外文翻译汽车差速器小齿轮轴的失效分析.docx

外文翻译汽车差速器小齿轮轴的失效分析.docx

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外文翻译汽车差速器小齿轮轴的失效分析

Failureanalysisofanautomobiledifferentialpinionshaft

Abstract

Differentialisusedtodecreasethespeedandtoprovidemomentincreasefortransmittingthemovementcomingfromtheenginetothewheelsbyturningitaccordingtothesuitableangleinvehiclesandtoprovidethatinnerandouterwheelsturndifferently.Piniongearandshaftattheentrancearemanufacturedasasinglepartwhereastheyareindifferentformsaccordingtoautomobiletypes.Mirrorgearwhichwillworkwiththisgearshouldbecomefamiliarbeforetheassembly.Incaseofanybreakdown,theyshouldbechangedasapair.Generally,inthesesystemsthereareweardamagesingears.Thegearinspectedinthisstudyhasdamageasaformofshaftfracture.

Inthisstudy,failureanalysisofthedifferentialpinionshaftiscarriedout.Mechanicalcharacteristicsofthematerialareobtainedfirst.Then,themicrostructureandchemicalcompositionsaredetermined.Somefractographicstudiesarecarriedouttoassesthefatigueandfractureconditions.

Keywords:

Differential;Fracture;Powertransfer;Pinionshaft

1.Introduction

Thefinal-drivegearsmaybedirectlyorindirectlydrivenfromtheoutputgearingofthegearbox.Directlydrivenfinaldrivesareusedwhentheengineandtransmissionunitsarecombinedtogethertoformanintegralconstruction.Indirectlydrivenfinaldrivesareusedattherearofthevehiclebeingeithersprungandattachedtothebodystructureorunsprungandincorporatedintherear-axlecasing.Thefinal-drivegearsareusedinthetransmissionsystemforthefollowingreasons[1]:

（a）toredirectthedrivefromthegearboxorpropellershaftthrough90°and,

（b）toprovideapermanentgearreductionbetweentheengineandthedrivingroad-wheels.

Invehicles,differentialisthemainpartwhichtransmitsthemovementcomingfromtheenginetothewheels.Onasmoothroad,themovementcomestobothwheelsevenly.Theinnerwheelshouldturnlessandtheouterwheelshouldturnmoretodotheturningwithoutlateralslippingandbeingflung.Differential,whichisgenerallyplacedinthemiddlepartoftherearbridge,consistsofpiniongear,mirrorgear,

differentialbox,twoaxlegearandtwopinionspidergears.

AschematicillustrationofadifferentialisgiveninFig.1.ThetechnicaldrawingofthefracturedpinionshaftisalsogiveninFig.2.Fig.3showsthephotographofthefracturedpinionshaftandthefracturesectionisindicated.

Indifferentials,mirrorandpiniongeararemadetogetusedtoeachotherduringmanufacturingandthesameserialnumberisgiven.Bothofthemarechangedonconditionthatthereareanyproblems.Inthesesystems,thecommondamageisthewearofgears[2–4].Inthisstudy,thepinionshaftofthedifferentialofa

minibushasbeeninspected.Theminibusisadieselvehicledrivenattherearaxleandhasapassengercapacityof15people.Maximumenginepoweris90/4000HP/rpm,andmaximumtorqueis205/1600Nm/rpm.Itstransmissionboxhasmanualsystem（5forward,1back）.Thedamagewascausedbystoppingandstartingtheminibusatatrafficlights.Inthisdifferential,entranceshaftwhichcarriesthepiniongearwasbroken.Variousstudieshavebeenmadetodeterminethetypeandpossiblereasonsofthedamage.

Theseare:

_studiescarriedouttodeterminethematerialoftheshaft;

_studiescarriedouttodeterminethemicro-structure;

_studiesrelatedtothefracturesurface.

ThereisacloserphotographofthefracturedsurfacesandfractureareainFig.4.Thefracturewascausedbytakingoutcircularmarkgearseeninthemiddleofsurfaces.

Fig.1.Schematicoftheanalyseddifferential.

Fig.2.Technicaldrawingoftheanalysedpinionshaft

Fig.3.Thepictureoftheundamageddifferentialpinionanalysedinthestudy

Fig.4.Photographsoffailedshaft

2.Experimentalprocedure

Specimensextractedfromtheshaftweresubjectedtovarioustestsincludinghardnesstestsandmetallographicandscanningelectronmicroscopyaswellasthedeterminationofchemicalcomposition.Alltestswerecarriedoutatroomtemperature.

2.1.Chemicalandmetallurgicalanalysis

Chemicalanalysisofthefractureddifferentialmaterialwascarriedoutusingaspectrometer.ThechemicalcompositionofthematerialisgiveninTable1.ChemicalcompositionshowsthatthematerialisalowalloycarburisingsteeloftheAISI8620type.

Hardenabilityofthissteelisverylowbecauseoflowcarbonproportion.Therefore,surfaceareabecomeshardandhighlyenduring,andinnerareasbecomestoughbyincreasingcarbonproportiononthesurfaceareawithcementationoperation.Thisisthekindofsteelwhichisgenerallyusedinmechanicalpartssubjecteddotorsionandbending.Highresistanceisobtainedonthesurfaceandhighfatigueendurancevaluecanbeobtainedwithcompressiveresidualstressbymakingthesurfaceharder[5–7].

Inwhichalloyelementsdistributethemselvesincarbonsteelsdependsprimarilyonthecompound-andcarbide-formingtendenciesofeachelement.Nickeldissolvesintheaferriteofthesteelsinceithaslesstendencytoformcarbidesthaniron.Siliconcombinestoalimitedextentwiththeoxygenpresentinthesteeltoformnonmetallicinclusionsbutotherwisedissolvesintheferrite.Mostofthemanganeseaddedtocarbonsteelsdissolvesintheferrite.Chromium,whichhasasomewhatstrongercarbide-formingtendencythaniron,partitionsbetweentheferriteandcarbidephases.Thedistributionofchromiumdependsontheamountofcarbonpresentandifotherstrongercarbide-formingelementssuchastitaniumandcolumbiumareabsent.Tungstenandmolybdenumcombinewithcarbontoformcarbidesifthereissufficientcarbonpresentandifotherstrongercarbide-formingelementssuchastitaniumandcolumbiumareabsent.Manganeseandnickellowertheeutectoidtemperature[8].

PreliminarymicrostructuralexaminationofthefaileddifferentialmaterialisshowninFig.5.ItcanbeseenthatthematerialhasamixedstructureinwhichsomeferriteexistprobablyasaresultofslowcoolingandhighSicontent.HighSicontentinthistypeofsteelimprovestheheattreatmentsusceptibilityaswellasanimprovementofyieldstrengthandmaximumstresswithoutanyreductionofductility[9].Ifthemicrostructurecannotbeinvertedtomartensitebyquenching,areductionoffatiguelimitisobserved.

Table1

Chemicalanalysisofthepiniongearmaterial（wt%）

FeCSiMnPSCrMoNi96.920.2350.2520.7860.0440.0160.4810.1510.517andfracturesurfaces.

Fig.5.Microstructureofthematerial（200·）.

ThereareareaswithcarbonphaseinFig.5（a）.ThereisthetransitionboundaryofcarburisationinFig.5（b）and（c）showsthematrixregionwithoutcarburisation.Asfarasitisseeninthesephotographs,thepiecewasfirstcarburised,thenthequenchingoperationwasdoneandthantempered.Thissituationcanbeunderstoodfromblindmartensiteplates.

2.2.Hardnesstests

ThehardnessmeasurementsarecarriedoutbyaMetTest-HTtypecomputerintegratedhardnesstester.Theloadis1471N.Themediumhardnessvalueoftheinteriorregionsisobtainedas43HRC.Microhardnessmeasurementshavebeenmadetodeterminethechanceofhardnessvaluesalongthecross-sectionbecauseofthehardeningofsurfaceareaduetocarburisation.TheresultsofVickershardnessmeasurementunderaloadof4.903NareillustratedinTable2.

2.3.Inspectionofthefracture

ThedirectobservationsofthepiecewithfracturedsurfacesandSEManalysesaregiveninthischapter.Thecrackstartedbecauseofapossibleprobleminthebottomofnotchcausedtheshafttobebrokencompletely.Thecrackstartedontheouterpart,aftersometimeitcontinuedbeyondthecentreandtherewasonlyalittlepartleft.Andthispartwasbrokenstaticallyduringsuddenstartingofthevehicleatthetrafficlights.Asacharacteristicofthefatiguefracture,therearetworegionsinthefracturedsurface.Theseareasmoothsurfacecreatedbycrackpropagationandaroughsurfacecreatedbysuddenfracture.ThesetworegionscanbeseenclearlyfortheentireproblemasinFig.4.Thefatiguecrackpropagationregioncoversmorethan80%ofthecross-section.

Table2

MicrohardnessvaluesDistancefromsurface（lm）50100200400Center

ValuesHV（4903N）588410293286263

Fig.

Fig.6.SEMimageofthefracturesurfaceshowingtheductileshear.

Fig.7.SEMimageofthefracturesurfaceshowingthebeachmarksofthefatiguecrackpropagation.

Shaftworksundertheeffectofbending,torsionandaxialforceswhichaffectrepeatedlydependingontheusageplace.Thereisasharpfilletatlevelonthefracturedsection.Forthisreason,stressconcentrationfactorsoftheareahavebeendetermined.Kt=2.4value（forbendingandtension）andKt=1.9value（fortorsion）havebeenacquiredaccordingtocalculations.Thesearequitehighvaluesforareasexposedtocombinedloading.

Theseobservationsandanalysisshowthatthepiecewasbrokenundertheinfluenceoftorsionwithlownominalstressesandmediumstressconcentration[10].

Thescanningelectronmicroscopyshowsthatthefracturehastakenplaceinaductilemanner（Fig.6）.Therearesomeshearlipsinthecrackpropagationregionwhichisaglueoftheplasticsheardeformations.Fig.7showsthebeachmarksofthefatiguecrackpropagation.Thedistancebetweenanytwolinesisnearly133nm.

3.Conclusions

Afaileddifferentialpinionshaftisanalysedinthisstudy.ThepinionshaftisproducedfromAISI8620lowcarboncarburisingsteelwhichhadacarburising,quenchingandtemperingheattreatmentprocess.Mechanicalproperties,microstructuralproperties,chemicalcompositionsand