英文资料.docx

英文资料.docx

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英文资料

SandCasting

Thefirststageintheproductionofsandcastingsmustbethedesignandmanufactureofasuitablepattern.Castingpatternsaregenerallymadefromhardwordandthepatternhastobemadelargerthanthefinishedcastingsizetoallowfortheshrinkagethattakesplaceduringsolidificationandcooling.Theextentofthisshrinkagevarieswiththetypeofmetaloralloytobecast.Forallbutthesimplestshapesthepatternwillbemadeintwoormorepiecestofacilitatemoulding.Ifahollowcastingistobemadethepatterndesignwillincludeextensionpiecessothatspacestoacceptthesandcorearemouldedintosand.Theseadditionalspacesinthemouldaretermedcoreprints.

Sandmouldsfortheproductionofsmallandmedium-sizedcastingsaremadeinamouldingbox.Themouldismadeintwoormorepartsinorderthatthepatternmayberemoved.

Thedraghalfofthemouldboxisplacedonaflatfirmboardandthedraghalfofhepatternplacedinposition.Facingsandissprinkledoverthepatternandthenthemouldboxisfilledwithmouldingsand.Thesandisrammedfirmlyaroundthepattern.Thisprocessoffillingandrammingmaybedonebyhandbutmouldproductionisautomatedinalargefoundrywiththemouldboxesmovingalongaconveyor,firstlytobefilledwithsandfromhoppersandthentopassundermechanicalhammersforramming.Whenrammingofthesandiscomplete,excesssandisremovedtoleaveasmoothsurfaceflushwiththeedgesofthemouldingbox.

Thecompleteddragisnowturnedoverandtheupper,orcope,portionofthemouldingboxpositionedoverit.Thecopehalfofthepatternisplacedinposition,correctalignmentbeingensuredbymeansofsmalldowelpins.Patternsforthenecessaryfeeder,runnerandrisersarealsoplacedsoastogiveanevendistributionofmetalintothemouldcavity.Therisersshouldcoincidewiththehighestreadilyescapefromthemould.Thesizesofrisersshouldbesuchthatthemetalinthemdoesnotfreezetoorapidly.Animportantfunctionofariseristoactasreservoirofliquidmetaltofeedsolidificationwithinthemould.Athincoatingofdrypartingsandissprinkledintomouldatthisstage.Thisistopreventthecopeanddragstickingtogetherwhenthecopehalfismoulded.Thecopeisnowfilledwithmouldingsandandthisisrammedfirmlyintoshapeinthesamemannerasinthemakingofthedrag.

Aftertherammingofsandinthecopeiscompletedthetwohalvesofthemouldingboxarecarefullyseparated.Atthisstageventingofthemouldingboxarecarefullyseparated.Atthisstageventingofthemouldcanbedone,ifnecessary,toincreasethepermeabilityofthemould.Afterventingthepatternsarecarefullyremovedfrombothcopeanddrag,andagateorgatesarecarefullycuttoconnecttherunnerchannelwiththemaincavity.Gatesshouldbesitedtoallowforentryintomouldwithaminimumofturbulence.Anyloosesandisgentlyblownawayandifacoreistobeuseditthecopeuponthedraganditisthenreadyforuse.Liquidmetalispouredsmoothlyintothemouldviathefeeder.Pouringceaseswhenliquidmetalappearsatthetopoftherisersandthefeederchannelisalsofull.

Whenthemetalthathasbeenpouredintoasandmouldhasfullysolidifiedthemouldisbrokenandcastingisremoved.Thecastingstillhastherunnerandrisersattachedtoitandtherewillbesandadheringtoportionsofthesurface.Runnersandrisersarecutoffandreturnedtothemeltingfurnace.Sandcoresarebrokenandadherentsandiscleanedfromthesurfacebyvibrationorbysandblastingwithdrysand.Anyfinsormetalflashformedatmouldpartinglinesareremovedbygrindingandthecastingsarethenreadyforinspection.

Engine

Anengineisadevicethatusestheenergyinafueltodowork.Theenergyinthechemicalsofthefuelisturnedintoheatenergy.Theheatisthenusedtomovethemetalpartsofamachine.Therearemanykindsoffuel.Mostenginesusegasoline,oil,kerosene,coal,orcoke.Theheatthatcomesfromburningthefuelmakesagasexpand.Thisexpandgasdrivespistonsorturbineblades.Thepistonsorturbinesturnshafts.Theturningshaftsmovegearsandotherwheels.Weusetheserotatingwheelsandshaftstomoveautomobiles,airplanes,andothertransport.Wecanalsousethemforpumping,drilling,digging,andothersuchactivities.

Earlyenginesburnedcoalorwoodtoheatwater.Thesteamwasusedtodrivesteamengine.Untilthemiddleofthiscentury,mostlocomotiveswerepoweredbysteam.Atthebeginningofthecentury,evensomeautomobileswererunonsteam.Westillusesteamengines,butmostofthemarebeingreplacedbymoreefficientengines.Todaywehavepowerfulgasolineanddieselenginestoworkforus.

Thesteamengineisanexternalcombustionengine.Thismeansthatfueliscombustion,orburned,outsidethecylinderthatproducespower.SincetheinventionofthesteamenginebyJamesWattin1769s,steamengineshadagreateffectontheindustrialrevolutionintheeighteenthcentury.Untilthemiddleofthetwentiethcentury,mostlocomotiveswerepoweredbysteam.Attheendofthecentury,evensomemobileswererunonsteam.Todaymostofthefunctionsofsteamenginehavebeentakenoverbyinternalcombustionenginesfueledbygasolineanddieseloil.However,therearemanysteamenginesinuseatsea.Steamisalsousedtogenerateelectricity.Thekindofsteamenginethatisusedtodaydoesnothavecylinders.Itisgenerallyasteamturbine.ThesteamturbinewasinventedbySirCharlesParsons.Steamturbinescanhandlesteamathigherpressuresthanpistonenginescan,andtheyaremorecompactthanpistonenginesofsimilarpower.

ThegasolineengineTheengineofmostautomobilesandsmallvehicleusegasolineasfuel.Thegasolineengineisacombustionengine.Thefuelisburnedincombustion（burning）chambersinsidetheengine.Thecombustionchambersareplacedatoneendofthecylinders.Pistonsmoveupanddowninthecylinders.Theyarepushedbythehotgasesfromtheburningfuel.Whenthefuelismixedwithairitburnssoquicklythatitexplodes.Thecombustionchambersandcylinderapartlikeabomb,theexplosionsimplykickshandagainsttheheadofthepiston.Itpushesitasfarasitcan.

Eachmovementofapistonupordowninitscylinderiscalledastroke.Mostgasolineenginesworkonafour-strokecycle.Thismeansthateachpistongoesupanddowntwiceforeachexplosion.Thatmakesfourmovementorstrokes.Thiscycleofeventsisrepeatedoverandoveragain.Onthefirstdownstroke,thepistonmovestothelowestpartofthecylinder.Amixtureofgasolinedropletsandairisdrawnintothecylinderaboveit.Nowthepistonmovesupagain.Thisisitssecondstroke.Itsqueezesthemixtureintoasmallspace.Anelectriclightsthemixture,anditexplodes.Thepistonisforcedownagainforitsthirdstroke.Thisiscalledthepowerstroke.Forthefourthstroke,thepistonmovestothetopagain.Thistimeitpushestheburntgasesoutofthecylinder.Thegasesleavetheengineasexhaustfumes.

Thefirstenginethatusedthefour-strokecyclewasmadeinabout1876.ItwasdesignedbyaGermanengineer,AugustOtto.Heusedcoalgas,notgasoline.ThefirstenginestoburngasolineweredevelopedbyKarlBenzandGottlieb.Thesetwomenwerefamousasautomobilepioneers.

Apistonsimplygoingupanddowncannotpushanautomobilealong.Itsmovementmustbechangedtoaturningmovement.Todothis,acrankshaftisused.Eachpistonoftheengineislinkedtopartofthecrankshaft.Eachpushitgivesmakestheshaftturn.Thespinningshaftpassesthepowerontotheautomobile'stransmissionsystem.Itusuallydoesthisthroughaheavyflywheel.Thetransmissionsystemtransmitspowertotheclutchandtothepropellershaft,throughagearbox.Thepropellershaftdrivestheroadwheelsbymeansofaxles.

Tokeepanautomobileenginegoing,thereneedtobeseveralsystems.Theremustbeafuelsystem.Thishastosupplygasolinetotheenginecylindersintherightamounts.Ithasalsotomixitwiththerightamountofair,sothatitwillexplodeproperly.Theremustbeanignitionsystem.Thishastoprovidesparkstoignitetheexplosivemixturefatexactlytherighttime.Therehastobeacoolingsystem,otherwisetheenginewouldoverheat.Thelubricationsystemmustkeepallthemovingpartsoiledandmovingfreely.Toomuchfrictioncauseswearofthemetalandmakestheengineoverheat.

TheengineunitGasolineenginehastwobasicparts.Theyarecalledthecylinderheadandthecylinderblock.Thecylinderblockismachinedfromsolidmetal.Themetalsusuallycastsidethecylinderblocksarethecylinders.Thewallsofthecylindershavetobeveryaccuratelymade,andarehighlypolished.Thepistonsthatmoveupanddowninthecylindersmustbeaccuratelymade,too.Theyhavespringybandsofmetalaroundthemtopresstightagainstthecylinderwallsandstopgasesleaking.Thebandsarecalledpistonrings.Theyareoftenmadeofaluminumalloyforstrengthandlightness.

Anenginemayhaveanynumberofcylinders.Theymaybearrangedinaline,orinoppositepairs.Theyareoftenarrangedina"V"shape.Inmanyairplaneswithpitonengines,thecylindersarearrangedinaringaroundthecrankshaft.

Thelowerpartofthecylinderblockiscalledthecrankcase.Thisiswherethecrankshaftlies.Thecrankshaftislinkedtoeachpitonbyaconnectingrod.Thecrankshaftismadeinasinglepiece.Itmustbetoughandaccuratelymachined.Itmayspinasmanyas6000timesaminute.Itchangesup-and-downmotion.Itdoesthisbymeansofcranks,oneforeachpiston.Thecranksaresetatdifferentang