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外文翻译--运动的综合,凸轮和齿轮.doc

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外文翻译--运动的综合,凸轮和齿轮.doc

外文翻译KinematicSynthesis,CamsandGearsMechanismsformthebasicgeometricalelementsofmanymechanicaldevicesincludingautomaticpackagingmachinery,typewriters,mechanicaltoys,textilemachinery,andothers.Amechanismtypicallyisdesignedtocreateadesiredmotionofarigidbodyrelativetoareferencemember.Kinematicdesign,orkinematicsyntheses,ofmechanismsoftenisthefirststepinthedesignofacompletemachine.Whenforcesareconsidered,theadditionalproblemsofdynamics,bearingloads,stresses,lubrication,andthelikeareintroduced,andthelargerproblembecomesoneofmachinedesign.Akinematiciandefinedkinematicsasthestudyofthemotionofmechanismsandmethodsofcreatingthem.Thefirstpartofthisdefinitiondealswithkinematicanalysis.Givenacertainmechanism,themotioncharacteristicsofitscomponentswillbedeterminedbykinematicanalysis.Thestatementofthetasksofanalysiscontainsallprincipaldimensionsofthemechanism,theinterconnectionsofitslinks,andthespecificationoftheinputmotionormethodofactuation.Theobjectiveistofindthedisplacements,velocities,accelerations,shockorjerksecondacceleration,andperhapshigheraccelerationsofthevariousmembers,aswellasthepathsdescribedandmotionsperformedbycertainelements.Inshort,inkinematicanalysiswedeterminetheperformanceofagivenmechanism.Thesecondpartofdefinitionmaybeparaphrasedintwoways1.Thestudyofmethodsofcreatingagivenmotionbymeansofmechanisms.2.Thestudyofmethodsofcreatingmechanismshavingagivenmotion.Ineitherversion,themotionisgivenandthemechanismistobefound.Thisistheessenceofkinematicsynthesis.Thuskinematicsynthesisdealswiththesystematicdesignofmechanismsforagivenperformance.Theareaofsynthesismaybegroupedintotwocategories.1.Typesynthesis.Giventherequiredperformance,whattypeofmechanismwillbesuitableGeartrainsLinkagesCammechanismsAlso,howmanylinksshouldthemechanismhaveHowmanydegreesoffreedomarerequiredWhatconfigurationiddesirableAndsoon.Deliberationsinvolvingthenumberoflinksanddegreesoffreedomareoftenreferredtoastheprovinceofasubcategoryoftypesynthesiscallednumbersynthesis.2.Dimensionalsynthesis.ThesecondmajorcategoryofkinematicsynthesisisbestdefinedbywayofitsobjectiveDimensionalsynthesisseekstodeterminethesignificantdimensionsandthestartingpositionofamechanismofpreconceivedtypeforaspecifiedtaskandprescribedperformance.Significantdimensionsmeanlinklengthsordistancesonbinary,ternary,andsoon,links,anglesbetweenaxis,camcontourdimensionsandcamfollowerdiameters,eccentricities,gearrations,andsoforth.Amechanismofpreconceivedtypemaybeaslidercranklinkage,afourbarlinkage,acamwithflatfollower,oramorecomplexlinkageofacertainconfigurationdefinedtopologicallybutnotdimensionally.Therearethreecustomarytasksforkinematicsynthesisfunctiongeneration,pathgenerationandmotiongeneration.Infunctiongenerationmechanismsrotationorslidingmotionsofinputandoutputlinksmustbecorrelated.Foranarbitraryfunctionxfy,akinematicsynthesistaskmaybetodesignalinkagetocorrelateinputandoutputsuchthattheinputmovesbyx,theoutputmovesbyxfyfortherange10nxxx.Inthecaseofrotaryinputandoutput,theanglesofrotationandarethelinearanalogsofxandyrespectively.Whentheinputlinkisrotatedtoavalueoftheindependentx,themechanisminablackboxcausestheoutputlinktoturntothecorrespondingvalueofthedependentvariablexfy.Thismayberegardedasasimplecaseofamechanicalanalogcomputer.Avarietyofdifferentmechanismscouldbecontainedwithintheblackbox.However,thefourbarlinkageisnotcapableoferrorfreegenerationofanarbitraryfunctionandcanmatchthefunctionatonlyalimitednumberofprecisionpoints.Itiswidelyusedinindustrybecausethefourbarlinkageidsimpletoconstructandmaintain.Inpathgenerationmechanismapointonafloatinglinkistotraceapathdefinedwithrespecttoafixedframeofreference.Ifthepathpointsaretobecorrelatedwitheithertimeorinputlinkpositions,thetaskiscalledpathgenerationwithprescribedtiming.Anexampleofpathgenerationmechanismsidafourbarlinkagedesignedtopitchabaseballortennisball.Inthiscasethetrajectoryofpointpwouldbesuchastopickupaballataprescribedlocationandtodelivertheballalongaprescribedpathwithprescribedtimingforreachingasuitablethrowvelocityanddirection.Therearemanysituationsinthedesignofmechanicaldevisesinwhichitisnecessaryeithertoguidearigidbodythroughaseriesofspecified,finitelyseparatedpositionsortoimposeconstraintsonthevelocityand/oraccelerationofthemovingbodyatareducednumberoffinitelyseparatedpositions.Motiongenerationorrigidbodyguidancemechanismrequiresthatanentirebodybeguidedthroughaprescribedmotionsequence.Thebodytobeguidedusuallyisapartofafloatinglink,ofwhichnotonlyisthepathofapointpprescribed,butalsotherotationofalinepassingthroughthepointandembeddedinthebody,.Forinstance,thelinemightrepresentacarrierlinkinaautomaticmachinerywhereapointlocatedonthecarrierlinkhasaprescribedpathwhilethecarrierhasaprescribedangularorientation.Prescribingthemovementofthebucketforabucketloaderidanotherexampleofmotiongenerationmechanisms,thepathoftipofthebucketiscriticalsincethetipmustperformascoopingtrajectoryfollowedbyaliftingandadumpingtrajectory.Theangularorientationofthebucketareequallyimportanttoensurethatloadisdumpedfromthecorrectposition.Acamisaconvenientdevicefortransformingonemotionintoanother.Thismachineelementhasacurvedorgroovedsurfacewhichmateswithafollowerandimpartsmotiontoit.Themotionofthecamusuallyrotationistransformedintofolloweroscillation,translation,orboth.Becauseofthevariouscamgeometriesandthelargenumberofcamandfollowercombinations,thecamisanextremelyversatilemechanicalelement.Althoughacamandfollowermaybedesignedformotion,path,orfunctiongeneration,themajorityofapplicationsutilizethecamandfollowerforfunctiongeneration.Themostcommoncamtypesaccordingtocamshapesarediskorplatetranslatingtwodimensionalorplanar,andcylindricalthreedimensionalorspatialcams.Followerscanbeclassifiedinseveralwaysaccordingtofollowermotion,suchastranslationoroscillationaccordingtowhetherthetranslationalstraightlinefollowermotionisradialofoffsetfromthecenterofthecamshaftandaccordingtotheshapeofthefollowercontactsurfacee.g.,flatface,roller,pointknifeedge,spherical,planarcurved,orspatialcurvedsurface.Inthecaseofadiskcamwitharadialinlinetranslatingrollerfollowerthesmallestcirclethatcanbedrawntangenttothecamsurfaceandconcentricwiththecamshaftisthebasecircle.Thetracerpointisapointatthecenteroftherollercenterandthenormaltothepitchcurve.Thepressureangleistheanglebetweenthedirectionofthepathoftherollercenterandthenormaltothepitchcurvethroughthecenteroftherollerandisthecomplementofthetransmissionangle.Neglectingfriction,thisnormaliscollinearwiththecontactforcebetweenthecamandfollower.Asinalinkage,thepressureanglevariesduringthecycleandisameasureoftheabilityofthecamtotransfermotiveefforttothefollower.Alargepressureanglewillproduceanappreciablelateralforceexertedonthestemofthefollower,which,inthepresenceoffriction,wouldtendtobindthefollowerintheguide.Numerousapplicationsinautomaticmachineryrequireintermittentmotion.Atypicalexamplewillcallforarisedwellreturnandperhapsanotherdwellperiodofaspecifiednumberofdegreeseach,togetherwitharequiredfollowerdisplacementmeasuredincentimetersordegrees.Thedesignersjobistolayoutthecamaccordingly.Thefirstdecisiontobemadeistochoosethecamfollowertype.Thespecifiedapplicationmaydictatethecombinationofthecamandfollower.Somefactorsthatshouldenterintothedecisionaregeometricconsiderations,dynamicconsiderations,environmentalconsiderationsandeconomicmatters.Onceatypeofcamandfollowerpairhasbeenselected,thefollowermotionmustbechosen.Therefore,thevelocity,acceleration,andinsomecasesfurtherderivativesofthedisplacementofthefollowerareofgreatimportance.Gearsaremachineelementsthattransmitmotionbymeansofsuccessivelyengagingteeth.Gearstransmitmotionfromonerotatingshafttoanother,ortoarackthattranslates.Numerousapplicationsexistinwhichaconstantangularvelocityratioorconstanttorqueratiomustbetransmittedbetweenshafts.Basedonthevarietyofgeartypesavailable,thereisnorestrictionthattheinputandtheoutputshaftsneedbeeitherinlineorparallel.Nonlinearangularvelocityratiosarealsoavailablebyusingnoncirculargears.Inordertomaintainaconstantangularvelocity,theindividualtoothprofilemustobeythefundamentallawofgearingforapairofgearstotransmitaconstantangularvelocityratio,theshapeoftheircontactingprofilesmustbesuchthatthecommonnormalpassesthroughafixedpointonthelineofthecenters.Anytwomatingtoothprofilesthatsatisfythefundamentallawofgearingarecalledconjugateprofiles.Althoughtherearemanytoothshapespossibleinwhichamatingtoothcouldbedesignedtosatisfythefundamentallaw,onlytwoareingeneralusethecycloidalandinvoluteprofiles.Theinvolutehasimportantadvantagesitiseasytomanufactureandthecenterdistancebetweenapairofinvolutegearscanbevariedwithoutchangingthevelocityratio.Thuschosetolerancesbetweenshaftsarenotrequiredwhenutilizingtheinvoluteprofile.Thereareseveralstandardgeartypes.Forapplicationswithparallelshafts,straightspurgear,parallelhelical,orherringbonegearsareusuallyused.Inthecaseofintersectingshafts,straightbevelofspiralbevelgearsareemployed.Fornonintersectingandnonparallelshafts,crossedhelical,worm,face,skewbevelorhypoidgearswouldbeacceptablechoices.Forspurgears,thepitchcirclesofmatinggearsaretangenttoeachother.Theyrollononeanotherwithoutsliding.Theaddendumistheheightbywhichatoothprojectsbeyondthepitchcirclealsotheradialdistancebetweenthepitchcircleandtheaddendumcircle.Theclearanceistheamountbywhichtheaddendumtoothheightbelowthepitchcircleinagivengearsexceedstheaddendumofitsmatinggear.Thetooththicknessisthedistanceacrossthetoothalongthearcofthepitchcirclewhilethetoothspaceisthedistancebetweenadjacentteethalongthearcofthepitchcircle.Thebacklashistheamountbywhichthewidthofthetoothspaceexceedsthethicknessoftheengagingtoothatthepitchcircle.

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