机械 外文翻译 外文文献 英文文献 轴承的摩擦与润滑.docx
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机械外文翻译外文文献英文文献轴承的摩擦与润滑
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Friction,LubricationofBearing
Inmanyoftheproblemthusfar,thestudenthasbeenaskedtodisregardorneglectfriction.Actually,frictionispresenttosomedegreewhenevertwopartsareincontactandmoveoneachother.Thetermfrictionreferstotheresistanceoftwoormorepartstomovement.
Frictionisharmfulorvaluabledependinguponwhereitoccurs.frictionisnecessaryforfasteningdevicessuchasscrewsandrivetswhichdependuponfrictiontoholdthefastenerandthepartstogether.Beltdrivers,brakes,andtiresareadditionalapplicationswherefrictionisnecessary.
Thefrictionofmovingpartsinamachineisharmfulbecauseitreducesthemechanicaladvantageofthedevice.Theheatproducedbyfrictionislostenergybecausenoworktakesplace.Also,greaterpowerisrequiredtoovercometheincreasedfriction.Heatisdestructiveinthatitcausesexpansion.Expansionmaycauseabearingorslidingsurfacetofittighter.Ifagreatenoughpressurebuildsupbecausemadefromlowtemperaturematerialsmaymelt.
Therearethreetypesoffrictionwhichmustbeovercomeinmovingparts:
(1)starting,
(2)sliding,and(3)rolling.Startingfrictionisthefrictionbetweentwosolidsthattendtoresistmovement.Whentwopartsareatastateofrest,thesurfaceirregularitiesofbothpartstendtointerlockandformawedgingaction.Toproducemotionintheseparts,thewedge-shapedpeaksandvalleysofthestationarysurfacesmustbemadetoslideoutandovereachother.Therougherthetwosurfaces,thegreaterisstartingfrictionresultingfromtheirmovement.
Sincethereisusuallynofixedpatternbetweenthepeaksandvalleysoftwomatingparts,theirregularitiesdonotinterlockoncethepartsareinmotionbutslideovereachother.Thefrictionofthetwosurfacesisknownasslidingfriction.Asshowninfigure,startingfrictionisalwaysgreaterthanslidingfriction.
Rollingfrictionoccurswhenrollerdevcesaresubjectedtotremendousstresswhichcausethepartstochangeshapeordeform.Undertheseconditions,thematerialinfrontofarollertendstopileupandforcestheobjecttorollslightlyuphill.Thischangingofshape,knownasdeformation,causesamovementofmolecules.Asaresult,heatisproducedfromtheaddedenergyrequiredtokeepthepartsturningandovercomefriction.
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.Thissystemisusedinthecrankcaseoflawn-mowerenginestolubricatethecrankshaft,connectingrod,andpartsofthepiston.
Alubricationsystemcommonlyusedinindustrialplantsisthepressuresystem.Inthissystem,apumponamachinecarriesthelubricanttoallofthebearingsurfacesataconstantrateandquantity.
Therearenumerousothersystemsoflubricationandaconsiderablenumberoflubricantsavailableforanygivensetofoperatingconditions.Modernindustrypaysgreaterattentiontotheuseoftheproperlubricantsthanatprevioustimebecauseoftheincreasedspeeds,pressures,andoperatingdemandsplacedonequipmentanddevices.
Althoughoneofthemainpurposesoflubricationisreducefriction,anysubstance-liquid,solid,orgaseous-capableofcontrollingfrictionandwearbetweenslidingsurfacescanbeclassedasalubricant.
Varietiesoflubrication
Unlubricatedsliding.Metalsthathavebeencarefullytreatedtoremoveallforeignmaterialsseizeandweldtooneanotherwhenslidtogether.Intheabsenceofsuchahighdegreeofcleanliness,adsorbedgases,watervapor,oxides,andcontaminantsreducefrictio9nandthetendencytoseizebutusuallyresultinseverewear;thisiscalled“unlubricated”ordrysliding.
Fluid-filmlubrication.Interposingafluidfilmthatcompletelyseparatestheslidingsurfacesresultsinfluid-filmlubrication.Thefluidmaybeintroducedintentionallyastheoilinthemainbearingofanautomobile,orunintentionally,asinthecaseofwaterbetweenasmoothtubertireandawetpavement.Althoughthefluidisusuallyaliquidsuchasoil,water,andawiderangeofothermaterials,itmayalsobeagas.Thegasmostcommonlyemployedisair.
Boundarylubrication.Aconditionthatliesbetweenunlubricatedslidingandfluid-filmlubricationisreferredtoasboundarylubrication,alsodefinedasthatconditionoflubricationinwhichthefrictionbetweensurfacesisdeterminedbythepropertiesofthesurfacesandpropertiesofthelubricantotherthanviscosity.Boundarylubricationencompassesasignificantportionoflubricationphenomenaandcommonlyoccursduringthestartingandstoppingoffmachines.
Solidlubrication.Solidsuchasgraphiteandmolybdenumdisulfidearewidelyusedwhennormallubricantsdonotpossesssufficientresistancetoloadortemperatureextremes.Butlubricantsneednottakeonlysuchfamiliarformsasfats,powders,andgases;evensomemetalscommonlyserveasslidingsurfacesinsomesophisticatedmachines.
Functionoflubricants
Althoughalubricantprimarilycontrolsfrictionandordinarilydoesperformnumerousotherfunctions,whichvarywiththeapplicationandusuallyareinterrelated.
Frictioncontrol.Theamountandcharacterofthelubricantmadeavailabletoslidingsurfaceshaveaprofoundeffectuponthefrictionthatisencountered.Forexample,disregardingsuchrelatedfactorsasheatandwearbutconsideringfrictionalonebetweenthesamesurfaceswithonlubricant.Underfluid-filmconditions,frictionisencountered.Inagreatrangeofviscositiesandthuscansatisfyabroadspectrumoffunctionalrequirements.Underboundarylubricationconditions,theeffectofviscosityonfrictionbecomeslesssignificantthanthechemicalnatureofthelubricant.
Wearcontrol.wearoccursonlubricatedsurfacesbyabrasion,corrosion,andsolid-to-solidcontactwearbyprovidingafilmthatincreasesthedistancebetweentheslidingsurfaces,therebylesseningthedamagebyabrasivecontaminantsandsurfaceasperities.
Temperaturecontrol.Lubricantsassistincontrollingcorrosionofthesurfacesthemselvesistwofold.Whenmachineryisidle,thelubricantactsasapreservative.Whenmachineryisinuse,thelubricantcontrolscorrosionbycoatinglubricatedpartswithaprotectivefilmthatmaycontainadditivestoneutralizecorrosivematerials.Theabilityofalubricanttocontrolcorrosionisdirectlyrelatlytothethicknessofthelubricantfilmremainingonthemetalsurfacesandthechermicalcompositionofthelubricant.
Otherfunctions
Lubricationarefrequentlyusedforpurposesotherthanthereductionoffriction.Someoftheseapplicationsaredescribedbelow.
Powertransmission.Lubricantsarewidelyemployedashydraulicfluidsinfluidtransmissiondevices.
Insulation.Inspecializedapplicationssuchastransformersandswitchgear,lubricantswithhighdielectricconstantsactsaselectricalinsulators.Formaximuminsulatingproperties,alubricantmustbekeptfreeofcontaminantsandwater.
Shockdampening.Lubricantsactasshock-dampeningfluidsinenergytransferringdevicessuchasshockabsorbersandaroundmachinepartssuchasgearsthataresubjectedtohighintermittentloads.
Sealing.Lubricatinggreasefrequentlyperformsthespecialfunctionofformingasealtoretainlubricantsortoexcludecontaminants.
Theobjectoflubricationistoreducefriction,wear,andheatingofmachineparswhichmoverelativetoeachother.Alubricantisanysubstancewhich,wheninsertedbetweenthemovingsurfaces,accomplishesthesepurposes.Mostlubricantsareliquids(suchasmineraloil,siliconefluids,andwater),buttheymaybesolidforuseindrybearings,greasesforuseinrollingelementbearing,orgases(suchasair)foruseingasbearings.Thephysicalandchemicalinteractionbetweenthelubricantandlubricatingsurfacesmustbeunderstoodinordertoprovidethemachineelementswithsatisfactorylife.
Theunderstandingofboundarylubricationisnormallyattributedtohardyanddoubleday,whofoundtheextrememlythinfilmsadheringtosurfaceswereoftensufficienttoassistrelativesliding.Theyconcludedthatundersuchcircumstancesthechemicalcomposition