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南昌航空大学科技学院学士学位论文1外文文献FrictionLubricationofBearingInmanyoftheproblemthusfar,thestudenthasbeenaskedtodisregardorneglectfriction.Actually,frictionispresenttosomedegreewhenevertwopartsareincontactandmoveoneachother.Thetermfrictionreferstotheresistanceoftwoormorepartstomovement.Frictionisharmfulorvaluabledependinguponwhereitoccurs.frictionisnecessaryforfasteningdevicessuchasscrewsandrivetswhichdependuponfrictiontoholdthefastenerandthepartstogether.Beltdrivers,brakes,andtiresareadditionalapplicationswherefrictionisnecessary.Thefrictionofmovingpartsinamachineisharmfulbecauseitreducesthemechanicaladvantageofthedevice.Theheatproducedbyfrictionislostenergybecausenoworktakesplace.Also,greaterpowerisrequiredtoovercometheincreasedfriction.Heatisdestructiveinthatitcausesexpansion.Expansionmaycauseabearingorslidingsurfacetofittighter.Ifagreatenoughpressurebuildsupbecausemadefromlowtemperaturematerialsmaymelt.Therearethreetypesoffrictionwhichmustbeovercomeinmovingparts1starting,2sliding,and3rolling.Startingfrictionisthefrictionbetweentwosolidsthattendtoresistmovement.Whentwopartsareatastateofrest,thesurfaceirregularitiesofbothpartstendtointerlockandformawedgingaction.Toproducemotionintheseparts,thewedgeshapedpeaksandvalleysofthestationarysurfacesmustbemadetoslideoutandovereachother.Therougherthetwosurfaces,thegreaterisstartingfrictionresultingfromtheirmovement.Sincethereisusuallynofixedpatternbetweenthepeaksandvalleysoftwomatingparts,theirregularitiesdonotinterlockoncethepartsareinmotionbutslideovereachother.Thefrictionofthetwosurfacesisknownasslidingfriction.Asshowninfigure,startingfrictionisalwaysgreaterthanslidingfriction.Rollingfrictionoccurswhenrollerdevcesaresubjectedtotremendousstresswhichcausethepartstochangeshapeordeform.Undertheseconditions,thematerialinfrontofarollertendstopileupandforcestheobjecttorollslightlyuphill.Thischangingofshape,knownasdeformation,causesamovementofmolecules.Asaresult,heatisproducedfrom南昌航空大学科技学院学士学位论文2theaddedenergyrequiredtokeepthepartsturningandovercomefriction.Thefrictioncausedbythewedgingactionofsurfaceirregularitiescanbeovercomepartlybytheprecisionmachiningofthesurfaces.However,eventhesesmoothsurfacesmayrequiretheuseofasubstancebetweenthemtoreducethefrictionstillmore.Thissubstanceisusuallyalubricantwhichprovidesafine,thinoilfilm.Thefilmkeepsthesurfacesapartandpreventsthecohesiveforcesofthesurfacesfromcominginclosecontactandproducingheat.Anotherwaytoreducefrictionistousedifferentmaterialsforthebearingsurfacesandrotatingparts.Thisexplainswhybronzebearings,softalloys,andcopperandtiniolitebearingsareusedwithbothsoftandhardenedsteelshaft.Theiolitebearingisporous.Thus,whenthebearingisdippedinoil,capillaryactioncarriestheoilthroughthespacesofthebearing.Thistypeofbearingcarriesitsownlubricanttothepointswherethepressuresarethegreatest.Movingpartsarelubricatedtoreducefriction,wear,andheat.Themostcommonlyusedlubricantsareoils,greases,andgraphitecompounds.Eachlubricantservesadifferentpurpose.Theconditionsunderwhichtwomovingsurfacesaretoworkdeterminethetypeoflubricanttobeusedandthesystemselectedfordistributingthelubricant.Onslowmovingpartswithaminimumofpressure,anoilgrooveisusuallysufficienttodistributetherequiredquantityoflubricanttothesurfacesmovingoneachother.Asecondcommonmethodoflubricationisthesplashsysteminwhichpartsmovinginareservoiroflubricantpickupsufficientoilwhichisthendistributedtoallmovingpartsduringeachcycle.Thissystemisusedinthecrankcaseoflawnmowerenginestolubricatethecrankshaft,connectingrod,andpartsofthepiston.Alubricationsystemcommonlyusedinindustrialplantsisthepressuresystem.Inthissystem,apumponamachinecarriesthelubricanttoallofthebearingsurfacesataconstantrateandquantity.Therearenumerousothersystemsoflubricationandaconsiderablenumberoflubricantsavailableforanygivensetofoperatingconditions.Modernindustrypaysgreaterattentiontotheuseoftheproperlubricantsthanatprevioustimebecauseoftheincreasedspeeds,pressures,andoperatingdemandsplacedonequipmentanddevices.Althoughoneofthemainpurposesoflubricationisreducefriction,any南昌航空大学科技学院学士学位论文3substanceliquid,solid,orgaseouscapableofcontrollingfrictionandwearbetweenslidingsurfacescanbeclassedasalubricant.VarietiesoflubricationUnlubricatedsliding.Metalsthathavebeencarefullytreatedtoremoveallforeignmaterialsseizeandweldtooneanotherwhenslidtogether.Intheabsenceofsuchahighdegreeofcleanliness,adsorbedgases,watervapor,oxides,andcontaminantsreducefrictio9nandthetendencytoseizebutusuallyresultinseverewearthisiscalledunlubricatedordrysliding.Fluidfilmlubrication.Interposingafluidfilmthatcompletelyseparatestheslidingsurfacesresultsinfluidfilmlubrication.Thefluidmaybeintroducedintentionallyastheoilinthemainbearingofanautomobile,orunintentionally,asinthecaseofwaterbetweenasmoothtubertireandawetpavement.Althoughthefluidisusuallyaliquidsuchasoil,water,andawiderangeofothermaterials,itmayalsobeagas.Thegasmostcommonlyemployedisair.Boundarylubrication.Aconditionthatliesbetweenunlubricatedslidingandfluidfilmlubricationisreferredtoasboundarylubrication,alsodefinedasthatconditionoflubricationinwhichthefrictionbetweensurfacesisdeterminedbythepropertiesofthesurfacesandpropertiesofthelubricantotherthanviscosity.Boundarylubricationencompassesasignificantportionoflubricationphenomenaandcommonlyoccursduringthestartingandstoppingoffmachines.Solidlubrication.Solidsuchasgraphiteandmolybdenumdisulfidearewidelyusedwhennormallubricantsdonotpossesssufficientresistancetoloadortemperatureextremes.Butlubricantsneednottakeonlysuchfamiliarformsasfats,powders,andgasesevensomemetalscommonlyserveasslidingsurfacesinsomesophisticatedmachines.FunctionoflubricantsAlthoughalubricantprimarilycontrolsfrictionandordinarilydoesperformnumerousotherfunctions,whichvarywiththeapplicationandusuallyareinterrelated.Frictioncontrol.Theamountandcharacterofthelubricantmadeavailabletoslidingsurfaceshaveaprofoundeffectuponthefrictionthatisencountered.Forexample,disregardingsuchrelatedfactorsasheatandwearbutconsideringfrictionalonebetweenthesamesurfaceswithonlubricant.Underfluidfilmconditions,frictionisencountered.Inagreatrangeof南昌航空大学科技学院学士学位论文4viscositiesandthuscansatisfyabroadspectrumoffunctionalrequirements.Underboundarylubricationconditions,theeffectofviscosityonfrictionbecomeslesssignificantthanthechemicalnatureofthelubricant.Wearcontrol.wearoccursonlubricatedsurfacesbyabrasion,corrosion,andsolidtosolidcontactwearbyprovidingafilmthatincreasesthedistancebetweentheslidingsurfaces,therebylesseningthedamagebyabrasivecontaminantsandsurfaceasperities.Temperaturecontrol.Lubricantsassistincontrollingcorrosionofthesurfacesthemselvesistwofold.Whenmachineryisidle,thelubricantactsasapreservative.Whenmachineryisinuse,thelubricantcontrolscorrosionbycoatinglubricatedpartswithaprotectivefilmthatmaycontainadditivestoneutralizecorrosivematerials.Theabilityofalubricanttocontrolcorrosionisdirectlyrelatlytothethicknessofthelubricantfilmremainingonthemetalsurfacesandthechermicalcompositionofthelubricant.OtherfunctionsLubricationarefrequentlyusedforpurposesotherthanthereductionoffriction.Someoftheseapplicationsaredescribedbelow.Powertransmission.Lubricantsarewidelyemployedashydraulicfluidsinfluidtransmissiondevices.Insulation.Inspecializedapplicationssuchastransformersandswitchgear,lubricantswithhighdielectricconstantsactsaselectricalinsulators.Formaximuminsulatingproperties,alubricantmustbekeptfreeofcontaminantsandwater.Shockdampening.Lubricantsactasshockdampeningfluidsinenergytransferringdevicessuchasshockabsorbersandaroundmachinepartssuchasgearsthataresubjectedtohighintermittentloads.Sealing.Lubricatinggreasefrequentlyperformsthespecialfunctionofformingasealtoretainlubricantsortoexcludecontaminants.Theobjectoflubricationistoreducefriction,wear,andheatingofmachineparswhichmoverelativetoeachother.Alubricantisanysubstancewhich,wheninsertedbetweenthemovingsurfaces,accomplishesthesepurposes.Mostlubricantsareliquidssuchasmineraloil,siliconefluids,andwater,buttheymaybesolidforuseindrybearings,greasesforuseinrollingelementbearing,orgasessuchasairforuseingasbearings.Thephysicalandchemicalinteractionbetweenthelubricantandlubricatingsurfacesmustbeunderstoodinordertoprovidethemachineelementswithsatisfactorylife.Theunderstandingofboundarylubricationisnormallyattributedtohardyanddoubleday,whofoundtheextrememlythinfilmsadheringtosurfaceswere
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