2 Screw Conveyor-libre

SCREW CONVEYOR BASIC DESIGN CALCULATION 

CEMA (Conveyor Equipment Manufacturer Association) Approach

HISTORY & APPLICATION 

HISTORY:

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APPLICATION:

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The first conveyor as such as designed by ARCHIMEDES (287 – 212 BC) for removing water from the ship and to raise water from river to farm land

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Screw conveyor s are bulk material transporting devices capable of handling a great variety of materials which have relatively good flowability .

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The next technological advancement of importance in screw conveyor design was patented on march 29, 1898 by Frank C. Caldwell.

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This construction is now known as the helicoid flight.

Flowability is defined in the CEMA material classification standard and denotes: “the degree of freedom of individual material particles to move past each other”

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This characteristic is important in a screw conveyor operation as the screw helix.

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The many advantages of screw conveyor is feasibility of nomerous feed and discharge of the material to be conveyed.

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Another used is the blending of several ingredients to make a finished product or to make a pre-mix for some product

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Screw conveyors may be operated horizontally, on an incline or vertically.

MATERIAL CODE & BULK MATERIAL CHARACTERISTICS 

MATERIAL CODE

A study has been made to define the characteristic of bulk materials in terms which are readily recognized.

Table 2-1 Material Classification Code Chart Major Class Density Size

Material Characteristics Included Bulk Density, loose Very Fine

Abrasiveness

Actual, lb/ft3 No. 200 sieve (0.0029") and under

A 200

No. 100 sieve (0.0059") and under

A 100

No. 40 sieve (0.016") and under

A 40

Fine

No. 6 sieve (0.132") and under

B6

Granular

1/2" and under

C1/2

3" and under

D3

7" and under

D7

over 16" to be spesified

D 16

X = actual maximum size

Dx

Irregular

Stringy, fibrous, cylindrical, slabs,etc

E

Very free flowing

flow function > 10

1

free flowing

4 < flow function < 10

2

average flowability

2 < flow function < 4

3

sluggish

flow function < 2

4

Midly abrasive Moderately abrasive

index 1 - 17 index 18 - 67

5 6

Extremely abrasive

index 68 - 416

7

Lumpy

Flowability

Code Designation

MATERIAL CODE & BULK MATERIAL CHARACTERISTICS 

BULK MATERIAL CHARACTERISTIC:

A study has been made to define the characteristic of bulk materials in terms which are readily recognized. The table shows in the first column the range of density that is usually experienced in handling material The material table is a guide only. The material code and the material factor (Fm) are based on experience of several conveyor manufacturers.

Table 2-2 Material Characteristic Material Ash, black ground

Weight lb/ft3

Material Code Component Series Mat'. Factor Vertical

105

105B635

1A-1B-1C

2.0

-

Ashes, coal, dry __ 1.2"

35 - 45

40C 1/246TY

3D

3.0

V

Ashes, coal, dry __3"

35 - 40

38D346T

3D

2.5

-

Cement, Clinker

75 - 95

85D336

3D

1.8

-

94

94A10026M

2D

1.4

V

Cement, Aerated (Portland)

60 - 75

68A 10026M

2D

1.4

V

Coal, Anthracite, sized__1/2"

49 - 61

55C 1/225 

2A-2B

1.0

-

Coal, Bitumious, mined

40 - 60

50D 335LNXY

1A-1B

0.9

-

Coal, Bitumious, slack

43 - 50

47C1/245T

2A-2B

0.9

-

Dolomite, crushed

80 - 100

90C 1/236

2D

2.0

-

Dolomite, lumpy

90 - 100

95D x36

2D

2.0

-

Fly Ash

30 - 45

38A4036M

3D

2.0

-

Gypsum, raw__1"

70 - 80

75D325

2D

2.0

Limestone, crushed

85 - 90

88Dx36

2D

2.0

Limestone, dust

55 - 95

75A 4046MY

2D

1.6 - 2.0

Cement, Portland

SELECTION OF CONVEYOR SIZE AND SPEED – 1 

MAXIMUM LUMP SIZE



The Allowable size of a lump in a screw conveyor is a function of the radial clearance between the outside diameter of the central pipe and the radius of the inside of the screw trough.

Table Maximum Lump Size Screw dia. (inches)

6 9 9 12 12 12 14 14 16 16 18 18 20 20

Pipe OD (inches)

2 3/8 2 3/8 2 7/8 2 7/8 3 1/2 4 3 1/2 4 4 4 1/2 4 4 1/2 4 4 1/2

Radial Clearance (inches)

2 5/16 3 13/16 3 9/16 5 1/16 4 3/4 4 1/2 5 3/4 5 1/2 6 1/2 6 1/4 7 1/2 7 1/4 8 1/2 8 1/4

Class 1

Class 2

Class 2

10% lumps

25% lumps

95% lumps Ratio

Ratio R=1.75

Ratio R=2.5

R=4.5

Max. Lump (inch.) Max. Lump (inch.) Max. Lump (inch.)

1 1/4 2 1/4 2 1/4 2 3/4 2 3/4 2 3/4 3 1/4 3 1/4 3 3/4 3 3/4 4 1/4 4 1/4 4 3/4 4 3/4

3/4 1 1/2 1 1/2 2 2 2 2 1/2 2 1/2 2 3/4 2 3/4 3 3 3 1/2 3 1/2

1/2 3/4 3/4 1 1 1 1 1/4 1 1/4 1 1/2 1 1/2 1 3/4 1 3/4 2 2

SELECTION OF CONVEYOR SIZE AND SPEED – 1 CONVEYOR SIZE & SPEED  In order to determine the size and speed of a screw conveor, it is necessary first to establish the material code number. This Code number controls the cross-sectional loading that should be used.  The capacity table (below) gives the capacities in ft3/hour at one revolution per minutes for various sized and various cross-sectional loading.  The basis for the Capacity Table is as follows 

= Where: C : Capacity [ft3/hour.rpm] Ds : Diameter of screw [inches] Dp : Diameter of pipe [inches] P : pitch of screw, [inches] K : percent trough loading

.   −     

SELECTION OF CONVEYOR SIZE AND SPEED – 2 

In order to determine the conveyor speed can be calculated by the formula:

=

.  .   .   .  

Where: N : Conveyor speed [rpm], (≤ recommended max. Rpm)

Q : Required capacity [ft3/h] C1 : Conv. Capacity at one rpm (see table below) CFo : Overload capacity factor [110% - 120%] CF1 : Conveyor pitch factor CF2 : Type of flight factor CF3 : Mixing paddle factor

Special Conveyor Pi tch Capacity Factor, CF1 Pitch

Standard Short Half Long

Description

CF1

Pitch = diameter of screw Pitch = 2/3 diameter of screw Pitch = 1/2 diameter of screw Pitch = 1 1/2 diameter of screw

1 1.5 2 0.67

Special Conveyor Fli ght Capacity Factor, CF2 Conveyor Loading

Type of Flight

Cut Flight Cut & Folded Flight Ribbon Flight

15%

30%

45%

1.95 NR 1.04

1.57 3.75 1.37

1.43 2.54 1.62

Special Conveyor Mixi ng Paddle Capacity Factor, CF3 o

Standard paddles per pitch set at 45  reverse pitch

Factor CF3 Compiled by Masda Ehsan

None 1

1 1.08

2 1.16

3 1.24

4 1.32

SELECTION OF CONVEYOR SIZE AND SPEED – 3 SCREW CONVEYOR CAPACITIES Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM A-15

45%

A-25

6 9

165 155

   

368 1,270

SCREW CONVEYOR CAPACITIES Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM

2.23

A-35

E-15

6

120

 

180

8.20

A-45

E-25

30%

9

100

 

545

1.49 5.45

E-35

12

90

 

1,160

12.90

E-45

B-15

12

145

 

2,820

19.40

B-35

B-25

14

140

 

4,370

31.20

B-45

14

85

 

1,770

20.80

6,060

46.70

C-35

16

80

 

2,500

31.20 45.00

C-15

16

C-25

130

 

18

120

 

8,120

67.60

C-45

18

75

 

3,380

20

110

 

10,300

93.70

D-15

20

70

 

4,370

62.50

164.00

D-25

24

65

 

7,100

109.00

24

100

 

16,400

D-35 D-45

Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM A-16

D-16

30%

6

60

 

90

Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM

1.49

A-17

D-17

15%

6

60

 

45

0.75 2.72

A-26

D-26

9

55

 

300

5.45

A-27

D-27

9

55

 

150

A--36

D-36

12

50

 

645

12.90

A-37

D-37

12

50

 

325

6.46

D-47

14

50

 

520

10.40

A-46

D-46

14

50

 

1,040

20.80

A-47

B-16

E-16

16

45

 

1,400

31.20

B-17

E-17

16

45

 

700

15.60

B-26

E-26

18

45

 

2,025

45.00

B-27

E-27

18

45

 

1,010

22.50

E-37

20

40

 

1,250

31.20

E-47

24

40

 

2,180

54.60

B-36

E-36

20

40

 

2,500

62.50

B-37

B-46

E-46

24

40

 

4,360

109.00

B-47

C-16

C-17

C-26

C-27

C-36

C-37

C-46

C-47

COMPONENT GROUP NORMAL SERVICE

HEAVY SERVICE

Component Group 1A, 1B and 1C

Component Group 2A, 2B, 2C and 2D

Regular flights and regular trough

Regular flights and he avy trough

Screw Dia.

Coupling Dia.

(inches)

(inches)

6 9 9 12 12 14 16 18 20 24

1 1/2 1 1/2 2 2 2 7/16 2 7/16 3 3 3 3 7/16

Thickness, US standard gauge or inches Trough Cover

16 ga. 14 ga. 14 ga. 12 ga. 12 ga. 12 ga. 12 ga. 10 ga. 10 ga. 10 ga.

16 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 12 ga. 12 ga. 12 ga.

Screw Dia.

Coupling Dia.

(inches)

(inches)

6 9 9 12 12 14 16 18 20 24

1 1/2 1 1/2 2 2 2 7/16 3 3 3 3 3 7/16

EXTRA HEAVY SERVICE Component Group 3A, 3B and 3D Regular flights and heavy trough Screw Dia.

Coupling Dia.

(inches)

(inches)

6 9 9 12 12 14 16 18 20

1 1/2 1 1/2 2 2 2 7/16 3 3 3 3

Thickness, US standard gauge or inches Trough Cover

14 ga. 10 ga. 10 ga. 3/16 3/16 3/16 3/16 3/16 3/16

16 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 12 ga. 12 ga

Thickness, US standard gauge or inches Trough Cover

14 ga. 10 ga. 10 ga. 3/16 3/16 3/16 3/16 3/16 3/16 3/16

16 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 12 ga. 12 ga. 12 ga.

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.) 

The horsepower required to operate a horizontal screw conveyor is based on proper installation, uniform and regular feed rate.

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The horse power requirement is the total of the horsepower to overcome conveyor friction (HPf ) and the horsepower to transport the material at specified rate (HP m)

  =

 =

 =

. .   .  

. . .    .  .     +   

Where: C : Equivalent design capacity [ft3/h],

e : Drive efficiency

L : Total length of conveyor [ft]

N : operating speed [rpm]

Fb : Hanger bearing factor

Fm : Material factor

Ff : Flight Factor

Fo : Power overload factor

Fd : Conveyor diameter factor

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.) Hanger Bearing Factor, Fb

Flight Factor, Ff  

Component Group Bearing Type Group A Ball Group B Babbit Bronze Wood Group C Plastic Nylon Teflon Group D Chilled hard iron Hardened alloy sleeve

Fb

1.0   1.7

2.0

Type of Flight

Standard Cut Flight Cut & Folded Flight Ribbon Flight

Conveyor Loading 15%

1.0   1.10 NR   1.05

30%

     

1.0 1.15 1.50 1.14

45%

   

1.0 1.20 1.70 1.20

95%

1.0 1.30 2.20 -

Paddle Factor, Fp

4.4

Standard paddles pe r pitch set at 45o reverse pitch

Fp

None

1

2

3

4

1

1.29

1.58

1.87

2.16

Screw Diameter Factor, Fd Screw Dia. (inch)

Fd

4 6 9 10 12 14 16 18 20 24

12 18 31 37 55 78 106 135 165 235

Compiled by Masda Ehsa n

Mechanical Efficiencies of Spe ed Reduction Mechanisms Type of Speed Reduction Mechanism

V-belts and sheaves Precision roller chain on cut tooth sprockets, open guard Precision roller chain on cut tooth sprockets, oil tights casing Single Reduction helical or herringbone eclosed gear reducer or gearmotor Double Reduction helical or herringbone eclosed gear reducer or gearmotor Triple Reduction helical or herringbone eclosed gear reducer or gearmotor Low ratio (upto 20:1 range) enclosed worm gear speed reducers Medium ratio (20:1 to 60:1 ratio range) enclosed worm gear speed reducers Gi gh rati o ( 60:1 to 100:1 rati o range ) e ncl ose d w orm ge ar spe ed re duce rs Note : Contact Gear Reducer Manufacturer for the exact Mechanical Efficie ncies

Approximate Efficiencies 0.94

0.93 0.94 0.95 0.94 0.93 0.9 0.7 0.5

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)

HORSEPOWER REQUIREMENT (INCLINED SCREW CONV.) 

ADVANTAGES: Inclined screw conveyor is often very desirable as it may solve a conveying problem with a minimum of equipment and occupy a minimum space

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DISADVANTAGES:

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

The capacity, or the maximum available capacity of a given screw conveyor decreases wit h increase of incline



The horse power per-unit capacity increases.

ADDITIONAL POWER to lift the material, the formula to calculate horsepower to lift the material as follow:

 =

 =

HPh: Horsepower to lift the material [HP] h : actual height of lift [ft] e : Drive efficiency

 . ()

  +  +  

Where: C : Equivalent design capacity [ft3/h]

 .  . 



PROBLEMS ASSOCIATED WITH INCLINED SCREW CONVEYOR 

Several things can be done to overcome many of the problems associated with inclined screw conveyor: 

Limit the use of standard screw components to inclines of less than 25 o, preferably not over 15o



Use close clearance between trough and screw



Increase the speed over that applicable for a horizontal screw conveyor of the same size

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Use short pitch screws, 2/3 or ½ pitch