1、08FTM02AGMA Technical PaperA Methodology forIdentifying DefectiveCycloidal ReductionComponents UsingVibration Analysis andTechniquesBy V. Cochran and T. Bobak,Sumitomo Drive TechnologiesA Methodology for Identifying Defective Cycloidal ReductionComponents Using Vibration Analysis and TechniquesVirgi
2、l Cochran and Todd Bobak, Sumitomo Drive TechnologiesThe statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractForseveralyearspredictivemaintenancehasbeengainingpopularityasa
3、methodforpreventingcostlyandtime-consuming machine breakdowns. Vibration analysis is the cornerstone of predictive maintenanceprograms,andtheequationsforcalculatingexpectedvibrationfrequenciesforbearingsandtoothedgearsetsare widely available. Cycloidal reducers present a special case due to the natu
4、re of their reductionmechanism. This paper will describe a method for utilizing vibration analysis in order to identify a defectivecycloidal ring gear housing, disc, and eccentric bearing.Copyright 2008American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314
5、October, 2008ISBN: 978-1-55589-932-53A Methodology for Identifying Defective Cycloidal Reduction Components UsingVibration Analysis and TechniquesVirgil Cochran and Todd Bobak, Sumitomo Drive TechnologiesIntroductionPredictive maintenance is becoming a popularmethod for preventing costly and time-co
6、nsumingequipmentfailure. Itsappliedmostoften toassem-bly lines, where the unexpected failure of a criticalmachine can lead to thousands of lost dollars perhour of downtime. Predictive maintenance viavibration analysis can indicate, before failureoccurs, whether internal machine components areworn an
7、d require replacement. The advanced noti-ficationthatpredictivemaintenanceprovidesallowsusers to repair their machines during scheduleddowntime, thereby preventing unexpected failuresduring production hours. The benefits are mainte-nance and spare parts cost reductions along withincreases in product
8、ion time. Product quality in-creases are also realized since machines can bemaintained in good condition consistently (1).Some vibration analysissoftware packagesincludebearing frequency and gear mesh information intheir databases, and by entering systeminformation the software provides the user wit
9、h theoperating frequencies of interest.Cycloidal reducers, however, are more complexthan toothed gear systems. They contain morerolling components than toothed gear sets, whichmakes calculating the expected operating frequen-cies difficult, see figure 1. However, predictivemaintenance can still be a
10、pplied to cycloidalreducers.The rotational frequencies of bearings and toothedgear sets are common items monitored by predic-tive maintenance practitioners. Bearing rotationalfrequencies canbe obtainedfrom bearingsuppliersor calculated, given the relevant system informa-tion. Frequencies suggested f
11、or monitoring are1shaft rpm for inner race faults and 1FTF(fundamental train frequency) for rolling elementfaults (2).A cycloidal eccentric bearing presents anuncommon case since both bearing races rotate.Cycloidal discs mesh with pins or rollers locatedalong the inner perimeter of the ring gear hou
12、sing.Asthediscsandthepins(orrollers)mesh,acontactfrequency is created which can be monitored. Seefigures 2 - 4. The user can calculate the cycloidaldisc mesh frequency by using:Cycloidal Disc Mesh FrequencyDMF=No.ofPinsOutput RPM60HzSlow speedshaft rollersCycloidaldiscsRing gearhousingEccentricbeari
13、ngFigure 1. Exploded view of cycloidal reduction components4Figure 2. Cycloidal eccentric bearingFigure 3. Cycloidal discFigure 4. Cycloidal disc meshes with pins orrollersPredictive maintenance procedureThe key to successful predictive maintenance pro-grams for cycloidal reducers and gearmotors is
14、torecord a baseline of a cycloidal units vibrationfrequenciesforuseareference assoon aspossibleafteraunitisplacedintoservice. Aslongastheunitis installed securely, aligned correctly, and itsoperating loads and conditions match those usedfor the unit selection, the resulting baseline will beaccurate.
15、A predictive maintenance program for cycloidalspeed reducers and gearmotors utilizing vibrationanalysis is implemented as follows:1. Install the unit according to the manufacturersinstructions,makingsuretheunitismountedse-curely and aligned correctly. A poor installationwill result in an inaccurate
16、baseline.2. Run the unit under nominal application condi-tions until it reaches operating temperature.3. Record a vibration measurement, and note theamplitudes of the eccentric cam and the eccen-tric bearing fundamental train frequencies.4. Store this measurement record for future refer-ence. This i
17、s the baseline against which allfuture measurements should be compared.5. Periodically record a vibration measurementand compare it against the baseline, checkingfor vibration amplitude increases. The mea-surement interval will depend on a unitsoperating loads and conditions. Its recom-mended to rec
18、ord measurements frequently atthe beginning of a predictive maintenanceprogram, and then adjust the interval accordingto the units maintenance requirements (3).6. Whenvibrationamplitudeincreasesarenotedatthe eccentric cam or the eccentric bearing fun-damental train frequencies, eccentric bearingwear
19、 may have begun. Decrease the vibrationmeasurement interval in order to compare theunits condition to its baseline more often.7. Oncevibrationamplitude increasesare notedathigher frequencies, schedule the unit for repair.Advanced wear has most likely begun, so theunit should be removed from service
20、in order toreplace worn internal components beforecatastrophic failure occurs.Test procedureA cycloidal reducer was returned which had suf-fered the typical damage associated with overloador inadequate lubrication: a worn eccentricbearing,pressuremarksinthesurfaceofthecycloidaldiscscenter holes, and
21、 burned grease. With theexception of burned grease, this damage also5occursattheendofaproperlyselectedunitsusefullife. Therefore,itwasselectedasthesubjectforthisstudy. The units eccentric bearing FTF wascalculated to be 11.20 Hz, and its cam frequencywasdeterminedtobe29.17Hz. ThecycloidalDMFwas calc
22、ulated as 14.75 Hz.A new, identical unit was assembled in order to ob-tain a baseline measurement. After obtaining thebaseline with a vibration data collector as shown infigure 5, the new units eccentric bearing, cycloidaldiscs, and ring gear assembly were exchanged forthose from the damaged unit in
23、 a progressive man-ner in order to simulate the progression of typicalwear. If a units eccentric bearing becomes wornand the unit continues to operate, wear progressesto the cycloidal discs. Further operation leads towearwithintheringgearassembly. Thewearspro-gression is due to the flow of metallic
24、particles be-tween the internal components by way of the lubri-cant.During the last phase, the worn components wereinstalled in the new unit individually in an attempt toisolate the operating frequencies characteristic ofthose components. The ring gear assembly fre-quencies were of particularinteres
25、t, forthey aredif-ficult to calculate.Testing ProcedureAll tests were conducted under full load.1. Thereturnedunitwasmeasuredwith avibrationanalyzer. This provided the data for the totalfailure condition.2. A new unit identical to the returned unit wasassembled.3. The new unit was measured with the
26、vibrationanalyzer to record a baseline.4. The new units eccentric bearing was ex-changed for the returned units worn eccentricbearing, and the vibrations of the new unit weremeasured. This provided data for the failuremode of eccentric bearing damage.5. Keeping the worn eccentric bearing in the newu
27、nit, the units cycloidal discs were exchangedfor the returned units discs. The vibrations ofthe new unit having the worn eccentric bearingand cycloidal discs were measured to obtaindata showing the next stage of wear.Vibration analyzerLoad unitVibration sensorTest unitFigure 5. Vibration measurement
28、 set-up66. With the worn eccentric bearing and discs in thenew unit, the returned units ring gear assemblywasinstalled,seefigure6. Thenewunitsvibra-tions were measured to obtain data showingthefinal level of progressive cycloidal failure ec-centric bearing failure combined with disc andring gear ass
29、embly damage.Figure 6. Cycloidal ring gear assembly7. Theneweccentricbearingandringgearassem-bly were returned tothe newunit. With theworncycloidaldiscsstillinthenewunit,thenewunitsvibrations were measured to obtain data show-ing how cycloidal disc wear contributes to thevibration graph.8. The cyclo
30、idal discs and ring gear assemblieswere exchanged between the units, so that thenewunithad thenew eccentricbearing, thenewcycloidal discs, and the worn ring gear assem-bly. The new unit was measured to obtain datashowing how ring gear assembly wear contrib-utes to the vibration graph.The load and te
31、st units were identical, and theinputmotors operated at a 1750 rpm input speed. A CSI(Computational Systems Incorporated) 2120singlechannel machinery analyzer, model A212001, wasused to measure the test units vibration. The ana-lyzers parameters were established as shown infigure 7.The vibration mea
32、surements were documentedus-ing CSIs MasterTrend RBMWare version 4.60,dated September 4, 2001.Figure 7. Vibration analyzer measurement parameters7ResultsItsimportanttonotethatthecycloidaloperatingfre-quencies a customer will see on a vibration graphwill depend on a units size and speed reduction ra-
33、tio. The graphs shown in figures 8 19 and 25 - 48are specific to the units we tested. The operatingfrequencies we measured are not applicable to allcycloidal reducers.Figures813showthevibrationgraphsacustomermight see if the customer were to measure a worncycloidal reducer. Figure 6 reveals some act
34、ivity.An evaluation of the vibrations in more detail, how-ever, reveals areas of concern. In figure 10 we seethe acceleration frequencies. The presence ofpeaksatthehigherfrequenciesindicatestheremayberollerbearingproblems. Sidebandsaccompany-ingthesepeaksareanotherindication. Atthispoint,the operati
35、ng frequencies of the eccentric bearingshould be examined.Figure 8. Waveform - returned unitFigure 9. Velocity frequency graph - returned unitFigure 10. Acceleration frequency graph - returned unit8The velocity frequencies were accessed with thevibration analysis software and the cursor wasplacedaro
36、und29Hz,theeccentricbearingcamfre-quency. Figure 11 shows the result. The peak ve-locity occurs at that frequency, and its second har-monic is not much less.The cursor was next placed around 11.20 Hz, theeccentricbearingsFTF,tocheckforeccentricbear-ing roller faults. Many harmonics were discovered,w
37、ith the higher ones having sidebands. See figure12 below.Finally, the cycloidal disc mesh frequency of 14.75Hz was checked. Figure 13 shows that harmonicsare present, but their velocity levels are low.If this unit were located in a noisy environment, itwould be difficult to hear its operation. Such
38、an en-vironment would make a baseline reference morevaluable. The customer in this case did not detectthe progression of wear, so the unit wasnt returneduntil it failed catastrophically after three months.Lets examine how a baseline measurement wouldhave been useful.Figure 11. Eccentric bearing cam
39、frequency graph - returned unitFigure 12. Eccentric bearing rollers frequency graph - returned unitFigure 13. Cycloidal disc mesh frequency graph - returned unit9Figures 14 19 show a baseline recorded from thenew unit. If customers record such a baseline im-mediately after installing a unit, then fu
40、ture mea-surements can be compared to it so that wear canbedetectedbeforecatastrophicfailureoccurs. Thisis especially useful in noisy environments, wheremachine component noise can be difficult to differ-entiate.Eachfigurehasacorrespondingoneamongfigures8 13. The comparison of figure 14 with figure
41、8shows that the returned unit creates much higheracceleration values than the new one. Clearly,somethinginthereturnedunit haschanged. Figure15vs.figure9andfigure 16vs. figure10 revealam-plitude increases and sidebands at the higher fre-quencies. These changes suggest defective ele-ments among the cy
42、cloidal reduction components.The same effects are noted when the componentsare evaluated by comparing figures 17 19 againstfigures 11 13. These observations signal thatsome amount of wear has occurred to all of them.Figure 14. Waveform - new unitFigure 15. Velocity frequency graph - new unitFigure 1
43、6. Acceleration frequency graph - new unit10Figure 17. Eccentric bearing cam frequency graph - new unitFigure 18. Eccentric bearing rollers frequency graph - new unitFigure 19. Cycloidal disc mesh frequency graph - new unitIn an overloaded or inadequately lubricated condi-tion, the reducer will over
44、heat. Undetected, theoverheating causes lubrication failure anddiscolorationofthebearingelements. Pitting,whichprogresses to flaking and sometimes spalling,results shortly thereafter. Figure 20 shows theappearance of the worn eccentric bearing that wasremoved from the returned unit.Once flakes from
45、the eccentric bearing are free toflow between the components via the lubricant, theflakes will create indentations in the contactsurfaces of the cycloidal disc center holes. Thereturned unit suffered such damage, as shown infigure21. Additionally,theringgearassemblycom-ponents will suffer wear. Figu
46、res 22 24 illustratethe Micropitting and abrasion that occurred to thereturned units components.Asdescribedinsteps4 - 6ofthe testingprocedure,theworncomponentswere substitutedinto thenewunitinamannerthatfollowstheprogressionofinter-nal wear. Figures 25 30 were obtained after thenew units eccentric b
47、earing was replaced with thewornunits eccentricbearing. Increases inall ofthefrequency amplitudes are noticeable.11a) Eccentric bearing rollersb)EccentricbearingcamFigure 20. Eccentric bearing damage due tooverload or inadequate lubricationFigure 21. Indentations in a cycloidal disccenter holeFigure
48、 22. Micropitting of the cycloidal disclobesFigure 23. Micropitting of the ring gear pinsFigure 24. Mild abrasion in the ring gear pin bores12Figure 25. Waveform - new unit with the worn eccentric bearing; see figures 8 and 14 forcomparisonFigure 26. Velocity frequency graph - new unit with the worn
49、 eccentric bearing; see figures 9and 15 for comparisonFigure 27. Acceleration frequency graph - new unit with the worn eccentric bearing; see figures10 and 16 for comparisonFigure 28. Eccentric bearing cam frequency graph - new unit with the worn eccentric bearing;see figures 11 and 17 for comparison13Figure 29. Eccentric bearing rollers frequency graph - new unit with the worn eccentric bearing;see figures 12 and 18 for comparisonFigure 30. Cycloidal disc mesh frequency graph - new unit with the worn eccentri
