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TribologyInternational40(2007)1270–1275EffectoflubricantviscositygradeonmechanicalvibrationofrollerbearingsR.Serrato,M.M.Maru,L.R.PadoveseMechanicalEngineeringDepartment,SaoPauloUniversity,BrazilAvenueProf.MelloMoraes2231CEP,05508-900Sa˜oPaulo,BrazilReceived11October2005;receivedinrevisedform22January2007;accepted30January2007Availableonline30March2007AbstractThisworkaimstocharacterizevibrationbehaviorofrollerbearingsasafunctionoflubricantviscosity.ExperimentaltestswereperformedinNU205rollerbearings,lubricatedwithmineraloilofthreedifferentviscositygrades(ISO10,32and68).Themechanicalvibrationwasdeterminedthroughtheprocessingandanalysisofbearingradialvibrationdata,obtainedfromeachofthelubricationconditions,during2hoftestrunfortemperaturestabilizationandunderseveralbearingshaftspeeds.Theappliedradialloadwas10%ofthebearingnominalload.Throughrootmeansquare(RMS)analysisofthevibrationsignals,itwaspossibletoidentifyspecificfrequencybandsmodulatedbythechangeinlubricantviscosity,whichwasrelatedtochangeinoilfilmthickness.r2007ElsevierLtd.Allrightsreserved.Keywords:Lubrication;Rollerbearing;Vibration;Viscosity1.IntroductionDependingonsomeaspects,lubricationinmechanicalsystemscanoccurindifferentregimes:fullfilm,mixedorboundarylubrication.Fullfilmlubricationcanbefurtherdividedintoelastic-hydrodynamiclubrication(EHL),whichoccursinnon-conformalcontactsunderhighpressure,andhydrodynamiclubrication(HD),occurringunderlowpressureandusuallyinconformalcontacts[1–3].AmongthegroupofmechanicalcomponentsoperatingunderEHLcondition,therearetherollingbearings.Thismachineelementtypeisoneofthosemoresensitivefordevelopmentoffaultsrelatedtolubricationdeficiency.Accordingtotechnicalpublicationsofrollingbearingmanufacturers[5,6],fromthetotaloffaultsfoundinthistypeofcomponent,50–80%arerelatedtodeficientlubrication,resultingfrominadequatelubricantuse,lackorexcessoflubricant,lubricantaging,andpresenceofsolidorliquidcontaminant.Inthefaceofhighpercentageofrollingbearingfailures,thedevelopmentoftechniquesfordetectionanddiagnosisoffaultsinrollingbearings,duetolubricationdeficiency,isafundamentalcontributiontothepreservationofmachineprecision.Intermsofmonitoringofrollingbearingperformance,vibrationmeasurementsareamongthemostusedtechni-ques.Nowadays,alotofworksondetectionoflocalizeddefectsinrollingbearingelementsthroughvibrationanalysiscanbefoundintheliterature[7–9];conversely,referencesonthedetectionoflubrication-inducedfaultsarestillfoundtobeless.Amongthese,thereisone[10]pointingoutthatwhenarollingbearingisinadequatelylubricated,itsvibrationresponseissimilartothatofasystemsubmittedtoarandomexcitation.Inthecaseofsystemswithlowdamping,thepredominantcomponentsofsuchresponsewouldcorrespondtothenaturalfrequenciesoftherollingbearing.Ontheotherhand,accordingtoBerry[11],frequencyspectraofvibrationsignalsforinadequatelubricationconditionarecharacter-izedbythreeorfourpeaksinthefrequencybandsfrom900to1600Hz,correspondingtonaturalfrequencybandsoftherollingbearing.Inaddition,heaffirmsthatthesefrequencybandsarealsoseenunderadequatelubricationcondition;althoughthevibrationmagnitudesaremuchsmallerinthiscase.ARTICLEINPRESS:10.1016/j.triboint.2007.01.025Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddresses:[e-mail](R.Serrato),[e-mail](M.M.Maru),[e-mail](L.R.Padovese).Inthesamecontext,anotherstudy[12]concludesthatvibrationenergyofabearingdependsonsurfaceirregula-rities,externalloadings,runningspeedandlubricantviscosity.Forinstance,itwasobservedinexperimentalteststhattheinfluencesoflubricantviscosityonvibrationresponsedependedonspeed:forabearingunderlargeloadandlowrunningspeeds,increaseinlubricantviscositycausesreductioninvibrationenergy.Incontrast,athighrunningspeeds,vibrationenergyishighwhenlubricantviscosityishigh.Furthermore,Massouros[13]citesthat,inplainbearingsunderboundarylubricationcondition,thejournalrotationresultsinimpactsamongthemicroasperitiesoftheslidingsurfacesatcontact,producingvibrations,bothnormalandtangential,totheslidingdirection.Allthementionedworksreinforcethealreadyknowncomplexityconcerningtheanalysisofvibrationphenom-enaoccurringinmechanicalcontacts.Thepresentworkintendstocontributeforimprovingknowledgeontherelationshipbetweenvibrationphenomenaandlubricationinbearings.Thisisdonebystudyinghowachangeinlubricantviscositycanaffectthemechanicalvibrationofarollingbearingbymeansofbasicproceduresofvibrationanalysis.2.MethodologyThetestedrollingbearingswereofNU205type,presentingthegeometryshowninFig.1.Mineraloilwithoutadditivewasusedaslubricant.Threedifferentviscositygradesweretested,ISO10(V1),ISO32(V2)andISO68(V3),withthepurposeofobtainingthevibrationresponserelatedtodifferentlubricationregimesinthebearingelementcontacts.Thetwohigherviscositygradesarerecommendedforbearingsoperatingintheselectedtestconditions,accordingtothemanufacturerscatalogue.TheISO10gradewasincludedinthestudysoastoforceacriticalconditiono