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enginemanagement(http://www.knowyourparts.com/technicalarticles/system/engine/)>TypesofEngineBearingDamage

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TypesofEngineBearingDamageBroughttoyoubyAASA(/brands/)

Figure1:Overplatedcopperalloybearinggougedbycastirondebris.Insetphotoshowsthemicroscopicdetailofthegouges.

Enginebearingsbothreducefrictionbetweentherotatingpartoftheengineandthestationarypartandsupportthecrank.Thebearingmaterialmustbeextremelystrongbecauseofthestressescausedbytheexplosionsinsidetheinternalcombustionengine.Reducingfrictionisaccomplishedinpartbythefactthatdissimilarmetalsslideagainsteachotherwithlessfrictionandwearthansimilarmaterialswill.

Figure2:Babbittbearingembeddedwithmachiningdebris.Theinsetphotoshowsmicroscopicdetailofthedebris.

Soanalloybearingmaterialdoesamuchbetterjobofkeepingasteelcrankshaftmovingthanasteelorcastironbearingdoes.

Althoughthematerialitselfmaygivetheenginebearingsomefrictionreducingproperties,itsperformanceisenhancedbyalubricantbetweenthemovingandstationarysurfaces.Anotherofthebearingsdutiesistoestablishandmaintainafilmofoil.

Thebearingsusuallydoaverygoodjobatkeepingthemovingpartsmovinghowever,whenabearingfails,theresultscanbecatastrophic.

Figure3:Thisaluminumbearingwasdamagedbyembedmentofglassbeads.Thisphotoshowstheextentofthedamage.

Evenwhentheyfail,itsusuallynotthebearingsfault.Withabitofinvestigation,anenginespecialistortechniciancandiscoverandeliminateoneofliterallydozensofreasonsforprematurewearorfailure.

DirtorDebrisDebris,suchasdirtordust,cancausesignificantdamagetoabearingsurface.Ifitsinthelubricationsystem,dirtwillusuallyleavecircumferentialscratchesandwilloftenremainlodgedinthebearingssurface.

Figure4:Foreignparticlesinthebearingliningmaybetheresultofimpropercleaningorafailuretoreplacethefilterandmayincluderoaddirtandsand.

Besuretoalwaysflushthelubricationsystemthoroughlybeforereassemblinganenginetoavoidenginebearingdamage.

Anotherwaydirtcanplayhavocisiftheenginecomponentsarentcompletelyclean.Aforeignparticletrappedbetweenthebearingbackandthehousingwillcausearaisedareainthebearing.

Thislittlehumpcanleadtocontactbetweenthebearingandthecrankjournal.Youshouldalwaysensurethebearingsareinstalledincleansurfaces.

InsufficientLubricationAcompletelackoflubricationinthecrankcaseusuallyleadstoseizureofthebearingandtotalfailureoftheengine.But,expertssayamorefrequentissueregardinglubricationissimplyinadequateoiling.Theabsenceofaproperoilfilmwillresultinmetaltometalcontact,occasionallytojustonebearingoroftentoanumberofthem.

Figure5:Lubricationisvital.Thisshowstheresultofadrystart.Thebearingsontheleft,furthestfromtheoilpumphavethemostwear.

Whenthebearingisdamagedbyoilstarvation,youllfindaveryshinysurfaceandevidenceofwiping.

Remember,thefailureoftheoilfilmcanbeseeninavarietyofwaysonthebearings.Checkonsuchthingsasblockedoilpassages,amalfunctioningoilpump,improperbearingselectionorinstallation,oilsealfailures,fueldilution(oftencausedbyblowbyoffuelandairpastthepistonrings)orfoamingoraeration,caused,ironically,byanoverfilledcrankshaft.

MisassemblySometimes,failuresaretheresultofsimpleinstallationerrors.Forexample,ifabearinghalfwithoutanoilholeisimproperlyputintoapositionwheretheholeisneeded,thatjournalwillreceivenolubrication.

Figure6:Lowoilsupplyoroilstarvationdoesntjustsoundbad,itdoesbadthingsinsidetheengine.

Othertypesofassemblyerrorsmayalsobeseen.Ifaconnectingrodormainbearingcapisinstalledinthewrongposition,orabearingisntsetintoplacesecurely,lubricationwillbeinsufficientandcausefailure.

Carefulinstallationproceduresare,ofcourse,criticalineveryaspectofenginebuildingcarelesserrorsarealwaysexpensive.

BearingCrushThetermcrushreferstotheoutwardforcecreatedbytheportionofthebearingthatextendsabovethehousingborewhenthebearinghalvesaresetintoplace.Thisextramaterialholdstheoutsidediameterofthebearingsfirmlyagainstthehousingborewhentheassemblyistorquedtospecification.

Byincreasingthesurfacecontactbetweenthebearingandconnectingrodhousingbore,crushminimizesbearingmovement,helpstocompensateforboredistortionandaidsinheattransfer.

Figure7:Toorichamixtureorblowbypastthepistonringcanleadtodilutionoftheoil.Thisdamagecanbeseenhere.

Insimpleterms,bearingcrushiswhatholdsthebearinginplace.Thinkofitasputting10poundsofsomethingintoafivepoundbag.Thetangorlocatortabontheshellthatfitsthesaddleisonlyforlocatingthebearingduringassembly.

Whencrushiscorrect,slightlyellipticalbearingshellsconformtoaperfectcirclewhentheyretorquedintoplace.Inthisway,thecrankshaftrotatesproperly.

However,whenthereisexcessivecrush,theadditionalcompressiveforcecausesthebearingtobulgeinwardatthepartinglinescausingsidepinch.

Excessivecrushmaybetheresultofanattempttoreduceoilconsumptionbyfilingdownthebearingcap,assemblingthebearingcapstootightlybyovertorquingthefastenersor,insomecases,usingtoofewshims.

Insufficientcrush,ontheotherhand,willresultinbearingsthatarenotheldsecurelyinthebore,andremainfreetomovebackandforthwithinthehousing.

Figure8:Excessivewearseennearthepartinglinesofupperandlowershellswascausedbybearingcapshift.Thisresultsinmetaltometalcontactandexcessivepressurecausingdeterioration.

Becausecontactbetweenthebearingbackandhousingboreisnecessaryforcooling,thisconditionmeansheattransferawayfromthebearingisimpeded,resultinginoverheatinganddeteriorationofthebearingsurface.

Insufficientcrushmaybecausedbytheimproperattempttoachieveabetterfitbyfilingthepartingfaces,bydirtorburrsholdingthebearingcapsopen,improperlytorquingthefastenersduringinstallation,improperlysizingthehousingboreor(ifneeded)usingtoomanyshimsintheassemblyprocess.

Figure9:Filletrideoccursiftheradiusofthefilletinthecornerofeachcrankjournalislargerthanneeded.Theedgesofthebearingcanthenrideonthosefilletsratherthanfittingneatlybetweenthem.

OtherCluestoBearingFailureShinyareaswillbeseenonthebackofthebearingduetoitsrubbingbackandforth.Insomecases,discolorationmaybeseenwhereoilhasworkeditswaybetweenthetwosurfacesandburned.

Overloadingmaybecausedbyvehicleoperatorerror.Excessiveidlingcanresultinanoilfilmthatcantsupporttheloadneeded.

Engineluggingcandistortthecrankcaseand/orcrankshaft,affectingtheconnectingrodand/ormainbearings.

Hotrodding,orforcingexcessiveloads,cansimilarlyaffectthebearings.Enginetuningandoperatingconditionsshouldalwaysbefollowedandappropriatebearingmaterialschosenfortheapplication.

Avehiclethatleaksoilhasitsshareofissues.But,somevehicleownerswhohavecarsortrucksthatdonotleakoilareinanevenmorepotentiallyserioussituation.Atleasttheleakerletsyouknowitneedsoiladdednowandthen,withfreshoilmaintainingitslevels.

Theenginethatseemstobeleakfreecanbeoverlooked,yetafteracertainamountoftime,theoilbeginstodegrade.Theacidsintheoilthenattackthebearingsurface.

Properselectionofbearingswillgoalongwaytowardasuccessfulenginebuild.Andcheckingassembledbearingclearancestomakesurethebearingsarenottootightortoolooseshouldalwaysbedoneasafinalchecktomakesureoilclearancesarewithinthedesiredrangefortheengine.

Figure10:Whenthereisexcessivecrush,theadditionalcompressiveforcecausesthebearingtobulgeinwardatthepartinglinescausingsidepinch.

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Attentiontothemachiningandmaterialselectionprocessesatthefrontendshouldhelpreducethechanceofbearingfailuredowntheroad.Butshouldthosemysteriousbearingfailuresoccur,youshouldeasilybeabletofindthecause.

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