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经典图书
Advantagesofcamsystems
1.Camsystemscanprovideawiderrangeoffunctionalrelationshipsbetweeninputandoutputthancanlinkagesystems.Inparticular,camscanproducetruedwells(seeSec.10.3),whereaslinkagescanonlyproduceapproximatedwells(seeSec.8.8).Camsalsocanproduceconstant-velocitymotionsoveraspecifieddisplacement.Beforechoosingacamsystemforagivenapplication,theengineershouldmakesurethatthespecialcapabilityofacamisreallyneeded.
2.Designingacamsystemtoprovideaprescribedfunctionisgenerallymoredirectandintuitivethandesigningacorrespondinglinkagesystem.Thishasbecomelessimportantascomputer-aidedlinkagesynthesissoftwarehascontinuedtobedeveloped.
3.Foragivenfunction,acamsystemalmostalwaysoccupieslessspacethandoesalinkagesystem.
Disadvantagesofcamsystems
1.Thecamfolloweralwayscontactsthecamsurfaceatasinglepointoralongasingleline.Therefore,theforcesbetweencamandfollowerarehighlyconcentratedratherthanbeingdistributedoveranarea,suchaswouldbethecaseinarevoluteorsliderjointinalinkage.Asaconsequence,forwearand/orvibrationreasons,morecarefulattentionmustbepaidincamsystemstosurfacehardness,surfacefinish,andlubrication.Whereasrevolutejointsinlinkagescanconsistofenclosedbearings,theexposednatureofcamsurfacescancauseproblemsofkeepingthosesurfacescleanandlubricated,sometimesrequiringtheenclosureoftheentirecamandfollowersystem.
2.Meansmustbeprovidedforkeepingthefollowerincontactwiththecamsurface.Thisusuallyinvolvestheuseofaspringoroftwocarefullycoordinatedcamsurfaces.Ineithercase,thetolerancesand/orcompliancesinthefollowersystemmustbetailoredcarefullytoaccommodateandopposetheforcesthatarereflectedtothefollowerbythemovingoutputmachinepartsandwhichcouldcauseseparationofthefollowerfromthecamsurface(s).
3.Athighspeeds,thetendencyofafollowertoseparatefromthecamsurfacecanresultinimpactsbetweenfollowerandcamand/orcanrequireveryhighspringloadstopreventseparation.Theseimpactsandloadsareconcentratedatasinglepointorlineofcontactbetweenfollowerandcams,asmentionedbefore.
4.Thecomplicatedshapeofcamsurfacescanrequiremoreexpensivemachiningthanisrequiredinthemanufactureoflinkagesystems.Notethatfollowersystemsofteninvolvesomelinks,sothattheuseofacamdoesnotalwayscompletelyreplacealinkage.
Principlesofinvolutegeartoothaction
Ifalengthofstringthatiswrappedaroundtheperipheryofacirculardiskisthengraduallyunwrapped,apointattheendofthatstringwillfollowacurvedpaththatisknownasaninvoluteofthecirclethatisrepresentedbythedisk.Figure11.12showsastringsbeingunwrappedfromaroundacirclec.Successivepositionsofthestringasitisunwrappedareshownasdashedlines,andthepathfollowedbytheendofthatstringisshownasthecurvei.Thatcurveisaninvoluteofthecirclec.Allotherinvolutesgeneratedonthatcirclewillbecongruentwiththecurvei:i.e.,theywillhaveshapesidenticaltothatofi.Indeed,thereisnothingspecialabouttheendofthestring,andifpointsthatarefixedtothestringandareequallyspacedalongitareconsidered,then,asthestringisunwoundformthecircle,thesepointswilltraceidenticalinvolutesthatareequallyspacedalongthestringandequallyspacedaroundthecircle. |
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发表于 2009-6-15 17:59:24
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