PAGE15液壓傳動第十講制動器力流體動力系統的優秀的特性之一是由電源產生，通過適當的控制和指導，并通過電線傳輸，就可以輕松轉換到幾乎任何類型的機械運動所需要用到的地方。使用一個合適的驅動裝置，可以獲得線性（直線）或者是旋轉運動。驅動器是一種轉換流體動力機械力和運動的裝置。缸、馬達和渦輪機是最常見的將流體動力系統應用于驅動設備的類型。這一章描述了各種類型的動作汽缸和他們的應用程序、不同類型的流體汽車和使用流體動力系統的渦輪機。汽缸制動汽缸是一種將流體動力轉換成線性或直線、力和運動的裝置。因為線性運動是沿著一條直線前后移動的往復運動。這種類型的制動器有時被稱為一個往復、或線性、電動機。由ram或活塞組成的汽缸在一個圓柱孔內操作。制動汽缸可以安裝，以便汽缸被固定在一個固定的結構，ram或活塞被連接到該機制來操作，或者是活塞和ram可能被固定到固定結構，汽缸附加到機械裝置來操作。制動汽缸氣動和液壓系統的設計和操作是類似的。一些變化的ram和活塞式制動汽缸的內容將在后面的段落中描述。沖壓式缸術語ram和活塞通常可以互換使用。然而，一個沖壓式缸通常被認為是一個截面積活塞桿超過一半的截面積活動元件。在大多數這種類型的制動汽缸中，桿和活動元件各占一半。這種類型的活動元件經常被稱為柱塞。沖壓式缸主要是用來推動而不是拉。一些應用程序需要ram的一部分在平坦的外部來推動或升降單位操作。其他應用程序需要一些機械裝置的附件，如一個U型夾或有眼螺栓。沖壓式缸的設計在很多其他方面不同，以滿足不同應用程序的要求。單作用千斤頂單作用千斤頂（如圖：10-1）試用力只在一個方向。流體定向的汽缸取代ram和他外部的彈性元件，將物體舉起放在上面。當流體壓力釋放后，由于沒有規定由流體動力縮回ram。物體的重量或者是一些機械設備，比如一個彈簧，迫使ram回到汽缸。這種流體能量就倒回到容器。單作用沖壓式汽缸通常用于液壓千斤頂。用于移動飛機的飛行甲板和機庫甲板航空母艦的升降機也使用這種類型的汽缸。在這些升降機中，汽缸是水平安裝的，通過一系列的電纜和捆運行升降機。流體壓力產生ram的外在能量使升降機運行。當從ram中釋放出流體壓力，升降機的重量迫使ram回到汽缸。反過來，這迫使流體回容器。雙作用千斤頂一個雙作用沖壓式缸如圖10-2。在這個汽缸中，兩個中風的ram都是由加壓流體產生的。它有兩個流體端口，每一個都在或是接近汽缸的兩端。針對流體壓力的封閉端缸擴展ram和應用力。撤銷ram和減少力、流體是指向截然相反的汽缸。一個四通換向閥是通常用于控制雙作用ram。當閥門定位來擴展ram，加壓流體進入端口A，作用于ram的地面，加強ram的外部力量。對于在液壓系統中的回流管，或者氣動系統中的大氣，通過控制閥門，高于ramFluidPowerNAVEDTRA14105CHAPTER10ACTUATORSOneoftheoutstandingfeaturesoffluidpowersystemsisthatforce,generatedbythepowersupply,controlledanddirectedbysuitablevaluing,andtransportedbylines,canbeconvertedwitheasetoalmostanykindofmechanicalmotiondesiredattheveryplaceitisneeded.Eitherlinear(straightline)orrotarymotioncanbeobtainedbyusingasuitableactuatingdevice.Anactuatorisadevicethatconvertsfluidpowerintomechanicalforceandmotion.Cylinders,motors,andturbinesarethemostcommontypesofactuatingdevicesusedinfluidpowersystems.Thischapterdescribesvarioustypesofactuatingcylindersandtheirapplications,differenttypesoffluidmotors,andturbinesusedinfluidpowersystems.CYLINDERSAnactuatingcylinderisadevicethatconvertsfluidpowertolinear,orstraightline,forceandmotion.Sincelinearmotionisaback-and-forthmotionalongastraightline,thistypeofactuatorissometimesreferredtoasareciprocating,orlinear,motor.Thecylinderconsistsofaramorpistonoperatingwithinacylindricalbore.Actuatingcylindersmaybeinstalledsothatthecylinderisanchoredtoastationarystructureandtheramorpistonisattachedtothemechanismtobeoperated,orthepistonorrammaybeanchoredtothestationarystructureandthecylinderattachedtothemechanismtobeoperated.Actuatingcylindersforpneumaticandhydraulicsystemsaresimilarindesignandoperation.Someofthevariationsofram-andpiston-typeactuatingcylindersaredescribedinthefollowingparagraphs.RAM-TYPECYLINDERSThetermsramandpistonareoftenusedinterchangeably.However,aram-typecylinderisusuallyconsideredoneinwhichthecross-sectionalareaofthepistonrodismorethanone-halfthecross-sectionalareaofthemovableelement.Inmostactuatingcylindersofthistype,therodandthemovableelementhaveequalareas.Thistypeofmovableelementisfrequentlyreferredtoasaplunger.Theram-typeactuatorisusedprimarilytopushratherthantopull.Someapplicationsrequiresimplyaflatsurfaceontheexternalpartoftheramforpushingorliftingtheunittobeoperated.Otherapplicationsrequiresomemechanicalmeansofattachment,suchasaclevisoreyebolt.Thedesignofram-typecylindersvariesinmanyotherrespectstosatisfytherequirementsofdifferentapplications.Single-ActingRamThesingle-actingram(fig.10-1)appliesforceinonlyonedirection.Thefluidthatisdirectedintothecylinderdisplacestheramandforcesitoutward,liftingtheobjectplacedonit.Sincethereisnoprovisionforretractingtherambyfluidpower,whenfluidpressureisreleased,eithertheweightoftheobjectorsomemechanicalmeans,suchasaspring,forcestherambackintothecylinder.Thisforcesthefluidbacktothereservoir.Thesingle-actingram-typeactuatingcylinderisoftenusedinthehydraulicjack.Theelevatorsusedtomoveaircrafttoandfromtheflightdeckandhangardeckonaircraftcarriersalsousecylindersofthistype.Intheseelevators,thecylindersareinstalledhorizontallyandoperatetheelevatorthroughaseriesofcablesandsheaves.Fluidpressureforcestheramoutwardandliftstheelevator.Whenfluidpressureisreleasedfromtheram,theweightoftheelevatorforcestherambackintothecylinder.This,inturn,forcesthefluidbackintothereservoir.Double-ActingRamAdouble-actingram-typecylinderisillustratedinfigure10-2.Inthiscylinder,bothstrokesoftheramareproducedbypressurizedfluid.Therearetwofluidports,oneatorneareachendofthecylinder.Fluidunderpressureisdirectedtotheclosedendofthecylindertoextendtheramandapplyforce.Toretracttheramandreducetheforce,fluidisdirectedtotheoppositeendofthecylinder.Afour-waydirectionalcontrolvalveisnormallyusedtocontrolthedouble-actingram.Whenthevalveispositionedtoextendtheram,pressurizedfluidentersportA(fig.10-2),actsonthebottomsurfaceoftheram,andforcestheramoutward.FluidabovetheramlipisfreetoflowoutofportB,throughthecontrolvalve,andtothereturnlineinhydraulicsystemsortotheatmosphereinpneumaticsystems.Normally,thepressureofthefluidisthesameforeitherstrokeoftheram.Recallfromchapter2thatforceisequaltopressuretimesarea(F=PA).Noticethedifferenceoftheareasuponwhichthepressureactsinfigure10-2.Thepressureactsagainstthelargesurfaceareaonthebottomoftheramduringtheextensionstroke,duringwhichtimetheramappliesforce.Sincetheramdoesnotrequirealargeforceduringtheretractionstroke,pressureactingonthesmallareaonthetopsurfaceoftheramlipprovidesthenecessaryforcetoretracttheram.TelescopingRamsFigure10-3showsatelescopingram-typeactuatingcylinder.Aseriesoframsisnestedinthetelescopingassembly.Withtheexceptionofthesmallestram,eachramishollowandservesasthecylinderhousingforthenextsmallerram.Theramassemblyiscontainedinthemaincylinderassembly,whichalsoprovidesthefluidports.Althoughtheassemblyrequiresasmallspacewithalltheramsretracted,thetelescopingactionoftheassemblyprovidesarelativelylongstrokewhentheramsareextended.Anexcellentexampleoftheapplicationofthistypeofcylinderisinthedumptruck.Itisusedtolifttheforwardendofthetruckbedanddumptheload.Duringtheliftingoperation,thegreatestforceisrequiredfortheinitialliftingoftheload.Astheloadisliftedandbeginstodump,therequiredforcebecomeslessandlessuntiltheloadiscompletelydumped.Duringtheraisecycle,pressurizedfluidentersthecylinderthroughportA(fig.10-3)andactsonthebottomsurfaceofallthreerams.Ram1hasalargersurfaceareaand,therefore,providesthegreaterforcefortheinitialload,Asram1reachestheendofitsstrokeandtherequiredforceisdecreased,ram2moves,providingthesmallerforceneededtocontinueraisingtheload.Whenram2completesitsstroke,astillsmallerforceisrequired.Ram3thenmovesoutwardtofinishraisinganddumpingtheload.Sometelescopingram-typecylindersareofthesingle-actingtype.Likethesingle-actingramdiscussedpreviously,thesetelescopingram-typecylindersareretractedbygravityormechanicalforce.Somehydraulicjacksareequippedwithtelescopingrams.Suchjacksareusedtoliftvehicleswithlowclearancestotherequiredheight.Othertypesoftelescopingcylinders,liketheoneillustratedinfigure10-3,areofthedouble-actingtype.Inthistype,fluid