VBX Application Note

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INSTRUMENTS

TECHNICAI NOTE

StrobeLightApplications ForMachinery Problem Solving B y J o nE . P a l m

ThisApplicatron Noteis basedon an arlicle thalorlglnally appearedin Sound& Vibralron Magazine, Sepiernber 1992

H?rdyInstrume.lslic. 3960Cate Fodlnada SanDiegoCA 92123 phone:61s?78_2900 F€r]6i9 276-6700

T E C H N I C ANLO T E StrobeLightApplicationsFor l\,4achinery P r o b l e mS o l v i n g ByJonE. Palm STROBELIGHTAPPLICATIONS FOR I,4ACHINERY PROBLEI\,I SOLV]NG

t.^ havebeenusedto study thebehavior of roratrns :tlgl:ls_fosct 1?ltattng :l!99:::91."'l macnrnery, vtbrahonof components oTstructures

and for balancng f6r a number o ' V e d r s J. u n c lo n a lv . s t r o b e o h t , c a nb c v e r yo t i l c . . r r .T r , . ' , * n y l f , " p n c er d n g Li 5 s 1

c'ecioc 'tn"rrii"o"iinil' bpsr t vou suirJo rortr^ ilii; Jil |?i?1iilt;..;il.;l;T?J"i" Today.rnost.strobe lightscornestandardwilh rechargeable baneriesrnakjngthem truly , portableand easy to use in the field.The idea beh nd the strobctigntrito fire a nich intensitylight_at a speciflcfrequencyor at a precisesuO_muttiple fr"equen( utu'"t mcasdredIn llashespar mi.rJtc1FpMr.lh,s ahowstn^ tightlo o. .un.nr;tn,ljo"l,,',)] r r a c n r T e Tr uVn nn 0 s p a a d everetnlurycoutdresutt. Beltson beltdrivenmachinery canbeexamined forirregular operation usinga strobe liqht. Page2

TECHNICAL NOTE

Forexample, in the caseof multiple belts,a loosebeltwillexhibitmoremotionthanbelts thathavethe properamountof tension. Thiscondition willcausevibration. The strobelioht is tunedto the beltspeed. A stfobelightcanbe usedto checkoverspeedtripson machinery. Thisis accomplished by ilrsttuningrunningspeedandthenfollowing speedas it is slowlyincfeased uniiltrip speedis reached. At thispointreiease thetuningknoba|d readthespeedon thedigiial 0r s p r a y .

WHAT CAN BE LEARNEDBY THE ADDEDADVANTAGEOF PHASE The abilityto rneasurephasefromthe vibrationsignaloffersthe user many advantages when tryingto diagnosemachineryproblems.Vibrationphaseis the vectorialdirectionof the predominant force responsible for machinevibraiion.Thinkof ii this wav; if a rotor couldcome out of its bearings,whichway would it go? Or if a machinecouldjump oFfof its mounting,which end wouldgo flrst?Vibrationphaseprovjdesthis type of informition.One phasereadinghas one bit of information that by itselfCoesnot offei much insictht abouta v . b r d l i ocno n dl i o n .B u t a g r o u po f p h a s er p a d t n gbs c qn s t o " p a i n a l p . c l u r el"o - trt s pu s p r For example,if you thoughtyou had an unbalancecondtion,phaseanalysiscouldreveal the problemwas actuallymisalignment.

SOl,4EBASICSABOUT PHASEfv1EASUREI\,4ENTS Phaseis a relativenteasurement. Placementof the sensordeterrnines the ohasereaclino. lvhen measuringradialvibrationin a verticaland horizontal directionon a bearing,the phasereadingswill show a 90 degreedifference(a 90 degreephaseshrft).Knowinqthis is used to the analystsadvantage.lt is importantto put the sensorback in the same ldcation when tryingto repeatmeasurements for trendingor when balancing. Sincephase.is b€ingread by obseryinga partof the machinethat is rotatingwiththe strobelight,it is importantto establishsome generalpractices. One is to take alonga tripodto hold.theslrobeif you are gong-tobe takinga lot of phasemeasurenrenls. lt makesthe job easierand it also allowsfor consistency. Of course,the markbeing observedmustbe unique.The mostcommonpartof the machineto observeis the end of a shaft.lt is importantto haveonlv one marKon lne sudace.if the end of the shafthas two keyvays,markone so jt cari be distinguished from the other. It is good practiceto have a standaJdconventionfor measufingphase.Whileyou are goingto want to relatephaseshiftfindingsin termsof degrees,it is a lot easie'rto rnake your-notes. in the.fle-ld in termsof the-clockface,esp€cially when you afe beginning. That is. 12:00o clockis 0 degreesa1d 6:-0,0 o'clockis 180 degrees.see Figu.e2.-lris v"eryeasy to recognizea ma.k appeafingat 4:30 as opposedlo tfing to relateto irslocatiordirecfly' rn degrees.

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TECHNICAL NOTE

274

27A

l ac

180

Fhas€ a.d Direcaio.oi R.tabon

Figure2 PhaseAngleandtheClockFacevs. Rotation When you are settng up for your referencereadingand you are lockedontothe vibration srgnar., rotatethe.phaseadJUstknob on the strobelighito bringthe markup to the 12:00 o clocl positron.Mdlc a I other phasereadingsrelativeto thislnitia conditionbut do not rotatethe phaseadjustknob lor relativereadings.N4ake good pictorialnotesrecording p,hasereadingsuslngthe clockface referenceas you proceed lt is a good ideato pui a "R" for radlaland an "A" for axialreadingsbelowthe clbckface for refjrenceafteryou ret!rn to the shop You can not maketoo many noteswhenyou are in thc field! Remembcr,the onlygood phasejs thatwhichoccursat runningspeed.We have not l p a r n e dl o . c o n s i s t e nirnl vr e , p r epl h a s . a t m u l t t p l . o, r r u n n i n gs p - q f , B e s u . ot h - \ t r o o ^ rgr.\ooo!!o s;gnat Do not usa Ihc havphasorsi1n"r lro fg,!l.ls lne once,oer.turnv,bralion retatrvepnJsemeasurements can be madewiththe slrobelightconneciedto the iey p h a s o rs i g n a l . ll you havea data.logger that recordsphaseand the strobelightyou are usinghas a phase referencesignaloutput,you can usethe data loggerto"reiordbothvib;ationlevel anopnase.

RADIALPHASEANALYSIS phaseme_asurements N4aking in the radialdirectjonon a bearingor bearingsupport a suspectedunbatance condition. When u'nbatance ii the !robtem :tJ'l]:tlf 9"nlrm pnasewt ::lloitow tre sensor.As the sensoflocalionis movedarourd the bea,irq rhe p-ase angle remainsthe same.This is becaJSerhe hea!ryspotis rotatingwiththe sna"rt Jro'ooei not changedJe to .otation.Sincethe strobelightis fiing basedon-theselsor see nq t-e

i fihaie T o y i ntS. r es e n s oi sr r o t . g o l rtgoc h a n gweh a irt s e e si nt e r m o lI 1nay ! 9 : pcnange 9 t 9 9 9Invv, braiton tevelbutnot phase. ir

Phasein tfs caseis thedelayi1 timeo. distance (measured in degrees) fro"nwl^enthe nea\rysoorpassesDelealntne senso.untilthesenso.canrespond to t1e forces produced. Thedelaycomesfromihe "mechanical path'thattheenergymusttravel Page4

TECHNICAL NOTE

throughto get to-t\ebaseo, thesensor.(Technically referred to as thetra.sferfuncton cnaraclertslcs ol tne mechantcal system.) ASlongaSt.e mechanical pathdoesnot cn,ange as lne sensorts moved,the phaseresponse in the previous paragraph willhold changesstowty_and contiiuoJsty it is duero a separarery l:l|?]rj::gl1:-"-^.-a"l-qe rorarnguroatancecompo-nent ot ihe shail.Thiscanbe causedby a loosecollaror irlst rng. ASlne snanrotateslhe toosecomponent rorates at a sliqhtJt lowerRpl\,4 distributrno unbata.rce around the shaft, adding and"su'btract.ng vuiitaiv ir,rri" lf^i99::-"9r!9l9rt r.olarlg1 cyce. Thisresufts in a consrantty changrigheavy'spot a-d c:l?gl^^^"i^o:.!':g-ggch a u s e s ,cao n ln u o u sy c h a - n g tpr gh a s ea n g l eT. h i sc o n dt i o nw i l la s of o l j o wl h e s " ^ s o ra s |ls tocal,on rschargedradallyon ihe beaf,ng. l!:j:l.91],9g"lon is cl-arged,if rnephasea'1gte atsochdngesso the sporren-c,rrs rflt. e s a m et o L ao n ( d o e sn o t . oo w t h es e n s o rt )h pb e a f i ncga pj s b e i n gJ o . c erd1o ' l l yo r e ' direction. Severemisalignrr-nr or a lockedcoupli.qcoulj causethis. P H A S EA N DU N B A L A N C E r^quired,lor balrncingTnese spced runnirgspeeovibrario,r Iht-:",!9 _"dl:!G,"re 'rqht.cJrb. LSAdforbaare strobe a-. no \inr p rt o.o-virj,.r sp^so110 !"_"i -"lo-,!n?_.l l lc pnase oara.tr can oc connected to a datacollector whichreadsvibration butncedsa phase.refcrence signal.Somedatacollectors havebuiltin balance[rogiamsior s ng e anrl d u a lp l a n eb a l a n c i n a . Balancing is a subjectof itsownso it willnot be covered extensively in thiswriting. Whatis coveredis somehintson howthe strobeljght - mjghtbe usedto helj und.,",onoso.c c h a r a c t e r i s toi cf asn u n b a l a n cper o b l e m . A l t a ,s. e f l i n qu p l n e s t r o b el g h t a n d t d ( i n ga v - n i c d lo r h o r , / o n t ar lo 6 j i 6 q, d e p p ' l d i nog^ thn b.Jring suppodstiffn{.ss,. movpthp aenso.to the beari.qon tre orhp.end lt _ ver," t m p o q a nrro h a e pt n . ,p o \ r l t o no r i ^ n t a l o no f l h e S d n s otrh e S j m e o 1 o o t hb e a r i n g \D . o nor movethe.strobelight,onlythe sensor.Checkthe phaseby observingihe positio-n of the mark.I\y'ake notesaccofdineto the followinq:

UNBALANCE B E A R l N GP H A S ER E L A T I O N S H I P S a m eP h a s e 180' Difference 90'Diffefence L e s st h a n9 0 ' More than 90"

StaticUnbalance C o u p l eU n b a l a n c e CombinedStaticand Couple Combined,moreStatic Combined,morecouple

Tf e solution,s for correciing thesetypesof unbalance prob.ems areexpla;1ed in manv

or-inba,ance yi:'i%";ln'!?e,?i:l 6re vou g'j?:tri:t?rvpe Hif Jii"':;1f.i?i:j: Page5

TECHNICAL NOTE

AXIALPHASEANALYSIS Axial.phase measurements canrevealusefulinformatjon aboutmachinery faults.A series at a,.bear,ng gfjjial T^eqs^L.em.gnts ta(enat thetop.rght.bottomard tefi(12:00.3:Od, otuuanov:uuposttans) wttltndtcate characteristics abouttheshaft.lf thephaseana,e staystnesameso the marklollowsthesensoras it is n-oved, the bea.inois wobbln-o (nonplanar mo,tion). A bentshaftor bowin theshaftdueto unbalance rsiorcingihe " bearingawayfromthe rnachine as the roroT rurns.

Axial phasemeasuTements can be usedto determinewhichmachinein a trainis misaligned. Phaseis measuredat each bearing.Set the strobelightto fead a convenient mdrk,on.arexpos_ed shattor couplnq. A corplirg is roi a good piacetLe qq1;nr.1.6 oyl I rs onen rnp orty p]ace.with lhe sensorat one end of lhe kain, adJUSt y':!-,!ty rhe pnasehnobto see the marka1 12:00o'clock.l"4ove the sensorto the nextbearihgat the oppositeend of the firstmachineand readthe phaseby the positionof the mark.-since the sensororientationisreversedan"inphase"cohditionputsthemat