会员注册 | 登录 | 微信快捷登录 支付宝快捷登录 QQ登录 微博登录 | 帮助中心 人人文库renrendoc.com美如初恋!
站内搜索 百度文库

热门搜索: 直缝焊接机 矿井提升机 循环球式转向器图纸 机器人手爪发展史 管道机器人dwg 动平衡试验台设计

   首页 人人文库网 > 资源分类 > DOC文档下载

外文翻译--The Basic Machines.doc

  • 资源星级:
  • 资源大小:45.50KB   全文页数:7页
  • 资源格式: DOC        下载权限:注册会员/VIP会员
您还没有登陆,请先登录。登陆后即可下载此文档。
  合作网站登录: 微信快捷登录 支付宝快捷登录   QQ登录   微博登录
友情提示
2:本站资源不支持迅雷下载,请使用浏览器直接下载(不支持QQ浏览器)
3:本站资源下载后的文档和图纸-无水印,预览文档经过压缩,下载后原文更清晰   

外文翻译--The Basic Machines.doc

TheBasicMachinesWhenaprehistoricmanorwomanusedasticktopryupastone,theleverwasinvented.Itisoneofthesixbasicmachines.Aleverisarigidpieceorbar,liketheearlypersonsstick,whichturnonapointcalledthefulcrum.Whenforceisappliedatasecondpoint,thatforceistransmittedtoathirdpointwhereitcanperformawork.Achildrensseesawisanexcellentexampleofalever.Thepointofbalanceonwhichtheseesawrestsisthefulcrumwhendownwardforceisappliedtooneend,theotherendrises.Theorganizeduseoflevergoesbackbeyondthebeginningofrecordedhistory.LeverswereprobablyusedtoraisethehugeblocksofstonefromwhichStonehengewasconstructed.Perhapsthestoneswereraisedbyusingtreetrunksasleversuntilthestonestoppledintoplace.Thewheelandaxleisthesecondbasictypeofmachine.Likethelever,thewheelgoesbacktoprehistorictimeswhensomeoneprobablydiscoveredthatitwaseasiertomoveheavyweightsbyslidingthemonlogsthanbycarryingthem.Theaxleisashaftonwhichawheelcanturnandthewheelandaxlecombinationmayhavefirstusedsometimearound3000B.C.forwaterraisingdevices.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.Thelinearoundwhichawheelrotatesisitsaxisgearcanchangeaxialmotion.Anotherkindofmechanismisthecam.Likethegear,itconsistsofapairofcomponentsthecamitselfistheinputmemberandthefolloweristheoutputmember.Thecamisattachedtoarotatingshaftittransmitsmotiontothefollower.Camcomesinmanydifferentshapesthereareheartshapedcams,cloverleafedcamsandothers.Bymeansofthesedifferentshapescamscanchangerotatingintoreciprocatingmotionorintooscillatingorvibratingmotion.Thefollowerisusuallyarodorshaft.Camscanalsotransmitexactmotionatspecialtimesinacycle.Theyarethereusefulwherethetimingofcomplexmotionsisimportant.Theyareinautomobileenginestoraiseandlowertenvalvesandinsewingmachinestocontrolthemovementsoftheneedle.Anotherkindofmechanismisknownasalinkageitisaseriesofatleastthreerodsorsolidlinksthatareconnectedbyjointsthatpermitthelinkstopivot.Whenonelinkisfixedtheotherlinkscanmoveonlyinpathsthatarepredetermined.Likecams,linkageareusedtochangethedirectionofmotion,totransmitdifferentkindsofmotion,ortoprovidevariationsintimingindifferentpartsofacyclebyvaryingthelengthsofthelinksinrelationtoeachother.Thespringisamechanismthatisusedinawidevarietyofmachinesitisfrequentlyanelastichelicalcoilthatreturnstoitsoriginalshapeafterbeingdistorted.Springesareessentialcomponentsinwatchesinsomecammechanismstheyholdthefollowerinplacetheyarefoundinscalesandtheyhelptocushionanautomobileride.Therearemanyvariationsonthebasiccoiledofspring,includingtheleafspringwhichismadeofstripsofelasticmaterialandspringsthatdependonthecompressionandexpansionofair.Aratchetisanotherpairedmechanismconsistingofawheelwithteethandapawlwhichdropsintothespacesbetweentheteeth.Theratchetmechanismisusedtopreventamotionfrombeingreversedortochangereciprocatingintorotarymotion.Thisisabriefintroductiontothecomplexworldofmachinecomponents.OpenLoopandClosedLoopControlOpenLoopControlSystemsThewordautomaticimpliesthatthereisacertainamountofsophisticationinthecontrolsystem.Byautomatic,itgenerallymeansthatthesystemisusuallycapableofadaptingtoavarietyofoperatingconditionsandisabletorespondtoaclassofinputssatisfactorily.However,notanytypeofcontrolsystemhastheautomaticfeature.Usually,theautomaticfeatureisachievedbyfeedingtheoutputvariablebackandcomparingitwiththecommandsignal.Whenasystemdoesnothavethefeedbackstructure,itiscalledanopenloopsystem,whichisthesimplestandmosteconomicaltypeofcontrolsystem.Unfortunately,openloopcontrolsystemlackaccuracyandversatilityandcanbeusedinnonebutthesimplesttypesofapplications.Consider,forexample,controlofthefurnaceforhomeheating.Letusassumethatthefurnaceisequippedonlywithatimingdevice,whichcontroltheonandoffperiodsofthefurnace.Toregulatethetemperaturetotheproperlever,thehumanoperatormustestimatetheamountoftimerequiredforthefurnacetostayonandthensetthetimeraccordingly.Whenthepresettimeisup,thefurnaceisturnedoff.However,itisquitelikelythatthehousetemperatureiseitheraboveorbelowthedesiredvalue,owingtoinaccuracyintheestimate.Withoutfurtherdeliberation,itisquiteapparentthatthistypeofcontrolisinaccurateandunreliable.Onereasonfortheinaccuracyliesinthefactthatonemaynotknowtheexactcharacteristicsofthefurnace.Theotherfactoristhatonehasnocontrolovertheoutdoortemperature,whichhasadefinitebearingontheindoortemperature.Thisalsopointstoanimportantdisadvantageoftheperformanceofanopenloopcontrolsystem,inthatthesystemisnotcapableofadaptingtovariationsinenvironmentalconditionsortoexternaldisturbances.Inthecaseofthefurnacecontrol,perhapsanexperienced

注意事项

本文(外文翻译--The Basic Machines.doc)为本站会员(英文资料库)主动上传,人人文库网仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对上载内容本身不做任何修改或编辑。 若此文所含内容侵犯了您的版权或隐私,请立即通知人人文库网([email protected]),我们立即给予删除!

温馨提示:如果因为网速或其他原因下载失败请重新下载,重复下载不扣分。

copyright@ 2015-2017 人人文库网网站版权所有
苏ICP备12009002号-5