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1、Lesson2GEARS1Gearsaredirect-contactbodies,operatinginpairs,thattransmitmotionandforcefromonerotatingshafttoanother,orfromashafttoaslide(rack),bymeansofsuccessivelyengagingprojectionscalledteeth.2Agearhavingtoothelementsthatarestraightandparalleltoitsaxisisknownasas
2、purgear.Aspurpaircanbeusedtoconnectparallelshaftsonly.Parallelshaftshowever,canalsobeconnectedbygearsofanothertypeandaspurgearcanbematedwithagearofadifferenttype.3Sincethepitchcirclesrollononeanother,thespacingoftheteethonthesecirclesonamatingpairmustbeequal.Thissp
3、acing,whichisknownasthecircularpitchpandisameasureoftoothsize,isthedistancebetweencorrespondingpointsonadjacentteeth,measuredonthepitchcircle.4Topreventjammingasaresultofthermalexpansion,toaidlubrication,andtocompensateforunavoidableinaccuraciesinmanufacture,allpow
4、er-transmittinggearsmusthavebacklash.Thismeansthatonthepitchcirclesofamatingpair,thespacewidthonthepinionmustbeslightlygreaterthanthetooththicknessonthegear,andviceversa.Oninstramentgears,backlashcanbeeliminatedbyusingagearsplitdownitsmiddle,onehalfbeingrotatablere
5、lativetotheother.Aspringforcesthesplitgearteethtooccupythefullwidthofthepinionspace.5Ifaninvolutespurpinionweremadeofrubberandtwisteduniformlysothattheendsrotatedabouttheaxisrelativetooneanother,theelementsoftheteeth,initiallystraightandparalleltotheaxis,wouldbecom
6、ehelices.Thepinionthenineffectwouldbecomeahelicalgear.1Helicalgearshavecertainadvantages;forexample,whenconnectingparallelshaftstheyhaveahigherload-carryingcapacitythanspurgearswiththesametoothnumbersandcutwiththesamecutter.Becauseoftheoverlappingactionoftheteeth,t
7、heyaresmootherinactionandcanoperateathigherpitch-linevelocitiesthanspurgears.Thepitch-linevelocityisthevelocityofthepitchcircle.Sincetheteethareinclinedtotheaxisofrotation,helicalgearscreateanaxialthrust.Ifusedsingly,thisthrustmustbeabsorbedintheshaftbearings.Theth
8、rustproblemcanbeovercomebycuttingtwosetsofopposedhelicalteethonthesameblank.Dependingonthemethodofmanufacture,thegearmaybeofthecontinuous-toothhe
