外文翻译--凸轮设计的基本内容.doc

1ELEMENTSOFCAMDESIGNHowtoplanandproducesimplebutefficientcamsforpetrolenginesandothermechanismsCamsareamongthemostversatilemechanismsavailableAcamisasimpletwo-memberdeviceTheinputmemberisthecamitself，whiletheoutputmemberiscalledthefollowerThroughtheuseofcams，asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesiredSomeofthecommonapplicationsofcamsareCamshaftanddistributorshaftofautomotiveengineProductionmachinetoolsAutomaticrecordplayersPrintingmachinesAutomaticwashingmachinesAutomaticdishwashersThecontourofhigh-speedcams(camspeedinexcessof1000rpm)mustbedeterminedmathematicallyHowever，thevastmajorityofcamsoperateatlowspeeds(lessthan500rpm)ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayoutIngeneral，thegreaterthecamspeedandoutputload，thegreatermustbetheprecisionwithwhichthecamcontourismachinedCamsinsomeformorotherareessentialtotheoperationofmanykindsofmechanicaldevices.Theirbest-knownapplicationisinthevalve-operatinggearofinternalcombustionengines,buttheyplayanequallyimportantpartinindustrialmachinery,fromprintingpressestoreapingmachines.Ingeneral,acamcanbedefinedasaprojectiononthefaceofadiscorthesurfaceofacylinderforthepurposeofproducingintermittentreciprocatingmotionofacontactingmemberorfollower.Mostcamsoperatebyrotarymotion,butthisisnotanessentialconditionandinspecialcasesthemotionmaybesemi-rotary，oscillatoryorswinging.Evenstraight-linemotionoftheoperatingmemberispossible,thoughthetermcammaynotbeconsideredproperlyapplicableinsuchcircumstances.Mosttextbooksonmechanicsgivesomeinformationonthedesignofcamsandshowexamplesofcamformsplottedtoproducevariousordersofmotion.Whereneithertheoperatingspeednorthemechanicaldutyisveryhigh,thereisagooddealoflatitudeinthenermissibledesignofthecamanditisonlynecessarytoavoidexcessivelysteepcontoursorabruptchangeswhichwouldresultinnoise,impactshock,andsidepressureonthefollower.But,withincreaseofeitherspeedorload,muchmoreexactingdemandsaremadeonthecam,callingforthemostcarefuldesignand,atveryhighspeed,theeffectofinertiaonthemovingpartsismostpronounced,sothatthefurtherfactorsofaccelerationandrateoflifthavetobetakenintoaccountandthesearerarelydealtwithinanydetailinthestandardtextbooks.Thedesignofthecamfollowerisalsoofgreatimportanceandbearsadefiniterelationtotheshapeofthecamitself.Thisisbecausethecamcannotmakecontactwiththefolloweratasinglefixedpoint.Surfacecontactisnecessarytodistributeloadandavoidexcesswear,thusthecamtransmitsitsmotionthroughvariouspointsoflocationonthefollower,dependingontheshapeof2thetwocomplementarymembers.Thecamsforoperatingi.c.enginevalvespresentspeciallydifficultproblemsindesign.Inthecaseofracingengines,boththeloadandspeedmayberegardedasextreme,becauseinmanyenginestherateatwhichthevalvescanbeeffectivelycontrolledisthelimitingfactorinengineperformance.Insomerespects,camdesignofminiatureenginesissimplifiedbyreasonoftheirlighterworkingparts(andconsequentlessinertia)butontheotherhand,workingfrictionisusuallygreaterandrotationalspeedsaregenerallyconsiderablyhigherthaninfull-sizepractice.Inthemanydesignsforsmallfour-strokeengineswhichIhavepublished,Ihavesoughttosimplifyvalveoperationandtoprovidedesignsforcamswhichcanbesimplyandaccuratelyproducedwiththefacilitiesoftheamateurworkshop.Numerousenginedesignswhichhavebeensubmittedtomebyreadershavecontainederrorsinthevalvegearandparticularlyinthecamsandinviewofprevalentmisconceptionsinthefundamentalprinciplesoftheseitems,IamgivingsomeadviceonthematterwhichItrustwillhelpindividualdesignerstoobtainthebestresultsfromtheirengines.Therehavebeenmanyenginesbuiltwithcamsofthoroughlybaddesignbutwhich,inspiteofthis,haveproducedresultsmoreorlesssatisfactorytotheirconstructors.Itmaybesaidthatwithincertainlimitsofspeedonecangetawaywithmurderbutinnocasecananengineperformefficientlywithbadlydesignedcams,orindeederrorsinanyofitsworkingdetails.Thisarticleisconcernedmainlywiththedesignofcamsforoperatingthevalvesofi.c.enginesand,inordertoavoidanyconfusionofterms,Fig.1showsthevariouspartsofacamofthistypeandexplainstheirfunctions.Thecircular,concentricportionofthecam,whichhasnooperativeeffect,isknownasthebasecircle:thehumyofthecam(shownshaded)isknownasthelobe,andtheflanksoneithersiderisefromthebasecircletothenose,whichisusuallyrounded.Liftmaybedefinedasthedifferencebetweentheradiusofthebasecircleandthatofthenose.theaneleenclosedbetweenthepointswheretheflanksjointhebasecircleistermedtheangularperiod,representingtheproportionofthefullcycleduringwhichthecamoperatesthevalvegear.InFig.2,typicalexamplesofcamsusedini.c.enginesareillustrated.Thetangentcam,A,hasdeadstraightflanks-whichasthenameimpliesformtangentstothebasecircle.Thistypeofcamiseasytodesignandproduce,thesimplestmethodofmachiningbeingbyacircularmillingprocessformingaconcentricsurfaceonthebasecircleandrunningstraightouttangentiallywheretheflanksstartandfinish.ItcanalsobeproducedbyfilingandIhaveinthepastdescribedhowtomakeitwiththeaidofarollerfilingrestinthelathe,inconjunctionwithindexinggeartolocatetheflankangles.Tangentcamscanonlyworkefficientlyinconjunctionwithaconvexcurvedfollower,asthisistheonlywayinwhichtheflankcanbebroughtprogressivelyandsmoothlyintoaction.Sometimeagoanenginewasdescribedhavingtangentcamsinconjunctionwithflatfollowers.Thiswasnotintendedforextremelyhighspeedandverylikelyproducedallthepowerrequiredofit,butitisquiteclearthattheflatfaceofthetangentcam.Onengagingtheflattappet-overthefulllengthoftheflankallatonce,mustproduceanabruptslappingactionwhichisnoisy,inefficientanddestructiveinthelongrun.Rollersareoftenusedasfollowerswithtangentcamsandaresatisfactoryinrespectoftheirshape,buttheideaofintroducingrollingmotionatthispointisnotasgoodasitseemsatfirstsight,becauseitmerelytransferstheslidingfrictiontoamuchsmallerarea-thatofthepivotpin.Itispossibleinsomecases,however,touseaballorrollerraceforthefollowerandthis,atanyrate,hasthemeritofdistributingandequalizingthewearingsurface.Tangentcamshavebeenusedwithacertaindegreeofsuccessforhigh-performance-engines3andwereatonetimepopularonracingmotorcycleengines,thoughusuallywithsomeslightmodificationofshape-often“designed”bythetunerwiththeaidof.aCarborundumslip!Theirmorecommonapplication,however,hasbeenongasandoilenginesrunningatrelativelyslowspeeds,wheretheyworkwellincontactwithrollersattachedtotheendsofthevalverockers.Camswithconvexflanksareextensivelyusedinmotorcarsandothermass-producedengines.Oneimportantadvantageinthisrespectisthattheyaresuitedtomanufactureinquantitybyacopyingprocessfromaccuratelyformedmastercams.Thefactthathat-basedtappetscanbeusedalsofavoursquantityproductionandtheycanbedesignedtoworkfairlysilently.Thecontouroftheflankcanbeplottedsothatviolentchangesintheaccelerationofthecamareavoidedand,moreimportantstill,thetappetwillfollowthecamonthereturnmotionwithoutanytendencytobounceorfloatatquitehighspeeds.Insuchcases,itmaybenecessarytointroducecompoundcurveswhichareextremelydifficulttocopyonasmallscale,butcamsmadewithflanksformmgtruecirculararcswillgivereasonablyefficientresults,andareveryeasilyproducedinanyscale:Concave-flankedcams.Comparativelyfewexamplesofconcave-flankedcams(Fig.2c)aretobeseennowadays,thoughtheyhavebeenusedextensivelyinthepastwiththeideaofobtainingthemostrapidopeningandclosingofthevalves.Theoretically,theycanbedesignedtoproduceconsant-acceleration,butinpracticetheyrendervalvecontrolverydifficultathighspeedandtheirfierceangleofattackproducesheavysidepressureonthetappet.Theconcaveflankmustalwayshaveasubstantiallygreaterradiusthanthefollower,oraslappingactionlikethatofatangentcamonaflatfollowerisproduced.Theshapeofthenoseinmosttypesofcamsisdictatedmainlybytheneedtodeceleratethefollowerassmoothlyaspossible.Itisonethingtodesignitinsuchawaythatidealconditionsareobtained,andquiteanothertoensureinpracticethatthefollowerretainsclosecontactwiththecam.Iftheradiusofthenoseistoosmall,thefollowerwillbounceandcomedownheavilyonthereturnflankofthecamand,.iftoogreat,valveopeningefficiencywillbereduced.Ofthethreetypesofcams,A,BandC,whichallhaveidenticallyequalliftandangularperiod,thelobeofBenclosesthesmallestarea,andonfirstsightitmightappearthatitistheleastefficientinproducingadequatevalveopening,ormeanliftarea,butowingtotheuseofaflatbasedtappet,itsliftcharacteristicsarenotverydifferentfromthoseofatangentcamwithround-basedtappet,andnotnecessarilyinferiortothoseofaconcave-flankcam.UnsymmetricalcamsItisnotcommontomakethetwoflanksofacamofdifferentcontourstoproducesomeparticularresultwhichthedesignermayconsiderdesirable.Insomecases,theobjectistoproducerapidopeningandgradualclosing,butsometimestheoppositeeffectispreferred.Whenallthingsareconsidered,however,mostattemptstomonkeyaboutwithcamformsleadtocomplicationswhichmayactuallydefeattheirownobject,atleastatreallyhighspeeds.Inmanyengines,particularlythoseofmotorcycles,thecamsoperatethevalvesthroughleversorrockerswhichmoveinanarcinsteadofinastraightline,asintheorthodoxmotorcartappet.Thismaybemechanicallyefficient,butitmodifiestheliftcharacteristicofthecam,asthepointatwhichthelattertransmitsmotiontothefollowervariesinrelationtotheradiusoftheleverarm,(Fig.3).Withthecamrotatinginaclockwisedirection,theeffectivelengthoftheleverwillbegreaterintheposition.4AduringvalveopeningthaninpositionBduringclosing,asindicatedbydimensionsXandY.Thisamountstothesameasusinganunsymmetricalcam,andintheexampleshown,wouldresultinslowopeningandrapidclosingofthevalve,orviceversaifeitherthedirectionofrotationofthecam,ortherelative“hand”ofthelever,isreversed.Theshorterthelever,thegreaterthediscrepancyintherateofmovement,Neithertheunsymmetricalcamformnorthepivotedleveriscondemnedasbaddesign,butIhavesoughttoavoidtheminmostoftheenginesIhavedesignedbecausetheyareacomplicatingfactorinwhatisalreadyaveryinvolvedproblem,andbykeepingtofairlysimplecamsandstraight-linetappets,onecanbeassuredthattherearenottoomanysnags.Theemploymentofcamswithflanksoftruecirculararchasenabledmetodevisemeansofproducingthemonthelathewithoutelaborateattachmentsand,whatismoreimportantstill,toproduceanentiresetofcamsforamulti-cylinderengineincorrectangularrelationtoeachotherbyequallysimplemeans.Thereisnodoubtwhateverthatthesemethodshaveenabledmanyengineconstructors(somewithoutpreviousexperience)totacklesuccessfullyaproblemwhichwouldotherwisehavebeenformidable,tosaytheleast.Manydesignershaveattemptedtoimprovevalveefficiencybydesigningcamswhichholdthevalveatmaximumopeningforaslongaperiodaspossible.Thisisdonebyprovidingdwellor,inotherwords,makingthetopofthelobeconcentricwiththecamaxisoverac