Vibration School

 

VIBRATION SCHOOL

What Is Vibration ? Vibration is a "back and forth" movement of a structure. It can also be referred to as a "cyclical" movement What Is Vibration Caused By ? Imperfections Imperfectio ns in the Machine: esi!n

ssembly

Manufacture

#peration

Installation

Maintenance

What re $ome Common Machine %roblems   &hat 'enerate Mechanical Vibration: Misali!nment

(nbalance

Worn belts ) pulleys

Bearin! efects

*ydraulic +orces

erodynamic +orces

,eaction +orces

,eciprocatin! +orces

Bent $hafts

,ubbin!

'ear %roblems

*ousin! istortion

Certain -lectrical %roblems

+rictional +orces

What re $ome Common Machine %roblems   &hat Amplify &hat  Amplify Mechanical Vibration But on/t Cause It0: ,esonance

1ooseness

Why o We Measure Vibration ? 2. ssess the condition primarily the bearin!s0 of a machine. By performin! this task effectively3 4e can eliminate catastrophic failures

 

due to component de!radation. 5. ia!nose the root causes0 of any e6cessive destructive0 vibration. By performin! this task effectively3 4e can e6tend the life of bearin!s and other components that are absorbin! the stresses and fati!uin! forces that are causin! the symptom symptom of  of e6cessive vibration. It should be understood that short7term event7based failures i.e. loss of lubrication3 sudden fracture of a component3 etc.0 are not protected a!ainst via any pro!ram that only collects data periodically. &he time bet4een onset and failure in these cases 7 4hich are rare 7 may take only minutes in e6treme cases03 hours3 days or 4eeks. +or e6ample3 many pro!rams are based on monthly data collection. ny event occurrin! durin! that month interval may cause failure prior to the ne6t data collection. &his is not a failure of the pro!ram or the technolo!y any more than drivin! a fork truck into a machine and destroyin! it is. &he !ood ne4s is that the vast ma8ority of potential and actual failures 4ill 9#& fall into this cate!ory and # lend themselves to bein! detected3 monitored and corrected throu!h a 4ell7 run vibration pro!ram. What oes &he &ransducer ctually etect ?

ctual Bearin! Movement:   -lliptical

 &ransducer Mounted Vertically "$ees" #nly Vertical Movement

 &ransducer Mounted *oriontally "$ees" #nly *oriontal Movement

 

What Vibration "Characteristics" o We Measure ?

 

mplitude &ells (s:

How Much Movement Much Movement #ccurs

What Vibration "Characteristics" o We Measure ?

+re;uency &ells (s:

How Often Often &he  &he Movement #ccurs *o4 many "cycles" in a period of time: a second or a minute0

What Vibration "Characteristics" o We Measure ?

 

%hase &ells (s:

In What Direction Is Direction Is &he Movement ,elative &o #ther 1ocations #n &he Machine t  'iven Moment In &ime0

What re "Conventions" ? Conventions are standards that you set or adopt that apply to every machine and application in the pro!ram. &hese conventions simplify trainin! of ne4 personnel and make sure everyone involved in the pro!ram is on the same pa!e. &here are at least  three  three basic conventions that should be set up. &hey are: 2. Bearin! 9umberin! "%ositions"0

 

5. %osition 9amin! ou can use other people/s ideas and conventions or develop one yourself that makes sense to your  people  people for your   machines.  &he !earbo6 sho4n here is a "double reduction" !earbo6 it has t4o separate !earmeshes0. &his !earbo6 has three ou may have applications that do not fall neatly into the =7 bearin! machine cate!ory. 1on! drive lines 4ith doens of bearin!s3 !earbo6es like the one sho4n on the previous pa!e3 multi7sta!e machines3 machines3 etc. may each re;uire their o4n uni;ue solution for bearin! namin!. In the case of a lon! drive line3 the bearin! may be named to coincide 4ith the piece of e;uipment alon! that drive line that bearin! is closest to. Conversely3 you may decide to rely strictly on position numbers in that case and not use position names at all. &erms such as "Intermediate $haft" may be used. &here is no sin!le3 universal namin! convention that 4ill apply to all machine confi!urations. ,emember the ob8ective:

Consistency Is The Key   

Common irection 9amin! Convention Conventions s irectional conventions are also of the utmost importance to set up and use.  simple3 common sense convention insures that 4homever is collectin! the data is a4are of the correct transducer location and direction. It also means the analyst3 if different than the data collection technician3 can analye the data 4ith confidence. &his convention !oes to the heart of one of the most important aspects of adata vibration pro!ram 7 the repeatability of readin!s from one collection to the ne6t

 

4hat !ood is a trend 4ithout !ood repeatability ?0. Its importance !oes hand7in7hand 4ith the importance of makin! sure the e6act transducer locations are clearly marked. &he convention be!ins 4ith #9- hard rule: 6ial irection is al4ays3 l4ays3 1W>$ parallel to shaft a6is 1et/s start 4ith horiontal3 direct drive machines. &hese machines are the most simple to define.  irection 7  7 ,uns alon! the a6is of the machine/s shaft 2. 6ial irection parallel to the shaft ) !round0.  5. Vertical irection 7 irection 7 &he shortest line possible connectin! the machine shaft and the machine base.  ou could3 dependin! on the proactivity of the pro!ram personnel3 eliminate a fe4 of the readin!s and rely on the others to tip you off that somethin! is 4ron!. >ou could then !o into full7blo4n anaysis mode and collect lots of data. (sin! that lo!ic3 the readin!s could be divided into "necessary" and "optional" readin!s as follo4s: 9ecessary ,eadin!s Ma6 +re; A of 1ines

(nits

irection %osition

2

25kcpm

=

Velocity

hori

Br! 2

5

@kcpm

=

-nvelopin!

hori

Br! 2

<

25kcpm

=

Velocity

hori

Br! 5

=

@kcpm

=

-nvelopin!

hori

Br! 5



@kcpm

=

Velocity

a6ial

Br! 5

&he necessary readin!s sho4n here monitor for early sta!e bearin! defects as 4ell as providin! !eneral information on machine health. In7depth analysis may be difficult3 dependin! on the specific problem3 and a problem developin! may re;uire more and better data be collected. #ptional  ,eadin!s Ma6 +re; A of 1ines

(nits

irection %osition

@ K

K5kcpm 25kcpm

2@ =

cceleration hori Velocity vertical

Br! 2 Br! 2



25kcpm

2@

Velocity

hori

Br! 5

J

25kcpm

=

Velocity

vertical

Br! 5

&he optional readin!s sho4n here monitor in additional planes vertical0 and for more specific problems 4ith hi!h fre;uency K5kcpm3 2@ lines0 and hi!h resolution 25kcpm3 2@ lines0 spectra.

 

atabase $etup:  +urther -6ample of $electin! %oint %arameters 1et/s look at a component other than the motor 7 a scre4 compressor note that 4e have already discussed 4hat is needed to ade;uately monitor the motor0. >ou 4ould need: 





&o collect a readin! 4ith an +ma6 of 25 cpm on each of the four bearin!s in order to monitor potential bearin! defects developin! and the common problems occurrin! at 263 56 and Lobe ass req. /%erodynamic0

teeth 6 rpm3 you 4ould 4ant to collect a readin! 4ith an +ma6 of ou 9eed &o o &o Be ble &o nalye &ime omain %lots It is e6tremely important to understand the limitations of the ++& and the unpredictability of the ++& process 4hen several problems are present simultaneously. &he time domain plot should be used 4henever applicable or in the presence of a stubborn or unusual problem. But there are three thin!s you must do to !et comfortable 4ith and !ood at analyin! time domain plots. 

6ractice

69ACTIC% 

 

69ACTIC%  Well3 that/s not entirely true 7 you also need to understand ho4 to set them up. We can/t help you 4ith the practice part but 4e can help you 4ith the setup.  

$ettin! (p &he %arameters +or   &ime omain %lot &o set up a time domain readin! can be a bit cumbersome. &his is mainly due to the fact that the setup is often done usin! ++& parameters such as +ma6 and lines of resolution. &his section 4ill first e6plain ho4 a time domain is set up and then provide some easy to use e6amples. &he time domain e;uivalent to +ma6 and lines of resolution are:   +ma6 G %eriod len!th of time sample bein! collected0  1ines of ,esolution G Bytes ho4 many pieces of data are collected to create the sample0  25 Bits is e;uivalent to 5 lines of resolution  25= bits is e;uivalent to = lines of resolution  5= bits is e;uivalent to  lines of resolution  =J@ bits is e;uivalent to 2@ lines of resolution  



o

o

o

o

s you may already kno43 a time domain plot is 8ust as susceptible to resolution limitations as an ++& is. +i!ures 23 5 and < are each from the same time domain plot 7 the latter t4o are oomed in on.

FI23RA 4

igure 2

igure 4

 

&he plot sho4n in +i!ure 2 7 an actual3 real7life time domain plot 7 4as collected 4ith 5= amplitude values the time domain e;uivalent of "lines of resolution"0. &he len!th of the time sample is .22= secs. +i!ure 5 sho4s a portion of the same time domain plot 4ith the sample reduced to .= seconds by oomin! in0. &his is done in the same manner as one 4ould oom in a an ++&. It still looks pretty !ood but 8ust as 4ith a spectrum3 oomin! in has done nothin! to improve the accuracy of the data. #nce collected3 you can never improve or in any 4ay chan!e the accuracy of any plot 7 time domain or spectrum. &he resolution is dictated by the parameters set up and cannot be altered after the fact . +i!ure < sho4s the same plot 4ith the sample reduced to only .2 seconds. It is no4 ;uite clear that the time domain plot is !enerated by compilin! a series of amplitude values and connectin! them 4ith lines 7 the same 4ay an ++& is !enerated. &his is R($& $ IM%#,&9& and must be stressed 8ust as much as 4ith the spectrum. Zoomin! in to this level does nothin! to improve the resolution and is about as helpful in vie4in! the bi! picture as lookin! at a forest 4ith your face 5 inches from a particular tree 4ould be 7 in other 4ords3 not helpful at all. $o ho4 do 4e set up a time domain readin! usin! ++& parameters from this information ?

$ettin! (p &he %arameters +or   &ime omain %lot

&he first thin! 4e need to do is fi!ure out ho4 lon! in seconds0 our time sample needs to be. *o4 do 4e do that ? Well3 it depends on 4hat 4e are tryin! to analye. 1et/s take a

 

machine rotatin! at our analyer or soft4are re;uires a time sample len!th and the number of data bits desired. If this is your option3 you/re about done. $imply choose .2 seconds or 2 msecs0 for the sample len!th or period and the correspondin! data bits for the number of amplitude values on the plot you 4ant 25 for 5 lines3 25= for = lines3 5= for  lines3 =J@ for 2@ lines0. It is recommended you collect either 5= or =J@ bits of data.  



>our analyer soft4are forces you to set the a bit readin! in ++& or parameters. In this case3 youup have more math to do. 

+or option 53 choosin! the number of lines of resolution 4e resolution 4e 4ant is strai!ht for4ard 7 simply select the number you 4ant. It is recommended that you use  lines l ines as the minimum and 4e 4ill use that in our e6ample. #nce you/ve decided on the desired resolution and you kno4 the time sample you 4ant3 use the follo4in! formula to find the +ma6 you must select:  +ma6 G A 1ines F &ime $ample  +ma6 X*ertY G  F .2 seconds G  *  





+ma6 XC%MY G 3 * 6 @ G =3 cpm 

If you 4ant 2@ lines 4ith the same len!th time sample3 you

 

4ould use:  +ma6 G 2@ F .2 seconds G 2@3 * 6 @ G J@3 cpm  

If you 4ant = lines 4ith the same len!th time sample3 you 4ould use:  +ma6 G = F .2 G =3 * 6 @ G 5=3 cpm  

9ote that you can !enerate the +ma6 in cpm directly by usin! the A lines 6 @ and dividin! it by the desired time sample:   lines 6 @ F .2 seconds G =3 F .2 G =3 cpm  

lso note that the shorter the time sample desired or !reater the resolution3 the hi!her the +ma6 selected.

$ettin! (p &he %arameters +or   &ime omain %lot 1et/s run another e6ample 4here 4e 4ant to capture 2 bearin! defect impacts on impacts on a shaft runnin! 25 rpm. Well3 first let/s convert to *: 25 cpm G 5 *. 9e6t3 4e need to kno4 the defect fre;uency. +or the e6ample3 4e 4ill use the very common outer race defect fre;uency found 8ust over