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DGZfPProceedingsBB90CDLecture46EWGAE2004469ThesourceofAcousticEmissionduringmeshingofspurgearsCheeKeongTan,DavidMbaSchoolofEngineering,CranfieldUniversity,Bedfordshire.MK430AL,UKTel4401234754681,Fax4401234751566.Emailc.k.tancranfield.ac.ukandd.mbacranfield.ac.ukAbstractConditionmonitoringofgearswithvibrationanalysisandSpectrometricOilAnalysisProgramsSOAParewidelyacceptedandusedinthemilitaryaviationindustry.WhilsttheliteraturetodatehasshowntheapplicationoftheAcousticEmissionAEtechniquetogearboxhealthmonitoringstillbeinitsinfancy,thereexistopportunitiestodevelopthetechniqueintoacomplementarydiagnostictool.Indevelopingsuchatool,itisimperativethatanunderstandingofthesourceofAEactivityisestablished.ThispaperpresentsexperimentalresultsthatexploretheAEsourceduringagearmeshandhasledtoquiteuniqueandsignificantobservations.KeywordAcousticEmission,asperitycontact,conditionmonitoring,geardefectdiagnosis,machinehealthmonitoring,rollingfriction,slidingfriction.LiteraturereviewsApplicationofvibrationanalysistogearfaultdiagnosisandmonitoringhasbeenwidelyinvestigatedanditsusageinindustryiswellestablished.Thisisparticularlyreflectedintheaviationindustrywherehelicopterengines,transmissionsystems,drivetrainsandrotorsystemshaveadoptedvibrationanalysisforhealthmonitoring.However,researchintheapplicationofAEtogearfaultdetectionandmonitoringislimited.Themostcommonfailuresencounteredinoperationalgearboxesincludemicropitting,pitting,scuffingandabrasivewear.Althoughtoothfractureandbendingfatiguearerare,thecriticalityofsuchafailurehasdrawnhugeattentionfromresearchers.InexaminationoftheAEtechniqueonthesefailuremodes,mostresearchershaveoptedforsimulatedpitdefects.Singh1,2,Tandon3andSiores4performedtheirexperimentsusingsimulatedpits,whilstToutountzakis5,Sentoku6andMiyachika7allowednaturaldefectssuchaspittingtooccurduringthetests.TheconclusiondrawnfromalltheseexperimentswereencouragingAEtechniquewasabletodetectbothseededandnaturaldefects.Amongtheseresearchers,onlyToutountzakisandSentokuemployedaslipringtotransmittheAEdatafromtherotatingAEsensortotheacquisitionsystems,therebyofferingadirecttransmissionpath.OthersmountedtheirAEsensorsonthebearingorgearboxcasingand,claimedsuccessfulidentificationofdefectivegears.ThepapersreviewedhaveillustratedthepotentialandviabilityoftheAEtechniqueinbecomingausefuldiagnostictoolinconditionmonitoringofgears.However,nonehaveDGZfPProceedingsBB90CDLecture46EWGAE2004470investigatedthesourceofAEactivityduringthegearmesh.AnunderstandingofthefundamentalAEsourcemechanisminmeshinggearsisofvitalimportanceindevelopingthistechnique.ObservationofAEburstsduringgearmeshAEdatawasrecordedduringsimulateddefectgeartests.AninterestingobservationwastheAEtransientsassociatedwiththegearmesh.Withasamplingrateof10MHzandrotationalspeedofapproximately742rpm,only16meshingteethsignatureswererecordedperacquisitionwindow.Figure1showsthetimedomainofanAEsignaturerecordedduringthetestsundertwodifferentloadconditionsclearlyshowingtheAEtransientresponseassociatedwithgearmeshingof16teeth.ThisAEtransientresponseledtheauthorstoinvestigatethepossiblesourceofAEactivityduetothemeshinggear.TheresultspresentedwereacquiredfromtheAEsensormountedonthepinionwheelandAEdatawascapturedwithacommercialdataacquisitioncardPACviaaslipring.00.0050.010.0150.020.0250.10.0500.050.1AmplitudevTimeseconds00.0050.010.0150.020.0250.10.0500.050.1TimesecondsAmplitudevFigure1TimedomainofanAEsignatureshowingclearlytheAEtransientresponseassociatedwithgearmeshingof16teethfortherotationalspeedof745rpmundervariousloadspreamplification20dBAESourceMechanismsFromtheobservationspresentedthusfar,theauthorsbelievetherearethreepossiblesourcesofAEactivityduringthegearmeshtoothresonance,secondarypressurepeakinlubricatedgearsand/orasperitycontacts.DGZfPProceedingsBB90CDLecture46EWGAE2004471ToothresonanceEstimationofthetoothresonancefrequencywasaccomplishedbymodellingthegeartoothascantileverbeamandspringmasssystem.Basedonthegeartoothgeometry,loadingconditionsandontheassumptionthattheloadistransmittedalongthepitchline,thetoothresonancefrequencywascalculatedat75kHz.Thisisbelowthefrequencyrange100to1200kHzoftheAEsensoremployed.Inactualgearsystems,alowermomentofinertiawouldbeexpectedresultinginalowernaturalfrequencythanthatcalculated.ThiseliminatedgeartoothresonanceasasourceofAEactivityduringgearmesh.SecondarypressurepeakinthelubricantTheotherpossiblesourceofAEmechanisminthemeshinggearscouldoccurasaresultofthepressuredistributionbetweenthegearteethsurfacesandthelubricatingoilfilm.Thispressuredistributionisstronglyinfluencedbythegearoperatingconditionssuchasload,speedandmaterialproperties,asdepictedinfigure29.Fromthisfigure,theoccurrenceofalocalpressurepeakfarinexcessoftheHertzainmaximumisobserved.Thishastheeffectofreducingthefilmthicknessatthepositionofthesecondarypressurepeak.Thedecreaseinfilmthicknessatthisparticularpositioncanbeabrupt,dependingonthesurfacevelocity.ThissuddenincreaseinpressureordecreaseinfilmthicknesscouldbeasourceofAEactivityalthoughtheauthorscannotprovethisatpresent.Itmaybeworthnotingthatloadhasnoinfluenceonthefilmthicknessorlevelofsecondarypeak.Iftheraisetimeforthesecondarypressurepeakisintheorderof0.8to10µs,thesecondarypressurepeakcouldgenerateAEtransientburstasobservedfromthegearmeshAEbursts.Thispostulationassumesisothermalconditionsunderpurerolling,howeverthisisnotthecasewithinthegearmeshwhererollingandslidingarebothexperienced.FrictionandAsperityContactsDuringthegearmeshsliding,rollingoracombinationofbothwilloccur.Asthegearteethsurfacesarelimitedtomanufacturingcapabilitiesapproximately2µmasperitycontactswilloccur10duringmeshingonalmostallgears,particularlyasthecalculatedoilfilmthicknessapproximately1µminthisinstanceislessthanthecompositeroughness.DGZfPProceedingsBB90CDLecture46EWGAE2004472Figure2Pressuredistributionsforacompressiblelubricantwithincreasingspeedsfromplots0to59.Duringexperiments,Smith11notedtransientshockpulsesduringthegearmesh.Itwasconcludedthattheseshockswereattributedtoasperitycontact.Whilstasingleasperitymodelwaspresentedastheprobablecauseoftheshocks,thelikelihoodofsuchscenarioinpracticeislimited,multiplecontactswillbepresent.However,itwasshownthatbasedonasperitywidthof5µmandslidingandrollingvelocitiesoftheorderof500mm/s,theraisetimeforsuchatransienteventwas10µs.ItmustbenotedthatthesensorsemployedbySmithhadanaturalfrequencyof50kHzandisoutsidetherangeofAE.Forthisparticularinvestigation,anasperitywidthofapproximately2µm,withapitchlinerollingvelocityoflessthan2000mm/s,canprovidearaisetimelessthan10µswhichwillbedetectedbytheAEsensoremployed.AsthegearmeshinvolvesrollingandslidingofmatinggearsitisworthnotingthatBonesetal8suggestedthatAEactivitywasmostlikelyduetoasperitycontactduringsliding.InrelatingAEtoslidingfrictionDornfeld12etalhaveshownthehighsensitivityofAEtoslidingspeedandappliedload.ItwasnotedthatthebasicmechanismforAEgenerationwastheelasticdeformationofthematerialatasperitycontacts.Therangeofsurfacefinishforthematerialsinvestigatedwasfrom1to4µmcomparablewiththegearstestedinthisstudy.ItwasnotedthatthestrengthofAEamplitudewasdependentonsurfaceroughness,inadditiontoslidingspeedandappliedload.TheobservationsofSmith11,Bones8andDornfeld12pointtoasperitycontactasthemainsourceofAEactivityforlubricatedanddrycontacts.AssuchconstanttemperaturetestswereperformedbytheauthorstoaidAEsourceidentificationduringgearmeshing.Sincetheoiltemperatureswerekeptconstant,theoilviscosityandfilmthicknesswillremainconstantduringtheexperiment.ItwasthoughtthatthismayconfirmtheauthorsviewthattheAEsourceisduetofriction.Thegearboxwasrunat745rpmwithaloadof220Nmfor5hoursatwhichtimetheoiltemperaturestabilised.ThegearboxwasbroughttoastopandadjustedtonoloadLocalpressurepeak
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