关于凸轮的外文翻译.docx

关于凸轮的外文翻译.docx

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关于凸轮的外文翻译

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ELEMENTSOFCAMDESIGN

Howtoplanandproducesimplebutefficientcamsforpetrolenginesandothermechanisms

Camsareamongthemostversatilemechanismsavailable．Acamisasimpletwo-memberdevice．Theinputmemberisthecamitself，whiletheoutputmemberiscalledthefollower．Throughtheuseofcams，asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesired．Someofthecommonapplicationsofcamsare

——Camshaftanddistributorshaftofautomotiveengine

——Productionmachinetools

——Automaticrecordplayers

——Printingmachines

——Automaticwashingmachines

——Automaticdishwashers

Thecontourofhigh-speedcams（camspeedinexcessof1000rpm）mustbedeterminedmathematically．However，thevastmajorityofcamsoperateatlowspeeds（lessthan500rpm）ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayout．Ingeneral，thegreaterthecamspeedandoutputload，thegreatermustbetheprecisionwithwhichthecamcontourismachined．

Camsinsomeformorotherareessentialtotheoperationofmanykindsofmechanicaldevices.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,dependingontheshapeofthetwocomplementarymembers.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.theaneleenclosedbetweenthepointswheretheflanksjointhebasecircleistermedtheangular‘period,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-enginesandwereatonetimepopularonracingmotorcycleengines,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,theycanbedesignedtoproduceconstant-acceleration,butinpracticetheyrendervalvecontrolverydifficultathighspeedandtheirfierceangleofattackproducesheavysidepressureonthetappet.Theconcaveflankmustalwayshaveasubstantiallygreaterradiusthanthefollower,oraslappingactionlikethatofatangentcamonaflatfollowerisproduced.

Theshapeofthenoseinmosttypesofcamsisdictatedmainlybytheneedtodeceleratethefollowerassmoothlyaspossible.Itisonethingtodesignitinsuchawaythatidealconditionsareobtained,andquiteanothertoensureinpracticethatthefollowerretainsclosecontactwiththecam.Iftheradiusofthenoseistoosmall,thefollowerwillbounceandcomedownheavilyonthereturnflankofthecamand,.iftoogreat,valveopeningefficiencywillbereduced.

Ofthethreetypesofcams,A,BandC,whichallhaveidenticallyequalliftandangularperiod,thelobeofBenclosesthesmallestarea,andonfirstsightitmightappearthatitistheleastefficientinproducingadequatevalveopening,ormeanliftarea,butowingtotheuseofaflatbasedtappet,itsliftcharacteristicsarenotverydifferentfromthoseofatangentcamwithround-basedtappet,andnotnecessarilyinferiortoth