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Bearings
ShellBearings
Thinshellbearingsareusedformostbearingapplicationsinthemainengine.Theyconsistofasteelbackingstripcoatedwithalayerofwhitemetal.Bearingsmaybebimetalortrimetal.typicalmaterialsaresteelbabbitt,steelbronzeorsteeltin/aluminium(tinaluminiumhasslightlygreaterloadbearingcapacitythanwhitemetalandmaintainsitsfatiguestrengthoveragreaterrangeoftemperatures.Thebearingmetalthicknessis0.5to3mm.AnOverlayof2040micrometersmaybeappliedtoimproveconformity.Thisisgenerallyaductilecoatingofleadandtin.InadditionnewbearingsmayhaveaflashlayerofafewmicronsoftintoprventoxidationAnintermediatelayermaybeusedbetweentheoverlayandmainbearingmetaltoavoiddiffusion.Thisisparticularlyfoundwherebearingloadsareverysuchasinthelowerhalfofthecrossheadbearing.Thelayerisgalvanicallyapplied.Comparedwiththetraditionalcastbearingtheyhaveanumberofadvantages.
Shellsareprefinishedthusallowingforquickerandeasierreplacement.ThebearingsaremadeunderstrictcontrolledconditionsgivingconsistenthighqualityproductsInmanycasesthetopandbottomhalvesareinterchangeableinanemergency.Thinlayerofwhitemetalcoolsquicklygivingfinegrainstructurewhichhashighstrengthandfatigueresistance.
Shellbearingssometimeshavealayerofcopperorbronzebetweenthesteelandthewhitemetalinordertoimproveadhesionofthewhitemetal.Thislayerwillalsoprovidesafeguardintheeventthatthewhitemetalbeingwormaway.Forcamshaftsshellbearingsarestillusedinpreferencetoballorrollerrace,theactionofthecamfollowersprovidesimpactloadingwhichcanmustbesupportedbythebearing.Ballorrollerracewouldtendtosufferfatigueorbrinellingdamage.Inadditiontothisreplacementissimplerwiththeplainbearing.
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Bearingwearmustbecheckedasthiscanallowthecamshafttodropthusalteringthetiming.WHITEMETALBEARINGCORROSIONWhitemetalsaretinbased,thatistheycontainahigherproportionoftinthenothercompounds.Atypicalcompositionmightbe86%tin,8.5%Antimony,5.5%lead.Inthepresenceofanelectrolytecorrosionofthetincanoccurformingextremelyhard,brittle,stannousandstannicoxides(mainlystannicoxideSn20)normallyinthepresenceofmoisture.Theseoxidesareusuallyofagreytogreyblackcolouredsurfacelayeronthewhitemetal,eitherinlocalpatchesorcompletelycoveringthebearing.Thehardnessofthisbrittleoxidelayercouldbeashighastwicethatofsteelandifitbecamedetached,possibleduetofatiguefailure,seriousdamagetobearingandjournalsurfacescouldoccur.Theformationoftheoxidelayerisaccompaniedbyanupwardgrowthfromthewhitemetal,whichcanconsiderablyreduceclearancesandcouldleadtooverheatingandseizureetc.Factorswhichappeartocontributetowardstheformationoftinoxidesare
Boundarylubricatione.gstartingconditionsSurfacediscontinuitiesConcentrationofelectrolytee.g.freshorsaltwaterorothercontaminationOiltemperatureStressesinthebearingmetal
Additivesinthelubricatingoilcanaddsomedegreeofprotectionascanefficientcentrifuging.StannicOxidebeingmuchharderthanthewhitemetalcausestwoproblems:
Preventsabsorptionofdirtparticles.Thisisnormallycarriedoutwhenabrasiveparticlesarestucktothesurfaceofthewhitemetal.LocaloverheatingandmeltingoccursandtheparticlefallsintothewhitemetalTheoxideisbrittleandcancrackwiththepieceedgeprojectingout(causingmachiningtypefailureofthrustcollarsespecially.)
Boththeseresultinscoringofthejournal,thesearenormallyconsideredasalowtemperaturetypeoffailure.Inadditionthepresenceofwaterintheluboilcancausetheoxidationofthemetalsinthebearingcausingthemetaltogrow.Thisreducesclearanceandcanleadtobearingfailure.
RollingElementBearings
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TheadvantagesanddisadvantagesofrollingbearingsAdvantage
LowstartingtorquecompactandreadytoinstallmachinemembersItisgenerallyeasiertolubricaterollingbearingsthanplainbearingsCombinedradialandthrustloadscanbecarriedreadilyinarollingbearingHighoverloadscanbecarriedforshortperiodswithoutundueoradverseeffectsRollingbearingsbecomenoisywhennearingtheendoftheirnaturallife.Runningclearancesislessthanplainbearings,preloadingcanreducedeflectionandincreaseaccuracy.StandardisationbyinternationalagreementmakesreplacementandservicingeasierBetterelectricalinsulationispossiblethanwithplainbearingsCanbemultifunctionale.g.combinedwaterpumpandfanonthesameshaft
DisadvantageMoreexpensivethanplainbearingswithmorepartstogowrongTheyarenoisierthanplainbearingsTheiruseathighspeedsandloadsarelimitedbycentrifugal,fatigueandbrinellingeffects.Plainbearingscanrunfasterandcarryheavierloads.Brinelling,frettingcanoccurduringstaticconditionswhenthebearingissubjectedtovibrationAllrollingbearingshaveafinitelife.IfalargenumberofaparticularbearingareproducedforaparticularapplicationapercentageofprematurefailurescanbeanticipatedRepairofrollingbearingsisalmostimpossibleinanaverageworkshop.Theentireunithastobereplacedwhenwornorfailed
RollingelementbearinguseElastohydrodynamiclubrication.Thatis,thesurfacematerialdeformstoassistwiththeformationandshapeoftheoilwedge.
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Astherollermovesoverthesurfacethelatterdeformsinapressurewave.Hertzianstressbuildupparalleltothesurfaceofthematerialinwhichdefectscanoccur.Defectsmayalsoformduetothepressurewave.Thisdefectsmayopenandclosefreelywiththepassingoftheelement.Shouldliquidenterthedefectthenhydrauliclockoccursandthedefectgrowstorelievethestress.Whenthesurfacedefectsjointhesubsurfacehydrauliclockingagainoccursandthesealsogrow.Eventuallythatportionofthematerialbecomeweakenedtoanextentaportionisdisplaced.Thisisageneralisationofthemechanismassociatedtopitting.AnothersignificantmechanismforthefailureofrollingelementbearingisspallingwheresubsurfacedefectsleadtothedetachmentofsectionsofmaterialThemosteffectivemethodfordetectionofbearingfailureisbyvibrationmonitoring.Thismaybebyobservingthevibrationvelocityhistory.Theoreticallythevibrationcharacteristicshouldhavewelldefinednodesatsuchfrequenciesasouterraceelementpass.Thisisofuseindeterminingthathighvibrationwithanassemblyisspecificallycausedbytherollingelementbearings
Rollingelementbearingstraditionallygeneratevibrationsoverawidefrequencyrange.Trendingofthisvibrationoveraperiodoftimewillallowestimationofthecurrentconditionofthebearing.Ageneralincreaseinthehighfrequencyvibrationisgenerallyassociatedwiththecreationofmicroscopiccracksandspallingtoosmallforhumaneyetodetect.AlthoughthisisnotconsidereddestructiveinitselfitisapreliminarystagethatleadstofurtherdegradationAlternatelythevibrationaccelerationlevelmaybemeasured.Inbothcasesdeterioratingbearingconditionsisindicatedbyincreasedvibrationlevels.OthermethodsincludebearingtemperaturemeasurementsandanalysisoflubricatingoilEarlydetectionofbearingfailurepreventsdamagesuchashousingfrettingandshaftdistortionduetooverheatingremedyofwhichfarexceedsthatofbearingreplacementonly.
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NormalLifeFailure
Shownisabearingdemonstratingnormalwearpatternforaverticallyloadedshaftwithrotatinginnerringandfixedouterrace.Thepatternontheouterraceisdisplaceddependingonthelineofactionoftheloading.
Althoughshownunifromtheactualwearcanbeofamorerandompatternandcarefulinterpretationisrequired.
PrematurefailureBearingfailurenotassociatedwithfatiguecanoccurduetoseveralreasonsbut
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mostcommonlymisalignment,overgreasingandcontamination
Overgreasing
Typically1/3rdoftheavailablespaceshouldbefiledwithgrease.Forsomeinstallationsaweightofgreaseisgivenbythemanufacturer.ContaminationContaminationisthemostcommoncauseofprematurefailure.Thisoccursusuallyduringbearinginstallationandtypicallyiscausedbypoorhousekeepingordirtloadedgrease.Excessivewearisindicatedbyadulledappearanceandindentedrunningsurface.Bearingsdamagedduetothistendtohaveindentations,andscratchesbothontheraceandtherollingelements.ThiseffectissometimesreferredtoasScoringMisalignment
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Issometimesidentifiedbyawidecontacttrackontheinnerraceandathinnernonperpendiculartrackontheouterrace.Misalignmentbyassmallamountas1/1000canleadtoseriousreductionsinlifeexpectancy
LubricationfailureIntheshorttermtheremovalofmanufacturingasperitiesleadstoahighlypolishedmirrorlikeappearance.Thismayprogresstoasurfacecrystallinewithdarklinesshowingacrystallinestructure.Thisiscausedbyincorrectspecificationforlubricationoroverheatingreducingviscosity
Generallyleadstooverheatingtoablue/blackcolourontheloadsurfacereducingtoastraw/goldcolorontheedgesofthebearingRusting
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Causedbywateringressintothebearingleadingtoredofblackpatchesonallsurfaces.Generallyassociatedwithinefficientsealingandpoorlubricantproperties.BrinellingThiscomesintwoformsTrueBrinelling
thiscausedwhentheelasticlimitoftheracematerialisexceededandapermanentdeformationoccurs.Associatedwithasharpimpactloadingsometimesoccurringduringpoorinstallationprocedures.ImpactsoccuratrollingelementpitchFalseBrinelling
Sometimesreferredtoas'washboarding'andiscausedbyrelativemovementoftheelementswithoutformationofanoilfilm.Haematitedepositsmaybefoundinthepits
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Moreassociatedwithhighbackgroundvibration.Damageappearanceisdistinctiveanddifferenttonormalpitting.Shownisthatseenwithballelements,rollerelementsarelinear.Bothareatelementpitchormultiplesofit.TheuseofqualityEPadditivescanreduceprogressofdamage.Itshouldbenotedthatfalsebrinellingcanalsoleadtoasimilarflutingappearancebutthepitsarebrighterandcontaincorrosionproducts,whereduetothepassageofelectricitythepitsaregenerallydark.Rollerbearingsaregenerallymoresusceptibletothistypeofdamagethusballbearingsarepreferredinhighvibrationareas,partsubmersioninoilbathalsoreducesthiseffect.Overloading
Leadstooverheatingandspalling(surfacematerialloss)oftherunningsurfacestypicallyinthedirectionofoverload.MayberemediedbymoreappropriateselectionofbearingsIndentations
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Typicallyunevencratersandpits.AssociatedwithcontaminatedorhardernedgreasebutalsooccurswithoillubricatedbearingscontainingweardebrisCracking
Associatedwithincorrectfit,unevenracesupportorsevereoverloadingPoorFitLoosefitThisleadstofrettingbetweenthecontactareasofthebearing/housingorbearing/shaft.Indicationthatthisisoccurringisbythepresenceofblackordarkredcolorationordepositsonthelandingareas.AsthisworsenstheinnerorouterracemaybegintocreeporrotaterelativetotheadjacentlandingareaTheinitiatorforthisisoftentheincreasedvibrationfromthebearingasitbeginstoreachtheendofitslife.Thisvibrationisdestructivebeforenoise,heatorvibrationlevelreachisdetectablewithouttheuseofaninstrument.Oncethefitislostinthismannerthebearingnolongercanofferreliableloadbearingcapacity.Theuseof'LoctiteBearingFit'cansolveminorfitproblems.Thetechniqueofraisingthesurface,forexampleusingacentrepunchhasverylimitedbenefit
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Tightfit
WheretheinterferencefitisgreaterthantheradialfitveryhighbearingtemperaturesaregeneratedduetooverloadingandprematurefailureresultsDiscolouredwidecontactareas,attheextremesleadstoveryheavyoverloading,hightemperaturesandcracking.MayleadtorestrictioninmovementoftherollingelementsandleadtoscuffingoftherunningareasthroughweldingCageWearAssociatedwithpoorlubricationorincorrectlyspecifiedbearingtype.Worsenstoapointthatthecagefails,theelementsmoveallowingtherelativepositionsoftheouterandinnerracestochange.SmearingThisiscausedbylackoflubricationandrelativeslidingmotionbetweentheelementsurfaces.Itiscommonlyseenontheendsofrollerbearingswheretheyhavebeensubjectedtoaxialforces.Thematerialisgenerallyheatedandsometransferoccursbetweentheparts.Surfacerehardeningcanoccurwithtearfracturesevident.Thiseffectmaybeseenwhererollersaresubjecttohighaccelerationswhenenteringtheloadzone(thisoccursastheelementsarenotdrivenoutwhenoutoftheloadzoneandcanberemediedbyreducingbearinginternalclearance),excessivepreloadingfortaperbearings,oreveninballbearingwhereloadistoolightinrelationtothespeedofrotation.Carefulselectionoflubricantcanavoidthisdamage.Rollerbearingsmaybesubjecttosmearingonassemblyduetopoorassemblyallowingtheelementsto'scrape'thesurfaceontheraceSmearingmayoccuronthenonrunningouteredgesoftheracesduetorelativemovementsbetweenhousing/shaftandthebearing.Thismayberemediedbyincreasingtheinterferencefit.ElectricaldischargeThiscauseseitherdeepcrateringorthinpitscontainingdarkproduct(referedtofluting).InstallationInstallationofrollingelementbearingsshouldbedoneinascleananenvironmentaspossible.Theleadingcauseofprematurebearingfailureiscontamination.Careshouldbetakennottocauseimpactladingtothebearingwhichmayleadtobrinelling.Itispreferablethatthebearingbepreheatedbyoilbathorinductionheaterbeforefit.WherenotpossiblethenthebearingshouldbeevenlypressedThecorrectquantityandtypeofgreaseshouldbeused.Althoughnotcommonitisnotunusualforbearingstoinitiallyrunwithhightemperaturesorincreasedload.ThismaybeassociatedtostressesleftoverfromwhenthebearingwaspressedontothelandingareasCareshouldbetakenthatthe
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bearingsarenotallowedtooverheat.ForsmallermotorsthismaybesuccessfullycompensatedbylighttappingoftheshaftIoppositedirectionsuntiland'tightness'isreduced.Forlargerassembliesasequenceofrunningtoalimitedtemperaturefollowedbyaperiodofcoolinggenerallyleadstoastabilizationandthennormalrunningtemperatures.ImprovingBearingLifeImprovedbearinglifemaybegainedbytheuseofbetterormoreappropriategreases.WherepossiblethegreaseshouldbereplacedtoremovewearparticlesandrestorelevelsofEPadditives.ForOillubricatedbearingstheuseofveryfinefiltrationisessentialItisunderstoodthat'Hybrid'bearingsconsistingofarelativelystandardsteelrollingelementbearingintowhichisinsertedasingleceramicelement.Theceramicelementisbelievedtopolishtherunningsurfaceremovingsurfacedefects.ForbearingusedinelectricalinstallationstheuseofTefloncoatingstotheouterracelandingsurfacepreventselectricaldischargethroughthebearings.
