齿轮失效分析.docx

齿轮失效分析.docx

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齿轮失效分析

HowtoAnalyzeGearFailures

RobertL.ErrichelloandJaneMuller,Geartech

Whenanimportantgearfailureoccurs,someonebecomesresponsibleforanalyzingthefailure,determiningitscauseandrecommendingasolution.Acompanycanselectitsownengineer,anoutsideconsultantorboth.Ifaconsultantiscalledin,thisshouldbedoneasearlyintheprocessaspossible.

Thoughsimilarproceduresapplytoanyfailureanalysis,thespecificapproachcanvarydependingonwhenandwheretheinspectionismade,thenatureofthefailureandtimeconstraints.

Whenandwhere.Ideally,theengineerconductingtheanalysisshouldinspectthefailedcomponentsassoonafterfailureaspossible.Ifanearlyinspectionisnotpossible,someoneatthesitemustpreservetheevidencebasedoninstructionsfromtheanalyst.

Ifasuitablefacilityfordisassemblingandinspectingthegearboxisnotavailableonsite,itmaybenecessarytofindanalternatelocationorbringthenecessaryequipmenttothesite.

Natureoffailure.Thefailureconditionscandeterminewhenandhowtoconductananalysis.Forexample,ifthegearsaredamagedbutstillabletofunction,thecompanymaydecidetocontinuetheiroperationandmonitortherateatwhichdamageprogresses.Inthiscase,samplesofthelubricantshouldbecollectedforanalysis,thereservoirdrainedandflushedandthelubricantreplaced.

Ifgearboxreliabilityiscrucialtotheapplication,thegearsshouldbeexaminedbymagneticparticleinspectiontoensurethattheyhavenocracks.Themonitoringphasewillconsistofperiodicallycheckingthegearsfordamagebyvisualinspectionandbymeasuringsoundandvibration.

Timeconstraints.Insomesituations,thehighcostofshuttingdownequipmentlimitsthetimeavailableforinspection.Suchcasescallforcarefulplanning.Forexample,dividingtasksbetweentwoormoreanalystsreducesthetimerequired.

PreparingforInspection

Beforevisitingthefailuresite,interviewacontactpersonlocatedatthesiteandexplainwhatyouneedtoinspectthegearboxincludingpersonnel,equipmentandworkingconditions.

Requestaskilledtechniciantodisassembletheequipmentunderyourdirection.But,makesurethatnoworkisdoneonthegearboxuntilyouarrive.Thismeansnodisassemblyorcleaning.Otherwise,awell-meaningtechniciancouldinadvertentlydestroyevidence.

Verifythatthegearboxdrawings,disassemblytoolsandadequateinspectionfacilitiesareavailable.

Askforasmuchbackgroundinformationaspossible,includingmanufacturer抯partnumbers,gearandbearingruntime（hr）,servicehistoryandlubricanttype.

Now,it抯timetoassembleyourinspectionequipment,includingitemssuchasamagnifyingglass,measuringtools,felttipmarkers,lubricantsamplingequipmentandphotographicequipment.Awell-designedsetofinspectionformsforthegearbox,gearsandbearingsshouldbeatthetopofyourprioritylist.

FailureInspection

Beforestartingtheinspection,reviewthebackgroundinformationandservicehistorywiththecontactperson.Theninterviewthoseinvolvedinthedesign,installation,operation,maintenanceandfailureofthegearbox.Encouragethemtotelleverythingtheyknowaboutthegearboxeveniftheyfeelitisnotimportant.

Aftercompletingtheinterviews,explainyourobjectivestothetechnicianwhowillbeworkingwithyou.Reviewthegearboxassemblydrawingswiththetechnician,checkingforpotentialdisassemblyproblems.

Visualexamination.Beforedisassemblingthegearbox,thoroughlyinspectitsexterior.Useaninspectionformasaguidetoensurethatyourecordimportantdatathatwouldotherwisebelostoncedisassemblybegins.Forexample,theconditionofsealsandkeywaysmustberecordedbeforedisassembly.Otherwise,itwillbeimpossibletodeterminewhenanydamagemayhaveoccurredtotheseparts.Geartoothcontactpatternsshouldbetakenbeforecompletelydisassemblingthegearbox.

Aftertheexternalexamination,disassemblethegearboxandinspectallinternalcomponents,bothfailedandundamaged.Examinecloselythefunctionalsurfacesofgearteethandbearingsandrecordtheircondition.Beforecleaningtheparts,lookforsignsofcorrosion,contaminationandoverheating.

Aftertheinitialinspection,washthecomponentswithsolventsandre-examinethem.Thisexaminationshouldbeasthoroughaspossiblebecauseitisoftenthemostimportantphaseoftheinvestigationandmayyieldvaluableclues.Alow-powermagnifyingglassandpocketmicroscopearehelpfultoolsforthisexamination.

Itisimportanttoinspectthebearingsbecausetheyoftenprovidecluesastothecauseofgearfailure.Forexample:

∙Bearingwearcancauseexcessiveradialclearanceorendplaythat

misalignsthegears.

∙Bearingdamagemayindicatecorrosion,contamination,electricaldischargeorlackoflubrication.

∙Plasticdeformationbetweenrollersandracewaysmayindicateoverloads.

∙Gearfailureoftenfollowsbearingfailure.

Geartoothcontactpatterns.（Completethisstepbeforedisassemblinggearboxcomponentsforinspection）.Thewayinwhichmatinggearteethcontactindicateshowwelltheyarealigned（Figure1）.Ifpractical,recordtoothcontactpatternsundereitherloadedorunloadedconditions.Forno-loadtests,painttheteethofonegearwithmarkingcompound.Then,rolltheteeththroughmeshsothecompoundtransfersthecontactpatterntotheunpaintedgear.Liftthepatternfromthegearwithscotchtapeandmountitonpapertoformapermanentrecord.

Forloadedtests,paintseveralteethononeorbothgearswithmachinist抯layoutlacquer.Runthegearsunderloadforasufficienttimetowearoffthelacquerandestablishthecontactpatterns.Photographthepatternstoobtainapermanentrecord.

Documentobservations.Describeallimportantobservationsinwriting,usingsketchesandphotographswhereneeded.Identifyandmarkeachcomponent（includinggearteethandbearingrollers）,soitisclearlyidentifiedinthewrittendescription,sketchesandphotographs.Itisespeciallyimportanttomarkallbearings,includinginboardandoutboardsides,sotheirlocationandpositioninthegearboxcanbedeterminedlater.

Describecomponentsinaconsistentway.Forexample,alwaysstartwiththesamepartofabearingandprogressthroughthepartsinthesamesequence.Thishelpstoavoidoverlookinganyevidence.

Concentrateoncollectingevidence,notondeterminingthecauseoffailure.Regardlessofhowobviousthecausemayappear,donotformconclusionsuntilalltheevidenceisconsidered.

Geargeometry.Theloadcapacityofthegearsetwillneedtobecalculatedlater.Forthispurpose,obtainthefollowinggeometrydata,eitherfromthegearsandgearhousingortheirdrawings:

∙Numberofteeth

∙Outsidediameter

∙Facewidth

∙Gearhousingcenterdistanceforeachgearset

∙Wholedepthofteeth

∙Tooththickness（bothspanandtoplandmeasurement）

Specimensforlaboratorytests.Duringtheinspection,youwillbegintoformulatehypothesesregardingthecauseoffailure.Withthesehypothesesinmind,selectspecimensforlaboratorytesting.Takebrokenpartsforlaboratoryevaluationor,ifthisisnotpossible,ensurethattheywillbepreservedforlateranalysis.

Oilsamplescanbeveryhelpful.But,aneffectivelubricantanalysisdependsonhowwellthesamplerepresentstheoperatinglubricant.Totakesamplesfromagearboxdrainvalve,firstdiscardstagnantoilfromthevalve.Thentakeasampleatthestart,middleandendofadraintoavoidstratification.Tosamplefromthereservoir,drawsamplesfromthetop,middleandnearthebottom.Examinetheoilfilterandmagneticplugforweardebrisandcontaminants.

Samplesfromtheoilstoragedrumorreservoircanuncoverproblemssuchasexcessivewaterintheoilduetoimproperstorage.

Haveyougotitall?

Beforeleavingthesite,makesurethatyouhaveeverythingneeded（completedinspectionforms,writtendescriptionsandsketches,photosandtestspecimens）forcompletingthefailureanalysis.

DetermineTypeofFailure

Nowit抯timetoexaminealloftheinformationanddeterminehowthegear（orgears）failed.

Severalfailuremodesmaybepresentandyouneedtoidentifywhichistheprimarymode,andwhicharesecondarymodesthatmayhavecontributedtofailure.Table1listssixgeneralclassesofgearfailuremodes,ofwhichthefirstfourarethemostcommon.Anunderstandingofthesefourcommonmodeswillenableyoutoidentifythecauseoffailure.

1.Bendingfatigue.Thiscommontypeoffailureisaslow,progressivefailurecausedbyrepeatedloading.Itoccursinthreestages:

∙Crackinitiation.Plasticdeformationoccursinareasofstressconcentrationordiscontinuities,suchasnotchesorinclusions,leadingtomicroscopiccracks.

∙Crackpropagation.Asmoothcrackgrowsperpendiculartothemaximumtensilestress.

∙Fracture.Whenthecrackgrowslargeenough,itcausessuddenfracture.

Asafatiguecrackpropagates,itleavesaseriesof揵eachmarks?

（visibletothenakedeye）thatcorrespondtopositionswherethecrackstopped（Figure2）.Theoriginofthecrackisusuallysurroundedbyseveralconcentriccurvedbeachmarks.

Mostgeartoothfatiguefailuresoccurinthetoothrootfillet（Figure3）wherecyclicstressislessthantheyieldstrengthofthematerialandthenumberofcyclesismorethan10,000.Thisconditioniscalledhigh-cyclefatigue.Alargepartofthefatiguelifeisspentinitiatingcracks,whereasashortertimeisrequiredforthecrackstopropagate.

Stressconcentrationsinthefilletoftencausemultiplecrackorigins,eachproducingseparatecracks.Insuchcases,crackspropagateondifferentplanesandmayjointofor