r s o n al hyg i en e may w e l l be s i g n i f i c a n t .

As these a r e improved,

i nc id e n c e o f s c r o t a 1 cancer can be e xpe ct e d t o decrease e v e n t u a l l y .

The long

l a t e n t p e r i o d f r o m f i r s t exposure t o d i a g n o s i s o f the cancer (10 t o 43 years) means t h a t any changes i n i n c i d e n c e cannot be observed q u i c k l y .

12.3.6

Eye I r r i t a t i o n

I n common w i t h a m u l t i t u d e o f commonly used m a t e r i a l s , eg. soapy water, many l u b r i c a n t s may cause some i r r i t a t i o n i f splashes e n t e r t h e eye.

W i t h the

m a j o r i t y o f p r o d u c t s t h i s w i l l n o t be more t h an v e r y s l i g h t , b u t some such as n e a t s o l u b l e o i l s , may, because t he y c o n t a i n a p p r e c i a b l e amounts o f s u r f a c e a c t i v e m a t e r i a l s such as soaps, be somewhat more i r r i t a t i n g .

12.4

ADDITIVE FACTORS

B e fo r e use i n l u b r i c a n t s , a d d i t i v e s a r e screened f o r t o x i c i t y and s k i n o r eye i r r i t a n c y .

T h i s i s e s s e n t i a l t o assess p o t e n t i a l hazards and determine any

required handling precautions during blending o f the f i n i s h e d l u b r i c a n t .

If i t

appears l i k e l y t h a t an a d d i t i v e may l e a d t o any increased hazard i n t h e blended l u b r i c a n t , f u r t h e r t e s t i n g may be done t o d e f i n e t h i s .

The e x t e n t o f p o s s i b l e

increased h a z a r d i n a f i n i s h e d f o r m u l a t i o n must be assessed t o decide whether o r n o t th e a d d i t i v e sho ul d be r e j e c t e d . I n t h e m a j o r i t y o f p rod uct s, a d d i t i v e s a r e minor i n g r e d i e n t s and t h e p o t e n t i a l hazards a r e e s s e n t i a l l y t ho se a s s o c i a t e d w i t h t h e base m i n e r a l o i l . a d d i t i v e c o n t e n t s a r e h i g h e r , eg.

Where

i n n e a t s o l u b l e o i l s and i n some engine l u b -

r i c a n t s , t h e f i n a l p r o d u c t may be more i r r i t a t i n g than t h e base m i n e r a l o i l .

I f use o f these a d d i t i v e s i s e s s e n t i a l f o r t e c h n i c a l performance reasons, t h e hazards must be c o n t r o l l e d by t h e i mp l eme nt ation o f adequate h a n d l i n g and use precautions.

A wide range of a d d i t i v e s i s used t o a c h i e v e improvements i n l u b r i c a n t p e r formance.

For each a d d i t i v e t ype , v a r i o u s chemical compounds have been found

t o be e f f e c t i v e . p o t e n t i a l hazards.

Very few o f these have been found t o p r e s e n t any s i g n i f i c a n t Among t ho se wh i ch have been t h e cause o f some concern a r e

l ea d compounds, o r t h o isomers o f phosphate e s t e r s , c h l o r i n a t e d naphthalenes, sodium n i t r i t e i n co mbi n at i on w i t h amines, sodium mercaptobenzothiazole and trichloroethylene.

12.4.1

Lead Compounds

One o f t h e e a r l y e f f e c t i v e methods of i mp roving a n t i w e a r and extreme pre s s u r e p r o p e r t i e s o f a l u b r i c a n t was t o i n c o r p o r a t e l e a d soap.

Although t h e r e

281 are no r e p o r t e d cases o f s u f f i c i e n t l e ad a b s o r p t i o n t o cause adverse h e a l t h e f f e c t s , Van Peteghem and Vos [13] r e p o r t e d s l i g h t increases i n b l o o d l e a d l e v e l s i n s t e e l m i l l employees w i t h f r e q u e n t o r prolonged s k i n c o n t a c t w i t h t h i s type o f l u b r i c a n t .

New a d d i t i v e t e chn ol o gy has enabled t h i s t y p e o f f o r m u l a t i o n

t o be l a r g e l y r e p l a c e d o v e r t h e l a s t few ye ars by unleaded l u b r i c a n t s .

12.4.2

Or th o Phosphates

A b s o r p t i o n o f o r t h o t r i c r e s y l phosphate has been shown t o cause c e n t r a l nervous system damage l e a d i n g t o neurornuscular problems and v a r i o u s stages o f paralysis.

The p ara isomer does n o t have t h i s e f f e c t and i s e s s e n t i a l l y i n e r t

p r o v i d i n g t h e c o n t e n t o f o r t h o isomer i s a t a v e r y low l e v e l .

!Suppliers o f

t r i c r e s y l phosphates have f o r many ye ars r e s t r i c t e d the o r t h o c o n t e n t t o l e s s than 1 p e r c e n t i n o r d e r t o a v o i d t h e p o s s i b i l i t y o f c e n t r a l nervous system effects

12.4.3

.

C h l o r i n a t e d Naphthalenes

C h l o r i n a t e d naphthalenes were used f o r a s h o r t p e r i o d many years ago as e f f e c t i v e extreme p r e s s u r e a d d i t i v e s i n c u t t i n g o i l s .

T h e i r use was discon-

t i n u e d when an a s s o c i a t i o n w i t h c h l o r a c n e o f t h e s k i n was found.

The types o f

c h l o r i n a t e d a d d i t i v e now used, eg. c h l o r i n a t e d p a r a f f i n s , do n o t cause t h i s effect.

12.4.4

Sodium N i t r i t e and Amines

Sodium n i t r i t e i n co mbi n at i on w i t h t r i and diethanolamines has been used f o r many y e a r s t o p r o v i d e s a t i s f a c t o r y a n t i c o r r o s i o n p r o p e r t i e s i n aqueous g r i n d i n g fluids,

and, a t lower c o n c e n t r a t i o n s ,

i n some s o l u b l e c u t t i n g o i l s .

R ecently,

small amounts o f n i t r o s a m i n e s , a t y p e o f ch emicalof which some a r e known t o be c a r c i n o g e n i c , have been found i n b o t h c o n c e n t r a t e s and d i l u t e d v e r s i o n s o f such p r o d u c t s [14,15].

Ni t ro sami n es a r e a l s o found i n many foods, d r i n k s and

cosmetics, a r e p r e s e n t i n t h e atmosphere o f c i t y s t r e e t s and a r e a l s o formed w i t h i n t h e body i t s e l f .

Assessment o f any increased hazard from g r i n d i n g f l u i d s

i s d i f f i c u l t because o f t h e problems o f e s t i m a t i n g exposure and e x t e n t o f a b s o r p t i o n i n t o t h e body.

Taking t h e w o r s t p o s s i b l e case,

i t appears t h a t

a b s o r p t i o n c o u l d approach t h a t f ro m foods, b u t t y p i c a l l y i t i s l i k e l y t o be v e r y much l e s s t h a n t h i s .

As a p r e c a u t i o n however, s u p p l i e r s have e l i m i n a t e d the

c o m b in a tio n o f n i t r i t e and amine exce pt i n some c r i t i c a l a p p l i c a t i o n s where i t has n o t y e t been p o s s i b l e t o m e t t h e t e c h n i c a l requirements w i t h a l t e r n a t i v e formulations.

282 12.4.5

Sodium Mercaptobenzothiazole

T h i s compound was used a t one t i m e a s a v e r y e f f e c t i v e c o r r o s i o n i n h i b i t o r i n aqueous l u b r i c a n t s .

I t was l a t e r i d e n t i f i e d as a s t r o n g s k i n s e n s i t i z e r and

i s no l o n g e r used i n a p p l i c a t i o n s where s k i n c o n t a c t i s l i k e l y .

12.4.6

Trichloroethylene

T r i c h l o r o e t h y l e n e was used w i d e l y as a non-flammable d i l u e n t i n open gear l u b r i c a n t s t o e n a b l e a h i g h l y v i s c o u s l u b r i c a n t f i l m t o be e a s i l y a p p l i e d t o gear t e e t h s u r f a c e s .

Exposure t o t r i c h l o r o e t h y l e n e vapour above t h e t i m e

weighted average TLV o f 100 ppm may be hazardous, c a u s i n g depression o f t h e c e n t r a l nervous system w i t h v i s u a l d i s t u r b a n c e s and l a c k o f c o - o r d i n a t i o n , t h e p o s s i b i l i t y o f damage t o t h e l i v e r and kidneys. t r i c h l o r o e t h y l e n e vapours has a l s o o c c u r r e d .

plus

Addiction t o s n i f f i n g

Such exposures on a c o n t i n u i n g

b a s i s a r e u n l i k e l y t o occur e x c e p t perhaps i n c o n f i n e d and p o o r l y v e n t i l a t e d spaces.

However, t o p r o v i d e a g r e a t e r margin o f s a f e t y ,

t r i c h l o r o e t h y l e n e can

be s u b s t i t u t e d w i t h a s l i g h t l y more expensive s a f e r a l t e r n a t i v e , 1.1.1

tri-

c h l o r o e t h a n e , which has a TLV o f 350 ppm. I t has a l s o been i n d i c a t e d t h a t t r i c h l o r o e t h y l e n e i s a c a r c i n o g e n i n animal tests.

However, t h e doses g i v e n by i n g e s t i o n i n t o t h e stomach were s o massive

compared t o p o s s i b l e human exposure t h a t t h e s i g n i f i c a n c e o f these t e s t r e s u l t s can be s e r i o u s l y questioned.

12.5

BACTERIA AND BlOClDES

Water based l u b r i c a n t s and m i n e r a l o i l l u b r i c a n t s contaminated w i t h w a t e r , eg. marine e n g i n e o i l s , can s u p p o r t t h e growth o f b a c t e r i a , y e a s t s and f u n g i . Growth does n o t n o r m a l l y occur i n p r o d u c t s which do n o t c o n t a i n water.

As s u p p l i e d t o

users, p r o d u c t s a r e n o r m a l l y f r e e o f b a c t e r i a , b u t c o n t a m i n a t i o n o c c u r s f r o m a number o f p o s s i b l e sources such as water f r o m engine c o o l i n g systems, t h e water used a s d i l u e n t , r e s i d u a l b a c t e r i a i n p l a n t c i r c u l a t i o n systems,

r e f u s e such a s

c i g a r e t t e s thrown i n t o t h e c o o l a n t , employees s p i t t i n g i n t o t h e p r o d u c t o r even from b a c t e r i a l contamination o f the a i r i n the p l a n t . The b a c t e r i a , y e a s t s o r f u n g i which grow i n aqueous c o o l a n t s o r l u b r i c a n t s contaminated w i t h water a r e n o t n o r m a l l y harmful t o humans.

Although concern

has been expressed t h a t b a c t e r i a l c o n t a m i n a t i o n may l e a d t o increased r e s p i r a t o r y o r s k i n i n f e c t i o n s , i n d u s t r i a l medical a d v i s e r s r e s p o n s i b l e f o r l a r g e metal machining p l a n t s have r e p o r t e d t h a t t h e y can f i n d no evidence f o r t h i s . types o f b a c t e r i a found a r e almost i n v a r i a b l y t h e non-pathogenic

The

t y p e which a r e

harmless t o humans a l t h o u g h v e r y o c c a s i o n a l l y a p o t e n t i a l l y h a r m f u l pathogenic t y p e may be i d e n t i f i e d .

283 For t e c h n i c a l reasons, aqueous c o o l a n t s ,

i t i s d e s i r a b l e t o c o n t r o l b a c t e r i a l growth.

With

t h i s i s ach i eve d e i t h e r by i n c l u s i o n o f small amounts o f b i o -

c i d e s i n t h e o r i g i n a l p r o d u c t o r by a d d i t i o n o f b i o c i d e s d u r i n g use.

By t h e i r

na tu r e , b i o c i d e s a r e mo de rat el y t o h i g h l y t o x i c by i n g e s t i o n and may be s k i n o r eye i r r i t a n t s i n t h e c o n c e n t r a t e d form.

T he r efore,

c o n t r o l l e d t o a v o i d i ncre ase d h e a l t h hazards.

t h e i r use must be c a r e f u l l y

I n the c o n c e n t r a t i o n s n o i m a l l y

used, and p r o v i d e d a p p r o p r i a t e h a n d l i n g p r e c a u t i o n s a r e observed, b i o c i d e s should p r e s e n t no hazard t o h e a l t h . "topping-up"

However, use o f excessive c o n c e n t r a t i o n s

may cause s k i n i r r i t a t i o n .

in

W i t h marine engine o i l s , b a c t e r i a l

problems can be c o n t r o l l e d by c o r r e c t o i l s e l e c t i o n and a p p r o p r i a t e o p e r a t i n g procedures [ 1 6 ] .

12.6

SYNTHETIC LUBRICANTS

A v a r i e t y o f chemical t ype s a r e used as s y n t h e t i c l u b r i c a n t s t o meet opera t i o n a l r e q u ir e m en t s which cannot be s a t i s f i e d adequately w i t h m i n e r a l o i l prod u c t s .

Types i n c l u d e v a r i o u s e s t e r s o f o r g a n i c f a t t y a c i d s , s i l i c o n e s , syn-

t h e t i c hydrocarbons such as p o l y o l e f i n e s , p o l y g l y c o l s and phosphate e s t e r s . There a r e no unusual h e a l t h hazards a s s o c i a t e d w i t h these. skin, s i m i l a r t o t h a t w i t h mineral o i l , pro lo n g e d c o n t a c t o ccurs.

D e f a t t i n g o f the

i s p o s s i b l e i n most cases i f repeated o r

I n t h e case o f phosphate e s t e r s , the use o f t h e

o r t h o isomer s h o u l d be avo i de d as i n d i c a t e d i n 1 2 . 4 . 2 .

12.7

USED AND RECLAIMED OR RE-REFINED OILS

There i s s t r o n g evi d en ce t h a t PCAH c o n t e n t o f m i n e r a l o i l based l u b r i c a n t s i nc r e a s e s d u r i n g use [ 1 7 ] . type o f a p p l i c a t i o n ,

The e x t e n t o f t h e increase appears t o depend on t h e

b e i n g up t o about t e n - f o l d f o r c u t t i n g o i l s and d i e s e l

eng in e o i l s , b u t perhaps one h u n d r e d - f o l d or more f o r g a s o l i n e engine o i l s and quenching o i l s .

Non-engine i n d u s t r i a l l u b r i c a n t s such as h y d r a u l i c , gear and

b e a r i n g o i l s would n o t be expected t o show any s i g n i f i c a n t PCAH increase d u r i n g use because o f t h e l i m i t e d t emp era t u re i n crea ses t o which they a r e subjected.

Much o f t h e in c r e ase i n e ng i ne o i l s appears t o a r i s e from g a s o l i n e combustion products

.

The s i g n i f i c a n c e o f these i n crea ses i n PCAH c o n t e n t i n r e l a t i o n t o any i nc r e a s e d s k i n cancer r i s k i s n o t c l e a r a t present f o r t h e reasons discussed i n 12.3.5.

I n t h e case o f c u t t i n g o i l s , c a l c u l a t i o n s based on o i l m i s t concent-

r a t i o n s a t t h e TLV o f

5

mg/m3 i n d i c a t e t h a t PCAH l e v e l s w i l l be o f the same order

o f magnitude as background atmospheric l e v e l s . m a i n t a i n e d below t h e TLV, risk.

Provided o i l m i s t l e v e l s a r e

i t appears t h e r e should be no s i g n i f i c a n t

increase i n

284 For e n v ir o n m en t a l c o n s e r v a t i o n reasons, t h e r e a r e a t t r a c t i o n s i n r e c l a i m i n g o r r e - r e f i n i n g used l u b r i c a n t s f o r f u r t h e r use.

In specific situations there

may a l s o be economic j u s t i f i c a t i o n f o r re cl amation.

A v a r i e t y o f processes may

be employed f r o m si mp l e c e n t r i f u g i n g and e a r t h f i l t r a t i o n t o a c i d t r e a t m e n t , r e d i s t i l l a t i o n and s o l v e n t e x t r a c t i o n . on t h e p o t e n t i a l hazards,

U n t i l further information i s available

i t i s co nsi d ere d i t would be prudent t o l i m i t use o f

such o i l s t o a p p l i c a t i o n s i n which t h e r e i s l i t t l e s k i n c o n t a c t u n l e s s i t i s c e r t a i n t h a t t h e o i l has o n l y been used where PCAH i n c r e a s e i s u n l i k e l y , o r has been t r e a t e d by a pro cess wh i ch w i l l remove PCAH, eg. s o l v e n t e x t r a c t i o n o f aroma t ic s

.

Used g a s o l i n e e ng i ne o i l s can c o n t a i n up t o about 1 per c e n t of l e a d [ 1 8 ] . T h i s o r i g i n a t e s m a i n l y f r o m l e a d a d d i t i v e s i n g a s o l i n e w i t h perhaps a minor c o n t r i b u t i o n fr o m wear o f e ng i ne p a r t s .

Repeated o r prolonged s k i n c o n t a c t w i t h

these o i l s may r e s u l t i n some i n crea sed a b s o r p t i o n o f l e a d i n t o t h e body. Used c u t t i n g o i l s u s u a l l y c o n t a i n sma l l metal c h i p s o r swarf w hich p r e s e n t an a d d i t i o n a l hazard t o t h e s k i n .

Many o f t he se metal p a r t i c l e s a r e n e e d l e - l i k e

shape and can cause m i c r o - l e s i o n s of t h e s k i n ,

in

l e a d i n g t o a general i r r i t a t i o n .

I t i s a l s o b e l i e v e d t h a t e n t r y o f swa rf i n t o t h e s k i n may d e s t r o y an e l e c t r o n e g a t i v e b a r r i e r beneath t h e s u r f a c e ove r about one square inch around t h e s i t e , thus a l l o w i n g o t h e r m a t e r i a l s such as t h e c u t t i n g f l u i d t o p e n e t r a t e i n t o t h e skin.

12.8 12.8.1

HEALTH AND SAFETY PRECAUTIONS Supplier's Responsibilities

S e c tio n 6 o f t h e H e a l t h and S a f e t y a t Mork A c t e t c .

1974 p l a c e s r e s p o n s i b i l -

i t i e s on s u p p l i e r s " t o ensure so f a r as i s reasonably p r a c t i c a b l e , t h a t t h e substance i s s a f e and w i t h o u t r i s k s t o h e a l t h when p r o p e r l y used" and t o make a v a i l a b l e i n f o r m a t i o n on any r e l e v a n t t e s t s and "about any c o n d i t i o n s necessary t o ensure t h a t i t w i l l be s a f e and w i t h o u t r i s k s t o h e a l t h when p r o p e r l y used." S u p p l i e r s a r e a l s o r e q u i r e d t o e l i m i n a t e o r minimise r i s k s t o h e a l t h and s a f e t y , as f a r as i s r e a son ab l y p r a c t i c a b l e . completely safe products;

S u p p l i e r s cannot be expected t o produce

indeed, i t can be s a i d t h a t t h e r e i s no such t h i n g

t h e r e a r e o n l y s a f e ways o f u s i n g a p rod uct .

-

Even pure w ater can be harmful i f

one d r i n k s t o o much o f i t and many p eo pl e w i l l s u f f e r s k i n problems i f t h e i r hands a r e immersed i n wat er f o r se vera l h ou rs d a i l y . For many y e a r s, r e p u t a b l e l u b r i c a n t s u p p l i e r s have been assessing t h e p o t e n t i a l hazards o f p rod uct s.

O f p a r t i c u l a r importance i s t h e assessment o f

t o x i c i t y o f p o s s i b l e a d d i t i v e s and r e j e c t i o n o f those w hich may l e a d t o s i g n i f i c a n t l y in c r e a s e d r i s k .

As i n d i c a t e d i n S e c t i o n 12.4, a number o f a d d i t i v e s

which may p r e s e n t hazards under some c o n d i t i o n s o f use have been r e p l a c e d i n r e c e n t y e a r s by a l t e r n a t i v e s a f e r m a t e r i a l s .

Care must always be e x e r c i s e d

285 however, t o ensure t h a t a m a t e r i a l w i t h a known r i s k i s n o t r e p l a c e d by a new m a t e r i a l w i t h unknown r i s k which may i n f a c t be much g r e a t e r . The H e a l t h and S a f e t y a t Work Act does n o t r e q u i r e s u p p l i e r s t o d i s c l o s e d e t a i l s o f p r o d u c t comp osi t i on s t o u sers.

W h i l s t most s u p p l i e r s w i l l make

broad c o m p o s i t i o n a l i n f o r m a t i o n g e n e r a l l y a v a i l a b l e , o r even d e t a i l e d informa t i o n on a c o n f i d e n t i a l b a s i s t o a u ser h e a l t h p r o f e s s i o n a l where necessary, f o r m u l a t i o n s a r e p r o p r i e t a r y i n f o r m a t i o n t o t h e i n d i v i d u a l manufacturer i n a c o m p e t i t i v e b u s iness wo rl d.

Co nsi d era bl e re search e x p e n d i t u r e may have been

i n c u r r e d i n d e v e l op i ng new a d d i t i v e s and i n s e l e c t i n g t h e b e s t combination o f a d d i t i v e s t o p r o v i d e a performance b e n e f i t i n a p a r t i c u l a r a p p l i c a t i o n .

Sup-

p l i e r s a r e n a t u r a l l y r e l u c t a n t t o i n crea se t h e p o s s i b i l i t y t h a t t h i s informa t i o n may pass t o t h e i r c o m p e t i t o r s .

I n any case, such i n f o r m a t i o n i s u s u a l l y

o f l i t t l e v a lu e t o t h e use r i n asse ssi ng p o t e n t i a l hazards. v a lu e i s t h e t o x i c o l o g i c a l

O f much g r e a t e r

i n f o r m a t i o n and t h e recommended h a n d l i n g p r e c a u t i o n s

based on t h i s i n f o r m a t i o n . Most s u p p l i e r s a l s o p r o v i d e use rs w i t h b o o k l e t s o r l e a f l e t s recommending safe h a n d l i n g p r e c a u t i o n s and r e v i e w i n g p o t e n t i a l h e a l t h hazards.

T y p i c a l recom-

mendations, t o g e t h e r w i t h p r a c t i c e s o f l a r g e user companies, have been reviewed by t h e I n s t i t u t e o f Petroleum and i n c o r p o r a t e d i n a Code o f P r a c t i c e f o r Metalwor k in g F l u i d s , p u b l i s h e d i n J u l y 1978 1191.

12.8.2

S k in P r o t e c t i o n

The b a s i c r e q ui re men t f o r a v o i d i n g s k i n problems i s t o minimise c o n t a c t . With t h e m a j o r i t y of l u b r i c a n t s , o ccasi o na l s k i n c o n t a c t f o r s h o r t p e r i o d s w i l l cause no problems.

Compliance w i t h t h e normal recommendation t o a v o i d prolonged

o r r e p e a te d s k i n c o n t a c t w i l l be s u f f i c i e n t t o p r e v e n t d e r m a t i t i s and s k i n i r r i t a t i o n problems.

The p r e c a u t i o n s needed t o a v o i d d e r m a t i t i s and s k i n i r r i -

t a t i o n w i l l a l s o p r e v e n t s k i n cancer.

Modern s o l v e n t r e f i n e d types o f m i n e r a l

o i l s ( o r e q u i v a l e n t ) a l s o mi n i mi se t h e r i s k o f s k i n cancer.

With t h e more

i r r i t a n t t y p e s o f p r o d u c t , such as k e r o s i n e and n e a t s o l u b l e o i l s , occasional v e r y s h o r t c o n t a c t i s u n l i k e l y t o cause problems, b u t i f any a p p r e c i a b l e c o n t a c t i s l i k e l y , s u i t a b l e p r o t e c t i v e measures sh ou ld be employed. Contact can be mi ni mi se d by u s i n g s u i t a b l e p r o t e c t i v e gloves and c l o t h i n g , b a r r i e r creams, and t h e p rop er i n s t a l l a t i o n and use o f splash guards on c u t t i n g machines.

P r o t e c t i v e c l o t h i n g which becomes contaminated w i t h o i l should be

changed f r e q u e n t l y and cl e an ed by any l a u n d e r i n g process ( d r y o r wet, o r a c o m b in a tio n o f b o t h ) which produces v i s u a l l y c l e a n garments. comes g r o s s l y contaminated, eg. by spray o r s p i l l a g e ,

I f c l o t h i n g be-

i t should be changed

immediately. S p e c ia l aprons a r e a v a i l a b l e , c o n s i s t i n g o f an impervious back w i t h a de ta c h a b le a b s o rbe nt f r o n t whi ch can be e a s i l y removed f o r c l e a n i n g .

The use o f

286 t h i s t y p e o f p r o t e c t i o n by t o o l s e t t e r s , who a r e p a r t i c u l a r l y l i k e l y t o be exposed t o heavy c o n t a m i n a t i o n i n l e a n i n g o v e r o i l y machines,

i s strongly

Use o f t h i s t y p e o f a pro n removes t h e t e m p t a t i o n t o stow o i l y rags

recommended.

o r t o o l s i n t r o u s e r po cket s, a p r a c t i c e whi ch c o u l d r e s u l t i n t h e s k i n i n t h e g r o i n a r e a b e in g i n p rol o ng ed c o n t a c t w i t h o i l - s o a k e d c l o t h i n g .

Sleeves o f

c l o t h i n g should be s h o r t o r r o l l e d up t o a v o i d c o n t i n u a l f r i c t i o n between o i l soaked c u f f s and t h e s k i n o f forearms and w r i s t s . wear o i l - s o a k e d c l o t h i n g " .

The golden r u l e i s

-

u n d e r c l o t h i n g which may become con t a mi na t e d f rom o i l - s o a k e d o v e r a l l s . c l o t h e s s h o u ld a l s o be changed f r e q u e n t l y . ination,

"Do n o t

I t sho ul d be remembered t h a t t h i s a p p l i e s a l s o * t o Under-

To h e l p minimise c l o t h i n g contam-

s e p a r a te l o c k e r f a c i l i t i e s f o r work and s t r e e t c l o t h e s a r e d e s i r a b l e

i n changing rooms. Employees who come i n t o c o n t a c t w i t h o i l should wash exposed s k i n a t t h e end o f any work p e r i o d , u s i n g warm w a t e r and soap, m i l d d e t e r g e n t o r p r o p r i e t a r y s k i n cleanser.

S t ron g soaps and d e t e r g e n t s and a b r a s i v e type soaps o r c l e a n s e r s

should be avoided.

K e rosi n e, p e t r o l and o t h e r degreasing s o l v e n t s should n o t be

used f o r c l e a n i n g t h e s k i n .

Hands sh ou l d a l s o be washed b e f o r e e a t i n g , d r i n k i n g

or smoking and b e f o r e and a f t e r u s i n g t h e l a v a t o r y .

E a s i l y a c c e s s i b l e washing

and t o i l e t f a c i l i t i e s sh ou l d be a v a i l a b l e and should be w e l l maintained. I d e a l l y , employees sh ou l d shower a t t h e end o f t h e work s h i f t t o remove a l l traces o f o i l from the skin.

I f rag s a r e used t o wipe o i l from t h e s k i n o r

machinery, a p l e n t i f u l su pp l y sho ul d be a v a i l a b l e so t h a t they can be changed frequently.

P r e f e r a b l y t h e y sh ou l d be o f a d i s p o s a b l e t y p e t o a v o i d t h e poss-

i b i l i t y o f a c c u mul a t i o n o f metal c h i p s and sw arf w hich may c u t o r s c r a t c h t h e skin. B a r r i e r creams a r e o f t e n used on t h e assumption t h a t they p r o t e c t t h e s k i n from d i r e c t contact w i t h o i l o r coolant.

Al t hough t h e i r e f f e c t i v e n e s s i s some-

what u n c e r t a i n , the y do have a p a r t t o p l a y i n m i n i m i s i n g d e r m a t i t i s problems by i n c r e a s i n g awareness o f t h e need f o r c a r e o f t h e s k i n .

They can a l s o make

eve n tu a l washing o f t h e s k i n more e f f e c t i v e , p a r t i c u l a r l y where " d i r t y " i n v o lv e d .

work i s

Use o f creams f r o m r e p u t a b l e s u p p l i e r s i s recommended s i n c e l o n g

exp e r ie n c e has enabled them t o a v o i d use o f components w hich may harm t h e s k i n . I t i s im p o r t a n t t o use t h e c o r r e c t t y p e o f cream f o r t h e type o f o i l i n v o l v e d

s i n c e a w a t e r r e s i s t a n t t y p e i nt en de d f o r use w i t h aqueous c o o l a n t s w i l l n o t g i v e e f f e c t i v e p r o t e c t i o n a g a i n s t m i n e r a l o i l based l u b r i c a n t s . Use o f a s k i n r e c o n d i t i o n i n g cream a f t e r work i s a l s o i m p o r t a n t t o h e l p r e p l a c e t h e n a t u r a l f a t s and o i l s removed f rom t h e s k i n by exposure t o l u b r i c a n t s and by washing.

T h i s i s a v e r y i mp ortant p a r t o f a s k i n c a r e programme

t o a v o i d d e r m a t i t i s problems.

I t i s p a r t i c u l a r l y important w i t h o l d e r em-

pl o y e e s who t e n d t o have d r i e r s k i n s , and i n t h e w i n t e r when low temperatures

281 and h u m i d i t y also tend t o cause dryness and c r a c k i n g o f t h e s k i n .

Male

employees a r e o f t e n r e l u c t a n t t o use a s k i n cream because o f t h e i r a s s o c i a t i o n w i t h t h e " s o f t f e m i n i n e touch" o f consumer a d v e r t i s i n g .

There i s no doubt t h a t

the common a s s o c i a t i o n o f m a s c u l i n i t y w i t h tough and rough hands which do n o t need s k i n creams has c o n t r i b u t e d t o numerous cases o f o c c u p a t i o n a l s k i n disease. E f f o r t s t o persuade more men t o use r e c o n d i t i o n i n g creams r e g u l a r l y would undoubtedly be w e l l rewarded i n reduced absence from work, o r need t o t r a n s f e r t o o t h e r j o b s , because o f d e r m a t i t i s .

S u i t a b l e creams a r e a v a i l a b l e f r o m r e p u t a b l e

b a r r i e r cream suppl i e r s . F i n a l l y , an i m p o r t a n t p a r t o f any s k i n p r o t e c t i o n programme i s t o ensure t h a t a l l employees who use o r a r e exposed t o any t y p e o f l u b r i c a n t o r metalworking f l u i d keep a c a r e f u l watch on a l l areas o f t h e i r s k i n and o b t a i n medical a d v i c e a t t h e f i r s t s i g n o f any a b n o r m a l i t y .

Medical a t t e n t i o n should be o b t a i n e d f o r

any c u t s and s c r a t c h e s as w e l l as d i s c o l o r a t i o n , soreness, warty growths.

itching, swelling o r

Awareness o f s k i n d i s o r d e r s and s k i n c a r e can be promoted by

d i s p l a y i n g and d i s t r i b u t i n g p o s t e r s and l e a f l e t s p u b l i s h e d by t h e H e a l t h and Safety E x e c u t i v e and by t h e o i l s u p p l i e r s .

Employees should be aware o f and

observe any s p e c i a l i n s t r u c t i o n s on p r o d u c t package l a b e l s o r i n t h e s u p p l i e r ' s product l i t e r a t u r e .

Under t h e H e a l t h and S a f e t y a t Work A c t ,

i t i s t h e respon-

s i b i l i t y o f t h e employer t o i n f o r m h i s employees o f any known o r p o t e n t i a l hazards t o h e a l t h and t o i n s t r u c t them on t h e a p p r o p r i a t e p r e c a u t i o n s t o be followed.

Constant reminders and proper s u p e r v i s i o n a r e necessary t o ensure

t h a t t h e contempt b r e d o f f a m i l i a r i t y does n o t o v e r r i d e prudence, o r t h e "problems o n l y happen t o o t h e r people" syndrome does n o t become predominant.

12.8.3

O i l M i s t and Vapour

Although exposure i s u n l i k e l y t o c r e a t e a hea t h hazard, c o n c e n t r a t i o n s o f o i l m i s t and vapour i n t h e p l a n t atmosphere shou d be minimised t o a v o i d an unpleasant environment.

The o i l m i s t c o n c e n t r a t on should be m a i n t a i n e d below

t h e Threshold L i m i t Value o f 5mg/m3 and p r e f e r a b y below 2.5mg/m3.

O i l mist i s

determined by use o f sampling pumps t o c o l l e c t m i s t on f i l t e r papers f o r a n a l y s i s by w e i g h i n g o r o t h e r methods.

The Occupational Hygiene Sub-committee o f t h e

I n s t i t u t e o f Petroleum A d v i s o r y Committee on H e a l t h has p u b l i s h e d d e t a i l s o f s u i t a b l e techniques [ 2 0 ] . atmosphere,

As a general guide,

i f m i s t can be seen i n t h e p l a n t

i t i s l i k e l y t o be above 5mg/m3.

I f t h e o i l m i s t l e v e l i s e x c e s s i v e , t h e f i r s t s t e p i s t o t r y t o reduce t h e amount generated. i s produced.

(1)

T h i s w i l l r e q u i r e an assessment o f t h e way i n which t h e m i s t

There a r e two b a s i c mechanisms by which m i s t i s formed:

I n some o p e r a t i o n s o i l may be atomised i n small d r o p l e t form. T h i s may occur

i n some h i g h speed c u t t i n g o p e r a t i o n s o r f r o m m i s t l u b r i c a t i o n systems through

288 over a p p l i c a t i o n o f m i s t o r poor r e c l a s s i f i c a t i o n o f t h e l u b r i c a n t .

In these

cases, m i s t c o n c e n t r a t i o n s may be reduced by adjustments t o t h e method o f appl i c a t i o n o r by m o d i f i c a t i o n t o t he co mpo si t i on o f t h e l u b r i c a n t .

I n metal

machining t h e r a t e , volume f l o w and p o i n t o f a p p l i c a t i o n can a f f e c t t h e degree o f misting.

Proper p o s i t i o n i n g o f sp l ash guards can c o n t r o l t h e escape o f o i l

m i s t i n t o t h e g e n era l p l a n t atmosphere.

S p e c i a l l y formulated a n t i - m i s t c u t t i n g

o i l s have s u c c e s s f u l l y reduced p l a n t m i s t l e v e l s i n some a p p l i c a t i o n s .

Mist

l u b r i c a n t f o r m u l a t i o n s r e q u i r e a c a r e f u l ba l ance between adequate m i s t i n g and reclassification properties.

The r a t e o f a p p l i c a t i o n o f t h e m i s t l u b r i c a n t and

t h e design o f t h e a p p l i c a t i o n system a r e a l s o important. ( i i ) V a p o r i s a t i o n o f l u b r i c a n t may be f o l l o w e d by condensation t o form small droplets o f o i l mist.

I n t h i s case i t may be p o s s i b l e t o p r o v i d e a d d i t i o n a l

c o o l i n g by i n c r e a s i n g t h e volume f l o w r a t e o f o i l a p p l i e d .

I t i s o f t e n thought

t h a t h i g h v e l o c i t y j e t o f o i l i s t h e most e f f e c t i v e means o f c o o l i n g whereas i n f a c t a low v e l o c i t y h i g h volume f l o w w i l l r e s u l t i n less o i l v a p o r i s a t i o n .

With

a h i g h v e l o c i t y j e t t h e r e may a l s o be a g r e a t e r tendency t o f o r m a t i o n o f o i l mi st.

I f f o r m a t i o n o f o i l m i s t and vapour cannot be e f f e c t i v e l y c o n t r o l l e d by app-

l i c a t i o n o r f o r m u l a t i o n changes, l o c a l exhaust v e n t i l a t i o n should be used. Systems i n c o r p o r a t i n g f i l t e r s t o remove o i l so t h a t c l e a n a i r can be r e t u r n e d t o t h e p l a n t atmosphere a r e co mmerci a l l y a v a i l a b l e .

For maximum e f f e c t i v e n e s s ,

th e exhaust hood sho ul d be l o c a t e d as c l o s e t o t h e p o i n t o f m i s t g e n e r a t i o n as poss i b l e

.

O i l m i s t i s sometimes generated by t he use o f a i r j e t s t o remove swarf from

machined p a r t s .

I f t h e r e i s no o t h e r way o f p e r f o r m i n g t h i s o p e r a t i o n , con-

s t r u c t i o n o f an e ncl o sure w i t h l o c a l exhaust v e n t i l a t i o n may be needed.

12.8.4

S k i n Cancer

I f good q u a l i t y s o l v e n t r e f i n e d o i l s ( o r t hose t r e a t e d adequately i n o t h e r ways t o reduce t h e a r o m a t i c c o n t e n t s ) a r e used i n a l l a p p l i c a t i o n s where any s i g n i f i c a n t s k i n c o n t a c t i s l i k e l y , s k i n cancer should n o t be a problem i n f u t u r e , p r o v i d e d t h a t p r e c a u t i o n s di scu ssed under Skin P r o t e c t i o n i n sec.12.8.2 a r e followed.

Because o f t h e l o ng l a t e n t p e r i o d f r o m i n i t i a l exposure t o

occ u r r e n c e ( o f t e n more t h an 20 yea rs) some cases a s s o c i a t e d w i t h prolonged o r rep e a t e d exposure t o p o o r l y r e f i n e d m i n e r a l o i l s many years ago, o r w i t h unsati s f a c t o r y hygiene p r a c t i c e s i n t h e p a s t , can s t i l l be expected t o a r i s e . However, f o l l o w i n g t h e i n t r o d u c t i o n o f s o l v e n t r e f i n e d o i l s i n c u t t i n g f l u i d f o r m u l a t i o n s and t h e i mp l eme nt at i on o f b e t t e r hygiene p r a c t i c e s , fewer cases as t ime passes.

t h e r e should be

289 Because changes o ccur i n co mpo si t i o n o f used o i l s , p a r t i c u l a r l y w i t h gasol i n e engine l u b r i c a n t s and quenching o i l s ,

i t would be prudent t o e x e r c i s e c a r e

w i t h these p r o d u c t s and a v o i d s k i n c o n t a c t as much as p o s s i b l e .

12.8.5

B a c t e r i a and B i o c i d e s

B a c t e r i a i n aqueous c o o l a n t s and m i n e r a l o i l l u b r i c a n t s contaminated w i t h water need t o be c o n t r o l l e d f o r t e c h n i c a l performance reasons. Section 12.5,

t h e r e i s no evidence o f h e a l t h hazards a s s o c i a t e d w i t h t h e o c c u r -

rence o f these b a c t e r i a .

However, a d d i t i o n o f b i o c i d e s t o c o o l a n t systems may

present hazards i n hand1 i n g these m a t e r i a l s . and t o x i c .

As i n d i c a t e d i n

T h e r e f o re,

Biocides a r e normally i r r i t a n t

t h e s u p p l i e r ' s h a n d l i n g recommendations should be c a r e -

f u l l y followed.

12.9

CONCLUSIONS

P r o v id e d t h a t use rs a r e aware o f p o t e n t i a l hazards and f o l l o w recommended h a n d l i n g p r a c t i c e s i n co mbi n at i on w i t h good personal and p l a n t hygiene standards, l u b r i c a n t s s h o u ld p r e s e n t n o undue h e a l t h r i s k s .

The major p o i n t s which need t o

be r e p e a t e d l y s t r e s s e d can be summarised as f o l l o w s : Use good q u a l i t y s o l v e n t r e f i n e d ( o r e q u i v a l e n t ) m i n e r a l o i l s i f t h e r e i s t o be s i g n i f i c a n t s k i n c o n t a c t . Ensure t h a t adequate i n f o r m a t i o n i s a v a i l a b l e t o enable products t o be used s a f e l y . Develop p r o p er awareness o f hazards t hro ugh t r a i n i n g , c a u t i o n a r y notices, supplier publications etc. A v o id r e p e a t ed or pro l on ge d s k i n c o n t a c t . Encourage good p erso na l h ygi e ne w i t h p roper s k i n c l e a n i n g p r a c t i c e s . Provide s u i t a b l e p r o t e c t i v e c l o t h i n g . Encourage p r o p e r use o f b a r r i e r and r e c o n d i t i o n i n g creams. Keep o i l m i s t c o n c e n t r a t i o n s w e l l below 5mg/rn3. E s t a b l i s h good o i l and machine maintenance p r a c t i c e s . Ob t a in e a r l y medical a d v i c e f o r any s k i n problems. Ensure good p r a c t i c e s a r e ma i nt ai n ed w i t h d i l i g e n t s u p e r v i s i o n .

REFERENCES

1 2 3

4 5 6

7

Wagner,W.D., e t . a l . , Am. Ind. Hyg. Assn. J. 1964, 5 , 158. Lushbaugh,C.C., e t . a l . , Arch. Ind. Hyg. 1950. 237. Wagner,W.D., e t . a l . , Unpublished r e s u l t s , USPHS, 1014 Broadway, C i n c i n a t t i , U.S.A. Decoufle,P., Ann. New York Acad. S ci . 1976, 94. Decoufle,P., J. Nat. Cancer I n s t . 1978, 1025. Drasche,H., e t . a l . , Z en t b l . Arb. Med. Arbschutz 1974, lo. Sanderson,J., O i l M i s t - Recent I n t e r e s t s i n Europe. Presented a t Esso Symp. on O i l M i s t and Ni t rosa mi ne s, Stockholm, March 1977.

1,

271, 5,

290 8 9 10

11 12 13

14 15 16

17 18 19 20

Leitch,A., B r i t . Med. J., 1922, 2, 1004. Bingham,E., and Horton,A.W., 'Advances i n B i o l o g y o f S k i n ' , Vol . V I I Carcinogenesis, 1966, Pergamon Press, New York. Medical Research Council, 'The Carcinogenic A c t i o n o f M i n e r a l O i l s : A Chemical and B i o l o g i c a l S t u d y ' . Special Report S e r i e s No. 306, 1968, H.M.S.O. London. J. Occ. Med. 1975, 1_z, 784. Scala,R.A., Wahlberg,J.E., Acta.Derrn. (Stockholm), 1974, 54, 471. Th. Van Peteghem and H. De Vos, B r i t . J. I n d . T e d . 1974, 2, 233. Zingmark,P.A. and Rapp,C. Ambio, 1977, 237. .NIDSH T e c h n i c a l Report ' C o n t r o l o f Exposure t o M e t a l w o r k i n g F l u i d s ' , February 1978, (Pub1 i c a t i o n No. 78-165). Technical B u l l e t i n , ' M i c r o b i o l o g i c a l Degradation o f L u b r i c a t i n g O i l s ' . Mobil O i l Co. L t d . , 1977. Thony,C., e t a l . , Arch. Mal. Prof de Med Trav e t Sec. SOC. ( P a r i s ) , 37. 1975, Clausen,J., and Rastogi,S.C., B r i t . J. Ind. Med. 1977, 208. I n s t i t u t e of Petroleum "Code o f P r a c t i c e f o r M e t a l w o r k i n g F l u i d s " , 1978, Heyden & Son L t d . , London. I n s t i t u t e o f Petroleum Occupational Hygiene Sub-committee, Ann. Occup. 293-297. Hyg. 1975,

6,

36,

2,

18,

291

13

EFFECTIVE CONTAMINATION CONTROL IN FLUID POWER SYSTEMS

J.B.SPENCER,

13.1

Manager, User Support D i v i s i o n , S p erry V i c k e r s .

INTRODUCTION

The s e l e c t i o n o f a f i l t e r and i t s p r o p e r l o c a t i o n i n a h y d r a u l i c system needs as much c a r e and t h e same l e v e l o f e x p e r t i s e as t h e s e l e c t i o n o f o t h e r components such as pumps, v a l v e s and c y l i n d e r s .

Many system designers l o o k no f u r -

t h e r t h a n t h e h y d r a u l i c equipment m a n u f a c t u r e r ' s catalogue f o r guidance, b u t u n f o r t u n a t e l y i t i s s t i l l common f o r h y d r a u l i c equipment manufactuers t o s p e c i f y one g e n e r a l l e v e l o f f i l t r a t i o n such as 25 micrometre, w i t h o u t r e g a r d t o the working p r e s s u r e , environment, o r d u t y c y c l e . a lower s t a n d a r d may be a ccep t a bl e ;

With c e r t a i n types o f equipment

an example o f t h i s being t h e e a r l i e r designs

of pumps, many o f whi ch g i v e l o n g t r o u b l e - f r e e s e r v i c e p r o t e c t e d o n l y by a 0.13 mm s t r a i n e r .

On t h e o t h e r hand, more modern equipment such as t h e m i n i a t u r i z e d

c o n t r o l s w i t h s m a l l e r cl e ara nce s t ha n many servo v a l v e s , w i l l need much h i g h e r standards o f p r o t e c t i o n . U s u a l l y t h e n e x t s t e p i s t o de ci de on t h e l o c a t i o n o f t h e f i l t e r and, again, th e g e n e r a l i z e d recommendations o f t h e f i l t e r manufacturer a r e o f t e n accepted w i t h o u t r e g a r d t o t h e p a r t i c u l a r system requirements.

F i n a l l y , the s i z e o f the

f i l t e r i s f i x e d and sometimes t h e o v e r r i d i n g c o n s i d e r a t i o n i s simply a d e s i r e t o match t h e f i l t e r p o r t s i z e t o t h e di a met er o f t h e a d j o i n i n g pipework.

T h i s may

w e l l ensure t h a t recommended v e l o c i t i e s a r e n o t exceeded b u t much more important and o f t e n o v e r lo oke d i s t h e f i l t e r e f f i c i e n c y and d i r t h o l d i n g c a p a c i t y . I t must be a d m i t t e d t h a t t h e h i t - o r - m i s s

approach described above o f t e n

appears t o a c h ie ve an a c c e p t a b l e r e s u l t , b u t w i t h over 70% o f h y d r a u l i c system f a i l u r e s known t o be due t o poor f l u i d c o n d i t i o n , t h e r e i s a c l e a r need f o r a more s y s t e m a t i c approach t o c o n t a m i n a t i o n c o n t r o l . The need has been a ccen t u at ed by t h e i n c r e a s i n g l y arduous c o n d i t i o n s under whic h systems o p e r a t e .

For example, a p r e s s u r e o f about 70 bar was common i n

i n d u s t r i a l h y d r a u l i c systems f o r many ye ars;

today,

140-210 bar systems a r e

292 commonplace and much o f t h e contaminant f o r m e r l y washed away i s now f o r c e d i n t o t h e c le a r a n c e s where i t does c o n s i d e r a b l e damage.

Smaller o i l r e s e r v o i r s mean

more r a p i d c i r c u l a t i o n and l e s s o p p o r t u n i t y f o r p a r t i c l e s t o s e t t l e o u t .

Higher

o p e r a t i n g temperatures r e s u l t i n t h i n n e r o i l , w hich i n some s i t u a t i o n s may g i v e l e s s p r o t e c t i o n a g a i n s t wear g i v i n g r i s e t o i ncreased contamination. I n t h e f a c e o f t h ese t re nd s,

t h e h y d r a u l i c equipment user wants improved

r e l i a b i l i t y and i t must be e a s i e r and l e s s c o s t l y f o r him t o achieve t h i s i f he works w i t h c l e a n o i l .

I t i s n o t d i f f i c u l t t o keep t h e o i l i n good c o n d i t i o n

p r o v i d e d t h e machine d esi g n i s r i g h t , and t h e rewards f o r doing so a r e b e t t e r r e l i a b i l i t y and l o ng er l i f e f rom b o t h t h e equipment and o i l .

13.1.1

A S y s te ma t i c Approach t o F i l t r a t i o n

To work towards t h e most e f f e c t i v e p r o t e c t i o n c o n s i s t e n t w i t h economy, we must f i r s t d e f i n e o u r aim.

I t i s n o t , as i s w i d e l y assumed, s i m p l y t o separate

o u t p a r t i c l e s l a r g e r t h an a c e r t a i n s i z e chosen a r b i t r a r i l y .

I n s t e a d , we must

ach ie v e s t a b l e l e v e l s o f c o n t a m i n a t i o n a c c e p t a b l e and a p p r o p r i a t e t o v a r i o u s p a r t s o f t h e system. For a s t a b i l i s e d c o n t a m i n a t i o n l e v e l , e c t e d by t h e system f i l t e r s ) .

' d i r t i n ' must equal ' d i r t o u t '

(coll-

' D i r t i n ' i s made up o f i n - b u i l t contaminant,

c o n t a m in a t io n i n t h e i n i t i a l charge o f o i l , and c o n t a m i n a t i o n drawn i n from t h e atmosphere t h r o u gh t h e a i r b r e a t h e r and c y l i n d e r s e a l s

-

a l l o f w hich c o n t r i b u t e

t o t h e g e n e r a t i o n o f p a r t i c l e s by t h e process o f wear. The e f f e c t o f t h e i n - b u i l t c o n t a m i n a t i o n must be c a r e f u l l y considered.

Inev-

i t a b l y i t w i l l be h i g h , even when c a r e i s taken i n t h e p r e p a r a t i o n o f pipework and m a n i f o l d b lo cks.

F l u s h i n g w i l l remove some i n i t i a l contaminant,

but there

a r e many systems where t h i s i s n o t done, and a t t h e f i r s t s t a r t - u p h i g h pressures a r e generated w i t h h i g h c o n t a m i n a t i o n l e v e l s p r e s e n t .

The r e s u l t i s u s u a l l y

r a p i d pump wear and v a l v e m a l f u n c t i o n , t h e f i r s t o f which w i l l almost c e r t a i n l y go u n d e t e c te d a t t h i s time.

A l l h y d r a u l i c systems should be run i n an o f f - l o a d

c o n d i t i o n u n t i l the desired contamination l e v e l i s a t t a i n e d . F i g u r e 1 shows a t y p i c a l r e l a t i o n s h i p between t h e design c o n t a m i n a t i o n l e v e l and t h e a c t u a l l e v e l p r i o r t o s t a r t - u p .

T h i s convenient method o f p r e s e n t a t i o n

f o l l o w s n a t u r a l l y f ro m t h e l o g a r i t h m i c d i s t r i b u t i o n o f p a r t i c l e s i z e t h a t occurs i n practice.

The r e l a t i v e sl o pe s o f t h e i n i t i a l and a c c e p t a b l e Contamination

l i n e s are a c l e a r p o i n t e r t o the type o f f i l t r a t i o n

needed.

The methods used t o d et ermi n e and c o n t r o l c o n t a m i n a t i o n l e v e l s w i l l be d i s cussed i n more d e t a i l l a t e r , b u t a t t h i s st ag e we can summarize t h e p r a c t i c a l and performance req ui re men t s o f t h e f i l t r a t i o n system as f o l l o w s : -

293

Initial contamination level

Fig. 1: Typical relationship between the desired design contamination level and the actual level prior to start up. It is essential that the system is flushed and run at no-load until the acceptable contamination level is achieved.

level

5 10 15 Parliclasire

(i)

25

50

- micrometres(lopa)

100

I t must be ca pa bl e o f red uci n g t h e i n i t i a l c o n t a m i n a t i o n t o t h e d e s i r e d l e v e l w i t h i n an a c c e p t a b l e p e r i o d o f t i me, w i t h o u t causing premature wear o r damage t o t h e h y d r a u l i c components.

(ii)

I t must be cap ab l e o f a c h i e v i n g and m a i n t a i n i n g t h e d e s i r e d l e v e l , i n c l u d i n g a s u i t a b l e f a c t o r o f s a f e t y t o c a t e r f o r a c o n c e n t r a t e d ingress which c o u l d o ccur; f o r example, when a system i s 'topped-up'.

( i i i ) The q u a l i t y o f maintenance a v a i l a b l e t h e end u s e r l o c a t i o n must be acknowledged. (iv)

F i l t e r s must be e a s i l y a c c e s s i b l e f o r maintenance purposes.

(v)

I n d i c a t i o n o f f i l t e r c o n d i t i o n t o s u i t t h e end u s e r ' s requirements must be p r o v id e d.

(vi)

I n c o n tin u ou s p roce ss p l a n t , f a c i l i t i e s must be p r o v i d e d t o a l l o w changing o f elements w i t h o u t i n t e r f e r i n g w i t h p l a n t o p e r a t i o n .

( v i i ) The f i l t e r s must p r o v i d e s u f f i c i e n t d i r t h o l d i n g c a p a c i t y f o r an a c c e p t a b l e i n t e r v a l between element changes. ( v i i i ) T h e i n c l u s i o n o f a f i l t e r i n t h e system must n o t produce u n d e s i r a b l e e f f e c t s on t h e o p e r a t i o n o f components,

e.g.

h i g h back pressures on

seal drains. (ix)

Sampling p o i n t s must be p r o v i d e d t o m o n i t o r i n i t i a l and subsequent l e v e l s

of c o n ta m i na t i o n.

294 13.2 13.2.1

DIRT INPUT

-

TYPES AND SOURCES OF CONTAMINATION

New O i l

A lt h o u g h o i l i s r e f i n e d and blended under r e l a t i v e l y c l e a n c o n d i t i o n s , i t i s u s u a l l y s t o r e d i n drums o r i n a b u l k t a n k a t t h e u s e r ' s f a c t o r y .

A t t h i s point

i t i s no lo n g e r c l e a n , because t h e f i l l i n g l i n e s c o n t r i b u t e metal and rubber

p a r t i c l e s and t h e drum always adds f l a k e s o f metal o r scale.

Storage tanks can

be a r e a l problem because wat er condenses i n them t o cause r u s t i n g and contamin a t i o n fr o m t h e atmosphere f i n d s i t s way i n unless s a t i s f a c t o r y . a i r

breather

f i l t e r s are f i t t e d . I f t h e o i l i s b e i n g s t o r e d under reasonable c o n d i t i o n s ,

t h e p r i n c i p a l contam-

i n a n t s on d e l i v e r y t o a machine w i l l be me t a l, s i l i c a , and f i b r e s .

With o i l s

from r e p u t a b l e s u p p l i e r s , sampling has shown average counts o f 30,000

t o 50,000

p a r t i c l e s above 5 mi crome t re p e r 100 m l , w i t h a r e l a t i v e l y low s i l t l e v e l . Using a p o r t a b l e t r a n s f e r u n i t o r some o t h e r f i l t r a t i o n arrangement,

i t i s possible t o

remove much o f t h e c o n t a m i n a t i o n p r e s e n t i n new o i l b e f o r e i t e n t e r s t h e system and i s ground down i n t o f i n e r p a r t i c l e s . I t must be s a i d i n pa ssi ng t h a t c o n t a m i n a t i o n a r i s i n g f r o m d e l i v e r y and s t o r age v a r i e s w i t h t h e i n d u s t r y .

For example, a i r c r a f t o p e r a t i o n g e n e r a l l y needs

h i g h standards o f c l e a n l i n e s s and f a i r l y q u i c k t u r n o v e r o f s t o r e s , whereas much l on g e r s t o r a g e p e r i o d s a r e t h e r u l e f o r ma ri ne systems and t h e environment may be more d i f f i c u l t t o c o n t r o l .

13.2.2

B u i l t - i n Con t a mi na t i o n

New machinery always has a c e r t a i n amount o f b u i l t - i n contamination.

Care i n

assembly and i n f l u s h i n g t h e system reduces t h i s , b u t never e l i m i n a t e s i t . T y p i c a l b u i l t - i n contaminants a r e b u r r s , c h i p s , m o is t u r e , p i p e dope, wel d s p l a t t e r ,

f l a s h , d i r t , dust, f i b r e , sand,

p a i n t s , and f l u s h i n g s o l u t i o n s .

The amount o f contaminant removed d u r i n g f l u s h i n g depends n o t o n l y on t h e e f f e c t i v e n e s s o f t h e f i l t e r used b u t a l s o t h e v e l o c i t y o f t h e f l u s h i n g f l u i d . Unless h i g h v e l o c i t i e s a r e a t t a i n e d , much o f t h e contaminant w i l l n o t be d i s lodged u n t i l t h e system i s i n o p e r a t i o n , w i t h component f a i l u r e t h e almost certain result.

I r r e s p e c t i v e o f t h e st a nd ard o f f l u s h i n g , an o f f - l o a d running-

i n p e r i o d s h o u ld be regarded as e s s e n t i a l .

Some b u i l t - i n contaminant,

such as

weld s c a l e , o f t e n remains i n t a c t u n t i l h i g h p r e s s u r e o i l i s f o r c e d between i t and t h e p a r e n t m e t a l , thus l o o s e n i n g i t .

13.2.3

Environmental Contamination

Contaminants f r o m t h e immediate surro un di n as can be i n t r o d u c e d i n t o a f l u i d power system.

On l a r g e i n s t a l l a t i o n s such as those w i t h i n steelw orks,

r e l a t i v e l y easy t o a s c e r t a i n t h e e nvi ro nme nt al c o n d i t i o n s , siderably.

it is

though they v a r y con-

For example, a coke oven system operates i n c o n d i t i o n s v e r y d i f f e r e n t

296

from a c o l d m i l l .

Sometimes t h e b e s t s o l u t i o n i s t o p r o t e c t t h e h y d r a u l i c

equipment by p r o v i d i n g a c l e a n room where maintenance can be c a r r i e d o u t under controlled conditions.

Unfortunately,

i t i s n o t uncommon t o see h y d r a u l i c power

sources exposed t o t h e w o r s t p o s s i b l e environment, w h i l e a l o n g s i d e t h e e l e c t r i c s are p r o t e c t e d by p r e s s u r i s e d and temperature c o n t r o l l e d cabins. I n most machine shops t h e r e l a t i v e l y l a r g e contaminant p a r t i c l e s o f 10-15 micrometres do n o t demand a h i g h st an da rd o f a i r f i l t r a t i o n , a l t h o u g h g r i n d i n g machines w i t h o u t e f f e c t i v e e x t r a c t i o n equipment can r e s u l t i n l o c a l i s e d p r o blems.

On t h e o t h e r hand, f o u n d r i e s and st on e q u a r r i e s demand a v e r y h i g h

standard o f f i l t r a t i o n because o f t h e a i r b o r n e a b r a s i v e p a r t i c l e s . The m o b i l e equipment f i e l d p r e s e n t s s p e c i a l problems because t h e o r i g i n a l manufacturer u s u a l l y s e l l s a st an da rd machine t o o p e r a t e i n a w ide v a r i e t y of environmental c o n d i t i o n s .

13.2.4

E n t r y P o i n t s f o r Environmental Con t amination

13.2.4.1

A i r breathers.

Very l i t t l e i n f o r m a t i o n appears t o be a v a i l a b l e on

what t h e f i l t e r w i l l a c t u a l l y ach i eve and p u r e l y nominal r a t i n g s a r e u s u a l l y specified.

There have been i nst an ces where t h e element has shrunk l e a v i n g a

f r e e passage f o r t h e a i r , wh i ch h i g h l i g h t s t h e need f o r more r i g i d e n g i n e e r i n g standards on t h i s t y p e o f p r o d u c t .

The amount o f a i r passing through t h e f i l -

whi ch means f o r example t h a t s i n g l e a c t i n g c y l i n -

t e r w i l l depend on d r a w - o f f ,

ders i n bad environments must r e s u l t i n a g r e a t e r ingress o f contaminant. I t i s encouraging t o see some ma nu f a ct u rers now o f f e r i n g b e t t e r grades of f i l t e r paper and t h a t i n c e r t a i n areas t h e combined f i l l e r / b r e a t h e r unacceptable.

has become

A sep ara t e b r e a t h e r i s more e f f i c i e n t and h e l p s t o r e l e a s e t h e

a i r w h i l e f i l l i n g t a kes p l a c e t hro ug h a s u i t a b l e gauze s t r a i n e r .

13.2.4.2

Power u n i t access p l a t e s .

I n some p l a n t s i t cannot be assumed t h a t

access p l a t e s w i l l always be re pl a ced , though h a p p i l y t h i s s t a t e o f a f f a i r s i s no t as common as i t once was.

I n power u n i t design, good s e a l i n g i s v i t a l , and

i n bad environments such items as s t r a i n e r s should n o t be p o s i t i o n e d i n s i d e t h e r e s e r v o i r i f access r e q u i r e s t h e r e f i t t i n g o f removable p l a t e s .

Other removable

items w i l l a l l o w i n g r e s s d u r i n g maintenance and good design p r a c t i c e should minimize t h i s .

13.2.4.3

Cylinder seals.

Wiper s e a l s cannot be 100% e f f e c t i v e i n removing

very f i n e contaminant f rom t h e c y l i n d e r rod. I f they were, t h e y would remove t h e o i l f i l m from t h e p i s t o n rod, producing a r e s u l t t h a t i s u s u a l l y diagnosed as a l e a k y seal. d r y r o d would q u i c k l y wear o u t t h e se al s.

I n any case, a completely

Where c y l i n d e r s remain extended i n a

296 h e a v i l y contaminated atmosphere c o n s i d e r a b l e q u a n t i t i e s o f f i n e p a r t i c l e s can g e t i n t o t h e system un l ess p r o t e c t i o n such as a b e l l o w s i s p r o v i d e d . I t has been shown t h a t c y l i n d e r p i s t o n r o d s e a l s n a t u r a l l y i n g r e s s about one p a r t i c l e o v er 10 mi cro met re f o r each square c e n t i m e t r e o f swept rod area. Wear o f s e a l s o r wi p ers can i n c r e a s e t h e i n g r e s s i o n r a t e c o n s i d e r a b l y .

Thus

i n bad ambient c o n d i t i o n s a 50mm di a met er r o d i n a 100mm b o r e c y l i n d e r , c y c l i n g a t a speed o f 12 metres p e r minute, c o u l d i n g r e s s about 20,000 p a r t i c l e s ove r 1Omicrometres e very mi nu t e , and t h i s q u a n t i t y c o u l d i n c r e a s e by a f a c t o r o f 100 f o r every 100 hours o f ru nn i ng .

13.2.5

Generated Co nt ami n at i on

Contamination i s c r e a t e d i n t e r n a l l y by t h e o p e r a t i o n o f a h y d r a u l i c system. The generated contaminants a r e p r o d u c t s o f wear, c o r r o s i o n , c a v i t a t i o n , and f l u i d breakdown,

i.e.

decomposition,

oxidation,

etc.

Experience shows t h a t i n

a system which has been c a r e f u l l y f l u s h e d and has f i l t e r e d o i l added t o t h e sealed r e s e r v o i r ( i n c o r p o r a t i n g an e f f e c t i v e b r e a t h e r ) , t h e c o n t a m i n a t i o n w i l l be m a i n l y system generated.

I f t h e i n i t i a l l e v e l i s n o t s a t i s f a c t o r y , t h i s induces wear w hich g r e a t l y a c c e l e r a t e s t h e b u i l d - u p o f generated contaminant.

13.3

EFFECTS OF TYPES AND S I Z E S OF PARTICLES

We know t h a t contaminant p a r t i c l e s a r e o f a l l shapes and s i z e s and t h a t t h e f i n e r t h e y a r e , t h e more d i f f i c u l t i t i s t o count them and t o determine t h e m a t e r i a l o f wh ic h t h e y a r e composed. a b r a s i v e and t h a t ,

However, we can say t h a t t h e m a j o r i t y a r e

i n t e r a c t i n g w i t h s u r f a c e p r o t r u s i o n s , t h e y plough and c u t

fragments f r o m a s u r f a c e .

T h i s wear accounts f o r about 90% o f f a i l u r e s due t o

c o n ta m in a tio n . F a i l u r e s a r i s i n g from contamination f a l l i n t o t h r e e categories:

13.3.1 v a lv e .

C a t a s t r o p h i c F a i l u r e o c c u r s when a l a r g e p a r t i c l e e n t e r s a pump o r For in s tan ce,

i f a - p a r t i c l e causes a vane t o jam i n a r o t o r s l o t , t h e

r e s u l t may w e l l be complete s e i z u r e o f t h e pump or motor.

I n a spool t y p e valve

a l a r g e p a r t i c l e t rap pe d a t t h e r i g h t p l a c e can s t o p a spool c l o s i n g c o m p l e t e l y . Another example of c a t a s t r o p h i c f a i l u r e o c c u r s when t h e p i l o t o r i f i c e of a v a l v e i s b lo c k e d by a l a r g e p a r t i c l e .

F i n e p a r t i c l e s can a l s o cause catastrop.hic

f a i l u r e , for i n s t a n c e i f a v a l v e f a i l s t o o p e r a t e due t o s i l t i n g .

13.3.2

I n t e r m i t t e n t F a i l u r e i s caused by contaminant on t h e s e a t o f a poppet

v a l v e w h ic h p r e v e n t s i t f ro m r e s e a t i n g p r o p e r l y .

I f t h e seat i s t o o hard t o

a l l o w t h e p a r t i c l e t o b e embedded i n t o i t , t h e p a r t i c l e may be washed away when t h e v a l v e i s opened ag ai n .

L a t e r , a n o t h e r p a r t i c l e may p r e v e n t complete c l o s u r e

297 o n l y t o be washed away when t h e v a l v e opens.

Thus a v e r y annoying t y p e o f i n t e r -

m i t t e n t f a i l u r e occurs.

13.3.3

D e g r a d at i o n F a i l u r e f o l l o w s wear, c o r r o s i o n ,

and c a v i t a t i o n e r o s i o n .

They cause increased i n t e r n a l leakage i n t h e system components, b u t t h i s condition i s often d i f f i c u l t t o detect. The e v e n t u a l r e s u l t , p a r t i c u l a r l y w i t h pumps, failure.

i s l i k e l y t o be c a t a s t r o p h i c

The p a r t i c l e s most 1 i k e l y t o cause wear a r e c l e a r a n c e - s i z e p a r t i c l e s

which j u s t pass t h r o u g h t h e cl e ara nce s between moving p a r t s ( F i g . 2 ) .

Particles of similar size to the clearances between moving parts cause the most abrasive wear. Larger particlescannot enter clearance,smaller particlespass through without contact.

a 0 Clearancesize particles interact with both faces simultaneously. Extra contaminationis generated by the disintegrationof the moving parts. Larger particlesare'ground up' by the interactionin the clearances.

8

F i g. 2

I n t e r a c t i o n o f moving p a r t s .

M a n u fa c t u r in g cl e ara nce s w i t h i n h y d r a u l i c components can be d i v i d e d i n t o two p r i n c i p a l zones,

i.e.

around 5 micrometres f o r h i g h pressure u n i t s , and 15-40

micrometres f o r low p r e s s u r e u n i t s .

The a c t u a l clearance may v a r y c o n s i d e r a b l y

depending on t h e t y p e o f u n i t and o p e r a t i n g c o n d i t i o n s i t sees.

Good component

d e s i g n i s im p o r t an t , t hu s m i n i m i s i n g t h e e f f e c t o f small clearances. We s h o u ld t h e r e f o r e l o o k a t t h e f a c t o r s a f f e c t i n g c r i t i c a l clearances and a l s o a t t h e t y p e o f f a i l u r e o c c u r r i n g i n v a r i o u s groups o f components.

13.3.4

Pumps

A l l h y d r a u l i c pumps have component p a r t s which move r e l a t i v e t o one another, s e p a r a t e d by a s mal l o i l - f i l l e d cl e ara nce .

G enerally, these components a r e

loaded toward each o t h e r by f o r c e s r e l a t e d t o pressure, and t h e p r e s s u r e always tend s t o f o r c e f l u i d t h rou gh t h i s cl e ara nce . As t h e f i n i t e l i f e o f most pumps i s determined by a v e r y small q u a n t i t y o f m a t e r i a l b e in g removed f r o m a few su rf a ces,

i t f o l l o w s t h a t i f the f l u i d w i t h i n

298 t h e c l e a r a n c e i s h e a v i l y contaminated, could occur.

r a p i d d e g r a d a t i o n and e v e n t u a l s e i z u r e

W i t h low p r e s s u r e u n i t s t h e d e s i g n p e r m i t s r e l a t i v e l y l a r g e c l e a r -

ances and c o n t a m i n a t i o n has l e s s e f f e c t .

A l s o a t t h e lower p r e s s u r e t h e r e i s

less force a v a i l a b l e t o d r i v e p a r t i c l e s i n t o c r i t i c a l clearances.

Increasing

t h e p r e s s u r e t h e r e f o r e i s o f major s i g n i f i c a n c e i n d e t e r m i n i n g t h e e f f e c t o f c o n t a m i n a t i o n on a pump. Another f a c t o r a f f e c t i n g clearances i s t h e o i l f i l m t h i c k n e s s , which i s a l s o related t o f l u i d viscosity.

An optimum v i s c o s i t y i s used f o r d e s i g n which p r o -

v i d e s good f i l m t h i c k n e s s t o s u p p o r t loads hydrodynamically b u t which i s a l s o low enough t o a l l o w adequate f i l l i n g o f t h e pump w i t h o u t c a v i t a t i o n .

I t i s gen-

e r a l l y found i n p r a c t i c e t h a t f i l t r a t i o n requirements become l e s s c r i t i c a l where h i g h e r v i s c o s i t i e s a r e used, and f o r t h i s reason t h e maximum v i s c o s i t y which i s c o m p a t i b l e w i t h t h e i n l e t c o n d i t i o n s should be chosen.

S i m i l a r l y , good tempera-

t u r e c o n t r o l w i l l be o f b e n e f i t i n t h i s r e s p e c t . The areas i n pumps p a r t i c u l a r l y s u b j e c t t o these c l e a r a n c e problems a r e : -

Gear pump

-

Tooth t o housing, gear t o s i d e p l a t e .

A x i a l p i s t o n pump

-

Shoe t o swashplate, c y l i n d e r b l o c k .

Vane pump

Vane t i p t o cam r i n g , r o t o r t o s i d e p l a t e .

F i g u r e s 3 , 4 and 5 i l l u s t r a t e t h e c r i t i c a l areas d i a g r a m m a t i c a l l y .

Clearance under vane tips depends on speed, pressure and viscosity

ote Some back flow occurs from hlgh to

Fig.3

C r i t i c a l clearances i n a vane pump.

299

In Low pressure

Minimum clearance

Fig.4

&

db Maximum clearance

C r i t i c a l c l e a r a n c e s i n a gear pump.

Where

P

p

= viscosity

=

pressure

Valve plate

Fig.5

C r i t i c a l c l e a r a n c e s i n an a x i a l p i s t o n pump. A l t h o u g h p s t o n c earance i s n o m i n a l l y f i x e d , a c t u a c l e a r a n c e v a r i e s w i t h e c c e n t r c i t y due t o l o a d and v i s c o s i t y .

I n many o f t h e f o r e g o i n g cases t h e cl e ara nces a r e e f f e c t i v e l y s e l f - a d j u s t i n g under o p e r a t i n g c o n d i t i o n s , smaller.

i . e . w i t h i n c r e a s i n g p r e s s u r e clearances become

Under adverse c o n d i t i o n s , and p a r t i c u l a r l y where t h e r e i s shock load-

i ng , t h i s in c r e a ses t h e v u l n e r a b i l i t y t o s m a l l e r contaminant p a r t i c l e s . where c le a r a n c e s a r e n o m i n a l l y f i xed , u nd er

Even

h i g h loads components may t a k e up

e c c e n t r i c p o s i t i o n s whi ch a g a i n makes them v u l n e r a b l e t o s m a l l e r p a r t i c l e s .

It

i s e x tr e m e ly d i f f i c u l t t o be p r e c i s e about e i t h e r t h e magnitude o f these c l e a r ances, p a r t i c u l a r l y under dynamic l o a d i n g , o r t h e e f f e c t o f d i f f e r e n t s i z e p a r t i c l e s i n t h e gaps. However, fr o m t h e d at a we do have and f ro m f i e l d experience gained t o date, we a r e a b l e t o suggest c o n t a m i n a t i o n l e v e l s which, an a c c e p t a b le l i f e f o r most pumps.

i f achieved, w i l l r e s u l t i n

These a r e presented i n t h e n e x t c h a p t e r ,

whic h d e a l s w i t h t h e s p e c i f i c a t i o n o f c o n t a m i n a t i o n l e v e l s . The u s e f u l l i f e o f a pump sho ul d end when i t no longer d e l i v e r s t h e r e q u i r e d o u t p u t a t a g i v e n s h a f t speed, d i s c h a r g e p ressure, and f l u i d temperature. guid e , 15-20% l o s s o f f l o w would i n d i c a t e t h e end o f t h e u s e f u l l i f e .

As a

A l l too

o f t e n d e g r a d a t i o n goes un de t e ct e d until,finally,catastrophic f a i l u r e occurs w i t h v a s t q u a n t i t i e s o f c o n t a m i n a t i o n b e i n g r e l e a s e d i n t o t h e system. owing such a f a i l u r e , t h e system i s n o t t h en p r o p e r l y cleaned,

If, foll-

t h e l i f e o f the

replacement pump w i 1 1 be reduced. I n t h e i n t e r e s t o f t h e end use r, t h e system designer should s p e c i f y t h e m i n i mum a c c e p t a b l e f l o w r a t e f r o m t h e pump t o achieve s a t i s f a c t o r y machine performance.

Means s h o u l d be p r o v i d e d f o r m o n i t o r i n g pump o u t p u t by i n s e r t i n g s u i t a b l e

instrumentation,

e i t h e r t e m p o r a r i l y o r pe rmanently, so t h a t r o u t i n e checks can

be c a r r i e d o u t t o reduce t h e r i s k o f c a t a s t r o p h i c f a i l u r e s . With p i s t o n u n i t s , i t i s u s u a l l y a s i m p l e m a t t e r t o measure case leakage, w hich can be a u s e f u l

gu id e t o pump c o n d i t i o n . Remember, t o t h e end u ser i t i s t o t a l c o s t s t h a t a r e i m p o r t a n t ; o f a low-cost pump may w e l l r e s u l t i n e xpe nsi ve downtime.

the f a i l u r e

I f , by t h e i n c l u s i o n

o f a f l o w meter, such a f a i l u r e can be a voi d ed, t h e i n i t i a l investment i n a flow meter would be f u l l y j u s t i f i e d .

13.3.5

Motors

What has been w r i t t e n about pumps a p p l i e s g e n e r a l l y t o motors o f s i m i l a r des i gn. I t must be remembered t h a t much o f the, contaminant passing through t h e pump

may be r e a c h in g t h e motor a l s o , where t h e r e w i l l be a s i m l l a r performance degradation.

I f , due t o wear, t h e v o l u m e t r i c e f f i c i e n c y o f t h e pump has f a l l e n t o 85%

o f i t s o r i g i n a l v a l u e and t h e v o l u m e t r i c e f f i c i e n c y o f t h e motor has f a l l e n t o , say, 90% o f o r i g i n a l ,

t h en t h e o v e r a l l v o l u m e t r i c e f f i c i e n c y o f t h e pump and

motor w i l l be down t o 0.85 x 0.9

76.5% o f t h e o r i g i n a l value.

For t h i s reason

301 c o n ta m in a tio n c o n t r o l i s p a r t i c u l a r l y i m p o r t a n t i n h y d r o s t a t i c transmissions t o p r o v i d e t h e necessary l e v e l o f f l u i d c l e a n l i n e s s .

13.3.6

D i r e c t i o n a l Valves

The r a d i a l c l e a r a n c e s p e c i f i e d between bore and spool i n most d i r e c t i o n a l v a lv e s i s i n t h e range o f

5

t o 13 mi cro met res.

As i s w e l l know, t h e p r o d u c t i o n

o f p e r f e c t l y round and s t r a i g h t bores i s e x c e p t i o n a l l y d i f f i c u l t , so i t i s unl i k e l y t h a t any spool w i l l l i e e x a c t l y c e n t r a l i n t h e clearance band. inal & i n .

I n a nom-

v a l v e , a good spool i s l i k e l y t o have l e s s than 2.5 micrometres

c 1 ea rance. I n an e l e c t r i c a l l y o pe rat ed v a l v e , shown i n F i g . 6 .

They a r e : Flow f o r c e s

Tank port

Cylinder Return Port

r--

Q MI

Spring force

t h e f o r c e s a c t i n g on t h e s o l e n o i d a r e

+

Pressure port

m

L---

Leakage carrying particles 1r---

r---

Inertia.

Tank port

l---

0 .

AP = Pressure drop across spool

+

1

I

Flow force FaAx AP

Friction

Cylinder pressure Port

Dotted lines show position of spool lands when closed.

Fi g . 6

+

Spring f o r c e

IIRll a

r---

Spring lorce

.

Va l ve spool c r i t i c a l clearances ( w i t h f l o w s and f o r c e s )

Flow, s p r i n g , and i n e r t i a f o r c e s a r e i n h e r e n t f a c t o r s b u t f r i c t i o n f o r c e s are, t o a g r e a t e x t e n t , dependent on f i l t r a t i o n .

I f t h e system i s h e a v i l y contamin-

a t e d w i t h p a r t i c l e s s i m i l a r i n s i z e t o t h e r a d i a l and d i a m e t r a l clearances, h i g h e r f o r c e s w i l l be needed t o move t h e s p o o l . An even worse s i t u a t i o n r e s u l t s f rom s i l t i n g , where contaminant i s forced i n t o t h e c le a r a n ces under p ressu re, e v e n t u a l l y l e a d i n g t o breakdown o f t h e o i l

f i l m and spool hang-up ( F i g . 7 ) . T h i s s i t u a t i o n o ccurs where v a l v e s s u b j e c t e d t o continuous p r e s s u r e a r e operated i n f r e q u e n t l y .

Such v a l v e s sh ou l d p r e f e r a b l y have l o c a l f i l t r a t i o n o f a

v e r y h i g h o r d e r i n t h e a d j a c e n t p r e s s u r e l i n e b u t due account should be taken o f p o s s i b l e p r e s s u r e surges generated d u r i n g component o p e r a t i o n .

The use o f

h i g h e f f i c i e n c y f i l t e r s as a s p e c i a l p r o t e c t i o n f o r s i n g l e u n i t s o r groups o f

302 u n i t s can r e s u l t i n t h e need f o r a v e r y h i g h d i r t c a p a c i t y i f t h e general l e v e l o f f i l t r a t i o n i n t h e system i s much lower.

One large particle can bridge gap

Working pressure

Break in oil film due to dirt concentration

Fig.7

/

1 Silt Build-up

V a l ve spool c r i t i c a l clearance. Eccentricity i s t h e normal c o n d i t i o n i n many cases.

Some idea o f t h e f o r c e s needed t o break t h i s spool hang-up,

compared w i t h

t h e f o r c e s a v a i l a b l e f rom t h e s o l e n o i d , can be gained from t h e example o f a nominal

&

i n . v a l v e o p e r a t i n g a t 210 b ar.

I f a v a l v e o f t h i s t y p e remains s e l -

ecte d i n t h e s p r i n g o f f s e t o r e ne rgi se d p o s i t i o n f o r a l e n g t h y p e r i o d o f time, s i l t i n g t a k e s p l a c e between spool and bo re t o produce t o t a l i m m o b i l i t y .

The

f o r c e needed t o overcome t h i s s t a t e has been found by experiment t o be o f t h e o r d e r o f 135 Newtons, b u t b o t h s p r i n g and s o l e n o i d c o u l d e x e r t o n l y 45 Newtons. Thus t h e e f f e c t o f t h e s i l t i s t o cause t o t a l system f a i l u r e .

13.3.7

P r e s s u re C o n t r o l s

H i g h l y a b r a s i v e p a r t i c l e s i n h i g h v e l o c i t y streams o f o i l erode t h e surfaces w i t h wh ic h they come i n t o c o n t a c t .

T h i s s i t u a t i o n i s common t o p r e s s u r e con-

t r o l l e r s , p a r t i c u l a r l y r e l i e f v a l v e s whi ch a r e s u b j e c t e d t o maximum system pre s s u r e drop and v e l o c i t i e s o f t h e o r d e r o f 30 m/s.

P i l o t c o n t r o l stages gen-

e r a l l y see low volumes a t h i g h v e l o c i t i e s and heavy c o n t a m i n a t i o n a f f e c t s b o t h t h e i r s t a b i l i t y and r e p e a t a b i l i t y . 13.3.8

Flow C o n t r o l s

The c o n t a m i n a t i o n t o l e r a n c e o f f l o w c o n t r o l v a l v e s w i l l depend v e r y much on the o r i f i c e c o n f i g u r a t i o n .

303 F i g u r e 8, f o r example, shows two o r i f i c e s which a r e o f en't r e l y d i f f e r e n t The groove t y p e (a) w i l l t o l e r a t e a h i g h

shape, a l t h o u g h having equal areas.

contamination l e v e l except when used a t low s e t t i n g , whereas

ype (b) i s much

more prone t o s i l t i n g a t a l l s e t t i n g s . With a l l types o f pressure-compensated f l o w c o n t r o l s , t h e performance o f t h e pressure r e d u c i n g element can be c o n s i d e r a b l y a f f e c t e d by c o n t a m i n a t i o n , i r r e s pective o f valve s e t t i n g .

Damage t o t h e m e t e r i n g o r i f i c e can a l s o occur, which

w i l l become p a r t i c u l a r l y apparent a t lower s e t t i n g s .

Note: Throttle profile gives orifice segments of equal area

/

Fig.8

\

Flow c o n t r o l v a l v e t h r o t t l e s e c t i o n s . P r o f i l e (b) i s more prone t o s i l t i n g .

Generally speaking, a l l s p o o l - t y p e c o n t r o l v a l v e s a r e a f f e c t e d by contaminat i o n i n t h e system, e s p e c i a l l y a t h i g h p r e s s u r e s .

The e f f e c t s a r e l i k e l y t o be

magnified i f p r e c i s e a x i a l p o s i t i o n i n g o f t h e spool i s necessary as, f o r example, i n pressure r e d u c i n g v a l v e s where l i m i t e d f o r c e s a r e a v a i l a b l e t o o p e r a t e t h e spool.

On t h e o t h e r hand, poppet v a l v e s , though a f f e c t e d by l a r g e p a r t i c l e s o f

contamination, tend t o be f a r more t o l e r a n t o f s i l t due t o t h e s e l f - c l e a r i n g action o f the seat,

13.3.9

However, e r o s i o n i s s t i l l l i k e l y t o o c c u r .

Summary

I t can be seen from t h e f o r e g o i n g t h a t an i n d i v i d u a l l a r g e p a r t i c l e a r r i v i n g a t the wrong p l a c e a t t h e wrong t i m e can cause c a t a s t r o p h i c f a i l u r e . Surfaces w i t h i n components should be separated by an o i l f i l m , o f which may be c o n t i n u a l l y changing.

the thickness

When t h i s gap i s b r i d g e d by contaminants,

wear w i l l occur thereby g e n e r a t i n g f u r t h e r p a r t i c l e s which may w e l l be ground i n t o many more s m a l l e r p a r t i c l e s .

F i n e p a r t i c l e s i n d i v i d u a l l y o r i n small

304

Fig.9

Suggested a c c e p t a b l e c o n t a m i n a t i o n l e v e l s f o r v a r i o u s h y d r a u l i c systems

Target Contamination Class t o CETOP RP70H

Suggested Maximum Particle Level

5um 15 vm

5vm

Sensitivity

Type o f System

15vm

Suggested Filtration Rating Bx > 75

13

9

4 000

250

Super critical

S i l t sensitive control system w i t h v e r y h i g h r e l i a b i l i t y . Laboratory o r aerospace.

1-2

15

11

16 000

1 000

Critical

High performance servo and h i g h p r e s s u r e l o n g l i f e systems, i.e. a i r c r a f t , machine t o o l s , e t c .

3-5

16

13

32 000

4 000

Very important

High q u a l i t y r e l i a b l e systems. General machine requirements

18

14

130 000

8 000

Important

General machinery and m o b i l e 2-15 systems. Medium p r e s s u r e , medium c a p a c i t y .

19

15

250 000 16 000

Average

Low p r e s s u r e heavy indust r i a l systems, or a p p l i c a t i o n s where l o n g l i f e i s not c r i t i c a l .

21

17

1 000 0 0 0 6 4 0 0 0

Hain

Low p r e s s u r e systems w i t h l a r g e c 1 earances.

10-12

5-25

25-40

Note: This graph assumes viscosity to be within recommended range.

I !I\,.* .

-..-. I ‘=-..I_ -----

Average hydraulic components including most pumps.

0 200

f

‘,Very sensitive t components

UI

i

-. * =..

I

I

I

.--------

7

p,yery tolerant components. I

’.

Figures relate to cumulative particle concentrations above5prnand 15pm respectively (see chapter 8).

Fig.10

Suggested c l e a n l i n e s s l e v e l f o r good l i f e

305 q u a n t i t i e s may n o t cause damage, b u t i f p rese nt i n l a r g e r c o n c e n t r a t i o n s may l ea d t o f a i l u r e t h rou gh s i l t i n g . The o b j e c t i v e must be t o o b t a i n t he most economic combination o f contamina t i o n c o n t r o l and co nt ami n at i on t o l e r a n c e f o r a g i v e n r e l i a b l e system l i f e under known performance and e nvi ro nme nt al c o n d i t i o n s . 13.4

SPECIFYING CONTAMINATION LEVELS

As s t a t e d p r e v i o u s l y , 25 micrometres i s a t y p i c a l general l e v e l o f f i l t r a t i o n , s p e c i f i e d w i t h o u t r e g a r d t o wo rki ng pre ssure , l o c a l environment o r d u t y c y c l e . We know f r o m e x p eri e nce t h a t under s i m i l a r en vironmental and o p e r a t i n a c o n d i t i o n s the e f f e c t i v e n e s s o f a 25 micrometre f i l t e r w i l l v a r y depending on i t s l o c a t i o n i n t h e system.

A l so , under steady f l o w c o n d i t i o n s as t h e pores tend t o c l o g the

f i l t r a t i o n performance may a c t u a l l y improve.

Under v a r y i n g o r i n t e r m i t t e n t

f l o w , however, t h e r e s u l t can be v e r y d i f f e r e n t because contaminant p a r t i c l e s a r e d i s l o d g e d f r o m t h e p ore s a l l o w i n g more f i n e p a r t i c l e s t o pass through. I t i s o b v io u s t h a t t h e c o n d i t i o n s o f use have t h e g r e a t e s t e f f e c t on t h e c ont a m in a t io n l e v e l o b t a i n e d w i t h a s p e c i f i c f i l t e r , which means t h a t i t i s gene r a l l y unwise t o o f f e r w i t h o u t q u a l i f i c a t i o n o r accept w i t h o u t q u e s t i o n a b l a n k e t recommendation o f , say t h e use o f a 25 micrometre f i l t e r .

For t h e user

i t means p u t t i n g h i s investment a t r i s k and f o r t h e manufacturer i t makes t h e a d m i n i s t r a t i o n o f w a r r a n t i e s ext reme l y d i f f i c u l t . From a wide spectrum o f f i e l d da t a , F i g . 9 summarises t h e l e v e l s o f contaminat i o n wh ic h a r e c on si de red a ccep t a bl e f o r most h y d r a u l i c systems. The d e c i s i o n o f whether a system i s c r i t i c a l w i l l depend l a r g e l y on t h e type o f components used and t h e system d esi g n p ressure.

F i g u r e 10 has t h e r e f o r e been

produced t o g i v e t h e recommended co nt ami n at i on l e v e l s w i t h knowledge o f the system p r e s s u r e and h a v i n g an al yse d t h e d i r t s e n s i t i v i t y o f the system componen t s.

13.5

SELECTING THE FILTER

B e f o r e a c h o i c e o f f i l t e r may be made t h e v a r i o u s r a t i n g s used by manufacture r s must be examined. 13.5.1

Nominal R a t i n g

S p e c i f i c a t i o n s MIL-F5504A and MIL-F5504B were e s t a b l i s h e d for d e t e r m i n i n g nominal r a t i n g s .

V e rsi on A d e f i n e s a 10 micrometre f i l t e r as b e i n g a b l e t o

remove 98% by w e i g h t o f a l l p a r t i c l e s o f t h e e l e c t e d contaminant (AC f i n e t e s t d u s t ) l a r g e r t h a n 10 micrometres a t a c e r t a i n h i g h c o n c e n t r a t i o n .

Version B

d e f i n e s a 10 m ic ro met re f i l t e r as b e i n g a b l e t o remove 95% by w eight o f 10-20 m i cr o m e t r e g l a s s beads a t a h i g h c o n c e n t r a t i o n .

Although l i t t l e use has been

made o f t h e s e p a r t i c u l a r s p e c i f i c a t i o n s , many manufacturers use s i m i l a r t e s t s t o p r o v i d e nominal r a t i n g s f o r t h e i r f i l t e r s .

306 Such t e s t s have two major l i m i t a t i o n s .

Firstly,

they do n o t l i m i t t h e maxi-

mum s i z e o f p a r t i c l e a l l o w e d t o pass through t h e element and from t e s t s i t has been found t h a t f i l t e r s meeting these requirements can pass p a r t i c l e s up t o 200 micrometres.

Secondly, t h e h i g h c o n c e n t r a t i o n o f contaminant added i s n o t t y p i -

c a l o f c o n d i t i o n s experienced i n a normal system.

I n practice, the particles

approach t h e f i l t e r i n small c o n c e n t r a t i o n s and those p a r t i c l e s t h a t a r e s m a l l e r than t h e mean p o r e s i z e pass r e a d i l y through t h e f i l t e r as l o n g as t h e f i l t e r medium remains reasonably c l e a n . For t h e reasons g i v e n above t h e r e i s a good case f o r d i s c o n t i n u i n g t h e use o f nominal r a t i n g s .

13.5.2

Absolute Rating

The NFPA F l u i d Power Glossary o f Terms d e f i n e s t h e a b s o l u t e r a t i n g as b e i n g t h e diameter o f t h e l a r g e s t h a r d s p h e r i c a l p a r t i c l e t h a t w i l l pass through a f i l t e r under s p e c i f i e d t e s t c o n d i t i o n s .

T h i s i s an i n d i c a t i o n o f t h e l a r g e s t

opening i n t h e f i 1 t e r - e w n t .

13.5.3

The Bubble T e s t

T h i s i s a t e s t used by manufacturers t o d e t e r m i n e t h e area o f g r e a t e s t porosity.

I t i s achieved by a p p l y i n g a i r p r e s s u r e t o t h e i n s i d e o f t h e f i l t e r e l e -

ment, which i s submerged i n a l i q u i d such as a l c o h o l , which wets t h e f i l t e r media.

The o p e r a t o r r o t a t e s t h e f i l t e r element a t each p r e s s u r e l e v e l and r e c -

o r d s t h e p r e s s u r e a t which t h e f i r s t stream o f bubbles e m i t t e d f r o m t h e f i l t e r element.

The t e s t can be c o n t i n u e d t o measure t h e p r e s s u r e s o f t h e second,

t h i r d , f o u r t h , e t c . l a r g e s t h o l e . By c o n t i n u i n g t o s l o w l y i n c r e a s e t h e p r e s s u r e , a p o i n t i s reached c a l l e d t h e 'open b u b b l e p o i n t ' a t which a i r bubbles appear o v e r t h e e n t i r e s u r f a c e o f t h e f i l t e r element;

t h i s i s a s i m p l i f i e d method o f

measuring t h e mean p o r e s i z e . I t i s claimed t h a t p r e c i s e r e s u l t s a r e n o t o b t a i n a b l e f r o m t h e b u b b l e t e s t ,

which i s u n f o r t u n a t e because such a s i m p l e t e s t would be i n v a l u a b l e .

As i t i s ,

i t s main use i s i n t h e q u a l i t y c o n t r o l o f elements t o ensure t h e r e i s no damage t o t h e media o r a bad s e a l .

13.5.4

Mean F i l t r a t i n g R a t i n g

T h i s i s a measurement o f t h e average s i z e of t h e pores o f t h e f i l t e r media. T h i s i s a v e r y s i g n i f i c a n t r a t i n g , s i n c e i t i s a measure o f t h e p a r t i c l e s i z e above which t h e f i l t e r s t a r t s b e i n g e f f e c t i v e . 'open bubble p o i n t ' method j u s t d e s c r i b e d .

I t can be measured u s i n g t h e

307 13.5.5

Multipass F i l t e r Test

T h i s t e s t , designed t o p r o v i d e a means o f d e s c r i b i n g t h e performance characteristics of a f i l t e r ,

i n v o l v e s t h e co nt i nu ous i n j e c t i o n o f a c o n t r o l l e d conta-

minant i n t o a t e s t system. filter,

As t h e contaminant can be removed o n l y by t h e t e s t

i t w i l l c o n t i n u e t o c i r c u l a t e i n t h e system unless i t i s captured.

The

s e p a r a tio n c a p a b i l i t y o f t h e t e s t f i l t e r i s monitored by a n a l y s i n g upstream and downstream f l u i d samples.

The d i r t h o l d i n g c a p a c i t y i s measured by t h e amount

i n grammes o f t e s t contaminant which can be added t o t h e system b e f o r e a s p e c i f i e d t e r m i n a l p r e s s u r e drop acro ss t h e f i l t e r i s reached. The mathematical r e l a t i o n s h i p whi ch d e s c r i b e s t h e t e s t i s developed from t h e following expression: Number o f p a r t i c l e s Number o f p a r t i c l e s Number o f downstream o f s i z e = o r i g i n a l l y o f s i z e + p a r t i c l e s > xpm > xpm injected o f s i z e >xpm 13.5.6

-

Number o f p a r t i c l e s removed o f s i z e >xpm

Beta R a t i o

The s e p a r a t i o n c h a r a c t e r i s t i c s a r e g i v e n by t h e Beta r a t i o , which i s d e f i n e d as f o l l o w s : o..

-

Number o f upstream p a r t i c l e s l a r g e r than xpm ~~

Number o f downstream p a r t i c l e s l a r g e r than xpm

A Beta r a t i o o f 1 i n d i c a t e s t h a t no p a r t i c l e contamination i s removed.

A

f i g u r e o f l e s s t ha n 1 i s c l e a r l y i m p o s s i b l e unless t h e f i l t e r i s u n l o a d i n g contaminants. For a f i l t e r e x h i b i t i n g a Beta r a t i o g r e a t e r than 1 , t h e downstream concent r a t i o n o f p a r t i c l e s above a g i v e n s i z e w i l l s t a b i l i z e t o g i v e an almost cons t a n t c o n t a m in a t io n l e v e l .

13.5.7

P r a c t i c a l C l a s s i f i c a t i o n o f F i l t e r Performance

Whatever fo r m at i s used by t h e f i l t e r ma nufacturer t o g i v e performance i n f o r mation on h i s p r odu ct ,

t h e degree o f f i l t r a t i o n p r o v i d e d w i l l b a s i c a l l y f a l l

i n t o one o f t h r e e c a t e g o r i e s , depending on t h e degree o f s i l t c o n t r o l .

Typical

data c o r r e s p o n d ing t o these c a t e g o r i e s a r e g i v e n i n Fig.12 though i t should be the m a n u f a c t u r e r ' s r e s p o n s i b i l i t y t o s t a t e i n t o w hich o f t h e t h r e e c l a s s i f i c a tions h i s products f a l l .

A t p r e s e n t t h e r e i s no u n i v e r s a l l y recognised s t idard

c l a s s i f i c a t i o n b u t work be i ng c a r r i e d o u t by v a r i o u s bodies should eventua 1 Y lead t o an i n t e r n a t i o n a l l y agreed d e f i n i t i o n . Two adverse f a c t o r s a f f e c t t h e a c t u a l performance o f f i l t e r s i n s e r v i c e namely p u l s a t i n g f l o w , and t h e sometimes u n c e r t a i n performance o f i n t e r n a l s e a ls and bypass v a l v e s .

308 Fig.12

D e f i n i t i o n o f practical c l a s s i f i c a t i o n categories Category Nomi na 1 Absolute Rating R a t i n g pm

S i l t Control

f to 1

3 to 5

P a r t i a l S i l t Control

3 to 5

10 t o 15

B10-15>75

10 t o 15

25 t o 40

625-40’75

No S i l t C o n t r o l (Chip removal)

13.5.7.1

Beta R a t i o

B3-5

’75

P u l s a t i n g Flow f o r c e s trirough t h e media those f i n e p a r t i c l e s w hich

would o t h e r w i s e lodge among t h e f i b r e s and between l a r g e r p a r t i c l e s a l r e a d y intercepted.

The e f f e c t may be compared w i t h a s i e v e h o l d i n g a m i x t u r e o f

stones, some l a r g e r and some s m a l l e r t ha n t h e openings i n t h e mesh;

when t h e

s i e v e i s s t a t i o n a r y , many o f t h e sma l l stones a r e r e t a i n e d b u t they f a l l through when t h e s i e v e i s shaken.

P u l s a t i n g f l o w t h e r e f o r e increases t h e p r o p o r t i o n o f

s i l t p a r t i c l e s i n t h e system downstream o f t h e f i l t e r , and t h i s i s r e f l e c t e d i n t h e performance curve o f t h e f i l t e r ( F i g . 1 3 ) .

13.5.7.2

Bypassing by i n t e r n a l o r e x t e r n a l v a l v i n g i s a c c e p t a b l e f o r many

systems s i n c e ,

i n a number o f passes, a l l t h e f l u i d e v e n t u a l l y goes through t h e

f i 1t e r . For f i l t e r s o t h e r t ha n t h ose designed f o r permanent bypassing i t would n o r m a l l y be expected t h a t t h e bypass be o p e r a t i v e o n l y when t h e element i s reaching t h e end o f i t s use f u l

life.

T h e r e f o r e t h e e f f e c t s o f premature opening o f t h e

bypass v a l v e o r a f a u l t y i n t e r n a l s e a l need t o be considered.

Since bypassing,

whatever t h e cause, does n o t d i s c r i m i n a t e between f i n e and coarse p a r t i c l e s , t h e e f f e c t i s t o weigh

t h e c o n t a m i n a t i o n p r o f i l e h e a v i l y a t t h e coarse end.

F i g u r e 14 shows t h e e f f e c t o f i n c r e a s i n g percentages o f bypass f l o w s , and i t w i l l be n o t e d t h a t even a t 0.1% bypass t h e maximum p a r t i c l e s i z e has almost doubled.

A t 1% i t has mare t h an t r e b l e d , and a t 10% i t has increased by a f a c -

tor o f five. The p r a c t i c a l s i g n i f i c a n c e o f byp assi n g and t h e need t o make p r o v i s i o n f o r i t depend on t h e l o c a t i o n o f t h e f i l t e r i n t h e system.

For example, on i n l e t

f i l t e r s a bypass v a l v e i s mandatory i n o r d e r t o p r o t e c t t h e pump from c a v i t a t i o n as th e f i l t e r becomes b l ocke d. Pressure f i l t e r s a r e i nt en de d t o p r o t e c t t h e system i f t h e r e i s c a t a s t r o p h i c f a i l u r e o f t h e pump.

I f such a f a i l u r e o ccurs when t h e f i l t e r i s bypassing,

th e n t h e p r o t e c t i o n i s n o n - e x i s t e n t .

There i s o b v i o u s l y a case f o r f i t t i n g non-

bypass p r e s s u r e f i l t e r s where t h e need f o r t h i s t y p e i s e s t a b l i s h e d .

An element

must be f i t t e d t h a t w i l l w i t h s t a n d t h e t o t a l system pressure, w hich increases the c o s t o f t h e f i l t e r .

Cl o gg i ng o f t h e element causes system Performance t o f a l l o f f and a r e l i a b l e i n d i c a t o r i s e s s e n t i a l t o g i v e e a r l y w arning o f t h i s .

309

-

1

106

105

r=-

Q filter

w)'

lnterupted flow performance.

103

102

10

Steady flow

1 102 103

50 70 90 120 m

Particle size

Fig.13

F i l t e r performance d e t e r i o r a t e s under i n t e r m i t t e n t o r p u l s a t i n g flow. The e f f e c t i s more marked w i t h f i n e r p a r t i c l e s which a r e forced through the media.

Fig.14

- micmmetns.

m

Influence o f by-pass leakage on the f i l t e r performance.

Where a bypass i s judged t o be permissible, the system designer must weigh c a r e f u l l y t h e e f f e c t s o f premature opening due t o c o l d s t a r t s and surge flows; the e l i m i n a t i o n o f surges, even those a t low pressure, w i l l reduce the tendency f o r t h e bypass t o open under normal o p e r a t i o n .

A t present t h e r e i s l i t t l e i n f o r m a t i o n a v a i l a b l e on the e x t e n t o f the adverse e f f e c t t h a t p u l s a t i n g f l o w has on f i l t e r performance.

Hopefully, Beta r a t i o s

w i l l e v e n t u a l l y be quoted which do r e l a t e t o c o n d i t i o n s which l i n e f i l t e r s a r e

subjected t o i n a c t u a l p r a c t i c e .

N a t u r a l l y , where stea,dy f l o w e x i s t s r e l a t i n g

more c l o s e l y t o l a b o r a t o r y t e s t c o n d i t i o n s , more accurate p r e d i c t i o n s o f f i l t e r performance can be made.

However, using the l i m i t e d knowledge a t o u r d sposal,

l i k e l y performance l e v e l s f o r each of our categories have been produced i n Fig. 15. On each graph a cumulative count o f both has been shown.

5

and 15 micrometre s i z e pa t i c l e s

The reason f o r using these f i g u r e s i s because o f t h e standard

set by the CETOP RP70H. For ease i n r e l a t i n g t o t h i s document,the range numbers a r e a l s o quoted. a d d i t i o n t o f i x e d f i g u r e s a t t h e two values t h e l i n e s a r e extended t o show expected trends f o r t y p i c a l f i l t e r s .

In

310

PARTIAL SILT REMOVER

SILT REMOVER

CUMIJLATIVECOUNT

CUMULATIVE COUNT SIZE pm

SIZE pm

25,000(18)

64.000(161

16.000(14)

N. B. Range number as per CETOP RP7OH shown in brackets.

N.B. Range number as per CETOP RP7OH shown in brackets.

22

^.

=r o l

105

I

E

h

a

m

i

3- 10‘ e

-a z0

0

i

10’

a 102

10

1

2

5

K)

152025

P*rtlCk

.I20

Fig.15A Assumed mean pore r a t i n g 3 micrometre. Curves show deterioration o f f i l t e r performance w i t h r e s p e c t t o p a r t i c l e s below t h i s s i z e under pulsating flow.

F i g . 1 5 9 Assumed mean p o r e r a t i n g 7 microm e t r e . Curves show d e t e r i o r a t i o n o f f i l t e r performance w i t h r e s p e c t t o p a r t i c l e s below t h i s s i z e under p u l s a t i n g f l o w .

311

CHIP REMOVER CUMULATIVE COUNT SIZE r m

IS

64.000 (16)

N.B. Range number as per CETOP RP70H shown in-brackets

Fig.15C Assumed mean p o r e r a t i n g 15 micrometre. Curves show d e t e r i o r a t i o n o f f i l t e r performance w i t h r e s p e c t t o p a r t i c l e s below t h i s s i z e under p u l s a t i n g f l o w . There i s l i t t l e o r no c o n t r o l o f 5 micrometre p a r t i c l e s .

312 Assuming t h e s e graphs r e l a t e t o performance o f t h e f i l t e r s u p p l i e d by t h e p a r t i c u l a r m a n u fa ct u rer chosen i t i s necessary t o show t h a t t h e actua ment when p l o t t e d s t a y s below t h e l i n e drawn f o r steady o r p u l s a t i n g As an.example,

requirelow.

i f we have a system w i t h a w orking p r e s s u r e o f , say

150 bar

u s i n g components w i t h average d i r t t o l e r a n c e , o u r c l e a n l i n e s s l e v e l graph (Fig.10)

t e l l s us

t h a t t h e d e s i r e d co nt ami n ation l e v e l should be somewhere

between 15/11 and 18/14. R e f e r r i n g now t o Fig.15a and b, we see t h a t under steady f l o w c o n d i t i o n s t h i s c o u l d be achieved by o u r p a r t i a l s i l t c o n t r o l f i l t e r , b u t i f p u l s a t i n g f l o w i s p r e s e n t a s i l t removal f i l t e r i s r e q u i r e d . I t can be assumed t h a t most p ressu res and r e t u r n l i n e f i l t e r s a r e s u b j e c t t o pulsating flow conditions. t i o n on f i l t e r l o c a t i o n .

The reasons a r e g i v e n i n g r e a t e r d e t a i l i n t h e secOn t h e o t h e r hand, o f f - l i n e systems have t h e b e n e f i t

of a steady f l o w r a t e c o n d i t i o n .

I t must be emphasised t h a t t h e onus i s v e r y much on t h e f i l t e r manufacturer t o i d e n t i f y t h e l i k e l y performance o f h i s p a r t i c u l a r media. 13.5.8

F i l t e r Sizing

I t i s c u r r e n t p r a c t i c e f o r man uf act ure rs t o s t a t e a f l o w r a t i n g a t a s p e c i f i c c l e a n p r e s s u r e drop (see F i g . 1 6).

W hi l e t h i s i s a g u i d e t o c a p a c i t y ,

i t may

w e l l be necessary f o r t h e system de si gn er t o p r o v i d e a d d i t i o n a l d i r t c a p a c i t y so

07 07

c-

I

Flow (I/mln) Oil vlscoslty 30 cSt. Element pressuredrop Is directly proportional to fluld viscosity.

Fig.16

T y p i c a l p r e s s u r e d rop s f o r c l e a n s i l t c o n t r o l and c h i p c o n t r o l elements o f s i m i l a r s i z e .

313 as t o ensure t h a t t h e end u s e r o b t a i n s an a cceptable element l i f e .

A l l too

o f t e n f i l t e r s s i z e d p u r e l y on f l o w r a t e have a s h o r t element l i f e .

I n choosing

l a r g e r f i l t e r c a p a c i t i e s g r e a t e r i n i t i a l expense may be i n c u r r e d , b u t t h i s i s almost c e r t a i n t o be re cove red i n reduced r u n n i n g c o s t s ,

i.e.

fewer element

changes, reduced l a b o u r c o s t s and reduced downtime. Co r r e c t f i l t e r s i z i n g n e c e s s i t a t e s r e l a t i n g t h e d i r t e n t e r i n g t h e f i l t e r t o t h e e f f e c t i v e element a rea and t h e maximum a l l o w a b l e p r e s s u r e drop.

The r e l a t -

i o n s h i p o f a r e a t o p r e s s u r e drop i s n o t simple, however, and f i l t e r i n l e t d i r t l e v e l s a r e r a r e l y known. There i s a l a b o r a t o r y t e s t ( t h e 'co mpa rat ive l i f e ' or ' d i r t c a p a c i t y ' t e s t ) which i s designed t o compare t h e d i r t h o l d i n g c a p a c i t i e s o f h y d r a u l i c f i l t e r s . An a r t i f i c i a l contaminant i s added a t a c o n s t a n t r a t e t o a c o n t i n u o u s l y r e c i r c l a t i n g o i l system and t h e r e s u l t a n t i n crea se i n d i f f e r e n t i a l pressure i s p l o t t e d a g a i n s t t h e w e i g h t o f contaminant added, as shown i n Fig.17.

The r e s u l t i n g

c u r v e has a c h a r a c t e r i s t i c f o r m which i s c o n s t a n t f o r a g i v e n f i l t e r media.

"I

[ 6 7

I

0 Total A.C. flne test dust added (grams). Fig.17

Typical d i r t capacity curve f o r hydraul i c f i l t e r element.

Fig.18

25

I

I

50 75 Percent dirt capacity.

1 100

Comparison o f . d i r t c a p a c i t i e s and d i f f e r e n t i a l pressures f o r two d i f f e r e n t f i l t e r s .

314 I n t h e f i r s t stages o f d i r t a d d i t i o n i t can be seen t h a t t h e p r e s s u r e drop increases s l o w l y , whereas l a t e r t h e p r e s s u r e d r o p increases v e r y r a p i d l y .

This

i l l u s t r a t e s why v e r y l i t t l e i s gained i n terms o f element l i f e by a l l o w i n g t h e f i l t e r t o operate a t a h i g h d i f f e r e n t i a l pressure a f t e r the 'knee' o f the curve The c u r v e also i l l u s t r a t e s t h e i r r e l e v a n c y o f a system

has been passed.

designer c o n c e r n i n g h i m s e l f s o l e l y w i t h c l e a n p r e s s u r e drops;

t h e more s i g n i f -

icant f a c t o r i s the d i f f e r e n t i a l pressure across the f i l t e r a f t e r a s p e c i f i e d amount o f d i r t has been added. Except w i t h non-bypass f i l t e r s , t h e maximum p r e s s u r e drop across t h e element i s u s u a l l y determined by t h e bypass v a l v e s e t t i n g .

The system d e s i g n e r must

also check t h a t t h e system performance i s m a i n t a i n e d w i t h t h e p r e s s u r e drop a t i t s maximum value.

I f f i l t e r s a r e o v e r s i z e d , t h e bypass may pass a s u b s t a n t i a l

flow without indicating. I t i s commonly thought t h a t t o o b t a i n l o n g e r s e r v i c e l i f e from a g i v e n f i l t e r

element w i t h o u t s a c r i f i c i n g f i l t e r e f f i c i e n c y , media area i n t o t h e f i l t e r envelope.

i t i s o n l y necessary t o pack more

T h i s i s n o t so s i n c e an optimum area

e x i s t s f o r a g i v e n envelope and i t i s d e t r i m e n t a l t o exceed t h i s a r e a . The comparative l i f e t e s t p r e v i o u s l y d e s c r i b e d i s used t o compare t h e d i r t h o l d i n g c a p a c i t y o f d i f f e r e n t f i l t e r elements, and i n Fig.18 we compare t h e d i r t c a p a c i t y o f two f i l t e r s o f i d e n t i c a l envelope s i z e .

The f i l t e r A has a lower

c l e a n p r e s s u r e drop than f i l t e r B because A has more area.

However, t h e optimum

area has been exceeded and t h e r e f o r e f i l t e r A would have a s h o r t e r l i f e than B f o r a given pressure d i f f e r e n t i a l . How c l o s e p a c k i n g reduces t h e e f f e c t i v e area i s i l l u s t r a t e d i n Fig.19 where t h e p l e a t s c l o s e up under p r e s s u r e and t h e small angles between them c l o g rapidly.

F a t i g u e f a i l u r e s can a l s o o c c u r when p r e s s u r e i s a p p l i e d .

As d a t a on

t h e r e s u l t s o f d i r t c a p a c i t y t e s t s a r e n o t r e a d i l y a v a i l a b l e , we must r e v e r t t o t h e m a n u f a c t u r e r ' s f l o w r a t i n g a t m s p e c i f i c c l e a n p r e s s u r e drop and use t h i s a s

a basis f o r assessing d i r t capacity. Fluid flow +

Fluid flow Effective areas

t

t under high flow

Pleats close up under pressure and small

Wider spacing gives smaller but more

angles between them clog rapidly, reducing the effectlve area

effective filter area

Fig.19A I n c r e a s i n g f i l t e r area w i t h i n a g i v e n package may a c t u a l l y reduce t h e d i r t h o l d i n g capac i t y .

medium

High stress under high flow conditions

Fig.198

t

J

Flow fatigue failure

Changes i n f l o w and p r e s s u r e drop cause t h e s i d e s o f element c o r r u g a t i o n s t o f l e x and t h e root t o stretch, giving rise t o f a t i g u e stresses.

315 Recapping, o u r o b j e c t i v e i s t o a c h i e v e a ' d i r t i n equals d i r t o u t ' c o n d i t i o n . The d e s i r e d d i r t c a p a c i t y o f t h e f i l t r a t i o n system w i l l t h e r e f o r e depend t o a l a r g e e x t e n t on o u r a b i l i t y t o c o n t r o l t h e d i r t i n p u t .

The d i r t i n p u t i s a

product o f i n b u i l t con t a mi na t i o n and i n gre ssed c o n t a m i n a t i o n which, produces system generated con t a mi na t i o n.

i n turn,

L e t us f i r s t examine each source i n

d e t a i l and t h e f a c t o r s c o n t r o l l i n g i t s i n p u t ( F i g . 2 0 ) . Contamination source

Dirt i n p u t

=

I n b u i l t : i n components p ip es, m a n i f o l d s , e t c .

Controller Good f l u s h i n g procedures, system n o t o pe rat ed on load u n t i l acceptable cont a m i n a t i o n l e v e l obtained.

plus P r e sen t i n i n i t i a l charge o f f l u i d

Integrity o f supplier. F l u i d stored under c o r r e c t c o n d i t i o n s ( e x c l u s i o n o f F1 u i d f i 1 t e r e d d i r t , condensation,etc.) during f i l l i n g .

plus lngressed t h rou gh a i r breather

An e f f e c t i v e a i r b r e a t h e r w i t h r a t i n g co mpatible w i t h degree o f f l u i d f i l t r a tion.

plus ln g re ssed d u r i n g f l u i d r e p l e ni sh men t

S u i t a b l e f i l l i n g p o i n t s which ensure some f i l t r a t i o n o f f l u i d b e f o r e e n t e r i n g r e s e r v o i r . T h i s t a s k undertaken by r e s p o n s i b l e personnel. Design should mi ni mi se t h e e f f e c t s .

plus lngressed d u r i n g maintenance plus lngressed t h rou gh c y l i n d e r ro d s e a l s

E f f e c t i v e wiper seals o r , i f airborne con t a mination, rods p r o t e c t e d by s u i t a b l e gaiters

.

plus F u r t h e r generated co nt am i n a t i o n produced as a r e s u l t o f t h e above and the s e v e r i t y o f the duty cycle.

F ig . 2 0

C o r r e c t f l u i d s e l e c t i o n and p r o p e r t i e s ( v i s c o s i t y and a d d i t i v e s ) maintained. Good system design m i n i m i s i n g e f f e c t s o f co nt amination present on system compon en t s.

The p r a c t i c a l st ep s wh i ch c o n t r o l C ontamination i n h y d r a u l i c systems.

Based on Fig.20 we w i l l now grade t h e c l e a n l i n e s s l e v e l o f a system between

1 and 7.

An example o f a grade 1 c l e a n system would be a c l e a n workshop w i t h

e f f e c t i v e c o n t r o l o v e r a l l con t a mi na t i o n i n gress.

A grade

7 d i r t y system would

be, say, a f o u n d ry w i t h l i t t l e o r no c o n t r o l over c o n t a m i n a t i o n i n g r e s s and a system o p e r a t i n g se vera l exposed c y l i n d e r s .

316 Figure 21 w i l l a s s i s t i n making a numerical assessment between these values based on t h e environment and t h e degree o f c o n t r o l over contamination.

Environmental Conditions Degree of Control

Good

Average

Bad

3

6

7

L i t t l e or no c o n t r o l over contamination ingression (many exposed c y l inders)

2

4

5

Some c o n t r o l over contamination ingression (few c y l inders).

1

2

3

Good c o n t r o l over contamination ingression ( g a i t e r e d c y l inders).

Fig.21

Assessment and c l a s s i f i c a t i o n o f system c l e a n l i n e s s l e v e l i n t o 7 grades.

We must now r e l a t e t h i s t o t h e e f f e c t i v e element area and the maximum a l l o w a b l e pressure drop.

The r e l a t i o n s h i p between area and pressure drop i s n o t

simple, but by using very broad approximations o f these values and assuming t h a t the manufacturers f l o w r a t i n g a t a s p e c i f i c c l e a n pressure drop i s a good guide t o d i r c t capacity,

the f o l l o w i n g s e l e c t i o n guides can be used:

To use this graph:

10

1. Decide on maximum acceptable pressure 2. Assess cleanliness grade in SyStem. from drop, this will depend on system requireFig. 22. Draw vertical line through inter. section of pressure drop and cleanliness ments or bypass pressure. Draw horizontal line through this value. grade line.

9

t

30

Bt

3. Read off multiplication factor where

u 7

-

vertical llne crosses horizontal axis. Multiply this factor by the actual flow rate at chosen pressure line location. Now select a filter element to handle this revlsed flow rate at a pressure drop of 1 bar (According to manufacturers clean rating).

2 3

Actual flow x multiplication factor = recommended filter capacity.

ia 6 a 5

?

!I4

i

2 1

1

3

2

4

Multipiiulion faclor.

Fig.22

Pressure l i n e f i l t e r s e l e c t i o n guide.

317

3'0

4\ \ \ \

Tousethisgraph: 1. Decide on maximumacceptable pressure 2. Assess cleanliness grade in system, from Fig. 22. Draw vertical line through interdrop, this will depend on system require ments or bypass pressure. Draw section of pressure drop and cleanliness horizontal line through this value. grade lines.

3. Read off multiplication factorwhere vertical line crosses horizontal axis. Multiply this factor by theactual flow rateat chosen return line location. Now select a filter element to handle this revised flow rate at a pressure drop of 0.3 bar(According to manufacturers clean rating). Actual flow x multiplication factor

= recommended filter capacity.

2

3

4

Mulliplhlion faclor.

Fig.23

Return l i n e f i l t e r s e l e c t i o n guide.

To use this graph: 1. Decideon maximumacceptable pressure 2. Assess cleanlinessgrade in system, from Fig. 22. Draw vertical line through interdrop, this will depend on system require ments or bypass pressure. Draw section of pressure drop and cleanliness horizontal line through this value. grade lines.

3. Read off multiplication lactor where vertical line crosses horizontal axis. Multiply this factor by theactual offline pump flow rate. Now select a filter element to handle this revised flow rate at a pressure drop of 0.3 bar(acc0rding to manufacturers clean rating). Note:Toachieve reasonable lifea minimum multiplication factor of 2 is recommended. Actual flow x multiplication factor

= recommendedfiltercapacity.

2 -

6

4

8

Multlpliutionfactor.

Fig.24

O f f l i n e f i l t e r s e l e c t i o n guide.

318 13.6

LOCATING THE FILTER

13.6.1

Pump I n l e t F i l t r a t i o n

C o r r e c t f i l l i n g o f t h e pump i s v i t a l i f s a t i s f a c t o r y o p e r a t i o n o f t h e hydraul i c system i s t o be achieved.

A l l t o o o f t e n i n s u f f i c i e n t a t t e n t i o n i s p a i d by

t h e d e s ig n e r t o t h e t o t a l pump i n l e t c o n f i g u r a t i o n and, as a r e s u l t , c a v i t a t i o n remains a m a jo r cause o f pump f a i l u r e . For t h i s reason i t i s w o r t h l o o k i n g once more a t t h e b a s i c p r i n c i p l e s o f pump f i l l i n g .

By f a r t h e most common method i s t o use atmospheric pressure

a c t i n g on t h e f l u i d s u r f a c e o f t h e r e s e r v o i r t o f o r c e f l u i d i n t o t h e pump i n l e t chambers.

For convenience, t h e pump i s o f t e n s i t e d above t h e f l u i d l e v e l

(Fig.25). From b a s i c h y d r a u l i c p r i n c i p l e s we know t h a t f o r f l o w t o t a k e p l a c e we must have a p r e s s u r e d i f f e r e n c e .

W i t h t h i s arrangement we r e l y on t h e mechanical

a c t i o n o f t h e pump t o c r e a t e a de pre ssi on a t i t s i n l e t .

I t i s usual f o r manu-

f a c t u r e r s t o q u o t e a maximum a l l o w a b l e d ep ression a t t h e pump i n l e t , w hich i s l i k e l y t o be o f t h e o r d e r o f 0.17 b a r .

Thus w i t h normal p r e s s u r e drops accoun-

t e d f o r , o n l y a v e r y smal l p r e s s u r e d rop can be t o l e r a t e d across t h e f i l t e r . For t h i s reason t h e s i z e and c o s t o f i n l e t f i l t e r s i s o f t e n g r e a t e r than, say, f i l t e r s i n the return l i n e .

Furthermore,

such low p r e s s u r e drops make s i l t

removal v i r t u a l l y i mpo ssi bl e . F i r e - r e s i s t a n t f l u i d s a r e v e r y s e n s i t i v e t o s u c t i o n pressures.

They have

h i g h e r s p e c i f i c g r a v i t i e s t ha n m i n e r a l o i l s , p a r t i c u l a r l y t h e s y n t h e t i c types. T h i s in c r e a s e s t h e p r e s s u r e drop t o t h e pump and a t t h e same t i m e demands a h i g h e r p r e s s u r e t o a c c e l e r a t e t h e f l u i d i n t o t h e pump.

Water g l y c o l s and w ater-

i n - o i l emulsions have a h i g h vapour p r e s s u r e and t h e pump i n l e t depression s h o u ld be l i m i t e d wherever p o s s i b l e t o h a l f t h e v a l u e f o r m i n e r a l o i l s , even when t e m p e r a t u r e i s l i m i t e d t o 5OoC.

With o r w i t h o u t i n l e t l i n e f i l t e r s i t i s

u s u a l l y e s s e n t i a l t o p r o v i d e a p o s i t i v e head a t t h e pump i n l e t when u s i n g these fluids

(Fig.26).

'There must beadepression here to'lift'theoil.

Atmospheric pressure

L l+

Atmospheric pressure

\

Pump is charged at positive pressure.

Fig.25

Ne g a t iv e head t a nk.

F i g.26

P o s i t i v e head tank.

319 I r r e s p e c t i v e o f t h e t y p e o f f l u i d , a p o s i t i v e head w i l l improve i n l e t c o n d i t i o n s because i t increases t h e f o r c e a v a i l a b l e t o c r e a t e t h e r e q u i r e d f l o w . For b o t h n e g a t i v e and p o s i t i v e head i n l e t arrangements, when s i z i n g i n l e t f i l t e r s we must ensure t h a t any f i l t e r (housing and element) and a s s o c i a t e d

1 ) pass t h e f u l l pump volume w i t h i n t h e p e r m i t t e d i n l e t depr-

pipework should:

ession f o r t h a t pump, and 2 ) p e r m i t a bypass f l o w t h a t i s s t i l l w i t h i n t h a t l i m i t when t h e f i l t e r element i s blocked.

( T h i s requirement o f t e n n e c e s s i t a t e s

t h e o p e r a t i o n o f t h e bypass f e a t u r e a t pressures as low as 0.085 b a r , a l e v e l a t which o p e r a t i o n i s seldom c o n s i s t e n t . )

A l l c a l c u l a t i o n s should t a k e i n t o account t h e e f f e c t o f h i g h e r v i s c o s i t y f l u i d , e.g.

a t c o l d s t a r t s , o t h e r w i s e c a v i t a t i o n w i l l occur.

The usual micrometre r a t i n g f o r i n l e t f i l t e r s i s 75 o r 150. a r e a v a i l a b l e i n c o r p o r a t i n g elements down t o 10 micrometre.

However, u n i t s

T h i s means l a r g e

housings and p r o b a b l e o v e r s i z i n g o f t h e i n l e t c o n f i g u r a t i o n . The 75 and 150 micrometre mesh elements w i l l remove most o f t h e p a r t i c l e s above t h e i r r a t i n g b u t a r e r e l a t i v e l y i n e f f e c t i v e i n removing a n y t h i n g s m a l l e r . The a d d i t i o n o f magnets w i l l remove some o f t h e f i n e m e t a l i i c p a r t i c l e s b u t t h e l o c a t i o n o f t h e magnets w i t h i n t h e f i l t e r must be such t h a t under no circumstances can accumulated contaminant break away, thereby passing a conglomerate i n t o t h e pump. An advantage o f t e n claimed f o r i n l e t f i l t e r s i s ease o f s e r v i c i n g . However, i n c o r r e c t re-assembly o f access covers can r e s u l t i n a i r i n g r e s s , which o f t e n goes undetected and can be harmful t o t h e system as d i r t . ,

I n l e t f i l t e r s a r e g e n e r a l l y used i n systems where maintenance procedures do

not prevent q u a n t i t i e s o f large p a r t i c l e s entering the reservoir, f o r instance d u r i n g topping-up.

P r o v i d i n g bypassing i s n o t o c c u r r i n g ,

from t h i s t y p e o f i n g r e s s i o n .

they p r o t e c t t h e pump

However, good r e s e r v o i r design t h a t i n c udes a

mesh screen o r b a f f l e can p r o v i d e an e q u a l l y a c c e p t a b l e c o n t a m i n a t i o n

eve1 a t

t h e pump i n l e t .

heir

Under these circumstances, s t r a i n e r s and f i l t e r s and

a s s o c i a t e d f i t t i n g s can be o m i t t e d f r o m t h e i n l e t l i n e , thus improving pump f i l l i n g conditions. I t i s an encouraging s i g n t h a t more d e s i g n e r s a r e p r o v i d i n g s e p a r a t e f i l l i n g

arrangements, u s u a l l y through some f o r m o f coarse f i l t e r t o reduce t h e need f o r l a s t - c h a n c e pump i n l e t p r o t e c t i o n .

I f t h e f i l l i n g requirements o f t h e pump a r e c r i t i c a l and supercharging i s necessary, as i s t h e case q u i t e o f t e n w i t h l a r g e v a r i a b l e - d i s p l a c e m e n t p i s t o n pumps,

i t i s common t o s i t e a f i l t e r between t h e supercharger and main pump.

For such a p p l i c a t i o n s ,

t h e same g u i d e l i n e s g i v e n f o r t h e s i z i n g o f p r e s s u r e o r

r e t u r n l i n e f i l t e r s can be used.

However, depressions can o c c u r when a v a r i a b l e

pump moves f r o m z e r o t o f u l l displacement, and t h e l i k e l y e f f e c t on f i l t e r p e r formance must be c a r e f u l l y considered.

320

13.6.2

Pressure L i n e F i l t r a t i o n

L e t us s t a r t by d i s c u s s i n g t h e l o c a t i o n o f t h e p r e s s u r e l i n e f i l t e r i n r e l a t i o n t o pump and r e l i e f v a l v e . F i g u r e 27 shows t h e p r e s s u r e f i l t e r l o c a t e d downstream o f t h e r e l i e f v a l v e . For t h e non-bypass t y p e t h e arrangement shown i n Fig.28

i s mandatory.

The

a c t u a l f l o w seen by t h e f i l t e r d u r i n g t h e o p e r a t i n g c y c l e depends on t h e system demand and d u r i n g o f f - l o a d

p e r i o d s t h e r e i s leakage f l o w o n l y i f b l o c k - c e n t r e

d i r e c t i o n a l v a l v e s a r e employed.

Naturally,

i f o f f - l o a d i n g i s achieved through

op e n - c e n t r e d i r e c t i o n a l v a l v e s t h e f i l t e r w i l l see f u l l pump o u t p u t f o r t h i s period.

Pressure filter

Q

Bypass

‘c

Pressure line filler

Rellef

Relief

.-+--A

Tank

Tank

Y ie_l

Strainer

Strainer

Tank

Fig.27

Pump

Tank

P r e s s u re l i n e f i l t r a t i o n w i t h bypass f i l t e r .

Fig.28

Pressure l i n e f i l t r a t i o n w i t h non-bypass f i l t e r .

To in c r e a s e t h e f l o w a cross t h e f i l t e r i t has become common p r a c t i c e t o l o c a t e t h e p r e s s u r e f i 1 t e r between pump and r e 1 i e f v a l v e (Fig.29).

The advo-

c a t e s o f t h i s arrangement p o i n t t o t h e f a c t t h a t t h e r e l i e f v a l v e i s p r o t e c t e d from pump generated d i r t .

A v a l i d p o i n t , however, i s t h a t t h i s generated d i r t

i s caused by pump wear which,

i n turn,

i s d i r e c t l y p r o p o r t i o n a l t o t h e conta-

m i n a t i o n l e v e l o f t h e f l u i d e n t e r i n g t h e pump i n l e t .

The pump i s o f t e n a c o s t l y

component and we should t h e r e f o r e d i r e c t more a t t e n t i o n t o reducing t h e contam i n a t i o n e n t e r i n g t h e pump t o a l e v e l w h i c h w i l l m i n i m i z e wear.

For t h i s

arrangement, a bypass i s mandatory and t h e r e must be an assurance from t h e f i l t e r m a n u fa c t ure r t h a t any f i l t e r m a l f u n c t i o n w i l l n o t r e s u l t i n excessive

321 pressure a t t h e pump o u t l e t .

Relief

Tank

,$!, Strainer

Tank

Fig.29

L o c a t i n g p ressu re f i l t e r b e f o r e r e l i e f v a l v e g i v e s c o n s t a n t f l o w t hro ug h f i l t e r .

Where v a r i a b l e - d i s p l a c e m e n t pumps a r e employed, c a r e f u l a n a l y s i s o f t h e f l o w seen by t h e f i l t e r must be made. pump which i s compensating ( i . e .

Take t h e c l a s s i c case o f a p r e s s u r e compensated low d i spl a cement a t maximum system o p e r a t i n g

pressure), where a low f l o w r a t e e x i s t s a l t h o u g h t h e pump i s s t i l l o p e r a t i n g a t pressure.

With a p r e s s u r e l i n e f i l t e r d i r t removal i s l i m i t e d .

Even when f l o w

demands a r e made t he se a r e 1 i m i t e d t o c y l i n d e r displacement. As i l l u s t r a t e d by Fig.15,

t h e f i l t e r e f f i c i e n c y depends upon t h e t y p e o f f l o w

i t sees, a l t h o u g h c u r r e n t l y no st an da rd t e s t e x i s t s f o r e v a l u a t i n g f i l t e r s when

they a r e s u b j e c t e d t o a b r u p t f l o w and p r e s s u r e changes and mechanical v i b r a t i o n . P r a c t i c e has shown t h e r e i s a r e d u c t i o n i n e f f i c i e n c y b u t i t s e x t e n t depends n o t o n l y on t h e q u a l i t y o f t h e a c t u a l f i l t e r media b u t a l s o on how w e l l t h i s media i s supported.

We must hope t h a t st a nd ard t e s t parameters can be agreed and a

Beta r a t i o quoted wh i ch r e l a t e s more c l o s e l y t o c o n d i t i o n s seen by a f i l t e r i n rea l h y d r a u l i c systems.

I n t h e f u t u r e i t w i l l a l s o be e s s e n t i a l t h a t t h e system

designer has i n f o r m a t i o n on t h e performance o f i n t e g r a l bypass v a l v e s under system o p e r a t i n g c o n d i t i o n s . I n a d d i t i o n t o mechanical v i b r a t i o n and a b r u p t f l o w and p r e s s u r e changes when valves a r e o p e r a te d, a p r e s s u r e l i n e f i l t e r i s a l s o s u b j e c t t o pump p u l s a t i o n s .

322 These e f f e c t s were demonstrated r e c e n t l y on a h y d r a u l i c component t e s t r i g where t h e s o - c a l l e d 10 micrometre nominal f i l t e r proved almost t o t a l l y i n e f f e c t i v e i n removing sub-5 micrometre contaminant. Because p r e s s u r e f i l t e r s have t o w i t h s t a n d t h e f u l l system p r e s s u r e w i t h adequate margin o f s a f e t y , t h e r e i s a tendency t o make these s m a l l , t h u s l i m i t i n t h e i r d i r t holding capacity.

It i s usual f o r manufacturers t o q u o t e a r a t e d

f l o w a t around 1 bar f o r bypass t y p e s and a t a s l i g h t l y h i g h e r p r e s s u r e f o r nonbypass t y p e s which i n c o r p o r a t e h i g h p r e s s u r e d i f f e r e n t i a l c a p a b i l i t y elements. I n summary,

i t can b e s a i d t h a t p r e s s u r e f i l t e r s a r e s u b j e c t e d t o c o n d i t i o n s

f a r removed from t h a t found i n a l a b o r a t o r y f i l t e r t e s t r i g , and because o f t h i s the designer's task i n assessing t h e r e s u l t a n t contamination i s extremely d i f f i cult.

The l e v e l s g i v e n i n F i g . 1 5 a r e based on f i e l d e x p e r i e n c e u s i n g good

q u a l i t y elements. Pressure f i l t e r s may w e l l be used i n c e r t a i n a p p l i c a t i o n s t o p r o t e c t t h e system should c a t a s t r o p h i c f a i l u r e s o f t h e pump occur, o r t o p r o v i d e s p e c i a l p r o t e c t i o n t o a s i n g l e u n i t o r group o f u n i t s .

An example o f t h e l a t t e r would

be a s e r v o v a l v e , where f a i l u r e m i g h t be e x t r e m e l y expensive, though i t should be noted t h a t a p r e s s u r e f i l t e r does n o t p r o t e c t a s e r v o v a l v e f r o m d i r t i n gressed t h r o u g h c y l i n d e r r o d s e a l s .

I n such cases a non-bypass f i l t e r should be

considered w i t h t h e assurance t h a t t o t a l element c o l l a p s e cannot o c c u r .

I f by-

pass types a r e employed, some means o f i n d i c a t i o n should be p r o v i d e d t o g i v e warning o f a p a r t i a l l y b l o c k e d element. From a maintenance p o i n t o f view, changing elements i n v o l v e s s t o p p i n g t h e system u n l e s s e x t e r n a l bypass v a l v i n g i s p r o v i d e d .

This operation o f t e n allows

f r e e a i r i n t o t h e system which must be c l e a r e d b e f o r e s a t i s f a c t o r y machine p e r formance can be o b t a i n e d .

13.6.3

Return L i n e F i l t r a t i o n

The usual r e t u r n l i n e f i l t r a t i o n arrangement shown i n F i g . 3 0 has a l l r e t u r n l i n e s passing through t h e f i l t e r .

D r a i n l i n e s f r o m pumps, motors, and c e r t a i n

v a l v e s should n o t be s u b j e c t e d t o p r e s s u r e surges emanating f r o m t h e system r e t u r n l i n e s and should r e t u r n s e p a r a t e l y t o tank. Where t h e r e a r e h i g h surges (e.g.

due t o u n c o n t r o l l e d decompression o r t h e

r a p i d a c c e l e r a t i o n o f t h e f l u i d column i n t h e r e l i e f v a l v e t a n k l i n e when t h i s v a l v e o p e r a t e s ) i t may be u n d e s i r a b l e t o pass t h e s e t h r o u g h t h e f i l t e r .

To

p r e v e n t c o l l a p s e o f t h e element due t o h i g h - v i s c o s i t y o i l , e.g. a t a c o l d s t a r t , o r when t h e element i s loaded w i t h d i r t , an i n t e r n a l bypass should be provided. When bypassing occurs under m i n i m u m f l o w and s u r g e c o n d i t i o n s t h e c i r c u l a t e d f l u i d should n o t be contaminated by d i r t a l r e a d y r e t a i n e d i n t h e f i l t e r .

The

t y p e o f surge experienced i n most r e t u r n l i n e s w i l l reduce t h e f i l t e r e f f i c i e n c y

323 and i n th e absence o f p r e c i s e data t h e f i l t e r should be s e l e c t e d u s i n g t h e p u l s a t i n g f l o w c o n d i t i o n f rom Fig.15. F u l l f l o w r e t u r n f i l t r a t i o n sho ul d be o f s u f f i c i e n t c a p a c i t y t o handle the maximum r e t u r n f l o w ( i n c l u d i n g any i n excess o f pump f l o w where, f o r example, unbalanced c y l i n d e r s a r e used) w i t h o u t t h e bypass opening.

Linear actuator

I I

I

valve

Directional valve

$0

Return line

Bypass

Strainer

filter

LLI Tank

Tank

Fig.30

B a s ic arrangement o f r e t u r n 1 i n e f i l t r a t i o n .

Careful assessment o f t h e f l o w a cross t h e f i l t e r i s necessary.

The comments

r e l a t i n g t o t h e use o f v a r i a b l e di sp l ace men t pumps w i t h a pressure f i l t e r again ap p ly , s i n c e w i t h p r e s s u r e compensated pumps the f i l t e r sees o n l y t h e volume d i s p l a c e d by t h e c y l i n d e r . For c e r t a i n systems p a r t i a l r e t u r n l i n e f i l t r a t i o n i s acceptable w i t h t h e bypass ( i n t e r n a l o r e x t e r n a l ) always p a s s i n g a percentage o f the flow .

With

t h i s arrangement much depends on t h e con t i n uo us r a t e o f f l o w which we can g e t acr o s s t h e f i l t e r .

S a t i s f a c t o r y performance can sometimes be achieved w i t h as

l i t t l e as 1 0 % p a ssi n g a cross t he a c t u a l element. To summarize, a r e t u r n l i n e f i l t e r does n o t p r o t e c t the system f r o m e n v i r o n -

mental d i r t e n t e r i n g t h e r e s e r v o i r v i a b r e a t h e r s o r d u r i n g topping-up.

However,

i f we s t a r t o f f w i t h a c l e a n r e s e r v o i r and t a k e p r e c a u t i o n s t o p r e v e n t the i ng r e s s o f e n v ir on men t a l c o n t a m i n a t i o n , then experience has shown t h a t e f f e c t i v e f i l t r a t i o n can be a chi e ved e con omi ca l l y w i t h r e t u r n l i n e f i l t e r s .

324 13.6.4

Off-Line F i t r a t i o n

I t has been s t a t e d t h a t t h e e f f e c t i v e n e s s o f f i l t e r s s ted i n b o t h pressure and r e t u r n l i n e s i s reduced by shocks, surges, p u l s a t i o n s

vibrations, etc. t o

an e x t e n t wh ic h depends on t h e t y p e o f media and how w e l l i t i s supported. Steady f l o w r e l a t i v e l y f r e e o f p r e s s u r e f l u c t u a t i o n s p r o v i d e s optimum f i l t e r performance.

The s i m p l e s t way o f a c h i e v i n g t h i s i s t o r e m v e t h e f i l t e r from

t h e main system and p l a c e i t i n an i n de pe nd ently powered c i r c u l a t i n g u n i t where i t s performance i s more p r e d i c t a b l e . Though connected t o t h e system r e s e r v o i r , a t y p i c a l arrangement i s shown i n Fig.31.

The l i k e l y c o n t a m i n a t i o n l e v e l f rom o u r t h r e e f i l t e r r a t i n g s under

steady f l o w c o n d i t i o n s i s g i v e n i n F i g . 1 5 , . a n d a o u r a b i l i t y t o p r o v i d e t h e c o r r e c t f l o w r a t e w i l l d et ermi n e whether o r n o t these l e v e l s can be achieved and ma i n t a ined

.

Fig.31

Layout o f o f f - l i n e f i l t r a t i o n system. Valves (a) and (b) can be p o s i t i o n e d so t h a t t h e u n i t w i l l p r o v i d e a f i l t e r e d f i l l o r di sch arg e.

Assuming reasonable standards o f e n g i n e e r i n g i n t h e design and b u i l d o f t h e system, t h e most r e l e v a n t f a c t o r s i n s e l e c t i n g t h e f l o w r a t e through t h e o f f l i n e f i l t r a t i o n a r e l i k e l y t o be environment and t a n k s i z e .

From f i e l d exper-

i e n c e t o d a t e t h e g u i d e l i n e s g i v e n i n F i g. 32 can be a p p l i e d and t h e f l o w r a t e

325 d e r i v e d can b e used t o d et ermi n e t h e s i z e o f t h e f i l t e r by t h e procedure p r e v iously outlined.

Flow r a t e ( l / m i n ) as percentage o f tank capacity ( I i t r e s )

Environment

5%

Good Average

10%

Bad

20%

Fig.32

Suggested f l o w r a t e s f o r o f f - 1 i n e f i l t r a t i o n .

With o f f - l i n e f i l t r a t i o n t h e d e s i g n e r ' s t a s k i s s i m p l i f i e d because he i s n o t governed by t h e f l o w and p r e s s u r e c h a r a c t e r i s t i c s o f t h e main system.

It

enables h im t o s e l e c t t h e b e s t f i l t e r and t h e b e s t f l o w through i t and then s e l e c t t h e s i z e t o a c h i e v e t h e d e s i r e d f re qu ency o f maintenance. Should t h e d e s i r e d c o n t a m i n a t i o n l e v e l n o t be achieved,

then c o r r e c t i o n s a r e

e a s i l y made t o t h e f l o w r a t e o r t y p e o f f i l t e r w i t h o u t i n any way a f f e c t i n g t h e de s ig n o f t h e main system. Furthermore, t h e o f f - l i n e

i n s t a l l a t i o n can be r u n

p r i o r t o s t a r t i n g t h e main system i n o r d e r t o c l e a n t h e o i l i n t h e t a n k and reduce t h e c o n t a m i n a t i o n l e v e l t h e pump w i l l see a t s t a r t - u p .

By t h e a d d i t i o n

o f s i m p l e v a l v e gear i t can be employed t o f i l t e r t h e i n i t i a l charge o f f l u i d and any used s u b seq ue nt l y i n topping-up.

I d e a l l y , i t should be l e f t running

c o n t i n u o u s l y t o p r o v i d e a complete t a n k o f c l e a n f l u i d ready f o r every s t a r t - u p . Unlike l i n e f i l t e r s ,

the o f f - l i n e

i n s t a l l a t i o n w i l l c o n t i n u e t o c l e a n up t h e

f l u i d when v a r i a b l e d e l i v e r y pumps a r e r u n n i n g a t minimum displacement. Being independent o f t h e main system o f f - l i n e f i l t e r s can be p l a c e d where they a r e most c o nve ni e nt f o r s e r v i c i n g . main system i s n o t a f f e c t e d ;

When element changes a r e necessary t h e

t h e o p e r a t i o n can be c a r r i e d o u t a t any t i m e

w i t h o u t s t o p p i n g o r i n t r o d u c i n g a i r i n t o t h e main system,

thus making t h e v e r y

minimum o f s e r v i c i n g s k i l l s a ccep t a bl e . Whether o r n o t o f f - l i n e f i l t r a t i o n can be t h e s o l e means o f f i l t r a t i o n depends on many f a c t o r s r e l a t i n g t o t h e c h a r a c t e r , q u a n t i t y and o r i g i n o f t h e c o n t a m in a t io n .

As i t i s p a r t i a l f i l t r a t i o n we must decide i f i t i s necessary

t o p r o t e c t i n d i v i d u a l o r groups o f components from s t r a y p a r t i c l e s l i k e l y t o cause a c a t a s t r o p h i c f a i l u r e . The f o r e g o i n g has a t t emp t e d t o show t h a t t h e r e i s a p l a c e f o r i n l e t , pressure, r e t u r n and o f f - l i n e f i l t r a t i o n , wh i ch i s borne o u t by f i l t e r manufacturers who once t o l d us t h a t p r e s s u r e f i l t e r s answered a l l o u r problems and who now o f f e r

a range o f r e t u r n l i n e f i l t e r s , and ma nu f a ct urers o f r e t u r n f i l t e r s who now o f f e r ranges o f p r e s s u r e f i l t e r s . S u r e l y what i s most i m p o r t a n t i s t o r e a l i s e t h a t l i k e a l l branches o f engine e r i n g , system f i l t r a t i o n e n g i n e e r i n g i n v o l v e s t a k i n g a r i s k . A s i n g l e p a r t i c l e

326

Fig. 3 3 A pictorial representation01 the bask liltration W4ualiOn.

327 i n t h e 5 t o 1 0 micrometre s i z e range, c o u l d cause system m a l f u n c t i o n i f i t

say,

a r r i v e d a t t h e wrong p l a c e a t a c r i t i c a l t i m e .

The machine t o o l designer may

w e l l have t h e b e n e f i t o f a p r o t o t y p e on which t o e v a l u a t e t h e f i l t r a t i o n system performance and make changes p r i o r t o t h e f i r s t p r o d u c t i o n b a t c h .

The designer

o f heavy equipment, say, f o r s t e e l w o r k s i s a f f o r d e d no such o p p o r t u n i t y ;

he

must be r i g h t f i r s t t i m e and may w e l l have t o use a l l t h e f i l t e r s e l e c t i o n p r o cedures a v a i l a b l e t o him i n o r d e r t o m i n i m i z e t h e r i s k o f expensive stoppages later.

That r i s k w i l l be w i t h us u n t i l (a) we have more r e a l i s t i c d a t a on f i l -

t e r performance, and (b) maintenance p r a c t i c e s a r e g r e a t l y improved. The message o f t h e p r e c e d i n g c h a p t e r s can b e s t be summarised d i a g r a m m a t i c a l l y (Fig.33).

This high1 ights the f a c t t h a t control over the i n b u i l t contamination

by a p p l y i n g good i n s t a l l a t i o n p r a c t i c e s and u s i n g a l l means a v a i l a b l e t o l i m i t t h e amount o f ingressed c o n t a m i n a t i o n , enables us t o c o n t r o l t h e system generated c o n t a m i n a t i o n t o an a c c e p t a b l e l e v e l . The l e v e l o f c o n t a m i n a t i o n e n t e r i n g t h e pump i s a c r i t i c a l f a c t o r and should be so c o n t r o l l e d t h a t i t p r e v e n t s t h e s o r t o f a c t i o n d e p i c t e d by a mangle. The d i f f i c u l t y i n c o n t r o l l i n g t h i s l e v e l by i n l e t f i l t e r s has been c l e a r l y s t a t e d . I n the main, t h e y should be c o n s i d e r e d o n l y f o r p r e v e n t i n g l a r g e p a r t i c l e s e n t e r i n g t h e pump and causing c a t a s t r o p h i c f a i l u r e . Downstream o f t h e pump t h e a b i l i t y o f a p r e s s u r e f i l t e r t o t r a p p a r t i c l e s w i l l be i n f l u e n c e d t o a l a r g e e x t e n t by any a b r u p t changes o f f l o w and p r e s s u r e , which have t h e e f f e c t o f d r i v i n g p a r t i c l e s t h r o u g h t h e f i l t e r i n g media.

We

know from p r a c t i c e t h a t bypass v a l v e s can m a l f u n c t i o n under c e r t a i n c i r c u m s t a n -

ces, and i t i s e s s e n t i a l t h a t t h e d e s i g n o f t h e f i l t e r p r e v e n t s any m i g r a t i o n o f c o n t a m i n a t i o n i f t h i s should happen. Ingress through c y l i n d e r r o d s e a l s , a l t h o u g h i n t h e s m a l l e r p a r t i c l e range, can n e v e r t h e l e s s add up t o s i g n i f i c a n t q u a n t i t i e s .

The performance o f any

r e t u r n l i n e f i l t e r w i l l , a g a i n , depend upon t h e magnitude o f changes i t sees i n terms o f f l o w and p r e s s u r e . O f f - l i n e f i l t r a t i o n enables optimum f i l t e r performance t o be accomplished, thus e n a b l i n g us t o c o n t r o l c o n t a m i n a t i o n l e v e l s more e f f e c t i v e l y .

Our a b i l i t y

t o remove c o n t a m i n a t i o n depends n o t o n l y on t h e f i l t e r micrometre r a t i n g b u t a l s o on t h e f l o w r a t e across i t , and we must p r o v i d e s u f f i c i e n t d i r t h o l d i n g c a p a c i t y t o ensure an element l i f e a c c e p t a b l e t o t h e end u s e r .

13.7

SUMMARY

Any examination o f t h e s u b j e c t o f c o n t a m i n a t i o n i n v o l v e s f o u r groups o f peop 1 e : (i)

The f l u i d manufacturer o r s u p p l i e r .

(ii)

The h y d r a u l i c equipment and f i l t e r manufacturers.

( i i i ) The manufacturer o f t h e machinery which uses t h e h y d r a u l i c equipment. ( i v ) The end u s e r o f t h e machinery.

328 Each o f these has a commercial r e s p o n s i b i l i t y t o supply equipment which w i l l perform i t s duty s a t i s f a c t o r i l y a t a reasonable p r i c e , and each must have some knowledge o f the c l e a n l i n e s s o f the working f l u i d . The f l u i d s u p p l i e r w i l l supply f l u i d as c l e a n as r e q u i r e d and w i l l charge accordingly. The h y d r a u l i c equipment manufacturer must advise t h e user to whom he i s s e l l i n g h i s equipment on the type o f f l u i d and i t s degree o f c l e a n l i n e s s best s u i t e d t o the h y d r a u l i c equipment he i s recommending f o r a s p e c i f i c a p p l i c a t i o n . The machinery manufacturer, who i s responsible t o the end user f o r supplying equipment, must be aware o f a l l the c o n d i t i o n s t o be met,

i n c l u d i n g supply of

f l u i d s , s e r v i c i n g f a c i l i t i e s , and the type o f r e l i a b i l i t y which has t o be maintained.

I t i s h i s p r e r o g a t i v e t o o f f e r h i s customer the best commercial propo-

s i t i o n and t h i s could mean, f o r example, e i t h e r an inexpensive throwaway u n i t which must be replaced r e l a t i v e l y f r e q u e n t l y , o r a more expensive item f o r which the o n l y service needed i s the occasional exchange o f a r e l a t i v e l y inexpensive component, such as a f i l t e r c a r t r i d g e . The end user has t o make the f i n a l judgement.

He needs t o be a b l e t o compare

the r e a l value o f each i n s t a l l a t i o n o f f e r e d t o him;

each user w i l l place d i f f e r

ent emphasis on the value o f each type o f equipment o f f e r e d .

To the end user

the value o f a pump i s i t s f i t n e s s f o r the purpose, how long i t w i l l perform i t s duty s a t i s f a c t o r i l y , and what the cost o f s e r v i c i n g w i l l be.

He i s n o t i n t e r -

ested i n how many 10 micrometre p a r t i c l e s a r e contained i n 100 m l o f h y d r a u l i c fluid. The u s e r ' s i n t e r e s t i s i n the l e a s t expensive f i l t e r t h a t w i l l p r o v i d e the required degree o f c l e a n l i n e s s .

I n making t h i s assessment, the o r i g i n a l o r

c a p i t a l c o s t o f the equipment has t o be balanced a g a i n s t the c o s t o f service. He may have t o compare, f o r example, the advantages o f f e r e d by a very expensive pump t h a t w i l l operate on ' d i r t y ' o i l w i t h those o f a low-cost pump p l u s f i l t e r .

I f i t i s t o m a i n t a i n i t s present h i g h i n t e g r i t y , the h y d r a u l i c equipment industry needs t o e s t a b l i s h more meaningful s p e c i f i c a t i o n s f o r d i f f e r e n t types of f i l t e r .

The s p e c i f i c a t i o n s must a l l o w the end user t o buy performance w i t h -

out necessarily having t o know how t h i s performance i s achieved.

The people

who need t o w r i t e these s p e c i f i c a t i o n s a r e those who know t h e a c t u a l r e q u i r e ments.

The r e s p o n s i b i l i t y must be d i v i d e d between manufacturers o f f i l t e r

elements, who should know what i s p o s s i b l e , and the manufacturers o f h y d r a u l i c equipment, who should know what i s needed.

These t w o groups should be a b l e t o

communicate i n meaningful terms.

A t present, there a r e no adequate techniques covering a l l aspects o f contami n a t i o n measurement i n h y d r a u l i c f l u i d s . a s t a r t i n the r i g h t d i r e c t i o n . n o t be 100% r i g h t f i r s t time;

This should n o t deter us from making

The c l e a n l i n e s s l e v e l c h a r t , f o r example, w i l l t h e r e w i l l be a c o n t i n u a l need t o review i t i n

329 the l i g h t o f new e xpe ri en ce. The e s c a l a t i n g c o s t o f equipment downtime and maintenance must encourage t h e end u s e r t o c o n s i d e r r u n n i n g c o s t s more c a v e f u l l y , f o r b o t h new and e x i s t i n g plant.

To be a b l e t o do t h i s he must a p p r e c i a t e more f u l l y the p a r t t h a t

c o n t a m i n a t i o n c o n t r o l p l a y s i n d e t e r m i n i n g t hose c o s t s f o r t h e h y d r a u l i c installation.

I t i s hoped t h a t t h i s volume has g i v e n a u s e f u l i n s i g h t t o t h e

r e a l n a t u r e o f t h e problem and has p o i n t e d t h e way t o a systematic approach and more c o s t - e f f e c t i v e s o l u t i o n s .

T h i s c h a p t e r has been e d i t e d by k i n d p ermission o f Sperry V i c k e r s from a more comprehensivb paper " E f f e c t i v e Contamination C o n t r o l i n F l u i d Power Systems",

w r i t t e n by J.B.

Spencer and C.

Balmer, p u b l i s h e d by Sperry Vickers.

330

1L1

SEALS FOR FLUID POWER EQUIPMENT PART ONE

B.D.HALLIGAN,

14.1

C Eng,MIMechE AMPRI, T e c h n i c a l Manager ( P r o d u c t A p p l i c a t i o n s ) James Walker & Co.Ltd.

INTRODUCTION

The f a c t t h a t a s e a l m a n u f a c t u r i n g i n d u s t r y e x i s t s a t a l l o r i g i n a t e s f r o m t he commercial o r p h y s i c a l i m p r a c t i c a b i l i t y o f a c h i e v i n g p e r f e c t and permanent m u t u a l i t y o f adjacent surfaces

-

be t h e y i n dynamic o r s t a t i c r e l a t i o n s h i p .

S e a l i n g d e v i c e s , wh i c h a r e u s u a l l y o f an e s s e n t i a l l y d e f o r m a b l e n a t u r e , p e r m i t t h e f l u i d power d e s i g n e r t o work w i t h i n e c o n o m i c a l l y s e n s i b l e c o n d i t i o n s o f

f i t , surface f i n i s h ,

t o l e r a n c e and f l u i d c l e a n l i n e s s a c r o s s t h e range o f temper-

a t u r e s and p r e s s u r e c o n d i t i o n s w i t h w h i c h he i s l i k e l y t o c o n te n d . The d e s i g n e r o f f l u i d power equipment has,

i n fact,

influenced trends i n the

s e a l i n g i n d u s t r y by g o i n g beyond t h e fundamental r e q u i r e m e n t o f i n t r o d u c i n g an e n v i r o n m e n t a l l y t o l e r a n t component f o r p r e v e n t i n g le a k a g e .

The q u e s t t o m i n i -

mise p r o d u c t i o n c o s t s has a c c e l e r a t e d t h e movement towards s e a l d e s i g n s embodyi n g i n t e g r a l b e a r i n g s and a n t i - e x t r u s i o n d e v i c e s c a p a b le o f s u p p o r t i n g s e a l s a g a i n s t s u b s t a n t i a l e x t r u s i o n gaps a t s i g n i f i c a n t p r e s s u r e s .

Overall i n the

c o n t e x t o f t hose s e a l s s p e c i f i e d f o r l i n e a r dynamic m o t i o n , a much a x i a l l y s h o r t e r s e a l has e v o l v e d wh i c h a l l o w s h x d r a u l i c c y l i n d e r s , f o r example, t o be produced t o more compact p r o p o r t i o n s w i t h t h e consequent s a v i n g i n raw m a t e r i a l c o s t s and i n t h e b u l k o f f l u i d power s u b - a s s e m b lie s . None o f t h i s i s bad.

However i t does mean t h a t a p r o l i f e r a t i o n o f s e a l

desi gns and m a t e r i a l s e x i s t s and t h o s e r e s p o n s i b l e f o r s e l e c t i o n a r e r e q u i r e d t o make t h e i r c h o i c e w i t h d i s c r e t i o n t o e n s u r e t h a t t h e most c o s t - e f f e c t i v e s o l u t i o n i s f oun d .

P i c k i n g a s e a l wh i c h f a c i l i t a t e s t h e achievement o f t h e

cheapest p r o d u c t i o n c o s t s f o r t h e component i n t o w h ic h i t f i t s i s seldom t h e most r easonabl e b a s i s f o r g u a r a n t e e i n g minimum leakage and maximum s e r v i c e a b i l i t y .

331 I t i s an u n f o r t u n a t e f a c t t h a t most f l u i d leakages a r e a t t r i b u t e d t o a f a u l t y seal, whereas i n v e s t i g a t i o n w i l l f r e q u e n t l y show t h a t t h e number o f occasions o f m a l f u n c t i o n due t o a f a u l t y p r o d u c t a r e few.

The r e a l source o f d i f f i c u l t y

u s u a l l y stems from i n c o r r e c t seal s e l e c t i o n , w o r k i n g c o n d i t i o n s which d i f f e r from those considered t o a p p l y , seal containments o u t s i d e s p e c i f i e d t o l e r a n c e s i n terms o f dimension o r f i n i s h , f a u l t y f i t t i n g , be sealed o r ,

importantly,

contaminants i n t h e f l u i d t o

personal i n t e r p r e t a t i o n o f 'leakage'

in quantitative

terms. To understand

t h e l i m i t a t i o n s o f f l u i d seal performance i t i s necessary t o

know a l i t t l e o f t h e m a t e r i a l s most t y p i c a l l y used, t h e o r i g i n s o f fundamental seal designs, how t h e y f u n c t i o n , and t h e i r r e f i n e m e n t t o t h e p r e s e n t s t a t e o f the a r t .

Fortunately,

t h e s e a l i s no l o n g e r an a f t e r - t h o u g h t b u t more o f t e n

receives t h e a t t e n t i o n i t p r o p e r l y deserves as a v i t a l i n t e r a c t i v e element i n any complete f l u i d power system.

14.2

MATERIALS

W i t h i n t h e scope o f t h i s c h a p t e r , a t t e n t i o n w i l l be c e n t r e d on s o l i d e l a s t o meric s e a l s , t y p i f i e d by t h e '0' r i n g f o r s t a t i c connections,

elastomer c o n t a i n -

i n g s e a l s such as c y l i n d e r packings based on p r o o f e d t e x t i l e , and those designs which employ p l a s t i c components f o r v a r i o u s f u n c t i o n s . The compounding o f e l a s t o m e r s i s regarded by many as b e i n g a b l a c k a r t and i t i s t r u e t o say t h a t w i d e l y d i f f e r i n g p r o p e r t i e s can be developed by v a r y i n g

t h e q u a n t i t y and t y p e o f rubber chemicals added t o t h e s t o c k rubber i n e i t h e r i n t e r n a l m i x e r s o r on m i x i n g m i l l s .

The raw m a t e r i a l s u p p l i e d t o t h e seal man-

u f a c t u r e r must u s u a l l y be processed t o t a i l o r t h e p h y s i c a l c h a r a c t e r i s t i c s r e quired f o r the sealing function.

Parameters such as t e n s i l e s t r e n g t h , e l o n g a t i o n

a t break, compression s e t , b r i t t l e p o i n t , and behaviour i n c o n t r o l f l u i d s a r e t y p i c a l o f t h o s e reviewed when assessing performance requirements and l a y i n g down compound s p e c i f i c a t i o n s . i n a d d i t i o n t o a p p l i c a t i o n s u i t a b i l i t y , p r o c e s s a b i l i t y i n mixing, extrusion and moulding sequences must be assured as must s a t i s f a c t o r y post-moulding operat i o n s such as d e - f l a s h i n g . T o m a i n t a i n c o n s i s t e n c y o f seal elastomers r e q u i r e s e x t e n s i v e l a b o r a t o r y

s u p p o r t and c o n t i n u o u s s t a n d a r d t e s t i n g o f t h e compound i t s e l f on a b a t c h - t o batch basis. Tables 14.1

-

1 4 . 3 a r e taken from t h e B r i t i s h Rubber M a n u f a c t u r e r s ' Associa-

t i o n 'Guide t o E l a s t o m e r i c S e a l s ' elastomers,

t o g i v e broad r e f e r e n c e t o most commonly used

t h e i r p r o p e r t i e s , f l u i d c o m p a t i b i l i t y and temperature r e s i s t a n c e .

332 TABLE 14.1 Polymers-types and General P r o p e r t i e s

NATURAL POLY I SOPRENE SYNTHETIC POLYISOPRENE Very good

(Natural Rubber-NR) (Pol y isoprene- I R) General physical p r o p e r t i e s , i n p a r t i c u l a r t e a r resistance, r e s i s t a n c e t o g l y c o l - e t h e r brake f l u i d s and vegetable o i l s .

Poor r e s i s t a n c e t o

Heat, weather, ozone and mineral o i l .

CHLOROPRENE (NEOPRENE-CR) Very good res i stance t o

Ozone, weathering and abrasion

STYRENE-BUTADIENE (SBR) Very good r e s i s t a n c e t o

Abrasion, g l y c o l - e t h e r brake f l u i d s and vegetable o i l s .

Poor r e s i s t a n c e t o

Mineral o i l and ozone.

ACRYLONITRILE-BUTADIENE ( N i t r i le-NBR)

Low, medium, and h i g h n i t r i l e s a r e a v a i l a b l e based on increasing a c r y l o n i t r i l e content which s i g n i f i c a n t l y a f f e c t s low temperature and f l u i d swell properties.

Very good r e s i s t a n c e t o

Mineral o i l , compression set and abrasion.

ISOBUTYLENE-ISOPRENE ( B u t y l - l I R )

CHLORO-ISOBUTYLENE-ISOPRENE (Chloro-Butyl) Very good r e s i s t a n c e t o Poor

POLYBUTADI ENE (Butad iene-BR) Very good r e s i s t a n c e t o

Poor resistance t o POLYSULPHIDE (Thiokol-T) Very good r e s i s t a n c e t o Poor

Tear, weather and gas permeation. T e n s i l e p r o p e r t i e s and mineral o i l resistance.

Tear, abrasion, low temperature, g l y c o l - e t h e r brake f l u i d s and vegetable oils. Offers high resilience. Water, ozone and mineral o i l .

Ozone, mineral o i l s , pet-roleum f u e l s and weather.

A l l other properties.

ETHYLENE PROPYLENE (EPR-EPM) ETHYLENE PROPYLENE TERPOLY MER (EPT-EPDM) Very good resistance t o Weather, ozone, heat, water, steam, g l y c o l - e t h e r brake f l u i d s and vegetable oils. ?oor resistance t o Mineral o i l . CHLOROSULPHONATED POLYETHYLENE (Hypa lon-CSM) Very good resistance t o

Water, ozone, abrasion, a c i d and weather.

333 TABLE 14.1

(contd.)

METHYL-VINYL SILOXANE ( S i l icone-VMQ) PHENYL-METHYL-VINYL SILOXANE ( S i 1 icone-PVMQ) Very good r e s i s t a n c e t o

H i g h and low temperature.

Poor r e s i s t a n c e t o

Abrasion,

TRI FLUOROPROPYL S I LOXANE ( F l u o r o s i 1 icone-FMQ) Very good POLYURETHANE D I - I SOCYANATE (Urethane-AU) Very good r e s i s t a n c e t o

tear

and t e n s i o n .

M i n e r a l o i l and f u e l r e s i s t a n c e . A l l o t h e r p r o p e r t i e s as VMQ. Mineral o i l , abrasion, tear, ozone and weather. O f f e r s h i g h modulus and t e n s i l e p r o p e r t i e s .

Poor

Moist heat resistance.

FLUORINATED HYDROCARBON ( F I uorocarbon-FPM) Very good r e s i s t a n c e t o

High temperature ( i n a i r and most o i l s ) , weather and p e t r o l e u m f u e l .

Poor

Tear s t r e n g t h .

POLYACRYLATE ( A c r y l ic-ACM) Very good

Heat, weather, m i n e r a l o i l and ozone r e s i s t a n c e . Water r e s i s t a n c e .

Po0 r

O f major i n t e r e s t t o t h e f l u i d power i n d u s t r y a r e : -

Acrylonitrile-butadiene ( N i t r i l e Probably upwards o f 80% o f based on n i t r i l e compositions. terms o f good compression s e t

-

NBR)

s e a l s s u p p l i e d t o t h e f l u i d power

ndustry are

The balance o f p r o p e r t i e s a v a i l a b e from NBR i n q u a l i t i e s , a b r a s i o n r e s i s t a n c e , m i era1 o i l

c o m p a t i b i l i t y , ease o f p r o c e s s i n g and low i n i t i a l c o s t favour t h i s c h o i c e . N i t r i l e compositions a r e n o t , however, c o m p a t i b l e w i t h f i r e - r e s i s t a n t

fluids

o f t h e phosphate e s t e r t y p e . Isobutylene-isoprene (Butyl

-

I IR)

S a t i s f a c t o r y i n s e r v i c e w i t h phosphate e s t e r f l u i d s b u t r a t h e r l i f e l e s s from a physical p o i n t o f view.

Not s u i t a b l e f o r m i n e r a l o i l s e r v i c e , n e i t h e r should

b u t y l s e a l s be smeared w i t h m i n e r a l o i l o r grease on f i t t i n g . E t h y l e n e p r o p y l e n e (EPR or

-

EPM)

E t h y l e n e p r o p y l e n e t e r p o l y m e r (EPT

-

EPDM)

Same s e r v i c e f u n c t i o n and l i m i t a t i o n s as b u t y l b u t g e n e r a l l y b e t t e r p h y s i c a l properties

.

F l u o r i n a t e d Hydrocarbon (Fluorocarbon

-

FPM)

The o n l y usual s e l e c t i o n where c o m p a t i b i l i t y w i t h m i n e r a l o i l and phosphate e s t e r i s required.

334 P ol yure t h an e (Urethane

-

Au)

Widely f a v o u r ed as a dynamic se al m a t e r i a l i n t h e US and i n Germany, p o l y ure t h a n e e x h i b i t s e x c e l l e n t a b r a s i o n r e s i s t a n c e and i s v e r y d u r a b l e i n s i t u a t i o n s whic h m ig h t a d v e r s e l y a f f e c t n i t r i l e comp osi tions o r even p r o o f e d f a b r i c s , e.g. passage o f s e a l l i p s o v e r p o r t s i n a c y l i n d e r w a l l . bility,

Poor low temperature f l e x i -

h y d r o l y s i s i n h o t wa t e r and a l o we r o p e r a t i o n a l temperature c e i l i n g a r e

l i m i t i n g factors. Other s e a l component m a t e r i a l s w o r t h y o f mention are:-

-

Textiles

woven c l o t h such as c o t t o n , asbestos,

t e r y l e n e and n y l o n

a r e used as t h e su bst rat um i n elastomer p r o o f e d f a b r i c pa cki ng s f o r medium and h i g h p r e s s u r e d u t i e s . Non-woven m a t e r i a l s such as p o l y e s t e r and polyamide a r e also available. The p r o o f i n g e l ast ome r can be v a r i e d t o s u i t b u t w i l l f r e q u e n t l y be based on NBR and/or c h l o r o p r e n e (Neoprene-CR)

-

PTFE

employed as a back-up r i n g m a t e r i a l f o r ' 0 ' r i n g s . Poor cre ep p r o p e r t i e s r e s t r i c t i t s use as a seal m a t e r i a l i n i t s own r i g h t un l ess e n e r g i s e d by an e l a s t o m e r i c component o r spring.

Nylon and acetal

-

main f u n c t i o n i s h ee l support f o r dynamic s e a l s o r as component b e a r i n g r i n g m a t e r i a l .

-

Hytrel

t h e r e g i s t e r e d t r a d e name o f a Du Pont range o f thermop l a s t i c p o l y e s t e r m a t e r i a l s w hich form a el a st o mers and t h e r m o p l a s t i c s .

b r i d g e between

Has s i m i l a r p r o p e r t i e s t o

p o l y u r e t h a n e b u t e x h i b i t s much b e t t e r low temperature f l e x ibility.

Used c u r r e n t l y as an a n t i - e x t r u s i o n element i n

c e r t a i n s p o o l - t y p e p i s t o n head s e a l s . When r e f e r r i n g t o rubber t e chn ol o gy as a ' b l a c k a r t ' t h e r e i s a t w o - f o l d interpretation.

On t h e one hand i t i s c e r t a i n l y a m a n u f a c t u r i n g area i n f l u e n c e d

by many v a r i a b l e s .

On t h e o t h e r ,

t h e end p r o d u c t used i n a s e a l i n g c o n t e x t i s

i n v a r i a b l y b l a c k i n c o l o u r due t o t h e use o f carbon b l a c k as a f i l l e r necessary f o r toughening the f i n i s h e d a r t i c l e .

Seal i d e n t i f i c a t i o n i s t h e r e f o e a major

problem u n l e s s housekeeping i s o f a h i g h o r d e r w i t h s e a l s p o s i t i v e l y separated and bagged i n s ma l l numbers h a v i n g f u l l s p e c i f i c a t i o n o f m a t e r i a l , s ze, and p a r t number c l e a r l y shown. Colour c o d in g i s one s o l u t i o n b u t t h e r e a l answer l i e s i n t h e use o f s e l f c o l o u r e d e la s to m e rs.

A t t h e moment a good de al o f development work

s being

.

335

TABLE 14.2

R-Recommended A-Acceptable

N-Not recommended

R

A

N

N

N

N

N

N

N

N

N

N

N

R

N

R

TABLE 1 4 . 3 OC

250

200

150

Temperature Range

OC

Any statement of temperature range f o r a given polymer must be qualified by consideration f o r the environment in which i t i s t o work. For example, service temperatures recommended f o r 1iquid applications are l i k e l y t o be s i g n i f i c a n t l y higher than those which would be appropriate t o dry heat s i t u a t i o n s . The values given in Table 3 r e l a t e t o typical f l u i d conditions. N.B. These a r e not continuous r a t i n g s .

W W W W Q, Q,

TTABLE A B i E 14.4 TYPES RECIPROCATING RECIPROCATING APPLICATIONS APPLICATIONS L I P TYPE

OF SEALS FOR RECIPROCATING, ROTARY AND STATIC APPLICATIONS APPLICATIONS APPLICATIONS

U R i n g (Also known as U P a c k i n g )

-

50116 e l a i t m - f o r p r e s ~ u r e up i t o 10 bar--gmd f i l m w r p i t y and l o * pressure sealing characterlst3csideal f o r a i r c y l i n d e r . Ymm t o abrasive *car. e x t r u ~ l ~and n f l u i d t v t t l n g a t hlgher hydraulic p r e s i u ~ e i . Can be f i t t e d in recerrrr r e q u i r i n g no sep*r*te dCLCII. l o l a d e l a i t m i w t h P l i i t i C heel-for OT~IIYTCI UP to 100 b a r d s 10lld rubbcr but hiqher oressure Capability due t o enhanced CItlYIlOn r e l l r t m c e .

350 bbai li I1I1 though though rohs't rohit Fabric-for pressures p r e i i u r e l up up ttoo 350 Fabric-for leis sseennssi itti ivvee Dprrooffiillee *w11 eitend Conitructions ooff leis ? I 1 extend Conitructions range. Frequently Frequently ffi itttteedd dwti thh an an adapt& adaptor rri inngg tthhi iss range. (as shorn shorn iinn the the aise&ly d 5 l m h l y sketcher) sketcher) or 01an an i inntteerrnnaal l (as arc available available s p ~ c a d c r . Spring Spring loaded loaded adaptors l d l p t o r r arc l li ipp spvcadcr. t o aupnent low QrCIIYlt real,ng I b 9 l l t y . Usually rcqulrr separate housing access such I S remyable cover p l a t e but I p c c i a I fabric U-nngr f o r indCcesItble 9movc1 d m available. Other materials include wlyurethdne. P l F t and leather. None o f t W I e designs a r e n o m l l y r e c m n d c d f O T shock loading. rams subject t o l a t e r a l t h r u s t . abrasive condit30nr o r high speed recIpToCation. U n n q 1 seal ~n one d i r e c t i o n only. A double-acting a r m n g m n t M i l l *quire tw ~ e d l it o be r i g ? d l y scpamted and each f a c i n g the p r c s ~ u r cIOUTCC.

AVAILABLE STANDARD STANDARD AVAILABLE MATERIALS MATERIALS

K

0

L

>

0)

L x t r e n r l y wide range of h y d r a d i c rervices-mst types of h y d r a u l i c Dress, r e c i p r o c a t i n g steam punpr, sludge p w s . h y d r a u l i c cylinder; * OilOil and water expanlion glandr, h y d r a u l i c valves.

N o r m l l y pmafed f a b r i c .

fa1 U p - r t m k i m l IlMl omving press rams. r e c i p r o c a t i n g Dmp l a m . l i f t s , cranes. h y d r a u l i c acCuMlators.

Proofed f a b r i c

Others i n c l u d e e i a s t M c r ,

'lFC Or leather

(a) S i n g l e L i p n

Heavy Duty L i p Packi ngs

"T

eU

o w ll-m

V Ring (When i n s e t form, more commonly known as Chevron

( b ) Two-Lips L

e

v

L

la)

Large diameter ram. u p - s t r o k i n g or d o n - s t r o k i n g (*ere f r i c t i o n a l r e s i s t a n c e i s n o t o f paralmunt importance) Wen where subjected t o Shock loads. e.g. i n f o r g i n g p r e i i e s ' h o r i z o n t a l vams of any diameter i n c l u d i n g tho%; o f e x t r w i o h ~ P C I I C I r a m s u b j e c t to I d t e m l t h r u s t : h i g h pressure' r e c i p i o c d t i n g pulps; t b s e heavy duty a p p l i c a t i o n s where packing housing and buIneI are n o t i n F i s t c l a l 5 condition.

337

I

w w -2

338 c a r r i e d o u t t o a ch i eve t h i s end w i t h o u t any s i g n i f i c a n t l o s s i n p h y s i c a l properties.

Coloured f l u o r o c a r b o n rubbers a r e a l r e a d y a v a i l a b l e .

14.3

SEAL DESIGNS

W i t h i n t h e boundaries s e t by f l u i d power systems t h e m a j o r i t y o f s e a l s employed can be c a t e g o r i s e d as l i p o r squeeze t y p e s . F i g u r e 1 i l l u s t r a t e s t h e s i m p l e s t forms u t i l i s e d f o r s t a t i c o r dynamic r e c i procating situations

-

the 'U'

r i n g and t h e '0' seal

-

and i n d i c a t e s how b o t h

r e l y on i n t e r f e r e n c e s t r e s s f o r a t l e a s t a component o f t h e i r f u n c t i o n a l opera t io n .

r--.

-

I.--'

INT

P

P

Fig.1

L i p and Squeeze Seals

Both a r e r e s p onsi ve t o system p r e s s u r e and w i l l generate a r a d i a l s t r e s s g r e a t e r than t h e p r e s s u r e t o be se al e d.

N e i t h e r r e l y on e x t e r n a l l y a p p l i e d

compression t o any r e a l degree as i n t h e case o f t h e s o f t packed gland.

This

i s p a r t i c u l a r l y r e l e v a n t t o squeeze s e a l s whi ch a r e u s u a l l y intended t o f l o a t a x i a l l y i n t h e i r housings. The q u a l i t y o f t h e s e a l i n g c o n t a c t a rea and t h e i n t e r f e r e n c e s t r e s s i n t h e same zone w i l l c o n t r o l low p r e s s u r e s e a l i n g e f f i c i e n c y .

These f a c t o r s p l u s

o v e r a l l s e a l geometry and co mpo si t i o n w i l l s e t t h e p o i n t a t w hich t i m e h y d r a u l i c response t a k e s o v e r f rom t h e manufactured i n t e r f e r e n c e c o n d i t i o n .

The h y d r a u l i c

component may become s i g n i f i c a n t a t p ressu res o f 40 bar f o r an e l a s t o m e r i c ' 0 ' r i n g o r a s h i g h as 200 b a r f o r a r i g i d p r o o f e d f a b r i c packing. S e a l i n g s l a c k o i l and low p ressu res i s g r i t y and i s t h e more d i f f i c u l t c o n d i t i o n . ure s i s a f e a t u r e o f se al containment,

i.e.

g e n e r a l l y a f u n c t i o n o f seal i n t e P r e v e n t i n g leakage a t h i g h pressp r e v e n t i n g e x t r u s i o n o r r a p i d wear

i f i n a dynamic d u t y . Tables 14.4 and 14.5 l i s t t y p i c a l l i p and squeeze types i n common and t r a d i t i o n a l usage.

(Courtesy o f ERMA).

339 Some p a r t i c u l a r p o i n t s t o n o t e on each c a t e g o r y : L i p seals

(i)

-

a r e s i n g l e - a c t i n g t o a p p l i e d pressure and must be separated

by a f i x e d component i n d o u b l e - a c t i n g d u t i e s so t h a t t h e h y d r a u l i c load f r o m t h e element under p r e s s u r e i s n o t passed on t o t h e t r a i l i n g element. For maximum s e r v i c e l i f e m u l t i - r i n g p acking s e t s a r e p r e f e r r e d where t h e s u c c e s s io n o f s e a l i n g edges ensures t h a t breakdown i s n o t sudden.

The use

o f s p l i t r i n g s i s a l s o e n t i r e l y f e a s i b l e w i t h most designs o f t h i s s o r t w i t h o u t s a c r i f i c i n g s e a l i n g performance t o any c r i t i c a l degree.

Savings i n

downtime w i l l be o bvi o us. ( i i ) Squeeze s e a l s

-

have t h e advantage o f b e i n g d o u b l e - a c t i n g and a r e u s u a l l y

housed more e con omi ca l l y t h an t h e i r co rresponding l i p b r e t h r e n .

The l i n e -

age fr o m t h e humble '0' r i n g t o t h e u n i t s e a l s i n contemporary use can be seen f r o m Ta bl e 14.5. Most s e a l s i n t h i s c a t e g o r y use a s i n g l e s e a l i n g zone and damage i n s e r v i c e w i l l be f o l l o w e d by more immediate breakdown than i n t h e case o f m u l t i - l i p p a c k in g s . The squeeze s e a l s a r e r a r e l y used i n s p l i t form a l t h o u g h they can be engineered so t o do. Tables 14.6 and 14.7 d e p i c t t h e e x t e n s i o n o f l i p and squeeze s e a l s t o r o t a r y and s t a t i c f u n c t i o n s . More advanced se al de si gn s a r e d e t a i l e d i n t h e n e x t - s e c t i o n .

14.4

TR IBOLOG I CAL CONS I DERAT I ONS

I t i s h e a r t e n i n g t o f i n d t h e s t u d y o f se al behaviour an e s s e n t i a l p a r t o f t r i b o l o g y seminars,

r e c e i v i n g as much a t t e n t i o n ,

and s u r f a c e topography.

indeed, as bearings,

lubricants

I n r e c e n t ye ars much company research and independent

s tu d y by o r g a n i s a t i o n s such as BHRA has been expended on d e f i n i n g s e a l i n g mechanisms

14.4.1

-

p a r t i c u l a r l y i n r e l a t i o n s h i p t o r e c i p r o c a t i n g motion. F i l m Conditions

A l l dynamic s e a l s r e l y on a co he ren t f l u i d f i l m under t h e i r c o n t a c t area i f t h e y a r e t o f u n c t i o n c o n s i s t e n t l y and p r e d i c t a b l y .

Such f i l m s may stem from

boundary l u b r i c a t i o n i n some modes and be t r u l y hydrodynamic i n o t h e r s .

f i l m w i l l v a r y i n t h i c k n e s s a c c o r d i n g t o se al p r o f i l e , pres s u r e , speed,

surface f i n i s h ,

t y p e o f f l u i d employed

On r e c i p r o c a t i n g d u t i e s i t m i g h t t y p i c a l l y v a r y from 0.25 r o t a r y s h a f t s v a l u es o f 0.6

-

The

interference stress, and i t s temperature.

-

3.0 microns.

On

1.0 mi cro n would u s u a l l y a p p l y .

Leakage f r o m r o t a r y s h a f t l i p s e a l s i s seldom e v i d e n t u n l e s s seal wear down o r u n d e r - l i p c r a c k i n g has t ake n p l a c e

-

a l l o t h e r f e a t u r e s b e i n g equal. However,

340

TABLE

14.5

RECIPROCATING APPLICATIONS

APPLICATIONS

AVAILABLE STANDARD MATERIALS

SQUEEZE TYPE

0-Seal

L i g h t and medium duty, pneumatic and h y d r a u l i c r e c i p r o c a t i n g services, e.g. small c y l i n d e r s , valve spools and stems, rod f i l m wiping.

P r i n c i p a l l y elastomer b u t polyurethane and PTFE a r e a l s o manufactured.

As 0-seal b u t b e t t e r resistance t o s p i r a l t w i s t .

Elastomer.

Pneumatic and h y d r a u l i c c y l i n d e r s p a r t i c u l a r l y where s e n s i t i v i t y i s essential, e.g. weighing machines, t e s t i n g equipment, e t c . P r i m a r i l y f o r p i s t o n heads but also available f o r gland d u t i e s .

Elastomer and PTFE ( p l a i n o r r e i n f o r c e d ) o r elastomer and proofed f a b r i c .

B a s i c a l l y developed f o r hydraulic cylinder rod and p i s t o n head d u t y p a r t i c u l a r l y i n mobile and i n d u s t r i a l h y d r a u l i c environments.

Elastomer/Plastic. Elastomer/Fabric,

Lobed Seal

Energized Sleeve Seal

Supported S i ngle-Acting Sea 1

m

Elastomer/Fabric/Plastic can a l s o be made i n polyurethane.

Supported Double-Acting Seal Developed f o r p i s t o n head duty i n h y d r a u l i c c y l i n d e r s associated w i t h mobile/ indus tr ia 1 hydraul ic industries.

Elastomer/Fabric,

Elastomer/Fabric/Plastic, Elastomer/Plastic.

341

TABLE 14.6 ROTARY APPLICATIONS

APPLICATIONS

AVAILABLE STANDARD MATERIALS

LIP TYPE

Retention o f l u b r i c a n t and exclusion o f f o r e i g n matter from bearings and c r i t i c a l surfaces.

SQUEEZE TYPE

Elastomer/metal, Elastomer/fabric. Spring loading t o l i p normally provided by t o r o i d a l spring o r f i n g e r spring.

The a p p l i c a t i o n o f squeeze-type seals t o r o t a r y d u t i e s requires specialised a t t e n t i o n . Consultation w i t h the seal manufacturer i s recommended.

TABLE 14.7 STATIC SEALS

APPLICATIONS

AVAILABLE STANDARD MATERIALS

GASKETS AND JOINTINGS

The most widely used form o f s t a t i c connection where external mechanical load i s available.

Elastomer, r e i n f o r c e d elastomer, proofed f a b r i c , elastomer bonded cork, compressed asbestos f i b r e and a wide v a r i e t y o f nonelastomer based compositions.

A wide range o f s t a t i c s e a l i n g duties. Cylinder end caps, autoclave doors, pressure vessels, couplings. etc.

Elastomer. proofed f a b r i c , leather, Polyurethane and PTFE.

A l l s t a t i c sealing duties which w i l l permit the use o f the selected 0-seal m a t e r i a l and which a r e o f a design which w i l l contain t h i s m a t e r i a l w i t h i n the confines o f the seal recess a t operating pressure.

Elastomer and PTFE.

LIP TYPE U Ring (Also known as U-Packinal _. . _. .,I

from comnon o r separate source SQUEEZE TYPE

342 i n r e c i p r o c a t i n g a p p l i c a t i o n s passage o f t h e s h a f t through t h e g l a n d w i l l c a r r y t h e o i l f i l m t o t h e atmosphere s i d e o f t h e s e a l .

ROD EMERGINGthick o i l film

Fig.2

ROD RETRACTINGseal energised p r e v e n t i n g t o t a l f i l m iturn

Cond t i o n o f Maximum C o l l e c t e d 0 1 F i l m .

F i g u r e 2 i s a diagrammatic r e p r e s e n t a t i o n o f t h e w o r s t f i l m t r a n s p o r t s i t u a t i o n i n v o l v i n g an emerging c y l i n d e r r o d w i t h s l a c k p r e s s u r e t o t h e g l a n d which r e t r a c t s w i t h t h e s e a l i n g element under l o a d .

I n t h i s mode t h e h e e l o f t h e s e a l

w i l l be e n e r g i s e d and can p r e v e n t t h e r e t u r n o f t h e t o t a l q u a n t i t y o f f i l m c a r r i e d by t h e rod.

The f i t t i n g o f an e f f e c t i v e w i p e r can a g g r a v a t e t h e c o n d i -

tion. To combat t h e emergent f i l m , c a r e f u l a t t e n t i o n i s s e a l geometry.

required t o pressure side

Designs such as t h a t shown i n F i g . 3 w i t h k n i f e - c u t s e a l i n g

edges and a s p e c i f i c r e l a t i o n s h i p between c o n t a c t edge and groove h e i g h t s have proved v e r y s u c c e s s f u l d e s p i t e t h e i r s h o r t a x i a l l e n g t h .

Fig.3

Minimum F i l m

-

Shallow Design

E q u a l l y e f f i c i e n t and h a v i n g t h e advantage o f more t h a n one s e a l i n g edge i s t h e concept shown i n F i g . 4 w h i c h i s a m a r r i a g e o f l i p and squeeze s e a l i n g principles.

343 STANDARD FABRIC CHEVRON GLAND RING

-

F A B R I C CHEVRON I N T E W l E D I A T E RING SPECIAL SYNTHETIC RUBBER HEADER RING

Fig.4

Minimum F i l m

-

M u l t i - L i p Design

F i g u r e 5 i l l u s t r a t e s t y p i c a l improvement i n performance a g a i n s t standard V - r i n g packing under o f f s e t l o a d c o n d i t i o n s and shows i t s a d a p t a b i l i t y even i n s p l i t form. LEAKAGE - cc/hr COMPARISON OF F A B R I C MULTI-LIP PACKING VERSUS S P L I T AN0 ENDLESS L O F I L M CHEVRON WHEN SUBJECTED TO EXAGGERATED ROD KNUCKLING S I Z E : 44.5mm x 60.3mm

(12" x 2:")

4-

GLAND BUSH CLEARANCE 0.5mm(.D20") TEST DURATION 500 HOURS

OIAMETRAL

( 2 ) S P L I T L O F I L M CHEVRON PACKIHG ( 3 1 ENDLESS L O F I L M CHEVRON PACKING

Fig.5

14.4.2

Comparative Seal Performance

Surface F i n i s h

I t i s n o t o n l y t h e numerical v a l u e o f s u r f a c e roughness which i s r e l e v a n t t o seal performance b u t a l s o t h e manner

i n which such a f i n i s h i s achieved.

I n t h e c o n t e x t o f f l u i d power equipment, honed o r r o l l e r b u r n i s h e d b a r r e l f i n i s h e s a r e recommended w i t h an average v a l u e o f between 0 . 4 t o 0.8 p m Ra. L a r g e r c y l i n d e r s should be f i n i s h e d t o 0.8

urn

Ra o r b e t t e r , i f p o s s i b l e , b u t i n

any e v e n t s h o u l d be no worse than 1.6 urn Ra. Rods should i n a l l circumstances be o f a s u r f a c e roughness 0.8 p m Ra o r better.

Many o f t h e s m a l l e r s i z e s w i l l be a v a i l a b l e from p r o p r i e t a r y r o d sup-

p l i e r s t o a s t a n d a r d f i n i s h o f 0.2 pm Ra. i a l , f u n c t i o n , and

All static

Type o f f i n i s h w i l l depend on mater-

t h e d e s i g n e r ' s e x p e r i e n c e o f s i m i l a r equipment.

housing areas may be f i n i s h e d i n t h e range 0.8 t o 1.6 um Ra f o r

p r o o f e d f a b r i c packings.and, p r e f e r a b l y , 0.4 t o 0.8 pm Ra For s m a l l e r h o u s i n g s a p p r o p r i a t e t o '0' r i n g s , e t c .

344 Any l o n g i t u d i n a l marking on rods o r c y l i n d e r s w i l l promote leakage and manuf a c t u r i n g processes posing such r i s k s , e.g. as-drawn tube, r e t r a c t i o n marks on r o l l e r - b u r n i s h i n g heads, should be most c a r e f u l l y monitored. Equally, the achievement o f t o o f i n e a f i n i s h 0.05 I.cm Ra seal.

-

-

perhaps i n t h e area 0.01

-

can prevent the establishment o f a coherent f l u i d f i l m under the

An e r r a t i c performance can f r e q u e n t l y f o l l o w and packing l i f e can be cur-

tailed.

Several p r a c t i c a l

cases have been demonstrated where the d e l i b e r a t e

i n t r o d u c t i o n o f a coarser f i n i s h has restored an adequate q u a l i t y o f sealing. I n any event, on most h y d r a u l i c i n s t a l l a t i o n s the advantages t o be gained by improving f i n i s h e s below 0.2/0.3 pm Ra a r e d i s p r o p o r t i o n a t e l y c o s t l y w i t h o u t o f f e r i n g t a n g i b l e performance improvement. 14.4.3

Seal F r i c t i o n

I n e s t i m a t i n g seal drag loads f o r c r i t i c a l a p p l i c a t i o n s , designers a r e faced w i t h t h e p r a c t i c a l problem o f f i n d i n g even a general idea o f a c t u a l values from seal manufacturers. There a r e r e a l d i f f i c u l t i e s i n e x t r a p o l a t i n g r e s u l t s based on l a b o r a t o r y equipment and applying values t o much l a r g e r p l a n t .

Equally, type o f l u b r i c a -

t i o n , choice o f packing, and degree o f l i p i n t e r f e r e n c e , whether as-moulded o r as a r e s u l t o f compression, a r e a l l q u a n t i t i e s which w i l l i n f l u e n c e the f i n a l result. Work c a r r i e d o u t i n t h i s area w i t h a view t o g i v i n g a n o t i o n a l allowance f o r f r i c t i o n takes due account of seal l e n g t h and expresses seal drag F r i c t i o n load where

=

DL 25.8

x [142

+

(0.8

X

load as :

R)] k g f

R = f l u d pressure (bar)

D z sea L = sea

(2) (2)

contact diameter contact l e n g t h

I t i s o f t e n d e s i r a b l e t o add a contingency allownace o f 15%.

As a r e s u l t , i t

may be seen t h a t a m u l t i - l i p packing o f 1000 mm c o n t a c t diameter and 50 m deep would r e q u i r e 0.2% approx. o f t h e t h r u s t developed on t h i s f u l l area diameter a t a pressure o f 200 bar. Figure 6 makes the p o i n t t h a t any elastomeric and t h e r e f o r e f l e x i b l e seal c o n s t r u c t i o n w i l l s u f f e r an increase i n contact band w i d t h w i t h increasing system pressure.

Even i n the case o f r e l a t i v e l y tough, h i g h modulus m a t e r i a l s

such as proofed f a b r i c o r polyurethane, pressures o f 200 bar o r over may be s u f f i c i e n t t o promote t o t a l a x i a l contact o f t h e seal w i t h the dynamic wear s u r f ace.

345

LOW

HIGH PRESSURE

PRESSURE

I izsi I I I I

CONTACT BAND AREA

Fig.6

I 1

is:!

CONTACT BAND AREA

Seal Contact Band Width

Interposing a loaded PTFE sleeve is an effective means o f limiting seal drag Figure 7 shows

dramatically despite substantial hydraulic pressure increases.

o n e form o f the composite principle utilising a rectangular section elastomeric energiser.

LOADED PTFE SLEEVE

Fig.7

RUBBER ENERGISER

Energised PTFE Sleeve Seal

346 The o r d e r o f f r i c t i o n c o n t r o l a v a i l a b l e by comparison t o comparably t e s t e d

'0' r i n g s and ' U ' r i n g s i s g i v e n i n F i g . 8 .

500

-

COMPARISON OF MOVING F R I C T I O N LOAD AT VARYING F L U I D PRESSURES

u-

.? 400 v

0 < 0

A

z

300

0 I-

$

200

LL -I

5 VI

100 'C' 0

50

100

200

150

250

300

350

O I L PRESSURE (BAR)

Fig. 8

Comparative F r i c t i o n C h a r a c t e r i s t i c s

The p a te n t e d d e s i g n shown i n F i g. 9

combines f r i c t i o n c o n t r o l w i t h maximum

f l l m wiping a b i l i t y f o r the rod s i t u a t i o n .

Fig. 9 E n e r g i s e d PTFE S l e e v e Seal with P r o f i l e d F i l m Wiping Edge

14.4.4

Type o f F l u i d

As f a r as dynamic s e a l s a r e concerned, t h e v a s t m a j o r i t y o f mineral-based o r phosphate e s t e r f l u i d s do n o t p r e s e n t l u b r i c i t y problems. One o f t h e most demanding t ype s o f d u t y f o r any dynamic seal i n v o l v e s p l a i n wat e r a t h i g h p r e ssure whether by d e s i g n o r by t o t a l l o s s o f s o l u b l e o i l content

341 i n a n o m i n a l l y l u b r i c a t e d system.

Hi g h s p e e d s a r e

particularly d i f f i c u l t to

s u s t a i n u n l e s s a minimum s o l u b l e o i l l e v e l o f 2% i s guaranteed. T e s t s conducted on t h r e e - t h r o w m i n i n g pumps o p e r a t i n g a t 250 b a r , 0 . 6 m/s, have shown t h a t f o r even a 2% s o l u b l e o i l c o n t e n t , average seal l i f e w i l l be extended by a f a c t o r o f f o u r by comparison w i t h u n t r e a t e d w ater.

Equally, a

s e p a r a t e l u b r i c a n t feed w i l l produce s i m i l a r l y d r a m a t i c improvements. The t e m p e r a t u re o f h y d r a u l i c f l u i d sho ul d n o t exceed 60°C

i f a t a l l possible,

as s i g n i f i c a n t l y f a s t e r s w e l l and s o f t e n i n g o f p r o o f e d f a b r i c s and s t r a i g h t polymers w i l l o c c u r above t h i s v a l u e .

To i l l u s t r a t e t h e p o i n t , t h e f o l l o w i n g

d a t a i s based on immersion t e s t i n g f o r seven days i n She 1 T e l l u s 27 m i n e r a l o i l . N i t r i l e proofed f a b r i c

Hig h n i t r i l e e l ast ome r

Softening o f

-

room t e mpe rature

+

0 . 7 % volume i n c r e a s e

6OoC 9OoC

2.6%

1 2O0C

3.3%

- 6OoC

0.8%

90°C

2.3%

1 2O0C

3.8%

1.2%

moulded f a b r i c m a t e r i a l u s u a l l y reduces i n t r i n s i c s t r e n g t h and

w i l l normally d i m i n i s h s e r v i c e l i f e .

14.4.5

Filtration

For t h e b u l k o f e l ast ome r p r o o f e d f a b r i c and s o l i d elastomer s e a l s f l u i d f i l t r a t i o n o f 25 mi cro ns sho ul d be p e r f e c t l y adequate from t h e s e a l i n g p e r f o r mance a s p e c t .

S pe ci al s e a l s such as those embodying PTFE wear faces w i l l bene-

f i t f r o m 10 m ic r on f i l t r a t i o n or b e t t e r .

I n e i t h e r event, f i l t r a t i o n equipment

o f f e r i n g a f i n e r c u t - o f f w i l l p r o b a b l y be s p e c i f i e d t o s u i t c o n t r o l v a l v e funct i o n s on a g i v e n pre ss o r h y d r a u l i c component. The m a j o r i t y o f c o n v e n t i o n a l h y d r a u l i c s e a l s w hich a r e t e s t e d in-house work i n c o n j u n c t i o n w i t h no more t ha n a co arse w i r e s t r a i n e r i n any h y d r a u l i c system. However, modern f i l t r a t i o n a i d s must be con si dered an advantage, p a r t i c u l a r l y i f t h e o p e r a t i o n o f t h e p l a n t i n v o l v e s produces a g g r e s s i v e r e s i d u e s .

14.4.6

A i r E n trai n men t

As c y c l e speeds become f a s t e r u r e s must f l u c t u a t e more q u i c k l y .

due t o i n c r e a s i n g work demands, system pressI n many

must be exhausted i n m i l l i s e c o n d s , e.g. moulding presses, e t c .

situations,

f u l l w orking pressure

d i e - c a s t i n g machines, p l a s t i c s i n j e c t i o n

I f a i r i s e n t r a i n e d i n t h e h y d r a u l i c f l u i d such r a p i d

decompressions can be e xcee di n gl y dangerous i f no a u t o m a t i c v e n t i n g i s a v a i l a b l e .

I f one c o n s i d e r s t h a t i n 1O;l o f h y d r a u l i c o i l a t 200 bar and 10°C i t i s p o s s i b l e t o d i s s o l v e n e a r l y 200 1 o f a i r , some i n d i c a t i o n o f t h e magnitude of

348 r i s k w i l l be apparent. The main problem r e l a t e s t o p i s t o n head s i t u a t i o n s where f l u i d c o l l e c t s between two opposed seals.

L i p packings a r e l a r g e l y s e l f - v e n t i n g b u t heavy

i n t e r f e r e n c e polyurethane cup r i n g s a r e o f t e n suspect i n t h i s d i r e c t i o n as they do not r e a c t s u f f i c i e n t l y q u i c k l y . Under no circumstances should two squeeze type seals, e.g.

'0' r i n g s , be

employed on a p i s t o n head as the a i r entrainment contingency can be aggravated by a proven phenomenon known as i n t e r - s e a l pressure whereby t h r e e o r f o u r times system pressure can be b u i l t up i n t h e annular clearance between the seals. Extrusion o f such seals

into t h e

a p p l i e d pressure can be noted i n t y p i c a l cases.

PRESSURE

-

3 TO 4 P

P

P

F i g . 10

I n the gland s i t u a t i o n , t h e r i s k s a r e n o t o f the same degree unless t h e r e i s r e s t r i c t e d access o f working f l u i d t o the packing v i a a s i n g l e p o r t i n the neck bush o r some s i m i l a r feature.

Adequate f l u i d access t o a l l seals should be

assured unless d e l i b e r a t e attempts a r e being made t o reduce t h e a c t u a l pressure a t t h i s p o i n t o r t o dampen pressure v a r i a t i o n s .

14.5

SELECTION

As so many seal designs w i l l apparently meet given c o n d i t i o n s and y e t be s i g n i f i c a n t l y d i f f e r e n t i n m a t e r i a l , s i z e , and p r i c e , the f l u i d power equipment designer can be f o r g i v e n f o r being confused. Table 14.8 i l l u s t r a t e s t h e v a r i a t i o n s t h a t could apply t o a given a c t u a t o r o f f i x e d rod, b a r r e l and s t r o k e dimensions and r e f l e c t s a survey made several years ago when eighteen quotations

were sought f o r a

double cushioning and a s t r o k e l e n g t h o f 18". v a r i e d between €175 and €24.50,

3"

dia. cylinder with

P r i c e s received a t t h a t time

y e t a l l purported t o do the same j o b .

Much w i l l depend on t h e u s e r ' s own experience and preference based on knowledge o f the a p p l i c a t i o n . consequence o f leakage,

his

Schedule maintenance periods, a c c e s s i b i l i t y ,

i n i t i a l cost and a v a i l a b i l i t y w i l l a l l p l a y t h e i r p a r t

i n s t e e r i n g t h e d e c i s i o n towards u l t i m a t e s e c u r i t y o r some o t h e r l e v e l o f costeffectiveness.

349

TABLE 14.8 UPPER

LOWER

UPPER

LOWER

UPPER

LOWER

350 I t i s s i g n i f i c a n t t h a t B r i t i s h S t e e l C o r p o r a t i o n a r e now s e t t i n g t h e i r own standards f o r c r i t i c a l c y l i n d e r d u t i e s and w i l l use m u l t i - l i p packing throughout. Such a s o l u t i o n would be e n t i r e l y u na ccep t a ble t o t h e manufacturer o f e a r t h moving

v e h i c l e s who would seek a more economic s o l u t i o n i n terms o f c y l i n d e r

s i z e and s e a l c o s t . Most s e a l m a nu f a ct u rers w i l l e r r on t h e s i d e o f c a u t i o n i f

t h e y a r e advised

o f in te n d e d s e r v i c e c o n d i t i o n s and i f i n doubt t h i s i s t h e p r e f e r r e d r o u t e t h a t t h e d e s ig n e r s h o u l d t a ke. Equally,

h a v i n g made h i s c h o i c e and b e i n g c o n f r o n t e d w i t h a range o f o v e r a l l

s i z e s f o r a g i v e n di a met er, t h e l a r g e s t se al s e c t i o n a v a i l a b l e should be taken n o t the smallest

-

l i p o r squeeze

-

o t h e r re qu i reme nt s a l l o w i n g .

The s m a l l e r t h e seal

-

-

whether

t h e f i n e r i s t h e wo rki ng t o l e r a n c e band o f i n t e r f e r e n c e and t h e

lower i s t h e c a p a b i l i t y f o r a bso rbi n g mi sal i g nment, v i b r a t i o n , adverse accumulat i v e tolerances,

etc.

This assertion applies equally

t o s t a t i c and dynamic

positions.

14.6

STORAGE BS 3754:1963

' S t o r a g e o f Vu l can i sed Rubber' was prepared under t h e a u t h o r i t y

o f t h e Rubber I n d u s t r y Standards Committee and i n c l u d e s t h e f o l l o w i n g recommend a t ions: "Most v u l c a n i s e d rub be rs change i n p h y s i c a l p r o p e r t i e s d u r i n g s t o r a g e and u l t i m a t e l y may become u n s e r v i c e a b l e , hardening,

f o r example, because o f excessive

softening, cracking, c r a z i n g o r o t h e r surface degradation.

These changes may be t h e r e s u l t of one p a r t i c u l a r f a c t o r o r a combination o f f a c t o r s , namely, t h e a c t i o n o f oxygen, ozone,

l i g h t , heat and humidity."

The d e l e t e r i o u s e f f e c t s o f t he se f a c t o r s may, however, be minimised by carefu

choice o f storage conditions.

14 6.1 14.6.1.1

Recommendations Temperature

The s t o r a g e t empe rat ure should be below 25OC and p r e f e r a b l y below 15OC.

At

temperatures exceeding 25OC c e r t a i n forms o f d e t e r i o r a t i o n may be a c c e l e r a t e d s u f f i c i e n t l y t o a f f e c t the u l t i m a t e service l i f e .

Sources o f heat i n s t o r a g e

rooms should be so a rran ge d t h a t t h e t emp era ture o f no s t o r e d a r t i c l e exceeds 25OC.

The e f f e c t s o f low t emp era t u re a r e n o t permanently

deleterious t o vul-

c a n is e d rubber a r t i c l e s b u t t h ey may become s t i f f e r i f s t o r e d a t low temperat u r e s and c a r e should be t a ken t o a v o i d d i s t o r t i n g them d u r i n g h a n d l i n g a t t h a t temperature.

When a r t i c l e s a r e t ake n f rom low temperature s t o r a g e f o r immediate

use t h e i r t e m p e r a t u re sh ou l d be r a i s e d t o a p p r o x i m a t e l y 3OoC throughout b e f o r e they a r e p u t i n t o s e r v i c e .

351

14.6.1.2

Hu m idi t y

M o i s t c o n d i t i o n s should be avo i de d; condensation does n o t

14.6.1.3

s t o r a g e c o n d i t i o n s should be such t h a t

o ccur.

Light

V u lc a n is e d r u b be r sh ou l d be p r o t e c t e d from l i g h t ,

i n p a r t i c u l a r d i r e c t sun-

l i g h t and s t r o n g a r t i f i c i a l l i g h t w i t h a h i g h u l t r a - v i o l e t c o n t e n t . a r t i c l e s a r e packed i n opaque c o n t a i n e r s , o f storage

14.6.1.4

Unless the

i t i s a d v i s a b l e t o cover any windows

rooms w i t h a r e d o r orange c o a t i n g o r screen. Oxygen and Ozone

Where p o s s i b l e , v u l c a n i s e d rubber sh ou l d be p r o t e c t e d from c i r c u l a t i n g a i r by wrapping, s t o r a g e i n a i r - t i g h t c o n t a i n e r s , o r o t h e r s u i t a b l e means;

this

p a r t i c u l a r l y a p p l i e s t o a r t i c l e s w i t h l a r g e s u r f a c e area t o volume r a t i o s , e.g. pro o fe d f a b r i c , c e l l u l a r ru bb er. As ozone i s p a r t i c u l a r l y d e l e t e r i o u s , s t o r a g e rooms should n o t c o n t a i n any equipment t h a t i s capable o f g e n e r a t i n g ozone,

such as mercury vapour lamps,

h i g h v o l t a g e e l e c t r i c a l equipment, e l e c t r i c motors, o r o t h e r equipment w hich may g i v e r i s e t o e l e c t r i c sparks o r s i l e n t e l e c t r i c a l discharges.

14.6.1.5

De f o r ma t i o n

V u lc a n is e d rubber should, wherever p o s s i b l e , be s t o r e d i n a r e l a x e d c o n d i t i o n f r e e from t e n s i o n , compression o r o t h e r d e f o r m a t i o n . Hig h q u a l i t y re qu i reme nt s f o r s t o r a g e and p e r i o d i c i n s p e c t i o n such as those s p e c i f i e d by th e Aero-Space

I n d u s t r y a r e o b t a i n a b l e from BS

2F.68:1963

"Recom-

mendations f o r t h e s t o r a g e and i n s p e c t i o n i n s t o r e o f v u l c a n i s e d rubber items".

14.7

ASSEMBLY

A lt h o u g h i n d i v i d u a l a p p l i c a t i o n s w i l l i n v o l v e p a r t i c u l a r f i t t i n g problems t h e r e a r e a number o f b a s i c p o i n t s o f good p r a c t i c e which,

i f observed, w i l l

c o n t r i b u t e t o optimum sea l performance: (i)

Check t h a t sea l i s o f c o r r e c t t yp e, p a r t number or s i z e and m a t e r i a l .

(ii)

Ensure t h a t se al i s i n undamaged c o n d i t i o n and clean.

( i i i ) Where p e r m i s s i b l e , smear t h e s e a l i n g edge o f dynamic s e a l s w i t h clean grease.

Con sul t any f i t t i n g i n s t r u c t i o n l a b e l p r o v i d e d by t h e manufac-

t u r e r t o a s c e r t a i n whether f u r t h e r grease a p p l i c a t i o n t o i n t e r - s e a l c a v i t i e s e t c . i s recommended. (iv)

Do n o t t r e a t f l a t g aske t su rf a ces w i t h any form o f j o i n t i n g p a s t e o r lubr i c a n t u n less i n s t r u c t e d so t o do, o t h e r w i s e t h e a b i l i t y o f the gasket t o g r i p t h e a d j a c e n t s e a l i n g faces may be impaired.

352 (v)

Clean a1

se al ho usi n g o r gasket s e a t i n g areas.

Check t h a t o t h e r s u r f a c e s

a d j a c e n t t o t h e passage o f t h e sea l on f i t t i n g a r e a l s o f r e e o f d i r t , s wa r f or o t h e r contaminants. (vi)

Check s e a l h ou si ng dimensions and s u r f a c e f i n i s h t o design recommendations.

( v i i ) I f a s e a l i s l i k e l y t o c o n t a c t t hre ad s, sharp

corners, p o r t s , c i r c l i p s o r

s i m i l a r c o n t i n g e n c i e s d u r i n g t h e assembly o p e r a t i o n then s u i t a b l e f i t t i n g a i d s must be pro vi de d.

The s l i g h t e s t n i c k o r t e a r on a c r i t i c a l edge o f

an e l a s t o m e r i c component w i l l reduce s e a l i n g i n t e g r i t y .

I f f r e q u e n t use

i s envisaged, n o n - m e t a l l i c f i t t i n g sl e eves can be o f advantage s i n c e damage t o a s i m i l a r m e t a l l i c d e v i c e can d u p l i c a t e t h e hazard t o t h e s e a l . ( v i i i ) D o n o t le a ve se al i n p a r t ' a s s e m b l y f o r any l e n g t h o f time i f s e a l i n g edges a r e s u b j e c t t o m i s a l i g n e d loads; f o r example rod seal f i t t e d t o c y l i n d e r w i t h r o d i n p o s i t i o n b u t no g l a n d bush f i t t e d . (ix)

I f a p p r o p r i a t e , a p p l y any post-assembly o p e r a t i o n recommended by s e a l manu f a c t u r e r , f o r example compression o f t h e p r e s c r i b e d amount i n an a d j u s t a b l e g la n d housing; r o t a r y d u t y; and so on.

run ni n g a t h a l f - s p e e d t o a s s i s t seal b e d d i n g - i n on a

f o l l o w i n g up f l a n g e b o l t s a f t e r a p e r i o d a t temperature,.

.. .

353

15

SEALS FOR FLUID POWER EQUIPMENT PART TWO ROTARY SHAFT LIP SEALS

E.D.HALLIGAN,

CEng, MIMechE, A M P R I . , T e c h n i c a l Manager (Product A p p l i c a t i o n s ) James Walker G Co.Ltd.

15.1

INTRODUCTION

For t h e purpose o f s e a l i n g l u b r i c a n t w i t h i n a b e a r i n g o r e x c l u d i n g f o r e i g n m a t t e r from b e a r i n g surfaces, t y p i c a l l y i n Fig.1,

t h e s p r i n g - l o a d e d r o t a r y s h a f t l i p s e a l , as shown

i s w i d e l y accepted.

The n a t u r e o f most designs p r e c l u d e s

t h e use o f t h i s t y p e o f seal f r o m o p e r a t i n g a t s i g n i f i c a n t p r e s s u r e s u n l e s s t h e s e a l i n g l i p i s adequately supported by a shaped p l a t e . much i n excess o f 2'.0

-

For p r e s s u r e c o n d i t i o n s

3.0 b a r , combined w i t h r o t a r y movement i t would be p r e -

f e r a b l e t o c o n s i d e r e i t h e r a compression p a c k i n g o r a r a d i a l f a c e mechanical s e a l as f i r s t c h o i c e .

Fig.1

Standard f a b r i c back s e a l w i t h r e t a i n i n g p l a t e .

3 54 DESIGN

15.2

A wide v a r i e t y o f l i p s e a l designs i s a v a i l a b l e i n terms o f o v e r a l l c o n s t r u c -

t i o n , m a t e r i a l , and l i p p r o f i l e .

Some a r e i n t e n d e d f o r f i t t i n g i n housings

which have no s e p a r a t e c o v e r - p l a t e and may be s u p p l i e d w i t h a r i g i d metal case t o which the s e a l i s bonded, t h e u n i t b e i n g a f o r c e - f i t

Fig.2

i n t h e housing,

(Fig.2).

T y p i c a l metal-encased s e a l .

Others have t h e advantage o f f l e x i b i l i t y by v i r t u e o f h a v i n g a p r o o f e d f a b r i c back ( F i g . 3 a ) w h i l s t r e q u i r i n g t h e p r o v i s i o n o f a r e t a i n i n g p l a ' t e t o n i p a x i a l l y the back o f t h e seal t o p r e v e n t leakage and o b v i a t e r o t a t i o n .

More recent dev-

elopments combine t h e m e r i t s o f f l e x i b i l i t y and s e l f - r e t e n t i o n

i n a metal-

supported a l l - r u b b e r

Fig.3a.

configuration (Fig.3b).

Standard f a b r i c back s e a l .

Fig.3b.

Moulded-in f l e x i b l e s t e e l band s e l f - r e t a i n i n g s e a l .

One o f the s e v e r a l advantages o f t h e f a b r i c back seal i s t h e f a c i l i t y w i t h which s p l i t s e a l s can be f i t t e d w i t h o u t r e d u c i n g performance which, cases,

i s o f t h e same o r d e r o f e f f i c i e n c y as an endless r i n g .

i n many

A rubber i n l a y

i s f r e q u e n t l y moulded i n t o t h e back o f such a seal t h r o u g h which the s p l i t i s effected.

The rubber abutment which r e s u l t s ensures good s e a l i n g across t h e

s p l i t portion. The p r o f i l e o f t h e l i p c o n t a c t area i s s u b j e c t e d t o each m a n u f a c t u r e r ' s design p h i l o s o p h y .

Some h a v e a knife-edge c o n t a c t band and r e l y on heavy as-

355 moulded l i p i n t e r f e r e n c e .

Others depend on t h e s p r i n g t e n s i o n t o u r g e t h e l i p

i n t o intimate shaft contact.

I n p r a c t i c e , a c a r e f u l balance o f i n t e r f e r e n c e ,

s p r i n g c h a r a c t e r i s t i c s and c o n t a c t band w i d t h must be a l l i e d t o knowledge o f the t y p e o f m a t e r i a l b e i n g used, b e a r i n g t ype, and c o n d i t i o n and o t h e r e n v i r o n mental c o n s i d e r a t i o n s such as t emp era t u re, f l u i d and,

i f any, pressure.

A t y p i c a l range o f se al p r o f i l e s i s shown i n F i g . 4 w hich a l s o i l l u s t r a t e s exp e d ie n t used when i n s u f f i c i e n t space i s a v a i l a b l e t o accommodate a p a i r o f seals,

i.e.

t h e s o - c a l l e d d u s t l i p de si gn .

Fig.4

15.3

T y p i c a l se al p r o f i l e s .

MATERIAL

N i t r i l e - b a s e s y n t h e t i c rubber compounds a r e w i d e l y employed as standard mate r i a l s by reason o f t h e i r c o m p a t i b i l i t y w i t h most l u b r i c a t i n g o i l s and greases and t h e i r r e s i s t a n c e t o wear.

There a r e , however, c e r t a i n r o l l i n g o i l s and

c o o l a n t s wh ic h may cause e x c e s s i v e s w e l l o r s h r i n k a g e o f such polymers and a f l u o r o c a r b o n r u b be r,

e.g.

V i t o n , may be a necessary s e l e c t i o n .

This i s p a r t i -

c u l a r l y t r u e o f some palm o i l s o l u t i o n s and o t h e r s o l u b l e types o f o i l . C o n s i d e r a t i o n f o r o p e r a t i o n a l t emp era t u re i s important and, i t i s t h e c o n d i t i o n a t t h e l i p o f t h e s e a l which i s paramount

t h e e n v ir o n m e n t a l t e mpe rat ure .

However,

-

i n t h i s respect, not necessarily

i t may be s a i d t h a t n i t r i l e s a r e f r e -

q u e n t l y used w i t h success f o r i n t e r m i t t e n t s e r v i c e temperatures o f 15OoC and a r e c o n t i n u o u s l y r a t e d f o r 120°C

i n lubricated conditions.

Above these values

e i t h e r a c r y l i c r ubb ers, f l u o r o c a r b o n o r s i l i c o n compounds would r e q u i r e i n v e s t i gation. Many

s i l i c o n e and n i t r i l e rubber s h a f t s e a l s a r e f i t t e d i n automotive a p p l i -

c a t i o n s f o r c r a n k s h a f t and gearbox s e a l i n g ,

i n which areas much use i s b e i n g

made o f grooved s e a l i n g s u r f a c e s which a r e designed t o produce a hydrodynamic

356 e f f e c t i n the

oil film

b e i n g sealed, e f f e c t i v e l y causing t h e o i l t o be pumped

away from t h e s eal l i p (see F i g . 5 ) .

F ig . 5

Metal i n s e r t se al w i t h hydrodynamic a i d .

D e t a i l e d m a t e r i a l recommendations a r e g i v e n i n Appendix 1 .

15.4

SEAL LUBRICATION

I n common w i t h most o t h e r forms o f dynamic s e a l , depend f o r t h e i r e f f i c i e n t and c o n s i s t e n t stable f l u i d f i l m .

r o t a r y s h a f t l i p seals

performance upon t h e presence o f a

The e s t a b l i s h m e n t o f t h i s f i l m , w hich w i l l t y p i c a l l y be i n

t h e o r d e r o f a few mi cro ns t hi ckn ess, may n o t o c c u r immediately, and i t i s n o t uncommon f o r a h i g h pe rcen t a ge o f wear t o occur on r o t a r y s h a f t seal l i p s during t h i s p e r i o d b e f o r e s t e a d y - s t a t e c o n d i t i o n s a r e achieved.

For t h i s reason, i t

i s always good p r a c t i c e t o a p p l y a l u b r i c a n t t o t h e seal l i p and i n t h e seal c a v i t y before f i t t i n g . Where s i n g l e s e a l s a r e housed a t each end o f t h e housing, t h e r e i s n o r m a l l y s u f f i c i e n t b e a r i n g l u b r i c a n t i n c o n t a c t t o p r o v i d e adequate l u b r i c a t i o n . Again, where two s e a l s a r e housed t o g e t h e r ,

i t i s o f t e n found t h a t w i t h b e a r i n g l u b r i -

c a n t on one s e a l and e i t h e r r o l l i n g f l u i d o r r o l l c o o l a n t on t h e o t h e r , f u r t h e r l u b r i c a t i o n i s unnecessary.

I n a l l cases t h e l i b e r a l a p p l i c a t i o n o f grease t o

t h e s e a l s on assembly w i l l ensure l u b r i c a t i o n from t h e b e g i n n i n g and,

i n some

i ns t a n c e s , t h i s w i l l be found s u f f i c i e n t t o l a s t from one f i t t i n g t o t h e n e x t . However, t h e r e a r e many s e a l i n g arrangements, e s p e c i a l l y on r o l l i n g m i l l be a r in g s , and

i n which two o r more s e a l s a r e f i t t e d t o g e t h e r i n t h e same housing

t h e r e i s th e danger t h a t a t l e a s t one w i l l r u n d r y u n l e s s l u b r i c a n t i s

s u p p l i e d from an e x t e r n a l source.

T h i s can b e s t be accomplished by d r i l l i n g a

h o l e t h r o u g h t h e chock c o n n e c t i n g w i t h an a n n u l a r groove i n t h e back o f t h e s e a l housing, as shown i n F i g. 6,

and u s i n g a s p e c i a l t y p e o f seal which has a

s e r i e s o f r a d i a l p o r t s i n t h e base f o r passage o f l u b r i c a n t t o t h e seal l i p . When i t i s d i f f i c u l t o r i m p o s s i b l e t o machine an annular groove i n t h e back o f t h e s e a l housing, shown i n F i g . 7 .

t h i s groove may be i n c o r p o r a t e d i n t h e s e a l s themselves, as

357

Fig.6

Seal l u b r i c a t i o n v i a i n s e a l base.

ports

Fig.7

Seal l u b r i c a t i o n v i a p o r t s and a n n u l a r groove i n seal base.

I n g e n e r a l , a good q u a l i t y m i n e r a l o i l o r grease i s s u i t a b l e f o r seal l u b r i c a t i o n , b u t molybdenised l u b r i c a n t s may be used t o advantage where a p p l i c a t i o n i s infrequent.

Care sh ou l d be taken t o ensure t h a t t h e grease o r l u b r i c a n t w i t h

which t h e s e a l

i s t o come i n t o c o n t a c t i s co mpatible.

are being

used w i t h d i u t e m i n e r a l a c i d s , b u t y l

For example, where s e a l s

compositions may w e l l be used.

T h i s m a t e r i a l i s l i k e l y t o s w e l l i n c o n t a c t w i t h m i n e r a l o i l o r grease and an a l t e r n a t i v e l u b r i c a n t w 1 1 be e s s e n t i a l .

15.5

SHAFT SURFACES

The s e a l i n g a re a o f t h e s h a f t sh ou l d be a f i n e ground f i n i s h o f 0.4 t o 0.8 pm (16

-

32 p i n . ) CLA or Ra f o r most a p p l i c a t i o n s , b u t f o r t h e h i g h e r speed range

i t i s recommended t h a t t h e s u r f a c e f i n i s h be improved t o 0.2

CLA o r Ra.

t o 0 . h pm ( 8 - 1 6 p I n )

I n a l l cases i t i s i m p o r t a n t t h a t t h e s h a f t s e a l i n g area be f r e e

from machining marks, de nt s, b u r r s and scra t ches. Where f l u o r o c a r b o n o r a c r y l i c

rubber s e a l s a r e b e i n g employed,

i t i s also

a d v i s a b l e t o use t h e f i n e r l e v e l o f f i n i s h i n d i c a t e d above i n o r d e r t o e l i m i n a t e p i c k - u p o f t h e s ea l m a t e r i a l .

I f l u b r i c a t i o n i s adequate and f r e e f rom a b r a s i v e c o n t a c t , unhardened m i l d s t e e l s h a f t s w i l l g e n e r a l l y g i v e s a t i s f a c t o r y r e s u l t s under normal o p e r a t i n g conditions.

However, a h a r d e r s h a f t m a t e r i a l i s t o be p r e f e r r e d f o r a p p l i c a t i o n s

where l u b r i c a t i o n i s poor, a b r a s i v e s a r e pre sent, o r speed and p r e s s u r e condit i o n s a r e p a r t i c u l a r l y arduous.

A f i n e machined f i n i s h i s s u i t a b l e f o r t h e housing bore.

358

15.6

FRICTION

The r u b b i n g f r i c t i o n o f t h e seal l i p on t h e s h a f t m a t e r i a l i n e v i t a b l y causes a h i g h e r l o c a l temperature than i s p r e s e n t i n t h e f l u i d b e i n g s e a l e d . e f f e c t w i l l be

This

due t o t h e i n t e r f e r e n c e o f t h e seal l i p on t h e s h a f t m a t e r i a l

and may be aggravated by inadequate l u b r i c a t i o n o r speeds i n excess o f t h a t f o r which a

p a r t i c u l a r seal was designed.

Assuming t h e system temperature t o be

w e l l w i t h i n t h e bounds o f t h e m a t e r i a l c a p a b i l i t y , a h i g h d i f f e r e n t i a l between l i p t i p temperature and f l u i d temperature w i l l be m a n i f e s t e d i n many cases by h a i r - l i n e scores i n t h e seal l i p c o - a x i a l w i t h t h e s h a f t .

Local c a r b o n i s a t i o n

o f t h e rubber compound w i l l a l s o be e v i d e n t . F i g u r e 8 i n d i c a t e s a s e t o f curves d e r i v e d by experiment, p r o j e c t i n g f r i c t i o n a l h o r s e power a g a i n s t s e a l d i a m e t e r f o r a c o n v e n t i o n a l r o t a r y l i p seal design o p e r a t i n g a t 500 r.p.m.

i n m i n e r a l l u b r i c a t i n g o i l a t d i f f e r e n t system

pressures.

300

LOO

500

600

700

800

900

Shaft diameter [ m m l Fig.8

F r i c t i o n a l horse power absorbed by r o t a r y s h a f t l i p s s e a l s , having c o n v e n t i o n a l l i p i n t e r f e r e n c e o p e r a t i n g a t 500 r e v h i n

359

15.7

SPEEDS

Many o f t h e f a b r i c - b a c k e d t y p e o f s e a l s t o g e t h e r w i t h those o f t h e m e t a l supported t y p e a r e w o r k i n g s a t i s f a c t o r i l y on 400 mm r o l l - n e c k d i a m e t e r s a t speeds o f up t o 25 m/s (5000 f t / m i n ) o v e r s u s t a i n e d w o r k i n g p e r i o d s . however,

many f a c t o r s such as s u r f a c e f i n i s h ,

There are,

e c c e n t r i c i t y , and l u b r i c a t i o n

which can l i m i t t h e maximum speed f o r which any seal i s s u i t a b l e , and manufact u r e r s should be c o n s u l t e d i n cases o f doubt.

15.8

ECCENTRICITY

Where p l a i n metal o r s y n t h e t i c c o m p o s i t i o n b e a r i n g s a r e b e i n g employed,

it

i s n o t uncommon t o f i n d t h a t t h e s h a f t i s n o t t r u l y c o n c e n t r i c w i t h t h e seal housing, owing t o b e a r i n g c l e a r a n c e and subsequent wear.

I n t h i s event,

it is

e s s e n t i a l f o r e f f i c i e n t s e a l i n g t h a t t h e seal l i p be capable o f f o l l o w i n g a l l s h a f t movement and,

indeed, on many l a r g e c o m p a r a t i v e l y slow-moving s h a f t s

e c c e n t r i c i t y values o f 2 . 5 mm have been s a t i s f a c t o r i l y accommodated.

Naturally,

t h e s e a l performance i n terms o f e c c e n t r i c i t y c a p a b i l i t y w i l l be speed and s h a f t diameter dependent. Where s p l i t s e a l s a r e f i t t e d , then p a r t i c u l a r c o n s i d e r a t i o n i s necessary t o t h e problem o f s h a f t e c c e n t r i c i t y , s i n c e t h e r e may be a tendency f o r a s p l i t seal t o open a t t h e j o i n .

15.9

PRESSURE

A l t h o u g h few r o t a r y s h a f t l i p s e a l s a r e spec f i c a l l y designed as s t a n d a r d compqnents t o accept s i g n i f i c a n t pressures, t h e use o f m e t a l - s u p p o r t i n g p l a t e s w i l l extend t h e u s e f u l n e s s o f t h i s t y p e o f s e a l

A t y p i c a l p r o f i l e i s shown ' i n

Fig.9.

Fig.9

Seal w i t h shaped s u p p o r t p l a t e f o r p r e s s u r e s up t o 3.0 b a r .

As a r e s u l t o f e x p e r i m e n t a l work on t h e s e a l i n g o f o i l - f i l l e d marine s t e r n glands

and manoeuvring t h r u s t e r s , a seal l i p p r o f i l e has been developed which

s a t i s f a c t o r i l y s u s t a i n s pressures up t o 4 . 0 b a r w i t h o u t t h e use o f a shaped support p l a t e .

The base must be f u l l y supported as i n d i c a t e d i n Fig.10.

360

Q Fig.10

New se al development f o r p ressures up t o 4.0 b a r w i t h o u t shaped sup po rt p l a t e .

Where c o n s t a n t a p p l i e d p r e s s u r e i s n o t a n t i c i p a t e d , ted t h a t g r e a s e - l u b r i c a t e d

i t i s f r e q u e n t l y sugges-

b e a r i n g s a r e equipped w i t h r e l i e f h o l e s and t h a t o i l -

l u b r i c a t e d b e a r i n g s have d r a i n s o f adequate s i z e .

D rains taken f r o m t h e ends

o f t h e b e a r i n g near t h e s e a l s w i l l h e l p t o d i s s i p a t e any l o c a l i s e d p r e s s u r e build - u p .

Where p o s s i b l e , st e ps sho ul d be t aken i n b e a r i n g d e s i g n t o p r e v e n t

escaping h i g h p r e s s u r e o i l i mp i ng i ng d i r e c t l y upon t h e s e a l s . I n some cases where t h e loss o f a sma l l amount o f grease i s unimportant,

a

simple r e l i e f system may be formed by f a c i n g s e a l s away f r o m t h e b e a r i n g and a l l o w i n g t h e s e a l l i p t o be l i f t e d under t h e i n f l u e n c e o f t h e l u b r i c a n t pressure.

15.10

CARE AND HANDLING

F l u i d s e a l s s ho ul d be handled a t a l l t i me s w i t h extreme c a r e s i n c e t h e l i f e o f b e a r in g s o r o t h e r c o s t l y machine p a r t s may depend upon t h e i r e f f i c i e n c y . A t t e n t i o n t o the f o l l o w i n g v i t a l p o i n t s w i l l a s s i s t i n ensuring t r o u b l e - f r e e operation during service.

15.10.1 (i)

Storage The s t o r e

sh ou l d have a c o o l , c l e a n and d r y atmosphere,

f r e e from

d u s t and g r i t . (ii)

Whenever p o s s i b l e , s e a l s should n o t be removed from t h e wrapping i n which they were s u p p l i e d , as t h i s p r o v i d e s p r o t e c t i o n and i d e n t i f i c a t i o n

( i i i ) Avoid u n t i d y s t a c k i n g as t h e w e i g h t may d i s t o r t t h e s e a l s a t t h e bottom o f the stack. (iv)

Seals should never be t h rea de d on w i r e o r s t r i n g as t h i s w i l l damage the l i p s .

361 15.10.2

(i)

H a n d lin g I t must be remembered t h a t seal l i p s a r e extremely v u l n e r a b l e t o damage

and t h e s m a l l e s t n i c k p r o v i d e s a p o t e n t i a l l e a k p a t h . (ii)

Seal r e i n f o r c i n g i n s e r t s , a l t h o u g h adequate f o r t h e i r d u t y , may deform under adverse h a n d l i n g o r s t a c k i n g .

( i i i ) Seals h avi n g met al o u t s i d e su rf a ces may damage o t h e r s e a l s , e s p e c i a l l y

i f the me t a l edges c o n t a c t t he ru bb er p a r t s o f n e i g h b o u r i n g seals. 15.10.3

Fitting

A h i g h p r o p o r t i o n o f f a i l u r e s and leakage o f o i l s e a l s i s due t o i n c o r r e c t f i t t i n g r e s u l t i n g i n damage t o b o t h se al and s e a l i n g s u r f a c e .

S t r i c t attention

t o th e f o l l o w i n g m a t t e r s i s e s s e n t i a l i f b e s t performance i s t o be obtained. (i)

Before f i t t i n g ,

t h e se al should be examined t o ensure t h a t i t i s clean

and undamaged. (ii)

The s e a l i n g l i p sho ul d be smeared w i t h s u i t a b l e c l e a n l u b r i c a n t . Seals used as d u s t e x c l u d e r s should be packed w i t h a compatible grease.

( i i i ) The s e a l i n g l i p , n o r m a l l y , sho ul d f a c e t h e f l u i d t o be sealed. When f i t t i n g ,

i t i s i m p o r t a n t t o ensure t h a t t h e s e a l i n g l i p i s n o t

damaged even by

the s l i g h t e s t n i c k , t h a t the s p r i n g i s c o r r e c t l y

l o c a t e d when i n p o s i t i o n , and t h a t t h e seal i s p r o p e r l y pressed home i n t o t h e ho usi n g rece ss. Examine t h e s h a f t which should be f r e e from a l l roughness and sharp edges and a v o i d pa ssi ng t h e s e a l i n g l i p over keyways, screw threads, o r shoulders.

S h a f t edges o r sho ul d ers should be w e l l rounded o r

chamfered, and where t h i s i s n o t p r a c t i c a b l e a f i t t i n g sleeve s l i g h t l y s h a f t w i t h a l e a d - i n t a p e r should be used.

l a r g e r t h an t h e

A c c o r d ing t o t h e t yp e o f assembly,

i t may be necessary e i t h e r t o f i r s t

p r e s s t h e se al i n t o t h e ho usi n g and subsequently o n t o t h e s h a f t o r , alternatively,

t o pass t he se al ove r t h e s h a f t and then press i t i n t o

t h e h o u si ng . I t i s p r e f e r a b l e f i r s t t o mount t h e seal on t h e s h a f t where circumstances p e r m i t , s i n c e t h i s a l l o w s o b s e r v a t i o n o f t h e l i p d u r i n g assembly. ( v i i ) The assembly sh ou l d n o t be al l o we d t o r e s t f o r any l e n g t h o f t i m e a t an in c o mpl e t e s t a t e o f f i t t i n g , where the w eight o f t h e s h a f t o r housing may be borne by t h e s e a l ,

r e s u l t i n g i n damage o r d i s t o r t i o n t o the

latter. ( v i i i ) W h e n p r e s s i n g t h e sea l i n t o t h e housing, a u n i f o r m p r e s s u r e should be e x e r t e d , p r e f e r a b l y by suitable tool.

means o f an a r b o r press i n combination w i t h a

The di a met er o f t h e t o o l should be s l i g h t l y s m a l l e r

than the diameter o f t h e h ou si ng by 0.1

t o 0.4 mm.

The o u t s i d e surface

362

of the seal can be smeared with a suitable lubricant in order to facilitate fitting. Care must be taken to ensure that the seal does not enter the housing recess in a tilted position, since this will cause damage to the outer surface.

SERVICE PROBLEMS AND THEIR SOLUTIONS

15.11

A number of fault-finding procedures follow which, if taken in sequence, should analyse the reason for a given difficulty with rotary shaft lip seals.

15.11.1

Unacceptable Leakage

This is almost always associated with oil lubricated bearings, since grease is not

a

difficult lubricant to seal. The term "unacceptable" can have wide

interpretation since an occasional drop of oil might be disastrous if it resulted in contamination of the product being handled by the machine concerned in such spheres as the textile, paper, or food industries, whereas it would probably remain unnoticed in a heavy industrial environment. Since the vast increase in

oil prices there is, however, a greater sensiti-

vity to oil losses in any form and leakage rates that hitherto have been ignored are now becoming regarded as unacceptable. When dealing with such complaints it is essential to discover the history of equipment concerned, and this broadly

falls into three categories:

(I)

New equipment recently commissioned, where sealing has been regarded

(11)

as unsatisfactory from the start. Equipment that has been in operation for a period of time and only recently has developed leakage problems.

( I l l ) Equipment that was satisfactory

during its first term of operation but

leakage has occurred after fitting replacement seals during routine maintenance or overhaul. Since trouble tracing is basically a process of eliminating of substantiating faults, the sequence of checks required would vary with each of the above

categories. In order to simp1 ify the procedures and'avoid

irrelevant investigations, the

recommended sequences for each of the above categories is defined by letter symbols to be used in conjunction with the attached fault-finding chart. Category ( ) Full checks in order as A, B, C ,

D and

E until fault is discovered.

Category ( I 1 A, B, C ( 1 ) ,

D ( 2 ) , and D ( 3 ) .

If faults as C ( 1 ) o r D(2) are exhibited, ascer-

tain Deriod o f service with seals. This should be calculated in terms of hours

363 o f r u n n i n g and r e l a t e d t o speed, t e m p e r a t u r e , and o t h e r e n v i r o n m e n t a l c o n d i t i o n s

A moderate speed w i t h good c l e a n l u b r i c a t i o n c o n d i t i o n s and ambient temperat u r e s would n o r m a l l y a n t i c i p a t e a seal

l i f e o f around 10,000 h o u r s .

H i g h speeds, p o o r l u b r i c a t i o n , e l e v a t e d temperatures,

o r p a r t i a l l y abrasive

media c o u l d reduce t h i s t o as l i t t l e as 2,000 h o u r s . The p r o b l e m may t h e r e f o r e be

s i m p l y t h a t o f b e i n g due f o r s e a l replacement.

I f f a u l t s as i n (D3) a r e i n e v i d e n c e , o b t a i n d e t a i l s o f a l l media i n c o n t a c t w i t h seal

( i n c l u d i n g any c l e a n i n g f l u i d s ) as a change o f l i p m a t e r i a l may be

necessary t o o b t a i n c o m p a t a b i l i t y . I f s e a l s d i s p l a y no f a u l t s , check f o r mechanical d e f e c t s as E ( 1 ) and E ( 2 ) . Category A,

B, C(

F a u l t - f i n d i n g Chart

(A)

I s leakage a c t u a l l y o c c u r r i n g f r o m t h e s e a l o r does i t stem f r o m such

sources as b e a r i n g c o v e r f l a n g e s and i s m e r e l y " c o l l e c t e d " housing,

g i v i n g a f a l s e impression?

by t h e s e a l

Check by w i p i n g c l e a n a l l a p p r o p r i a t e

a r e a s and r u n machine t o a s c e r t a i n leakage s o u r c e . (B)

I s l e a k a g e f r o m around s e a l back o r f r o m t h e l i p a l o n g t h e s h a f t ?

wiping (C) I f O / D (1)

b o t h c l e a n and o b s e r v i n g w h i l e machine i s leakage

-

Check by

running.

check t h e f o l l o w i n g :

Is s e a l a good f i t i n h o u s i n g o r i s i t s l a c k ? (On s p l i t s e a l s a s l a c k s e a l w i l 1 d i s p l a y a gap between s e a l e n d s ) .

( 2 ) I f h o u s i n g b o r e i s c o r r e c t s i z e t h e n s e a l O / D dimension i s suspect i f s l a c k i n housing. ( 3 ) I f s e a l i s good f i t i n h o u s i n g , check f o r damage on h o u s i n g b o r e .

(4) Check

h o u s i n g d e p t h t o e n s u r e s e a l i s b e i n g a x i a l l y compressed - i f

applicable. (0 )

If

leakage a l o n g s h a f t , check t h e f o l l o w i n g :

( 1 ) S h a f t s i z e , s u r f a c e f i n i s h , s h a f t damage a t c o n t a c t a r e a . ( 2 ) If ( 1 ) O . K . , ing.

check c o n d i t i o n o f s e a l i n g l i p f o r h a r d e n i n g and/or c r a c k -

I f e i t h e r i n e v i d e n c e , t h e n speed o r t e m p e r a t u r e c o n d i t i o n s a r e

probably incompatible w i t h seal m a t e r i a l .

( 3 ) I f l i p i s s o f t o r s w o l l e n t h i s i s u s u a l l y an i n d i c a t i o n o f chemical i n c o m p a t i b i l i t y w i t h t h e media i n c o n t a c t w i t h t h e s e a l .

(4) I f

( 2 ) and ( 3 ) O.K.,

check s e c t i o n w i d t h o f s e a l w i t h s p r i n g f i t t e d .

364 T h i s s h o ul d be a t l e a s t nominal s e c t i o n +1% immediately on removal and i n c r e a s i n g t o nominal

+3% a f t e r

one hour i n f r e e s t a t e .

S p r i n g may be

shortened by up t o 5% o f i t s o r i g i n a l l e n g t h i f s e c t i o n appears inadequate. (E)

I f checks

S e c t i o n measurement sho ul d be average o f

4

e q u i - d i s t a n t readings.

(C) and (D) do n o t r e v e a l any f a u l t s t h e problem may be due t o

mechanical c o n d i t i o n s and t h e f o l l o w i n g should be checked:

( 1 ) S h a f t t o ho usi n g co nce nt r c i t y s h a f t and h ou si ng b o r e a t

4

-

check by means o f c a l i p e r s between

p o i n t s around p e r i p h e r y .

V a r i a t i o n s o f more

t h a n 0.3 mm r e q u i r e f u r t h e r i n v e s t i g a t i o n . (2) I f s m a l l e s t c a l i p e r measurement o c c u r s between bottom o f s h a f t and housing, t h i s may i n d i c a t e b e a r i n g wear w i t h r e s u l t a n t dynamic e c c e n t r i city.

(Applicable t o horizontal shafts only).

( 3 ) I f b e a r i n g i s O.K. t he n ho usi n g o f f s e t may be r e s p o n s i b l e .

Seal housings

a r e n o r m a l l y c e n t r a l i s e d w i t h t h e s h a f t by means o f a machined r e g i s t e r w i t h th e b e a r i n g h ou si ng .

Where t h i s f e a t u r e

i s n o t i n c o r p o r a t e d then

i t may-be p o s s i b l e t o c e n t r a l i s e t h e housing by s l a c k e n i n g t h e b o l t s and repos i t on i ng .

(4)

I f (11,

( 2 ) and (3) a r e blameless t he n t h e f o l l o w i n g r e q u i r e s i n v e s t i -

gation:

I f bear ngs a r e o i l p r e s s u r e l u b r i c a t e d i s t h e r e adequate drainage t o prevent pressure build-up against the seal?

I f equipment op era t e s on a c o n s t a n t o i l l e v e l p r i n c i p l e , wheels o r b a l l - j o u r n a l

a r e t h e r e gear-

b e a r i n g s i n c l o s e p r o x i m i t y t o t h e seal causing

o i l turbulence or flooding? Where

the l a t t e r s i t u a t i o n exists,

t h e housing lands should be o n l y

m a r g i n a l l y l a r g e r t ha n s h a f t d i ame t e r i n o r d e r t o form a b a f f l e o r , alternatively,

a b a f f l e p l a t e f i t t e d between b e a r i n g and seal housing.

A temporary remedy can be made by u s i n g a 2.5 mm CAF gasket a t t h e bottom o f

the housing, t h e I / D o f which sh ou l d be s h a f t diameter p l u s 0.5 nnn maximum.

A

f u r t h e r gasket o f equal t h i c k n e s s sh ou l d be f i t t e d underneath the r e t a i n i n g p l a t e t o restore the correct s e a l s (see Fig .1 1 and F i g . 1 2 ) .

amount o f a x i a l compression i n t h e case o f r e t a i n e d

365

Fig.11

Showing how t o o l a r g e b o r e d i a m e t e r o f s e a l h o u s i n g l a n d p e r m i t s h i g h v e l o c i t y o i l impingement on s e a l .

B a f f l e p l a t e between b e a r i n g reduce r i s k o f o i l leakage.

and s e a l o r c l o s e - f i t t i n g s e a l would

.A.F gasket .5mm thick

Fig.12

Showing how f i t t i n g o f C.A.F. g a s k e t s can p r o v i d e a temporary remedy f o r s i t u a t i o n i n F i g . 1 1 .

( W i t h acknowledgement t o t h e l a t e Mr.Ray B l a d w i n - S e n i o r A d v i s o r y E n g i n e e r , rotary shaft l i p seals

-

who c o m p i l e d much o f t h e d a t a p r e s e n t e d h e r e ) .

366 APPENDIX 1

ROTARY SHAFT L I P SEAL F(ATERIAL RECOMMENDATION CHARTS

The accompanying c h a r t s show v a r i o u s l u b r i c a n t s and r o l l i n g f l u i d s t h a t a r e commonly used i n t h e metal r o l l i n g i n d u s t r y .

Some o f these a r e n o t c h e m i c a l l y

c o m p a t i b l e w i t h t h e standard n i t r i l e rubbers used i n r o t a r y s h a f t l i p seal p r o d u c t i o n , and i t i s t h e r e f o r e necessary t o employ an a l t e r n a t i v e rubber compound t o achieve s a t i s f a c t o r y r e s u l t s i n terms o f s e a l e f f i c i e n c y and l i f e .

Seal L i p s The rubber compound i n t h e column headed " 1 s t choice"

i s t h e grade l e a s t

a f f e c t e d by t h e media concerned and should be s e l e c t e d wherever p o s s i b l e .

In

most i n s t a n c e s a second c h o i c e i s g i v e n and t h i s rubber can be used w i t h o u t serious detriment t o the seal. Where f l u o r o c a r b o n rubber (denoted by t h e l e t t e r 'A') appears as t h e o n l y c h o i c e , please n o t e t h a t none o f t h e c o n v e n t i o n a l commercial grade rubbers can be used as an a l t e r n a t i v e .

I n i n s t a n c e s where f l u o r o c a r b o n rubber appears as

t h e second c h o i c e i t i s u s u a l l y f o r economic reasons,

i n d i c a t i n g t h a t i t i s only

m a r g i n a l l y s u p e r i o r t o t h e commercial grade compound g i v e n as f i r s t c h o i c e , and t h e small advantage would o f t e n n o t j u s t i f y t h e a d d i t i o n a l c o s t . Seal Backs Although w i t h some media t h e seal back

f a b r i c m a t e r i a l s used f o r c o n s t r u c t i o n o f t h e

s u f f e r l i m i t e d v o l u m e t r i c change,

the effects are not detrimental t o

s e a l i n g e f f i c i e n c y as t h i s p a r t o f t h e s e a l i s f u l l y housed and n o r m a l l y a x i a l l y restrained. Selection o f Materials In r o l l i n g - m i l l applications,

r o t a r y s h a f t l i p seals a r e normally required t o

seal n o t o n l y a g a i n s t b e a r i n g l u b r i c a n t s b u t a l s o r o l l i n g f l u i d s ,

be taken t o s e l e c t a m a t e r i a l which i s c o m p a t i b l e w i t h both.

and c a r e should

T h i s may i n v o l v e

some compromise by s e l e c t i n g a second c h o i c e m a t e r i a l f o r e i t h e r t h e l u b r i c a n t o r the r o l l i n g f l u i d ,

i n o r d e r t o o b t a i n a reasonable degree o f c o m p a t i b i l i t y

w i t h both. I n d u a l s e a l i n g arrangements employing back-to-back

s e a l s , should o n l y one o f

t h e media n e c e s s i t a t e f l u o r o c a r b o n r u b b e r , t h e opposing s e a l c o u l d be made w i t h a l e s s c o s t l y c o m p a t i b l e grade o f rubber p r o v i d i n g t h a t c a r e i s taken i n i d e n t i f i c a t i o n and f i t t i n g . For convenience, rubbers, and

l e t t e r symbols a r e used i n t h e c h a r t s t o i n d i c a t e v a r i o u s

t h e key t o these i s g i v e n :

367 KEY TO MATERIAL CODES Code

Base r u b b e r compound

F I uorocarbon High n i t r i l e Medium h i g h n i t r i l e N i t r i le N i t r i l e with graphite N i t r i l e w i t h p a r a f f i n wax

RECOMMENDED SEAL LIP MATERIALS FOR USE WITH BEARING OILS AND GREASES Seal l i p m a t e r i a l s Brand Name o r Number 1 s t Choice

2nd Choice

B.P.

HCT 80

C

E

B.P.

Sperno 350 HB 'I 450 HB

C

D

B.P.

Energol GR 125-XP

C

0

E

C a s t r o l 98

C

CLP 1 1 4

C

E

C a l y s o l Grease

C

A

C

Duckhams Zero O i l

D

EP 69 O i l

C

D

EP 80 Gear L u b r i c a n t

D

C

Esso E s t i c 65

C

A

Esso Nuray 146

C

A

Esso Pen-0-Led

D

C

EP3

Fuchs MR 40

C

F

Mobi 1 Exu 66/25

C

D

C

F

C

E

C

E

" II

Vacuol i n e ' A A ' 'I 25 x 25R

I,

I'

Heavy

C

E

I'

Mobi l u b e H.D.90

C

E

"

Mobi l e x EP2 Grease

C

F

OM 100 O i l

C

A

Regal Gear O i l

C

B

S h e l l O i l 1624 and 1611

B

C

,I

II

I

EE"

"

Carnea

C

B

"

Faunus ' B '

B

A

I'

Macoma

D

C

"

Nassa 78

C

A

Seal l i p m a t e r i a l s f o r b e a r i n g o i l s and greases (contd.) Seal l i p m a t e r i a

S

Brand Name o r Number 1 s t Choice

2nd C Dice

C

A

Teressa

C

A

"

T i v e l a 75

C

D

"

V i t r e a 75

C

B

"

R e t i n o x Grease

A

C

She1 1 Telona 945 'I

'I

Donax 17

C

B

I'

Aeroshel 1 Grease No.7

C

A

Oil

C

D

Ucon

A l o - J idac

A

Caster O i l ( w i t h w a t e r ) I, I' ( w i t h methyl a t e d

A

B

C

E

C imcoo 1

E

A

Cirncool E 5

B

C

Castrol

A

B

B

A

spirit)

Coolage SL

Croda D4 A Lubrotex

'I

1;

7804

A

Dasco 900

B

A

Esso Somentor N35

C

A

I,

,I

'I

w20

A

6

I'

33 and N60

A

C

Univis ~ 5 8

C

B

Emulsion KF 81

A

-

Germ K i n e t r o l FR3A

A

-

G u l f Mineral Seal

C

D

I'

Cut O i l

C

D

"

93

C

D

B

A

I'

Houghton Permasol

k

B

H u i l e de Laminage 102 P & 982 R

B

C

Hydr i t

B

D

KF 61 R o l l i n g Oil

A

D

Lubricor ' T I

B

E

"

S u l f o n a ED

369 Seal l i p m a t e r i a l s f o r r o l l c o o l a n t s and r o l l i n g o i l s ( c o n t d . )

Seal l i p m a t e r i a l s Brand Name o r Number

1 s t Choice

Mobil Generex 56, 57 and 404D 11 11

"

22 and 24

11

322

A

B

C

B

C

D

P r o s o l 66

A

B

44

B

A

33

A

Solvac 11

A

-

B

A

11

II

'I

2nd Choice

~

~~~

11

11

a00 and "BB"

C

B

M i r o b o 415A

B

A

Ocut G

C

A

Palm O i l

B

A

Quaker T i n n o l 12

E

D

B

D

B

A

B

A

V a c t r a "HH"

I'

11

109

I1

'I

I,

Quakerol 41 ( w i t h S h e l l Carnea 31) 43, 82, a7 M and

88-182 M I'

Qwerl 506

Rol l u p 200 S h e l l Dromus

B

B

D

B

A

B

A

C

B

'I

1270/1307

B

A

I'

1076

D

C

Tayol 316 Emulsion

B

A

Texaco Texatherm 320

A

C

S t e r n o l PL106 and PL107

T r e l l u b 12 A

B

A

Wyrol H40

C

A

370

I(;

SEALS FOR FLUID POWER EQUIPMENT PART THREE COMPRESSION PACKINGS

B.D. HALLIGAN, C.Eng, MIMechE, AMPRI Technical Manager (Product Appl i c a t i o n s ) James Walker and Co. L t d .

16.1

THE PACKED GLAND

Compared t o t h e f i n i t e q u a l i t i e s o f f e r r o u s m e t a l s f o r example, t h e essenti a l l y deformable n a t u r e o f s e a l i n g m a t e r i a l s has i n t r o d u c e d a measure o f v a r i a b i l i t y t h a t causes many commentators t o l o o k on f l u i d s e a l i n g technology as an a r t r a t h e r than a science.

I f t h i s i s t r u e , and manufacturers o f mechan

i c a l f a c e s e a l s would be b u t one area o f v a l i d o b j e c t i o n , then t h e f i e l d o f compression packings i s , a r g u a b l y , t h e b l a c k e s t area o f t h a t a r t . Regarded as an anachronism i n a p e r i o d o f h i g h t e c h n o l o g i c a l achievement, compression packings show no s i g n s o f l o s i n g s i g n i f i c a n t ground i n terms o f p r o d u c t i o n q u a n t i t i e s as new and improved t y p e s p r o l i f e r a t e b o t h i n Europe and elsewhere.

To understand t h i s s i t u a t i o n r e q u i r e s some a p p r e c i a t i o n o f t h e

fundamental mode o f o p e r a t i o n o f t h e a d j u s t a b l e g l a n d o r s t u f f i n g box shown i n Fig. 1.

SYSTEM __c PRESSURE

Fig.1

COMPRESSIVE FORCE &THROUGH GLAND SPIGOT

f

h

Compression Packing

t

Y

371 T h is may be f i l l e d w i t h s p l i t pa cki ng r i n g s chosen from a v a r i e t y o f m a t e r i a l s and c o n s t r u c t i o n s , de scri be d elsewhere, which a r e persuaded to r e a c t a g a i n s t a s h a f t , whether r o t a r y o r r e c i p r o c a t i n g , t o t h e e x t e n t t h a t t h e r a d i a l f o r c e developed exceeds t h e p r e s s u r e t o be sealed. Such a p r i n c i p l e c o u l d n o t be m r e elementary and i t s i n t r i n s i c value c o u l d be f u r t h e r q u e s t i o n e d as pa cki ng s i n t h i s c a t e g o r y used f o r r o t a t i n g o r r e c i p r o c a t i n g equipment r e l y on a c o n t r o l l e d leakage f o r l o n g - t e r m l u b r i c a t i o n purposes,

i f t h e y a r e t o s u r v i v e f o r an adequate p e r i o d .

The c o n t i n u e d j u s t i f i c a t i o n for t h e compression packing might appear obscure a g a i n s t such a background b u t t h e r e can be no doubt t h a t c e r t a i n areas o f appl i c a t i o n e x i s t where no reasonable s u b s t i t u t e i s a v a i l a b l e .

16.1.1

Pumps

Many reasoned and w e l l - r e s e a r c h e d papers have been p u b l i s h e d t o support mechanical s e a l s a g a i n s t s o f t p acki n g and v i c e versa.

There i s no doubt t h a t

th e former have sup pl a nt ed packed g l an ds as o r i g i n a l equipment on the m a j o r i t y o f rotodynamic pumps f o r a v a r i e t y o f process and s e r v i c e f l u i d s b u t they a r e o p e r a t i n g parameters and c o s t c o n s i d e r a t i o n s w hich w i l l f r e q u e n t l y d i c t a t e t h e choice o f s o f t p a cki ng . Table 16.1 compares t h e r e l a t i v e a t t r i b u t e s o f t h e two contenders i n b a s i c terms. I n g e n e r a l i t may be s a i d t h a t , u n l e s s ze ro leakage i s an a b s o l u t e p r i o r i t y , compression p a c k i n gs w i l l r e t a i n an i m p o r t a n t p o s i t i o n wherever r e g u l a r maintenance i s a v a i l a b l e and t h e f o l l o w i n g c o n s i d e r a t i o n s a p p l y :

-

s i m p l i c i t y i n g l a n d d esi g n and a n c i l l a r y equipment ease o f f i t t i n g f l e x i b i l i t y o f su pp l y and spares f o r p l a n t u t i l i z i n g many d i f f e r e n t t y p e s and s i z e s o f pump h a n d l i n g a wide v a r i e t y o f f l u i d s

-

f r e q u e n t a b i l i t y t o c a t e r f o r adverse c o n d i t i o n s w i t h o u t e l a b o r a t e precautions

16.1.2

Valves

I f any doubt e x i s t s r e g a r d i n g s e l e c t i o n on pumps then a much more obvious

c h o i c e o f s o f t p a cki ng a p p l i e s t o t h e v a l v e scene. ment, ease o f f i t t i n g and,

The r e l a t i v e l a c k o f move-

i n t h i s case, l a c k o f leakage requirement f o r l u b -

r i c a t i o n purposes p l u s t h e most d e c i s i v e advantage o f low c o s t , a r e f a c t o r s whic h i d e a l l y r e l a t e t o compression packings. There a r e a r e as where moulded e l a s t o m e r i c s e a l s p r e s e n t a reasonable a l t e r n a t i v e b u t even t h e most e x o t i c compounds would seldom be used above 250°C u n l e s s r e i n f o r c e d by asbestos f a b r i c .

-

w

4 N

TABLE 16.1

Compa r i son I n i t i a l Cost

Re1 i a b i 1 i t y

S o f t Packing

Mechan ica 1 Sea 1

o f t h e o r d e r o f 1 O : l i n f a v o u r o f s o f t packing depending on s i z e and a p p l i c a t i o n APPROXIMATELY EQUAL ample warning o f impending f a i l u r e l i t t l e or no warning o f with p o s s i b i l i t i e s f o r correction end o f u s e f u l l i f e w i t h p o s s i b i l i t y o f sudden complete f a i l u r e

-

Installation

e s s e n t i a l l y simple - r e q u i r i n g no s p e c i a l s k i l l s i f c o r r e c t procedure adopted

s k i l l e d f i t t i n g required p r e c i s e l y defined environment and assembly

Maintenance

r e g u l a r and r e q u i r i n g experience

zero

Spares

f a c i l i t y f o r s t o c k i n g l e n g t h form m a t e r i a l o r complete pre-formed s e t s a t r e l a t i v e l y low c o s t

spare seal components must c o s t can be be a v a i l a b l e substantial

S h af t Wear

can be c o n s i d e r a b l e ; s h a f t sleeves reduce replacement c o s t s

nil

-

f r i c t i o n losses s l i g h t l y h i g h e r w i t h s o f t packing Op era t i n g Costs

leakage losses zero w i t h mechanical s e a l s b u t p o s i t i v e w i t h s o f t packing as l u b r i c a t i o n o f s e a l i n g r i n g s i s e s s e n t i a l

373 16.2

OPERATING PRINCIPLES

By comparison t o t h e se al t yp es d escri b ed i n t h e o t h e r papers e l a s t o m e r i c l i p and squeeze s e a l s

-

-

particularly

compression packings respond t o a p p l i e d

pre s s u r e i n i n v e r s e p r o p o r t i o n to t h e hardness o f t h e i r c o n s t r u c t i o n and r e l y on an e x t e r n a l f o r c e t o produce t h e r a d i a l p ressure r e q u i r e d f o r e f f e c t i v e sealing.

The method o f g e n e r a t i n g t h a t f o r c e can v a r y b u t u s u a l l y (and p r e f e r -

a b l y ) i n v o l v e s a b o l t e d q l a n d s p i g o t as shown i n Fig.1 where c o n t r o l l e d a x i a l movement i s e a s i l y ach i eve d by a dj u st me nt o f t h e r e t a i n i n g n u t s o r studs. Spring l o a d i n g i s sometimes used i n i n a c c e s s i b l e s i t u a t i o n s b u t such a p r o v i s i o n l a c k s t h e f i n e c o n t r o l demanded by some packing types and has a l i m i t e d range o f l o a d c a p a b i l i t y . W h i l s t t h e s e a l i n g f o r c e can be a d j u s t e d t o c a t e r f o r s e r v i c e wear, care must be t a k e n t o a v o i d o v e r compression which w i l l lead t o excessive f r i c t i o n , s h a f t wear and premature p acki n g f a i l u r e . To in c r e a s e d e n s i t y and d i s s i p a t e he at , s o f t packings i n v a r i a b l y contain. l u b r i c a n t s , l o s s o f which, t hro ug h e xcessi ve compression o r o v e r - h e a t i n g i n s e r v i c e , w i l l r e s u l t i n pa cki ng volume l o s s w i t h subsequent r e d u c t i o n i n t h e e f f e c t i v e s e a l i n q r e a c t i o n and c o r r e s p o n d i n g l y i n c r e a s i n g leakage r a t e s .

By

l i m i t i n g compression t o a p o i n t where s l i g h t c o n t r o l l e d leakage i s obtained, adequate l u b r i c a t i o n o f t h e dynamic s u r f a c e s i s ensured and over-compression o f th e p a c k in g avoided.

However, where l u b r i c a t i o n i s a problem

gl a n d c o o l i n g i s r e q u i r e d

-

-

o r a degree o f

a l a n t e r n r i n g can be i n c o r p o r a t e d i n t o t h e gland

area f o r t h e d i s t r i b u t i o n o f a d d i t i o n a l l u b r i c a n t / c o o l a n t (Fig.2a).

The p o s i t i o n

o f a l a n t e r n r i n g w i l l depend on t h e n a t u r e o f t h e a p p l i c a t i o n b u t , s i n c e t h e pa c k in g r i n g s n e a r e s t t h e g l a n d s p i g o t do most o f t h e work, t h e a d d i t i o n a l f l u i d should u s u a l l y be i n t r o d u c e d near t o t h a t area.

ADDITIONAL LUBRICANT/COOLANT

I

RING

DISTRIBUTION PORTS

BARRIER FLUID

374 I f i t i s e s s e n t i a l t h a t t h e f l u i d b e i n g pumped does n o t escape t o atmosphere

(e.g.

a t o x i c medium) t h e l a n t e r n r i n g may serve t o i n t r o d u c e a b a r r i e r f l u i d

a t a p r e s s u r e o f 0.5 t o 1 b a r above t h a t t o be sealed ( F i g . 2 b ) .

Similarly,

where t h e r e i s a r i s k o f severe a b r a s i v e wear t o t h e packing, a f l u s h i n !

fluid

may be i n t r o d u c e d throuoh t h e l a n t e r n r i n g ( F i g . 2 c ) . For a p p l i c a t i o n w i t h n e c a t i v e pump p r e s s u r e s ( i . e .

s u c t i o n ) a supply o f t h e

medium b e i n g sealed can be made through t h e l a n t e r n r i n g t o p r e v e n t a i r - d r a w i n g (Fig.2d).

SUPPLY OF SEALED MEDIUM TO PREVENT AIR-DRAWING

FLUSHING FLU I D

I

I

I f extreme temperatures a r e t o be encountered,

i t i s unlikely that cooling

through t h e l a n t e r n r i n g w i l l be s u f f i c i e n t and recourse must be made t o i n t e r n a l c o o l i n g o f t h e g l a n d housing and s h a f t t o reduce t h e temperature a t t h e gland t o a v a l u e w i t h i n t h e p a c k i n g ' s c a p a b i l i t i e s .

Conversely, when d e a l i n g

w i t h media which c r y s t a l l i z e o r congeal when c o o l (e.g.

sugars, t a r s , e t c . ) ,

t h e p a c k i n g w i l l f a c e r a p i d d e s t r u c t i o n u n l e s s g l a n d h e a t e r s o r a steam j a c k e t e d arrangement a r e employed t o r e s t o r e t h e f l u i d s t a t e b e f o r e s t a r t i n g up. I t should always be remembered t h a t t h e i n c l u s i o n o f a l a n t e r n r i n g i n t o t h e gland area i n v a r i a b l y c o m p l i c a t e s assembly and can p r o v i d e a p o s s i b l e source o f shaft scoring;

t h e y should, t h e r e f o r e , o n l y be c o n s i d e r e d when t h e n a t u r e o f

the a p p l i c a t i o n a b s o l u t e l y demands t h e i r presence.

16.3

GLAND DESIGN

A t t h i s juncture,

few i n t e r n a t i o n a l standards e x i s t t o d e f i n e housing design

f o r s o f t packings b u t t h e dimensions shown i n Table 16.2 should be s a t i s f a c t o r y f o r most a p p l i c a t i o n s .

Housing depths w i l l v a r y w i t h i n d i v i d u a l circumstances,

such as t h e i n c l u s i o n o f a l a n t e r n r i n g , b u t f i v e r i n g s o f square s e c t i o n packings a r e u s u a l l y recommended f o r t h e average,

uncomplicated d u t y .

375 TABLE 16.2

Suggested h ou si ng w i d t h s i n r e l a t i o n t o s h a f t diameters. A l l dimenZions i n mm.

A l l p a c k in g s e xce pt expanded g r a p h i t e Shaft Diameter

Housing Width

Expanded g r a p h i t e Sh aft Diameter

Housing Width

up t o 12

3

up t o 18

3

above 12 t o 18

5

above 18 t o 75

75 t o 150 150 and above

5 7.5

18 t o 25

6.5

25 t o 50

8

50 t o 90

10

90 t o 150

12.5

150

15

10

Other d e s ig n c o n s i d e r a t i o n s wort hy o f n o t e , b u t o f t e n overlooked, can be summarized a s :(i)

The p r o v i s i o n o f an adequate t a pe red ' l e a d i n ' a t t h e mouth o f t h e g l a n d t o f a c i l i t a t e e n t r y o f t h e pa ckina and t o o b v i a t e t h e r i s k o f damage i n t h e assembly o p e r a t i o n .

A minimum o f 15" x 6.5mm u s u a l l y

r e p r e s e nt s good p r a c t i c e . (ii)

The p r o v i s i o n o f a reasonable s u r f a c e f i n i s h on adjacent metal p a r t s p a r t i c u l a r l y t he dynamic su rf a ce. w i l l o c cu r;

The b e t t e r the f i n i s h t h e l e s s wear

0.4um (16 p i n ) CLA o r Ra on the s h a f t and 1.6 pm

(54 u i n ) CLA o r Ra on t h e s t u f f i n g box bore should be i d e a l f o r most applications.

The use o f s h a f t sleeves can g i v e c o n s i d e r a b l e main-

tenance advantage when c o n s i d e r i n g t h e q u e s t i o n o f s u r f a c e f i n i s h . (iii)

The danger o f extreme r u n n i n g cl e ara nces a t t h e g l a n d on t h e s p i g o t si d e.

-

particularly

I n those e x c e p t i o n a l cases where excessive

c le a r a n c e i s u na voi d ab l e, t h e p acki n g should be p r o t e c t e d by an independent r i n g o f s u i t a b l y r o b u s t m a t e r i a l o r c o n s t r u c t i o n which reduces t h e c l e a r a n c e t o a minimum. (iv)

An a llo wa nce for e n t r y o f t h e g l a n d s p i g o t w e l l i n t o the gland area; c e r t a i n l y t o an e x t e n t t h a t exceeds s u b s t a n t i a l l y t h e depth o f the ta p e r e d l e a d i n .

The l e n g t h o f s p i g o t s e l e c t e d must a l s o c a t e r f o r

p a c k in g compression, r e s u l t i n g f rom gland adjustment.

Typical e n t r y

l e n g t h s sho ul d be a t l e a s t two t i mes packing s e c t i o n .

For packings

o f s o f t e r c o n s t r u c t i o n , maximum l e n g t h should be provided.

With

modern p acki n g m a t e r i a l s , b e v e l l e d glands a r e seldom an advantage and can a c t u a l l y promote movement o f t h e s e a l i n g r i n g on t h e s p i g o t s i d e i n t o t h e l i v e cl ea ran ce.

-

376 (v) (vi)

The need t o a v o i d e x c e s s i v e s h a f t misalignment o r whip. The p r o v i s i o n o f adequate s h a f t support.

The p a c k i n g must

n o t be used a s a b e a r i n g .

16.4

PACKING CONSTRUCTION AND MATERIALS

16.4.1

Fibre Material

Mineral

asbestos

Vegetable

cotton

Synthetic

aramid

flax

glass

jute

graphite filament

ramic

graphite f o i l

sisal

nylon polytetrafluoroethylene (PTFE) rayon

16.4.2 Dry

Lubricants Metals

graphite

aluminium f o i l

talc

copper f o i l and w i r e

molybdenum d i s u l p h i t e Wet

l e a d f o i l and w i r e

mica

brass w i r e monel w i r e

tallow

inconel w i r e

castor o i l s t r a i g h t mineral lubricating o i l

stainless steel wire Elastomers

n a t u r a l and s y n t h e t i c

petrolatum

sol i d f r a c t i o n s p a r a f f i n wax soaps s i l i c o n e grease PTFE d i s p e r s i o n s 16.4.3

Construction

The p r i n c i p a l forms o f c o n s t r u c t i o n s f o r f i b r o u s compression packings a r e : (i)

Braided

I n d i v i d u a l yarns a r e b r a i d e d tube o v e r tube and squared o f f .

The d e n s i t y o f t h i s t y p e o f c o n s t r u c t i o n i s h i g h and i d e a l f o r many valve applications. (ii)

Plaited

M u l t i p l e y a r n s a r e interwoven i n p l a i t e d bundles i n such a

fashion that the d i r e c t i o n o f f i b r e f o l l o w s the periphery o f the packing r i n g .

The n a t u r a l c h a r a c t e r i s t i c o f t h i s c o n s t r u c t i o n i s more

s u i t e d t o c e n t r i f u g a l pump a p p l i c a t i o n s than v a l v e s e r v i c e a l t h o u g h t h e i n h e r e n t f l e x i b i l i t y o f t h e f o r m i s p o p u l a r w i t h some users.

Fig.3 (iii)

Basic p a c k i n g c o n s t r u c t i o n s Cross P l a i t

A l l t h e y a r n s a r e i n t e r l o c k i n g and pass d i a g o n a l l y through

t h e p a c k i n g t o p r o v i d e a f i r m c o n s t r u c t i o n o f c o n s i s t e n t d e n s i t y and shape.

Used e x t e n s i v e l y f o r s y n t h e t i c y a r n packings f o r v a l v e s and

pumps. (iv)

Composite asbestos p l a s t i c

This packing category i s o f f a i r l y recent

o r i g i n and i n c l u d e s those t y p e s based upon a b r a i d e d and r e i n f o r c e d asbestos j a c k e t e n c l o s i n g a ' p l a s t i c ' c o r e . t h e s c i e n t i f i c sense,

Although i n a c c u r a t e i n

t h e t e r m ' p l a s t i c ' c o n v e n i e n t l y d e s c r i b e s those

many m i x t u r e s o f asbestos f i b r e and l u b r i c a n t , b o t h m i n e r a l and s o l i d , from which r e a d i l y deformable p a c k i n g m a t e r i a l may be made.

This

p a c k i n g i s w i d e l y accepted f o r d i f f i c u l t v a l v e s e a l i n g d u t i e s .

A l 1 t h e f i b r e - b a s e d c o n s t r u c t i o n s d e s c r i b e d here a r e f r e q u e n t l y r e i n f o r c e d w i t h metal.

T h i s a p p l i e s p a r t i c u l a r l y t o asbestos'based

p r o d u c t s where t h e use

o f metal w i r e i n t h e y a r n can e x t e n d t h e s e r v i c e c a p a b i l i t y o f t h e packing t o

800°C and beyond.

But f o r t h i s f e a t u r e , even t h e b e s t q u a l i t y n o n - m e t a l l i c

asbestos y a r n p a c k i n g would be r e s t r i c t e d t o temperatures o f about 315°C maximum.

A l l o f t h e l u b r i c a n t s shown a r e used i n c o n j u n c t i o n w i t h f i b r e packings o f d i f f e r e n t s o r t s and a r e a p p l i e d by d i p p i n g , c o a t i n g , soaking, nation, dusting, etc.

vacuum impreg-

The prime o b j e c t i s maximum l u b r i c a n t r e t e n t i o n .

Fre-

q u e n t l y , s e v e r a l t r e a t m e n t s and r e p e a t processes a r e employed t o achieve t h i s end.

318

F o i l wrapped deformable asbestos core.

F o i l c r i n k l e d , t w i s t e d and folded upon i t s e l f .

Corrugated f o i l , concertina wound.

Fig.4

T y p i c a l metal f o i l based packing c o n s t r u c t i o n

Fig.5

D o u b l e - b e v e l l e d , e l a s t o m e r p r o o f e d f a b r i c packing f o r a b r a s i v e d u t i e s

379 I n t h e f i e l d o f compression packings, e l a s t o m e r s a r e n o t w i d e l y used a l t h o u g h some b r a i d e d packings do employ y a r n s t h a t a r e t r e a t e d w i t h a rubber p r o o f i n g t o render them more s u i t e d t o d i f f i c u l t wet a p p l i c a t i o n s such as condensate d u t y . Rings o f square o r r e c t a n g u l a r s e c t i o n compression packing, manufactured from folded, r o l l e d o r l a m i n a t e d e l a s t o m e r p r o o f e d c l o t h , a r e s t i l l p o p u l a r f o r r e l a t i v e l y slow moving, lower p r e s s u r e r e c i p r o c a t i n g pumps h a n d l i n g water o f LP steam. One p a r t i c u l a r design, w i t h a moulded, d o u b l e - b e v e l l e d s e c t i o n , made f r o m semi-metallic

rubberised yarn,

i s p a r t i c u l a r l y e f f e c t i v e on r o t a r y a p p l i c a t i o n s

dealing w i t h v i s c o u s media which s o l i d i f y when t h e pump i s i d l e and cause damage t o conventional p l a i t e d packings on r e s t a r t i n ? f r o m c o l d .

T h i s moulded p a c k i n g

i s a l s o s u i t a b l e f o r d u t i e s i n v o l v i n g s o l i d s and a b r a s i v e s (See F i 0 . 5 ) .

16.4.4

Additional Materials

In h i g h performance r o t a r y and r e c i p r o c a t i n g packings, t h r e e m a t e r i a l s which may be regarded as i m p o r t a n t a d d i t i o n s t o t h e seal m a n u f a c t u r e r s ' armoury have become a v a i l a b l e i n t h e l a s t two decades and p l a y a h i q h l y s i g n i f i c a n t p a r t i n extending t h e f r o n t i e r s o f performance o f t h e t r a d i t i o n a l s o f t p a c k i n g : ( i ) PTFE

P o l y t e t r a f l u o r o e t h y l e n e y a r n s p r o v i d e s o f t packings f o r s e r v i c e s

where c o r r o s i v e media a r e b e i n g handled o r freedom f r o m c o n t a m i n a t i o n i s an e s s e n t i a l requirement.

A s e m i - r i g i d f l u o r o c a r b o n p l a s t i c , PTFE i s unique i n possessing almost complete chemical r e s i s t a n c e w i t h i n i t s temperature range which, f i e l d o f a p p l i c a t i o n , spans t h e c r y o g e n i c area t o 250°C.

in this

Another major

advantage r e f e r s t o i t s v e r y low c o e f f i c i e n t o f f r i c t i o n . L u b r i c a t e d p l a i t e d PTFE y a r n packings a r e s u i t a b l e f o r r o t a r y s u r f a c e speeds up t o 8 m/s and a r e a l s o f i n d i n g i n c r e a s i n g acceptance on h i g h speed, h i g h pressure, m u l t i - r a m r e c i p r o c a t i n g pumps. I n s o l i d f o r m t h i s m a t e r i a l i s n o t a c c e p t a b l e as a compression p a c k i n g due t o poor creep p r o p e r t i e s and l a c k o f r e s i l i e n c e .

However, s o l i d

j u n k r i n g s o r spacers i n PTFE a r e o f t e n used t o enhance p a c k i n g p e r formance on arduous pump d u t i e s . ( i i ) Aramid f i b r e

C r o s s p l a i t y a r n packings made f r o m aramid f i b r e , u s u a l l y

o f a d i s t i n c t i v e y e l l o w c o l o u r , a r e becoming i n c r e a s i n g l y p o p u l a r f o r a v a r i e t y o f pump and v a l v e s e r v i c e s h i t h e r t o s a t i s f i e d by PTFE

l u b r i c a t e d asbestos packings.

380 The y a r n has h i g h t e n s i l e s t r e n g t h ,

e x c e l l e n t r e s i l i e n c e , thermal

s t a b i l i t y up t o 250°C and i s r e s l s t a n t t o a wide range o f chemicals. Aramid f i b r e p acki n gs a r e s u i t a b l e f o r speeds up t o 15 m/s and g i v e im p r e s s iv e r e s u l t s w i t h a b r a s i v e media. ( i i i) G r a p h i t e ( a ) Yarn p a cki ng s i n t h i s m a t e r i a l a r e a development f o r r o t a r y pump a p p l i c a t i o n s and p r o v i d e p o s s i b i l i t i e s f o r e x t e n d i n g t h e range o f t h e packed g l a n d beyond bo un da ri es h i t h e r t o e s t a b l i s h e d .

A h i g h c o e f f i c i e n t o f t h erma l c o n d u c t i v i t y ,

low f r i c t i o n and r e s i s t a n c e

t o chemical a t t a c k a r e t h e u s e f u l c h a r a c t e r i s t i c s o f t h i s m a t e r i a l . Temperatures up t o 400°C may be co nsi d ered. I f a good performance i s t o be o b t a i n e d , then c l o s e a t t e n t i o n must be p a i d t o mechanical c o n d i t i o n s such as s h a f t run o u t and f i n i s h .

Care

i n f i t t i n g and r u n n i n g - i n i s a l s o mandatory. ( b ) Expanded g r a p h i t e f o i l i s t h e r e c e n t and dramatic a p p l i c a t i o n o f graphite, p a r t i c u l a r l y i n the context o f valve applications. g r a p h i t e m a t e r i a l s combine t h e w e l l - e s t a b l i s h e d characteristics,

Expanded

thermal and f r i c t i o n

l o n g a s s o c i a t e d w i t h t h e c o r r e c t l y developed use o f

carbon based p r o d u c t s , w i t h a unique f l e x i b i l i t y and r e s i l i e n c e .

The

a t t r i b u t e s o f t h i s e x f o l i a t e d f o rm o f g r a p h i t e bear r e c o r d i n g .

-

e x c e l l e n t r e s i s t a n c e t o compression s e t r e s u l t i n g i n l i t t l e l o s s o f r a d i a l gland f o r c e o r flange seating s t r e s s over long periods (see Table 16.3)

-

no l o s s o f v o l a t i l e s even a t h i g h t emperature thus m i n i m i s i n g frequency o f g l an d ad j ust men t

-

h i g h t e mpe rat ure c a p a b i l i t y p a r t i c u l a r l y i n n o n - o x i d i s i n g environments

-

-

h i g h thermal c o n d u c t i v i t y low f r i c t i o n p r o p e r t i e s

-

self lubricating

e x c e p t i o n a l l y low c h l o r i d e c o n t e n t no adhesion o r c o r r o s i o n problems fire-safe

381 TABLE 16.3 Compress i b i 1 it y / r e c o v e r y ASTM F36-66 Procedure H (Major lo a d 7/mm2)

Expanded graphite lmm t h i c k 1 .o g/cc

(1000 l b f / i n Z )

Expanded graphite 2mm t h i c k 1.0 g/cc

compressibility

39%

33%

recovery

20%

25%

40 N/mm2

40 N/mm2

Stress r e l a x a t i o n BS 1832:1972 Temperature 300°C f o r 16 h r s I n i t i a l s t r e s s o f 40 N/mm2 Residual s t r e s s

N.B.

10 N=l k g f 1 N/mmz=lO kgf/cm2 approx.

Fig.6 shows an i n t e r e s t i n g comparison o f performance on a t e s t g l a n d between h a l f r i n g s o f expanded g r a p h i t e and a l u b r i c a t e d asbestos y a r n packing.

Not

only d i d t h e fo r me r r e q u i r e fewer g l a n d ad j ustments d u r i n g t h e p e r i o d o f t e s t i n g but t h e average leakage r a t e was much l e s s

-

t o the p o i n t o f r u n n i n g v i r t u a l l y

dry f o r p r o t r a c t e d p e r i o d s . Rather t h a n use t h e t a pe f o r m o f expanded g r a p h i t e which i s p r i m a r i l y a usef u l maintenance e x p e d i e n t , moulded r i n g s t o a s e l e c t e d and c o n t r o l l e d d e n s i t y should be t h e f i r s t c h o i c e f o r pump and v a l v e glands. A lt h o u g h more c o s t l y t ha n c o n v e n t i o n a l pa cking m a t e r i a l s , economies o f r a d i a l w i d t h and number o f r i n g s used a r e f e a s i b l e q u i t e a p a r t f r o m t h e performance advantage l i k e l y t o be d e r i v e d f rom t h e use o f expanded g r a p h i t e . One cannot le ave m a t e r i a l s w i t h o u t s p e c i a l r e f e r e n c e t o t h e v i t a l r o l e played by asbestos

-

a much d e n i g r a t e d m i n e r a l f i b r e w i t h o u t w hich economic and prac-

t i c a l s o l u t i o n s t o many s e a l i n g problems would n o t be f e a s i b l e .

Although under-

s ta n d a b le , i n an age o f c o r r e c t awareness o f h e a l t h and s a f e t y m a t t e r s , t h e o v e r - r e a c t i o n a g a i n s t asb est os has r e v e a l e d many i n c o n s i s t e n c i e s .

Motor manuf-

a c t u r e r s may p r o h i b i t i t s use as a p l a n t maintenance m a t e r i a l b u t c o n t i n u e t o use asbestos i n a b r a k e - l i n i n g and c l u t c h - f a c i n g r o l e where r e s i d u a l dust i s evident.

Some u sers may seek t o l i m i t i t s a p p l i c a t i o n i n a safe form as a v a l v e

packing o r gasket b u t p e r p e t u a t e i t s s p e c i f i c a t i o n f o r f i r e - p r o o f p o s i t i o n s and r o o f i n g where t h e m i n e r a l i s c u t i n a d r y form. Hazards e x i s t b u t adherence t o b a s i c a d v i c e on h a n d l i n g asbestos w i l l r e s u l t i n a s e n s i b l e balance between p r o h i b i t i o n and p r a c t i c a l i t y .

In t h i s context,

users o f asbestos based pump and v a l v e pa cki ngs, gaskets o r a l l i e d components might heed, t o advantage, t h e statement i ssu ed by t h e Asbestos I n f o r m a t i o n Committee t o t h e B r i t i s h Valve M a n u f a c t u r e r s ' A s s o c i a t i o n , appended t o these no t e s.

a copy o f which i s

D I S T I L L E D WATER TEMPERATURE OF MEDIUM

TEST CONDITIONS:

- 9OoC - 3.45

PRESSURE OF MEDIUM

bar

U x p a n d e d graphite rings ( i n halves)

-

Lubricated asbestos yarn rings ( s p l i t )

I000 900 800

700 600

‘9L

500 400 300

200 100

0

2.5 m/s

5 m/s

7 . 5 m/s

12.5 m/s

10 m/s P

SHAFT SURFACE SPEED

2.5 2.0

I .5 1 .o

0.5

0

I--

---

I - - - -

.J

& 7.5 m/s

10 m/s

12.5 m/s

SHAFT SURFACE SPEED

I

5

10

1

15 TOTAL TEST TIME-HOURS

20

25

30

383

16.4.5

Selection

The most v e x i n g q u e s t i o n as, f o r many d u t i e s , exist.

so many reasonable a l t e r n a t i v e s

Much w i l l depend on p erso na l e xpe ri en ce, frequency o f maintenance,

o r i g i n a l cost l e v e l , contamination considerations, size, e t c . I n t h e case o f a man uf act ure r pro du ci ng l a r g e q u a n t i t i e s o f v a l v e s o r pumps t o standard dimensions, t h e r e i s much t o be s a i d f o r purchasing sets o r r i n g s r a t h e r t h a n l e n g t h form packing.

W i t h t h e t echniques a v a i l a b l e , packing can be

s u p p lie d ready f o r immediate f i t t i n g w i t h s u b s t a n t i a l r e d u c t i o n i n t h a t o v e r a l l c o s t r e p r e s e n te d by r e c e i v i n g l e n g t h f orm t h a t must be c u t t o s i z e by s k i l l e d personnel.

T h i s economy i s n o t c o n f i n e d t o t h e l a r g e manufacturers b u t i t i s

they who w i l l e n j o y t h e g r e a t e r advantage. On t h e o t h e r hand,

i n many i n st a nce s, t h e problem o f s t o c k i n g r i n g s o r s e t s

ta ilo r - m a d e f o r an assortment o f v a l v e s v a r y i n g i n o r i g i n , can prove i n t o l e r a b l e .

type and dimensions

For t he se cases, t h e r e i s a c l e a r need f o r t h e v e r s a t -

i l i t y o f p a c k in g i n l e n g t h form.

Comparable w i t h t h i s s o l u t i o n i s t h e expedient

provided by those pa cki ng s o f p l a s t i c n a t u r e t h a t a r e a v a i l a b l e i n loose form but t h i s advantage must be weighed a g a i n s t t h e l a b o u r c o s t i n t h e c a r e f u l f i t t i n g r e q u i red. The q u a n t i t y o f p acki n g t o use.and i t s s i z e f o r a g i v e n a p p l i c a t i o n r e l i e s l a r g e l y on t h e e xp eri e nce o f t h e u ser/ man uf acturer i n t h e type o f d u t y being performed, o r on l i a i s o n w i t h a p acki n g s u p p l i e r a t t h e design stage. l a t t e r c o u r s e o f a c t i o n i s always f avo ure d i f any doubt e x i s t s ,

The

s i n c e an exact

knowledge o f t h e c a p a b i l i t i e s and l i m i t a t i o n s o f t h e m a t e r i a l employed can be found o n l y w i t h those s p e c i a l i s t s r e s p o n s i b l e f o r compounding and p r o d u c t i o n . F iv e r i n g s o f square s e c t i o n p acki n g a r e o f t e n accepted as a s u f f i c i e n t number f o r t h e average u nco mpl i ca t e d d u t y b u t t h e r e a r e many pump a p p l i c a t i o n s where t h e presence o f a l a n t e r n r i n g o r s i m i l a r c o n s i d e r a t i o n may d i c t a t e a greater q u a n t i t y . The a p p r o p r i a t e pa cki ng s e c t i o n t o use i n r e l a t i o n t o diameter i s open t o a degree o f i n d i v i d u a l p r e f e r e n c e b u t broad recommendations a r e shown i n Table 1 6 . 2 . To g i v e an idea o f t h e c a p a b i l i t i e s o f t he v a r i o u s m a t e r i a l s and c o n s t r u c t i o n s o f s o f t p a cki ng s which a r e r e a d i l y a v a i l a b l e , r e f e r e n c e may be made t o :

Table 16.5

-

co mpa rat i ve speed performance

Table 16.6

-

co mpa rat i ve temperature performance

Table 1 6 . 7

-

co mpa rat i ve c o s t i n d i c a t i o n .

Table 16.4

s u i t a b i l i t y i n d i f f e r e n t media/speed and temperature l i m i t s

N . B . The statement o f speed and t emp era t u re l i m i t s f o r a g i v e n m a t e r i a l should n o t be c o n s t r u e d as meaning t h a t a pa cki ng w i l l be s u i t a b l e f o r d u t i e s where such maxima a r e j o i n t l y encountered.

384 TABLE 16.4

SYMBOLS

0

-*

Recommended Consult Corrosion inhibitor included

solvents

RECIPROCATING, ROTARY PUMPS & VALVES Lubricated aluminium foil.

1

540

Lubricated braided asbestos

I

350

I

Lubricated plaited asbestos.

315

Plaited, lubricated asbestos impregnated with PTFE dispersion.

290

Plaited. lubricated asbestos impregnated with PTFE disoersion but with no additional lubricant.

290

PTFE impregnated asbestos and glass fibre yarns with suitable lubricant.

I 2 9 0

PTFE yarn impregnated with PTFE dispersion and inen lubricant. Soft lead based foil wrapped round lubricated asbestos COW.

I I 1

7.5

I

CI

lo

20

10

;“t I 8

1 7 5

250

8

260

12

Hydrocarbon resistant lubricated plaited asbestos.

1

I

7

I

Lubricated plaited cotton.

( 9 0 1

7

I

Cross plan aramd hbre yams

I

200

250

I

n

~

Puregraphite foil with no volatile additives

15

10

D

O

RECIPROCATING PUMPS & VALVES Monel wire reinforced asbestoscoverwith plastic core.*

I

Synthetic rubber bonded braided asbestos with brass wire reinforcement.

I

1 0

0

310

Lubricated plaited flax.

VALVES ONLY

480

90

I I T

Constructed from a jacket of asbestos reinforced with inconel wire braided ouer a resilient asbestos core. Lubricated braided asbestos with monel Self-lubricating fibrous asbestos with flake Qraphiteor mica. Lubricated braided asbestosand brasswirereinforced.

540

1

510

PTFE yarn impregnated with PTFE dispersion and inen lubricant but with no additional lubricant.

2 50

Unsintered PTFE cord gland seal for rapid valve packing.

250

I

1 0

0

T A B L E 16.5 MAXIMUM ROTARY SPEEDS FOR PUMP PACKINGS

MATERIAL

I

0

I

ROTARY SPEED I d s )

5

10

15 m/s

20

25

30

T A B L E 16.6 MAXIYUM SERVICE TEMPERATURES OF PUMP PACKINGS MATERIAL

TABLE 16.7 TYPICAL RELATIVE COSTS OF PUMP PACKINGS

MATERIAL

I

RELATIVE COST

I

388 16.4.6

Fitting

I t i s o f t e n assumed t h a t u n s k i l l e d l a b o u r can be used t o repack pump glands b u t t h i s i s t r u e o n l y so l o n g as u n s k i l l e d i s n o t equated w i t h unaware.

Fam-

i l i a r i t y w i t h t h e f o l l o w i n g i d e a l procedure w i l l be more t h a n r e p a i d i n terms o f t r o u b l e f r e e p a c k i n g performance. Where l e n g t h f o r m i s used: ( i ) S p i r a l l y wrap t h e m a t e r i a l around a r o d o f diameter e q u i v a l e n t t o t h e pump s h a f t . ( i i ) Cut t h e r e q u i r e d number o f r i n g s c l e a n l y t o o b t a i n good b u t t - j o i n s . See Fig.7. ( i i i ) Proceed as f o r pre-formed s p l i t p a c k i n g r i n g s .

Fig.7 Where pre-formed s p l i t p a c k i n g r i n g s a r e used: ( i ) C a r e f u l l y remove o l d p a c k i n g ( i n c l u d i n g where a p p r o p r i a t e t h e p a c k i n g on t h e f a r s i d e o f a l a n t e r n r i n g ) . ( i i ) Thoroughly c l e a n a l l s u r f a c e s t h a t w i l l c o n t a c t t h e p a c k i n g and, where p e r m i t t e d , smear w i t h o i l .

Gland and neck bushes, s h a f t s u r f a c e and

b e a r i n g s s h o u l d a l s o be checked f o r s i g n s o f wear and r e c t i f i e d as necessary. ( i i i ) Place f i r s t r i n g o v e r t h e s h a f t by opening t o an ' S ' c o n f i g u r a t i o n t o ensure t h a t bending e f f e c t s a r e spread o v e r t h e whole r i n g .

Fig.8

See Fig.8.

389 ( i v ) I n s e r t f i r s t r i n g i n t o s t u f f i n g box and l i g h t l y bed i n w i t h a s p l i t (wooden) d i s t a n c e p i e c e and g l a n d s p i g o t .

With p l a i t e d packing t h e ' V '

f o r m a t i o n on t h e o u t s i d e di a met er o f t h e r i n g should be p o i n t i n g i n the d i r e c t i o n o f s h a f t r o t a t i o n .

F ig .9 (a)

See Fig.9.

P l a i t e d p acki n g

Eig.g(b)

F o i l wrapped packing

(v) Repeat ( i i ) and ( i v ) w i t h remainder o f r i n g s e n s u r i n g t h a t each r i n g

i s f i r m l y s ea t e d and t h a t t h e b u t t j o i n s a r e staggered by a t l e a s t 90".

N.B.

The

ing s must be f i t t e d i n d i v i d u a l l y and under no circumstances

s h o u ld complete s e t s be f i t t e d as a u n i t . ( v i ) When t h e r e q u i s i t e number o f r i n g s have been f i t t e d , u n t i l t h e s h a f t o r s p i n d l e t o r q u e i n creases. p u l l up t o f i n g e r t i g h t n e s s o n l y .

t i g h t e n gland nuts

Then s l a c k o f f g l a n d and

( I f pump i s t o be s t o r e d b e f o r e use

leave g l a n d s l a c k so t h a t pa cki ng r e s i l i e n c e i s n o t impaired). ( v i i ) Running i n pumps

10 t o 15 minutes.

Prime c a s t i n g and r u n pump up t o o p e r a t i n g speed f o r

I f pump i s n o t f i t t e d w i t h g l a n d c o o l i n g , a c o l d

water spray o v e r t h e g l a n d ho usi n g w i l l a v o i d e x c e s s i v e heat b u i l d - u p d u r i n g t h i s stage.

I f n o leakage occurs,

pressure and s l a c k e n g l a n d f u r t h e r .

s t o p pump, v e n t c a s i n g

Repeat u n t i l leakage s t a r t s .

The c o n t r o l l e d leakage, e s s e n t i a l f o r l u b r i c a t i o n purposes, can then be o b t a i n e d by r u n n i n g t h e pump and e v e n l y t i g h t e n i n g the g l a n d n u t s i n increments o f 2 f l a t s u n t i l ap pro xi mately one drop e v e r y few seconds i s o b ta in e d .

Ap pro xi mat el y 15 mi n ut es should be l e f t between successive

adjustments.

DO NOT OVERTIGHTEN.

( v i i i ) Where lo o s e f o r m m a t e r i a l i s used f o r v a l v e s proceed as ( i ) and ( i i ) and tamp p a c k in g i n t o a dense homogeneous mass p r o g r e s s i v e l y f i l l i n g the housing t o t h e r e q u i r e d degree.

390 ( i x ) Because o f t h e danger o f c o r r o s i o n t h r o u g h e l e c t r o l y t i c a c t i o n , packings c o n t a i n i n g g r a p h i t e should be avoided on v a l v e s o r pumps w i t h s t a i n l e s s o r chrome s t e e l stems.

T h i s r i s k i s most a c u t e when t h e p a c k i n g remains

i n t h e g l a n d d u r i n g s t o r a g e and i s p a r t i c u l a r l y aggravated by t h e presence o f m o i s t u r e .

16.4.7

F a u l t Finding

A m a j o r advantage o f compression packings i s t h a t breakdown i s r a r e l y sudden o r c a t a s t r o p h i c b u t r a t h e r a m a t t e r o f a g r a d u a l b u i l d up o f leakage u n t i l an unacceptable l e v e l i s reached.

Normally, c o n s i d e r a b l e l i f e can be achieved by

c o n t r o l l i n g leakage w i t h f u r t h e r t i g h t e n i n g o f t h e g l a n d n u t s (N.B.

t h e seepage

o f f l u i d which a c t s as a l u b r i c a n t f o r t h e p a c k i n g on r o t a r y a p p l i c a t i o n s should n o t be confused w i t h leakage and t h e r a t e o f one drop e v e r y few seconds should be m a i n t a i n e d ) .

However,

i f o t h e r than r o u t i n e maintenance o r j u s t p l a i n

' f a i r wear and t e a r ' a r e suspected as t h e cause o f leakage and t h e need f o r re-packing,

then t h e f o l l o w i n g h i n t s c o u l d w e l l prove u s e f u l .

( i ) Confirm t h a t t h e p a c k i n g i s r a t e d as s u i t a b l e f o r t h e a p p l i c a t i o n . ( i i ) I f one o r more r i n g s a r e m i s s i n g f r o m t h e s e t , check f o r e x c e s s i v e neck bush c l e a r a n c e a l l o w i n g e x t r u s i o n o f r i n g s i n t o t h e system. I f t h e t o p r i n g has e x t r u d e d between t h e g l a n d f o l l o w e r , a n t i - e x t r u s i o n

r i n g s c o u l d a v o i d replacement o f metal p a r t s . (iii)

I f t h e p a c k i n g ' s r a d i a l t h i c k n e s s appears d i m i n i s h e d i n one o r more

p l a c e s , check f o r an u n d e r s i z e s h a f t o r b a d l y worn b e a r i n g s which c o u l d cause s h a f t whip o r s p i n d l e wobble. ( i v ) I f r a d i a l s e c t i o n o f p a c k i n g d i r e c t l y beneath t h e s h a f t

i s reduced or

premature leakage o c c u r s a l o n g t h e t o p o f t h e s h a f t , check f o r m i s a l i g n m e n t o f s h a f t c e n t r e t o s t u f f i n g box bore. ( v ) I f p a c k i n g i s worn on t h e o u t e r diameter, check f o r loose r i n g s or r i n g s r o t a t i n g w i t h t h e s h a f t due t o i n s u f f i c i e n t g l a n d load. ( v i ) I f p a c k i n g r i n g s have b u l g e s on t h e i r r a d i a l faces,

the adjacent r i n g

was p r o b a b l y c u t t o o s h o r t , c a u s i n g p a c k i n g under p r e s s u r e t o be f o r c e d i n t o t h e gap a t t h e j o i n t . ( v i i ) I f p a c k i n g n e a r e s t g l a n d s p i g o t shows e x c e s s i v e d e f o r m a t i o n w h i l s t o t h e r r i n g s a r e i n f a i r c o n d i t i o n , t h e s e t was p r o b a b l y i n c o r r e c t l y i n s t a l l e d and s u b j e c t e d t o e x c e s s i v e g l a n d tightening.. N.B.

O v e r - t i g h t e n i n g i s u s u a l l y t h e g r e a t e s t s i n g l e cause o f premature p a c k i n g f a i l u r e .

391 ( v i i i ) I f t h e cause o f your p a r t i c u l a r problem i s s t i l l n o t apparent, g i v e equal a t t e n t i o n t o I.D.

and O.D.

leakage and check f o r a rough s t u f f i n g

box bore b e f o r e seeking s p e c i a l i s t a d v i c e .

16.4.8

Standardization

In t h e i n t e r e s t s o f s t o c k c o n t r o l i t i s c l e a r l y an advantage t o r a t i o n a l i s e the v a r i e t y o f packings used i n any p l a n t t o t h a t minimum number which w i l l e f f e c t i v e l y c a t e r f o r a l l t h e c o n d i t i o n s l i k e l y t o be encountered. no o b j e c t ,

If cost i s

then t h e r e a r e s i n g l e , s o p h i s t i c a t e d m a t e r i a l s and c o n s t r u c t i o n s

t h a t w i l l go some way towards s a t i s f y i n g most demands, b u t i t i s d o u b t f u l i f cost e f f e c t i v e n e s s c o u l d be j u s t i f i e d .

Far b e t t e r t o compromise on a small

number o f r e p u t a b l e p r o d u c t s developed f o r t h e areas i n q u e s t i o n , e.g.

pumps,

valves, e t c . There a r e few standards a p p l y i n g t o compression packings on a n a t i o n a l o r i n t e r n a t i o n a l b a s i s , a l t h o u g h many i n d i v i d u a l companies and o r g a n i z a t i o n s have domestic standards which have,

i n many cases, been t h e s u b j e c t o f c o l l a b o r a t i o n

between user and packing manufacturer. BS 4371 : 1968 s p e c i f i e s minimum standards f o r l u b r i c a t e d p l a i t e d c o t t o n , lubricated p l a i t e d f l a x ,

l u b r i c a t e d p l a i t e d o r b r a i d e d asbestos, d r y w h i t e non-

m e t a l l i c p l a i t e d o r b r a i d e d asbestos, p l a i t e d o r b r a i d e d asbestos, m e t a l l i c wire r e i n f o r c e d ,

i n d u r a t e d asbestos,

and l u b r i c a t e d f i b r o u s asbestos and g i v e s

guidance on l i m i t i n g o p e r a t i n g parameters f o r these c o n s t r u c t i o n s . Where packings a r e r e q u i r e d f o r s e r v i c e w i t h p o t a b l e water i n t h e Water A u t h o r i t y d i s t r i b u t i o n system (which covers r e s e r v o i r t o t a p ) , o n l y those mate r i a l s which have gained a N a t i o n a l Water Council Approval may be used.

Such

products have been t e s t e d t o e s t a b l i s h t h a t t h e y produce no c o l o u r , t a s t e o r t u r b i d i t y , a r e n o n - t o x i c and w i l l n o t support m i c r o b i a l growth. S t a t u t o r y Instruments 1978 No. 1927 "The M a t e r i a l s and A r t i c l e s i n Contact w i t h Food R e g u l a t i o n s 1978" r e q u i r e d t h a t compression p a c k i n g m a t e r i a l s , f o r example,

I'

... do

o r l i k e l y t o be,

n o t t r a n s f e r t h e i r c o n s t i t u e n t s t o foods w i t h which they a r e , i n contact,

i n q u a n t i t i e s which c o u l d -

( i ) endanger human h e a l t h o r ( i i ) b r i n g about a d e t e r i o r a t i o n i n t h e o r g a n o l e p t i c (sensory q u a l i t y ) o f such food o r an unacceptable change i n i t s n a t u r e , substance, o r qua1 i t y . " Such r e g u l a t i o n s i n e v i t a b l y r e s t r i c t t h e range o f a v a i l a b l e m a t e r i a l s and lubricants.

C o n s u l t a t i o n w i t h t h e s u p p l i e r i s recommended t o e s t a b l i s h p r e -

f e r r e d grades.

392

16.4.9

The F u t u r e

The research c u r r e n t l y b e i n g undertaken as a d i r e c t r e s u l t o f t h e a n t i asbestos lobby may produce glass/PTFE/graphite match e x i s t i n g m a t e r i a l s .

o r ceramic c o n s t r u c t i o n s which

However, many problems remain t o be s o l v e d i n p r o -

ducing a general purpose p r o d u c t t h a t can compete w i t h asbestos i n terms o f l u b r i c a n t r e t e n t i o n , absorbency o r d u r a b i l i t y a t comparable c o s t . There can be no doubt t h a t g r a p h i t e f o i l p r o d u c t s w i l l , because o f t h i s s i t u a t i o n , see g r e a t e r acceptance d e s p i t e h i g h i n i t i a l c o s t . r e p o r t s a r e e x t r e m e l y encouraging and,

Performance

i r r e s p e c t i v e o f c u r r e n t market con-

s i d e r a t i o n s , t h e m a t e r i a l deserves t o succeed on i t s own m e r i t s .

393 APPENDIX 1 COPY OF STATEMENT ISSUED BY THE ASBESTOS INFORMATION CENTRE,

40 PICCADILLY, LONDON W1V 9PA, TO THE BRITISH VALVE MANUFACTURERS ASSOCIATION, 3 PANNEL COURT, CHERTSEY STREET, GUILDFORD, SURREY,

GU1 4EU

ON 3 0 t h APRIL 1980. SAFETY OF ASBESTOS GLAND PACKINGS AND GASKETS C r y s o t i l e ( w h i t e ) asbestos f i b r e i s a b a s i c c o n s t i t u e n t o f v a l v e packings and gaskets because i t combines i n one m a t e r i a l s o f t n e s s ,

r e s i l i e n c e , absorption

p r o p e r t i e s , s t r e n g t h as a r e i n f o r c e m e n t and, where r e q u i r e d , h e a t r e s i s t a n c e . Asbestos i s o n l y a r i s k t o h e a l t h i f i t s d u s t i s i n h a l e d .

Valve packings and

gaskets based on c h r y s o t i l e ( w h i t e ) asbestos a r e s a f e t o handle by v a l v e users and by maintenance e n g i n e e r s .

A good s t a n d a r d o f i n d u s t r i a l hygiene should be

observed when h a n d l i n g o r u s i n g p r o d u c t s which c o n t a i n asbestos. The A d v i s o r y Committee on Asbestos i n t h e i r i n t e r i m statement and f i n a l r e p o r t p u b l i s h e d i n October 1979 recommend t h a t asbestos d u s t should be k e p t t o t h e lowest p r a c t i c a b l e l e v e l and c a l l f o r a 1 f i b r e / m l c o n t r o l l i m i t .

The r e p o r t

f u r t h e r s t a t e s t h a t t h e presence o f c h r y s o t i l e i s u n l i k e l y t o have produced any m a t e r i a l i n c r e a s e i n t h e r i s k o f l u n g cancer i n t h e general p o p u l a t i o n o r any a p p r e c i a b l e number o f cases o f mesothelioma, and t h e same i s c e r t a i n l y t r u e o f asbestosis.

Gland packings and gaskets w i l l n o t c r e a t e d u s t l e v e l s i n

excess o f t h e c o n t r o l l i m i t , g i v e n normal usage and maintenance. ' L u b r i c a t e d ' packings a r e vacuum impregnated w i t h m i n e r a l o i l s and greases, g r a p h i t e and o t h e r l u b r i c a n t s , and do n o t e m i t d u s t . ' D r y ' p a c k i n g s a r e n o r m a l l y t r e a t e d w i t h a dust-suppressant which s i g n i f i c a n t l y reduces d u s t e m i s s i o n so t h a t i n normal use, n o t be expected t o p r e s e n t a hazard.

i n c l u d i n g maintenance, they would

They a r e n o t commonly used i n v a l v e glands.

'Hard' packings and gaskets (compressed asbestos f i b r e ) a r e made f r o m a comb i n a t i o n o f asbestos, rubber and o t h e r f i l l e r s .

The asbestos f i b r e i s locked

i n t o t h e rubber m a t r i x and w i l l n o t n o r m a l l y c r e a t e d u s t i n use o r maintenance.

I f however, these m a t e r i a l s a r e s u b j e c t e d t o g r i n d i n g r o r o t h e r a b r a s i v e p r o cesses, p r e c a u t i o n s should be taken t o a v o i d i n h a l i n g any dust which may be emitted. ' S o f t ' o r moulded packings a r e n o r m a l l y made f r o m r u b b e r i s e d asbestos c l o t h , and t h e asbestos i s sealed w i t h i n t h e rubber c o a t i n g , so t h a t they a l s o a r e s a f e i n use and maintenance.

394 C r o c i d o l i t e ( b l u e ) asbestos No c r o c i d o l i t e

( b l u e ) a s b e s t o s has been used i n p a c k i n g s and g a s k e t s manuf-

a c t u r e d i n t h e UK f o r s e v e r a l years. however,

incorpo ra te b l u e asbestos.

Some i m p o r t e d p a c k i n g s and g a s k e t s may When l u b r i c a t e d t h e s e p a c k i n g s a r e s a f e

t o handle but r e s p i r a t o r y p r o t e c t i o n w i l l

be needed when t h e y a r e i n a d r y

state. Remova 1 Care s h o u l d be taken when removing o l d p a c k i n g s w h i c h may have l o s t t h e i r lubricants. i.e.

They s h o u l d be damped and t h e n removed w i t h t h e c o r r e c t t o o l s ,

packing e x t r a c t o r s .

Label 1 i n g Any p a c k i n g s o r g a s k e t s w h i c h r e q u i r e s p e c i a l h a n d l i n g p r e c a u t i o n s c a r r y a warning l a b e l .

395

17

CENTRALISED LUBRICATION SYSTEMS DESIGN

J.G.MERRETT,

17.1

Managing D i r e c t o r , E n g i n e e r i n g and General Equipment L t d .

INTRODUCTION

W h i l s t we a r e k e p t c o m p a r a t i v e l y w e l l informed o f t h e l a t e s t developments i n o i l and grease technology and t h e v i t a l r o l e i t p l a y s i n i n d u s t r i a l and commerc i a l applications,

v e r y l i t t l e has been w r i t t e n about t h e e q u a l l y v i t a l "Centra-

I i s e d L u b r i c a t i o n Systems" and some o f t h e methods a v a i l a b l e t o Engineers by which grease and o i l may be t r a n s m i t t e d t o t h e p o i n t o f l u b r i c a t i o n . I n our s o c i e t y where energy, machinery and l a b o u r a r e now (1981) expensive commodities,

i t i s c l e a r t h a t i n t h e p a s t i n s u f f i c i e n t a t t e n t i o n has been p a i d

t o the d i r e c t and i n d i r e c t losses o f energy, occasioned by wear and f r i c t i o n and t o t h e savings o f m a t e r i a l s .

However,

i n 1977 a government f i n a n c e d Ameri-

can Report suggested t h a t $16.25 b i l l i o n p.a.

( a t 1976 v a l u e s ) c o u l d be saved

by a "Strategy f o r Energy Conservation through T r i b o l o g y "

U.K.

[l].

Converted i n t o

(1980) v a l u e s , t h i s i n d i c a t e s an e q u i v a l e n t s a v i n g o f energy through t r i -

bology i n excess o f €1;

b i l l i o n p.a.

I n s h o r t , c o r r e c t l y s e l e c t e d l u b r i c a n t s and t h e i r methods o f a p p l i c a t i o n by C e n t r a l i s e d Systems can e f f e c t s i g n i f i c a n t savings, e.g. machine t o o l s , conveyors, cranes, r o l l i n g m i l l s , b l a s t furnaces, b a l l m i l l s , sugar machinery, paper m i l l s , heavy m o b i l e p l a n t , e t c . t o v a r y i n g degrees, a l l r e q u i r e t h e a p p l i c a t i o n o f lubricants.

A c o r r e c t l y designed and i n s t a l l e d C e n t r a l i s e d L u b r i c a t i o n

System i s t h e e n g i n e e r ' s insurance a g a i n s t some o f t h e severe t r i b o l o g i c a l problems, i . e .

f r i c t i o n and wear, which would o t h e r w i s e occur i f t h e p l a n t and

machinery were i n a d e q u a t e l y l u b r i c a t e d . U n f o r t u n a t e l y , a l l t o o o f t e n i n t h e supply of p l a n t and machinery t h e s e l e c t i o n o f t h e l u b r i c a n t and C e n t r a l i s e d L u b r i c a t i o n equipment a r e considered a t a l a t e stage i n t h e manufacture o f t h e p l a n t which, coupled w i t h t h e c o n f l i c t i n g i n t e r e s t s o f machinery b u i l d e r s , can r e s u l t i n u n s u i t a b l e l u b r i c a t i o n equipment being s e l e c t e d .

L i k e w i s e , t h e l u b r i c a t i o n equipment supply companies have a

r e s p o n s i b i l i t y , n o t o n l y t o know t h e i r own p r o d u c t , b u t a l s o t o a p p r e c i a t e t h e t r i b o l o g i c a l requirements o f t h e p l a n t and machinery which r e q u i r e s t o be

396 lubricated.

F a i l u r e i n t h i s area i n v a r i a b l y r e s u l t s i n the p l a n t user being

p l a c e d i n t h e u n e n v i a b l e p o s i t i o n o f h a v i n g t o a p p l y , a t an e a r l y stage, f o r a d d i t i o n a l c a p i t a l t o r e c t i f y new p l a n t o r , more o f t e n t h a n n o t , t o l i v e w i t h t h e problems and r e c t i f y as and when t h r o u g h a maintenance budget

-

both o f

which the p l a n t u s e r would be t h e f i r s t t o agree i s e n t i r e l y u n s a t i s f a c t o r y . T h i s paper endeavours t o deal w i t h some o f these f a c t o r s a f f e c t i n g t h e c h o i c e o f l u b r i c a n t and l u b r i c a t i n g equipment, t h e b a s i c elements o f t h e machinery t o be l u b r i c a t e d , and t h e c o n d i t i o n s under which i t o p e r a t e s .

17.2

POINTS OF LUBRICATION

Bearings, t h e e s s e n t i a l components o f p l a n t and machinery, may be g e n e r a l l y grouped i n t o j o u r n a l , t h r u s t , conveyor c h a i n p i n and l i n k , a n t i - f r i c t i o n , ways and crane r a i l s .

slide-

Each wear s u r f a c e must be t r e a t e d s e p a r a t e l y w i t h r e g a r d

t o l u b r i c a n t and l u b r i c a t i o n t e c h n i q u e . The l u b r i c a t i o n requirement o f a p l a i n j o u r n a l b e a r i n g i s t h e p r o v i s i o n o f an adequate and c o n s t a n t f l o w o f l u b r i c a n t o f s p e c i f i e d v i s c o s i t y t o g i v e a f l u i d f i l m o f high-load bearing capacity.

The j o u r n a l b e a r i n g has i n h e r e n t l y a

convergence between t h e s h a f t and t h e b e a r i n g .

When r e l a t i v e m o t i o n takes p l a c e

a f i l m o f l u b r i c a n t i s induced between t h e s u r f a c e s , e f f e c t i v e l y s e p a r a t i n g them. Bearings employed t o absorb t h r u s t and p r e v e n t misalignment of s h a f t s v a r y g r e a t l y i n t y p e and l u b r i c a t i o n requirement, w h i l s t a n t i - f r i c t i o n b e a r i n g s r e q u i r e l e s s l u b r i c a n t than p l a i n b e a r i n g s .

Most a v a i l a b l e formulae d e a l i n g

w i t h t h e a p p l i c a t i o n o f grease t o these b e a r i n g s t r e a t speed as an i m p o r t a n t factor.

For small a n t i - f r i c t i o n b e a r i n g s such as those employed i n l i g h t l y

loaded f r a c t i o n a l horse-power motors, t o o much grease can be damaging. I n such cases, recommended l u b r i c a t i o n i n t e r v a l s o f up t o s e v e r a l years have been e s t a blished.

Chain p i n s and l i n k s p r e s e n t m a j o r c r i t i c a l wear p o i n s on f l o o r and

overhead ( i n c l u d i n g Power and Free) conveyors.

I n t h e automobi e i n d u s t r y ,

c h a i n l e n g t h s o f s e v e r a l hundred metres, h a v i n g thousands o f

PO

nts requiring

l u b r i c a t i o n , a r e commonplace ( F i g . 1 ) . Slideway and crane r a i l l u b r i c a t i o n r e q u i r e s t h e r i g h t l u b r i c a n t and t h e r i g h t a p p l i c a t o r (see Fig.10,

S e c t i o n 17.6.1).

Too l i t t l e l u b r i c a n t r e s u l t s i n

r a p i d wear; e x c e s s i v e l u b r i c a n t can be a hazard t o l i f e o r l i m b . Every case i s d i f f e r e n t , y e t i n e v e r y case i t i s c r i t i c a l t h a t t h e l u b r i c a n t i s a p p l i e d i n l i n e w i t h the requirements o f t h a t component,

b o t h w i t h r e g a r d t o mechanical

wear and t o energy conservancy.

17.3

SELECTING THE LUBRICANT

-

O I L OR GREASE?

I n modern machinery l u b r i c a t i o n ,

l u b r i c a n t s and t h e means o f t h e app i c a t i o n

must be considered t o g e t h e r . The b e s t l u b r i c a n t w i l l s e r v e no u s e f u l purpose i f i t i s n o t a p p l i e d a t t h e

397

Fig.1

Excessive p i n and l i n k wear i n a conveyor c h a i n . A 3mm p i n wear p e r p i n on a 732111 conveyor increases t h e c h a i n l e n g t h by 23 metres.

r i g h t t i m e , a t t h e r i g h t p l a c e , and i n t h e c o r r e c t q u a n t i t y .

Conversely, t h e

best l u b r i c a t i o n equipment, a p p l y i n g l u b r i c a n t q u i t e c o r r e c t l y , w i l l be o f l i t t l e use i f t h e l u b r i c a n t i t feeds i s u n s u i t a b l e f o r t h e d u t y i t has t o perform. L u b r i c a n t s g e n e r a l l y , e i t h e r o i l s o r greases, a r e s u p p l i e d by o i l companies supported by s p e c i a l i s t o i l and grease m a n u f a c t u r i n g companies, who w i l l p r o v i d e advice on any p a r t i c u l a r l u b r i c a n t and a p p l i c a t i o n as w e l l as on t h e c h o i c e o f lubricants i n general.

The s u p p l i e r o f t h e l u b r i c a t i o n equipment w i l l a l s o

o f t e n be a b l e t o a d v i s e , e s p e c i a l l y on l u b r i c a t i o n d e s i g n problems, o r a c t as an intermediary

.

The s u b j e c t o f l u b r i c a n t s i s t r e a t e d h e r e o n l y i n i t s v e r y broadest terms under t h r e e headings:"OIL"

o r GREASE";

when t o use one and when t h e o t h e r .

LUBRICATING GREASE;

t h e types and how t o s e l e c t them.

LUBRICATING OILS; types and how t o s e l e c t them.

17.3.1

O i l o r Grease? When t o Use One and When t h e Other

Movement between two d r y s u r f a c e s causes heat and wear.

The purpose o f

i n t r o d u c i n g a l u b r i c a n t between t h e two s u r f a c e s i s t o reduce f r i c t i o n , heat, and wear.

O i l has t h e f o l l o w i n g advantages: and i t scavenges.

A t t h e same t i m e ,

i t flows,

i t penetrates,

i t removes heat,

i t has l i m i t e d s e a l i n g q u a l i t i e s and poor

' s t a y i n g ' power, t h e r e f o r e r e q u i r e s more f r e q u e n t r e p l e n i s h i n g than grease.

398 L u b r i c a t i n g grease, a s e m i - s o l i d p l a s t i c - l i k e m a t e r i a l , has e x c e l l e n t s e a l ing characteristics,

possesses good ' s t a y i n g ' power,

i.e.

i t adheres t o surfaces

more r e a d i l y and l o n g e r t h a n o i l and i t can be an e x c e l l e n t c o r r o s i o n p r e v e n t a tive.

Unlike o i l ,

i t i s a poor conductor o f heat and a poor scavenger;

n o t f l o w o r spread e a s i y where c o n t a m i n a t i o n by

-

i t does

t h e l a t t e r p r o p e r t y can, however, be an advantage

u b r i c a n t s must be avoided such as i n f o o d and b o t t l i n g

and t h e t e x t i l e i n d u s t r es and where t h e l u b r i c a n t i s used as a s e a l i n g medium. Where an a p p l i c a t i o n e n t a i l s HEAT REMOVAL, o i l i s t h e r e f o r e g e n e r a l l y t h e choice.

I t can be appl ed i n t h e f o r m o f a l i q u i d o r as an o i l m i s t ,

m i c r o - f o g comprising an o i l - a i r m i x t u r e .

i.e.

I n severe cases o f h e a t removal, o i l

can be r e c i r c u l a t e d and, d u r i n g t h e c i r c u l a t i o n ,

i t can be c o o l e d and cleaned.

Large t u r b i n e b e a r i n g s and f a s t gear t r a i n s a r e t y p i c a l examples where o i l c i r c u l a t i n g systems a r e used and l a r g e amounts o f o i l a r e c i r c u l a t e d . For a p p l i c a t i o n s where heat removal i s n o t a p r o b l e m , b u t LUBRICATION o r t h e PREVENTION OF INGRESS OF DIRT, WATER, DUST and OTHER EXTRANEOUS MATTER i s l u b r i c a t i n g grease can be used.

-

I t s a p p l i c a t i o n ranges from heavy m i l l b e a r i n g s

and s l i d e s t o t e x t i l e machinery, e t c .

I t i s a l s o t h e most s u i t a b l e where a p p l i -

c a t i o n o f t h e l u b r i c a n t i s r e q u i r e d a t LONG INTERVALS. N e a r l y a l l B a l l and R o l l e r Bearings, except those i n r e s p e c t o f which heat removal i s e s s e n t i a l , a r e g e n e r a l l y grease l u b r i c a t e d .

Where DIRTY/DUSTY and

WET CONDITIONS e x i s t , grease

s generally preferable.

l u b r i c a t i o n makes grease t h e

p r e f e r r e d ' l u b r i c a n t f o r the vast m a j o r i t y o f

The convenience o f grease

r o l l i n g bearings. G e n e r a l l y , o i l l u b r i c a t i o n i s employed i n t h e r e l a t i v e l y few cases where i t i s n o t p o s s i b l e t o t a k e advan age o f t h e m e r i t s o f grease as a r o l l i n g b e a r i n g

1u b r i c a n t . Where motion i s INTERMITTENT o r OSCILLATING, grease i s o f t e n t h e more s u i t able lubricant. Summarising, where heat conveyance away f r o m r u b b i n g s u r f a c e s o r p e n e t r a t i o n o f t h e l u b r i c a n t i s o f importance, o r t h e scavenging f u n c t i o n i s necessary, o i l i s p r e f e r a b l e , w h i l s t l u b r i c a t i n g grease can and,

i n most cases, should be

considered as t h e p r e f e r r e d l u b r i c a n t f o r slow moving machinery,

long l i f e

l u b r i c a t i o n , and where d i r t and d u s t has t o he kept o u t o f t h e b e a r i n g s , s l i d i n g surfaces, etc.

17.3.2

L u b r i c a t i n g Grease

-

The Types and How t o S e l e c t Them

Greases designed f o r l u b r i c a t i o n a r e e s s e n t i a l l y a m i x t u r e o f m i n e r a l o i l and t h i c k e n e r , a c c o r d i n g to t h e a p p l i c a t i o n requirement.

I n t h e most w i d e l y

used modern grease t h e t h i c k e n e r i s a m e t a l l i c soap, u s u a l l y o f l i t h i u m o r calcium, w i t h t h e q u a n t i t y o f o i l a d j u s t e d t o g i v e a s o l i d , s e m i - s o l i d o r semi f 1u i d consistency.

399 The soap f i b r e s form a s t r u c t u r e t h a t r e t a i n s t h e o i l , t h e dimensions and arrangement o f t h e f i b r e s v a r y i n g a c c o r d i n g t o t h e metal and t h e f a t t y a c i d from which t h e soap was made.

The q u a n t i t y , dimensions and d i s t r i b u t i o n o f t h e

f i b r e s a r e t h e main parameters c o n t r o l l i n g t h e s t a b i l i t y and f l o w p r o p e r t i e s o f this lubricating material. One o f t h e most i m p o r t a n t p h y s i c a l f e a t u r e s o f grease i s s t i f f n e s s ( f o r s i s t e n c y ) which i s i n d i c a t e d by a t e s t t h a t measures t h e d e p t h t h a t t h e cone s i n k s i n t o a sample o f grease.

The d e p t h measured i s a t e n t h o f a m i l l i m e t r e ,

and r e f e r r e d t o as t h e degree o f p e n e t r a t i o n .

F i g u r e 2 shows one c l a s s i f i c a -

t i o n system f o r greases. Grade Number.

445

000 OD 0 1 2 3

400

355 310 265 220

4 5 6 Fig.2

Worked p e n e t r a t i o n a t 25'C

175 130

85

-

-

-

-

-

475 430 385 340 295 250 205 160 115

Description Very f l u i d Fluid Semi - f 1 u i d Very s o f t Soft Semi-firm Firm Very f i r m Hard

NLGl c l a s s i f i c a t i o n o f t h e c o n s i s t e n c y o f greases.

Although lithium-based greases today s a t i s f y a v a s t number o f l u b r i c a t i o n requirements f o r cranes, conveyors, f o r g i n g presses, continuous c a s t i n g p l a n t s , r o l l i n g mills, etc.,

t h e r e a r e a number o f s i t u a t i o n s r e q u i r i n g l u b r i c a n t s w i t h

s p e c i a l p r o p e r t i e s , f o r example, t h e a b i l i t y t o w i t h s t a n d h i g h temperatures such as those encountered i n power s t a t i o n t u r b i n e s , bakery oven conveyors, e t c . These l a t t e r greases i n c l u d e t h e c l a y - t h i c k e n e d and o t h e r s o l i d - t h i c k e n e d compounds which w i t h s t a n d c o n s i d e r a b l y h i g h e r temperatures, e s p e c i a l l y when employed w i t h h i g h temperature s y n t h e t i c l u b r i c a n t s , e.g.

polyglycol, s y n t h e t i c esters,

and s i l i c o n e s . The upper temperature l i m i t a t which any grease may be used i s dependent p a r t l y on t h e t y p e o f t h i c k e n e r , p a r t l y on t h e f l u i d and i t s r e q u i r e d s e r v i c e life.

Higher o p e r a t i n g temperatures have t h e e f f e c t o f s h o r t e n i n g t h e l u b r i -

c a n t ' s s e r v i c e l i f e and r e d u c i n g p e r m i s s i b l e o p e r a t i n g speeds.

I t i s therefore

convenient t o express t h e working l i m i t s o f a grease i n terms o f b e a r i n g speeds and temperatures, as shown i n Fig.3.

T h i s diagram shows t h a t l i t h i u m , sodium

and calcium-based greases have upper temperature l i m i t s o f 130, 110 and 7OoC r e s p e c t i v e l y , and t h a t clay-based and calcium-complex greases can be used a t up t o 150°C.

400

Fig.3

17.3.3

Lubricating 011s

Working l i m i t s o f m i n e r a l o i l greases

-

Types and How t o S e l e c t Them

Vegetable and animal o i l s a r e e x c e l l e n t l u b r i c a n t s , b u t have s h o r t l i f e as they o x i d i s e and tend t o go r a n c i d ; as a r e s u l t , t h e i r main use i s f o r 'one s h o t ' a p p l i c a t i o n s such as f o r f o r g i n g o r as a d d i t i v e s f o r m i n e r a l o i l s . G e n e r a l l y , t h e o r i g i n o f t h e o i l determines i t s use ( F i g . 4 )

O i l Origin

ADD1

ication

Mineral O i l

e.g. p e t r o l e u m base f o r general l u b r i c a t i o n o f mechanical p a r t s such as engines, gears and g e n e r a l e n g i n e e r i n g p l a n t .

Vegetable O i l

e.g. c a s t o r , palm, and rape seed o i l s f o r s p e c i a l a p p l i c a t i o n s where h i g h l u b r i c i t y i s d e s i r a b l e such as k i l n s , bakery ovens.

Animal O i l

e.g. sperm o r o t h e r f i s h o i l s from sheep wool f o r a p p l i c a t i o n s such as k i l n s , bakery ovens.

Synthetic O i l

e.g. glycol d e r i v a t i v e s and d i e s t e r f o r extreme h i g h temperature. Fig.4

Origin of Oils

401 F i g u r e 5 i l l u s t r a t e s how v i s c o s i t y o f o i l s change w i t h temperature, becoming t h i n n e r when they a r e heated, b u t they do n o t change v i s c o s i t y a t t h e same r a t e . The r a t e o f v i s c o s i t y change w i t h temperature i s r e f e r r e d t o as the ' v i s c o s i t y index'.

-40-30-20 -10 0 10 20 30 $0 50 60 70 80 90XX)11O1201X)14015MM)710

degrees fahrenheit

Fig.5

17.3.3.1

V i s c o s i t y vs Temperature f o r two o i l s h a v i n g v i s c o s i t y indexes o f 0 and 100 r e s p e c t i v e l y .

The p r o p e r t i e s o f t h e o i l must be c a r e f u l l y considered when d e s i g n i n g

a l u b r i c a t i o n system, e.g.

conveyor chains passing through a p r e - t r e a t m e n t oven

may reach a temperature o f 180/200°C.

O i l s w i t h s p e c i a l i n c l u s i o n s such as

g r a p h i t e o r molybdenum d i s u l p h i d e i n g l y c o l as used i n bakery ovens must have p r o v i s i o n f o r a g i t a t i o n o r r e c i r c u l a t i o n w i t h i n t h e l u b r i c a t o r s t o r a g e tank t o prevent s e t t l i n g o u t o f t h e g r a p h i t e o r molybdenum. Bakery ovens g e n e r a l l y use molybdenum d i s u l p h i d e i n g l y c o l w h i l s t f o r extreme p r e s s u r e l o a d i n g c o n d i t i o n s on Power and Free t r o l l e y s , as i n the c a r i n d u s t r y , chains and t r o l l e y wheels r e q u i r e s p e c i a l l u b r i c a n t s , h a v i n g a h i g h adhesion c h a r a c t e r i s t i c - a s w e l l as i m p a r t i n g r u s t - p r o o f i n g and w a t e r - p r o o f i n g .

402

17.3.3.2

A f u r t h e r s p e c i a l i s t l u b r i c a n t i s rape seed o i l ;

f a t t y vegetable o i l .

t h i s i s a low a c i d

Because o f i t s r e l a t i v e l y h i g h f l a s h - p o i n t

i t i s suitable

f o r h i g h temperature work when r e f i n e d and g i v e n a g r a p h i t e i n c l u s i o n . uses a r e f o r continuous c a s t i n g moulds, f o r g i n g , metals.

Typical

and as a c u t t i n g o i l f o r harder

Here a g a i n , i t s s p e c i a l p r o p e r t i e s must be considered when p r o v i d i n g

t h e l u b r i c a t i n g means.

17.3.4

Summarising L u b r i c a n t S e l e c t i o n

For t h e f i n a l s a t i s f a c t o r y s e l e c t i o n o f l u b r i c a n t t h e f o l l o w i n g f a c t o r s must t h e r e f o r e be determined.

The c o n s t r u c t i o n and m a t e r i a l s employed i n t h e compon-

e n t s t o be l u b r i c a t e d , o p e r a t i n g speeds and l o a d i n g , l i f e requirement o f l u b r i c a n t and machine, o p e r a t i n g environment, e.g.

t h e presence o f w a t e r , steam,

chemicals o r a b r a s i v e m a t e r i a l s i n t h e atmosphere; and l a s t , b u t n o t l e a s t , t h e method o f a p p l i c a t i o n , t h e t y p e o f system t o be employed, t h e diameter of p i p e runs, e t c .

17.4

PIPE DIAMETER vs

FLOW CHARACTERISTICS

To determine t h e l u b r i c a n t f l o w r a t e and volume,

t h e l e n g t h and diameter o f

t h e p i p e l i n e s should be c a r e f u l l y c a l c u l a t e d t o ensure t h a t t h e l u b r i c a n t can s a t i s f a c t o r i l y reach t h e wear p o i n t s a t t h e extreme ends o f t h e p i p e l i n e s .

The

f o l l o w i n g t a b l e summarises some p r a c t i c a l r e s u l t s f r o m t e s t s t o prove optimum measurements and e n d - o f - l i n e p r e s s u r e , u s i n g a l i t h i u m - b a s e d grease o f No.2 consistency a t

15OoC. Nominal bore (mm)

50 38 25 19 17.5

Applied pressure (kPa)

P r e s s u r e drop (kPa/m)

1100

36.1 61.5 144.7 210.3

1 a75

4410 6410

LUBRICATION REQUIREMENTS FOR PLAIN BEARINGS

For normal w o r k i n g c o n d i t i o n s i t has been found t h a t t h e amount o f grease required f o r p l a i n bearings i s equivalent t o a layer o f O . l m m b e a r i n g area (0.1 x d x L) p e r hour o f b e a r i n g o p e r a t i o n .

on t h e developed

Figure

6 illustrates

a c h a r t f o r c a l c u l a t i n g t h e grease requirements f o r p l a i n b e a r i n g s . p l e t o c a l c u l a t e t h e grease r e q u i r e d f o r a b e a r i n g o f long,

i n t e r s e c t diameter and l e n g t h a s a t

*

As an exam-

75mm diameter by 250mm

f o l l o w l i n e o f a r r o w and where i t

i n t e r s e c t s t h e t o p s c a l e , t h i s i n d i c a t e s t h e amount o f grease r e q u i r e d , b e i n g i n t h i s example 6 g m o r 0.21 oz p e r hour.

403

D

LENGTH OF BEARING L

Fig.6

17.6

Grease requirements f o r a p l a i n b e a r i n g .

SELECTING THE RIGHT TYPE OF LUBRICATION SYSTEM

L u b r i c a t i o n systems f o r p l a n t and equipment, whatever t h e a p p l i c a t i o n , i n d i v i d u a l design.

t h e b e a r i n g requirements and t h e d e s i g n e r s ' a p p l i c a t i o n s p e c i f i c a t i o n . regard t o t h e human element, chapter deal w i t h :

17.6.1

require

They must be f u n c t i o n a l and c o r r e c t l y engineered t o s a t i s f y

they must be f o o l - p r o o f .

With

Systems i n c l u d e d i n t h i s

Grease, O i l and Micro-Fog.

Grease L u b r i c a t i o n Systems

A l l C e n t r a l i s e d Grease L u b r i c a t i o n Systems a r e o f t h e n o n - r e c i r c u l a t i n g t y p e and o p e r a t e on t h e t o t a l loss p r i n c i p l e .

They a r e b a s i c a l l y d i v i d e d i n t o D i r e c t

Feeding systems and I n d i r e c t Feeding systems, o t h e r w i s e r e f e r r e d t o as L i n e systems, as i l l u s t r a t e d i n F i g . 7 .

17.6.1.1

D i r e c t Feeding Systems a r e those where t h e volume o u t p u t o f t h e D i r -

e c t Feeding l u b r i c a t i n g pump i s p o s i t i v e ;

t h e pumping p l u n g e r s and means o f

m e t e r i n g t h e o u t p u t t o i n d i v i d u a l wear p o i n t s b e i n g i n c o r p o r a t e d i n t h e l u b r i c a t i n g pump.

T h e r e f o r e D i r e c t Feeding systems o p e r a t e on a P o s i t i v e Volume P r i n -

ciple,

t h e y i n t r o d u c e a metered volume o f l u b r i c a n t i n t o t h e pipes, and

i.e.

s i n c e t h i s volume i s n o t a f f e c t e d by p r e s s u r e i n t h e p i p e s , t h e pumps can work c o n t i n u o u s l y a g a i n s t h i g h back pressures.

- -I

I

GREASE SYSTEMS TOTAL LOSS SYSTEMS

DIRECT FEEDSYSTEMS

INDIRECT FEED M T E M S

I

PR0GRESSI:E MSTEMS

I

I

Y DUAL L I N E

FIG.7

FUNDAMENTAL LUBRICATION

DIVISION OF C E N T R A L I S E D GREASE SYSTEMS.

PARALLEL SVSTEMS

405 D i r e c t Feeding systems can be d i v i d e d i n t o those where each o u t l e t has i t s own m e t e r i n g p l u n g e r (F i g. 8)

and t ho se where a s i n g l e moving plunger d i s t r i b u t e s

p r o g r e s s i v e l y t h e metered amounts o f grease i n t o t h e v a r i o u s o u t l e t s ( F i g . 9 ) . N o r m a lly , d i r e c t f e e d i n g l u b r i c a t o r s o b t a i n t h e i r pumping a c t i o n e i t h e r by means o f moving p l u n g e r s , combined w i t h a system o f p o r t i n g , o r by the use o f p l u n g e r s i n c o n n ect i on w i t h s p r i n g - l o a d e d b a l l v a l v e s .

The l a t t e r type should

be a v o id e d i n t h e case o f d i r t y o r d u s t y c o n d i t i o n s as any i n g r e s s o f e x t r a n eous m a t t e r may l od ge i n t h e s e a t i n g o f t h e b a l l v a l v e and render t h e l u b r i c a t o r inoperative. I t f o l l o w t h a t where a l u b r i c a t o r i s d r i v e n by a moving p a r t o f a machine, an amount o f l u b r i c a n t r e q u i r e d r e l a t i v e t o t h e movement o f t h e s h a f t i n t h e bearing bein g l u b r i c a t e d , a d i r e c t f e e d i n g l u b r i c a t o r should be used (Fig.10) l u b r i c a t o r d r i v e n v i a t h e d i r e c t d r i v i n g elements, e.g.

w i t h the

E c c e n t r i c D r i v e s , Throw

Plates, o r O f f s e t P i n Drives.

17.6.1.2

I n d i r e c t o r L i n e Systems have t h e pumping and m e t e r i n g elements geo-

g r a p h i c a l l y separated;

t he y a r e connected by means o f one o r s e v e r a l p i p e l i n e s .

The a c t i o n i s h y d r a u l i c ,

t h e pump s e r v i n g as a p r e s s u r e c r e a t i n g u n i t f o r pumping

grease i n t o a p i p e l i n e whi ch i s t h e r e b y s e t under pressure.

As t h e pressure

i nc r e a s e s , so t h e v a r i o u s m e t e r i n g elements e j e c t t h e i r charges e i t h e r progressi v e l y , o r a c c o r d i n g t o t h e back pre ssure s a g a i n s t which t h e y operate. D u r in g normal o p e r a t i o n s each element, h aving g i v e n up i t s metered amount,

blocks i t s e l f and w i l l n o t pass any f u r t h e r l u b r i c a n t t o t h e p o i n t s o f a p p l i c a tion.

When a l l elements have g i v e n up t h e i r metered charge, a r a p i d r i s e i n

pre s s u r e o c c u r s i n t h e main l i n e .

U t i l i z i n g e i t h e r t h i s i n c r e a s e i n pressure a t

t h e pump o r a t t h e end o f t h e l i n e , a s i g n a l i s g i v e n i n d i c a t i n g t h a t t h e l u b r i c a t i n g phase i s completed. Thereupon t h e main l i n e has t o be d ep ressurized, w hich i s u s u a l l y e f f e c t e d by opening i t t o t h e grease sup pl y u n i t , e.g.

the reservoir.

Depending on the

t y p e o r make o f system, t h e p l u n g e r s o f t h e m e t e r i n g v a l v e s a r e then r e s e t t o p e r m i t t h e i r f u r t h e r o p e r a t i o n , o r t h e y a r e a l r e a d y s e t f o r another a p p l i c a t i o n phase wh ic h moves them back i n t o t h e i r o r i g i n a l p o s i t i o n , thereby completing a system c y c l e .

The method and mechanics o f r e s e t t i n g depend on t h e p a r t i c u l a r

ty p e o f system; a l s o whether one complete c y c l e o f t h e system i n v o l v e s one o r two a p p l i c a t i o n ( l u b r i c a t i o n ) phases. The fundamental d i v i s i o n o f ' l i n e systems' p a r a l l e l t y p e s o f systems.

i s t h a t between p r o g r e s s i v e and

I n t h e p r o g r e s s i v e system t h e l u b r i c a n t must pass

thro u g h t h e m e t e r i n g elements o r v a l v e s p r o g r e s s i v e l y ,

i.e.

o n l y a f t e r having

a c t u a t e d t h e f i r s t element t o f e ed l u b r i c a n t t o t h e p o i n t o f a p p l i c a t i o n w i l l t h e l u b r i c a n t be passed t o t h e second element, and so f o r t h .

T h i s i s i n con-

t r a s t t o t h e o p e r a t i o n o f t h e p a r a l l e l system, on w hich t h e m e t e r i n g u n i t s a r e

YOl>INNO> OlNW

m

YOU3NN03 a 3 a ~ 33 1 1 ~ 1 s

SNOIWINNOJ L l l A n O

YJMOllOJON 3AVH Sl3aOW (110)V l

*I

:S)ltlOM 1 1 MOH

NOllV3l11!fll 01Slll11lN33 311VWOlflV 11Oj 1102V3111!111 0334 1331110 90P

407

408

Application: Lubiicat/onoluplo I W p ~ ) i n l ~ . ~ l l h g ~ e a l e o r o s . pafl(IcUlar1yon presses. machine lools, packagmg machinery

Introducingcentral lubrication in stages:

Design: A m a n pipe l m m a simple fillinp 01 lubricalion pump loads lo the dislrtbulor; the quantities of lubricant delivered are diitiibuled by the. d w i b u l o r 10the outlels in a parlicular pie. arranged sequence If required. the Iubricanl lrom the distributor can be led lo other distrtbulors lor lurlhet redislribulion

11 . .tg.: L u b r m i c o nof not more lhan 20 points by means 01 a grease nipple screwed into the dislributor andsgreasegun whichisoperaled untiltheflow indjcatoc shows tihat the lubr#calionoperation has been compleletl.

2nd .lago: A hand pump and dlslrlbUIorSin Series. WhlCh distribute the lubr~csnllo the lubrication point6 in a specified manner.

Fig. 11.

Examples o f Progressive Systems.

Fig. 12.

Progressive Lubrication on a plate shear.

409 a c t u a t e d as p r e s s u r e i n c r e a s e s i n t h e main l i n e . Each group i s f u r t h e r d i v i d e d i n t o systems u t i l i z i n g one l i n e o n l y and systems u t i l i z i n g two l i n e s , t h e f o r m e r b e i n g c a l l e d s i n g l e - l i n e systems, t h e l a t t e r d u a l - l i n e systems.

The most p o p u l a r t y p e s o f systems used a r e :

P r o g r e s s i v e Systems, o p e r a t e o n p r e s s u r e / v o lu m e c y c l e s ;

i n t h e i r case t h e

i n c r e a s i n g p r e s s u r e c r e a t e d by t h e pump a c t u a t e s a m e t e r i n g v a l v e w h ic h , h a v i n g g i v e n up i t s s e t amount o f l u b r i c a n t . a l l o w s t h e g r e a s e t o pass i n t o t h e main l i n e leading t o the next metering valve.

When s u f f i c i e n t p r e s s u r e has been

b u i l t up, t h e v a l v e i s a c t u a t e d and l u b r i c a n t a l l o w e d t o f l o w t o t h e n e x t m e te r i n g v a l v e , and so o n p r o g r e s s i v e l y , u n t i l i t r e t u r n s t o t h e l u b r i c a t o r o r where, when s u f f i c i e n t p r e s s u r e has been b u i l t up, a r e v e r s i n g v a l v e i s a c t u a t e d w h ic h reverses t h e f l o w o f t h e grease.

The s e l e c t i o n o f p r o g r e s s i v e systems i s depen-

dent upon t h e number o f p o i n t s t o be l u b r i c a t e d .

F i g u r e 1 1 i l l u s t r a t e s some

o p t i o n s a v a i l a b l e , and F i g . 1 2 shows a t y p i c a l i n s t a l l a t i o n o f a p r o g r e s s i v e l u b r i c a t i o n system on a P l a t e Shear. Dual L i n e Systems ( P a r a l l e l ) , o p e r a t e o n t h e same b a s i c p r i n c i p l e , v i z : t h e m o t o r i z e d l u b r i c a t i n g pump ( F i g . 1 3 )

f o r c e s g r e a s e i n t o one o f two main f e e d

l i n e s i n w hi ch a r e p l a c e d a number o f d u a l - l i n e m e t e r i n g e le m e n ts , each o u t l e t

M U orised BS-A h

p

SC-A Diffaeraial R c Un

MODE L BSA

Fig.13

Dual l i n e system.

410

I_

I

I_

Line 2 pressurising

Line 1 weuurising

2oo

Interval

I

Fig.14

end of line 1 b 2

Dual l i n e system p r e s s u r e .

66

Main Line 2

Fig.15

2

Dual l i n e m e t e r i n g element.

D u a l - l i n e m e t e r i n g elements o p e r a t e h y d r a u l i c a l l y w i t h o u t t h e use o f b a l l s and s p r i n g s o r check v a l v e s .

A c o n t r o l p i s t o n d i r e c t s t h e supply o f grease t o

e i t h e r s i d e of a m e t e r i n g p i s t o n which i s f i t t e d w i t h an i n d i c a t o r p i n , e n a b l i n g easy i n s p e c t i o n o f o p e r a t i o n a t each p o i n t on t h e system.

R e g u l a t i n g screws

f i t t e d t o each i n d i c a t o r housing p e r m i t adjustment down t o 20% o f maximum o u t p u t . The p o s i t i o n o f t h e c o n t r o l p i s t o n ( 2 ) and t h e feed p i s t o n (6) a r e shown i n ' A ' a f t e r the f i r s t p a r t o f the d u a l - l i n e cycle.

P r e s s u r i s e d l u b r i c a n t f r o m main

411 l i n e 1 has moved o v e r c o n t r o l p i s t o n ( 2 ) and then a t t h e upper s i d e o f t h e feed

( 6 ) , d i s p l a c i n g i t and d i s c h a r g i n g a measured q u a n t i t y o f l u b r i c a n t v i a c ro s s p o r t i n g (7) and a cross spool (1) t o o u t l e t ( 8 ) . P o s i t i o n ' B ' i s t h e second

piston

p a r t o f t h e d u a l - l i n e c y c l e and p r e s s u r i s e d l u b r i c a n t from main l i n e 2 has moved c o n t r o l p i s t o n ( 2 ) and d i s p l a c e d t h e m e t e r i n g p i s t o n (6) thus d i s c h a r g i n g a measured s h o t o f l u b r i c a n t v i a cross p o r t (5) and across spool ( 3 ) t o o u t l e t ( 4 ) . For e x tr e m e ly d i r t y and a b r a s i v e environments such as those found i n a B l a s t Furnace, P i g Caster o r Coal P r e p a r a t i o n and Washery P l a n t , i t i s a d v i s a b l e t o have t h e d u a l l i n e m e t e r i n g elements housed i n toughened g l a s s - f r o n t e d p r o t e c t i o n boxes, s i m i l a r t o t h a t shown i n F i g. 16 .

F i g. 16

P r o t e c t i o n box.

412 17.6.1.3

Comparison o f P a r a l l e l Systems - S i n g l e l i n e and D u a l - l i n e .

B o th systems d e p r e s s u r i z e t h e l i n e , b u t i n t h e case o f t h e S i n g l e L i n e System th e r e s e t t i n g o f t h e p l u n g e r s i n t h e m e t e r i n g elements i s u s u a l l y e f f e c t e d by s p r i n g p r e s s u r e , whereas i n t h e Dual L i n e system. when main feed l i n e No.2 i s p r e s s u r i s e d , a second s e r i e s o f dual l i n e m e t e r i n g elements d i s c h a r g e l u b r i c a n t t o the points o f application.

I n d o i n g s o , t hey r e s e t t h e dual l i n e m e t e r i n g

elements i n main l i n e No.1, w i t h which t he y f orm an i n t e g r a l u n i t .

17.6.1.4

S e l e c t i n g Grease L u b r i c a t i o n Systems.

Wherever p o s s i b l e , l u b r i c a t i o n systems sh ould be avoided w hich use equipment i n c o r p o r a t i n g s p r i n g s and v a l v e s , p a r t i c u l a r l y where t h e l u b r i c a t i o n systems a r e r e q u i r e d t o o p e r a t e i n arduous and a b r a s i v e environments. I n g e n e r a l p r a c t i c e , t h e m a j o r i t y o f C e n t r a l i s e d L u b r i c a t i o n Systems used a r e e i t h e r o f t h e ' D i r e c t Feed' o r ' P a r a l l e l - D u a l be in g o p e r a t e d ' m a n u a l l y ' o r ' a u t o m a t i c a l l y ' .

L i n e ' type;

b o t h a r e capable o f

The c h o i c e o f Grease L u b r i c a t i o n

Systems i s u s u a l l y d i c t a t e d by t h e number o f p o i n t s o f a p p l i c a t i o n and t h e i r l o c a t i o n , always b e a r i n g i n mind t h a t f o r utmost r e l i a b i l i t y and minimal maintenance D i r e c t Feed Systems sho ul d be s e l e c t e d wherever i t i s p r a c t i c a l . o f explanation,

By way

a P a r a l l e l - D u a l L i n e system can c y c l e and y e t f a i l t o d e l i v e r

l u b r i c a n t t o some of t h e p o i n t s o f a p p l i c a t i o n w hich i s o n l y e v i d e n t i f t h e o p e r a t o r v i s u a l l y examines t h e p o s i t i o n o f every dual l i n e element i n d i c a t o r pin. Q u i t e o f t e n t h i s i s i m p r a c t i c a l because o f elements b e i n g l o c a t e d i n hazardous areas

o n l y b e in g a c c e s s i b l e when t h e p l a n t and machinery a r e shut down.

T h i s cannot o c c ur w i t h D i r e c t Feed pumps, f o r t h e y w i l l o n l y o p e r a t e e f f e c t i v e l y providing

the points o f application are receiving lubricant;

course, broken f ee d-p i pe s

barring, o f

(which can a p p l y t o b o t h types o f system).

I n prac-

t i c e , a D i r e c t Feed pump can u s u a l l y c a t e r f o r up t o 40 p o i n t s o f a p p l i c a t i o n .

17.6.1.5

L u b r i c a t i o n o f P l a i n o r Sleeve Bearings.

These b e a r i n g s , p a r t i c u l a r l y t h ose o v e r 4" diameter, r e q u i r e as near c o n t i n uous l u b r i c a t i o n as i s p o s s i b l e .

I t i s n o r m a l l y recommended t h a t b e a r i n g s of

t h i s t y p e be f e d by D i r e c t Feeding l u b r i c a t o r s d r i v e n from t h e moving s h a f t . T h i s ensures l u b r i c a n t i s f e d t o t h e b e a r i n g s when t h e s h a f t i s i n o p e r a t i o n and no l u b r i c a n t i s f e d t o t h e b e a r i n g when t h e s h a f t i s i n o p e r a t i v e .

17.6.1.6

Lubrication o f Anti-Friction

( B a l l and R o l l e r ) Bearings.

A n t i - f r i c t i o n b ea ri ng s r e q u i r e c o n s i d e r a b l y s m a l l e r q u a n t i t i e s o f l u b r i c a n t tha n p l a i n b e a r in gs.

Except i n t h e case o f l a r g e a n t i - f r i c t i o n b e a r i n g s , they

w i l l n o t n o r m a l l y be l u b r i c a t e d c o n t i n u o u s l y . Where a n t i - f r i c t i o n be ari n gs a r e c l o s e t o g e t h e r , a hand-operated o r t i m e c l o c k c o n t r o l l e d d i r e c t f e e d i n g pump may be used.

Where they a r e spaced over

413 some d i s t a n c e ,

a l i n e system, e i t h e r hand-operated o r t i m e c l o c k c o n t r o l l e d ,

is

preferred. For l a r g e a n t i - f r i c t i o n b e a r i n g s and t h ose i n s t a l l a t i o n s where t h e g r e a t e s t r e l i a b i l i t y o f D i r e c t Feeding systems i s d e s i r a b l e

b u t t h e number o f p i p e s

should be k e p t low, d i r e c t f e e d i n g pumps w i t h P o s i t i v e D i v i d e r s may be used, s p l i t t i n g v o l u m e t r i c a l l y metered amounts o f l u b r i c a n t independent o f v a r y i n g back p r e s s u r e s . Very f a s t o p e r a t i n g a n t i - f r i c t i o n b e a r i n g s such as those running a t 1400 rpm However, o v e r - g r e a s i n g w i l l do no harm t o l a r g e

should n o t be f e d c o n t i n u o u s l y .

s l o w - r u n n in g a n t i - f r i c t i o n b e a r i n g s where q u i t e o f t e n t h e grease i s used as a s e a l a n t , p r e v e n t i n g d i r t and o t h e r f o r e i g n m a t t e r a n t i - f r i c t i o n bearings

-

t o e n t e r t h e b ea ri ng .

-

the greatest destroyer o f

Therefore i t i s essential t h a t

t h e grease i s k e p t c l e a n a t a l l t i mes, w i t h t h e l u b r i c a t i n g pump c o n t a i n e r o r r e s e r v o i r b o t to m f i l l e d v i a a grease keg o r b u l k grease s t o r a g e system.

17.6.2

O i l L u b r i c a t i o n Systems

O i l L u b r i c a t i o n Systems s e r v e two purposes:

t o l u b r i c a t e and/or c o o l .

On

many a p p l i c a t i o n s . p a r t i c u l a r l y i n t h e absence o f h i g h ambient temperature o r where t h e h e a t g en era t e d i n t h e b e a r i n g s o r t h e gears i s n o t g r e a t . t h e removal o f h e a t by t h e o i l need n o t f e a t u r e as a sep arate c o n s i d e r a t i o n i n t h e s e l e c t i o n o f t h e o i l c i r c u l a t i o n system.

T h i s can be arranged on t h e b a s i s o f l u b r i c a t i o n

c o n s i d e r a t i o n s a l on e.

i n t h e case o f many o t h e r a p p l i c a t i o n s , the

However,

c o o l i n g p r o p e r t i e s o f t h e o i l a r e o f g r e a t importance. O i l systems may t h e r e f o r e be grouped under t h r e e main headings:Group 1

Systems designed f o r l u b r i c a t i o n on a t o t a l loss basis.

Group 2

Systems designed f o r l u b r i c a t i o n and w i t h a small amount o f heat removal.

Group

3

Systems designed f o r l u b r i c a t i o n where an a p p r e c i a b l e degree o f c o o l i n g i s a l s o r e q u i r e d because o f operating conditions.

Group 1 and 2 systems v a r y a c c o r d i n g t o t h e t y p e o f machine and i t s l u b r i c a t i o n requirements.

Systems o f t h e t o t a l l o s s t y p e may be operated e i t h e r manually,

m e c h a n ic a lly , o r mo t o ri sed , whereas systems o f t h e t y p e which c o l l e c t t h e used o i l and r e c i r c u l a t e i t must be a u t o m a t i c .

V arious combinations o f these systems

can be employed, and t h e f o l l o w i n g a r e some t y p i c a l examples: 17.6.2.1

Group 1

- Total

loss systems designed f o r l u b r i c a t i o n purposes o n l y .

I n t h i s t y p e o f system t h e l u b r i c a n t , a f t e r l u b r i c a t i n g t h e b e a r i n g s o r gears, i s n o t used a g a in. pumps.

The group c o n s i s t s o f manual, mechanical o r motor operated

The f o r m e r g e n e r a l l y a p p l i e s on sma l l items o f p l a n t , e.g.

machine t o o l s ,

414 mechanical h a n d l i n g equipment, j i g s and f i x t u r e s , presses, e t c .

These systems

may be f u r t h e r s u b - d i v i d e d i n t o D i r e c t P o s i t i v e Systems or P o s i t i v e S p l i t Systems. D i r e c t P o s i t i v e Systems u s u a l l y comprise one o r more d i f f e r e n t i a l p l u n g e r ty p e o i l l u b r i c a t o r s , e.g.

Se ct i o ns 6.1.1

(F ig.9)

shows t h e o p e r a t i o n o f a man-

u a l l y o p e r a te d 8 - o u t l e t gre ase pump whi ch i s a l s o a d a p t a b l e f o r o i l , w i t h Fig.17 i l l u s t r a t i n g a t y p i c a l a p p l i c a t i o n on a pre ss l u b r i c a t i n g t h e slidew ays and crosshead.

For a p p l i c a t i o n s where a ut oma t i c l u b r i c a t i o n i s r e q u i r e d , mechanical

l u b r i c a t o r s h a v ing u s u a l l y 28 t o 32 pumping u n i t s may be f i t t e d ( F i g . 1 8 ) .

These

l u b r i c a t o r s may be d r i v e n e i t h e r m e c h a n i c a l l y t h r o u g h a r a t c h e t from t h e machine be in g l u b r i c a t e d o r by geared e l e c t r i c motor.

Each pump u n i t can be r e g u l a t e d

fro m z e r o t o maximum, t o f e e d m i n u t e p r e c i s e q u a n t i t i e s o f o i l t o t h e p o i n t s o f lubrication application.

Fig.17

D i r e c t p o s i t i v e system.

415

Fig.18

P o s i t i v e S p l i t System.

D i r e c t p o s i t i v e system.

T h i s t y p e o f system i s used where q u a n t i t i e s o f o i l

per a p p l i c a t i o n p o i n t a r e g r e a t e r t h a n can be s u p p l i e d by t h e p r e v i o u s l y described d i r e c t f e e d i n g p l u n g e r system.

I t comprises o f one o r more small h i g h

p r e s s u r e pumps f i t t e d w i t h i n t e g r a l r e l i e f v a l v e s , and s u p p l i e s l u b r i c a n t t o t h e b e a r i n g s through volume d i v i d e r s .

The d i v i d e r s may be used e i t h e r t o i n -

crease t h e number o f p o i n t s o r t o m o d i f y t h e q u a n t i t i e s f e d t o t h e l u b r i c a t i o n points.

416

Fig.19

17.6.2.2

P o s i t i v e s p l i t system.

Group 2 - Systems designed f o r l u b r i c a t i o n w i t h s m a l l amount o f

heat removal.

T h i s t y p e o f system s u p p l i e s t h e l u b r i c a n t t o t h e b e a r i n g s and

r e t u r n s i t under g r a v i t y through t h e r e t u r n p i p e s and/or drainways t o t h e s u p p l y tank f o r r e c i r c u l a t i o n .

T h i s group can a l s o be s u b - d i v i d e d i n t o D i r e c t P o s i t i v e

Systems o r P o s i t i v e S p l i t Systems.

The former c o n s i s t s o f t h e m e c h a n i c a l l y

operated d i f f e r e n t i a l p l u n g e r t y p e o i l l u b r i c a t o r as d e s c r i b e d i n Group 1.

417 T y p i c a l a p p l i c a t i o n s i n c l u d e paper machines, l a r g e k i l n s , o r g e n e r a l l y where a l a r g e number o f b ea ri ng s a r e t o be f e d p o s i t i v e l y w i t h an a d j u s t a b l e feed. Systems o f t h i s t y p e r e q u i r e a minimum amount o f maintenance and a t t e n t i o n .

The

l u b r i c a t i o n r e s e r v o i r s o f c o n t a i n e r s can be kept f i l l e d e i t h e r by a header t a n k s u p p l y i n g s e v e r a l l u b r i c a t o r s , o r each l u b r i c a t o r can be f i t t e d w i t h s p e c i a l b u i l t - i n s u c t i o n pumps whi ch su pp l y t h e c o n t a i n e r w i t h l u b r i c a n t from t h e main s u p p ly tank.

Each l u b r i c a t o r pump u n i t ( o u t l e t ) can be connected d i r e c t t o t h e

l u b r i c a t i o n p o i n t s o r t o a p o s i t i v e volume d i v i d e r , depending on t h e number o f feeds and t h e l u b r i c a t i o n re qu i reme nt s o f t h e p o i n t s . The l u b r i c a n t i s r e t u r n e d under g r a v i t y t o t h e main supply tank through d r a i n ways o r r e t u r n p i p e s (which can be a rran ge d w i t h b e a r i n g sump l e v e l c o n t r o l dev ic e s ) f o r r e c i r c u l a t i n g t o t h e l u b r i c a t o r c o n t a i n e r s o r header t a n k by means of s u c t i o n pumps, as d escri b ed , o r t o t h e t a n k by a f l o a t - c o n t r o l l e d gear pump (Fig.20).

-

Discharge f i l t e r

I

I b

d

L

&

6

6

b

d

b

b

b

b

1

b b b

b b d

\ l b

/

b

O

b

l

d

b

b b

Reservoir

F ig . 20

System w i t h smal l amount o f heat removal

As i n t h e case o f t h e p r e v i o u s group,

t h e l u b r i c a t o r can be d r i v e n e i t h e r

d i r e c t f r o m t h e machine b e i n g l u b r i c a t e d o r f i t t e d w i t h i t s own motor,

Each

pump can be f e d e i t h e r f ro m t h e machine sump o r from a separate d r a i n and supply tank.

The system i s u s u a l l y i n s t a l l e d on machine t o o l s , sugar machinery, gear

boxes, p r i n t i n g machines, and spe ci al -p urp ose machinery.

17.6.2.3

Group

3

-

Systems designed f o r l u b r i c a t i o n c o o l i n g .

Where t h e r e i s c o n s i d e r a b l e ambient h ea t o r where t h e power t r a n s m i t t e d by t h e p a r t b e in g l u b r i c a t e d c r e a t e s a h i g h degree o f h e a t , t h e c o o l i n g f u n c t i o n o f t h e l u b r i c a n t assumes g r e a t importance.

I t i s necessary i n such cases t o

ap p ly s u f f i c i e n t l u b r i c a n t t o e x t r a c t t h e h eat and t o m a i n t a i n t h e bearings o r gears a t an optimum t emp era t u re.

Arrangements must a l s o be made f o r r e t u r n i n g

t h e l u b r i c a n t t o a sup pl y t a n k f o r c o o l i n g , f i l t r a t i o n , and r e c i r c u l a t i o n , between t h e l u b r i c a t i o n equipment man uf act urer, t h e p l a n t designers, and t h e operators. Such a system u s u a l l y comprises a l a r g e o i l r e s e r v o i r o r s t o r a g e tank; motor d r i v e n pump ( n o r m a l l y a gear t y p e pump a d j a c e n t t o o r on t h e s t o r a g e t a n k ) ; coolers;

f i l t e r s ; p r e s s u r e gauges; a l a r m and f l o w c o n t r o l equipment; t o g e t h e r

w i t h t h e necessary v a l v e s and i n t e r c o n n e c t i n g pipework. and s e l f - c o n t a i n e d w i t h a c a p a c i t y o f 3 8 c c / s e c

The system may be simple

t o 750cc/sec

complex system c a pa bl e o f d e l i v e r i n g s e v e r a l l i t r e s / s e c .

p e r minute, o r a

These systems can be

p r o v i d e d w i t h s i m p l e o r e l a b o r a t e f l o w c o n t r o l , w arning d e v i c e s , and o t h e r i n s t r u m e n t a t i o n a c c o r d i n g t o t h e needs o f t h e i n s t a l l a t i o n (Fig.21).

Fig.21

T y p i c a l example o f l u b r i c a t i n g and c o o l i n g system packaged u n i t f o r t h e l u b r i c a t i o n o f rubber machinery.

419 17.6.3

Micro-Fog L u b r i c a t i o n Systems

Aerosol l u b r i c a t i o n i s t h e g e n e r i c term f o r o i l m i s t o r M i c r o - f o g systems which have been used s u c c e s s f u l l y f o r o v e r t w e n t y y e a r s .

Compared w i t h Centra-

l i s t e d Grease o r O i l l u b r i c a t i o n , a M i c r o - f o g system, t o p e r f o r m t h e same t a s k , r e q u i r e s l e s s l u b r i c a n t and energy and t h e i n i t i a l c o s t i s r e l a t i v e l y low. i s a l s o a h i g h l y f l e x i b l e system,

It

r e a d i l y i n s t a l l e d o n t o e x i s t i n g p l a n t as w e l l

as a t t h e new machine stage.

17.6.3.1

Working P r i n c i p l e s (Fig.22) o u t 1 i n e s t h e e s s e n t i a l and a u x i 1 i a r y

elements o f t h e M i c r o - f o g system and p r o v i d e s a g u i d e t o some o f t h e more common areas o f a p p l i c a t i o n .

During operation,

t h e system produces c o n t i n u o u s l y a dense

c o n c e n t r a t i o n of m i c r o p a r t i c l e s o f o i l which a r e conveyed i n a ' d r y ' f o g i n a low p r e s s u r e d i s t r i b u t i o n system.

On r e a c h i n g t h e p o i n t of a p p l i c a t i o n t h e

' d r y ' f o g i s passed t h r o u g h r e c l a s s i f i e r s , which a r e r e a l l y m e t e r i n g and condens i n g o r i f i c e s , so as t o a c c u r a t e l y f e e d an e x a c t q u a n t i t y o f l u b r i c a n t t o s u i t the operating conditions.

M i cro-foq

1u b r i c a t i o n

lubrication 1u b r i c a t i o n

Fig.22

M i c r o - f o g system.

420 To ensure t h a t t h e f o g reaches t h e r e c l a s s i f i e r d r y ,

the d i s t r i b u t i o n piping

i s sized t o allow o i l p a r t i c l e s t o travel along the piping a t a v e l o c i t y less tha n 7.3 metres/sec,

which i s sl o w enough t o p r e v e n t condensation.

l en c e i n t h e r e c l a s s i f i e r s causes t h e o i l p a r t i c l e s t o 'w et o u t '

The t u r b u -

i n t o the l i n e

l e a d i n g t o o r d i r e c t o n t o t h e b e a r i n g s u r f a c e s where they then form a p r o t e c t i v e f i l m of oil. In o r d e r t o c r e a t e ' d r y '

f og , o i l i s f i r s t drawn i n t o a compressed a i r stream

as i t passes t h r o ug h a V e n t u r i l o c a t e d on t o p o f t h e l u b r i c a t i o n c o n t r o l u n i t (Fig.23).

O i l p a r t i c l e s o f a p p r o x i m a t e l y 0.002mm

i n diameter a r e c o l l e c t e d i n

t h e a i r stream and can be t r a n s p o r t e d l o ng d i s t a n c e s i n t h e d r y c o n d i t i o n .

Lubricator head A i r bypass a d j u s t i n Low pressure w i t c h

lligh pressure s w i t c h Electrical conduit ent Auxiliary requlatcr

Supply t o pressur j e t reclassifier

Waste pipe e x i t

/

Fig.23

Lubrication control unit.

Since M i c r o - f o g systems r e q u i r e no r e t u r n l i n e s they can be designed t o easy i n s t a l l a t i o n and assembly;

hence, l o w c o s t i n s t a l l a t i o n , w i t h o u t t h e problems

a s s o c i a t e d w i t h s i n g l e - l i n e s e r i e s o i l c i r c u l a t i n g systems w hich i n c o r p o r a t e q u i t e s o p h i s t i c a t e d and r e l a t i v e l y expensive v a l v e b l o c k s t h a t have a b u i l t - i n self-reversing operation.

A l t ho ug h such systems may r e q u i r e no separate reverse

and r e c y c l e v a l v e o r v e n t i n g phase d u r i n g t h e l u b e c y c l e , they do have, however, a d i s t i n c t d is a d va nt ag e because t h e y r e q u i r e a m u l t i p l i c i t y o f system t u b i n g (Fig.20). Moreover, Once a M i c r o - f o g system has been i n s t a l e d i t i s much less demandi n g t h a n o t h e r a ut o mat i c l u b e systems.

The system

s e a s i l y assembled,

using a

b u i l d i n g b l o c k approach, whi ch i s designed f o r easy s e r v i c i n g , r e p a i r s and general maintenance.

421 17.6.3.2

O i l Quality.

C o r r e c t o i l s e l e c t i o n i s i m p o r t a n t as some o i l s i n c o r p o r a t e polymers w hich suppress a e r o s o l p r o p e r t i e s , w h i l e h e a v i e r g rade o i l s may r e q u i r e h e a t i n g up t o between 4Oo/45OC

t o a t t a i n t h e v i s c o s i t y f o r maximum o u t p u t . For a l l normal

purposes t h e c o n t r o l u n i t s w i l l p e r f o r m w e l l when w orking w i t h o i l s up t o 700 c e n t i s t o k e s a t 20°C. Summarised l u b r i c a t i n g o i l re qu i reme nt s f o r a s a t i s f a c t o r y M i c r o - f o g system are:(i)

Good a e r o s o l p r o p e r t i e s .

(ii)

Low r a t e o f co nd en sat i on t hro ug h p i p e s .

( i i i ) Low l e v e l o f s t r a y i n g by t h e p a r t i c l e s . (iv)

A h i g h degree o f r u s t i n h i b i t i o n .

(v)

Absence o f c l o g g i n g t en de nci e s a t t h e v e n t u r i n o z z l e o r any polymer precipitation.

17.6.3.3

Compressed A i r

2

A c c e p t in g t h a t most i n d u s t r i a l compressed a i r i s s u p p l i e d a t 7 bar (100 l b / i n ) 2 i n a M i c r o - f o g l u b r i c a t i o n system i t has t o be reduced t o about 2 b a r (30 l b / i n 1. Dur in g i t s passage t h r o u g h t h e v e n t u r i o r i f i c e on t h e c o n t r o l u n i t , a p r e s s u r e

2

dro p o f 0 . 7 bar (10 I b / i n ) t ake s pl a ce.

17.6.3.4

System Design Con si de rat i o ns

To c a l c u l a t e t h e l u b r i c a t i o n re qu i reme nt s o f bearings, an e m p i r i c a l f a c t o r r e f e r r e d t o as a ' l u b r i c a t i o n u n i t '

(L.U.)

has been evolved,

e n a b l i n g a l l moving

s u r f a c e s r e q u i r i n g l u b r i c a n t t o be co nve rt e d t o t h e i r e q u i v a l e n t L.U. The amount o f l u b r i c a n t main

rating.

and bra nch l i n e p i p e bores and r e c l a s s i f i e r nozzles

may t h e n be s i z e d t o p r o v i d e t h e c o r r e c t amount o f l u b r i c a n t a t each l u b r i c a t i n g point.

I n t h i s manner, a n t i - f r i c t i o n b ea ri ngs, j o u r n a l bearings, s l i d e s , gears,

c h a in s , and o t h e r we ari n g s u r f a c e s r e q u i r i n g l u b r i c a t i o n can a l l be converted t o e q u i v a l e n t L.U.

r a t i n g s and se rved by a p p r o p r i a t e l y s i z e d M i c r o - f o g l u b r i c a t i o n

systems.

17.6.3.5

Some T y p i c a l A p p l i c a t i o n s

F i g u r e 24 i l l u s t r a t e s a t h r e e S t ran d Aluminium F o i l M i l l o p e r a t i n g a t 1000 t o 1500 m e t r e s / m in u t e w i t h two 1000 L.U. g e n e r a t i n g heads ( t h i r d a c t s as a standby) s e r v i n g t h e m i l l s t a c k and e x i t a n c i l l a r i e s , w i t h a s e p a r a t e 300 L.U. u n i t serving the entry a n c i l l a r i e s .

2.5 l i t r e s p e r w o r k i n g hour. motor oears.

generating

The t o t a l amount o f o i l used i s l e s s than

F i g u r e 25 i l l u s t r a t e s t h e l u b r i c a t i o n o f v i b r a t o r

422

423

i

I

"i" 'i' 0-

I

Fig.25

17.6.4

Lubrication o f vibrator motor gears.

Check List

Based o n the foregoing, prior to actual selection of the appropr ate lubricant and associated equipment, it is advisable to draw up a check 1 st of known The following list, in simplified form, is for guide purposes only and can naturally be elaborated upon to suit the spec fic needs o f facts and requirements.

the designer o r plant engineer. Specification o f Plant t o be lubricated: Type of plant and machinery to be lubricated. Industrial Application, including operating environmental conditions, e.g. is i t dirty, abrasive, wet, hot, etc.? Surfaces, sizes and speeds to be lubricated, e.g. Bearings (plain), Bearings (roller), Grease (type), Slideways, Chains, etc. Number and Location of lubrication Points (Fixed) and (Moving) and proposed site (if known) for lubricator enabling assessment of pipe and flexes: Frequency plant and machinery operators and whether lubrication needs to be continuous, semi-continuous, or intermittent.

424

-

L u b r i c a n t t ype and s p e c i f c a t i o n S e r v i ces a v a i l a b l e i n t h e P l a n t

-

Grease, O i l , o r M i c r o - f o g ,

etc

e l e c t r i c , pneumatic.

H e a l t h and S a f e t y a spe ct s e.g. normal, flameproof, a c c e s s i b i l i t y any h i s t o r y o f p r e v i o u s l y t r i e d l u b r i c a n t s / e q u i p m e n t . Method proposed f o r f i l l i n g t h e l u b r i c a t i o n t a n k / r e s e r v o i r , e.g. manual, semi-automatic, b u l k storage. Recommended L u b r i c a t i o n Equipment: Why? E s tima t e d performance w i t h any known Case Studies and References. Econom ic s

.

Spares and S e rvi ce. Sometimes equipment s e l e c t i o n i s an i n e v i t a b l e compromise as a r e s u l t o f c o n f l i c t i n g l u b r i c a t i o n re qu i reme nts;

i n such circumstances i t i s

im p o r t a n t t o a p p r e c i a t e a l l t h e f a c t s and t o subsequently gauge p l a n t performance a c c o r d i n g l y . 17 7

SUMMARY

T h i s c h a p t e r has a t t emp t e d t o c l a r i f y t h e more g e n e r a l l y accepted methods o f l u b r i c a t i o n , where necessary i l l u s t r a t i n g a c t u a l examples o f p l a n t and l u b r i c a t i o n equipment.

I t i s n o t i n any way i n t e nded t o i n f e r t h a t t h i s i s t h e o n l y

l u b r i c a t i o n equipment a v a i l a b l e ; from Trade J o u r n a l s , e t c .

o f co urse , t h e r e a r e o t h e r s r e a d i l y determined

L i k e w i s e , t h e r e a r e many more s e l e c t items o f l u b r i -

c a t i o n equipment t a i l o r - m a d e t o meet t h e needs o f s p e c i a l i s t p l a n t . For example, Overhead and F l o o r Conveyors which can t r a v e l up t o 50 metres/ m in u te , u n l e s s e f f e c t i v e l y l u b r i c a t e d (and cl eaned where t h e environment demands) can wear and s e i z e , r e s u l t i n g i n c o s t l y stoppages. l n i t i a l y , c o n v e n t i o n a l s t a t i c l u b r i c a t o r s were used where a m i x t u r e o f a i r and o i l

-

not only

and now more r e c e n t l y j u s t o i l

- was

shot over a gap, w hich r e s u l t e d

n t h e f a i l u r e t o a de qu at el y l u b r i c a t e (Fig.1)

b u t a l s o caused d r i p -

page w i t h c o n s e qu en t i a l p rod uct c o n t a m i n a t i o n and h e a l t h hazards. These problems were c o m p l e t e l y overcome by i n t r o d u c i n g a range o f s p e c i a l purpose l u b r i c a t o r s ;

(F i g. 26 ) shows one such example.

The c o n c l u s i o n i s t o d et ermi n e a l l t h e f a c t s r e g a r d i n g t h e p l a n t t o be l u b r i c a t e d and t o t he n e v a l u a t e t h e l u b r i c a n t s and l u b r i c a t i o n equipment ava i 1a b le .

425

Fig.26

A dog c h a i n assembly engages t h e conveyor c h a i n which moves t h e o i l - d i s p e n s i n g n o z z l e p l a t e s i n and o u t o v e r t h e p i n l i n k s . O i l i s f o r c e d t h r o u g h each n o z z l e .

REFERENCES

1 2

3 4

5

Mechanical L u h r i c a t i o n o f E.O.T. Cranes by D r . H . P e t e r J o s t and P e t e r W. Murray. A F u l l y Automatic B u l k H a n d l i n g L u b r i c a t i o n System f o r a S i n t e r P l a n t by G. W i l l i a m s . An E n g i n e e r i n g Approach t o t h e s e l e c t i o n o f C e n t r a l i s e d Grease L u b r i c a t i o n Systems by D r . H. P e t e r J o s t . Modern B r i t i s h and European Steelworks L u b r i c a t i o n Developments by D r . H. P e t e r J o s t . E!ew M i s t L u b r i c a t i o n Concepts f o r Tapered R o l l e r Bearinos used on High C.H. West and Speed R o l l i n g M i l l Back-up by R o l l s by \!.E.McCoy, P.E. W i l k s .

426

6

7 8 9 10

Aerosol L u b r i c a t i o n Systems - t h e i r c o n t r i b u t i o n t o savings i n o p e r a t i n g and maintenance c o s t s by R.E. K n i g h t and J.G. M e r r e t t . Micro-Fog L u b r i c a t i o n f o r b e a r i n g e f f i c i e n c y by J.G. M e r r e t t . Automatic L u b r i c a t o r s and Cleaners Increase Conveyor L i f e by J.G. M e r r e t t . Automatic L u b r i c a t i o n o f Chain and Conveyor Systems by R.M. Dombroski. The James C l a y t o n L e c t u r e - "Energy Saving through T r i b o l o g y " by D r . H. P e t e r J o s t and D r . J. S c h o f i e l d .

421

18 R.A.

ON CONDITION MAINTENANCE

COLLACOTT, Ph.D.,

Director Head

18.1

-

-

B.Sc(Eng)

,

F. I .Mar. E . ,

F. I .Mech.E.

UK Mechanical H e a l t h M o n i t o r i n g Group

F a u l t D i a g n o s i s Centre, L e i c e s t e r P o l y t e c h n i c .

INTRODUCTION

Maintenance c a r r i e d o u t when r e q u i r e d a f t e r a s i g n i f i c a n t d e t e r i o r a t i o n i n a component as i n d i c a t e d by a sensor o r m o n i t o r e d parameter i s c a l l e d o n - c o n d i t i o n ma i n tenance. I f a person, when v i s i t i n g t h e d o c t o r f o r a h e a l t h - c h e c k was i n v i t e d t o have t h e i r body opened up t o see whether ' e v e r y t h i n g was c o r r e c t ' a most u n s a t i s f a c t o r y s t a t e o f a f f a i r s would e x i s t .

S i m i l a r l y , t o open up a machine i n o r d e r

t o check t h a t i t i s a l r i g h t i s j u s t as u n s a t i s f a c t o r y good can be done t o a machine i n t h i s way.

-

much more harm than

A c c o r d i n g l y i t i s l o g i c a l t o use

d i a g n o s t i c techniques t o assess t h e ' h e a l t h ' o r c o n d i t i o n o f p l a n t and machi n e r y i n j u s t t h e same way as a medical d o c t o r uses symptoms and a i d s t o assess t h e c o n d i t i o n o f human beings

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and v e r y o f t e n s i m i l a r o r i d e n t i c a l equipment

i s used b o t h by t h e medical d o c t o r and d i a g n o s t i c engineer.

18.2

BACKGROUND

O n - c o n d i t i o n m o n i t o r i n g i s a l r e a d y a c t i v e l y and e f f e c t i v e l y used i n many industries

-

i n a i r c r a f t , nuclear reactors, steel m i l l s , petroleum r e f i n e r i e s ,

s h i p s , power g e n e r a t i o n e t c .

I t i s a technology which has r a p i d l y e v o l v e d

d u r i n g t h e p a s t 5 t o 10 years by u s i n g methods and techniques which have been developed s i n c e 1945. H i s t o r i c a l l y , t h e c h a i n o f e v o l u t i o n i s shown i n Fig.1. 1750

1915 1940 1950 1960 1970

... steam engines ... steam t u r b i n e s ... steam t u r b i n e s ... n u c l e a r r e a c t o r s ... space v e h i c l e s ... ... ... ... ...

Figure 1

-

s i m p l e c o n t e n t s gauges, simple governor p r e s s u r e , tempe rature, c o n t e n t s gauges e t c . automatic c o n t r o l s robotry m i n u t i a r i s a t i o n , remote t e l e m e t r y computer m i n u t i a r i s a t i o n , microprocessors

H i s t o r i c a l chain o f e v o l u t i o n of c o n d i t i o n monitorinq

428

18.3

MANAGEMENT OF CONDITION MONITORING

There a r e t h r e e e f f e c t i v e stages i n t h e management o f an o n - c o n d i t i o n maintenance system which go hand-in-hand w i t h a change i n o r g a n i s a t i o n whereby maintenance i n v o l v e s t h e r u n n i n g o f ( i ) a d i a g n o s t i c c a p a b i l i t y ,

( i i ) a repair

team. The t h r e e stages i n d i a g n o s t i c management a r e :

1)

F a i l u r e Modes and E f f e c t s A n a l y s i s

-

whereby t h e whole/complete p l a n t i s analysed s y s t e m a t i c a l l y t o determine which p a r t s a r e c r i t i c a l and need t o be m o n i t o r e d , a l s o t o a p p r a i s e t h e i r t y p i c a l f a i l u r e cause

2)

M o n i t o r i n g Technique S e l e c t i o n and Sensor A p p r a i s a l

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whereby t h e most e f f e c t i v e method i s chosen

3)

L i m i t Decision

-

whereby t h e amount o f d e t e r i o r a t i o n which can be p e r m i t t e d i s decided upon

A f u l l account o f a l l t h r e e stages i s g i v e n i n r e f e r e n c e

18.4

[l].

FAILURE MODES AND EFFECTS ANALYSIS

A t y p i c a l example o f t h e way i n which t h i s can be done was e x p l a i n e d by

Venton and Harvey [ 2 ] ,

B r i d g e s [ 3 ] and mentioned by Davies [ 4 ] , i n essence i t

i s t h e use o f i n f o r m a t i o n t o p r e p a r e a numerical assessment o f t h e o r d e r o f e s s e n t i a l i t y o f v a r i o u s components (sub-systems) w i t h i n a p l a n t o r machinery. A t i t s most elementary t h i s may be prepared by a s i m p l e ' D e l p h i ' a n a l y s i s ;

at

i t s more complex i t may i n v o l v e an i n t e g r a t e d d a t a - a p p r a i s a l method u s i n g f a i l u r e r a t e data which can be o b t a i n e d f o r d i f f e r e n t components

-

such f a i l u r e

r a t e data can be o b t a i n e d f r o m a number o f e s t a b l i s h e d d a t a banks [ 5 ] .

A t y p i c a l ' s t a r t i n g ' a p p r a i s a l f o r a marine power p l a n t m i g h t be t h e i n t e r dependency c h a r t f o r a l l sub-systems as shown i n Fig.2.

T h i s a l r e a d y shows t h a t

f o r t h e p a r t i c u l a r m i s s i o n f o r which t h i s a p p r a i s a l was made, h i g h r e l i a b i l i t y o f e l e c t r i c a l g e n e r a t i o n was i m p o r t a n t

-

a s t a t e o f a f f a i r s which was c o n f i r m e d

by a b a s i c PHASE CRITICALITY ANALYSIS and remained unchanged when allowance was made f o r t h e i n h e r e n t s a f e t y t h r o u g h a HAZARDS AND R I S K S ANALYSIS. The causes o f p o t e n t i a l f a i l u r e need t o be e s t a b l i s h e d . be d e r i v e d from h i s t o r i c a l r e c o r d s

-

Again, t h i s must

i t i s i m p o r t a n t t o make, m a i n t a i n and

analyse r e c o r d s i n o r d e r t o p r o v i d e adequate i n f o r m a t i o n upon which t o choose t h e c o r r e c t m o n i t o r i n g methods.

I n a t y p i c a l a n a l y s i s by W i l k i n s o n and

K i l b o u r n [61 t h e f a i l u r e r a t e d a t a o f Table 18.1 was used t o determine t h e best arrangement o f an e l e c t r i c a l g e n e r a t i n g standby p l a n t t o choose t h e most re1 i a b l e system.

429 I nter-dependen t System System

(1)

2) T r a n s m i s s i o n

3) o i l

(2)

(3) ( 4 ) ( 5 ) ( 6 ) (7) ( 8 ) (9)

x

1 ) Main e n g i n e

x

X

(10)

x

(11)

x

(12) (13)

x

X X

fuel

4 ) Compressed

X

X

air

x

5) E l e c t r i c a l

x

x

x

genera t i o n

6) S t e e r i n g

X

gear

7) Deck

X

X

machinery

8) Sea w a t e r

X

9 ) B i I g e and

X

ballast

10) V e n t i l a t i o n

X

11) Exhaust

X

12) C o n t r o l s

13)

Fig.2

x

X

Steam

x

System i n t e r - d e p e n d e n c y A n a l y s i s

As a consequence o f p a s t o p e r a t i o n a l e x p e r i e n c e - e i t h e r by an i n d i v i d u a l user o r f r o m a m a n u f a c t u r e r o r some d a t a c e n t r e such as t h a t p r o v i d e d by L l o y d s R e g i s t e r o f S h i p p i n g o r t h e UK Atomic Energy A u t h o r i t y (Systems Re1 i a b i l i t y S e r v i c e ) - i t i s p o s s i b l e t o p r e p a r e FAULT TREES such as t h a t shown i n

F I ~ [. 7 ~] .

D e t a i l e d e x a m i n a t i o n o f such f a u l t t r e e s w i l l

i d e n t i f y t h e poss-

i b l e p r i m a r y and secondary causes o f f a i l u r e s and t h e l i k e l y symptoms they w i l l produce. F a i l u r e symptoms a r e t h e p h y s i c a l consequences o f a m a l f u n c t i o n s i t u a t i o n .

I f f o r example one c o n s i d e r s b e a r i n g f a i l u r e s t h e r e a r e two q u i t e d i f f e r e n t p h y s i c a l e f f e c t s between r o l l e r element b e a r i n g s and p l a i n b e a r i n g s .

With

r o l l e r e l e m e n t b e a r i n g s e x p e r i e n c e shows t h a t f a i l u r e s a r e a s s o c i a t e d w i t h l o c a l s u r f a c e d e f e c t s , c o n s e q u e n t l y o p e r a t i o n o f d e f e c t i v e b e a r i n g s s e t s up u l t r a h i g h f r e q u e n c y shock waves whi&

can be most e f f e c t i v e l y diagnosed by shock

p u l s e methods p o s s i b l y a i d e d by s t a t i s t i c a l a n a l y s i s by a method c a l l e d 'kurtosis'.

On t h e o t h e r hand, p l a i n b e a r i n g s wear i n a u n i f o r m w h o l e - s u r f a c e

method w i t h t h e r e s u l t t h a t c l e a r a n c e s a r e changed and some i n t e r f e r e n c e ( r u b b i n g ) may ensue;

f o r such c o n d i t i o n s , methods o f c l e a r a n c e measurement by

i n d u c t i v e p i c k - u p s or v i b r a t i o n measurement or even d e b r i s a n a l y s i s may be appropriate.

For t h e two c l a s s e s o f b e a r i n g s d i f f e r e n t sensor systems w i l l be

430

L

aao u

m

FIGURE 3

Typical FAULT TREE for a bearing failure analysis

c

-

PROBLEM IIEFINITION

EVALUATION OF

RMA

EVALUATION

FUhCTlONAL BLOC DIALRAMS ALTERNATIVE SYSTEMS

OBJECTIVES, REOUIREMENTS

I

AN0

I

ANALYSIS REVIEW

t i 1 TRADE-OFF

I I I

OF

ENVIRONMEIITAL

I

OF

FORMULATION Of A N A I Y Y ,

FORMULATION

I

PROGHAM

STUDIES

OUANTITATIVE RMA - ANALYSIS

I

A N A l YSI5

OF RELIABILITY BLOC

DIAGRAMS

CHARACTERISTICS

I

+I

FORMUI.ATION

OF MATHEMATICAL

MODELS

J

f

.1

FAlLlJRE DATA COLLECTION

I

JI CORRECTIVE MAINTENANCE

e

ANALYSIS

f

MAINTENANCE

t

PREDICTION

FAULT TREE ANALYSIS

DATA COLLEC

FIGURE 4

I

I

I

OF

R M A - CHARACTERISTICS

ANALYSIS REVIEW

r TRADE-OFF STUDIES

L

Reliability analysis

1

sequence

I

diagram

-

I

1 1

432 ava i 1 a b I e

.

Thus t h e w h o l e i n i t i a l a n a l y t i c a l s t a g e c a n be r e p r e s e n t e d b y a number o f p r o c e d u r e s w h i c h h a v e been f u l l y s e t o u t i n F i g . 4

TABLE 18.1

[7].

F a i l u r e / r e p a i r data Repair Rate, Repa i r s / h

Mean Time t o Repair, h ( 2 men)

T u r b o - a l t e r n a t o r and c o n t r o l s : a l t e r n a t o r end t u r b i n e end

0.08197 0.122

12.2 8.2

D i e s e l a l t e r n a t o r and c o n t r o l s : a l t e r n a t o r end d i e s e l end

0.08197 0.05051

12.2 19.8

T u r b o - a l t e r n a t o r s y s t e m sea w a t e r c i r c u l a t i n g pump

0.1

10.0

D i e s e l a l t e r n a t o r sea w a t e r c i r c u l a t i n g pump

0.1493

6.7

Condenser

0.1667

6.0

A i r ejector

0.1136

8.8

E x t r a c t i o n pump

0.09434

B o i l e r f e e d pump

0.1429

Waste h e a t e c o n o m i s e r - t y p e b o i l e r

0.03636

27.5

O i l fired boiler

0.05+:

20 . O ”

Composite b o i l e r c i r c u l a t i n g pump

0.09259

10.8

Components

:l:

10.6 7.0

Estimated

To e n s u r e t h a t t h e c h a r a c t e r i s t i c s o f e a c h f a i l u r e / d e f e c t m a l f u n c t i o n i t i s n e c e s s a r y t o be a b l e t o r e c o g n i s e t h e c h a r a c t e r i s t i c s

-

which i s o n l y obtained

through extensive defect r e c o g n i t i o n experience.

18.5

MONITORING TECHNIQUE SELECTION

There a r e b a s i c a l l y f o u r techniques f o r t h e m o n i t o r i n g o f p l a n t and machinery d e t e r i o r a t i o n : 1.

dynamic methods

-

in p a r t i c u l a r v i b r a t i o n monitoring but also i n c l u d i n g t h e u s e o f a i r - b o r n e sounds

433 2.

inspection/integrity

-

s u r v e i l l a n c e methods

which o r i g i n a t e d w i t h n o n - d e s t r u c t i v e t e s t i n g techniques b u t have now extended t o i n c l u d e leak-testing;

odour i d e n t i f i c a t i o n ;

corrosion

m o n i t o r i n g and s t r e s s wave emission

3. contaminant i n s p e c t i o n

-

as a means o f i d e n t i f y i n g wear d e b r i s and r e l a t i n g i t b o t h q u a n t i t a t i v e l y and q u a l i t a t i v e l y t o i t s source

4. trends analysis

-

e f f e c t i v e l y data l o g g i n g e i t h e r as s t r a i g h t sensor o u t p u t s as f o r example temperatures, pressures and speeds o r i n a c o o r d i n a t e d form u s i n g such parameters as s p e c i f i c f u e l consumpt i o n r a t e o r even ' d e l t a s

'

of variations

f r o m the norm

18.5.1

V i b r a t i o n Monitoring

T h i s i s a w e l l e s t a b l i s h e d technique r a n g i n g i n s o p h i s t i c a t i o n from t h e use o f j u d i c i o u s l y p l a c e d d i a l gauges i n c o n j u n c t i o n w i t h stroboscopes t o broadband a n a l y s i s , narrow band a n a l y s i s , a u t o - c o r r e l a t i o n , o t h e r h i g h l y instrumented techniques [8].

s i g n a l a v e r a g i n g and

Most m o n i t o r i n g a p p l i c a t i o n s a r e

s a t i s f a c t o r i l y d e a l t w i t h by means o f : ( i ) p r o x i m i t y probes and pick-ups w i t h p o s s i b l y a cathode ray tube ( c r t ) display

( ii ) seismic v e l o c i t y transducers o r p i e z o - e l e c t r i c accelerometers o u t p u t t i n g to either (a) broad-band ( o v e r a l I ) v i b r a t i o n meters (b) v i b r a t i o n spectrum a n a l y s e r s ( p a r t - o c t a v e o r narrow-band width) ( i i i ) waveform a n a l y s e r s

18.5.1.1

P r o x i m i t y Probes

The o r b i t moved through a s h a f t which i s loose i n i t s b e a r i n g s y e t s u b j e c t e d t o t h e i n f l u e n c e o f v a r i o u s f o r c e s can be observed by u s i n g two p r o x i m i t y probes p l a c e d a t a 90" r e l a t i v e a n g l e and t h e i r o u t p u t s l e d t o an X - Y p l o t t e r o r a CRT.

The r e s u l t i n g d i s p l a y , Fig.5,

can be used t o measure t h e a c t u a l amount

o f o r b i t a l e c c e n t r i c i t y (and t h u s t o determine whether ' b e a r i n g w i p e ' i s 1 i k e l y t o o c c u r ) a l s o t h e shape o f t h e o r b i t i s r e l a t e d t o t h e t y p e o f d e f e c t f o r c e 90

that the source o f troub/e cdn be estab/ished.

434

PICKUP

Fi g .5

S h a ft o r b i t a l a n a l y s i s

18.5.1.2

Seismic V e l o c i t y Transducers/Accelerometers/Vibration Meters/Spectrum A n al yse rs

T h i s i s th e deve l op i ng are a o f i n t e r e s t i n ' s t r a i g h t ' v i b r a t i o n a n a l y s i s . V e l o c i t y t r a n s d u cers a r e most a p p l i c a b l e a t t he lower frequency ranges; erometers a r e e f f e c t i v e a t t h e h i g h e r f req ue ncy ranges;

accel-

a t intermediate

fre q u e n c ie s (around 500 Hz = 500 x 60 = 3000 cpm) e i t h e r transducer i s a p p l i c ab le . With o v e r a l l (wide) bandwidth v i b r a t i o n a n a l y s i s a transducer p i c k s up a s i g n a l d e r i v e d f rom a l l t h e c o n s t i t u e n t f r e q u e n c i e s and t h i s i s measured by the meter

-

which a c c o r d i n g l y t e l l s whether t h e v i b r a t i o n i s i n c r e a s i n g i n

strength,

i e . t h e system i s d e t e r i o r a t i n g .

To t e l l what i s d e t e r i o r a t i n g i n a machine i t i s necessary t o measure t h e v i b r a t i o n s i g n a l produced by each c o n s t i t u e n t component. r e c o r d i n g t h e fr equ en cy 'sp ect rum' f o r t h e machine.

T h i s i s done by

Thus each component w i l l

ge n e r a te v i b r a t i o n s a t a p a r t i c u l a r f req ue ncy ( i t s ' d i s c r e t e ' frequency) and when p l o t t e d as i n Fig.6 produce a ' s p i k e ' on t h e graph a t t h a t frequency. I f the ' s p i k e '

incre ase s w i t h succeeding s p e c t r a i t w i l l mean t h a t a defect

de v e lo p in g i n t h a t p a r t i c u l a r component.

is

To know t h e values f o r t h e d i s c r e t e

fr e q u e n c ie s f o r d i f f e r e n t components i t i s necessary t o make frequency c a l c u l ations o f various kinds

-

i n any event machine designers make these c a l c u l a t i o n s

when machines a r e designed and such i n f o r m a t i o n can be o b t a i n e d when t h e machines a r e b e i ng purchased. Table.

Typical frequencies are given i n the f o l l o w i n g

435

5

FREQUENCY

Fig.6

T y p i c a l frequency spectrum

TABLE 18.2

D i s c r e t e Frequency C a l c u l a t i o n s

V i b r a t i o n Type

Frequency Equation

Simple harmonic

f

Pendu 1 um ( s imp1e )

f = L 9

Pendulum (compound)

f = -1

= -1-

A

2n M

2n L

Bar, u n i f o r m l y loaded, f i x e d b o t h ends Shaft, t o r s i o n a l o s c i l l a t i o n s , s i n g l e f l y w h e e l

f =

w

L2

1 2 271 I

Ball bearing

-

m a l f u n c t i o n o f o u t e r race

nN f = - (1 2

Ball b e a r i n g

-

m a l f u n c t i o n o f i n n e r race

f =

Ball b e a r i n g

-

defective b a l l

D f = N(d

Gear t e e t h

-

d -cos D

B)

nN d ( 1 + - cos 2 D

B)

f = N

tooth defect

ih2)

L!.

- 3.57

f

where, A = s t i f f n e s s o f system M = mass o f system g = acceleration o f g r a v i t y L = l e n g t h o f pendulum h = distance,c.g. t o p i v o t , compound pendulum k = r a d i u s o f g y r a t i o n about c.g., compound pendulum !i= l e n g t h o f b a r E = m o d u l u s o f E l a s t i c i t y (Young’s Modulus) I = second mment of are a o f s e c t i o n about n e u t r a l a x i s w = load p e r u n i t l e n g t h a p p l i e d t o bar T = t o r s i o n a l s t i f f n e s s o f s h a f t CJ .9

gh

2n (k2

1

d -C O S ~8 ) D

t

= shear modulus = p o l a r second moment of area p e r s h a f t = p o l a r moment o f i n e r t i a o f f l y w h e e l = number o f b a l l s i n b e a r i n g N = s h a f t speed (rev/min) d = b a l l diameter D = b a l l p i t c h c i r c l e diameter N1= gearwheel speed (rev/min) t = number o f t e e t h on gearwheel.

G J I n

436 When u s i n g v i b r a t i o n s sensors p a r t i c u l a r c a r e must be taken i n s e l e c t i n g t h e i r mounting p o s i t i o n and i n methods by means o f which they a r e a t t a c h e d . Some v i b r a t i o n d e f e c t s o n l y show up i n t h e r a d i a l d i r e c t i o n , o t h e r s o n l y p r o duce major e f f e c t s i n t h e a x i a l d i r e c t i o n .

I t i s t h e r e f o r e t o be recommended

t h a t 3 sensors be used a t each l o c a t i o n so t h a t t h e v i b r a t i o n s can be e s t a b l i s h e d i n two p e r p e n d i c u l a r r a d i a l p o s i t i o n s as w e l l as t h e a x i a l d i r e c t i o n .

A study o f t h e e f f e c t s o f v a r i o u s d e f e c t s i n d i c a t e s t h e f o l l o w i n g v i b r a t i o n a l characteristics:

TABLE 18.3

V i b r a t i o n C h a r a c t e r i s t i c s o f D e f e c t i v e Components

unbalance

... ..

misalignment

o c c u r s a t s h a f t speed

........

a t s h a f t speed (sometimes 3 o r 4 x )

........

"

p l a i n bearing loose i n housing

...

worn gears

f a u l t y be1 t drive

...

..

........

occurs a t t o o t h mesh frequency = rpm x no. t e e t h

occurs a t b e l t frequency

mechan c a l looseness e l e c t r c a l induced

18.5.1.3

occurs a t 1 / 2 o r 1 / 3 s h a f t speed

... ...

..

.....

in a radial direction i n a r a d i a l o r more dominantly i n an a x i a l d ir e c t ion

in a radial direction in a radial or axial d ir e c t ion in a radial direction

2 x s h a f t speed a t synchronous frequency

........

should disappear o f f power

Waveform A n a l y s i s

The shape o f t h e v i b r a t i o n waves and t h e g e n e r a l i n t e r a c t i o n o f superimposed v b r a t i o n s can be s t u d i e d by p a s s i n g t h e v i b r a t i o n s i g n a l through a time-doma n r e c o r d e r such as an u l t r a - v i o l e t i n genera

(UV) r e c o r d e r .

Such methods a r e

l i m i t e d t o v e r y low frequency ( s t r u c t u r a l ) v i b r a t i o n s and some

transient studies.

18.5.2

I n s p e c t i o n / l n t e g r i t y Surveillance

Methods t o determine t h e presence o f f l a w s which have been adopted f r o m n o n - d e s t r u c t i v e ( n d t ) t e s t i n g methods i n c l u d e t h e f o l l o w i n g : (i)

Dye p e n e t r a t i o n

-

which r e v e a l s c r a c k s a s small as 0.025 pm t o t h e naked eye

431

-

( i i ) Flux t e s t i n g

magnetic m a t e r i a l s magnetised t o r e v e a l t h e presence o f c r a c k s when t h e s u r f a c e i s spread w i t h magnetic p a r t i c l e s / p o w d e r

(iii)

E l e c t r i c a l resistance

-

by which two probes a r e moved o v e r t h e s u r f a c e w i t h an e l e c t r i c a l p.d. them;

between

c r a c k s a l t e r t h e r e s i s t a n c e and

therefore the passing current (iv)

Eddy c u r r e n t t e s t i n g

-

c u r r e n t s induced i n a m a t e r i a l ( n o t n e c e s s a r i l y magnetic) and c r a c k s l o c a t e d by a search c o i l

(v)

Ultrasonic testing

-

whereby c r a c k s a r e l o c a t e d by t h e r e f l e c t i o n o f u l t r a s o n i c waves propagated i n t o the m a t e r i a l ;

t h i s method i s

d e v e l o p i n g i n t o one o f t h e p r i m a r y n d t techniques (vi)

-

Radiographic

by which X-rays p e n e t r a t e t h e s u r f a c e and l o c a t e hidden c r a c k s .

I f one adds t o t h i s c a t a l o g u e o f techniques t h e newer ones which have been d e v e l o p e d e x c l u s i v e l y f o r i n s p e c t i o n / s u r v e i l l a n c e purposes and add

t o t h i s leak

d e t e c t i o n and c o r r o s i o n m o n i t o r i n g , a v a s t a r r a y o f p o s s i b l e methods i s p r e sented.

An i n d i c a t i o n o f t h e range o f these i s g i v e n i n t h e f o l l o w i n g t a b l e :

TABLE 18.4

C l a s s i f i c a t i o n s o f I n t e g r i t y S u r v e i l l a n c e Techniques

Acoustic/ultrasonic

-

Electr ica1

-

Magnetic

-

hysteresis;

Radiography

-

beta-ray backscatter;

Thermal

-

infra-red; compounds

Visual

-

holography;

s t r e s s wave emission;

ultrasonic;

c a p a c i t a n c e ; corona d i s c h a r g e ; c o r r o s i o n probe; eddy c u r r e n t ; microwave; resistance particles;

prints X-ray;

s u r f a c e impedance;

gamma-ray;

neutron

thermographic

borescopes; C.C.T.V.; dye-penetrant; holography p h o t o - e l e c t r o n emission;

438 TABLE 18.5

Some f u r t h e r d e s c r i p t i o n s

Beta-ray B a c k s c a t t e r

t o determine l a y e r t h i c k n e s s ; c o m p o s i t i o n o f aggregates

assess t h e

Capacitance

c r a c k d e t e c t i o n , bond d e f e c t s i n n o n - m e t a l l i c materials

Corrosion-probe

m a t e r i a l loss by c o r r o s i o n measurement

Dye Penetrant

crack penetration using v i s i b l e o r fluorescent dyes

Eddy Current

surface d e f e c t s d e t e c t e d by e l e c t r o m a g n e t i c induction

F l u x Sensors

uses r e s i d u a l o f induced magnetic f l u x p e r t u r ba t i o n s

Magnetic H y s t e r e s i s

measures magnetic changes due t o t h e presence o f faults

Magnetic P a r t i c l e s

s o l i d magnetic p a r t i c l e m i g r a t i o n i n t h e presence o f an a p p l i e d magnetic f i e l d l o c a t e s surface defects

Magnetic P I i n t s

a s t r i p p a b l e p a i n t f i l m under magnetic f i e l d e f f e c t s which i l l u s t r a t e s s u r f a c e d e f e c t s i n f e r romagne t ic mate r ia 1 s

Microwave

infra-red

O p t i c a l Aids

borescopes, f i b r e o p t i c s , CCTV

P h o t o e l e c t r o n Emission

t h e d e t e c t i o n o f spontaneous e l e c t r o n emissions f r o m p l a s t i c a l l y deformed s u r f a c e s

Radiography

p e n e t r a t i o n t o show up d e f e c t i v e and b u r n t - o u t parts

Resistivity

c r a c k o r bond t e s t i n g by e l e c t r i c a l r e s i s t a n c e measurement

S t r e s s Wave Emission

a method f o r l o c a t i n g t o p o s i t i o n and s e v e r i t y o f d e f e c t s i n metal s u r f a c e s and s t r u c t u r e s

U1 t r a son i cs

which uses t r a n s m i s s i o n c h a r a c t e r i s t i c s o f u l t r a - h i g h frequency s o n i c waves t o l o c a t e d e f e c t s .

18.5.3

i n s p e c t i o n o f non-conducting m a t e r i a l s

Contaminant A n a l y s i s

Moving c o n t a c t between t h e m e t a l l i c components o f any mechanical system i s accompanied by wear, which r e s u l t s i n t h e g e n e r a t i o n o f wear d e b r i c p a r t i c l e s . I n a l u b r i c a t e d system these p a r t i c l e s a r e i n suspension i n t h e c i r c u l a t i n g oil.

Under normal c o n d i t i o n s t h e r a t e o f wear i s low and p a r t i c l e s formed a r e

v e r y small,

The s i z e and r a t e o f g e n e r a t i o n o f these p a r t i c l e s increase as t h e

r a t e o f wear increases.

By i d e n t i f y i n g and measuring these m e t a l l i c p a r t i c l e s ,

t h e s u r f a c e f r o m which t h e p a r t i c l e s were worn can be i d e n t i f i e d and t h e r a t e of wear can be determined t o be normal o r abnormal.

439 Techniques f r e q u e n t l y used i n o i l c o n d i t i o n m o n i t o r i n g a r e (i) (ii) (iii) (iv) (v)

Magnetic p l u g i n s p e c t i o n S p e c t r o m e t r i c O i l A n a l y s i s Procedure (SOAP) Ferrography P a r t i c l e Counting ( f o r h y d r a u l i c f l u i d s ) Patch T e s t i n g

A t y p i c a l magnetic p l u g , F i g . 7 ,

i n c o r p o r a t e s a n o n - r e t u r n v a l v e so t h a t i t

can be i n s e r t e d i n t o a p i p e - l i n e and/or withdrawn w i t h o u t loss o f f l u i d . D e b r i s which has been trapped by such a magnetic p l u g can be measured w i t h a magnetometer t o determine t h e amount t h a t has been c o l l e c t e d ; recorded t o determine t h e c o l l e c t i o n t r e n d .

t h i s may then be

A t t h e same time, examination under

a s i m p l e microscope and comparison w i t h Debris R e c o g n i t i o n Drawings/Photographs makes i t p o s s i b l e t o t e l l t h e component from which t h e major amount o f wear m a t e r i a l has a r i s e n , as shown i n Fig.9. When spectroscopy was f i r s t i n t r o d u c e d as a chemical a n a l y t i c a l i n s t r u m e n t

100 years ago,

i t b r o u g h t about a r e v o l u t i o n i n c h e m i s t r y .

I t s advantages were

n o t m e r e l y t h a t spectroscopy was a s e n s i t i v e d e t e c t o r b u t t h a t t h e i n s t r u m e n t c o u l d d e t e c t and measure the q u a n t i t y o f an element p r e s e n t i n t h e sample i n dependently o f how t h e element was i n c o r p o r a t e d i n a compound.

The flame o r

spark o f t h e e m i s s i o n o r a b s o r p t i o n apparatus broke down t h e compounds and each element d i s p l a y e d i t s i n d i v i d u a l s e t o f spectrum l i n e s , Fig.10.

Nothing was

occluded, t h i s was t h e fundamental d i f f e r e n c e f r o m a l l p r e v i o u s l y e x i s t i n g methods of chemical a n a l y s i s . When t h e advantages o f o i l a n a l y s i s were f i r s t r e a l i s e d a t t h e t i m e o f t h e i n t r o d u c t i o n o f d i e s e l locomotives t o t h e r a i l r o a d s , t h e emission spectrograph was adapted t o o i l a n a l y s i s , t h e t h e o r y b e i n g t h a t a r a p i d increase o f a m e t a l l i c element i n t h e l u b r i c a t i n g o i l would imply t h a t a p a r t made o f t h a t element was wearing r a p i d l y .

A i r d i r t , assembly o r r e p a i r d e b r i s , system wear

metals, c o r r o s i o n p r o d u c t s and c o o l a n t water i n h i b i t o r s a r e some of t h e mate r a i l s which may be detected. E l emen t

wavelength (Angstrom u n i t s )

A l umi n ium

3092

Copper

3247

Ch rom i um

3579

Iron

3720

Lead

2833

Sod i um

589 0

Tin

2354

440

Fig.7

Magnetic p l u g ( c h i p d e t e c t o r )

VALVE OPEN-

FLOW

VALVE CLOSED

MAGN

O-RING SEALS BAYONET PINS

Fig.8

Operating p r i n c i p l e o f t h e TEDECO/Muirhead V a c t r i c Magnetic Plug/ Chip D e t e c t o r

B a l l Debris rounded ' r o s e - p e t a l ' r a d i a l l y s p l i t shape

-

-

(b) Track D e b r i s rounded, s u r f a c e break up, c r i s s cross scratches

-

Rol l e r D e b r i s (d) Gear Tooth D e b r i s g e n e r a l l y c u r l e d and i r r e g u l a r shape, g r e y rectangular, p a r a l l e l l i n e s s u r f a c e as s p l a s h o f across width solder Magnetic p l u g d e b r i s source r e c o g n i t i o n

441

40

30

1

D

-

-

20 -

10

IB

I d

0

L9-J-J WAVE- LENGTH

Fip.10

TABLE 18.6

I

C

1 3800

(ANGSTRON UNITS 1

T y p i c a l s p e c t r o s c o p i c spectrum

Element d e t e c t e d and f r e q u e n t source Element

Sources

Aluminium, S i l i c o n Borax, potassium,

Dust and A i r b o r n e d i r t sodium

Coolant i n h i b i t o r r e s i d u e s

Calcium, Sodium Chromium, cooper, tin, zinc

S a l t water r e s i d u e s iron,

lead

Barium, calcium, magnesium, phosphorus, z i n c

Wear, c o r r o s i o n o r r e s i d u a l assembly d e b r i s Engine o i l a d d i t i v e s

Some manufacturers i n c o r p o r a t e s p e c i a l elements i n t o d i f f e r e n t p a r t s o f an engine t o a c t as t r a c e r s so t h a t t h e i r presence i n a sample p r o v i d e s an unambiguous i n d i c a t i o n o f t h e source o f t r o u b l e .

SOAP has developed t o t h e p o i n t where t h e m a j o r i t y o f m i l i t a r y s e r v i c e s , r a i l r o a d s and a i r l i n e s o p e r a t e s p e c t r o g r a p h i c o i l a n a l y s i s as a s t a n d a r d p r o cedure f o r d e t e c t i n g problem areas, schedule o v e r h a u l s and r o u t i n e o i l mon it o r i n g .

44 2 I t m ig h t be tho ug ht t h a t wear p a r t i c l e s c o u l d be examined by v i e w i n g a f i l t e r and measuring t h e number o f p a r t i c l e s .

U n f o r t u n a t e l y f o r most cases

o t h e r t h a n h y d r a u l i c systems t h i s i s n o t p o s s i b l e . c e l l a n e o u s mass o f p a r t i c l e s a r e found.

I f o i l i s f i l t e r e d a mis-

Large and small p a r t i c l e s a r e p i l e d

one on t h e o t h e r so t h a t i t i s n o t p o s s i b l e t o determine t h e i r c h a r a c t e r i s t i c s , size distribution etc.

I n f a c t , f i l t e r e d d e p o s i t s g i v e t h e impression o f being

"just dirt".

01L SAMPLE

COLLECTOR RECEPTACLE

Fig.11

Schematic diagram o f 'F erro gra ph ' ( t h e s t r e n g t h o f the magnetic f i e l d i s g r e a t e s t a t t h e b ot t o m o f t h e s l i d e . )

The m o n i t o r i n g o f wear p a r t i c l e s on t h e F errograph, Fig.11, hand,

i s a t e c h ni qu e i n which most t h i n g s a r e occluded.

on the o t h e r

One o f i t s most v a l -

ua b le c h a r a c t e r i s t i c s stems f ro m t h e f a c t t h a t i t does n o t see every p a r t i c l e and,

i n fact,

ig nore s e v e r y t h i n g exce pt wear metal p a r t i c l e s i n the o i l .

The

s e p a r a t i o n and a n a l y s i s o f wear p a r t i c l e s f o r s i z e d i s t r i b u t i o n , type o f m e t a l , p h y s i c a l shape, c r y s t a l s t r u c t u r e e t c .

i s o f t e n a much more s e n s i t i v e i n d i c a t o r

of t h e wear s i t u a t i o n than a d i r e c t vi e w o f t h e worn surface.

Characteristic

d e b r i s f r o m modes o f f a i l u r e have been d i scussed by S c o t t i n a p r e v i o u s chapter on wear.

443

A l l wear d e b r i s a n a l y s i s t ech ni q ue s a r e p a r t i c l e s i z e dependent and any p a r t i c u l a r t e c h n iq ue i s s e n s i t i v e o n l y t o a s p e c i f i c range o f p a r t i c l e s i z e s . Fig.12 summarises t he e f f i c i e n c y o f these t echniques as a f u n c t i o n o f p a r t i c l e size [g].

ISOA

0

Fig.12

Ferrography

Magnetic plug

10 100 Particle size (microns)

1000

E f f i c i e n c y o f v a r i o u s sensors as a f u n c t i o n o f p a r t i c l e s i z e

The c le a n 1 in e ss re qu i reme nt s o f h y d r a u l i c systems have become more c r i t i c a l i n r e c e n t years.

Systems employing e l e c t r o h y d r a u l i c servo v a l v e s i n n u m e r i c a l l y

c o n t r o l l e d machine t o o l s , h i g h pre ssure systems where pump o r v a l v e clearances may be as s m a ll as 0.5 micrometre a r e p a r t i c u l a r examples.

Analysing p a r t i c -

u l a t e c o n t a m i n a t i o n f o r these cases a r e u s u a l l y c a r r i e d o u t by p a r t i c l e count methods u t i l i s i n g e i t h e r microscopes o r a ut omatic c o u n t e r s . Microscope c o u n t i n g methods (ASTM F312, F 3 1 3 , ARP 598A, I P

2751, a r e p r o -

cedures which a r e con si de red u n s u i t a b l e f o r p a r t i c l e s s m a l l e r than 5 microns. These methods s i z e and co un t s t a t i s t i c a l l y p a r t i c l e s r e t a i n e d on a membrane surface a f t e r t h e f l u i d sample has been f i l t e r e d .

A p a r t f r o m c o u n t i n g being v e r y

time consuming, t h e r e can be v a r i a t i o n when d i f f e r e n t people count t h e same

444 slide.

Automatic c o u n t e r s a r e however now a v a i l a b l e u t i l i s i n g scanning com-

p u t e r s and TV screens. Other in s tr u m ent s u s i n g t he p r i n c i p l e o f l i g h t i n t e r c e p t i o n count p a r t i c l e s suspended i n a l i q u i d f r o m e i t h e r a sample b o t t l e o r d i r e c t f r o m a h y d r a u l i c system.

As these i nst rume nt s o p e r a t e on t h e blockage o f l i g h t p r i n c i p l e , they

measure the p r o j e c t e d a rea o f a p a r t i c l e and then r e c o r d the diameter o f a c i r c l e o f e q u i v a l e n t area.

U n f o r t u n a t e l y t h ey cannot d i f f e r e n t i a t e between

s o l i d p a r t i c l e s and a i r bubbles. The t y p i c a l c o l o u r o f c o n t a m i n a t i o n i n any g i v e n h y d r a u l i c system remains f a i r l y constant.

The darkness o f t h e p a r t i c u l a t e d i s c o l o u r a t i o n o f a f i l t e r

t h e r e f o r e a rough i n d i c a t i o n o f t h e c l e a n l i n e s s o f the t e s t f l u i d .

is

T h i s Patch

Test procedure i s however o n l y g e n e r a l l y a p p l i c a b l e t o gross l e v e l s o f contami n a t io n .

18.5.4

Trends A n a l y s i s

T h i s i s l i t t l e more t h an t h e d at a l og ge r w i t h a memory and t r e n d a n a l y s i s capability.

The most el e men t a ry f o r m i s t h e h a n d w r i t t e n watch l o g w hich i s

inspected and analysed.

As i n p r a c t i c e , r e a l use i s o n l y made o f t h e l o g a f t e r

a f a i l u r e has o c curre d, t h e modern tendency i s t o use sensors t o i n p u t t o a general d a ta system which scans t h e m o n i t o r p o i n t s and produces a r e g u l a r p r i n t out.

I n more advanced form, a computer i s capable o f combining many o f t h e

i n p u t s and e s t a b l i s h i n g t r e n d s which, by mathematical m o d e l l i n g , can be associ a t e d w i t h t h e changes whi ch would occu r f o l l o w i n g s p e c i f i c d e f e c t s and cons e q u e n t ly when t h e system i s i n t e r r o g a t e d ,

i t can s t a t e (through a l o g i c

process) t h e p l a n t and component whi ch i s d e t e r i o r a t i n g ;

w i t h preset-1 i m i t s

such a system w i l l even produce a st at eme nt o f t h e ' u n e x p i r e d l i f e ' remaining. Such advanced t r e n d s m o n i t o r s a r e more commonly known as 'performance' m o n ito r s .

18.6

DETERIORATION LIMITS

Most c l a s s i f i c a t i o n s o c i e t i e s and b od i es o f a s i m i l a r n a t u r e have establ i s h e d s p e c i f i c a t i o n s and codes o f p r a c t i c e which d e f i n e t h e d e t e r i o r a t i o n l i m i t s which s h oul d be a l l o w e d b e f o r e c o r r e c t i v e a c t i o n i s taken.

Typically,

l i m i t s which have been e s t a b l i s h e d f o r d e t e r i o r a t i o n i d e n t i f i e d by v i b r a t i o n methods in c lu d e :

V D I Code o f P r a c t i c e 1056 (October 1964) D I N 45665 (November 1967) B S 4675:

1971

IS0 2372/3 I t does seem t h a t f o r v i b r a t i o n l i m i t s , most people i n general e r r

on

the

445 side o f c o n s i d e r a b l e c a u t i o n a l t h o u g h i n p r a c t i c e , t h e l i m i t must depend upon a large range o f environmental f a c t o r s s p e c i f i c t o each i n s t a l l a t i o n . I t i s o n l y by experience,

b o t h p e r s o n a l and t h a t o f o t h e r t h a t a c t i v e

decision l i m i t s can be reached.

I n t h e whole f i e l d o f c o n d i t i o n m o n i t o r i n g ,

developments a r e o c c u r r i n g a t such a r a p i d speed t h a t o n l y an o r g a n i s a t i o n such as the UK Mechanical H e a l t h M o n i t o r i n g Group through i t s r e g u l a r seminar/ symposia and courses i s i t p o s s i b l e t o a c q u i r e a l l t h e i n f o r m a t i o n consultants

-

-

known t o

needed t o implement and manage an o n - c o n d i t i o n maintenance system.

REFERENCES

1

Collacott,R.A. 'Mechanical Fau t D i a g n o s i s and C o n d i t i o n M o n i t o r i n p ' Chapman & H a l l , London 1977. 2 Venton ,A. D. F. and Harvey ,B. F. R e l i a b i l i t y assessment i n machinery system d e s i g n ' Proc. 1.Mech.E. 973. 3 Bridges,D.C. 'The a p p l i c a t i o n o f r e l i a b i l i t y t o t h e design o f s h i p ' s machinery' Trans. 1.Mar.E. 86, P a r t 6 1974. 4 Davies,A.E. ' P r i n c i p l e s and p r a c t i c e o f a i r c r a f t powerplant maintenance' Trans. 1.Mar.E. 85 P a r t 6 1973. 5 Collacott,R.A. 'Data Sources f o r Re1 i a b i l i t y S t a t i s t i c s ' UKM P u b l i c a t i o n s L t d . , 92 London Road, L e i c e s t e r LE2 OQR 1976. 6 Wilkinson,H.C. and Ki1bourn.D.F. 'The d e s i g n o f s h i p ' s machinery i n s t a l l a t i o n s ' S h i p p i n g World and S h i p b u i l d e r , August 1971. 7 Mathieson,Tor-Chr. ' R e l i a b i l i t y e n g i n e e r i n g i n s h i p machinery p l a n t design' Report IF/R.12 U n i v e r s i t y o f Trondheim,N.I.T. 1973. 8 Col l a c o t t ,R.A. ' V i b r a t i o n M o n i t o r i n g and D i a g n o s i s ' George Godwin L i m i t e d 1978. 9 Pocock,G. ' Introduction t o Ferrography' Symposium on Ferrography, B r i t i s h I n s t . Non D e s t r u c t i v e T e s t i n g , London 1979.

446

I!)

THE TRIBOLOGY OF METAL CUTTING

E. M. TRENT

Department o f I n d u s t r i a l M e t a l l u r g y , U n i v e r s i t y o f Birmingham, Birmingham 815 2TT

19.1

INTRODUCTION

The e f f i c i e n c y o f metal c u t t i n g o p e r a t i o n s i s v e r y l a r g e l y c o n t r o l l e d by t h e behaviour o f t h e work m a t e r i a l and t h e t o o l m a t e r i a l a t t h e i n t e r f a c e between them near t h e c u t t i n g edge o f t h e t o o l .

Metal c u t t i n q i s c a r r i e d o u t on all

the m a jo r c l a s s e s o f m e t a l s and a l l o y s produced commercially.

The machining

o p e r a t i o n s such as b o r i n g , d r i l l i n g , t a p p i n g , t u r n i n q , m i l l i n g , p l a n i n g and sawing a r e v e r y v a r i e d i n c h a r a c t e r .

Ob j ect s as small as instrument p a r t s o r

as l a r g e as i n d u s t r i a l b o i l e r s a r e machined t o remove excess m a t e r i a l and t o ge n e r a t e t h e necessary shapes w i t h t h e r e q u i r e d p r e c i s i o n , b u t , however v a r i e d , a l l machining o p e r a t i o n s have c e r t a i n f e a t u r e s i n common.

19.2

METAL CUTTING PHENOMENA

A l l machining o p e r a t i o n s i n v o l v e t h e use o f one o r more t o o l s o f a wedge shape w i t h a c u t t i n g edge, whi ch remove a t h i n l a y e r f r o m t h e s u r f a c e o f a l a r g e r body as shown i n Fig.1.

The t o o l s a r e always moved a s y m m e t r i c a l l y w i t h

re s p e c t t o t h e wedge a n g l e s o t h a t t h e l a y e r removed

-

t h e " chip"

-

bears

a g a i n s t and moves ove r one s u r f a c e o f t h e wedge, known as t h e "rake face" o f the t o o l .

The t o o l i s so shaped t h a t t h e f r e s h l y c u t metal s u r f a c e does n o t rub

a g a i n s t t h e o t h e r f a c e o f t h e wedge tool.

-

t h e "cl earance face" o r " f l a n k "

o f the

(Fig.1).

The l a y e r o f metal removed i s f i r s t p l a s t i c a l l y deformed by a shearing a c t i o n , r o u g h l y a l o n g a p l a n e A-B

i n Fig.l.,

e x t e n d i n g f r o m t h e t o o l edge A t o

t h e p o s i t i o n B where t h e c h i p sep ara t e s f rom t h e undeformed work m a t e r i a l . i n v o l v e s severe shear s t r a i n

-

This

usually a natural s t r a i n o f a t least 2 but often

v e r y much h i g h e r as can be seen f r o m F i g. 2.

which shows a s e c t i o n t h r o u g h t h e

forming c h i p d u r i n g t h e c u t t i n g o f copper.

The shear plane i s under h i g h com-

p r e s s i v e s t r e s s and t h e t r i - a x i a l s t r e s s c o n d i t i o n i s such t h a t w i t h most d u c t i l e m e t a l s and a l l o y s t h e h i g h shear s t r a i n can be s u s t a i n e d w i t h o u t f r a c t u r e s o t h a t a c ont i nu ou s c h i p i s formed, as w i t h t h e example o f copper.

With

m e t a ls and a l l o y s o f low d u c t i l i t y , however, o r under c u t t i n g c o n d i t i o n s where t h e compressive s t r e s s on t h e shear p l a n e i s low, t h e c h i p may be broken i n t o

447

CLEARANCE FACE

Fig.1

Fig.2

TOOL

Features o f Metal C u t t i n g

S e c t i o n through c h i p and b a r o f h i g h p u r i t y copper a f t e r "quick-stop". Machined a t 122111m i " - ' .

448 small fragments.

A d i s c o n t i n u o u s c h i p i s formed, f o r example, when c u t t i n g

c a s t i r o n o r a f r e e c u t t i n g brass. The c h i p moves across t h e r a k e s u r f a c e o f t h e t o o l away f r o m t h e c u t t i n g edge and breaks c o n t a c t w i t h t h e t o o l s u r f a c e a t some p o s i t i o n C, u s u a l l y r a t h e r i l l defined.

Always t h e c o n t a c t l e n g t h A-C

o f t h e l a y e r b e i n g removed ( t l

10 times t h e feed tl.

i n Fig.1.)

i s g r e a t e r than t h e o r i g i n a l t h i c k n e s s

-

t h e "feed"

-

A-C

i s o f t e n 5 o r even

The work and t o o l m a t e r i a l s a r e i n c o n t a c t a t t h e

c u t t i n g edge and u s u a l l y f o r a s h o r t d i s t a n c e down t h e c l e a r a n c e f a c e o r f l a n k o f the t o o l .

The l e n g t h o f c o n t a c t i n t h i s r e g i o n A-D

than on t h e rake f a c e A-C.

The c l e a r a n c e a n g l e (Fig.1.)

i s u s u a l l y much s h o r t e r which i s u s u a l l y about

6" t o 15" r e s t r i c t s t h e l e n g t h of c o n t a c t on t h i s s u r f a c e , b u t d u r i n g c u t t i n g t h e most common f o r m o f wear i s one i n which a s u r f a c e i s worn on t h e t o o l nearly p a r a l l e l t o the d i r e c t i o n o f c u t t i n g

-

t h e " f l a n k wear land"

T h i s worn s u r f a c e , and hence t h e l e n g t h o f c o n t a c t A-D,

i n Fig.1.

tends t o i n c r e a s e w i t h

c u t t i n g t i m e b u t , t o a v o i d t o t a l t o o l f a i l u r e , t h e f l a n k wear should n o t be a l l o w e d t o become t o o l a r g e b e f o r e t h e t o o l i s reground o r replaced.

19.3 19.3.1

CONDITIONS AT THE TOOL-WORK INTERFACE Tool Forces and Stresses

The f o r c e s a c t i n g on t h e t o o l a r e ( 1 ) t h a t r e q u i r e d t o shear t h e work material over t h e area o f t h e shear p l a n e A-8 (Fig.1.)

and ( 2 ) t h a t r e q u i r e d t o move t h e

c h i p across t h e t o o l rake f a c e c o n t a c t r e g i o n A-C. m a t e r i a l o v e r t h e f l a n k A-D

The f o r c e t o move t h e work

i s small compared w i t h t h e o t h e r f o r c e s and can be

neglected i n a f i r s t approximation.

Tool dynamometers have been developed and

used t o measure t h e f o r c e s a c t i n g on t h e t o o l i n two d i r e c t i o n s d i r e c t i o n o f c u t t i n g Fc and i n t h e d i r e c t i o n o f t h e f e e d F f .

-

i n the

I n mast c u t t i n g

o p e r a t i o n s t h e f o r c e s a c t i n g on t h e t o o l v a r y f r o m a few k i l o g r a m s t o a few hundred k i l o g r a m s . The c u t t i n g f o r c e Fc a c t s n e a r l y normal t o t h e r a k e f a c e o f t h e t o o l and e x e r t s a l a r g e l y compressive s t r e s s on t h i s s u r f a c e .

The f e e d

f o r c e F f i s almost always s m a l l e r than t h e c u t t i n g f o r c e ( t y p i c a l l y 40-60% o f t h e c u t t i n g f o r c e ) and e x e r t s a s h e a r i n g s t r e s s on t h e t o o l s u r f a c e . While t h e f o r c e s a c t i n g on t h e t o o l can be measured w i t h accuracy, even t h e mean v a l u e o f the s t r e s s a c t i n g on t h e c o n t a c t area between work m a t e r i a l and t o o l i s d i f f i c u l t t o d e t e r m i n e because t h e a r e a o f c o n t a c t i s d i f f i c u l t o r impossible t o measure e x a c t l y .

The s t r e s s e s a r e n o t e v e n l y d i s t r i b u t e d on t h e

c o n t a c t area and i t i s n o t easy t o determine t h e s t r e s s d i s t r i b u t i o n on t h i s area.

The general c h a r a c t e r o f t h e s t r e s s d i s t r i b u t i o n on t h e rake f a c e o f a

c u t t i n g t o o l , however,

i s now g e n e r a l l y accepted t o be t h a t suggested by Zorev

111 and shown d i a g r a m m a t i c a l l y i n Fig.3.

The compressive s t r e s s a c t i n g on t h e

rake face i s a t a maximum a t o r c l o s e t o t h e c u t t i n g edge and d i m i n i s h e s t o z e r o a t t h e end o f t h e c o n t a c t area.

The maximum compressive s t r e s s near t h e edge

449

(COMPRESSIVE

STRESS

* ul

CHIP SHEAR STRESS

'b-\ (DISTANCE

, ,

Fig.3

I

'

0

I

'FROM 'CUT

/

S t r e s s d i s t r b u t i o n i n c u t t i n g t o o l ( a f t e r Zorev)

2,000

1,500

Z

-0 4-

5

1,000

CJl

A,

C .c U 0

'

0,

500

I.?

0

50

100

150

200

Cutting speed rn min?

Fig.4

Forces a c t i n g on a t o o l as f u n c t i o n o f c u t t i n g speed. Depth o f c u t 1.25mm. Feed 0.25mm/rev.

450 i s o f t e n t w i c e t h e mean s t r e s s on t h e area o f c o n t a c t .

The shear s t r e s s on t h e

rake f a c e i s more u n i f o r m l y d i s t r i b u t e d as shown i n Fig.3. The v a l u e s o f t h e compressive s t r e s s near t h e edge a r e h i g h r e l a t i v e t o t h e y i e l d stress o f the material being c u t

-

t h e work m a t e r i a l .

For example, i n

c u t t i n g s t e e l t h e compressive s t r e s s near t h e edge may be o f t h e o r d e r o f -2 1500 N mm A major requirement o f a s a t i s f a c t o r y t o o l m a t e r i a l i s t h u s h i g h

.

y i e l d s t r e s s i n compression, and t h e V i c k e r s o r Rockwell hardness v a l u e s a r e u s u a l l y taken as an i n d i c a t i o n o f t h i s p r o p e r t y .

The most commonly used c u t t i n g

t o o l m a t e r i a l s a r e hardened h i g h speed s t e e l and cemented c a r b i d e .

The minimum

hardness o f c u t t i n g t o o l m a t e r i a l s i n common use i s 750 HV (62 Rockwell C). Although t h e r e a r e few r e l i a b l e d a t a f o r t h e s t r e s s e s on t h e c o n t a c t area o f t o o l s i n real c u t t i n g operations,

i t i s c e r t a i n t h a t they are r e l a t e d t o the

y i e l d s t r e s s o f t h e work m a t e r i a l .

Approximate values f o r t h e mean s t r e s s

a c t i n g normal t o t h e r a k e f a c e o f a t u r n i n g t o o l under a standard s e t o f c u t t i n g c o n d i t i o n s a r e shown i n Table 19.1 f o r d i f f e r e n t work m a t e r i a l s . TABLE 19.1

Mean compressive s t r e s s on c o n t a c t area

Work m a t e r i a l

Compressive S t r e s s

-2

N mm

I ron Steel (medium carbon) T i taniurn Copper 70/30 brass Lead

340 770 570

310 420 14

For t h e c u t t i n g o f m a t e r i a l s o f v e r y h i g h y i e l d s t r e n g t h , p a r t i c u l a r l y h e a t t r e a t e d s t e e l s and n i c k e l base a l l o y s , t h e usual s t e e l and cemented c a r b i d e t o o l s may be inadequate because t h e s t r e s s imposed by t h e work m a t e r i a l i s h i g h enough t o deform t h e c u t t i n g t o o l edge even a t v e r y low c u t t i n g speed where t h e t o o l edge temperature i s low.

I t i s g e n e r a l l y c o n s i d e r e d i n a machine shop

t h a t h i g h speed s t e e l t o o l s cannot be used t o c u t s t e e l s w i t h hardness h i g h e r than 350 HV (36 Rc) and t h a t t h e c u t t i n g o f s t e e l w i t h hardness o v e r 550 HV (53 Rc) becomes v e r y d i f f i c u l t even w i t h cemented c a r b i d e t o o l s .

For t h e mach-

i n i n g o f f u l l y hardened s t e e l i t has been more usual t o r e s o r t t o g r i n d i n g u s i n g s i l i c o n c a r b i d e , aluminium o x i d e o r bonded diamond wheels, o r t o shape by e l e c t r o d i s c h a r g e machining (EDM) o r e l e c t r o chemical machining (ECM). Recently t h e i n t r o d u c t i o n o f new c u t t i n g t o o l m a t e r i a l s w i t h s t i l l h i g h e r y i e l d strength tools

-

-

i n c l u d i n g compacted p o l y c r y s t a l l i n e diamond and c u b i c boron n i t r i d e

has made t h e c u t t i n g o f f u l l y hardened s t e e l s , h i g h e r s t r e n g t h n i c k e l -

based a l l o y s , and o t h e r v e r y h a r d m a t e r i a l s , a more f e a s i b l e p r o p o s i t i o n f o r i n d u s t r i a l shaping o p e r a t i o n s .

451 19.3.2

C u t t i n g Speed

One o f t h e most i m p o r t a n t parameters i n metal c u t t i n g i s t h e v e l o c i t y a t which the work m a t e r i a l passes t h e c u t t i n g edge

-

t h e c u t t i n g speed.

This

varies g r e a t l y i n i n d u s t r i a l o p e r a t i o n s f r o m almost zero, f o r example near t h e centre o f a d r i l l , t o 300 m min-'

o r even h i g h e r .

research workers measuring t o o l f o r c e s found t h a t ,

Rather t o t h e i r s u r p r i s e i n g e n e r a l , these f o r c e s do

not i n c r e a s e as t h e c u t t i n g speed i s r a i s e d .

I n many cases t h e f o r c e s decrease, -1 p a r t i c u l a r l y i n t h e speed range up t o 65 m min as shown f o r example i n Fig.4. This has been shown t o be t r u e f o r a wide range o f work m a t e r i a l s and c u t t i n g conditions.

The f o r c e s decrease m a i n l y because t h e a r e a o f c o n t a c t between t o o l

and work decreases as c u t t i n g speed i s r a i s e d .

A l t h o u g h t h e r e have been no v e r y

d e t a i l e d s t u d i e s o f t h e s t r e s s a c t i n g on t h e t o o l s u r f a c e as a f u n c t i o n o f c u t t i n g speed, t h e r e i s no evidence t o i n d i c a t e t h a t t h e s t r e s s a c t i n g on t h e t o o l i s r a i s e d as c u t t i n g speed i s increased. C u t t i n g speed i s o f p a r t i c u l a r importance i n r e l a t i o n t o t h e economics o f machining.

The c o s t o f machining o p e r a t i o n s i s reduced by i n c r e a s i n g t h e r a t e

of metal removal, and t h e main i n c e n t i v e t o t h e development i n machining i n t h e l a s t hundred y e a r s has been t h e r e d u c t i o n o f t h e v e r y h i g h c o s t s by t h e use o f new machines and t o o l s capable o f machining a t increased r a t e s .

In the c u t t i n g

o f h i g h m e l t i n g p o i n t m e t a l s and a l l o y s t h e l i f e o f t h e c u t t i n g t o o l becomes p r o g r e s s i v e l y s h o r t e r as t h e c u t t i n g speed i s r a i s e d u n t i l t h e c o s t o f r e p l a c i n q worn o u t t o o l s more than outweighs t h e advantages o f h i g h e r speed.

I t has been

the a b i l i t y o f t h e c u t t i n g t o o l t o w i t h s t a n d t h e c o n d i t i o n s a t t h e t o o l edge which has l i m i t e d t h e r a t e o f machining o f s t e e l and c a s t i r o n .

The development

and commercial use f i r s t of h i g h speed s t e e l s and then o f cemented c a r b i d e s has enabled c u t t i n g speeds t o be r a i s e d by a f a c t o r o f about 20 t i m e s compared w i t h carbon s t e e l t o o l s and t h e r e a r e s t i l l many o p e r a t i o n s i n which t o o l l i f e i s the f a c t o r l i m i t i n g t h e r a t e o f metal removal. In g e n e r a l as t h e c u t t i n g speed i s r a i s e d n e i t h e r t h e f o r c e s a c t i n g on t h e

t o o l n o r t h e s t r e s s e s on t h e area o f c o n t a c t a r e increased.

The energy expended

i n metal c u t t i n g , however, increases a p p r o x i m a t e l y i n p r o p o r t i o n t o t h e c u t t i n g speed,

i f o t h e r c o n d i t i o n s remain c o n s t a n t .

T h i s energy i s c o n v e r t e d i n t o heat

near t h e c u t t i n g edge, and r a i s e s t h e temperature o f t h e t o o l , r e d u c i n g i t s y i e l d s t r e s s and i n c r e a s i n g t h e r a t e o f t o o l wear.

I t i s t h i s r i s e i n temper-

a t u r e which l i m i t s t h e a b i l i t y o f t h e t o o l s t o w i t h s t a n d i n c r e a s i n g c u t t i n g speed.

19.3.3

The g e n e r a t i o n o f temperatures i n metal c u t t i n g must now be considered. Heat i n Metal C u t t i n g

I n metal c u t t i n g energy i s expended i n t o two main r e g i o n s (1) a l o n g t h e shear p l a n e A-B

(Fig.1.)

where t h e work m a t e r i a l i s sheared t o form t h e c h i p ,

and (2) a t t h e r a k e surface o f t h e t o o l where t h e c h i p i s moved across t h e contact area.

452 The energy expended i n s h e a r i n g t h e work m a t e r i a l t o f o r m t h e ch p m a i n l y r e s u l t s i n r a i s i n g t h e temperature o f t h e c h i p and almost a l l o f t h s h e a t i s c a r r i e d o u t o f t h e system when t h e c h i p breaks c o n t a c t w i t h t h e t o o

.

Since any

one element o f t h e c h i p i s i n c o n t a c t w i t h t h e t o o l f o r o n l y a v e r y s h o r t time

-

-

t y p i c a l l y a few m i l l i - s e c o n d s

o n l y a small p r o p o r t i o n o f t h i s h e a t c o u l d be I t i s probable,

conducted i n t o t h e t o o l under t h e most f a v o u r a b l e c o n d i t i o n s .

as w i l l be shown, t h a t a l l t h e h e a t i n t h e body o f t h e c h i p i s c a r r i e d o u t o f t h e system i n most cases.

A small p r o p o r t i o n o f t h e h e a t generated on t h e

shear p l a n e i s conducted back i n t o t h e body o f t h e workpiece.

The energy ex-

pended on t h e shear p l a n e i s n o r m a l l y t h e lar’gest p a r t o f t h e t o t a l energy o f cutting

-

o f t e n o f t h e o r d e r o f 75 t o 8 0 % o f t h e t o t a l .

The temperature o f t h e

c h i p i s o f t e n r a i s e d t o 200-350°C when c u t t i n g s t e e l o r o t h e r h i g h m e l t i n g point materials. I t i s t h e s m a l l e r p o r t i o n o f t h e t o t a l energy o f c u t t i n g - t h a t expended i n

moving t h e c h i p o v e r t h e t o o l

-

temperatures a t t h e t o o l / w o r k

i n t e r f a c e and t h e c o n d i t i o n s a t t h i s i n t e r f a c e

must t h e r e f o r e be considered.

which i s r e s p o n s i b l e f o r t h e g e n e r a t i o n o f h i g h

The h i g h compressive s t r e s s normal t o t h e t o o l

r a k e s u r f a c e has a l r e a d y been emphasised.

The mean s t r e s s on t h e c o n t a c t area

i s always much h i g h e r than t h e s t r e s s e s n o r m a l l y encountered a t moving i n t e r faces i n e n g i n e e r i n g systems.

The v e r y h i g h s t r e s s e s a l o n e would r e s u l t i n t h e

area o f r e a l c o n t a c t between t h e two s u r f a c e s b e i n g a much h i g h e r p o r p o r t i o n o f t h e apparent c o n t a c t a r e a than is usual f o r s l i d i n g s u r f a c e s , and would t e n d t o promote s e i z u r e .

Other f a c t o r s a r e a l s o f a v o u r a b l e t o s e i z u r e .

The t o o l i s

c o n t i n u a l l y c u t t i n g i n t o clean metal, being brought i n t o contact w i t h surfaces f r e e f r o m o x i d e o r o t h e r l a y e r s which i n h i b i t t h e e x t e n s i o n o f c o n t a c t areas i n many s l i d i n g s i t u a t i o n s .

The c l e a n work m a t e r i a l f l o w s c o n t i n u o u s l y o v e r t h e

t o o l s u r f a c e i n one d i r e c t i o n , sweeping away o x i d e o r o t h e r l a y e r s i n i t i a l l y p r e s e n t on t h e t o o l , which have l i t t l e chance t o r e - f o r m .

Relatively high

temperatures generated a t t h e i n t e r f a c e i n c r e a s e atomic a c t i v i t y and t h i s a l s o tends t o promote s e i z u r e .

19.3.4

Seizure o f t h e Tool/Work

Interface

When these c o n d i t i o n s a r e considered,

i t i s not surprising t o f i n d that

s e i z u r e between the t o o l and work s u r f a c e s i s commonly observed on c u t t i n g tools

[Z]. The process o f f r i c t i o n w e l d i n g i s o f t e n c a r r i e d o u t under con-

d i t i o n s l e s s severe than those encountered i n metal c u t t i n g .

For example,

j o i n t s can be made by f r i c t i o n w e l d i n g a t p r e s s u r e s o f 75 N mm-’ speeds o f 50 m min-’, 750 N mm-’

sound

and p e r i p h e r a l

whereas i n metal c u t t i n g s t r e s s e s on t h e c o n t a c t area o f

o c c u r and h i g h e r speeds a r e o f t e n used.

That s e i z u r e occurs a t t h e

t o o l / w o r k i n t e r f a c e i s confirmed by numerous m e t a l l o g r a p h i c o b s e r v a t i o n s , o f which t h r e e examples w i l l be g i v e n here.

453 Fig.5.

shows a p o l i s h e d and e t c h e d s e c t i o n t h r o u g h t h e rake f a c e o f a

cemented c a r b i d e t o o l and a d h e r i n g work m a t e r i a l a f t e r c u t t i n g s t e e l a t 1 0 0 m -1 min I t shows t h e work m a t e r i a l i n c o n t a c t w i t h t h e t o o l s u r f a c e n o t j u s t a t

.

the tops o f t h e a s p e r i t i e s b u t a t a l l t h e h i l l s and v a l l e y s o f t h e s u r f a c e on a micro scale.

To say t h a t these s u r f a c e s a r e s e i z e d t o g e t h e r means t h a t s l i d i n g

as normally conceived, w i t h t h e two s u r f a c e s separated by a f l u i d f i l m o r i n contact o n l y a t t h e a s p e r i t i e s ,

i s not possible.

The two s u r f a c e s a r e mechan-

i c a l l y i n t e r l o c k e d and/or m e t a l l u r g i c a l l y bonded o v e r t h e whole o r a l a r g e p a r t of the i n t e r f a c e .

That m e t a l l u r g i c a l bonding i s o f t e n i n v o l v e d i s shown by

examples, such as those shown i n Figs.6 and

7, i n which, a f t e r s t o p p i n g c u t t i n g

by p r o p e l l i n g t h e t o o l r a p i d l y f r o m t h e c u t t i n g p o s i t i o n , t h e c h i p remains f i r m l y adherent t o t h e t o o l o r separates f r o m t h e t o o l a t some p o s i t i o n remote from the i n t e r f a c e ,

Fig.5

l e a v i n g a l a y e r o f work m a t e r i a l welded t o t h e t o o l surface.

S e c t i o n t h r o u g h c a r b i d e t o o l w i t h adherent work m a t e r i a l ( w h i t e ) a f t e r cutting steel. Shows s e i z u r e c o n d i t i o n s a t i n t e r f a c e .

Seizure i s n o r m a l l y t h o u g h t o f as a c o n d i t i o n where a mechanism ceases t o f u n c t i o n , as when a b e a r i n g s e i z e s , b u t i n m e t a l c u t t i n g t h e s e i z e d area i s small, t h e r e i s adequate power t o c o n t i n u e c u t t i n g and t h e t o o l i s s rong enough t o r e s i s t t h e s t r e s s e s imposed by s e i z u r e c o n d i t i o n s .

Movement c o n t nues by This gives

shear in t h e work m a t e r i a l i n a r e g i o n a d j a c e n t t o t h e t o o l s u r f a c e . r i s e t o two main s o r t s o f c o n d i t i o n s near t h e i n t e r f a c e .

The f i r s t

ayers o f

work m a t e r i a l s e i z e d t o t h e t o o l a r e s e v e r e l y work hardened and shea transferred t o the next layers.

i s then

I n t h i s way a body o f "dead m e t a l " may be

b u i l t up, adherent t o t h e t o o l m a t e r i a l , which p e r s i s t s f o r l o n g p e r i o d s o f cutting.

Fig.8 shows an example o f t h i s f e a t u r e , known as a " b u i l t - u p

edge".

The b u i l t - u p edge reaches a s t a b l e s t a t e and t h e s i z e and shape depending on the work m a t e r i a l and t h e c u t t i n g c o n d i t i o n s .

I t i s a dynamic s t r u c t u r e w i t h

454

Fig.6

S e c t i o n through h i g h speed s t e e l t o o l and adherent c h i p ( a u s t e n i t i c s t a i n l e s s s t e e l ) a f t e r machining a t 3 0 m min-1.

Fig.7

S e c t i o n throuqh h i g h speed s t e e l t o o l and adherent c h i p fragment a f t e r c u t t i n g low carbon s t e e l a t 107111min-'.

fragments b e i n g c o n t i n u o u s l y added and broken away.

The s h e a r i n g a c t i o n l e a d i n g

t o c h i p f o r m a t i o n may t a k e p l a c e a t a d i s t a n c e o f 300 urn o r more from t h e t o o l surface. A b u i l t - u p edge i s o f t e n formed when c u t t i n g a l l o y s c o n t a i n i n g more than one

phase, such a s s t e e l , c a s t i r o n and a l p h a - b e t a b r a s s

[31.

With such m a t e r i a l s

a b u i l t - u p edge o c c u r s when c u t t i n g a t r e l a t i v e l y low speeds, b u t disappears

455

Fig.8

B u i l t - u p edge formed d u r i n g c u t t i n g low carbon s t e e l a t 15m min

when speed o r f e e d a r e r a i s e d .

-1

.

When c u t t i n g pure m e t a l s and s o l i d s o l u t i o n s a t

almost any speed, and when c u t t i n g two phase a l l o y s i n t h e h i g h e r range o f speeds,

s e i z u r e c o n d i t i o n s a r e observed t o e x i s t a t most o f t h e i n t e r f a c e , b u t

t h e b u i l t - u p edge i s absent.

Movement o f t h e work m a t e r i a l t a k e s p l a c e by

shear c o n c e n t r a t e d i n t o a v e r y t h i n l a y e r a d j a c e n t t o t h e t o o l s u r f a c e u s u a l l y o f t h e o r d e r o f 25-50 um i n t h i c k n e s s . Fig.2.

An example of such a l a y e r i s seen i n

f o r t h e c u t t i n g o f copper and a l a y e r when c u t t i n g a low carbon s t e e l i s

seen a t h i g h m a g n i f i c a t i o n i n Fig.9.

I n t h i s l a y e r t h e work m a t e r i a l behaves

more l i k e a v e r y v i s c o u s l i q u i d than a normal metal and t h e l a y e r i s termed a "flow-zone".

I n t h e flow-zone

l o 4 t o l o 5 p e r second

-

t h e r a t e o f shear s t r a i n i s e x t r e m e l y h i g h

-

and t h e amount o f s t r a i n i s so extreme t h a t o r i g i n a l

s t r u c t u r a l f e a t u r e s (such as p e a r l i t e and f e r r i t e i n s t e e l ) a r e c o m p l e t e l y destroyed.

There i s good evidence t h a t , w i t h i n t h e flow-zone,

dynamic recovery

456 and/or r e c r y s t a l l i s a t i o n a r e t a k i n g p l a c e and t h e behaviour o f t h e m a t e r i a l i s a k i n t o i t s behaviour i n h o t w o r k i n g p r o c e s s e s .

Fig.9

19.3.5

Flow zone a t under s u r f a c e of c h i p , a d j a c e n t t o r a k e f a c e of t o o l , formed d u r i n g c u t t i n g low carbon s t e e l a t 63m min-'.

C u t t i n g Tool Temperatures

The energy expended i n deforming t h e work m a t e r i a l i n t h e flow-zone volume o f metal deformed,

per u n i t

i s much h i g h e r than on t h e shear p l a n e and t h e temp-

e r a t u r e s generated i n t h e flow-zone a r e t h e r e f o r e h i g h e r . t h e flow-zone and t h e t o o l i s v e r y good.

The c o n t a c t between

As has been demonstrated t h e r e i s

continuous m e t a l l i c c o n t a c t i n many cases and heat f l o w s r e a d i l y across t h e boundary t o heat t h e t o o l .

I t i s t h e heat generated i n t h e flow-zone a t t h e

i n t e r f a c e between t o o l and work m a t e r i a l which i s t h e main h e a t source r a i s i n g t h e temperature o f t h e t o o l and c r e a t i n g t h e c o n d i t i o n s under which c u t t i n g t o o l s a r e worn. The r e g i o n s o f t h e t o o l s which a r e heated t o h i g h temperature a r e v e r y l o c a l i s e d and w i t h i n these r e g i o n s temperature g r a d i e n t s a r e v e r y steep, b u t i t i s p o s s i b l e t o study t h e temperature d i s t r i b u t i o n i n some d e t a i l f o r c e r t a i n c o n d i t i o n s o f c u t t i n g by o b s e r v a t i o n o f t h e changes i n s t r u c t u r e o r hardness o f s t e e l t o o l s i n those p a r t s o f t h e t o o l s heated by c u t t i n g a c t i o n above t h e i r tempering temperature.

Fig.10 shows, f o r example, t h e temperature g r a d i e n t s i n

457 a h i g h speed s t e e l t o o l used t o c u t a l o w c a r b o n s t e e l a t a speed o f 76m m i n - l a t a f e e d o f 0.25 mm p e r r e v .

T h i s i s c h a r a c t e r i s t i c o f t h e t y p e o f temperature

d i s t r i b u t i o n f o u n d t o o c c u r i n t o o l s u s e d t o c u t s t e e l u n d e r c o n d i t i o n s where a flow-zone o c c u r s a t t h e i n t e r f a c e . t o o l edge was r e l a t i v e l y l o w

-

Fig.10

shows t h a t t h e t e m p e r a t u r e n e a r t h e

i n t h i s c a s e u n d e r 650°C

-

but there i s a high

t e m p e r a t u r e r e g i o n j u s t o v e r l m m f r o m t h e edge i n t h e d i r e c t i o n o f c h i p f l o w , where,

i n t h i s example,

t h e t e m p e r a t u r e a t t h e i n t e r f a c e was o v e r 800°C.

It i s

f o r t u n a t e t h a t i n c u t t i n g s t e e l and many o t h e r a l l o y s a t h i g h speed t h e r e g i o n o f h i g h e s t t e m p e r a t u r e i s a t a d i s t a n c e f r o m t h e edge where t h e c o m p r e s s i v e s t r e s s o n t h e t o o l i s a maximum ( F i g . 3 ) .

As t h e c u t t i n g speed i s r a i s e d t h e maximum t e m p e r a t u r e o n t h e r a k e f a c e o f t h e tool

i n c r e a s e s r a p i d l y , w h i l e t h e t e m p e r a t u r e n e a r t h e edge i s i n c r e a s e d

more s l o w l y .

The y i e l d s t r e s s o f t h e t o o l m a t e r i a l d e c r e a s e s w i t h r i s i n g tem-

p e r a t u r e and, a s c u t t i n g speed i s r a i s e d , t h e t e m p e r a t u r e a t t h e edge may r e a c h a v a l u e where t h e y i e l d s t r e s s o f t h e t o o l i s r e d u c e d b e l o w t h e c o m p r e s s i v e s t r e s s e x e r t e d by t h e w o r k m a t e r i a l .

The t o o l edge i s t h e n p l a s t i c a l l y deformed

and t h i s l e a d s t o a r a p i d r i s e i n t h e r a t e o f h e a t g e n e r a t i o n a t t h e c u t t i n g edge.

The t o o l t h e n f a i l s c a t a s t r o p h i c a l l y u s u a l l y w i t h i n a f e w seconds.

This

i s t h e m a i n mechanism w h i c h s e t s t h e u p p e r l i m i t t o t h e r a t e o f m e t a l removal w h i c h c a n be a c h i e v e d w i t h h i g h speed s t e e l t o o l s (and, a t a h i g h e r l e v e l o f speeds, w i t h cemented c a r b i d e t o o l s ) when c u t t i n g s t e e l and o t h e r h i g h m e l t i n g point alloys.

Fig.10

T e m p e r a t u r e g r a d i e n t s i n t o o l used t o c u t l o w c a r b o n s t e e l a t 76m m i n - l , 0.25mm/rev f e e d .

458 19.3.6

S1 i d i n g a t t h e TooI/\,dork

Interface

Many o f t h e p e c u l i a r and c h a r a c t e i i s t i c f e a t u r e s o f machining o p e r a t i o n s a r i s e from t h e unusual f e a t u r e o f s e i z u r e a t t h e i n t e r f a c e between t o o l and work m a t e r i a l , b u t these c o n d i t i o n s do n o t e x i s t under a l l c u t t i n g c o n d i t i o n s and on a l l p a r t s o f t h e c o n t a c t area.

The model o f s e i z u r e which has been g i v e n i s t o o

s i m p l i f i e d and must be c o r r e c t e d . A t v e r y low speeds s e i z u r e may n o t o c c u r .

S i m i l a r l y a t t h e p e r i p h e r y o f the

c o n t a c t r e g i o n , even a t h i g h r a t e s o f metal removal, t h e r e i s good evidence t o show t h a t s l i d i n g t a k e s p l a c e a t t h e i n t e r f a c e by a t y p e o f s t i c k - s l i p process. Thus a s e c t i o n through t h e o u t e r edge o f a s t e e l c h i p o f t e n shows a segmented c h i p w i t h a p e r i o d i c s t r u c t u r e a t t h e i n t e r f a c e (Fig.11.) s l i p action.

indicating a stick-

The c e n t r e o f t h e same c h i p shows a flow-zone d e m o n s t r a t i n g

seizure a t t h i s p a r t o f the i n t e r f a c e (Fig.12).

That s l i d i n g o c c u r s i n these

p e r i p h e r a l r e g i o n s may be a t t r i b u t e d t o two main f a c t o r s (1) lower compressive s t r e s s near a f r e e s u r f a c e o f t h e c h i p and ( 2 ) access o f atmospheric oxygen t o t h e i n t e r f a c e a t t h i s p o s i t i o n , r e d u c i n g t h e tendency t o m e t a l l i c bonding. Since t h e mechanisms o f wear may be v e r y d i f f e r e n t under c o n d i t i o n s o f s e i z u r e and s l i d i n g ,

i t i s u s e f u l t o have i n mind a model o f t h e r e a i o n s where

s e i z u r e and s l i d i n g o c c u r most u s u a l l y . o n a c u t t i n g t o o l .

Fig.13 shows such a

map f o r a simple t u r n i n g t o o l .

Fig.11

S e c t i o n through o u t e r edge o f c h i p a f t e r c u t t i n g medium carbon s t e e l a t h i g h speed. Shows s t i c k - s l i p a c t i o n a t i n t e r f a c e .

459

Fig.12

19.4

S e c t i o n t h r o u g h c e n t r e o f same c h i p as Fig.11. Shows flow-zone a t i n t e r f a c e c h a r a c t e r i s t i c o f seizure.

CUTTING TOOL WEAR

While t h e upper l i m i t t o t h e r a t e o f metal removal when c u t t i n g s t e e l o r o t h e r h i g h m e l t i n g p o i n t a l l o y s i s determined by t h e a b i l i t y o f t h e t o o l t o w i t h stand t h e c u t t i n g s t r e s s e s a t e l e v a t e d temperatures, t h e l i f e o f t h e t o o l decreases as t h e c u t t i n g speed i s r a i s e d b e f o r e t h i s l i m i t i s reached.

A t lower

speeds t h e shape o f t h e t o o l i s changed by one o r more o f a number o f d i f f e r e n t wear mechanisms u n t i l i t can no l o n g e r c u t e f f i c i e n t l y .

Fig.14 shows diapram-

m a t i c a l l y on a model t u r n i n g t o o l t h e l o c a t i o n o f t h e c h i e f wear f e a t u r e s observed. "Flank wear" on t h e c l e a r a n c e f a c e o f t h e t o o l o f t e n increases s t e a d i l y w i t h t i m e o f c u t t i n g u n t i l , when a c r i t i c a l amount o f wear i s reached, t h e tempera t u r e on t h i s s u r f a c e s t a r t s t o r i s e r a p i d l y and t o o l f a i l u r e may be sudden. The c r i t i c a l amount o f f l a n k wear v a r i e s under d i f f e r e n t c o n d i t i o n s .but i t may be between 0.4 and 1.5 mm.

To a v o i d complete f a i l u r e , which may be expensive,

t o o l s a r e n o r m a l l y reground o r r e p l a c e d b e f o r e t h e c r i t i c a l wear i s reached. Flank wear may o c c u r a t any c u t t i n g speed b u t t h e wear r a t e increases w i t h

460

X

X

,L I

INCOMPLETE SEIZURE

SECTION X - X

Fig.13

Diagram o f t u r n i n g t o o l showing r e g i o n s o f s e i z u r e and o f s l i d i n g a t the tooI/work i n t e r f a c e

/

/

ACCELERATED WEAR IN REGION OF 1 . DEFORMAT1ON OR SLIDING

RAKE FACE

CRATER

WtAK

$4 ffw

ACCELERATED WEAR IN REGION OF SLIDING

FLANK OR CLEARANCE FACE

Fig.14

Diagram showing wear f e a t u r e s on t u r n i n g t o o l

461 speed as t h e u l t i m a t e l i m i t f o r t h e t o o l m a t e r i a l i s approached.

In the region

o f h i g h speed c u t t i n p t h e r a t e o f f l a n k wear and t h e t o o l l i f e o f t e n f o l l o w t h e r e l a t i o n s h i p g i v e n by T a y l o r [ 4 ] f o r t o o l l i f e i n r e l a t i n g t o c u t t i n g speed Vt"

V t

=

-

c

c u t t i n g speed c u t t i n g t i m e t o f a i l u r e o r t o some s t a n d a r d amount o f wear

n and C

-

c o n s t a n t s f o r a g i v e n t o o l and work m a t e r i a l

" C r a t e r wear"

i s t h e term used f o r a groove o r c r a t e r worn on the rake face

o f t h e t o o l , u s u a l l y a t some d i s t a n c e f r o m t h e c u t t i n g edge ( F i g . 1 4 ) .

Cratering

wear i s c h a r a c t e r i s t i c a l l y observed on t o o l s used a t h i g h c u t t i n g speeds and t h e r a t e o f c r a t e r wear increases as t h e c u t t i n g speed approaches t h e u l t i m a t e l i m i t f o r the t o o l material.

As t h e c r a t e r becomes deeper i t weakens t h e t o o l edge

and may l e a d t o f r a c t u r e o f t h e edge and t o o l f a i l u r e . "Flank wear" and " c r a t e r wear" a r e d e s c r i p t i v e terms and t h e words do n o t imply d i s t i n c t wear mechanisms.

The mechanisms o f wear w i l l now be discussed.

Where s e i z u r e c o n d i t i o n s o c c u r a t t h e t o o l / w o r k i n t e r f a c e a t l e a s t f o u r d i f f e r e n t mechanisms o f wear have been observed and these w i l l be c o n s i d e r e d f i r s t .

Wear

under c o n d i t i o n s o f s l i d i n g a t t h e i n t e r f a c e w i l l be c o n s i d e r e d s e p a r a t e l y .

19.4.1

Abrasion

The a b r a s i v e a c t i o n o f h a r d phases i n t h e work m a t e r i a l , such as o x i d e s o r c a r b i d e s may c o n t r i b u t e t o t h e wear o f c u t t i n g t o o l s .

Abrasion i s , however,

p r o b a b l y n o t a m a j o r cause o f wear under s e i z u r e c o n d i t i o n s u n l e s s t h e p a r t i c l e s o f t h e h a r d phases a r e l a r g e , e.g.,

g r e a t e r than 40 pm, o r p r e s e n t i n v e r y h i g h

c o n c e n t r a t i o n s , as t h e y may be f o r example on t h e s u r f a c e o f c a s t i n g s .

Even

w i t h h i g h speed s t e e l t o o l s t h e a b r a s i v e a c t i o n o f d i s p e r s e d , f i n e h a r d p a r t i c l e s i s p r o b a b l y small because, under s e i z u r e c o n d i t i o n s they r a r e l y impinge on t h e t o o l s u r f a c e i n such a way as t o remove t o o l m a t e r i a l .

W i t h h a r d e r t o o l mat-

e r i a l s such as c a r b i d e s o r diamond, few i f any p a r t i c l e s i n t h e work m a t e r i a l a r e h a r d e r t h a n t h e t o o l s and a b r a s i v e a c t i o n i s l e s s l i k e l y .

The hardness of

c u t t i n g t o o l m a t e r i a l s i s o f more s i g n i f i c a n c e as a measure o f t h e i r a b i l i t y t o w i t h s t a n d h i g h compressive s t r e s s than as a measure o f t h e i r r e s i s t a n c e t o abrasion.

19.4.2

Surface Shearing

When c u t t i n g h i g h e r m e l t i n g p o i n t m e t a l s a t h i g h speeds, t h e i n t e r f a c e temp e r a t u r e , p a r t i c u l a r l y on t h e rake f a c e o f t h e t o o l ( F i g . l O ) ,

may be very h i g h ,

so t h a t t h e y i e l d s t r e s s o f t h e s t e e l t o o l i s reduced t o a v e r y low value i n a

small volume of metal a t t h e i n t e r f a c e .

T h i n l a y e r s o f t h e t o o l m a t e r i a l may

then be sheared away by t h e work m a t e r i a l bonded t o t h e t o o l s u r f a c e .

Fig.15

462 shows an example o f t h i s wearing a c t i o n i n which a c r a t e r i s b e i n g worn on t h e rake face o f a h i g h speed s t e e l t o o l when c u t t i n g carbon s t e e l a t h i g h speed. This wear mechanism i s u s u a l l y observed o n l y where t h e i n t e r f a c e temperature i s above 800°C on s t e e l t o o l s .

It i s one o f t h e mechanisms o f wear r e s p o n s i b l e

f o r c r a t e r wear on s t e e l t o o l s and a l s o f o r t h e f i n a l stages o f f l a n k wear j u s t before complete t o o l f a i l u r e .

T h i s mechanism causes r a p i d t o o l wear.

I t has

not been observed t o o c c u r on cemented c a r b i d e t o o l s .

Fig.15

S e c t i o n through t o o l used t o c u t low carbon s t e e l a t h i g h speed. Showing f o r m a t i o n o f c r a t e r on r a k e f a c e by s h e a r i n g o f h i g h speed t o o l i n r e g i o n of h i g h temperature.

19.4.3

D i f f u s i o n and i n t e r a c t i o n

Under c o n d i t i o n s o f c u t t i n g where t h e t o o l and work s u r f a c e s a r e m e t a l l u r g i c a l l y bonded, t h e t o o l shape can be changed by a process o f d i f f u s i o n and i n t e r a c t i o n between t h e two m a t e r i a l s .

I n t h e s i m p l e s t s i t u a t i o n atoms from

the t o o l m a t e r i a l may d i f f u s e i n t o t h e work m a t e r i a l f l o w i n g o v e r t h e s u r f a c e and be c a r r i e d away by i t

-

i.e.,

t h e t o o l m a t e r i a l i s d i s s o l v e d i n t o t h e work

m a t e r i a l by a process o f t h e same c h a r a c t e r as t h a t of a b l o c k o f s a l t b e i n g d i s s o l v e d by a stream o f water r u n n i n g o v e r i t s s u r f a c e .

Diffusion i s a highly

temperature dependent process and d i f f u s i o n wear o c c u r s a t an a p p r e c i a b l e r a t e o n l y a t r e l a t i v e l y h i g h c u t t i n g speeds where t h e i n t e r f a c e temperature i s h i g h . When c u t t i n g s t e e l s w i t h h i g h speed s t e e l t o o l s wear by a d i f f u s i o n mechanism i s p r o b a b l y s i g n i f i c a n t o n l y where t h e i n t e r f a c e temperature exceeds 650°C. I t has been shown t h a t , even a t m o d e r a t e l y h i g h c u t t i n g speeds o f 25-30 m min

-1

when c u t t i n g s t e e l , temperatures o f 790°C and o v e r o f t e n o c c u r a t p a r t s of t h e

463 i n t e r f a c e , and t h e r e i s a r a p i d r i s e i n t e m p e r a t u r e w i t h f u r t h e r increments i n speed. Simple d i f f u s i o n and a v a r i e t y o f i n t e r a c t i o n s depending on t h e chemical c o m p o s i t i o n s and m e t a l l u r g i c a l s t r u c t u r e s o f t h e t o o l and work m a t e r i a l s , o c c u r across the i n t e r f a c e .

I t i s p r o b a b l e t h a t wear based on a t o m i c i n t e r a c t i o n s i s

t h e most i m p o r t a n t mechanism c h a n g i n g t h e shape o f h i g h speed s t e e l , cemented c a r b i d e o r diamond c u t t i n g tools i n m a c h i n i n g t h e h i g h e r m e l t i n g p o i n t m e t a l s and a l l o y s a t h i g h speeds. Fig.16 shows a s e c t i o n t h r o u g h t h e c r a t e r e d s u r f a c e o f a h i g h speed s t e e l t o o l w i t h a d h e r i n g work m a t e r i a l .

The wear p r o c e s s was one o f d i f f u s i o n ;

t o o l s u r f a c e shows no s i g n s o f p l a s t i c d e f o r m a t i o n .

the

There i s good e v i d e n c e t h a t

wear by d i f f u s i o n i s r e s p o n s i b l e f o r most c r a t e r and f l a n k wear on h i g h speed s t e e l t o o l s where t h e i n t e r f a c e t e m p e r a t u r e i s above

650°C b u t below t h e tem-

perature required f o r the shearing action.

Fig.16

S e c t i o n t h r o u g h h i g h speed s t e e l t o o l i n worn c r a t e r a f t e r c u t t i n g Interface characteristic low a l l o y s t e e l a t 18m m i n - ’ f o r 38 m i n u t e s . o f d i f f u s i o n wear

464 C u t t i n g t o o l m a t e r i a l s h a v e been d e v e l o p e d e m p i r i c a l l y and m o s t h a v e been developed f o r t h e machining o f s t e e l s i n c e t h i s i s t h e main market f o r c u t t i n p tools

[5].

The f i r s t cemented c a r b i d e s p r o d u c e d w e r e a l l o y s o f t u n g s t e n c a r b i d e

and c o b a l t (WC-Co).

These a r e v e r y s u c c e s s f u l f o r t h e c u t t i n g o f n o n - f e r r o u s

m e t a l s and c a s t i r o n a t speeds much h i g h e r t h a n c a n be a c h i e v e d w i t h s t e e l t o o l s because t h e WC-Co a l l o y s have h i g h e r y i e l d s t r e s s and c a n r e s i s t t h e s h e a r i n g a c t i o n a n d d e f o r m a t i o n a t h i g h t e m p e r a t u r e s , a n d a l s o b e c a u s e t h e WC-Co a l l o y s do n o t r e a c t w i t h t h e s e w o r k m a t e r i a l s a t t h e h i g h c u t t i n g speeds. For t h e c u t t i n g o f s t e e l h o w e v e r , t h e WC-Co a l l o y s a r e n o t so s u c c e s s f u l ,

s i n c e r a p i d c r a t e r i n g wear i n p a r t i c u l a r o n t h e r a k e f a c e o f t h e t o o l s g i v e s a v e r y s h o r t l i f e f o r t o o l s used a t speeds n o t much h i g h e r t h a n t h o s e u s e d w i t h h i g h speed s t e e l t o o l s .

Cemented c a r b i d e t o o l s w e r e s u c c e s s f u l f o r c u t t i n g o f

s t e e l a t h i g h e r speeds o n l y when a s m a l l p r o p o r t i o n ( 5 t o 20%) o f T i c o r TaC was added t o t h e WC-Co a l l o y s .

These " s t e e l

c u t t i n g g r a d e s " o f cemented c a r b i d e a r e

a b l e t o m a c h i n e s t e e l a t more t h a n t h r e e t i m e s t h e speed w i t h WC-Co a l l o y s because t h e r a t e o f w e a r , p a r t i c u l a r l y t h e c r a t e r wear o n t h e r a k e f a c e , much l o w e r .

i s so

I t i s now known t h a t t h e i r s u c c e s s must be a t t r i b u t e d t o t h e much

l o w e r r a t e o f s o l u t i o n o f TIC o r TaC i n s t e e l a t e l e v a t e d t e m p e r a t u r e s , compared w i t h t h a t o f WC.

The r a t e o f c r a t e r i n g

in particular

i s a f f e c t e d because i t i s

on t h e r a k e f a c e o f t h e t o o l , away f r o m t h e c u t t i n g edge, temperatures a t t h e tool/work

t h a t the highest

i n t e r f a c e a r e located (Fig.10).

The r a t e o f f l a n k

wear a l s o i s g e n e r a l l y r e d u c e d b y t h e i n c l u s i o n o f T I C a n d TaC i n t h e t o o l m a t e r i a l s when c u t t i n g s t e e l

[5].

I n a more r e c e n t d e v e l o p m e n t t h e r a t e o f wear h a s been d e c r e a s e d s t i l l f u r t h e r b y c o a t i n g cemented c a r b i d e t o o l s w i t h t h i n l a y e r s o f s o l u t i o n r e s i s t a n t The s u b s t a n c e s used f o r t h e c o a t i n a s a r e TIC, T i N o r H f N , w h i c h a r e

materials.

deposited on t h e s u r f a c e o f t h e t o o l s a s l a y e r s known as c h e m i c a l v a p o u r d e p o s i t i o n ("CVD'I).

5

t o 10

jm

t h i c k by a process

These c a r b i d e s a n d n i t r i d e s a r e

t o o b r i t t l e t o be u s e d b y t h e m s e l v e s a s c u t t i n g t o o l s ,

b u t as t h i n l a y e r s w i t h

v e r y f i n e g r a i n s i z e s u p p o r t e d b y t h e t o u g h e r cemented c a r b i d e s u b s t r a t e t h e y w i t h s t a n d w e l l t h e s t r e s s e s o f m a c h i n e shop o p e r a t i o n s , and t h e y a r e l e s s r e a d i l y dissolved i n the steel flowing over the t o o l surfaces. t o o l I i f e may be p r o l o n g e d b y a f a c t o r o f 1.5

to

3

cemeiited c a r b i d e s , o r c u t t i n g speeds 30 t o 60 m m i n

With these coated t o o l s

t i m e s compared w i t h u n c o a t e d

-1

h i g h e r c a n be used f o r t h e

same t o o l l i f e when c u t t i n g s t e e l . Even more r e s i s t a n t t o s o l u t i o n i n s t e e l a t h i g h t e m p e r a t u r e s i s a l u m i n i u m oxide.

A1 0

2 3

( a l u m i n a ) has a h i g h y i e l d s t r e s s a t h i g h t e m p e r a t u r e and t o o l

t i p s a r e s i n t e r e d o r h o t p r e s s e d f r o m a l u m i n a powder. as " c e r a m i c

tools")

A l q 0 3 t o o l t i p s (known

o f h i g h d e n s i t y a n d f i n e g r a i n s i z e c a n be used f o r c u t t i n g

s t e e l and c a s t i r o n a t speeds a s h i g h a s 600-700 m m i n - ' wear.

w i t h v e r y low r a t e s of

These t o o l s , h o w e v e r , a r e l a c k i n g i n t o u g h n e s s and t h e i r u s e i n c u t t i n g

465 s t e e l i s r e s t r i c t e d f o r t h i s reason t o a v e r y small p r o p o r t i o n o f machine shop o p e r a t i o n s on s t e e l . h i g h speed.

They can be more w i d e l y used f o r c u t t i n g c a s t i r o n a t v e r y

Recently cemented c a r b i d e t o o l s have been p u t on the market w i t h

t h i n c o a t i n g s o f A1203 d e p o s i t e d by CVD and these a r e being assessed f o r t h e c u t t i n g o f b o t h s t e e l and c a s t i r o n . Reaction between t o o l and work m a t e r i a l a l s o l i m i t s t h e speeds used when c u t t i n g s t e e l and n i c k e l - b a s e d a l l o y s w i t h diamond t o o l s .

Cubic boron n i t r i d e

i s a s y n t h e t i c m a t e r i a l made by t h e same t y p e o f u l t r a h i g h pressure process used f o r t h e p r o d u c t i o n o f s y n t h e t i c diamond. diamond and, a l t h o u g h i t i s less hard,

I t has a s i m i l a r s t r u c t u r e t o

i t can be used i n c u t t i n g s t e e l s and

n i c k e l - b a s e d a l l o y s a t h i g h e r speeds because i t r e a c t s w i t h them l e s s r e a d i l y than does diamond a t e l e v a t e d temperatures

161.

Thus, under t h e s e i z u r e c o n d i t i o n s which p r e v a i l i n many metal c u t t i n g o p e r a t i o n s , t h e l i f e o f t h e c u t t i n g t o o l s i s o f t e n c o n t r o l l e d by processes o f d i f f u s i o n and i n t e r a c t i o n between t o o l and m a t e r i a l a t h i g h i n t e r f a c e tempera t u r e s when c u t t i n g m a t e r i a l s o f h i g h m e l t i n g p o i n t a t h i g h speeds.

19.4.4

Attrition

I f , under c o n d i t i o n s o f s e i z u r e , t h e temperature-dependent wear processes o f

s h e a r i n g and d i f f u s i o n were t h e o n l y ones r e s p o n s i b l e f o r changing t h e t o o l shape, t o o l l i f e might be expected t o be almost i n f i n i t e a t low c u t t i n g speed. A t low speeds,

however, t o o l s a r e f r e q u e n t l y worn by another mechanism which can

be c a l l e d " a t t r i t i o n " .

Sections through t h e edge o f t o o l s used a t low speeds

o f t e n show t h a t t h e t o o l has been worn by a mechanism i n v o l v i n g b r e a k i n g away from t h e t o o l s u r f a c e o f fragments o f m i c r o s c o p i c s i z e Fig.17.

f o r a h i g h speed s t e e l t o o l .

-

as shown f o r example i n

Such a wear mechanism has been observed

w i t h almost a l l c l a s s e s o f t o o l m a t e r i a l a f t e r c u t t i n g a t low speed, and as t h e speed i s r a i s e d i t becomes l e s s i m p o r t a n t .

I n many cases b o t h d i f f u s i o n and

a t t r i t i o n wear a r e observed on t h e same worn s u r f a c e s . A t t r i t i o n wear seems t o be most severe when t h e machine t o o l l a c k s r i g i d i t y , when v i b r a t i o n o c c u r s o r when t h e r e a r e pronounced i r r e g u l a r i t i e s i n t h e work material.

I t i n v o l v e s an i n t e r m i t t e n t a c t i o n i n which small fragments o f the

t o o l a r e t o r n away t o leave c h a r a c t e r i s t i c a l l y rough worn s u r f a c e s .

High speed

s t e e l t o o l s a r e more r e s i s t a n t t o a t t r i t i o n than a r e cemented c a r b i d e s and the l i f e o f h i g h speed s t e e l t o o l s i s o f t e n l o n g e r a t low c u t t i n g speeds t h a n . t h a t o f cemented c a r b i d e s f o r t h i s reason.

T w i s t d r i l l s f o r example a r e most

commonly made f r o m h i g h speed steels, n o t o n l y because t h e y a r e cheaper but because,

i n many a p p l i c a t i o n s t h e l i f e i s l o n g e r than t h a t o f cemented c a r b i d e

d r i l l s and performance i s more c o n s i s t e n t .

When cemented c a r b i d e t o o l s a r e used

466

Fig.17

S e c t i o n t h r o u g h c u t t i n g edge of h i g h speed s t e e l t o o l a f t e r c u t t i n g s t e e l a t 20m min Worn s u r f a c e c h a r a c t e r i s t i c o f a t t r i t i o n wear

'.

i n t h e low speed range where a t t r i t i o n wear i s dominant, t h e r a t e o f wear i s v e r y dependent on t h e c a r b i d e g r a i n s i z e .

F i n e g r a i n e d cemented c a r b i d e s a r e

much more r e s i s t a n t t o a t t r i t i o n wear than coarse g r a i n e d ones, and WC-Co a l l o y s a r e more r e s i s t a n t t h a n t h e s t e e l c u t t i n g c a r b i d e grades and a r e o f t e n used t o c u t s t e e l a t low speed f o r t h i s reason. Fig.13 shows d i a g r a m m a t i c a l l y those p a r t s o f a t u r n i n g t o o l where s e i z u r e and s l i d i n g a r e most l i k e l y t o o c c u r d u r i n g c u t t i n g .

Rather f r e q u e n t l y more

r a p i d wear i s observed i n t h e s l i d i n g r e g i o n s than a t t h e s e i z e d s u r f a c e s .

For

example, Fig.18 shows a deep tongue o f wear on a t o o l i n t h e s l i d i n g wear r e g i o n where t h e o u t e r edge o f t h e c h i p crossed t h e c u t t i n g edge o f t h e t o o l .

The wear

r a t e was many times h i g h e r a t t h i s p o s i t i o n than i n t h e a d j a c e n t s e i z e d r e g i o n . High r a t e s o f wear a r e o f t e n observed a l s o a t t h e nose o f t h e t o o l i n t u r n i n g o p e r a t i o n s , where t h e o t h e r edge o f t h e c h i p crosses t h e c u t t i n g edge.

467

Fig.18

Clearance f a c e o f t o o l used t o c u t low carbon s t e e l . Shows adherent work m a t e r i a l and deeply worn qroove a t o u t s i d e o f c u t , c h a r a c t e r i s t i c o f wear by s l i d i n g a c t i o n

Such in c r e a s e d wear r a t e i n t h e s l i d i n g r e g i o n s does n o t always occur, and t h e c o n d i t i o n s i n f l u e n c i n g t h e s e v e r i t y o f wear have n o t y e t been s t u d i e d i n sufficient detail.

I t seems most p r o b a b l e t h a t t h e wear i n t h e s l i d i n g r e g i o n s

i n v o l v e s r e a c t i o n between t h e t o o l and work s u r f a c e s and oxyaen o f the atmosphere, which has access t o t h i s p a r t o f t h e i n t e r f a c e .

One p o s s i b l e wear

mechanism i s t h e f o r m a t i o n of o x i d e l a y e r s on t h e tool and the removal o f these p e r i o d i c a l l y by t h e s t i c k - s l i p a c t i o n o f t h e s l i d i n g c h i p (Fig.11).

The worn

s u r fa c e s i n t h e s l i d i n g r e g i o n s a r e n o r m a l l y v e r y smooth and t h i s s o r t o f wear on c a r b i d e t o o l s may be a l most as r a p i d as on s t e e l t o o l s , so t h a t a b r a s i o n by h a r d p a r t i c l e s , a l t h o u g h p o s s i b l e i n t h i s r e g i o n , mechanism o f wear i n most o p e r a t i o n s .

i s p r o b a b l y n o t the main

The r a t e o f s l i d i n g wear may’be i n -

fl ue n c e d by t h e use of c u t t i n g l u b r i c a n t s .

19.5

COOLANTS AND LUBRICANTS

A d i s c u s s i o n o f t r i b o l o g y i n metal c u t t i n g would n o t be complete w i t h o u t c o n s i d e r i n g t h e i n f l u e n c e of c o o l a n t s and l u b r i c a n t s .

The c u t t i n g t o o l i s o f t e n

f l o o d e d w i t h a f l u i d d e s c r i b e d e i t h e r as a c o o l a n t o r as a l u b r i c a n t . types a r e used

-

Two main

water-based f l u i d s c o n t a i n i n g o i l and o t h e r a d d i t i v e s i n sus-

pension o r s o l u t i o n and m i n e r a l o i l s w i t h o r w i t h o u t extreme pressure a d d i t i v e s (ma in ly s u l p h u r and c h l o r i n e c o n t a i n i n g substances).

The f u n c t i o n o f t h e w ater-

468 based f l u i d s i s m a i n l y as a c o o l a n t w i t h t h e o i l p r e s e n t m a i n l y t o p r e v e n t c o r r o s i o n , w h i l e t h e o i l based f l u i d s have r e l a t i v e l y poor c o o l i n g c a p a c i t y and a r e used m a i n l y f o r t h e i r l u b r i c a n t a c t i o n . C o o l a n t s a r e o f t e n necessary t o reduce t h e t e m p e r a t u r e o f machine, t o o l and w o r k p i e c e i n o r d e r t o promote e f f i c i e n t o p e r a t i o n and t o m a i n t a i n d i m e n s i o n a l tolerances.

P r a c t i c a l e x p e r i e n c e shows t h a t a s t r o n g f l o w o f c o o l a n t can a c t

t o i n c r e a s e t o o l l i f e when c u t t i n g a t h i g h speeds, o r t o p e r m i t t h e use o f h i g h e r c u t t i n g speeds.

The d i r e c t i n f l u e n c e o f a c o o l a n t on t h e maximum t e m p e r a t u r e

generated a t t h e t o o l / w o r k

[71.

interface i s usually rather s l i g h t

e r a t u r e i s generated i n t h e f l o w - z o n e a t t h e r a k e f a c e ( F i g . 1 0 ) .

The tempCoolant cannot

p r e v e n t t h e g e n e r a t i o n o f h e a t a t t h i s s u r f a c e and can a c t o n l y t o steepen t h e t e m p e r a t u r e g r a d i e n t s and reduce t h e volume o f t o o l m a t e r i a l h e a t e d t o h i g h t e m p e r a t u r e , b u t i t can have l i t t l e i n f l u e n c e on t h e maximum t e m p e r a t u r e on t h e rake f a c e .

A c t i n g n e a r t h e c u t t i n g edge t h e c o o l a n t can be more e f f e c t i v e and

t h e m a j o r c o o l i n g e f f e c t i s p r o b a b l y t h a t o f r e d u c i n g t h e t e m p e r a t u r e near t h e edge, t h u s i n c r e a s i n g t h e y i e l d s t r e n g t h o f t h e t o o l t o p r e v e n t l o c a l deforma t i o n , and a l s o r e d u c i n g t h e r a t e o f d i f f u s i o n wear on t h e f l a n k o f t h e t o o l . From what has been s a i d about s e i z u r e a t t h e t o o l / w o r k u n l i k e l y t h a t any l u b r i c a n t ,

i n t e r f a c e , i t seems

i n e i t h e r gaseous o r l i q u i d form, can p e n e t r a t e t o

t h a t p a r t o f t h e i n t e r f a c e where s e i z u r e o c c u r s .

L u b r i c a n t s can, however, a c t

e f f e c t i v e l y i n t h e p e r i p h e r a l r e g i o n s where s l i d i n g o c c u r s a t t h e i n t e r f a c e . By p e n e t r a t i n g f r o m t h e p e r i p h e r a l r e g i o n s t h e y may be a b l e t o r e s t r i c t t h e area o f complete s e i z u r e and t h u s t o reduce t h e f o r c e s a c t i n g on t h e t o o l .

Force

measurements have shown r e d u c t i o n s i n b o t h c u t t i n g f o r c e and f e e d f o r c e caused by t h e use o f c o o l a n t s and l u b r i c a n t s a t low c u t t i n g speed.

Many t e s t s have

-1

i n d i c a t e d t h a t t h e i n f l u e n c e o f l u b r i c a n t s i s g r e a t e s t a t speeds below 30 m min w h i l e t h e y have v e r y l i t t l e l u b r i c a t i n g e f f e c t o v e r 60 m min

-1

.

The most

e f f e c t i v e l u b r i c a n t s i n m e t a l c u t t i n g a r e t h o s e w i t h extreme p r e s s u r e a d d i t i v e s , s u g g e s t i n g t h a t s u c c e s s f u l l u b r i c a t i o n i n v o l v e s t h e f o r m a t i o n o f e a s i l y sheared s u r f a c e l a y e r s when t h e l u b r i c a n t s come i n t o c o n t a c t w i t h f r e s h l y exposed m e t a l s u r f a c e s on t h e work m a t e r i a l . When a b u i l t - u p edge i s formed, c o o l a n t s and ' l u b r i c a n t s a r e o f t e n e f f e c t i v e i n g r e a t l y reducing i t s size.

A l a r g e b u i l t - u p edge i s o f t e n r e s p o n s i b l e f o r

v e r y p o o r s u r f a c e f i n i s h and one of t h e most i m p o r t a n t f u n c t i o n s o f a c u t t i n g l u b r i c a n t i s t o improve t h e s u r f a c e where t h i s i s a r e q u i r e m e n t o f t h e s u r f a c e b e i n g machined.

Often,

i n t h i s r e s p e c t , water-based

l u b r i c a n t s , and even w a t e r

i t s e l f , a r e e f f e c t i v e i n r e d u c i n g t h e s i z e o f t h e b u i l t - u p edge. For many c u t t i n g o p e r a t i o n s l u b r i c a n t s a r e e s s e n t i a l .

A t h i g h speeds t o o l

l i f e may be improved by c o o l a n t a c t i o n , b u t t h e use o f l u b r i c a n t s a t l o w e r speeds t o improve s u r f a c e f i n i s h i s n o t always e f f e c t i v e i n r e d u c i n g t h e r a t e o f wear.

I n f a c t tool wear r a t e i s o f t e n i n c r e a s e d by t h e a c t i o n o f l u b r i c a n t s i n

469 t h e s l i d i n g r e g i o n s where i t can p e n e t r a t e .

T h i s a c c e l e r a t i o n o f w e a r may occur

w i t h b o t h s t e e l and cemented c a r b i d e t o o l s .

High r a t e of wear i n r e g i o n s o f I n many e n g i n e e r i n g mech-

s l i d i n g a t t h e i n t e r f a c e has a l r e a d y been discussed. anisms a major f u n c t i o n of

l u b r i c a n t s i s t o p r e v e n t s e i z u r e between moving p a r t s .

I n t h e case o f metal c u t t i n g o p e r a t i o n s , t h e e l i m i n a t i o n o f s e i z u r e i s n o t t h e o b j e c t i v e o f t h e use o f l u b r i c a n t s .

The e l i m i n a t i o n o f s e i z u r e i n many cases

c o u l d r e s u l t i n a d i s a s t r o u s i n c r e a s e i n t h e r a t e o f t o o l wear.

I n most

c u t t i n g o p e r a t i o n s s e i z u r e between t o o l and work m a t e r i a l i s a normal and desirable condition,

r a t h e r than a hazard t o be avoided.

The main u s e f u l fun-

c t i o n s o f c u t t i n g f l u i d s a r e t o reduce temperature and thus increase c u t t i n g e f f i c i e n c y , t o reduce c u t t i n g f o r c e s ,

t o improve s u r f a c e f i n i s h , and t o h e l p

c l e a r c h i p s away f r o m t h e c u t t i n g t o o l . There i s one way i n which s e i z u r e between tool and work m a t e r i a l can be modified,

i f n o t e l i m i n a t e d , t o t h e advantage o f c e r t a i n c u t t i n g o p e r a t i o n s , and

t h i s i s t o i n c l u d e w i t h i n t h e work m a t e r i a l a phase which may i n t e r p o s e i t s e l f between t o o l and work d u r i n g t h e c u t t i n g o p e r a t i o n . f r o m w i t h i n t h e work m a t e r i a l can renew i n t e r f a c i a l

Such a substance a p p l i e d l a y e r s as they a r e swept

away by t h e f l o w o f t h e work m a t e r i a l o v e r t h e t o o l .

Under c e r t a i n c o n d i t i o n s

such substances can be d e s c r i b e d as p e r f o r m i n g t h e f u n c t i o n s o f an i n t e r n a l l u b r i c a n t and a r e g e n e r a l l y known as free-machining a d d i t i v e s . Manganese s u l p h i d e i n s t e e l may a c t i n t h i s way and, when t h e s t e e l c u t t i n g grades o f c a r b i d e a r e used, t h e s u l p h i d e s from t h e s t e e l w i l l o f t e n form an intermediate layer a t the seized tool/work interface.

C e r t a i n calcium-aluminium

s i l i c a t e s i n s t e e l , which l i k e manganese s u l p h i d e a r e p l a s t i c a l l y deformed when t h e s t e e l i s sheared, a l s o f u n c t i o n i n t h e same way as t h e s u l p h i d e s .

Silicate

l a y e r s a r e formed on s t e e l - c u t t i n g c a r b i d e t o o l s a t t h e i n t e r f a c e when c u t t i n g a t h i g h speeds and a r e o f t e n e x t r e m e l y e f f e c t i v e i n r e d u c i n g t o o l wear r a t e and p e r m i t t i n g t h e use o f h i g h e r c u t t i n g speeds. Sulphides i n copper a l s o t e n d t o f o r m a t h i n l a y e r a t t h e t o o l / w o r k

interface

where t h e i r main f u n c t i o n seems t o be t o reduce t h e a r e a o f c o n t a c t on the t o o l rake f a c e and t h u s g r e a t l y t o reduce t h e c u t t i n g f o r c e s which a r e e x t r e m e l y h i g h when c u t t i n g h i g h - c o n d u c t i v i t y copper.

The a d d i t i o n o f l e a d t o brass r e s u l t s i n

an a c c u m u l a t i o n o f l e a d a t t h e b r a s s / t o o l cutting.

i n t e r f a c e under many c o n d i t i o n s o f

The g r e a t e s t b e n e f i t which t h e l e a d c o n f e r s i s t h e b r e a k i n g up o f the

brass c h i p s i n t o small fragments which a r e e a s i l y disposed o f i n h i g h speed a u t o m a t i c machining. O f a l l t h e aspects o f machining t h e f u n c t i o n s o f c u t t i n g f l u i d s as c o o l a n t s

and l u b r i c a n t s have p r o b a b l y r e c e i v e d t h e l e a s t a t t e n t i o n from s c i e n t i f i c research.

There i s much t o be l e a r n e d c o n c e r n i n g t h e ways i n which they a c t t o

achieve t h e r e s u l t s f o r which t h e y a r e used by p r a c t i c a l m a c h i n i s t s .

470 REFERENCES 1

2

3 4

5 6

7

Zorev,N.N., I n t e r n a t i o n a l Research i n P r o d u c t i o n E n g i n e e r i n g (1963), A.S.M.E., P i t t s b u r g h , p.42. Trent,E.M., I . S . I . Report N0.94., 1967, p . 1 1 . W i l l iarn5,J.E. and Rollason,E.C., J., I n s t . , Met., (19701, 144. Boothroyd,G., "Fundamentals of Metal M a c h i n i n g and Machine Tools", McGraw-Hi 1 1 (1975). Trent,E.M., "Metal C u t t i n g " , 1977, B u t t e r w o r t h s . Hibb5,L.E. and Wentorf,R.E. J r . , 8 t h Plansee Seminar ( 1 9 7 4 ) , Paper No.42. Smart,E.F. and Trent,E.M., Proc., 1 5 t h I n t . , Conf., M.T.D.R., (1975) 187.

98,

471

v ( ) ROLE OF LABORATORY TEST MACHINES U

PROFESSOR F.T.

BARWELL, H.H.

JONES

Department o f Mechanical Engineering, U n i v e r s i t y C o l l e g e o f Swansea.

20.1

THE EXPERIMENTAL METHOD

The e s t a b l i s h m e n t o f f a c t s and t h e u n d e r s t a n d i n g o f r e l a t i o n s h i p s between p h y s i c a l phenomena can o n l y be based on experience,

t h e e x p e r i m e n t a l method

should t h e r e f o r e c o n s i s t o f s t r u c t u r i n g e x p e r i e n c e s o as t o produce unambiguous answers t o c e r t a i n q u e s t i o n s . meaning,

For s t r u c t u r e d experience o r experiment t o have

i t must reproduce t h e circumstances s u r r o u n d i n g t h e occurrence o f t h e

phenomena under study.

Otherwise t h e r e s u l t s , though perhaps i n t e r e s t i n g , w i l l

be i r r e l e v a n t t o t h e purpose o f t h e i n v e s t i g a t i o n . T r i b o l o g i c a l i n v e s t i g a t i o n s a r e b e s t c a r r i e d o u t under s e r v i c e c o n d i t i o n s on f u l l s c a l e apparatus b u t t h i s i s seldom p o s s i b l e .

I n f o r m a t i o n may o f t e n be

r e q u i r e d i n advance o f t h e c o n s t r u c t i o n o f t h e machine i n v o l v e d , measurements may n o t be p o s s i b l e i n t h e o p e r a t i o n a l environment ( i n space f o r example) and f u l l s c a l e t e s t i n g may be t o o c o s t l y e s p e c i a l l y when t e s t s t o d e s t r u c t i o n a r e required.

L a b o r a t o r y t e s t methods have t h e r e f o r e been e v o l v e d which serve a

number o f v i t a l f u n c t i o n s i n r e l a t i o n t o t h e p r a c t i c e o f e n g i n e e r i n g . The t e s t i n g o f l u b r i c a n t s by measurement o f t h e i r p h y s i c a l and chemical p r o p e r t i e s i s w e l l developed (See Standard Methods o f T e s t i n g Petroleum and i t s Products,

I n s t i t u t e o f Petroleum and ASTM) b u t t h e c o m p l e x i t y o f t h e r e q u i r e -

ments o f many e n g i n e e r i n g a p p l i c a t i o n s i s such t h a t s a t i s f a c t o r y performance cannot always be p r e d i c t e d f r o m such t e s t s and i t i s necessary t o s i m u l a t e service conditions.

Thus, f o r a p r o d u c t t o be approved f o r supply t o t h e U . S .

Army f o r l u b r i c a t i o n o f I.C.

engines,

i t has t o be s u b m i t t e d t o a s e r i e s o f

engine t e s t s and has t o s a t i s f y c e r t a i n s p e c i f i e d c r i t e r i a .

Once approval has

been g i v e n , b u l k s u p p l i e s may be accepted on t h e b a s i s o f s u f f i c i e n t p h y s i c a l and chemical t e s t i n g t o ensure c o n s i s t e n c y o f c o n s t i t u t i o n . Many i n d u s t r i a l processes i n v o l v e a c o n s i d e r a b l e q u a n t i t y o f raw m a t e r i a l and f u l l s c a l e t e s t i n g may be i n c o n v e n i e n t o r expensive.

I t may n o t always be

p o s s i b l e t o o b t a i n access t o t h e i n t e r a c t i n g s u r f a c e s o f i n t e r e s t and i t may be d e s i r e d t o e x p l o r e t h e e f f e c t o f a v a r i a b l e o v e r a g r e a t e r range than would be p o s s i b l e w i t h e x i s t i n g machinery.

I n these circumstances, t h e c o n s t r u c t i o n o f

s p e c i a l l a b o r a t o r y machines may be e s s e n t i a l .

472

T r i b o l o g i c a l s i t u a t i o n s can g e n e r a l l y be reduced t o t h e c o n s i d e r a t i o n o f i n t e r a c t i n g s u r f a c e s o f g i v e n c o m p o s i t i o n and shape w i t h a p p r o p r i a t e i o a d i n g and r e l a t i v e m o t i o n t o g e t h e r w i t h l u b r i c a n t and e n v i r o n m e n t , and i t i s t h e r e f o r e a t t r a c t i v e t o p r o v i d e s p e c i a l t e s t machines w h i c h p r e s e n t no a p p a r e n t resemblance t o any p r a c t i c a l machine, b u t w h i c h reproduce s u f f i c i e n t l y a c c u r a t e l y the t r i b o l o g i c a l conditions involved.

A number o f machines a r e a v a i l a b l e

c o m m e r c i a l l y which p u r p o r t t o do t h i s and p r o v i d e d c a r e i s taken t o ensure t h e r e l e v a n c e o f t h e t e s t c o n d i t i o n s t o t h e a p p l i c a t i o n , t h e y may produce much useful information. F i n a l l y where an e f f o r t i s b e i n g made t o expand knowledge o f t h e fundamentals u n d e r l y i n g t r i b o l o g i c a l a c t i o n , s p e c i a l equipment i s n e c e s s a r y t o e x t e n d t h e range o f o b s e r v a t i o n and t o i s o l a t e p a r t i c u l a r v a r i a b l e s .

I t i s o f t e n found

t h a t a p p a r a t u s d e v i s e d f o r fundamental r e s e a r c h i s p a r t i c u l a r l y u s e f u l f o r applied investigations.

20.2 20.2.1

LUBRICANT TYPE APPROVAL TESTING MACHINES Engine T e s t s

A great deal o f l u b r i c a n t t e s t i n g i s c a r r i e d o u t i n or der t o s a t i s f y the r e l e v a n t s p e c i f i c a t i o n s f o r e n g i n e o i l , t y p i c a l examples a r e DEF-2101-D U n i t e d Kingdom and MIL-L-46152 TABLE 20.1

Engine T e s t ( s )

P r o p e r t i e s under t e s t

-

P e t t e r W1 gasoline

O i l oxidation, bearing corrosion, lacquer format ion

P e t t e r AV1 diesel

Detergency and h i g h t e m p e r a t u r e stability

O l d s m o b i l e V-8 sequence I I B - gasoline

Low t e m p e r a t u r e r u s t i n g and deposits

F o r d V-8 sequence VC - gasoline

B u i l d up o f d e p o s i t s due t o i n t e r m i t t e n t low temperature operat ion

O l d s m o b i l e V-8 sequence I I I C - gasoline

High temperature o x i d a t i o n

CRC L-38 (CLR) - gasoline

B e a r i n g c o r r o s i o n and shear s t a b i l i t y of multigrade5

C a t e r p i 1 l a r TH - diesel

Ring s t i c k i n g wear and a c c u m u l a t i o n o f deposits

-

MIL-L-46152

i n the United States.

Summary o f T y p i c a l Engine T e s t s

Specification DEF-2101-D

i n the

473

Fig.1

P e t t e r A v l Rig

Fig.2

Fig.3

Caterpillar r i g

Piston examination

474 The main c r i t e r i o n o f acceptance o f a l u b r i c a n t which has been s u b j e c t e d t o t y p e approval t e s t i n g i s t h e c o n d i t i o n o f t h e p i s t o n a f t e r t e s t .

This i s rated

by a panel o f e x p e r t s who assess such f.actors as t h e freedom o f t h e r i n g s , a minimum o f s c u f f i n g o f t h e p i s t o n crown and a minimal amount o f carbon i n t h e t o p r i n g groove.

There should a l s o be no carbon d e p o s i t i n t h e lower r i n g

grooves and t h e p i s t o n s k i r t should be e n t i r e l y f r e e o f l a c q u e r . As w i l l have been g a t h e r e d f r o m p r e v i o u s c h a p t e r s , t h e a c t i o n o f l u b r i c a n t a d d i t i v e s i s complex and t h e r e may be s i g n i f i c a n t i n t e r a c t i o n s o f an u n d e s i r a b l e nature.

For example, some a d d i t i v e s l e a d t o c o r r o s i o n o f c o p p e r - l e a d b e a r i n g s

and i n p e t r o l - e n g i n e s l e a d e d - f u e l s may a f f e c t t h e n a t u r e o f p i s t o n d e p o s i t s ("grey p a i n t " ) .

A d d i t i o n a l t e s t s a r e t h e r e f o r e r e q u i r e d which were o r i g i n a l l y

c a r r i e d o u t f o r 36 hours i n a f o u r c y l i n d e r C h e v r o l e t engine.

The copper-lead

b e a r i n g s o f t h e t e s t e n g i n e were weighed and examined i n o r d e r t o assess c o r r o s i o n and t h e p i s t o n s examined t o assess any d e p o s i t .

A P e t t e r \ / I . spark-

i g n i t i o n engine has been s u b s t i t u t e d f o r t h e C h e v r o l e t e n g i n e i n t h e U.K. engine, t h e CRC L-38,

and an

has been s p e c i a l l y developed f o r l u b r i c a n t a p p r o v a l t e s t s

i n the U.S.A. Although i t may appear a t f i r s t s i g h t t h a t t h e use o f an a c t u a l engine i s a s t r a i g h t f o r w a r d means f o r assessing t h e q u a l i t y o f an o i l , t h e t e s t c o n d i t i o n s are very c r i t i c a l .

The c o m p o s i t i o n o f t h e f u e l , t h e t i m i n g o f t h e v a l v e s , t h e

c o n d i t i o n s o f t h e i n j e c t i o n system a l l may a f f e c t t h e performance o f a l u b ricant.

I n p a r t i c u l a r , t h e exhaust arrangements can markedly a f f e c t r e s u l t s

[la.

20.2.2

Gear T e s t s

The i n t r o d u c t i o n o f h y p o i d gears i n back a x l e s was made p o s s i b l e o n l y by t h e a p p l i c a t i o n o f extreme p r e s s u r e l u b r i c a n t s and t h e maintenance o f adequate l u b r i c a n t q u a l i t y i s essential t o t h e i r successful functioning. qua1 i f y l u b r i c a n t s t o t h e U . K .

(CS 3000) and U.S.A.

(MIL-L-2105C)

Tests t o specifications

a r e c a r r i e d o u t on a c t u a l gears i n a t e s t arrangement o f t h e t y p e shown i n Fig.4.

The l o a d c a r r y i n g and extreme p r e s s u r e c h a r a c t e r i s t i c s o f gear l u b r i c -

a n t s i n a x l e s under c o n d i t i o n s o f high-speed, by low-speed,

low-torque o p e r a t i o n , f o l l o w e d

h i g h - t o r q u e o p e r a t i o n should s a t i s f a c t o r i l y p r e v e n t t h e occ-

u r r e n c e o f gear t o o t h r i d g i n g , r i p p l i n g , p i t t i n g , w e l d i n g , e x c e s s i v e wear, o r o t h e r s u r f a c e d i s t r e s s o r t h e f o r m a t i o n o f o b j e c t i o n a b l e d e p o s i t s when t e s t e d on b o t h u n t r e a t e d and phosphate t r e a t e d gear assemblies i n accordance w i t h t h e s p e c i f i e d procedures. The p r i m a r y a c t i o n o f t h e l u b r i c a n t a d d i t i v e i s t o c o n f e r E.P. on t h e l u b r i c a n t .

properties

T h i s i m p l i e s p r e v e n t i o n o f s c u f f i n g by a c t i v a t i o n o f t h e

c h e m i c a l l y a c t i v e a d d i t i v e s by t h e occurrence o f instantaneous temperature

475

Fig.4

Low speed high torque test r i g hypoid gear and dynamometer installation

Fig.5

Ridging t y p e gear failure

416

Fig.5

Examples o f g e a r damage

477

Fia.6

Rippling type near f a i l u r e

Fig.7

I.A.E.

h i g h speed g e a r r i g

478 " f l a s h e s " between p a i r s o f gears.

The High-speed,

Low-torque t e s t i s used t o

assess t h i s behaviour u s i n g equipment w h i c h embodies an a u t o m o b i l e r e a r - a x l e assembly.

Evidence o f f a i l u r e i s p r o v i d e d by t h e i n i t i a t i o n o f s c u f f i n g .

TABLE 20.2

Summary o f t h e F u l l A x l e Test o f MIL-L-2105C

Test

and CS 3000

Requ i remen t s

Equipment

Moisture Corrosion 7 days

CRC L-33 Federal method 5326

Maximum o f 5% vapour phase c o r r o s i o n o f cover p l a t e , no corrosion o f functional parts.

High Speed Shock Load Test

CRC L-42 Federal method 6507 lP.234/69

Score p r e v e n t i o n equal t o or b e t t e r than RG 1 1 0 o r CRC 10/90 r e f e r e n c e o i l .

Low Speed High Torque Test

CRC L-37 Federal method 6506 I P 232 Proc. 8.

No gear s u r f a c e d i s t r e s s o r deposit formation allowable.

When c h e m i c a l l y a c t i v e a d d i t i v e s a r e used, t h e r e i s always t h e r i s k t h a t they w i l l a c t under c o n d i t i o n s and i n a manner which i s n o t d e s i r e d . t y p e f a i l u r e may o c c u r as i n F i g . 5 and " r i p p l i n g "

example, " r i d g i n g " Fig.6.

The "Low-speed

For as i n

High-torque'' t e s t i s used t o ensure t h a t l u b r i c a n t s a r e

f o r m u l a t e d t o p r e v e n t these t y p e s o f f a i l u r e s f r o m o c c u r r i n g under s e r v i c e conditions. L u b r i c a n t s f o r use i n spur gears a r e u s u a l l y t e s t e d f o r t h e i r a n t i - s c u f f i n g p r o p e r t i e s i n machines which embody means f o r power c i r c u l a t i o n .

One o f t h e

o r i g i n a l machines i n t h i s c l a s s was designed by Mansion and i s known as t h e I.A.E.

machine.

The gears under t e s t a r e geared t o g e t h e r ,

p a r a l l e l s h a f t s , and loaded by t h e " l o c k i n g - u p " s t r a i n i n the shafts.

supported on two

o f a predetermined t o r s i o n a l

The gear s e t s a r e assembled i n a "back-to-back"

relation-

s h i p so t h a t power c i r c u l a t e s c o n t i n u o u s l y and t h e d r i v i n g motor i s o n l y r e q u i r e d t o make up t h e l o s s e s o c c u r r i n g w i t h i n t h e system. Another machine which employs t h e power c i r c u l a t i n g p r i n c i p l e i s t h e "Ryder" machine.

In t h i s d e s i g n one s e t o f gears a c t s as t e s t s e c t i o n and t h e o t h e r i s

o f the single-helical

configuration.

Load i s t h e n a p p l i e d by means o f an a x i a l

f o r c e which i s c o n v e r t e d i n t h e h e l i c a l - g e a r s t o a t o r q u e t e n d i n g t o t w i s t t h e s h a f t thereby l o a d i n g t h e t e s t gears. The F.Z.G.

Machine (Fig.8)

a l s o employs i n v o l u t e gears on t h e p o w e r - c i r c -

ulating principle.

A l l t h e above machines r e q u i r e gears as t e s t p i e c e s and, because each t e s t must be t a k e n t o d e s t r u c t i o n , t h e t e s t procedures a r e v e r y expensive.

There i s

a s t r o n g tendency t h e r e f o r e t o employ disc-machines wherein t h e r e l a t i v e amount

419

Test gears

Lubr i c a n t T

Fig.8

FZG Gear o i l t e s t

o f r o l l i n g and s l i d i n g can be a d j u s t e d t o correspond w i t h t h e events i n t h e meshing c y c l e o f a spur gear.

20.2.3

Simulation o f I n d u s t r i a l Situations

Many i n d u s t r i a l processes r e q u i r i n g l u b r i c a t i o n do n o t correspond t o t h e c o n d i t i o n s o f t h e t y p e approval t e s t o f l u b r i c a n t s , and i t i s necessary t o i n v e s t i g a t e t h e e x i s t i n g t r i b o l o g i c a l s i t u a t i o n t a k i n g i n t o account t h e n a t u r e o f t h e i n t e r a c t i n g m a t e r i a l s , t h e i n d u s t r i a l environment and t h e a p p l i e d f o r c e s and n o t i o n s . I n t h i s circumstance,

i t i s f r e q u e n t l y necessary t o c o n s t r u c t a s p e c i a l

machine t o reproduce w i t h i n t h e l a b o r a t o r y t h e s p e c i f i c c o n d i t i o n s surrounding t h e i n d u s t r i a l problem. An example o f such a machine i s shown i n Fig.9

[2].

This consists o f a

machine designed and c o n s t r u c t e d a t t h e Swansea T r i b o l o g y Centre t o s i m u l a t e the c o n d i t i o n s d e t e r m i n i n g t h e a c t i o n o f t h e s i d e g u i d e s of r o l l i n g m i l l s .

The

s i d e g u i d e s , n o r m a l l y made o f bronze, a r e a c t e d on by t h e edges o f t h e s t e e l s t r i p undergoing c o l d - r o l l i n g which t h e y r e s t r a i n f r o m unwanted sideways movement.

They a r e s u b j e c t e d t o much wear ( F i g . g c ) ,

and moreover, t h e n o n - f e r r o u s

m a t e r i a l t r a n s f e r r e d f r o m t h e guide t o t h e s t e e l was o b j e c t i o n a b l e t o t h e customers o f t h e m a t e r i a l . A c c o r d i n g l y a guide member was f i t t e d w i t h s t r a i n - g a u g e s and t h e r m i s t o r s and s u b j e c t e d t o normal o p e r a t i o n so t h a t t h e f o r c e s a c t i n g on t h e guide c o u l d be determined t o g e t h e r w i t h t h e o p e r a t i o n a l temperature.

The machine i l l u s t r a t e d

480

Fig.g(a)

R o l l i n g M i l l w i t h s i d e guides

i n F i g . g ( b ) was then designed t o embody a c o n t i n u o u s s t e e l b e l t t o r e p r e s e n t t h e c o l d s t r i p beimg r o l l e d i n t h e f u l l s c a l e apparatus and a member which resembled a s i d e guide and which was f o r c e d a g a i n s t t h e edge o f t h e s t r i p w i t h a f o r c e which was determined on t h e b a s i s o f t h e s t r a i n - g a u g e measurements. A f t e r a s e r i e s o f t e s t s had c o n f i r m e d t h a t t h e wear mode o b t a i n e d i n t h e l a b o r a t o r y corresponded e x a c t l y w i t h t h a t observed i n p r a c t i c e , a s e r i e s o f a l t e r n a t i v e m a t e r i a l s was i n v e s t i g a t e d .

I t was concluded t h a t t h e s u b s t i t u t i o n o f

n o d u l a r c a s t - i r o n f o r t h e bronze presented s e v e r a l advantages as f o l l o w s : (a) The r a t e o f wear was reduced (b) The m a t e r i a l was l e s s expensive t o p r o c u r e (c) The c o n t a m i n a t i o n o f t h e p r o d u c t w i t h n o n - f e r r o u s m a t e r i a l was avoided. T h i s example demonstrates t h e p o s s i b i l i t y o f study o f i n d u s t r i a l problems u s i n g

1 a bora t o r y met hods.

20.3

ANALYSIS INTO SYSTEM ELEMENTS AS THE BASIS FOR SELECTION OF LABORATORY TESTS

T r i b o l o g i c a l s i t u a t i o n s encountered i n d i f f e r e n t machines may p r e s e n t c e r t a i n f e a t u r e s i n common i n s o f a r as t h e y a l l embody s u r f a c e s i n r e l a t i v e motion.

481

Fig.9(b)

E x p e r i m e n t a l R i g f o r S i m u l a t i n q S i d e Guide Wear

Fig.g(c)

S i m u l a t e d worn s i d e g u i d e

482 However,

i t w i l l be apparent t h a t t h e n a t u r e and c o n f i g u r a t i o n o f s u r f a c e s may

v a r y w i d e l y as between one machine and another as w i l l t h e modes o f f o r c e and motion.

Therefore t h e r e can be no s i n g l e l a b o r a t o r y r i g which can r e p r e s e n t a l l

tribological situations.

However,

i t i s a l s o t r u e t h a t many machines possess

f e a t u r e s which a r e s u f f i c i e n t l y a l i k e t o j u s t i f y t h e c o n s t r u c t i o n o f t e s t machines f o r t h e purpose o f e v a l u a t i n g l u b r i c a n t s o r m a t e r i a l s o f c o n s t r u c t i o n f o r use t h e r e i n . Great c a r e i s necessary however t o analyse p r e c i s e l y t h e n a t u r e o f t h e t r i b o l o g i c a l s i t u a t i o n s i n v o l v e d i n o r d e r t o s e l e c t t h e c o r r e c t c o n d i t i o n s t o be a p p l i e d i n t h e t e s t machine.

A s c i e n t i f i c a t t i t u d e i s necessary and meaningless

j a r g o n such as " f i l m - s t r e n g t h ' ' o r " l u b r i c i t y "

should be avoided.

(An example o f

a l l o w i n g j a r g o n t o t a k e o v e r i s g i v e n by t h e i n v e s t i g a t o r who s u b j e c t e d t o an

E.P.

t e s t a l u b r i c a n t r e q u i r e d f o r a process imposing h i g h p r e s s u r e a t v e r y low

speed). The f i r s t s t e p i n t h e a n a l y t i c a l process must be t o c h a r a c t e r i z e t h e machine element under study w i t h r e s p e c t t o i t s p o s i t i o n i n t h e k i n e m a t i c c h a i n ; a "Higher P a i r " o r a "Lower

Pair"?

is it

O f t h e lower p a i r s , t h e s l i d i n g p a i r , F i g .

l O ( a ) i s encountered on machine slideways and t h e "Revolute the basis o f the m a j o r i t y o f bearings.

Pair" Fig.lO(b)

is

A p a r t f r o m t h e "screw1' p a i r (which i s a

combination o f t h e s l i d i n g and r e v o l u t e p a i r s ) a l l o t h e r p a i r s a r e h i g h e r p a i r s and a r e so c h a r a c t e r i s e d because t h e y must accommodate r e l a t i v e motion which i s p a r t l y s l i d i n g and p a r t l y t u r n i n g . From t h e p o i n t o f vie!+ o f T r i b o l o g y , t h e i m p o r t a n t d i s t i n c t i o n between lower and h i g h e r p a i r s i s t h a t the former a l l o w c o n t a c t t o be made throughout t h e f u l l e x t e n t o f a s u r f a c e , whereas t h e l a t t e r o n l y a l l o w " p o i n t "

o r "1 ine" c o n t a c t .

Some common examples o f h i g h e r p a i r s a r e shown i n F i g . l O ( d - 9 ) . a r e s a i d t o be "conformal"

and h i g h e r p a i r s a r e "counterformal".

Thus lower p a i r s I t w i l l be

apparent t h a t the p r o f o r m a l d i f f e r e n c e between d i s p e r s e d and c o n c e n t r a t e d cont a c t s w i l l be r e f l e c t e d i n t h e design o f t h e i n t e r a c t i n g components,

in their

m a t e r i a l s o f c o n s t r u c t i o n and i n t h e p r o p e r t i e s r e q u i r e d o f any l u b r i c a n t s applied.

[3].

Fig.11 i l l u s t r a t e s t h e c l a s s i f i c a t i o n o f some common machine

elements i n t o conformal and c o u n t e r f o r m a l c o n f i g u r a t i o n s as w e l l as i n d i c a t i n g t h e mode o f damage c h a r a c t e r i s t i c o f each a p p l i c a t i o n . To be s u c c e s s f u l , l a b o r a t o r y t e s t i n g machines must be s i m p l e and must employ e a s i l y manufactured t e s t p i e c e s .

They must however reproduce t h e c o n d i t i o n s o f

thermal and s t r e s s i n t e n s i t y t o which i t i s a n t i c i p a t e d t h a t t h e l u b r i c a t e d system w i l l be s u b j e c t e d i n s e r v i c e .

The f i r s t broad c l a s s i f i c a t i o n must be

i n t o lower and h i g h e r p a i r s . There a r e r e l a t i v e l y few simple machines a v a i l a b l e commercially f o r t e s t i n g l u b r i c a n t s i n lower p a i r s because, as f a r as l u b r i c a t i o n proper i s concerned,

/

I

v)

.I

Q

U

Q,

L

+ z ,

c 9

(d) Gear tooth contact

lei

&I(

bearmg

IfJ Taper roller

l g J Wheel on roil

bearing

Classification of machine elements into higher and lower pairs

483

FIGURE 10

Nature of Contact

Characteristic Mode of Damage

Lower pair (Conformal 1 Fixtures subject to

Higher pair (Counterformal1

Gears

Hertzian

I

Wheel on rail

Y

~

FIGURE 11

Classification of mode of damage of common machine elements

485 t h i s i s u s u a l l y hydrodynamic and t h e o n l y p r o p e r t y r e q u i r e d o f t h e l u b r i c a n t i s i t s v i s c o s i t y wh i ch can e a s i l y be measured by p h y s i c a l methods.

The i m p o r t a n t

p r o p e r t i e s o f t h e l u b r i c a n t i n t he se a p p l i c a t i o n s a r e r e l a t e d t o such f a c t o r s as detergency o r a n t i - c o r r o s i o n and f u l l s c a l e t e s t s a r e necessary t o safeguard against undesirable i n t e r a c t i o n s . For t e s t i n g m a t e r i a l - l u b r i c a n t co mbi n at i on s under c o n d i t i o n s o f p u r e s l i d i n g , a very c o n v e n ie n t arrangement c o n s i s t s o f t h r e e c y l i n d r i c a l p i n s w hich a r e h e l d i n an upper member w i t h t h e i r axes p a r a l l e l and t h e assembly i s loaded a g a i n s t the s i d e o f an a n n u l a r t e s t p i e c e which i s r o t a t e d about i t s g e n e r a t i n g a x i s . The "Cygnus" (Fig.13)

machine i s o f t h i s t ype .

Both t h e Almen (Fig.13)

and t h e Falex

machines a r e b a s i c a l l y 1 i n e - c o n t a c t machines b u t a c e r t a i n degree o f

c o n f o r m i t y u s u a l l y a r i s e s from.wear o f t h e t e s t pieces.

These two machine types

present t h e common f e a t u r e t h a t two i d e n t i c a l t e s t p i e c e s a r e f o r c e d a g a i n s t a r o t a t i n g member from d i a m e t r i c a l l y opposed d i r e c t i o n s , on t h e s p i n d l e .

i n t e r n a l r a d i u s some 1.5 x 10 which i s 0.35 mm (0.25 "Falex"

a r e "V"

mechanical.

thus balancing the f o r c e

The Almen t e s t p i e c e s f orm segments o f a c y l i n d e r having an

shaped.

-4m

(0.006

i n ) i n d i ame t e r.

i n ) g r e a t e r than t h a t o f t h e s p i n d l e The corresponding t e s t p i e c e s o f t h e

Loading on t h e "Almen"

i s h y d r a u l i c and the "Falex"

R e s u l t s a r e d i f f i c u l t t o e v a l u a t e because o f t h e l o a d i n g methods

u s u a l l y employed [ 4 ] . As r e g a r d s h i g h e r p a i r s ,

t h e r e i s a wi de v a r i e t y o f machines w hich a p p l y

s l i d i n g under c o u n t e r f o r m a l c o n d i t i o n s .

These have g e n e r a l l y been developed t o

assess t h e q u a l i t y o f ge ar l u b r i c a n t s w i t h p a r t i c u l a r r e f e r e n c e t o t h e i n h i b ition o f scuffing. (Stanhope-Seta)

One o f t h e most su ccessf ul o f these has been t h e S h e l l

f o u r b a l l machine (Fig.14).

The problem o f o b t a i n i n g t e s t

pieces wh ic h a r e o f u n i f o r m f i n i s h and m e t a l l u r g i c a l c o n d i t i o n has been s o l v e d by u s i n g b a l l s s e l e c t e d i n batches f r o m commercial p r o d u c t i o n .

Three o f these

b a l l s a r e clamped t o f orm a n e s t i n t o whi ch t h e f o u r t h b a l l ( h e l d i n a chuck) i s forced and r o t a t e d .

The b a l l s a r e 12.7 mm (0.5

i n ) i n diameter and t h e r o t a t -

i on a l speed o f t h e s p i n d l e t o which t h e chuck i s a t t a c h e d i s 1450-1500 r e v min-'. Load may b e a p p l i e d i n increments up t o 800 kg. s e v e r a l procedures,

[5].

The machine may be used i n

t h e most common o f wh i ch i s t h e "Wear-scar

diameter method",

A number o f t e s t s ( u s u a l l y 20) a r e each made on a f r e s h s e t o f b a l l s , load

be in g in c r e a s e d between each t e s t t h e d u r a t i o n o f which i s one minute.

The d i a -

meter o f t h e wear s c a r a pp are nt a f t e r each t e s t i s measured and p l o t t e d a g a i n s t load on l o g a r i t h m i c paper.

Duri n g t h e t e s t s a t t h e lower loads wear i s n e g l i g -

i b l e b u t s u f f i c i e n t r u b b i n g a c t i o n t ake s p l a c e t o leave a measurable mark which when measured and p l o t t e d , g i v e s r i s e t o a s t r a i g h t l i n e w hich c l o s e l y p a r a l l e l s t h a t o b t a i n e d by c a l c u l a t i n g t h e d i ame t e r o f t h e H e r t z i a n c o n t a c t between the loaded b a l l s .

W i t h most l u b r i c a n t s a l o a d i s reached a t w hich a sharp r i s e i n

486

Specimen arrangement

FIG.12

CYGNUS F R I C T I O N AND WEAR TEST MACHINE

Brass lockina Almen blocks

m

at

F I G . 13

w + # j L Falex blocks

Exploded view of pin and

FALEX LUBRICANT TEST MACHINE

V blocks

Cutaway view throughout sample pan

487

Male ball chuck Fourth ball (rotates)

-0

Ball pot lock ring -

(stationary ) Ball Dot insert Ball pot Calibrated arm

Ball pot _ _ mounting disc

Top boll rotates

Ball and flats

Load

Four ball machine

FIGURE 14

Cone and cylinder

488 diameter occurs.

This value i s u s u a l l y s u f f i c i e n t l y d e f i n i t e t o characterise a

l u b r i c a n t and i s c a l l e d t h e i n i t i a l s e i z u r e load.

I n some cases however, r e -

covery may o c c u r a l t h o u g h t h e b a l l s o f t e n become welded t o g e t h e r a t t h e h i g h e r loads.

Another method o f e v a l u a t i o n i s based on t h e t i m e e l a p s i n g under a con-

s t a n t load b e f o r e t h e occurrence o f a sudden i n c r e a s e i n f r i c t i o n . One disadvantage o f t h e f o u r b a l l machine i s t h a t t h e m a t e r i a l o f t h e t e s t p i e c e s , b a l l b e a r i n g s t e e l , i s n o t r e p r e s e n t a t i v e o f m a t e r i a l s w i t h which t h e l u b r i c a n t may be r e q u i r e d t o i n t e r a c t .

A c c o r d i n g l y t h e c e n t r a l b a l l may be

r e p l a c e d by a c o n i c a l l y ended p i e c e and t h e f i x e d b a l l s by small c y l i n d e r s arranged t o form a t r i a n g l e i n a h o r i z o n t a l p l a n e , Fig.14. The Timkin Machine (Fig.15)

embodies a l i n e r a t h e r than a p o i n t c o n t a c t .

The

r o t a t i n g element, formed f r o m t h e r i n g o f a t a p e r r o l l e r b e a r i n g a c t s on a stationary rectangular steel block.

F r i c t i o n can be measured.

Most t e s t i n g machines a r e so arranged t h a t t h e wearing s u r f a c e s a r e i n cont i n u o u s o r repeated c o n t a c t so as t o o b l i t e r a t e t h e i n i t i a l m a n i f e s t a t i o n s o f surface f a i l u r e .

A machine was a c c o r d i n g l y designed a t M.E.R.L.

(now N.E.L.)

which c o n s i s t e d o f two c y l i n d e r s which, i n a d d i t i o n t o r o t a t i o n , c o u l d be t r a v ersed one r e l a t i v e t o t h e o t h e r so t h a t t h e c o n t a c t zone was made t o c o n t i n u a l l y embody f r e s h m a t e r i a l (Fig.16). I n a l l t h e aforementioned machines, o n l y s l i d i n g t a k e s p l a c e whereas i n many mechanisms, place.

i n v o l u t e gears f o r example, r o l l i n g as w e l l as s l i d i n g can t a k e

Disc inachines such as t h e "Amsler"

( a l s o t h e M e r r i t t and S.A.E.

machines) have d i s c s which a r e loaded edgewise t o p r o v i d e v a r i o u s c o m b i n a t i o n s o f r o l l i n g and s l i d i n g ( F i g . 1 7 ) .

-

P i t t i n g t y p e f a i l u r e i s u s u a l l y a s s o c i a t e d w i t h r o l l i n g c o n t a c t and can be simulated i n the laboratory [ 6 ] using a simple m o d i f i c a t i o n o f the four b a l l machine ( F i g . 1 8 ) .

I n s t e a d o f t h e t h r e e lower b a l l s b e i n g clamped i n t o p l a c e

they a r e a l l o w e d t o r o t a t e w i t h i n a s p e c i a l l y designed b a l l r a c e [ 6 ] . i s d e s i r e d t o i n v e s t i g a t e t h e p i t t i n g behaviour o f a s p e c i a l s t e e l ,

Where i t

it i s

p o s s i b l e t o s u b s t i t u t e a c o n i c a l l y ended t e s t p i e c e f o r t h e c e n t r a l b a l l a l t h o u g h t h e t h r e e f r e e b a l l s must be r e t a i n e d .

20.4

A h i g h speed v e r s i o n i s a l s o a v a i l a b l e .

EQUIPMENT FOR B A S I C RESEARCH

The i n v e s t i g a t i o n s pursued i n b a s i c r e s e a r c h l a b o r a t o r i e s may appear t o be u n r e l a t e d t o p r a c t i c e by reason of t h e a r t i f i c i a l c o n d i t i o n s o f t e n imposed i n o r d e r t o e l u c i d a t e some fundamental r e l a t i o n s h i p .

The c l o s i n g o f t h e communic-

a t i o n gap between t h e fundamental i n v e s t i g a t o r and t h e engineer whose p r a c t i c e

lies i n m a n u f a c t u r i n g i n d u s t r y f o r example, has been made e a s i e r by t h e d e v e l opment o f t h e p r a c t i c e o f m o d e l l i n g complex systems u s i n g computers.

Thus i n

Fig.19 t h e r e c t a n g l e marked A r e p r e s e n t s a real. machine-element s u b j e c t e d t o t h e environmental c o n d i t i o n s and l o a d i n g w i t h i n t h e i n d u s t r i a l environment.

Its

489

Lubricant sample

+ Load

Fig.15

Timken t e s t

Fig.16

N.E.L.

c ro ssed c y l i n d e r machine

490

Fig.17

Amsler machine

lL

F

Fig.18

Rolling four ball machine

491

MATHEMATICAL

Fig.19

LABORATORY EXPERIMENTS

E/

The r o l e o f the computer model i n r e l a t i n g b a s i c research t o engineering p r a c t i c e

response t o these c o n d i t i o n s can be determined by a methodical s e r i e s o f t e s t s , r e s u l t s o f which a r e r e p r e s e n t e d by r e c t a n g l e 8.

Assuming t h a t n o t a l l i s w e l l

o r t h a t some development i n p r o d u c t o r p r o d u c t i o n c a p a c i t y i s r e q u i r e d ,

i t may

be necessary t o f o r m u l a t e some p r e d i c t i o n s o f behaviour l y i n g o u t s i d e t h e range o f p r e v i o u s experience.

The system can then be represented by a s e r i e s o f

e q u a t i o n s l i n k e d t o g e t h e r t o form a mathematical model o f t h e machine as indi c a t e d by r e c t a n g l e C.

S p e c i f i c a t i o n s f o r a p p l i e d c o n d i t i o n s and l o a d i n g s would

be f e d i n and t h e computer would produce s o l u t i o n s t o t h e e q u a t i o n s represented by r e c t a n g l e D .

These s o l u t i o n s would p r o v i d e p r e d i c t i o n s o f t h e magnitude o f

o u t p u t q u a n t i t i e s which a r e r e p r e s e n t e d by t h e arrows p o i n t i n g t o the l e f t from

D t o B.

The arrows p o i n t i n g t o t h e r i g h t f r o m €3 t o D r e p r e s e n t t h e r e s u l t s o f

actual t e s t .

I f t h e r e i s agreement between measured o u t p u t s and c a l c u l a t e d pre-

d i c t i o n s , t h e model can be s a i d t o be complete.

I f t h e r e i s divergence, the

model must be r e f i n e d and a d j u s t e d u n t i l a c c e p t a b l e agreement r e s u l t s .

Once

t h i s agreement has been a t t a i n e d , t h e model may be used t o p r e d i c t the behaviour o f an i n f i n i t e v a r i e t y o f machine elements and a p p l i e d c o n d i t i o n s p r o v i d e d they l i e w i t h i n t h e range o f e q u a t i o n s embodied i n t h e model. Attempts t o c o n s t r u c t r e a l i s t i c mathematical models however, f r e q u e n t l y r e v e a l gaps i n understanding o f t h e p h y s i c a l system i n v o l v e d which can o n l y be f i l l e d by c a r e f u l l y c o n t r o l l e d l a b o r a t o r y experiments such as those which were necessary t o e l u c i d a t e t h e n a t u r e and o p e r a t i o n a l r e l a t i o n s h i p s o f e l a s t o hydrodynamic l u b r i c a t i o n (as i n d i c a t e d a t E ) .

Such experiments u s u a l l y form the

b a s i s o f advanced t e s t methods made necessary by developing p r a c t i c e .

492 REFERENCES Cree,J.C. ( 1 9 5 3 ) C a t e r p i l l a r L1 a n d C h e v r o l e t L4 t e s t p r o c e s s , Symposium o n E n g i n e T e s t i n g o f L u b r i c a t i n g O i l . I n s t i t u t e o f Petroleum R o y l a n c e , B . J . (1977) The A p p l i c a t i o n o f E x i s t i n g Knowledge t h e S o l u t i o n o f I n d u s t r i a l T r i b o l o g y Problems. P r o c . I . Mech. E. I n t h e Press. B a r w e l l ,F.T. (1979) ' B e a r i n g Systems - P r i n c i p l e s a n d P r a c t i c e ' Clarendon, Oxford. Meckleburg,K.R. (1975) 'Forces i n the Falex c o n f i g u r a t i o n ' . Trans. ASLE., V01.18, pp. 97-104. Extreme P r e s s u r e P r o p e r t i e s : F r i c t i o n a n d blear T e s t s : Four B a l l I P 239/77. The I n s t i t u t e o f P e t r o l e u m . Standards f o r Machine. Petroleum and i t s p r o d u c t s . 'The e f f e c t o f l u b r i c a n t p i t t i n g B a r w e l 1 , F . T . a n d S c o t t , D . (1956) f a i l u r e o f b a l l b e a r i n g s ' E n g i n e e r i n g , Vo1.182, pp. 9-12.

493

GLOSSARY

Terms and D e f i n i t i o n s

-

Abrasion

Wear by displacement o f m a t e r i a l caused by h a r d p a r t i c l e s o r hard

protuberances.

-

Absolute V i s c o s i t y

-

Additive

see v i s c o s i t y .

A m a t e r i a l added t o a l u b r i c a n t f o r t h e purpose o f i m p a r t i n g new

p r o p e r t i e s o r o f enhancing e x i s t i n g p r o p e r t i e s .

Adhesive Wear

-

Wear by t r a n s f e r e n c e o f m a t e r i a l f r o m one s u r f a c e t o another

d u r i n g r e l a t i v e motion, due t o t h e process o f s o l i d - p h a s e welding.

-

Anti-Wear A d d i t i v e

Area o f Contact

-

An a d d i t i v e used t o reduce wear.

The area o f c o n t a c t between two s o l i d s u r f a c e s i s d e s c r i b e d

i n two ways. ( i ) Apparent Area:

t h e area o f c o n t a c t d e f i n e d by t h e boundaries

o f t h e macroscopic i n t e r f a c e o f t h e bodies ( i i ) Real Area:

t h e sum o f t h e l o c a l areas t r a n s m i t t i n g i n t e r f a c i a l

f o r c e d i r e c t l y between t h e bodies.

Asperities

B,O

Life

-

-

The s m a l l s c a l e i r r e g u l a r i t i e s on a s u r f a c e .

see r a t i n g l i f e .

,

L

B a b b i t t Metal

-

A n o n - f e r r o u s b e a r i n g a l l o y , e i t h e r t i n o r l e a d based con-

s i s t i n g o f v a r i o u s amounts o f copper, antimony, t i n and l e a d .

Base Stock ( o i l )

-

R e f i n e d p e t r o l e u m o i l used i n t h e p r o d u c t i o n o f l u b r i c a n t s

and o t h e r products.

The base s t o c k may be used a l o n e or blended w i t h

o t h e r base s t o c k s and/or a d d i t i v e s .

Bearing

-

A support o r guide by means o f which a moving p a r t i s p o s i t i o n e d

w i t h r e s p e c t t o t h e o t h e r p a r t s o f a mechan’ism.

494 Bearing Area - The p r o j e c t e d b e a r i n g l o a d c a r r y i n g area when viewed i n t h e d i r e c t i o n o f the load.

Beilby layer

-

An amorphous l a y e r o f deformed metal and o x i d e p a r t i c l e s formed

by p o l i s h i n g .

Blending

-

The process o f m i x i n g m i n e r a l o i l s t o o b t a i n d e s i r e d viscous

properties.

Boundary L u b r i c a t i o n

-

A c o n d i t i o n o f l u b r i c a t i o n i n which t h e f r i c t i o n and wear

between two s u r f a c e s i n r e l a t i v e m o t i o n a r e determined by t h e p r o p e r t i e s o f t h e s u r f a c e s , and by t h e p r o p e r t i e s o f t h e l u b r i c a n t o t h e r than b u l k viscosity.

Brinelling

-

I n d e n t a t i o n of t h e s u r f a c e o f a s o l i d body by repeated l o c a l

impact o r impacts, o r by s t a t i c o v e r l o a d .

-

Cavitation Erosion

Wear o f a s o l i d body moving r e l a t i v e l y t o a l i q u i d i n a

r e g i o n o f c o l l a p s i n g vapour bubbles which cause l o c a l h i g h impact pressure

tempe r a t u r e s

0r

Centre L i n e Average (CLA)

-

.

An E n g l i s h measure o f s u r f a c e topography r e p r e s -

e n t i n g t h e average d e p a r t u r e o f a l i n e p r o f i l e o f t h e s u r f a c e f r o m t h e centre line.

Channe i n g

-

The tendency o f a grease t o f o r m a channel by w o r k i n g down a

b e a r i n g o r d i s t r i b u t i o n system, l e a v i n g shoulders t o a c t as a r e s e r v o i r and s e a l .

Clearance R a t i o

-

In a bearing, the r a t i o o f r a d i a l clearance t o shaft radius.

C o e f f i c i e n t of F r i c t i o n

-

The r a t i o o b t a i n e d by d i v i d i n g t h e t a n g e n t i a l f o r c e

r e s i s t i n g m o t i o n between two b o d i e s by t h e normal f o r c e p r e s s i n g these bodies t o g e t h e r .

Composite Bearing M a t e r i a l

-

A s o l i d m a t e r i a l composed o f a c o n t i n u o u s o r

p a r t i c u l a t e s o l i d l u b r i c a n t phase dispensed t h r o u g h o u t a l o a d b e a r i n g m a t r i x t o p r o v i d e c o n t i n u o u s replenishment o f s o l i d l u b r i c a n t f i l m s as wear o c c u r s , and e f f e c t i v e h e a t t r a n s f e r f r o m t h e f r i c t i o n s u r f a c e .

495 Corrosion I n h i b i t o r

-

A d d i t i v e s f o r p r o t e c t i n g l u b r i c a t e d surfaces a g a i n s t

chemical a t t a c k .

They may

be p o l a r compounds w e t t i n g the metal s u r f a c e

p r e f e r e n t i a l l y , or they may a bso rb t h e w ater t o form a w a t e r - i n - o i l e m u ls io n

-

o n l y t h e o i l touches t h e metal.

Some c o r r o s i o n i n h i b i t o r s

combine c h e m i c a l l y w i t h t h e metal t o g i v e a n o n - r e a c t i v e surface.

-

C o r r o s i v e Wear

A process i n which chemical or e l e c t r o c h e m i c a l r e a c t i o n w i t h

t h e environment predominates.

-

Cutting Fluid

A f l u i d applied t o a c u t t i n g tool t o assist i n the c u t t i n g

o p e r a t i o n by c o o l i n g ,

-

Det e r g e n t A d d i t i v e s

l u b r i c a t i n g o r o t h e r means.

compounds which, when blended w i t h l u b r i c a t i n g o i l s ,

d i s p e r s e t he d e t e r i o r a t i o n p r o d u c t s f r o m t h e f u e l and l u b r i c a n t , esp e c i a l l y t ho se formed under h i g h t emp erature c o n d i t i o n s , and, t h u s , m in im is e t h e f o r m a t i o n o f d e p o s i t s l i a b l e t o cause p i s t o n - r i n g s t i c k i n g or o t h e r t r o u b l e .

Di s p e r s a n t A d d i t i v e s

-

compounds which, when blended w i t h l u b r i c a t i n g o i l s ,

m a i n t a i n t h e p r o d u c t s o f combustion f rom t h e fuel i n a f i n e l y dispersed s t a t e , and t h e r e b y mi ni mi se sludge f o r m a t i o n and f i l t e r b l o c k i n g , p a r t i c u l a r l y i n g a s o l i n e engines o p e r a t i n g under c o l d c o n d i t i o n s .

Drop p o i n t

-

temperature a t which a drop o f grease o r o t h e r petroleum product

f i r s t detaches i t s e l f f ro m t h e main b u l k o f m a t e r i a l when a sample i s s t e a d i l y heated under p r e s c r i b e d c o n d i t i o n s .

Duty Parameter

-

A di me nsi o nl e ss number whi ch i s used t o e v a l u a t e t h e p e r -

formance o f b ea ri ng s.

Dynamic V i s c o s i t y

Eccentricity Ratio

-

see: V i s c o s i t y .

-

In a b e a r i n g , t h e r a t i o o f t h e e c c e n t r i c i t y t o t h e r a d i a l

c l earance.

Elasto-hydrodynamic L u b r i c a t i o n

-

A c o n d i t i o n o f l u b r i c a t i o n i n which t h e

f r i c t i o n and f i l m t h i c k n e s s between two bodies i n r e l a t i v e motion a r e determined by t h e e l a s t i c p r o p e r t i e s o f t h e bodies,

i n combination w i t h

t h e v is c o us p r o p e r t i e s o f t h e l u b r i c a n t a t t h e p r e v a i l i n g pressure, temperature and r a t e o f shear.

496 Embeddability

-

The a b i l i t y o f a b e a r i n g m a t e r i a l t o embed harmful f o r e i g n

p a r t i c l e s and reduce t h e i r tendency t o cause s c o r i n g o r a b r a s i o n .

-

Emulsion

A d i s p e r s i o n o f g l o b u l e s o f one l i q u i d i n a n o t h e r i n which i t i s

insoluble.

EP (Extreme p r e s s u r e ) A d d i t i v e

-

A chemical substance c o n t a i n i n g one o r more

elements, e s p e c i a l l y s u l p h u r , c h l o r i n e o r phosphorus, a b l e t o r e a c t w i t h metal s u r f a c e s t o g i v e i n o r g a n i c f i l m s o f h i g h m e l t i n g p o i n t .

The

presence o f these f i l m s h i n d e r s w e l d i n g and s e i z u r e and t h u s p r e v e n t s s c u f f i n g and s c o r i n g , p a r t i c u l a r l y i n gears o p e r a t i n g under h i g h l o a d conditions.

-

Erosion

E r o s i v e wear i s loss o f m a t e r i a l f r o m a s o l i d s u r f a c e due t o

r e l a t i v e m o t i o n i n c o n t a c t w i t h a f l u i d which c o n t a i n s s o l i d p a r t i c l e s .

F a t t y Acids

-

Long c h a i n o r g a n i c a c i d s which o c c u r n a t u r a l l y as t h e i r g l y c e r i d e

e s t e r s i n animal and v e g e t a b l e o i l s and f a t s .

Filler

-

A substance such as l i m e , t a l c , mica and o t h e r powders, added t o

grease t o i n c r e a s e i t s c o n s i s t e n c y or t o an o i l t o i n c r e a s e v i s c o s i t y .

Flash P o i n t

-

The l o w e s t temperature a t which t h e vapour of a l u b r i c a n t can be

i g n i t e d under s p e c i f i e d c o n d i t i o n s .

Flash Temperature

-

The maximum l o c a l temperature generated a t some p o i n t o f

c l o s e approach i n a s l i d i n g c o n t a c t .

Flexure Pivot

-

A t y p e o f b e a r i n g f o r l i m i t e d movement i n which t h e moving

p a r t s a r e guided by f l e x u r e o f e l a s t i c members r a t h e r than by r o l l i n g o r s l i d i n g surfaces.

Fretting

-

The removal o f e x t r e m e l y f i n e p a r t i c l e s f r o m b e a r i n g s u r f a c e s due t o

the i n h e r e n t adhesive f o r c e s between t h e s u r f a c e s p a r t i c u l a r l y under t h e c o n d i t i o n o f small a m p l i t u d e v i b r a t i o n .

F r e t t i n g Corrosion

Friction

-

-

A f o r m o f f r e t t i n g i n which chemical r e a c t i o n predominates.

The r e s i s t i n g f o r c e t a n g e n t i a l t o a common boundary between two

bodies when, under t h e a c t i o n o f an e x t e r n a l f o r c e , one body moves o r tends t o move r e l a t i v e t o t h e s u r f a c e o f t h e o t h e r .

491

-

F r i c t i o n Polymer

An amorphous o r g a n i c d e p o s i t which i s produced when c e r t a i n

m e t a l s a r e rubbed t o g e t h e r i n t h e presence o f o r g a n i c l i q u i d s o r gases.

-

Galling

A severe f o r m o f s c u f f i n g a s s o c i a t e d w i t h gross damage t o t h e surfaces

or failure.

Grease

-

The use o f t h i s f o r m should be avoided.

A l u b r i c a n t composed o f an o i l t h i c k e n e d w i t h a soap o r o t h e r t h i c k e n e r

t o a semi-solid o r s o l i d consistency.

A lime-based grease i s prepared

f r o m a l u b r i c a t i n g o i l and Calcium soap.

Sodium, Barium, L i t h i u m and

Aluminium based greases a r e a l s o used.

Hydraulic F l u i d

-

A f l u i d used f o r t r a n s m i s s i o n o f h y d r a u l i c p r e s s u r e o r a c t i o n ,

not necessarily involving lubricant properties.

May be o i l , water o r

synthetic ( f i r e r e s i s t a n t ) 1 iquids.

-

Hydrodynamic L u b r i c a t i o n

A system o f l u b r i c a t i o n i n which t h e shape and

r e l a t i v e m o t i o n o f t h e s l i d i n g s u r f a c e s causes t h e f o r m a t i o n o f a f l u i d f i l m h a v i n g s u f f i c i e n t p r e s s u r e t o separate t h e s u r f a c e s .

-

Hydrostatic Lubrication

A system o f l u b r i c a t i o n i n which t h e l u b r i c a n t i s

s u p p l i e d under s u f f i c i e n t e x t e r n a l p r e s s u r e t o separate t h e opposing s u r f a c e s by a f l u i d f i l m .

Initial Pitting

-

Surface f a t i g u e o c c u r r i n g d u r i n g t h e e a r l y stages o f o p e r a t i o n

o f gears, a s s o c i a t e d w i t h removal o f h i g h l y s t r e s s e d l o c a l areas and running-in.

-

Journal

That p a r t o f a s h a f t o r a x l e which r o t a t e s o r o s c i l l a t e s r e l a t i v e l y

t o a r a d i a l bearing.

Kinematic V i s c o s i t y

L,o

-

life

Lacquer

-

-

See: V i s c o s i t y .

See: R a t i n g L i f e .

Hard,

lustrous, varnish-like,

o i l i n s o l u b l e d e p o s i t which tends t o

f o r m on t h e p i s t o n s and c y l i n d e r s o f i n t e r n a l combustion engines.

Load C a r r y i n g Capacity

-

The maximum l o a d t h a t a s l i d i n g o r r o l l i n g system can

s u p p o r t w i t h o u t f a i l u r e o r t h e wear exceeding t h e d e s i g n l i m i t s f o r the particular application.

498

-

Lubricant

Any substance i n t e r p o s e d between two s u r f a c e s i n r e l a t i v e motion

f o r t h e purpose o f re du ci ng t h e f r i c t i o n o r wear between them.

-

M i l d Wear

A form o f wear c h a r a c t e r i s e d by removal o f m a t e r i a l i n v e r y small

fragments.

Non-Newtonian V i s c o s i t y

-

The ap pa ren t v i s c o s i t y o f a m a t e r i a l

i n which t h e

shear s t r e s s i s n o t p r o p o r t i o n a l t o t he r a t e o f shear.

Oil

-

A l i q u i d o f ve ge t a bl e , an i mal , m i n e r a l o r s y n t h e t i c o r i g i n f e e l i n g

sl i p p e r y t o t h e touch.

-

Oiliness

That p r o p e r t y o f a l u b r i c a n t t h a t produces low f r i c t i o n under

c o n d i t i o n s o f boundary l u b r i c a t i o n .

The lower t h e f r i c t i o n , t h e g r e a t e r

the o i l i n e s s .

-

O i l M i s t (Fog)

An o i l atomised w i t h t h e a i d o f compressed a i r and then con-

veyed by t h e a i r i n a low-pressure d i s t r i b u t i o n system t o m u l t i p l e points o f lubricant

Pitting

-

application.

Any removal o r displacement o f m a t e r i a l r e s u l t i n g i n the f o r m a t i o n o f

surface c a v i t i e s .

P l a i n B e a r in g

-

Any si mpl e s l i d i n g t yp e o f b e a r i n g as d i s t i n g u i s h e d from f i x e d -

pad, p iv o t ed -pa d o r r o l l i n g - t y p e b ea ri ngs.

Porous B e a r in g powders,

-

A b e a r i n g made f rom porous m a t e r i a l such as compressed metal

t h e po res a c t i n g e i t h e r as r e s e r v o i r s f o r h o l d i n g , o r passages

f o r supplying l u b r i c a n t .

-

Pour P o i n t

The l o we st t emp era t u re a t which a l u b r i c a n t can be observed t o

f l o w under s p e c i f i e d c o n d i t i o n s .

PTFE

-

P o l y t e t r a f l u o r e t h y l e n e , a polymer h a v i n g o u t s t a n d i n g l o w - f r i c t i o n p r o p e r t i e s ove r a wide t emp era t u re range.

PV F a c t o r

-

The p r o d u c t o f b e a r i n g p ressu re and s u r f a c e v e l o c i t y .

499

-

Rating L i f e

The f a t i g u e l i f e i n m i l l i o n s o f r e v o l u t i o n s o r hours a t a g i v e n

o p e r a t i n g speed which 90 p e r c e n t o f a group o f s u b s t a n t i a l y i d e n t i c a l r o l l i n g element b e a r i n g s w i l l s u r v i v e under a g i v e n load.

he 90 per

c e n t r a t i n g 1 i f e i s f r e q u e n t l y r e f e r r e d t o as llL,o-l i f e " or "B,o-l

Redwood V i s c o s i t y

-

ife".

A comrnerical measure o f v i s c o s i t y expressed as t h e t i m e i n

seconds r e q u i r e d f o r 50 c u b i c c e n t i m e t e r s o f a f l u i d t o f l o w through a t u b e o f 10 mm l e n g t h and 1 . 5 mm d i ame ter a t a g i v e n temperature.

Root Mean Square Hei g ht (RMS)

-

An American measure o f s u r f a c e topography

r e p r e s e n t i n g t h e average d e p a r t u r e o f a l i n e p r o f i l e of t h e s u r f a c e f r o m a mean l i n e .

SAE

-

S o c i e t y o f Automotive Engineers.

Sa y b o lt V i s c o s i t y

-

A commerical measure o f v i s c o s i t y expressed as t h e time i n

seconds r e q u i r e d f o r 60 c u b i c c e n t i m e t e r s o f a f l u i d t o f l o w through t h e o r i f i c e o f t h e Standard Sa ybo l t U n i v e r s a l Viscometer a t a g i v e n temp e r a t u r e under s p e c i f i e d c o n d i t i o n s .

-

Sc o r in g

The f o r m a t i o n o f severe s c r a t c h e s i n t h e d i r e c t i o n o f s l i d i n g .

Scratching

Scuffing

-

-

The f o r m a t i o n o f f i n e scrat che s i n t h e d i r e c t i o n o f s l i d i n g

L o c a l i s e d damage caused by t h e o ccurrence o f solid-phase w elding

between s l i d i n g su rf a ces, w i t h o u t l o c a l s u r f a c e m e l t i n g .

-

Severe Wear

A f o r m o f wear c h a r a c t e r i s e d by removal o f m a t e r i a l i n r e l a t i v e l y

l a r g e fragments.

Soap

-

I n l u b r i c a t i o n , a compound formed by t he r e a c t i o n o f a f a t t y ' a c i d w i t h a metal o r metal compound.

Solid Lubricant

-

Any s o l i d used as a powder o r t h i n f i l m on a s u r f a c e t o

p r o v i d e p r o t e c t i o n f r o m damage d u r i n g r e l a t i v e movement, and t o reduce f r i c t i o n and wear.

Spalling

-

Stick-Slip

S e p a ra t i o n o f p a r t i c l e s f ro m a s u r f a c e i n t h e form o f f l a k e s .

-

A r e l a x a t i o n o s c i l l a t i o n u s u a l l y a s s o c i a t e d w i t h decrease i n the

c o e f f i c i e n t o f f r i c t i o n as t h e r e l a t i v e v e l o c i t y increases.

500 Synthetic Lubricant

-

A l u b r i c a n t produced by s y n t h e s i s r a t h e r than by e x t r a c t i o n

o r r e fin e me nt .

-

Th in F i l m L u b r i c a t i o n

A c o n d i t i o n o f l u b r i c a t i o n i n which t h e f i l m t h i c k n e s s o f

t h e l u b r i c a n t i s such t h a t t h e f r i c t i o n between the s u r f a c e s i s d e t e r mined by t h e p r o p e r t i e s o f t h e s u r f a c e s as w e l l as by t h e v i s c o s i t y o f the l u b r i c a n t .

T o t a l A c i d Number (TAN)

-

The q u a n t i t y o f base, expressed i n terms o f t h e

e q u i v a l e n t number o f m i l l i g r a m s o f po t assium h y d r o x i d e t h a t i s r e q u i r e d t o n e u t r a l i s e a l l a c i d i c c o n s t i t u e n t s p r e s e n t i n 1 gram o f sample.

T o t a l Base Number (TBN)

-

The q u a n t i t y o f a c i d , expressed i n terms o f t h e

e q u i v a l e n t number o f m i l l i g r a m s o f p ot assium h y d r o x i d e t h a t i s r e q u i r e d t o n e u t r a l i s e a l l b a s i c c o n s t i t u e n t s p r e s e n t i n 1 gram o f sample.

Va r n is h

-

A d e p o s i t r e s u l t i n g f rom t h e o x i d a t i o n and/or p o l y m e r i s a t i o n o f f u e l s ,

l u b r i c a t i n g o i l s , o r organic c o n s t i t u e n t s o f bearing m a t e r i a l s .

Viscosity

-

That b u l k p r o p e r t y o f a f l u i d , s e m i - f l u i d o r s e m i - s o l i d substance

which causes i t t o r e s i s t f l o w . V i s c o s i t y i s d e f i n e d by t h e e q u a t i o n dv dx

q = -r/-

T i s t h e shear s t r e s s ,

v t h e v e l o c i t y , ds t h e t h i c k n e s s o f an element

measured p e r p e n d i c u l a r t o t h e d i r e c t i o n of f l o w ; t h e r a t e o f shear.

dv/ds i s known a s

V i s c o s i t y i n t h e normal, t h a t i s Newtonian sense

i s o f t e n c a l l e d dynamic o r a b s o l u t e v i s c o s i t y .

Kinematic o r s t a t i c

v i s c o s i t y i s t h e r a t i o o f dynamic v i s c o s i t y t o d e n s i t y a t a s p e c i f i e d temperature and pre ssure .

V i s c o s i t y Index ( V I )

-

A r b i t r a r y s c a l e used t o show t h e magnitude o f v i s c o s i t y

changes w i t h t emp era t u re i n l u b r i c a t i n g o i l s and o t h e r products.

Wear

-

The removal of m a t e r i a l from s u r f a c e s i n r e l a t i v e motion, n o r m a l l y by a b r a s io n , adhesion o r c o r r o s i o n .

-

Wedge E f f e c t

Wettability

-

The e s t a b l i s h m e n t o f a p r e s s u r e wedge i n a l u b r i c a n t .

A term used t o i n d i c a t e t h e ease w i t h which a l u b r i c a n t w i l l

spread o r f l o w o v e r a b e a r i n g su rf a ce.

501 ZDDP

-

I n i t i a l s f o r z i n c dialkyl-dithiophosphate, which i s w i d e l y used a s an extreme p ressu re agent.

I t i s a l s o an e f f e c t i v e o x i d a t i o n i n h i b i t o r

b u t should n o t b e used i n mechanisms w i t h s i l v e r bearings.

Acknowledgement.

Permission t o qu ot e terms and d e f i n i t i o n s r e c e i v e d f r o m

The I n s t i t u t e o f Petroleum and t h e OECD.

502

AUTHOR INDEX

Numbers u n d e r l i n e d g i v e t h e page on which t h e complete r e f e r e n c e i s l i s t e d , o t h e r numbers r e f e r t o t h e page number on which t h e a u t h o r ( o r h i s work) i s mentioned i n t h e t e x t .

221 2

Amateau,M.F.,

210

Archard,J.F.,

13

Attwood,D.G.,

3

Aubert,F.,

lo

220 222

Babichev, 17

2

Bailey,J.M.,

18

Barwell,F.T.,

1

3s

471 482 488 492 13

lo 13 2 Bickerman,J.J., 14 2 Bingham,E., 278 290 4

Beilby,G.,

Block.H.,

12

Booser,E.R., Boothroyd,G., Bosma,R.,

60

Czichos,H.,

4 10

130

Davis,T.A., Oawson,P.H.,

17 29 130

Decoufle,P.,

277

Devine,M.J.,

3

289

290 4 lo

426

Dombroski,R.M.,

1 5 10 11 130

Duckworth,W.E.,

221 fi 14 2

206

Dumbleton,J.H.,

5

12 15 28

Orasche,H.,

289

9

11 13 13

Bridges,D.C.,

428

Brown,T.W.F.,

64

25 2 8 0 Eastham,D.R.,

3 10 130 221

E l w e l l ,R.C.,

445

Engel , P . A . ,

18

Evans,F.C.,

217

Eyres ,A. R.,

275

2

Buckley,D.H.,

4

Bunshah,R.F.,

220

, 130

277

28

79 221

222

428 433

428 445

Davies,A.E.,

Dowson,O.,

2

30 427

492

130

47 49 78 131

Braithwaite,E.R.,

Burke ,A. E.

474

Dobychin,M.N.,

Brainard,W.A.,

Bryce,J.B.,

24

79

Bowden,F.P., Bowen,E.R.,

lo

Crooks,C.S.,

2 23 2 15 2 60 79 461

130

284 290

De Vos,H., 281

7

Blackwell ,J.,

3

Cree,J.C.,

2

14 15 17 25

Bates,T.R.,

Chynoweth,A.,

Collacott,R.A.,

7

Bartenev,G.M.,

fi 52 78 130

5

Clausen,J.,

329

Balmer,C.,

Churchill,J.R.,

Cameron,A.,

130 78

47

3

221

222

fi

Fein,R.S.,

5

Fidler,F.,

17 2 9 130

445

503 Forrester,P.G.,

130 131 208

?21

'0

F o r s y t h , I., 5

Kilbourn,D.F.,

Fowle,T. I., 184 193

196

King,M.,

4 5 58 72 75 78 79 131

Gass,H.,

220 222

Glasser,W.A., Godet,M.,

4

Kruschov,M.M.,

17

9

Lancaster,J.K.,

6

11 217 221

Halling,J.,

330 353 370

lo 221 222 5 1 1

3

Hancock,B.T., Harvey,B.F.,

5

7

11 220 222

2

Hjertzen,D.G.,

132

H o l l igan,P.T.,

9

Hother-Lushington,S.,

Jahanrnir,S.,

13

2

2

2 2

72

Jones,M.H.,

471

3

Justusson,W.M.,

2 lo 1 3

Lund,J.W.,

50 52

to 222 206

28

2

45 47 58 72 75 78 79

Mathieson,T.C.,

429

445

425 7

2 23 2

29

Mear,R.B.,

130

Mear5,D.C..

28

30 485

492

395 426

--

Mills,G.H.,

21 25 27 29 30

Milne,A.A.,

15

3

Ming Feng, I., 13 15 28 Moes,H., 60

2

Moore,M.A.,

221

27

78 277

Merrett,J.G.,

2 3 9 10 11 395 425 426 220

Ludema,K.C.,

Meckleburg,K.R.,

5

Jones,G.J.,

49

130

130 21

McDonald,D.,

132

Johnson,K.L.,

290

McCullagh,P.J.,

a

20

Jarvis,R.A.,

3 8

McCoy,W.E.,

Jarnes,R.D., Jarnieson,D.T.,

Ling,F.F.,

Martin,F.A., 17

130

Hunter,M.S.,

278

Lushbaugh,C.C.,

278 290

Horton,A.W.,

Leitch,A.,

Loy,B.,

Hinterrnan,H.E.,

2 2 lo

75

Lee,C.S.,

Love,P.,

130 14

221

206

Lloyd,K.A.,

2

5

Higginson,G., Hiley,R.W.,

11

7

Leak,D.A.,

Linkinhoker,C.L.,

9

465

223 267

Laventev,V.V.,

Lehrke,W.D.,

445 fl

428

Hastings,G.W., Hibbs,L.E.,

Lansdown,A.R.,

2

20

Halligan,B.D.,

Hirst,W.,

222

220

Grunberg,L.,

Juntz,R.S.,

lo lo 14 9

4

K r a g e l s k y , I .V.,

2

18

Gregory,J.C.,

Jost,H.P.,

426

130

Godfrey,D.,

Jones,M.P.,

3

13

Knight,R.E., Kornbalov,V.S.,

lo 210 221 4

Georges,J.M.,

428

lo

4

Kirk,J.A., Garner,D.A.,

266 445

Kennedy-Srnith,R.,

17

9

Morris,J.A.,

130

Murray,P.W.,

395

425

504 Neale,M.J.,

lo 31 131

2

Newman,A.O.,

64

Nicholson,O.W., Nonnen,F.A.,

80

Smal lheer,C.V.,

210 221

Smart,E.F.,

lo

2

Smi th,A. I . , Soul ,D.M.,

130 P h i l l i p s , R . , 206 221 Pinkus,O., 52 78

242

Spencer,J.B.,

222 291 329

Sridharan,P.,

13

Spalvins,T.,

Perkins,C.A.,

443 445

Pocock,G.,

130 266 468 470 216 221

Shone,E.B.,

79

221

Sternlicht,B.,

130 217 221 Przbyszewski,J.S., 221 222

Pratt,G.C.,

Summers-Smith,O.,

Tabor,D., 2

Rabinowicz,E.,

284

Thony,C.,

-

454 470

Rollason,E.C.,

Ruff,W.,

25

Ryman,F.O.,

479

Tourret,R.,

23

3

221 221 446

216 215

492

-

78

Van Peteghem,T.,

-

278 289 279

Walsh,W.,

1 3 4 5 7 -10 11 13 14 15 17 18 20 21 23 24 25 27

-

3

205 211 212 216 218 221 488 492 -

3

Sharp,W.F.,

219

222 221

222

290

lo

Welsh,N.C.,

131

18 19

15

2

2

465

470

425

Westcott,V.C.,

3

289

Waterhouse,R.B.,

West,C.H.,

290

445

279

Warriner,J.F.,

Wentorf ,R.E.,

-

14 18 20 29

Sherbiney,M.A.,

277

Wahlberg,J.E.,

2 10 426

281 428

Wagner,W.D.,

2

Scott,H.H.,

452 464

468 470

lo

14

2

Venton,A.D.F.,

Schofield,J.,

Shen,C.,

290 lo

283

30

Sanderson,J.,

Scott,O.,

78

Tilley,G.P.,

Trent,E.M.,

49

Scala,R.A.,

lo

52

Tremain,G.R.,

130

Roylance,B. J.,

Saka,N.,

3

130

Roehner,T.G.,

5 1_1 4 5

Thomson ,K.K.,

Richardson,R.C.D.,

Rose,A.,

Thomas,T.R.,

fi

5

Reynolds,O.,

222

221

Tevazwark,J.L.,

290

8

13

8

221 130

216

Teer,D.G.,

2

281

12 15

Taylor,C.M.,

130

Rastogi ,S.C., Reda,A.A.,

Tait,J.,

3 13 a

3

Rafique,S.O., Rapp,C.,

8

221

29

13

Swanson,T.D.,

15

216

130

Quayle,J.P.,

Raask,E.,

78

52

to 13

1 4

Suh,N.P.,

4 9

13 15 24

25 28 29 30

505

-

428 445

Wilkinson,H.C., Wilks,P.E.,

425 425

W i 1 1 iams,G., Williams,J.E.,

454 470 fi 80 130 218 222 205 221 23 9

7

Wilson,R.W., Woldman,N.E., Wright,E.P.,

17 18

Wright,K.H.R.,

Zingmark,P.A.,

Zorev,N.N.,

3

2

221

206

Zakay,V.F.,

Zlatin,L.,

11

5

Williams,D.F.,

281

290

lo

-

448 470

506 f r i c t i o n m o d i f i e r 246 264 hazards 275-290 head 280

SUBJECT INDEX

metal c u t t i n g 262 467 metal d e a c t i v a t o r 247 pour p o i n t depressant 247 257 r u s t i n h i b i t o r 246 254

Abel method 244

t a c k i n e s s 247

Abrasion

v i s c o s i t y index improver 247 250 255

d e f i n i t i o n 17 493 economics 3 Ab r a s iv e Wear 3 12 17 c u t t i n g t o o l s 214 461 467

Adhesion wear t h e o r y 13 493 A i r entrainment 347

A1 ignment, gears 184-187 Almen Weiland t e s t 261

damage 17

Amides 250

e f f e c t o f hardness 17

Analysis

gears 194 202

Auger e l e c t r o n spectroscopy 4

p a r t i c l e s 296

i on spectroscopy 4

p l a i n b e a r in gs 81 97 207

microprobe 4

r e s i s t a n c e 206 213

scanning e l e c t r o n microscopy 4

r o l l i n g b e a r i n g s 173

s p e c t r o g r a p h i c o i l 8 24 439

s e a l s 334 374

x r a y photon

4

Accelerometers 434

A n t i c h a t t e r a d d i t i v e 265

A c i d i t y 244

A n t i c o r r o s i o n a d d i t i v e 276

A c i d tr e a tm e n t 275

A n t i foam a d d i t i v e 247

Add i t ives

A n t i o x i d a n t a d d i t i v e 232 246 251 252

276

a n t i c h a t t e r 265 a n t i c o r r o s i o n 276

A n t i squawk a d d i t i v e 265

a n t i foam 247

A n t i wear a d d i t i v e 231 246 260 276 281

a n t i o x i d a n t s 232 246 251 252 276 a n t i squawk 265 a n t i wear 231 246 260 276 281 493 b i o c i d e s 276 289 c o r r o s i o n i n h i b i t o r 246 251 253 254

495 d e t e r g e n t s 243 245 246 249 276 495 d i s p e r s a n t s 243 249 251 276 495 e m u l s i f i e r s 247 255 264 276 extreme p r e s sure 231 247 258 259

276 467 474 496 e f f e c t on gears 191 193 200 201 e f f e c t on p l a i n b e a r i n g s 1 1 2 e f f e c t on s c u f f i n g 15

493 Area o f c o n t a c t a pparent 452 493 r e a l 452 493 A r t h r i t i s 28 Asbestos packings 377 378 h e a l t h 393 A s p e r i t i e s 493 r o l e i n wear 13 A t t r i t i o n 465 Aus f orming 206

507 B a c t e r i a 282 289 Bad bonding 89 B a r r i e r cream 286 Bearing area 494 materials

Bear ings p l a i n journal heat balance 57 l u b r i c a t i o n requirements 402 m a t e r i a l s e l e c t i o n 43

aluminium a l l o y 83 87

maximum p r e s s u r e 43

aluminium l e a d 210

maximum temperature 71

aluminium t i n 83 210

minimum c l e a r a n c e 65

b a b b i t 82 493

non l a m i n a r 72

copper l e a d a l l o y s 83 86 208 474

performance 39 65

l e a d bronze 83 86 208

plastic

n y l o n 217

porous 32 88

7 217

o v e r l a y 82 84 208

power loss 67 70

phosphor bronze 83 88

p r o f i l e bore 72

p l a s t i c 217

pv f a c t o r 34 498

p o l y a c e t a l 218

s p e c i f i c l o a d r a t i n g 66

porous 88 498

s u r f a c e f i n i s h 47

p t f e 215-218

r o l l i n g element

s i l i c o n bronze 88

a p p l i c a t i o n s 166

s i l v e r 88

c l e a n i n g 171

t h e r m o p l a s t i c 217

c 1 earances 144

thermoset 217

c o r r e c t i o n f a c t o r 146

t r i m e t a l 82 88

damage see f a i l u r e s

w h i t e metal 82 208 Bearings

dimensions 144 d i s m a n t l i n g 168

f l e x u r e 32 36 42

f a t i g u e l i f e 35 139

h y d r o s t a t i c 35 37

f i t s 163

magnetic 32

f r i c t i o n 150

p l a i n journal

grease 152

b e a r i n g l o a d c a p a c i t y 54 61

l o a d c a r r y i n g c a p a c i t y 139

c l e a r a n c e 65

l u b r i c a t i o n 151 151

d e s i g n l i m i t s 46

m a t e r i a l 211

e f f e c t o f bearing temperature 48 60 g r o o v i n g 56 misalignment 67 o i l f i l m s t a b i l i t y 50 64 o i l f i l m t h i c k n e s s 60 o i l o x i d a t i o n 50 s u r f a c e roughness 47 f a i l u r e s see f a i l u r e s f l u i d f i l m 35

mounting 167 n o i s e 159 o i l m i s t 158 o i l s e l e c t i o n 156 performance 39 s e a l s 157 s e l e c t i o n 132 shock p u l s e 163 speed l i m i t 1 4 5 148 149

508 type

a x i a l p i s t o n pump 298

angular contact 134

gear pump 298

b a l l t h r u s t 138

r a d i a l 45 67

c y l i n d r i c a l r o l l e r 135

r a t i o 494

double row a n g u l a r c o n t a c t 134

spool v a l v e 301

d o u b l e row r o l l e r 135

vane pump 298

double row s p h e r i c a l 137

C l e v e l a n d method 244

n e e d l e 136

Cloud p o i n t 235

s e l f a l i g n i n g 133

Cold c r a n k s i m u l a t o r 255

s i n g l e row deep groove 133

Composites

s p h e r i c a l r o l l e r t h r u s t 138

bearing design data 6

t a p e r r o l l e r 136

bearing material 6

r u b b i n g 34

b e a r i n g performance 6

l i f e 140

m a t e r i a l a p p l i c a t i o n 207

m a t e r i a l s e l e c t i o n 210 pv f a c t o r s 41

Compression p a c k i n g see Seals C o n d i t i o n m o n i t o r i n g 427-445

B e i l b y l a y e r 454

Conradson method 244

Beta r a t i o 307

Consumption o i l 267

Bioc i d e s 282 289

C o n t a c t a r e a 184 452 493

Boroscope 437

Contaminant a n a l y s i s 438

Brake m a t e r i a l

7

Contamination

B r i n e l l i n g 178 494

b u i l t i n 292 294

B r o o k f i e l d v i s c o m e t e r 255

c o n t r o l 315

B u i l t up edge 454

e f f e c t on d i r e c t i o n v a l v e s 301

Bypass see f i l t r a t i o n

f l o w c o n t r o l s 302 g e a r s 194

Cams 216

l u b r i c a n t s 234

C a r b u r i s i n g 210 216 219

motors 300

C a r c i n o g e n i c compounds 279

p r e s s u r e c o n t r o l 302

Case h a r d e n i n g 210 219

pumps 297

Cavitation

v a l v e s p o o l s 301

e r o s i o n 101 117-121 397 494

e n v i r o n m e n t a l 294

m a t e r i a l r e s i s t a n t 10 19

g e n e r a t e d 296

s u r f a c e damage 19

l e v e l s 304

Centre l i n e average 494

seals

Ceramics 6 213

s p e c i f i c a t i o n 305

Channeling 494

sources 294 315

C h e l a t i n g agents 254 Chemical vapour d e p o s i t i o n Chromium p l a t i n g 216 Clearance c r i t i c a l 297 299

347

Coolant

7 220 464

467

Copolymers 250

509 Corrosion

Dye p e n e t r a t i o n 436

7

ceramics

Dynamic l o a d r a t i n g 139

copper l e a d a l l o y s 107 i n h i b i t o r 251 253 254

E c c e n t r i c i t y r a t i o 46 51 495

p l a i n b e a r i n g s 102-117

Eddy c u r r e n t t e s t 437

r o l l i n g b e a r i n g s 181

E l a s t o hydrodynamic l u b r i c a t i o n 5 495

r e s i s t a n c e 102-117 see a l s o f a i l u r e s C u t t i n g f l u i d s 6 262 C u t t i n g speeds 449 451 C u t t i n g t o o l s 215 a lu m in iu m o x i d e 450 464 boron n i t r i d e 450 465

polymers 217 traction 5 E l e c t r i c discharge 119 181 r e s i s t a n c e method 437 E l e c t r o chemical machining 450 d i scharge machining 450 Embeddabi 1 it y 81

ceramic 215 464

Emulsions 237 255 282 318

cemented c a r b i d e s 215 450 464

Engine t e s t s 472

c o s t s 451

Eros i o n

h i g h speed t o o l s t e e l 451 464

b e a r i n q damage 117-121

m a t e r i a l for 450 464

ceramics

s i l i c o n c a r b i d e 450

economics 2

wear o f

7 215 459-467

7

p r e s s u r e c o n t r o l 302 steam 38

Debris a n a l y s i s 24 419 c u t t i n g wear 25 26

wear 12 496 see a l s o f a i l u r e s Extreme pressure a d d i t i v e 232 258 259 496

f a t i g u e 27 440

c u t t i n g o i l s 262

la m in a r wear 25 440

e f f e c t on s c u f f i n g 14

r u b b i n g wear 25 26

e f f e c t on p l a i n b e a r i n g s 112

s p h e r i c a l wear 25

e f f e c t on gears 191 193 200 201

s y n o v i a l f l u i d 28 Del ami na t i o n t h e o r y 13 De lp h i a n a l y s i s 428

t u r b i n e o i l s 262 see a l s o a d d i t i v e s Eye i r r i t a t i o n 280

Dermat it i s 277 D e t e r i o r a t i o n l i m i t 444 De t e r g e n t s 243 245 246 249

Fa i 1 u r e s Gears

D i s p e r s a n t s 243 249 251

a b r a s i v e wear 194 202

D i t h i o c a r b o n a t e s 253

alignment 184-187

Di th io p h o s p h a te s 252 253

broken t e e t h 197

Drawing 263

f a t i g u e p i t t i n g 189 197 198 474 498

D r i l l i n g 263 Drop p o i n t 495

foaming 204

Duty parameter 495

f r e t t i n g 202

510 g a l l i n g 191 497

see a l s o f a i l u r e s

g r o o v i n g 188

F a u l t t r e e 429

n o i s e 203

Ferrography 9 25 439 442

o v e r h e a t i n g 203

F i l l e r 496

r i d g i n g 188 474 475

Filtration

r i p p l i n g 201 474 476

e f f e c t o f f i r e r e s i s t a n t f l u i d s 318

s c o r i n g 476

emulsions 318

s c u f f i n g 191 199 474 476

magnets 319

v i b r a t i o n 203 p l a i n bearings a l l o y i n g 125

o f f l i n e 317 324 p r e s s u r e l i n e 316 320 pump i n l e t 318 327

bad bonding 89

r e t u r n l i n e 317 322

bad f i t t i n g 96

s e a l s 347

c a v i t a t i o n 117-121

Fi 1t e r

c o r r o s i o n 102-117

a b s o l u t e r a t i n g 306

e l e c t r i c a l d i s c h a r g e 119

a i r b r e a t h e r 294 295

e r o s i o n 117-121

b e t a r a t i o 307

e x c e s s i v e l e a d 91

bubble t e s t 306

extraneous p a r t i c l e s 97-101

bypass 308 309 320

f a t i g u e 123

c l a s s i f i c a t i o n 307

gas c a v i t i e s 91

d i r t c a p a c i t y 313

o v e r s i z e cuboids 91

e f f i c i e n c y 310 321

thermal c y c l i n g 125

gears 194

v i s c o s i t y 126

l o c a t i o n 318

w i r e wool 122

mean r a t i n g 306

r o l l ing bearings

m u l t i p a s s t e s t 307

a b r a s i v e 173

nominal r a t i n g 305

cage 175

o f f l i n e 317 324

c o r r o s i o n 181

p r e s s u r e l i n e 316 320

e l e c t r i c c u r r e n t 181

p u l s a t i n g f l o w 308

f a l s e b r i n e l l i n g 178

r e t u r n l i n e 317 322 324

i n c o r r e c t mounting 174

s e l e c t i o n 305-317

smearing 177

s i l t c o n t r o l 308 312

v i b r a t i o n 178 wear 172 Falex t e s t 261

s i z i n g 312 F i r e r e s i s t a n t f l u i d 234 318 F l a m m a b i l i t y 234

False b r i n e l l i n g 178

F l a s h p o i n t 235 244 496

Fatigue

F1ash temperature 496

f r e t t i n g 18 p l a i n b e a r i n g s 123 r o l l i n g c o n t a c t 20

F l e x u r e b e a r i n g 32 36 496 performance 42 Flow zone 456

511 F l u i d e r o s i o n 19 Flushing 292 294

Hazards a d d i t i v e s 280

Flux t e s t i n g 437

b a c t e r i a 282 289

Four b a l l machine 232 261 485 487

b i o c i d e s 282 289

Frequency spectrum 435

d e r m a t i t i s 277

Fretting

eye i r r i t a t i o n 280

d e f i n i t i o n 18 496

h e a l t h 545

gears 202

o i l m i s t 277 287

wear d e b r i s 18

o i l vapour 278

see f a i l u r e s

r e c l a i m e d o i l 283

F r i c t i o n m o d i f i e r 264

r e - r e f ined o i 1 283

F r i c t i o n w e l d i n g 452

s c r o t a 1 cancer 279

FZG t e s t machine 261 478

s k i n cancer 278 s y n t h e t i c l u b r i c a n t s 283

G a l l i n g 191 497

H e a l t h and S a f e t y a t Work A c t 284

Gears 184-204

Holography 437

a l ignment 184

H y d r o s t a t i c b e a r i n g 32 34 35

c o n t a c t a r e a 184 f a i l u r e s see f a i l u r e s

IAE t e s t 261

f a t i g u e p a r t i c l e s 25

l n f r a r e d technique 437

f i l t r a t i o n 194

I n s p e c t i o n techniques 436

l u b r i c a t i o n systems 195

I r o n p r i n t i n g 99

m a t e r i a l s e l e c t i o n 210 o i l v i s c o s i t y f o r 193 194

Jost report 1

s c u f f i n g 191 199 t e s t s 261 478

K u r t o s i s 429

Graphite 239 240 242 380 Grease a p p l i c a t i o n 396

Lacquer 472 474 497 Lubricant

c l a s s i f i c a t i o n 399 497

a n t i f r i c t i o n p r o p e r t i e s 231

r o l l i n g b e a r i n g 152

a n t i wear p r o p e r t i e s 231

s e l e c t i o n 224 396

e s t e r s 235

temperature l i m i t s 224 4 0 1

extreme p r e s s u r e p r o p e r t i e s 231

Grooving 188

l i m i t s o f o i l 224

Hardenab i 1 i t y

l i m i t s o f s o l i d 224

l i m i t s of grease 224 e f f e c t o f chromium 211

m i n e r a l o i l 226

e f f e c t o f vanadium 211

polyphenol e t h e r s 236

Hardness

s e l e c t i o n 223-241 400 467

t o o l s t e e l 214

s i l i c o n e s 235

e f f e c t on f a t i g u e s t r e n g t h 207

s o l i d 239

512 v i s c o s i t y 226 229 L u b r i c a t i o n systems

c u t t i n q t o o l t e m p e r a t u r e 456 d i f f u s i o n 462

c o o l i n g 418

e f f e c t o f h e a t 451

d u a l l i n e 409

effect

o f s l i d i n g 458

d i r e c t f e e d 403

e f f e c t o f speed 451

d i r e c t p o s i t i v e 414

e f f i c i e n c y 446

gas 239

f e e d 448

g e a r s 195

f l a n k wear 447

i n d i r e c t l i n e 405

f l o w zone 455

m i c r o f o g 419

h e a t g e n e r a t i o n 451

o i l 413

l u b r i c a n t s 467

p o s i t i v e s p l i t 415

q u i c k s t o p method 447

p r o g r e s s i v e 408

r a k e 447

s e l e c t i o n 403 412

s e i z u r e 452 458

t o t a l loss 413

s u r f a c e s h e a r i n g 461 t e m p e r a t u r e g r a d i e n t 457

Machine e l e m e n t s

tool force

h i g h e r p a i r 482 483 l o w e r p a i r 482 483 M a i n t e n a n c e o n c o n d i t i o n 427-445

448

t o o l wear 459 Molybdenum d i s u l p h i d e 239 240 242 Monitoring techniques

M a g n e t i c b e a r i n g 32

c o n t a m i n a n t a n a l y s i s 8 429 433 438

M a g n e t i c p l u g 24 439 440

s p e c t r u m a n a l y s i s 434

Mater i a 1

t r e n d a n a l y s i s 433

a b r a s i o n r e s i s t a n c e 206

v i b r a t i o n a n a l y s i s 8 429 432 433

c o m p o s i t e 206

w a v e f o r m a n a l y s i s 436

c o r r o s i o n r e s i s t a n c e 206 e l e v a t e d t e m p e r a t u r e 206 207 f i b r e r e i n f o r c e d 206 207 p l a s t i c bearing

7

s e l e c t i o n 205-222 Metal c u t t i n g

Niemann FZG t e s t 261 N i t r i d i n g 210 N o i s e r o l l i n g b e a r i n g s 159 Non d e s t r u c t i v e t e s t i n g 436 dye p e n e t r a t i o n 436

a b r a s i o n 461

e d d y c u r r e n t 436

a t t r i t i o n 465

e l e c t r i c a l r e s i s t a n c e 436

b u i l t up edge 454

f l u x 436

chip

4 4 7 454

r a d i o g r a p h i c 436

c l e a r a n c e a n g l e 447

u l t r a s o n i c 436

c l e a r a n c e f a c e 447 c o o l a n t s 467

O i l d e g r a d a t i o n 270

c r a t e r wear 460 c u t t i n g edge 447 c u t t i n g speed

447 451

f i l m i n s t a b i l i t y 50 64 f i l m w h i r l 72 m i s t 158 277 287 498

513 o x i d a t i o n l i m i t 50

Re-ref i n i n g

s t a n d a r d t e s t s 244

a c i d c l a y 272

vapours 278 287

economics 273

O i l i n e s s 498 Ov e r la y b e a r i n g s 208 O x i d a t i o n r e s i s t a n c e 6 207

Matthys process 273 Ri dg i ng gears 188 474 475 R i p p l i n g gears 201 474 496 R o l l i n g bearings see b e a r i n g s

Packed g l a n d 370 Particles t y p e o f wear 24 440 e f f e c t on f l u i d power systems 296 P a r t i c l e c o u n t i n g 439 Patch t e s t 439 Pensky M a r t i n Method 244 Phenates 248 Phenoles 252 Phosphonates 245 Piston r i n g

Rolling contact fatigue c r a c k p r o p a g a t i o n 23 damage 20 d e f i n i t i o n 20 e f f e c t o f environment 20 e f f e c t o f l u b r i c a n t 23 e f f e c t o f m a t e r i a l 23 p a r t i c l e s 27 Root mean square h e i g h t 499 Rubbing bearings see b e a r i n g s Rust i n h i b i t o r 246 254 Ryder t e s t machine 261

m a t e r i a l s e l e c t i o n 215 wear 216 P i t t i n g 189 197 198 474 498 P l a i n b e a r i n g s see be ari n gs Plastics

7

Porous b e a r i n g s 88 498 Pour p o i n t 235 244 257 498 P r o f i l e b o r e b e a r i n g s 72 PTFE 6 216 239 240 242 Pumps e f f e c t o f contaminant 297 298

S a f e t y l u b r i c a n t 275-290 Sal i c y l a t e s 249 S a ybo lt v i s c o s i t y 499 S co ri ng 476 499 S c u f f i n g 14 cams and tappets 216 e f f e c t o f extreme pressure a d d i t i v e 15 fatigue particles gears 191 199 474 476

PV f a c t o r 34 498

i n c i p i e n t 15

Ra d ia l c l e a r a n c e 45 144

p i s t o n r i n g s 215

Rad io g r a p h ic t e s t 437

s u r f a c e changes 1 5

mechanism o f 14 499

Ramsbottom method 244

Seal l u b r i c a t i o n 356 357

R a t i n g l i f e 499

Sea 1 s

Reaming 263

compression packings 370-394

Reclaimed o i l s a f e t y 283

a b r a s i v e wear 374

Reclamation 9 271

c o m p r e s s i b i l i t y 381

Redwood v i s c o s i t y 499

c o s t 372 387

R e l i a b i l i t y a n a l y s i s 431

f a u l t s 390

514 f i t t i n g 388

lobed 340

f o r pumps 371

0

f o r v a l v e s 371

s i n g l e a c t i n g 340

g la n d d e s ign 374 375 env i ronmen t 384

331 340

Seals r o t a r y l i p 353-369 absorbed power 358

h e a l t h a s pe ct s 391

design 354

i n s t a l l a t i o n 372

e c c e n t r i c i t y 359

leakage 382

f a u l t f i n d i n g 362

ma i ntenance 372

f i t t i n g 361

m a t e r i a l s 376-382

f r i c t i o n 358

packed g la n d 370

hand1 ing 361

r e l i a b i l i t y 372

m a t e r i a l s e l e c t i o n 367-369

r o l l i n g b ea ri ng s 157

maximum pressure 359

s e l e c t i o n 383

maximum speed 359

s h a f t wear 372

s e r v i c e problems 362-365

speed 385

s h a f t s u r f a c e 357

s t a n d a r d i s a t i o n 391 s u r f a c e f i n i s h 375 temperature 386 Seals l i p 338 339 assembly 351

storage 360 Seals s t a t i c compression packing 353 370-394 gasket 341

1 i p 341

c o n t a c t a r e a 338 345

r a d i a l f a c e mechanical 353

e f f e c t o f d e forma t i o n 351

squeeze 341

h u m i d i t y 351

S h e l l f o u r b a l l machine 232 262 485 387

1 i g h t 351

Shock p u l s e measurement 163

oxone 351

S i l t i n g 303

oxygen 351

S i l t removal 308 312

temperature 350

Skin cancer 278 288

energ is e d 345

Skin p r o t e c t i o n 285

f r i c t i o n 344

S o l i d l u b r i c a n t s 6 239 499

leakage 339

S o l vent e x t r a c t i o n 276

m a t e r i a l s e l e c t i o n 331-338

Soap 499

m u l t i l i p 343

S p a l l i n g 213 499

performance 343

Spectroscopy

s e l e c t i o n s 348

Auger e l e c t r o n 4

squeeze 338 339

x r a y photon 4

s to r a g e 350

scanning i o n 4

s u r f a c e roughness 343

ion s c a t t e r i n g 4

Sea 1 s r e c i p r o c a t i ng

Sp ectrographic o i l a n a l y s i s 8 24 439

double a c t i n g 340

Spectrum a n a l y s e r s 434

e n e r g is e d s le eve 340 346

S t i c k s l i p 467 499

515 S t r i b e c k c u r v e 227

S h e l l f o u r b a l l 232 261 485 487

Succinimides 250

Timken 261 488

Sulphonates 245

Thermography 437

Surface f i n i s h

Th iophosphonates 245

e f f e c t on o i l f i l m 48 Surface t r e a t m e n ts

T hre shold l i m i t value 277 T oo l s see c u t t i n g t o o l s

b o r o n i s i n g 219

T o t a l a c i d number 500

c a r b u r i s i n g 210 216 219

T o t a l base number 500

case h a r d e n in g 210 219

T o x i c i t y 235 276 280 282

chemical vapour d e p o s i t i o n 7 220 464 chromosing 219

amines 281 c h l o r i n a t e d napthalenes 281 l e a d compounds 280

chromium p l a t i n g 216 e l e c t r o d e p o s i t i o n 219 flame h a r d e n ing 219 h a r d s u r f a c i n g 219 ion implantation 7 io n n i t r i d i n g 221 i o n p l a t i n g 7 221 molybdenum p l a t i n g 216 n i t r i d i n g 219 220 Noskuff 220

o r t h o phosphates 281 sod ium mercaptobenzothiazole 282 sodium n i t r i t e t r i c h 1 o r o e t h y l ene 282

Trend a n a l y s i s 444 T r ib o l ogy handbook 2 10 module 1 report 1 2 units 1

phosphating 219

T u rbi ne o i l 262

plasma spray 219 s h o t peening 219

U l t r a s o n i c t e s t 437

s i l i c o n i s i n g 219 s u i f BT 219

V i b r a t i o n a n a l y s i s 8 434

Sulphinuz 220

Viscosity

Synovial f l u i d 28

a b s o l u t e 226 600

S y n t h e t i c l u b r i c a n t s 276 500

index 229 500 index improver 229 255

Takiness 247

k i n e m a t i c 226 500

Tappet m a t e r i a l 216

p ressure r e l a t i o n s h i p 231

Tes t machines

temperature r e l a t i o n s h i p 229 401

Almen Weiland 261 485 486 h s l e r 488

Waveform a n a l y s i s 436

c r o s s c y l in d e r 488 Cygnus 485 486

Wea r

Falex 261 486

a b r a s i v e s 194 202

I A E 261 478 Neimann FZG 26

Water based f l u i d s 247 255 264 467

adhesive 12 17 478

chemical 12

516 c u t t i n g t o o l s 215 459 d e b r i s a n a l y s i s 8 24 429 mechanical 12 p i s t o n r i n g 215 r e s i s t a n c e t o a b r a s i o n 213 r e s i s t a n t m a t e r i a l s 213 W e t t a b i l i t y 500 White m e t a ls 82 208 Wire wool f a i l u r e 122

ZDDP 501