机械制造 毕业设计 外文英文文献 翻译 齿轮和齿轮传动Word文件下载.docx

机械制造 毕业设计 外文英文文献 翻译 齿轮和齿轮传动Word文件下载.docx

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　　　Gearcostincreasessharplywithdemandsforhighprecisionandaccuracy.Soitisimportanttoestablishtolerancerequirementsappropriatefortheapplication.Gearsthattransmitheavyloadsorthanoperateathighspeedsarenotparticularlyexpensive,butgearsthatmustdobotharecostly.

　　　Silentgearsalsoareexpensive.Instrumentandcomputergearstendtobecostlybecausespeedordisplacementratiosmustbeexact.Attheotherextreme,gearsoperatingatlowspeedinexposedlocationsarenormallytermednocriticalandaremadetominimumqualitystandards.

　　　Fortoothforms,size,andquality,industrialpracticeistofollowstandardssetupbytheAmericanGearManufacturesAssociationAGMA.

Toothform

　　　StandardspublishedbyAGMAestablishgearproportionsandtoothprofiles.Toothgeometryisdeterminedprimarilybypitch,depth,andpressureangle.

　　　Pitch：

StandardspitchesareusuallywholenumberswhenmeasuredasdiametralpitchP.Coarse-pitchgearinghasteethlargerthan20diametralpitch?

usually0.5to19.99.Fine-pitchgearingusuallyhasteethofdiametralpitch20to200.

　　　Depth:

Standardizedintermsofpitch.Standardfull-depthhaveworkingdepthof2/p.Iftheteethhaveequaladdendaasinstandardinterchangeablegearstheaddendumis1/p.Stubteethhaveaworkingdepthusually20%lessthanfull-depthteeth.Full-depthteethhavealargercontractratiothanstubteeth.Gearswithsmallnumbersofteethmayhaveundercutsothantheydonotinterferewithoneanotherduringengagement.Undercuttingreduceactiveprofileandweakensthetooth.

　　　Matinggearswithlongandshortaddendumhavelargerload-carryingcapacitythanstandardgears.Theaddendumofthesmallergearpinionisincreasedwhilethatoflargergearisdecreased,leavingthewholedepththesame.Thisformisknowasrecess-actiongearing.

　　　PressureAngle:

Standardanglesareand.Earlierstandardsincludea14-pressureanglethatisstillused.Pressureangleaffectstheforcethattendstoseparatematinggears.Highpressureangledecreasesthecontactratioratioofthenumberofteethincontactbutprovidesatoothofhighercapacityandallowsgearstohavefewerteethwithoutundercutting.

　　　Backlash:

Shortestdistancesbetweenthenon-contactingsurfacesofadjacentteeth.

　　　GearsarecommonlyspecifiedaccordingtoAGMAClassNumber,whichisacodedenotingimportantqualitycharacteristics.Qualitynumberdenotetooth-elementtolerances.Thehigherthenumber,thecloserthetolerance.Number8to16applytofine-pitchgearing.

　　　Gearsareheat-treatedbycase-hardening,through-hardening,nitriding,orprecipitationhardening.Ingeneral,hardergearsarestrongerandlastlongerthansoftones.Thus,hardeningisadevicethatcutstheweightandsizeofgears.Someprocesses,suchasflame-hardening,improveservicelifebutdonotnecessarilyimprovestrength.

　　　Designchecklist

　　　Thelargerinapairiscalledthegear,thesmalleriscalledthepinion.

　　　GearRatio:

Thenumberofteethinthegeardividebythenumberofteethinthepinion.Also,ratioofthespeedofthepiniontothespeedofthegear.Inreductiongears,theratioofinputtooutputspeeds.

　　　GearEfficiency:

Ratioofoutputpowertoinputpower.includesconsiderationofpowerlossesinthegears,inbearings,andfromwindageandchurningoflubricant.

　　　Speed:

Inagivengearnormallylimitedtosomespecificpitchlinevelocity.Speedcapabilitiescanbeincreasedbyimprovingaccuracyofthegearteethandbyimprovingbalanceoftherotatingparts.

　　　Power:

Loadandspeedcapacityisdeterminedbygeardimensionsandbytypeofgear.Helicalandhelical-typegearshavethegreatestcapacitytoapproximately30,000hp.Spiralbevelgeararenormallylimitedto5,000hp,andwormgearsareusuallylimitedtoabout750hp.

　　　Specialrequirements

　　　Matched-SetGearing:

Inapplicationsrequiringextremelyhighaccuracy,itmaybenecessarytomatchpinionandgearprofilesandleadssothatmismatchdoesnotexceedthetoleranceonprofileorleadfortheintendedapplication.

　　　ToothSpacing:

Somegearsrequirehighaccuracyinthecircularofteeth.Thus,specificationofpitchmayberequiredinadditiontoanaccuracyclassspecification.

TheAMGAstandardsrecommendbacklashrangestoprovideproperrunningclearancesformatinggears.Anoverlytightmeshmayproduceoverload.However,zerobacklashisrequiredinsomeapplications.

　　　QuietGears:

Tomakegearsasquitaspossible,specifythefinestpitchallowableforloadconditions.Insomeinstances,however,pitchiscoarsenedtochangemeshfrequencytoproduceamorepleasant,lower-pitchsound.Usealowpressureangle.Useamodifiedprofiletoincluderootandtiprelief.Allowenoughbacklash.Usehighqualitynumbers.Specifyasurfacefinishof20in.orbetter.Balancethegearset.Useanonintegralratiosothatthesameteethdonotrepeatedlyengageifbothgearandpinionarehardenedsteel.Ifthegearismadeofasoftmaterial,anintegralratioallowsthegeartocold-workandconformtothepinion,therebypromotingquietoperation.Makesurecriticalareatleast20%apartfromoperatingspeedingorspeedmultiplesandfromfrequencyoftoothmesh.

　　　Multiplemeshgear

　　　Multiplemeshreferstomovethanonepairofgearoperatinginatrain.Canbeonparallelornonparallelaxesandonintersectionornonintersectingshafts.Theypermithigerspeedratiosthanarefeasiblewithasinglepairofgears.

　　　Seriestrains:

Overallratioisinputshaftspeeddividedbyoutputspeed,alsotheproductofindividualratiosateachmesh,exceptinplanetarygears.Ratioismosteasilyfoundbydividingtheproductofnumbersofteethofdrivengearsbytheproductofnumbersofteethofdrivinggears.

　　　Speedincreaserswithstep-upratherthanstep-downratiosmayrequirespecialcareinmanufacturinganddesign.Theyofteninvolvehighspeedsandmaycresteproblemsingeardynamics.Also,frictionalanddragforcesaremagnifiedwhich,inextremecases,mayleadtooperationalproblems.

　　　EpicyclicGearing:

Normally,agearaxisremainsfixedandonlythegearsrotates.Butinanepicyclicgeartrain,variousgearsaxesrotateaboutoneanthertoprovidespecializedoutputmotions.Withsuitableclutchseandbrakes,anepicyclictrainservesastheplanetarygearcommonlyfoundinautomatictransmissions.

　　　Epicyclictrainsmayusespurorhelicalgears,externalorinternal,orbevelgears.Intransmissions,theepicyclicorplanetarygearsusuallyhavemultipleplanetstoincreaseloadcapacity.

　　　Inmostcases,improvedkinematicaccuracyinagearsetdecreasesgearmeshexcitationandresultsinlowerdrivenoise.Gearsetaccuracycanbeincreasedbymodifyingthetoothinvoluteprofile,bysubstitutinghigherqualitygearingwithtightermanufacturingtolerances,andbyimprovingtoothsurfacefinish.However,ifgearmeshexcitationgeneratersresonancesomewhereinthedrivesystem,nothingshortofa“perfect”gearsetwillsubstantiallyreducevibrationandnoise.

　　　Toothprofilesaremodifiedtoavoidinterferenceswhichcanresultfromdeflectionsinthegears,shafts,andhousingasteethengageanddisendgage.Ifthesetoothinterferencesarenotcompensatedforbyprofilemodifications,gearsloadcapacitycanbeseriouslyreduced.Inaddition,thedrivewillbenoisierbecausetoothinterferencesgeneratehighdynamicloads.Interferencestypicallyareeliminatedbyrelivingthetoothtip,thetoothflank,orboth.Suchprofilemodificationsareespeciallyimportantforhigh-load,high-speeddrives.Thegraphofsoundpressurelevelvstipreliefillustrateshowtoothprofilemodificationscanaffectoveralldrivenoise.Ifthetipreliefislessthanthisoptimumvalue,drivenoiseincreasesbecauseofgreatertoothinterference;

agreateramountoftipreliefalsoincreasenoisebecausethecontactratioisdecreased.

　　　Tightermanufacturingtolerancesalsoproducequietiergears.Tolerancesforsuchparametersasprofileerror,pitchAGMAqualitylevel.Forinstance,thegraphdepictingSPLvsbothspeedandgearqualityshowshownoisedecreasesexample,noiseisreducedsignificantlybyanincreaseinaccuracyfromanAGMAQn11qualitytoanAGNAQn15quality.However,formostcommercialdriveapplications,itisdoubtfulthattheresultingsubstantialcostincreaseforsuchanaccuracyimprovementcanbejustifiedsimplyonthebasisofreduceddrivenoise.

　　　Previously,itwasmentionedthatgearsmusthaveadequateclearancewhenloadedtopreventtoothinterferenceduringthecourseofmeshing.Tipandflankreliefarecommonprofilemodificationsthatcontrolsuchinterference.Gearsalsorequireadequatebacklashandrootclearance.Noiseconsiderationsmakebacklashanimportantparametertoevaluateduringdrivedesign.Sufficientbacklashmustbeprovidedunderallloadandtemperatureconditionstoavoidatightmesh,whichcreatesexcessivelyhighnoiselevel.Atightmeshduetoinsufficientbacklashoccurswhenthedriveandcoastsideofatoothareincontactsimultaneously.Ontheotherhand,gearswithexcessivebacklashalsoarenoisybecauseofimpactingteethduringperiodsofnoloadorreversingload.Adequatebackla