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1、GearsLessonOneChapterOneMechanicsComponentsSpurandhelicalgears.Agearhavingtoothelementsthatarestraightandparalleltoitsaxisisknownasaspurgear.Aspurpaircanbeusedtoconnectparallelshaftsonly.Parallelshafts,however,canalsobeconnectedbygearsofanothertype,andaspurgearcanbematedwithagearofadiffer
2、enttype(Fig.1-1).Fig1-1SpurgearsAddendumcircle齿顶圆Dedendumcircle齿根圆Pitchcircle节圆Basecircle基圆RootcircleFlank齿侧Chordalpitch弦间距splitgearTopreventjammingasaresultofthermalexpansion,toaidlubrication,andtocompensateforunavoidableinaccuraciesinmanufacture,allpower-transmittinggearsmusthavebacklas
3、h.Thismeansthatonthepitchcirclesofamatingpair,thespacewidthonthepinionmustbeslightlygreaterthanthetooththicknessonthegear,andviceversa.Oninstrumentgears,backlashcanbeeliminatedbyusingagearsplitdownitsmiddle,onehalfbeingrotatablerelativetotheother.Aspringforcesthesplitgearteethtooccupythef
4、ullwidthofthepinionspace.Helicalgearshavecertainadvantages;forexample,whenconnectingparallelshaftstheyhaveahigherloadcarryingcapacitythanspurgearswiththesametoothnumbersandcutwiththesamecutter.Becauseoftheoverlappingactionoftheteeth,theyaresmootherinactionandcanoperateathigherpitch-lineve
5、locitiesthanspurgears.Thepitch-linevelocityisthevelocityofthepitchcircle.Sincetheteethareinclinedtotheaxisofrotation,helicalgearscreateanaxialthrust.Ifusedsingly,thisthrustmustbeabsorbedintheshaftbearings.Thethrustproblemcanbeovercomebycuttingtwosetsofopposedhelicalteethonthesameblank.Dep
6、endingonthemethodofmanufacture,thegearmaybeofthecontinuous-toothherringbonevarietyoradouble-helicalgearwithaspacebetweenthetwohalvestopermitthecuttingtooltorunout.Double-helicalgearsarewellsuitedfortheefficienttransmissionofpowerathighspeeds.Helicalgearscanalsobeusedtoconnectnonparallel,n
7、on-intersectingshaftsatanyangletooneanother.Ninetydegreesisthecommonestangleatwhichsuchgearsareused.Inordertoachievelinecontactandimprovetheload-carryingcapacityofthecrossed-axishelicalgears,thegearcanbemadetocurvepartiallyaroundthepinion,insomewhatthesamewaythatanu
