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机械外文翻译--基本的螺旋锥齿轮.doc

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机械外文翻译--基本的螺旋锥齿轮.doc

TheBasicsofSpiralBevelGears1GearingPrinciplesCylindricalandStraightBevelGearsThepurposeofgearsistotransmitmotionandtorquefromoneshafttoanother,Thattransmissionnormallyhastooccurwithaconstantratio,thelowestpossibledisturbancesderivedfromastraightrackwithstraighttoothprofile.Aparticulargear,rollingintherackwithconstantcenterdistancetotherack,requiresinvoluteflanksurfaces.Ashapingtoolwiththeshapeofrackcanmachineagearwithaperfectinvoluteflankform.Figure1showsacylindricalgearrollinginarack.Inthecaseofasingleindexfacemillingmethod,thetoothleadfunctioniscircular,asthebladeinthecutterperformsacircularmotion,whilethegeneratinggearrestsinafixedangularposition.Thetoothprofilingbetweenthecutterandthegeneratinggeardoesnotrequireanyrotationofthegeneratinggear.Thevirtualgeneratinggearisformedbythecutterheadinanongeneratingprocess.InFigure3,therotatingbladesinthecutterheadcanbeunderstoodtorepresentonetoothofthegeneratinggear.Asexplainedearlier,thegeneratinggearisthebevelgearequivalentofthestraightrackforgeneratingacylindricalgeartooth.Thepinionslotproducedinthatwayhastwodefects.First,theprofilewillnotallowrollingbetweenpinionandgeneratinggearcomparetotherackandcylindricalgeartoothinFigure1.Second,thepinionslotdoesnothavetheproperdepthalongthefacewidth.Assoonastheteethhaveaspiralangleandtheslotinclinestoanangleonanaxialplane,theteethwindaroundtheworkgearbody.Inafixedangularposition,justtheheelsection,forexample,iscuttotheproperdepth.Therollmotionrotatesthevirtualgeneratinggearandtheworkgearwiththeproperratiowhiletheyareengagedsimilartothelinearmotionoftherack,Figure1,inconjunctionwiththegearrotation.Thatprocedurewasformachiningoneslot.Tomachinethenextslot,thecutterwithdraws,andtheworkindexesonepitch.ThespiralangleistheinclinationangleofthecurvedtoothtangenttotheradiusvectorfromtheintersectionpointofpinionandgearaxisseeFigure4.Becauseofthecurvedshapeofthetoothlength,differentpointsalongthelacewidthhavedifferentspiralangles.Thenominalspiralangleofthespiralbevelgearorpinionistheanglemeasuredfromthecenterofthetooth.Itispossibletouseabevelgeneratinggearthatisidenticaltotileringgear.Filepinionisinthatcasegeneratedbyrollingwiththebevelgeneratinggear,andthegearismanufacturedsimplybyplungingthecuttertofulldepthwithoutrollingnongeneratedformcutting.AstraighttoothbevelgearsethascontactlinesthatareparalleltothepitchlineFigure5,top.Thefirstcontactbetweenageneratinggeartoothandapiniontoothstarts,forexample,intherootandmovesduringtherotationofthetwomatingmembersalongthepathofcontactstraightuptothetop.Thecontactlinesrepresentthemomentarycontactbetweenthetwoflanksinmesh.Withaspiralbevelgearsetthecontactlinesareinclinedrelativetothepitchlineorientation.Unlikethecontactlinesofthestraightbevelgearset,thecontactlinesofthespiralbevelgearsethavedifferentlengths.Thebottomoffigure5showsthemovementofthecontactfromheeltoptotoeroot.Theveryshortcontactlengthincreasesfromthebeginningoftherolltowardsthecenterofthefacewidthandreducesastherollapproachestheexitatthetoeend.Thecontactlinesbetweenpinionandgeneratinggearareidenticaltothecontactlinesbetweencutterbladesandpinionflanks.2SingleIndexProcessFaceMillingInasingleindexprocess,justoneslotiscutatatime.Forthenongeneratedmemberonly,thecutterrotatesandisfedintotheworkgeartothefulldepth.Afterreachingthefulldepth,thecutterwithdrawsandtheworkindexesonepitchtothenextdesiredslotpositionFigure6,rightside.Theprocessrepeatsuntilallslotshavebeenmachined.Theresultingflankleadfunctionisacirculararc.Machiningageneratedmemberisdonebyplungingattheheelrollpositionfirst.Afterplunging,therollmotionbegins,andgeneratingoftheflanksfromheeltotoeoccurs.Theflankleadfunctionforafacemilled,generatedgearisacirculararcthatiswoundaroundaconicalsurface.Themanufacturing.ofafacemilledbevelgearpairispossibleinafivecutprocessorinacompletingprocess.Thefivecutprocessconsistsofthefollowingfiveindependentoperations1.Gearroughingalternateroughingblades,2.Gearfinishingalternatefinishingblades,3.Pinionroughingalternateroughingblades,4.Pinionfinishingconvexinnerbladesonly,and5.Pinionfinishingconcaveouterbladesonly.Acompletingprocessusestwocombinedoperations1.Gearroughingandfinishingalternateroughingfinishingbladesand2.Pinionroughingandfinishingalternateroughingfinishingblades.3ContinuousIndexingProcessFaceHobbingAcontinuouscuttingprocessconsistsofcontinuousrotationsandafeedmotiononly.Whileanouterandaninnerblademovethroughaslotoftheworkgear,theworkgearisrotatingintheoppositedirection,Therelationofthecutterrpmandtheworkrotationisequivalenttotheratiobetweenthenumberofworkgearteethandthenumbercutterheadbladegroupsstarts.Theresultingflankleadfunctionisanepicycloid.Theeffectivecuttingdirectionofthebladesinthecutterheadisnotperpendiculartothecutterradiusvectorlikeinthesingleindexingprocess.Thebladesaremovedinthecutterheadtangentiallytoanoffsetpositiontoaccommodatethecorrectorientationwithrespecttothecuttingmotionvector.Thepitchpointsonthecuttingedgeofinnerandouterbladehaveanidenticalradius.Therightslotwidthisachievedwiththeangulardistancebetweentheouterbladefirstandthefollowinginnerblade.TheleftportionofFigure6showsthekinematicrelationshipandtheorientationofthebladesrelativetocutterandcuttingmotion.Balancingofthetooththicknessesbetweenpinionandgearcanonlyberealizedbydifferentradiiofinnerandouterbladepitchpoints,sincethespacingbetweenthebladesisgivenbythecutterheaddesignandthereforeremainsconstant.WhileonebladegrouplikeshowninFigure6ismovingthroughoneslot,theworkrotatesintheoppositedirection,suchthatthenextbladegroupentersthenextslot.Thatway,alltheslotsaroundtheworkgeararecutataboutthesametime.Thefeedmotiontofeedthecrittertothefulldepthpositionisthereforeslowerthaninthesingleindexprocess.Anongeneratedworkgearisfinishedwhenthefulldepthpositionisreached.Togetthehighestpossiblespacingaccuracy,adwelltimeisappliedtothenongeneratedmember.Theaimofthedwellmotionistoalloweachbladetomoveoncemoretoeachslot,whichtakesNslotstopassby,whereNisthenumberofcutterstartstimesthenumberof.gearteeth.Nisequivalenttoasmanyringgearrevolutionsasthecutterhasstarts.Forageneratedpinion,arollmotionfollowstheplungingcycleinthecenter,rollpositionthecutterdoesnotcutthefulldepthyet.Therollmotionafterplungingmovesthecuttingactiontotheheelbothplungingandrollingtoheelispartoftheroughingcycle.Attheheelrollposition,thecutteradvancestothefulldepthposition,thecutterrpmincreasestoafinishingsurfacespeed,andaslowrollmotionfromheeltotoefollows.Whenarrivingatthetoeendrollposition,allteethofthegeneratedworkgearorpinionarefinishedseeFigure7.4HeatTreatmentofBevelGearsHeattreatmentfollowsthesoftcuttingoperation.Thegenerallyusedlowcarbonsteelhastobecarburizedonthesurface,bycasehardeningforexample.Theheattreatmentisfinishedwiththequenchingoperationthatprovidesasurfacehardnessintherangeof60to63RcRockwellC.Thepinionmaybe3Rcharderthantheringgeartoequalizethewearandreducetheriskofscoring.Thecorematerialstayssofterandmoreductile,withahardnessintherangeof30to40Rc.Thedistortionsfromheattreatmentarecriticaltothefinalhardfinishingoperation.Thekindofheattreatmentfacilitysaltbath,furnaceorcontinuousfurnace,aswellasthedifferencesbetweenthechargesofblankmaterial,hasasignificantinfluenceOnthegeardistortion.Thegear,whichismostlyshapedlikearing,losesitsflatnessitgetsafacerunout.viathehardeningprocedure.Thepinion,inmostcases,isshapedlikealongshaftthatlosesitsstraightnessradialrunout.Inadditiontotheblankbodydistortions,heattreatingcausesadistortionoftheindividualteeth.Thespiralanglechanges,theflanklengthcurvatureisreducedandthepressureanglechanges.Toachievethebestresults,attentionhastobepaidtoprocessingandhandlingofthepartsthroughthefurnace.5HardFinishingofBevelGearsThefinalmachiningoperationafterheattreatmentshouldprovideagoodsurfacefinishandremoveflankdistortions.Themostcommonprocessusedislapping.Pinionandgeararebroughtintomeshandrolledunderlighttorque.Toprovideanabrasiveaction,amixtureofoilandsiliconcarbideispouredbetweentheteethFigure8.Arelativemovementofpinionandgearalongtheiraxesandamovementinoffsetdirectioniscreated,suchthatthecontactareamovesfromtoetoheelandbacknumeroustimes.Thelappingprocessimprovesthesurfacefinish,leavesadesirablemicrostructureontheflanksurfacesandremovesheattreatdistortionstosomeextent.Themetalremovalisnotuniforminthedifferentflanksectionsandvariesfromsettoset,sincethepinionandgearareusedastoolstohardfinisheachother.Thisisthereasonwhylappedsetshavetobebuiltasapairlappedpinionsandgearsarenotinterchangeable.Thelappingsurfacestructureisnotorientedinthedirectionofthecontactlines,thusprovidingagoodhydrodynamicoilfilmbetweenthecontactareas.Thelappingstructurealsotendstodeliversidebandsinanoisefrequencyanalysis,whichmakesthegearsetappeartorollmorequietly.Duringthelappingprocess,apinionandageararealwaysmachinedandfinishedatthesametime.Thetimetolapapairisequaltoorshorterthanthetimetomachineonememberusinganotherfinishingmethod.Therefore,lappingisoftencalledthemosteconomicalbevelgearfinishingprocess.Anotherfinishingoptionisgrindingofbevelgears,whichislimitedtofacemilledsingleindexbevelgears.ThegrindingwheelenvelopsasinglesideoranalternatecompletingcutterFigure9.Todaystechnologydoesnotallowtheuseofagrindingtoolinacontinuousindexingprocess.Theadvantageofgrindingisthemanufacturingofanaccurateflanksurfacewithapredeterminedtopography.Theprocessallowstheconstantlyrepeatedproductionofequalpans.Buildinginpairsisnotnecessary.Lappedpairsusedinvehiclesrequireanoilchangeafterthefirst1,000milesbecauseofabrasiveparticlesintroducedtothetoothsurfacesduringlapping.Afurtheradvantageofgrindingoverlappingisthatsuchanoilchangeisunnecessarywithgroundspiralbevelgears.Aprocessbetweenlappingandgrindingwithrespecttosurfacespeedandrelativemotionishoning.Honingtrialsonbevelgearshavebeendone,buttheyhaventbeenprovensuccessful.Skivingisahardcuttingprocess.Atoolmaterialsuchascarbideorboroncarbonitrideisusedontilecuttingedge.Thecuttingmachinesetupisidenticaltothatforsoftmachining.Thebladepointdimensioniswiderthantheoneforsoftcutting,suchthata0.005inchuniformstockremovalperflanktakesplace.Skivingdeliversahighqualitypartaccuracyandafinesurfacefinish.Skivingisappliedtosmallbatchesofmostlylargergearsets,Theadvantageofskivingistheuseofthesamecutterheadonlywithdifferentbladesandthepossibleuseofthesamecuttingmachine.Thatmakestheinvestmentinmachinesandtoolsaminimum.6SomeBevelGearConventionsTheexpressionbevelgearsisusedasageneraldescriptionforstraightandspiralbevelgearsaswellashypoidgearsets.Iftheaxesofthepinionandgeardonotintersectbuthaveadistanceinspace,thegearsetiscalledahypoidgearset.Thenameisderivedfromthehyperbolicshapeofthepitchcones.Forsimplification,theblanksarestillmanufacturedwithaconicalshape.Theconvexgearflankrollswiththeconcavepinionflank.Thispairofflanksiscalledthedriveside.Thedirectionofrotationwherethoseflankscontactthepiniondrivesiscalledthedrivedirection.Thedrivesidedirectionisalwaysusedinvehiclestodrivethevehicleforward.Thereversedirectionissubsequentlycalledthecoastsidevehiclerollsdownhill,footisoffthegaspedal,wheelsdrivetheengine.Insomecases,thecoastsideisusedtodrivethevehicle,butitisstillcalledthecoastside.Easeoffisthepresentationofflankformcorrectionsappliedtopinionand/orgear.Theeaseofftopographyindefinedintheringgearcoordinatesystem,regardlessofwherethecorrectionsweredonepinion,gearorboth.Protuberanceisaprofilereliefintherootareaoftheflank,whichpreventsflank

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