外文翻譯--The Basic Machines.doc

TheBasicMachinesWhenaprehistoricmanorwomanusedasticktopryupastone,theleverwasinvented.Itisoneofthesixbasicmachines.Aleverisarigidpieceorbar,liketheearlypersonsstick,whichturnonapointcalledthefulcrum.Whenforceisappliedatasecondpoint,thatforceistransmittedtoathirdpointwhereitcanperformawork.Achildrensseesawisanexcellentexampleofalever.Thepointofbalanceonwhichtheseesawrestsisthefulcrum；whendownwardforceisappliedtooneend,theotherendrises.Theorganizeduseoflevergoesbackbeyondthebeginningofrecordedhistory.LeverswereprobablyusedtoraisethehugeblocksofstonefromwhichStone-hengewasconstructed.Perhapsthestoneswereraisedbyusingtreetrunksasleversuntilthestonestoppledintoplace.Thewheelandaxleisthesecondbasictypeofmachine.Likethelever,thewheelgoesbacktoprehistorictimeswhensomeoneprobablydiscoveredthatitwaseasiertomoveheavyweightsbyslidingthemonlogsthanbycarryingthem.Theaxleisashaftonwhichawheelcanturnandthewheelandaxlecombinationmayhavefirstusedsometimearound3000B.C.forwater-raisingdevices.Itusefortransportationevolvedwiththedomesticationofthehorse.Warchariotswerethetanksofancienttimesandwagonswerethetrucks.Thethirdbasicmachineisthepulley.Thissimpledevicewasusedinancienttimesfortaskssuchasraisingwaterfromwellsorstreams.Apulleycontainedinahousingiscalledablock.Whenafixedblocktowhichaweightisattached,downwardpullontheropewillraisetheweight.Thedeviceiscalledablockandtackle.Thethreeremainingbasicmachinesaresorelatedtooneanotherthattheyaresometimesgroupedtogether.Theyarethewedge,theinclinedplane,andthescrew.Thewedgeisatrianglewithtwochiefsurfacesthatmeetinasharpangle.Wedgesareusedforsplittingopenorpushingapart.Theywereusedfromveryearlytimesforcuttingwood,aswithanaxe.Anailisafamiliarformofthewedge.WehavealreadymentionedtheinclinedplaneastheprobablemethodemployedbytheEgyptiansformanipulatingintoplacethehugeblocksofstoneinthepyramids.Earlymenandwomenknewthataweightcouldbepusheduptoahillorarampofearthwithlesseffortthanwouldberequiredtomovethesameweightvertically.Theinclinedplaneisanimportantfactorthatconcernscivilengineerswhendesigninghighwaysorrailroads.Themechanicalengineermorefrequentlyusesthescrew,aspiralformoftheinclinedplane.Thefigurethatresultsfromwrappingthelineofaninclineplanearoundacylinderiscalledahelix.Essentiallyallmachinesarevariationsorcomponentsthatofthesixbasictypes.Thereareanumberofdifferentkindsofmechanismsorcomponentsthattransmitmotionorchangeitinonewaytoanother.Modernmachinesandtheircomponentshavebecomesocomplexthatabranchofthescienceofmechanicscalledkinematicsevolvedofmostmodernmachines,itistheirmechanismsthatgivethemtheirgreatversatilityandflexibility.Gearsplaysuchanimportantpartinmachinesthattheyhavebecomethetheyhavebecomethesymbolformachinery.Theyarewheelwithteeththatengageormeshwitheachothersothattheyworkinpairstotransmitorchangemotion.Theyarefrequentlyusedtoreduceorincreasethespeedofamotionandtheycanalsochangethedirectionofmotion.Thelinearoundwhichawheelrotatesisitsaxis；gearcanchangeaxialmotion.Anotherkindofmechanismisthecam.Likethegear,itconsistsofapairofcomponents；thecamitselfistheinputmemberandthefolloweristheoutputmember.Thecamisattachedtoarotatingshaft；ittransmitsmotiontothefollower.Camcomesinmanydifferentshapes-thereareheart-shapedcams,clover-leafedcamsandothers.Bymeansofthesedifferentshapescamscanchangerotatingintoreciprocatingmotionorintooscillatingorvibratingmotion.Thefollowerisusuallyarodorshaft.Camscanalsotransmitexactmotionatspecialtimesinacycle.Theyarethereusefulwherethetimingofcomplexmotionsisimportant.Theyareinautomobileenginestoraiseandlowertenvalvesandinsewingmachinestocontrolthemovementsoftheneedle.Anotherkindofmechanismisknownasalinkage；itisaseriesofatleastthreerodsorsolidlinksthatareconnectedbyjointsthatpermitthelinkstopivot.Whenonelinkisfixedtheotherlinkscanmoveonlyinpathsthatarepredetermined.Likecams,linkageareusedtochangethedirectionofmotion,totransmitdifferentkindsofmotion,ortoprovidevariationsintimingindifferentpartsofacyclebyvaryingthelengthsofthelinksinrelationtoeachother.Thespringisamechanismthatisusedinawidevarietyofmachines；itisfrequentlyanelastichelicalcoilthatreturnstoitsoriginalshapeafterbeingdistorted.Springesareessentialcomponentsinwatches；insomecammechanismstheyholdthefollowerinplace；theyarefoundinscalesandtheyhelptocushionanautomobileride.Therearemanyvariationsonthebasiccoiledofspring,includingtheleafspringwhichismadeofstripsofelasticmaterialandspringsthatdependonthecompressionandexpansionofair.Aratchetisanotherpairedmechanismconsistingofawheelwithteethandapawlwhichdropsintothespacesbetweentheteeth.Theratchetmechanismisusedtopreventamotionfrombeingreversedortochangereciprocatingintorotarymotion.Thisisabriefintroductiontothecomplexworldofmachinecomponents.Open-LoopandClosed-LoopControlOpen-LoopControlSystemsThewordautomaticimpliesthatthereisacertainamountofsophisticationinthecontrolsystem.Byautomatic,itgenerallymeansthatthesystemisusuallycapableofadaptingtoavarietyofoperatingconditionsandisabletorespondtoaclassofinputssatisfactorily.However,notanytypeofcontrolsystemhastheautomaticfeature.Usually,theautomaticfeatureisachievedbyfeedingtheoutputvariablebackandcomparingitwiththecommandsignal.Whenasystemdoesnothavethefeedbackstructure,itiscalledanopen-loopsystem,whichisthesimplestandmosteconomicaltypeofcontrolsystem.Unfortunately,open-loopcontrolsystemlackaccuracyandversatilityandcanbeusedinnonebutthesimplesttypesofapplications.Consider,forexample,controlofthefurnaceforhomeheating.Letusassumethatthefurnaceisequippedonlywithatimingdevice,whichcontroltheonandoffperiodsofthefurnace.Toregulatethetemperaturetotheproperlever,thehumanoperatormustestimatetheamountoftimerequiredforthefurnacetostayonandthensetthetimeraccordingly.Whenthepresettimeisup,thefurnaceisturnedoff.