兰州交通大学毕业设计（论文）1GearsandgeardriveGearsarethemostdurableandruggedofallmechanicaldrives.Theycantransmithighpoweratefficienciesupto98%andwithlongservicelives.Forthisreason,gearsratherthanbeltsorchainsarefoundinautomotivetransmissionsandmostheavy-dutymachinedrives.Ontheotherhand,gearsaremoreexpensivethanotherdrives,especiallyiftheyaremachinedandnotmadefrompowermetalorplastic.Gearcostincreasessharplywithdemandsforhighprecisionandaccuracy.Soitisimportanttoestablishtolerancerequirementsappropriatefortheapplication.Gearsthattransmitheavyloadsorthanoperateathighspeedsarenotparticularlyexpensive,butgearsthatmustdobotharecostly.Silentgearsalsoareexpensive.Instrumentandcomputergearstendtobecostlybecausespeedordisplacementratiosmustbeexact.Attheotherextreme,gearsoperatingatlowspeedinexposedlocationsarenormallytermednocriticalandaremadetominimumqualitystandards.Fortoothforms,size,andquality,industrialpracticeistofollowstandardssetupbytheAmericanGearManufacturesAssociation(AGMA).ToothformStandardspublishedbyAGMAestablishgearproportionsandtoothprofiles.Toothgeometryisdeterminedprimarilybypitch,depth,andpressureangle.Pitch：StandardspitchesareusuallywholenumberswhenmeasuredasdiametralpitchP.Coarse-pitchgearinghasteethlargerthan20diametralpitch–usually0.5to19.99.Fine-pitchgearingusuallyhasteethofdiametralpitch20to200.Depth:Standardizedintermsofpitch.Standardfull-depthhaveworkingdepthof2/p.Iftheteethhaveequaladdenda(asinstandardinterchangeablegears)theaddendumis1/p.Stubteethhaveaworkingdepthusually20%lessthanfull-depthteeth.Full-depthteethhavealargercontractratiothanstubteeth.Gearswithsmallnumbersofteethmayhaveundercutsothantheydonotinterferewithoneanotherduringengagement.Undercuttingreduceactiveprofileandweakensthetooth.Matinggearswithlongandshortaddendumhavelargerload-carryingcapacitythanstandardgears.Theaddendumofthesmallergear(pinion)isincreasedwhilethatoflarger兰州交通大学毕业设计（论文）2gearisdecreased,leavingthewholedepththesame.Thisformisknowasrecess-actiongearing.PressureAngle:Standardanglesare020and025.Earlierstandardsincludea14-02/1pressureanglethatisstillused.Pressureangleaffectstheforcethattendstoseparatematinggears.Highpressureangledecreasesthecontactratio(ratioofthenumberofteethincontact)butprovidesatoothofhighercapacityandallowsgearstohavefewerteethwithoutundercutting.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.DesignchecklistThelargerinapairiscalledthegear,thesmalleriscalledthepinion.GearRatio:Thenumberofteethinthegeardividebythenumberofteethinthepinion.Also,ratioofthespeedofthepiniontothespeedofthegear.Inreductiongears,theratioofinputtooutputspeeds.GearEfficiency:Ratioofoutputpowertoinputpower.(includesconsiderationofpowerlossesinthegears,inbearings,andfromwindageandchurningoflubricant.)Speed:Inagivengearnormallylimitedt...
