外文翻译--润滑和轴颈轴承.doc -- 5 元

附录英语翻译LUBRICATIONANDJOURNALBEARINGS1IntroductionAbearingcanbedefinedasamemberspecificallydesignedtosupportmovingmachinecomponents.Themostcommonbearingapplicationisthesupportofarotatingshaftwhichistransmittingpowerfromonelocationtoanotheroneexampleisthecrankshaftbearingsofautomaticengineanotherexampleistheshaftbearingsusedalltypesofelectricmotors.Sincethereisalwaysrelativemotionbetweenabearinganditsmatingsurface,frictionisinvolved.Inmanyinstances,suchasthedesignofpulleys,brakesandclutches,frictionisdesirable.However,inthecaseofbearings,thereductionoffrictionisoneofprimeconsiderationsfrictionresultsinlossofpower,generationofheatandwearofmatingsurfaces.Journalandantifrictionbearingsarethetwogeneraltypesofbearingsexistence.Journalbearingsoperatewithslidingcontact,whereasantifrictionbearingsexperiencepredominantlyrollingcontact.Theamountofslidingfrictioninjournalbearingsdependsonthesurfacefinishes,materials,slidingvelocitiesandthetypeoflubricantused.Theprinciplemotionretardingeffectinantifrictionbearingsiscalledrollingresistanceratherthanrollingfriction.Thisissobecausetheresistanceofmotionisessentiallyduetothedeformationoftherollingelementsand,hence,itisnotaslidingphenomenon.Antifrictionbearingswillbeinchapter1.4.Toreducetheproblemsassociatedwithslidingfrictioninjournalbearings,alubricantisusedinconjunctionwithcompatiblematingmaterials.Whenselectingthelubricantandmatingmaterials,onemusttakeintoaccountbearingpressures,temperaturesandrubbingvelocities.Theprinciplefunctionofthelubricantinslidingcontactbearingsistopreventphysicalcontactbetweentherubbingsurfaces.Thusthemaintenanceofanoilfilmundervaryingloads,speedsandtemperatureistheprimeconsiderationinslidingcontactbearings.2TheoryofFrictionFrictionistheresistanceonepartexertsonasecondpartwhenrelativeslidingmotionoccursorisattempted.Thusfrictiontakesplacewhenevertwosurfacesrubtogether.Thecauseoffrictionistheinevitableinterlockingofthetinyirregularitiesofthetwomatingsurfaces.Aforceisrequiredtodeformthetinypeaksandvalleystopermitmotion.WhenablockofweightWrestsonahorizontalfixedsurface,aforcePisappliedtotheblock.Initially,Pequalszero,butitsvalueconstantlyincreasesasafunctionoftime.Duetofriction,aforceFiscreatedbetweenblockandfixedsurface.Thedirectionofthefrictionalforce,F,isoppositethatofP,becausefrictionalwaysopposesmotionorattemptedmotion.Alsonotethatthenormalforce,N,actingperpendiculartothematingsurfaceisequalandoppositetotheweight,W,oftheblock.RollingContactBearingsTheconcernofamachinedesignerwithballandrollerbearingsisfivefoldasfollowsalifeinrelationtoloadbstiffness,i.e.deflectionsunderloadcfrictiondwearenoise.Formoderateloadsandspeedsthecorrectselectionofastandardbearingonthebasisofloadratingwillbecomeimportantwhereloadsarehigh,althoughthisisusuallyoflessmagnitudethanthatoftheshaftsorothercomponentsassociatedwiththebearing.Wherespeedsarehighspecialcoolingarrangementsbecomenecessarywhichmayincreasefrictionaldrag.Wearisprimarilyassociatedwiththeintroductionofcontaminants,andsealingarrangementsmustbechosenwithregardtothehostilityoftheenvironment.Becausethehighqualityandlowpriceofballandrollerbearingsdependsonquantityproduction,thetaskofthemachinedesignerbecomesoneofselectionratherthandesign.Rollingcontactbearingsaregenerallymadewithsteelwhichisthroughhardenedtoabout900HV,althoughinmanymechanismsspecialracesarenotprovidedandtheinteractingsurfacesarehardenedtoabout600HV.Itisnotsurprisingthat,owingtothehighstressesinvolved,apredominantformoffailureshouldbemetalfatigue,andagooddealofworkisbasedonacceptedvaluesoflifeanditisgeneralpracticeinthebearingindustrytodefinetheloadcapacityofthebearingasthatvaluebelowwhich90percentofabatchwillexceedalifeofonemillionrevolutions.Notwithstandingthefactthatresponsibilityforthebasicdesignofballandrollerbearingsrestswiththebearingmanufacturer,themachinedesignermustformacorrectappreciationofthedutytobeperformedbythebearingandbeconcernednotonlywithbearingselectionbutwiththeconditionsforcorrectinstallation.Thefitofthebearingracesontotheshaftorontothehousingsisofcriticalimportancebecauseoftheircombinedeffectontheinternalclearanceofthebearingaswellaspreservingthedesireddegreeofinterferencefit.Inadequateinterferencecaninduceserioustroublefromfrettingcorrosion.Theinnerraceisfrequentlylocatedaxiallybyabuttingagainstashoulder.Aradiusatthispointisessentialfortheavoidanceofstressconcentrationandballracesareprovidedwitharadiusorchamfertoallowspaceforthis.Wherelifeisnotthedeterminingfactorindesign,itisusualtodeterminemaximumloadingbytheamounttowhichabearingwilldeflectunderload.Thustheconceptofstaticloadcarryingcapacityisunderstoodtomeantheloadthatcanbeappliedtoabearing,whichiseitherstationaryorsubjecttoslightswivelingmotions,withoutimpairingitsrunningqualitiesforsubsequentrotationalmotion.Thishasbeendeterminedbypracticalexperienceastheloadwhichwhenappliedtoabearingresultsinatotaldeformationoftherollingelementdiameter.Thiswouldcorrespondtoapermanentdeformationof0.0025mmforaball25mmindiameter.Thesuccessfulfunctioningofmanybearingsdependsuponprovidingthemwithadequateprotectionagainsttheirenvironment,andinsomecircumstancestheenvironmentmustbeprotectedfromlubricantsorproductsofdeteriorationofthebearingdesign.Moreover,sealswhichareappliedtomovingpartsforanypurposeareofinteresttotribologistsbecausetheyarecomponentsofbearingsystemsandcanonlybedesignedsatisfactorilyonthebasisoftheappropriatebearingtheory.Notwithstandingtheirimportance,theamountofresearcheffortthathasbeendevotedtotheunderstandingofthebehaviorofsealshasbeensmallwhencomparedwiththatdevotedtootheraspectsofbearingtechnology.NumericalControlOneofthemostfundamentalconceptsintheareaofadvancedmanufacturingtechnologiesisnumericalcontrolNC.PriortotheadventofNC,allmachinetoolsweremanuallyoperatedandcontrolled.Amongthemanylimitationsassociatedwithmanualcontrolmachinetools,perhapsnoneismoreprominentthanthelimitationofoperatorskills.Withmanualcontrol,thequalityoftheproductisdirectlyrelatedtoandlimitedtotheskillsoftheoperator.Numericalcontrolrepresentsthefirstmajorstepawayfromhumancontrolofmachinetools.Numericalcontrolmeansthecontrolofmachinetoolsandothermanufacturingsystemsthroughtheuseofprerecorded,writtensymbolicinstructions.Ratherthanoperatingamachinetool,anNCtechnicianwritesaprogramthatissuesoperationalinstructionstothemachinetool.Foramachinetooltobenumericallycontrolled,itmustbeinterfacedwithadeviceforacceptinganddecodingtheprogrammedinstructions,knownasareader.Numericalcontrolwasdevelopedtoovercomethelimitationofhumanoperators,andithasdoneso.Numericalcontrolmachinesaremoreaccuratethanmanuallyoperatedmachines,theycanproducepartsmoreuniformly,theyarefaster,andthelongruntoolingcostsarelower.ThedevelopmentofNCledtothedevelopmentofseveralotherinnovationsinmanufacturingtechnology1.Electricaldischargemachining.2.Lasercutting.3.Electronbeamwelding.Numericalcontrolhasalsomademachinetoolsmoreversatilethantheirmanuallyoperatedpredecessors.AnNCmachinetoolcanautomaticallyproduceawidevarietyofparts,eachinvolvinganassortmentofwidelyvariedandcomplexmachiningprocesses.Numericalcontrolhasallowedmanufacturerstoundertaketheproductionofproductsthatwouldnothavebeenfeasiblefromaneconomicperspectiveusingmanuallycontrolledmachinetoolsandprocesses.Likesomanyadvancedtechnologies,NCwasborninthelaboratoriesoftheMassachusettsInstituteofTechnology.TheconceptofNCwasdevelopedintheearly1950swithfundingprovidedbytheU.S.Airforce.Initsearlieststages,NCmachineswereabletomakestraightcutsefficientlyandeffectively.However,curvedpathswereaproblembecausethemachinetoolhadtobeprogrammedtoundertakeaseriesofhorizontalandverticalstepstoproduceacurve.Theshorteristhestraightlinesmakingupthesteps,thesmootheristhecurve.Eachlinesegmentinthestepshadtobecalculated.Thisproblemledtothedevelopmentin1959oftheAutomaticallyProgrammedToolsAPTlanguage.ThisisaspecialprogramminglanguageforNCthatusesstatementssimilartoEnglishlanguagetodefinethepartgeometry,describethecuttingtoolconfiguration,andspecifythenecessarymotions.ThedevelopmentoftheAPTlanguagewasamajorstepforwardinthefurtherdevelopmentofNCtechnology.TheoriginalNCsystemswerevastlydifferentfromthoseusedtoday.Themachineshadhardwiredlogiccircuits.Theinstructionalprogramswerewrittenonpunched