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1、TheBasicsofSpiralBevelGears1GearingPrinciplesCylindricalandStraightBevelGearsThepurposeofgearsistotransmitmotionandtorquefromoneshafttoanother,Thattransmissionnormallyhastooccurwithaconstantratio,thelowestpossibledisturbancesderivedfromastraightrackwithstraighttoothprofile.Aparticulargear,rollingint
2、herackwithconstantcenterdistancetotherack,requiresinvoluteflanksurfaces.Ashapingtoolwiththeshapeofrackcanmachineagearwithaperfectinvoluteflankform.Figure1showsacylindricalgearrollinginarack.Inthecaseofasingleindexfacemillingmethod,thetoothleadfunctioniscircular,asthebladeinthecutterperformsacircular
3、motion,whilethegeneratinggearrestsinafixedangularposition.Thetoothprofilingbetweenthecutterandthegeneratinggeardoesnotrequireanyrotationofthegeneratinggear.Thevirtualgeneratinggearisformedbythecutterheadinanon-generatingprocess.InFigure3,therotatingbladesinthecutterheadcanbeunderstoodtorepresentonet
4、oothofthegeneratinggear.Asexplainedearlier,thegeneratinggearisthebevelgearequivalentofthestraightrackforgeneratingacylindricalgeartooth.Thepinionslotproducedinthatwayhastwodefects.First,theprofilewillnotallowrollingbetweenpinionandgeneratinggear(comparetotherackandcylindricalgeartoothinFigure1).Seco
5、nd,thepinionslotdoesnothavetheproperdepthalongthefacewidth.Assoonastheteethhaveaspiralangleandtheslotinclinestoanangleonanaxialplane,theteethwindaroundtheworkgearbody.Inafixedangularposition,justtheheelsection,forexample,iscuttotheproperdepth.Therollmotionrotatesthevirtualgeneratinggearandtheworkgea
6、rwiththeproperratiowhiletheyareengaged(similartothelinearmotionoftherack,Figure1,inconjunctionwiththegearrotation).Thatprocedurewasformachiningoneslot.Tomachinethenextslot,thecutterwithdraws,andtheworkindexesonepitch.Thespiralangleistheinclinationangleofthecurvedtoothtangenttotheradiusvectorfromthei
7、ntersectionpointofpinionandgearaxis(seeFigure4).Becauseofthecurvedshapeofthetoothlength,differentpointsalongthelacewidthhavedifferentspiralangles.Thenominalspiralangleofthespiralbevelgearorpinionistheanglemeasuredfromthecenterofthetooth.Itispossibletouseabevelgeneratinggearthatisidenticaltotileringg
8、ear.Filepinionisinthatcasegeneratedbyrollingwiththebevelgeneratinggear,andthegearismanufacturedsimplybyplungingthecuttertofulldepthwithoutrolling(non-generatedformcutting).Astraighttoothbevelgearsethascontactlinesthatareparalleltothepitchline(Figure5,top).Thefirstcontactbetweenageneratinggeartoothan
9、dapiniontoothstarts,forexample,intherootandmovesduringtherotationofthetwomatingmembersalongthepathofcontactstraightuptothetop.Thecontactlinesrepresentthemomentarycontactbetweenthetwoflanksinmesh.Withaspiralbevelgearsetthecontactlinesareinclinedrelativetothepitchlineorientation.Unlikethecontactlineso
10、fthestraightbevelgearset,thecontactlinesofthespiralbevelgearsethavedifferentlengths.Thebottomoffigure5showsthemovementofthecontactfromheeltoptotoeroot.Theveryshortcontactlengthincreasesfromthebeginningoftherolltowardsthecenterofthefacewidthandreducesastherollapproachestheexitatthetoeend.Thecontactli
11、nesbetweenpinionandgeneratinggearareidenticaltothecontactlinesbetweencutterbladesandpinionflanks.2SingleIndexProcess-FaceMillingInasingleindexprocess,justoneslotiscutatatime.Forthenon-generatedmemberonly,thecutterrotatesandisfedintotheworkgeartothefulldepth.Afterreachingthefulldepth,thecutterwithdra
12、wsandtheworkindexesonepitchtothenextdesiredslotposition(Figure6,rightside).Theprocessrepeatsuntilallslotshavebeenmachined.Theresultingflankleadfunctionisacirculararc.Machiningageneratedmemberisdonebyplungingattheheelrollpositionfirst.Afterplunging,therollmotionbegins,andgeneratingoftheflanksfromheel
13、totoeoccurs.Theflankleadfunctionforafacemilled,generatedgearisacirculararcthatiswoundaroundaconicalsurface.Themanufacturing.ofafacemilledbevelgearpairispossibleinafive-cutprocessorinacompletingprocess.Thefive-cutprocessconsistsofthefollowingfiveindependentoperations:1.Gearroughing(alternateroughingb
14、lades),2.Gearfinishing(alternatefinishingblades),3.Pinionroughing(alternateroughingblades),4.Pinionfinishingconvex(innerbladesonly),and5.Pinionfinishingconcave(outerbladesonly).Acompletingprocessusestwocombinedoperations:1.Gearroughingandfinishing(alternateroughingfinishingblades)and2.Pinionroughing
15、andfinishing(alternateroughingfinishingblades).3ContinuousIndexingProcess-FaceHobbingAcontinuouscuttingprocessconsistsofcontinuousrotationsandafeedmotiononly.Whileanouterandaninnerblademovethroughaslotoftheworkgear,theworkgearisrotatingintheoppositedirection,Therelationofthecutterrpmandtheworkrotati
16、onisequivalenttotheratiobetweenthenumberofworkgearteethandthenumbercutterheadbladegroups(starts).Theresultingflankleadfunctionisanepicycloid.Theeffectivecuttingdirectionofthebladesinthecutterheadisnotperpendiculartothecutterradiusvector(likeinthesingleindexingprocess).Thebladesaremovedinthecutterhea
17、dtangentiallytoanoffsetpositiontoaccommodatethecorrectorientationwithrespecttothecuttingmotionvector.Thepitchpointsonthecuttingedgeofinnerandouterbladehaveanidenticalradius.Therightslotwidthisachievedwiththeangulardistancebetweentheouterblade(first)andthefollowinginnerblade.TheleftportionofFigure6sh
18、owsthekinematicrelationshipandtheorientationofthebladesrelativetocutterandcuttingmotion.Balancingofthetooththicknessesbetweenpinionandgearcanonlyberealizedbydifferentradiiofinnerandouterbladepitchpoints,sincethespacingbetweenthebladesisgivenbythecutterheaddesignandthereforeremainsconstant.Whileonebl
19、adegroup(likeshowninFigure6)ismovingthroughoneslot,theworkrotatesintheoppositedirection,suchthatthenextbladegroupentersthenextslot.Thatway,alltheslotsaroundtheworkgeararecutataboutthesametime.Thefeedmotiontofeedthecrittertothefulldepthpositionisthereforeslowerthaninthesingleindexprocess.Anon-generat
20、edworkgearisfinishedwhenthefulldepthpositionisreached.Togetthehighestpossiblespacingaccuracy,adwelltimeisappliedtothenon-generatedmember.Theaimofthedwellmotionistoalloweachbladetomoveoncemoretoeachslot,whichtakesNslotstopassby,whereNisthenumberofcutterstartstimesthenumberof.gearteeth.Nisequivalentto
21、asmanyringgearrevolutionsasthecutterhasstarts.Forageneratedpinion,arollmotionfollowstheplungingcycleinthecenter,rollposition(thecutterdoesnotcutthefulldepthyet).Therollmotionafterplungingmovesthecuttingactiontotheheel；bothplungingandrollingtoheelispartoftheroughingcycle.Attheheelrollposition,thecutt
22、eradvancestothefulldepthposition,thecutterrpmincreasestoafinishingsurfacespeed,andaslowrollmotionfromheeltotoefollows.Whenarrivingatthetoe(endrollposition),allteethofthegeneratedworkgearorpinionarefinished(seeFigure7).4HeatTreatmentofBevelGearsHeattreatmentfollowsthesoftcuttingoperation.Thegenerally
23、usedlowcarbonsteelhastobecarburizedonthesurface,bycasehardeningforexample.Theheattreatmentisfinishedwiththequenchingoperationthatprovidesasurfacehardnessintherangeof60to63Rc(RockwellC)Thepinionmaybe3RcharderthantheringgeartoequalizethewearandreducetheriskofscoringThecorematerialstayssofterandmoreduc
24、tile，withahardnessintherangeof30to40Rc.ThedistortionsfromheattreatmentarecriticaltothefinalhardfinishingoperationThekindofheattreatmentfacility(saltbath，furnaceorcontinuousfurnace)，aswellasthedifferencesbetweenthechargesofblankmaterial，hasasignificantinfluenceOnthegeardistortionThegear,whichismostly
25、shapedlikearing，losesitsflatness(itgetsafacerun-out)viathehardeningprocedureThepinion，inmostcases，isshapedlikealongshaftthatlosesitsstraightness(radialrunout)Inadditiontotheblankbodydistortions，heattreatingcausesadistortionoftheindividualteethThespiralanglechanges，theflanklengthcurvatureisreducedand
26、thepressureanglechangesToachievethebestresults，attentionhastobepaidtoprocessingandhandlingofthepartsthroughthefurnace5HardFinishingofBevelGearsThefinalmachiningoperationafterheattreatmentshouldprovideagoodsurfacefinishandremoveflankdistortions.Themostcommonprocessusedislapping.Pinionandgeararebrough
27、tintomeshandrolledunderlighttorque.Toprovideanabrasiveaction,amixtureofoilandsiliconcarbideispouredbetweentheteeth(Figure8).Arelativemovementofpinionandgearalongtheiraxesandamovementinoffsetdirectioniscreated,suchthatthecontactareamovesfromtoetoheelandbacknumeroustimes.Thelappingprocessimprovesthesu
28、rfacefinish，leavesadesirablemicro-structureontheflanksurfacesandremovesheattreatdistortionstosomeextent.Themetalremovalisnotuniforminthedifferentflanksectionsandvariesfromsettoset,sincethepinionandgearareusedastoolstohardfinisheachother.Thisisthereasonwhylappedsetshavetobebuiltasapair；lappedpinionsa
29、ndgearsarenotinterchangeable.Thelappingsurfacestructureisnotorientedinthedirectionofthecontactlines,thusprovidingagoodhydrodynamicoilfilmbetweenthecontactareas.Thelappingstructurealsotendstodeliversidebandsinanoisefrequencyanalysis,whichmakesthegearsetappeartorollmorequietly.Duringthelappingprocess,
30、apinionandageararealwaysmachinedandfinishedatthesametime.Thetimetolapapairisequaltoorshorterthanthetimetomachineonememberusinganotherfinishingmethod.Therefore,lappingisoftencalledthemosteconomicalbevelgearfinishingprocess.Anotherfinishingoptionisgrindingofbevelgears,whichislimitedtofacemilled(single
31、index)bevelgears.Thegrindingwheelenvelopsasinglesideoranalternatecompletingcutter(Figure9).Todaystechnologydoesnotallowtheuseofagrindingtoolinacontinuousindexingprocess.Theadvantageofgrindingisthemanufacturingofanaccurateflanksurfacewithapredeterminedtopography.Theprocessallowstheconstantlyrepeatedp
32、roductionofequalpans.Buildinginpairsisnotnecessary.Lappedpairsusedinvehiclesrequireanoilchangeafterthefirst1,000milesbecauseofabrasiveparticlesintroducedtothetoothsurfacesduringlapping.Afurtheradvantageofgrindingoverlappingisthatsuchanoilchangeisunnecessarywithgroundspiralbevelgears.Aprocessbetweenl
33、appingandgrindingwithrespecttosurfacespeedandrelativemotionishoning.Honingtrialsonbevelgearshavebeendone,buttheyhaventbeenprovensuccessful.Skivingisahardcuttingprocess.Atoolmaterialsuchascarbideorboroncarbonitrideisusedontilecuttingedge.Thecuttingmachinesetupisidenticaltothatforsoftmachining.Theblad
34、epointdimensioniswiderthantheoneforsoftcutting,suchthata0.005-inchuniformstockremovalperflanktakesplace.Skivingdeliversahighqualitypartaccuracyandafinesurfacefinish.Skivingisappliedtosmallbatchesofmostlylargergearsets,Theadvantageofskivingistheuseofthesamecutterhead(onlywithdifferentblades)andthepos
35、sibleuseofthesamecuttingmachine.Thatmakestheinvestmentinmachinesandtoolsaminimum.6SomeBevelGearConventionsTheexpressionbevelgearsisusedasageneraldescriptionforstraightandspiralbevelgearsaswellashypoidgearsets.Iftheaxesofthepinionandgeardonotintersectbuthaveadistanceinspace,thegearsetiscalledahypoidg
36、earset.Thenameisderivedfromthehyperbolicshapeofthepitchcones.Forsimplification,theblanksarestillmanufacturedwithaconicalshape.Theconvexgearflankrollswiththeconcavepinionflank.Thispairofflanksiscalledthedriveside.Thedirectionofrotationwherethoseflankscontactthepiniondrivesiscalledthedrivedirection.Th
37、edrivesidedirectionisalwaysusedinvehiclestodrivethevehicleforward.Thereversedirectionissubsequentlycalledthecoastside(vehiclerollsdownhill,footisoffthegaspedal,wheelsdrivetheengine).Insomecases,thecoastsideisusedtodrivethevehicle,butitisstillcalledthecoastside.Ease-offisthepresentationofflankformcorrectionsappliedtopinionand/orgear.Theease-offtopographyindefinedintheringgearcoordinatesystem,regardlessofwherethecorrectionsweredone(pinion,gearorboth).Protuberanceisaprofilereliefintherootareaoftheflank,whichpreventsflank
機(jī)械外文翻譯--基本的螺旋錐齒輪.doc
