机电一体化专业英语教学课件 作者 黄星 夏玉波 Unit 4.ppt

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1、HydraulicMachineryandForgingEquipmentPartITechnicalandPracticalReadingPassageAHydraulicMachineryHydraulicmachineryismachinesandtoolswhichusefluidpowertodowork.Inthistypeofmachine,high-pressureliquid—calledhydraulicfluid—istransmittedthroughoutthemachinetovarioushydraulicmotorsandhyd

2、rauliccylinders.Hydraulicmachineryisoperatedbytheuseofhydraulics,wherealiquidisthepoweringmedium.Pneumatics,ontheotherside,isbasedontheuseofagasasthemediumforpowertransmission,generationandcontrol.ForceandtorquemultiplicationAfundamentalfeatureofhydraulicsystemsistheabilitytoapplyfo

3、rceortorquemultiplicationinaneasywaywithouttheneedofmechanicalgearsorlevers,eitherbyalteringtheeffectiveareasintwoconnectedcylindersortheeffectivedisplacementbetweenapumpandmotor.Boththeseexamplesareusuallyreferredtoasahydraulictransmissionorhydrostatictransmissioninvolvingacertainh

4、ydraulic"gearratio".HydrauliccircuitsOpencentercircuitsusepumpswhichsupplyacontinuousflow.Theflowisreturnedtotankthroughthecontrolvalve'sopencenter;thatis,whenthecontrolvalveiscentered,itprovidesanopenreturnpathtotankandthefluidisnotpumpedtoahighpressure.Closedcentercircuitssupplyfu

5、llpressuretothecontrolvalves,whetheranyvalvesareactuatedornot.Thepumpsvarytheirflowrate,pumpingverylittlehydraulicfluiduntiltheoperatoractuatesavalve.Thevalve'sspoolthereforedoesn'tneedanopencenterreturnpathtotank.Multiplevalvescanbeconnectedinaparallelarrangementandsystempressureis

6、equalforallvalves.OpenandclosedcircuitsOpen-loop:Pump-inletandmotor-return(viathedirectionalvalve)areconnectedtothehydraulictank.Thetermloopappliestofeedback;themorecorrecttermisopenversusclosed"circuit".Closed-loop:Motor-returnisconnecteddirectlytothepump-inlet.Tokeepuppressureonth

7、elowpressureside,thecircuitshaveachargepumpthatsuppliescooledandfilteredoiltothelowpressureside.Closed-loopcircuitsaregenerallyusedforhydrostatictransmissionsinmobileapplications.Advantages:Nodirectionalvalveandbetterresponse,thecircuitcanworkwithhigherpressure.Thepumpswivelanglecov

8、ersbothpositiveandn