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外文翻译--凸轮设计的基本内容.doc

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外文翻译--凸轮设计的基本内容.doc

1ELEMENTSOFCAMDESIGNHowtoplanandproducesimplebutefficientcamsforpetrolenginesandothermechanismsCamsareamongthemostversatilemechanismsavailable.Acamisasimpletwomemberdevice.Theinputmemberisthecamitself,whiletheoutputmemberiscalledthefollower.Throughtheuseofcams,asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesired.SomeofthecommonapplicationsofcamsareCamshaftanddistributorshaftofautomotiveengineProductionmachinetoolsAutomaticrecordplayersPrintingmachinesAutomaticwashingmachinesAutomaticdishwashersThecontourofhighspeedcamscamspeedinexcessof1000rpmmustbedeterminedmathematically.However,thevastmajorityofcamsoperateatlowspeedslessthan500rpmormediumspeedcamscanbedeterminedgraphicallyusingalargescalelayout.Ingeneral,thegreaterthecamspeedandoutputload,thegreatermustbetheprecisionwithwhichthecamcontourismachined.Camsinsomeformorotherareessentialtotheoperationofmanykindsofmechanicaldevices.Theirbestknownapplicationisinthevalveoperatinggearofinternalcombustionengines,buttheyplayanequallyimportantpartinindustrialmachinery,fromprintingpressestoreapingmachines.Ingeneral,acamcanbedefinedasaprojectiononthefaceofadiscorthesurfaceofacylinderforthepurposeofproducingintermittentreciprocatingmotionofacontactingmemberorfollower.Mostcamsoperatebyrotarymotion,butthisisnotanessentialconditionandinspecialcasesthemotionmaybesemirotary,oscillatoryorswinging.Evenstraightlinemotionoftheoperatingmemberispossible,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,camdesignofminiatureenginesissimplifiedbyreasonoftheirlighterworkingpartsandconsequentlessinertiabutontheotherhand,workingfrictionisusuallygreaterandrotationalspeedsaregenerallyconsiderablyhigherthaninfullsizepractice.InthemanydesignsforsmallfourstrokeengineswhichIhavepublished,Ihavesoughttosimplifyvalveoperationandtoprovidedesignsforcamswhichcanbesimplyandaccuratelyproducedwiththefacilitiesoftheamateurworkshop.Numerousenginedesignswhichhavebeensubmittedtomebyreadershavecontainederrorsinthevalvegearandparticularlyinthecamsandinviewofprevalentmisconceptionsinthefundamentalprinciplesoftheseitems,IamgivingsomeadviceonthematterwhichItrustwillhelpindividualdesignerstoobtainthebestresultsfromtheirengines.Therehavebeenmanyenginesbuiltwithcamsofthoroughlybaddesignbutwhich,inspiteofthis,haveproducedresultsmoreorlesssatisfactorytotheirconstructors.Itmaybesaidthatwithincertainlimitsofspeedonecangetawaywithmurderbutinnocasecananengineperformefficientlywithbadlydesignedcams,orindeederrorsinanyofitsworkingdetails.Thisarticleisconcernedmainlywiththedesignofcamsforoperatingthevalvesofi.c.enginesand,inordertoavoidanyconfusionofterms,Fig.1showsthevariouspartsofacamofthistypeandexplainstheirfunctions.Thecircular,concentricportionofthecam,whichhasnooperativeeffect,isknownasthebasecirclethehumyofthecamshownshadedisknownasthelobe,andtheflanksoneithersiderisefromthebasecircletothenose,whichisusuallyrounded.Liftmaybedefinedasthedifferencebetweentheradiusofthebasecircleandthatofthenose.theaneleenclosedbetweenthepointswheretheflanksjointhebasecircleistermedtheangularperiod,representingtheproportionofthefullcycleduringwhichthecamoperatesthevalvegear.InFig.2,typicalexamplesofcamsusedini.c.enginesareillustrated.Thetangentcam,A,hasdeadstraightflankswhichasthenameimpliesformtangentstothebasecircle.Thistypeofcamiseasytodesignandproduce,thesimplestmethodofmachiningbeingbyacircularmillingprocessformingaconcentricsurfaceonthebasecircleandrunningstraightouttangentiallywheretheflanksstartandfinish.ItcanalsobeproducedbyfilingandIhaveinthepastdescribedhowtomakeitwiththeaidofarollerfilingrestinthelathe,inconjunctionwithindexinggeartolocatetheflankangles.Tangentcamscanonlyworkefficientlyinconjunctionwithaconvexcurvedfollower,asthisistheonlywayinwhichtheflankcanbebroughtprogressivelyandsmoothlyintoaction.Sometimeagoanenginewasdescribedhavingtangentcamsinconjunctionwithflatfollowers.Thiswasnotintendedforextremelyhighspeedandverylikelyproducedallthepowerrequiredofit,butitisquiteclearthattheflatfaceofthetangentcam.Onengagingtheflattappetoverthefulllengthoftheflankallatonce,mustproduceanabruptslappingactionwhichisnoisy,inefficientanddestructiveinthelongrun.Rollersareoftenusedasfollowerswithtangentcamsandaresatisfactoryinrespectoftheirshape,buttheideaofintroducingrollingmotionatthispointisnotasgoodasitseemsatfirstsight,becauseitmerelytransferstheslidingfrictiontoamuchsmallerareathatofthepivotpin.Itispossibleinsomecases,however,touseaballorrollerraceforthefollowerandthis,atanyrate,hasthemeritofdistributingandequalizingthewearingsurface.Tangentcamshavebeenusedwithacertaindegreeofsuccessforhighperformanceengines3andwereatonetimepopularonracingmotorcycleengines,thoughusuallywithsomeslightmodificationofshapeoftendesignedbythetunerwiththeaidof.aCarborundumslipTheirmorecommonapplication,however,hasbeenongasandoilenginesrunningatrelativelyslowspeeds,wheretheyworkwellincontactwithrollersattachedtotheendsofthevalverockers.Camswithconvexflanksareextensivelyusedinmotorcarsandothermassproducedengines.Oneimportantadvantageinthisrespectisthattheyaresuitedtomanufactureinquantitybyacopyingprocessfromaccuratelyformedmastercams.Thefactthathatbasedtappetscanbeusedalsofavoursquantityproductionandtheycanbedesignedtoworkfairlysilently.Thecontouroftheflankcanbeplottedsothatviolentchangesintheaccelerationofthecamareavoidedand,moreimportantstill,thetappetwillfollowthecamonthereturnmotionwithoutanytendencytobounceorfloatatquitehighspeeds.Insuchcases,itmaybenecessarytointroducecompoundcurveswhichareextremelydifficulttocopyonasmallscale,butcamsmadewithflanksformmgtruecirculararcswillgivereasonablyefficientresults,andareveryeasilyproducedinanyscaleConcaveflankedcams.ComparativelyfewexamplesofconcaveflankedcamsFig.2caretobeseennowadays,thoughtheyhavebeenusedextensivelyinthepastwiththeideaofobtainingthemostrapidopeningandclosingofthevalves.Theoretically,theycanbedesignedtoproduceconsantacceleration,butinpracticetheyrendervalvecontrolverydifficultathighspeedandtheirfierceangleofattackproducesheavysidepressureonthetappet.Theconcaveflankmustalwayshaveasubstantiallygreaterradiusthanthefollower,oraslappingactionlikethatofatangentcamonaflatfollowerisproduced.Theshapeofthenoseinmosttypesofcamsisdictatedmainlybytheneedtodeceleratethefollowerassmoothlyaspossible.Itisonethingtodesignitinsuchawaythatidealconditionsareobtained,andquiteanothertoensureinpracticethatthefollowerretainsclosecontactwiththecam.Iftheradiusofthenoseistoosmall,thefollowerwillbounceandcomedownheavilyonthereturnflankofthecamand,.iftoogreat,valveopeningefficiencywillbereduced.Ofthethreetypesofcams,A,BandC,whichallhaveidenticallyequalliftandangularperiod,thelobeofBenclosesthesmallestarea,andonfirstsightitmightappearthatitistheleastefficientinproducingadequatevalveopening,ormeanliftarea,butowingtotheuseofaflatbasedtappet,itsliftcharacteristicsarenotverydifferentfromthoseofatangentcamwithroundbasedtappet,andnotnecessarilyinferiortothoseofaconcaveflankcam.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,ortherelativehandofthelever,isreversed.Theshorterthelever,thegreaterthediscrepancyintherateofmovement,Neithertheunsymmetricalcamformnorthepivotedleveriscondemnedasbaddesign,butIhavesoughttoavoidtheminmostoftheenginesIhavedesignedbecausetheyareacomplicatingfactorinwhatisalreadyaveryinvolvedproblem,andbykeepingtofairlysimplecamsandstraightlinetappets,onecanbeassuredthattherearenottoomanysnags.Theemploymentofcamswithflanksoftruecirculararchasenabledmetodevisemeansofproducingthemonthelathewithoutelaborateattachmentsand,whatismoreimportantstill,toproduceanentiresetofcamsforamulticylinderengineincorrectangularrelationtoeachotherbyequallysimplemeans.Thereisnodoubtwhateverthatthesemethodshaveenabledmanyengineconstructorssomewithoutpreviousexperiencetotacklesuccessfullyaproblemwhichwouldotherwisehavebeenformidable,tosaytheleast.Manydesignershaveattemptedtoimprovevalveefficiencybydesigningcamswhichholdthevalveatmaximumopeningforaslongaperiodaspossible.Thisisdonebyprovidingdwellor,inotherwords,makingthetopofthelobeconcentricwiththecamaxisoveracertainangulardistanceinthecentreofitslift.Todothis,however,itisnecessarytomaketheflanksexcessivelysteep,thusproducingheavysidethrustonthetappet,andmakingcontrolathighspeedmoredifficult,Fig.4A.Alittleconsideration,however,willshowthatthesameresultcanbeachieved,withmuchlessmechanicaldifficulty,byliftingthevalvesomewhathigherataneasierrate,asshownatB.Thisavoidstheneedforsuddenaccelerationanddecelerationofthetappetandpromotesflowefficiencyofthevalve.Theshadedportionsofthetwocamsshowthedifferencesintheareaofthelobe,showingthatnothingisreallygainedbythedwell.FactorsinefficiencyHighvalveliftisadesirablefeature,butonlyifitcanbeobtainedwithoutmakingextradificultiesincontrollingthevalve.Themaximumportareaofavalveisobtainedwhentheliftisequaltoonefourthoftheseatdiameter,butowingtothebafflingeffectonthevalvehead,ahigherliftisbetterforflowefficiencyifitispracticable.Largediametervalveswillobviouslyreleaseandadmitgasefficientlybuttheyaremoredifficulttocontrolandkeepcoolathighspeedthansmallervalves.Anotherpointisthattheexhaustvalveisrequiredtoopenagainstahighcylinderpressure,andthelargeritisthemoretheloadimposedonthecam,quiteapartfromthespringload.

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