液压支 架课程设计 毕业设计 英语翻译Word格式.docx

液压支 架课程设计 毕业设计 英语翻译Word格式.docx

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Thepowerinputsystem,thepowertransmissionsystem,thecontroldevicesandthepoweroutputportion.thepowerinputsegmentusuallyconsidersofpumpsandaccumulators.thepowertransmissionsystemincludestubing,tubingconnectorss,swiveljointsandflexiblehose.Flowcontrolvalves,pressurecontrolvalves,andrelivevalvesmakeupthecontrolelements.Thepoweroutputportionusuallyconsistsofrotaryandoscillstorymotorsorlinearactuators.Becausealmostallprecticalhydraulicsystemscanbedividedintothesrsamefourbasicsegments,theywillbeconsideredindetaillater.

Parallelingtheimprovementinhydraulicfluidsandothermaterialshasbeenanincreaseinthecomplexityandsophisticationodhydraulicsystems.Mostearlyhydraulicsystemswerenomorecomplexthanthesysteminanautomobilehydraulicjack.Thejackcontainsahandoperatedpumpwhichdrawsoilfromareservoir.oilispumpedpastacheckornon-returnvalveintoalinearactuator.Theoilflowingintotheactuatorforcesthepistonbacktothereservoir.

Throughtheuseofpowerdrivenpumpsanddirectionalcontrolvalvesthesimplesystemshavegrownsothatanumberofloadscanbehandledindependentlyorsequentiallyformthesamepowersource.Powerhydrualicsisapoweramplifierinwhichasmalleffortappliedtoacontroldeviceresultsinalargeamountofworkperformedonaload.Withboostservos,hydraulicpoweraidstheoperator’seffort.Furtherrefinementhasledtotheirreversibleservo-mechanicinwhichtheoperatoe’seffortisseparatedfromtheloadbytherigidityoftheoilintheactuatingcylinderormotor

ASCABTAGESIFHYSRAULICSYSTEMS

Whatarethereasonsforusinghydraulicpowerinheavydutyindustrial,mobileandaircraftdesigns?

Eventhroughpneumaticcontrolsystemspercededhydraulicservos,andelectricalpowerisinmorewidespreadisethanhydraulic,systemswhichrequirehighpioweramplificationorwhich,mustpackgreaterergyintosmallspacearealmostinvariablyhydraulic.

Ifhydraulicpowertransmissionandamplificationweretobecomparedwithpneumatic,electricalandmechanicalsysyems,thesearesomeofthereasonsforspecifyinghydraulicpower.

Hydraulicmotatorspossessanextremelyhighratiooftorquetoinertia.Becauseofthis,verylittleoftheavaialabletorqueisdissipatedinacceleratingthemotor;

mostofthemotors,forexample,varyfromabout30000radianspersecondsquaredforlargersizesat3000psito340000radianspersecondsquardforsmallermotors.Noothervalves.Highersupplypressuresdevelopaccordinglyhigheraccelerationrates.

Hydraulicmotorsdevelopnocounterelectro-motiveforceasdoelectricalmotors.Therefore,theaccelationrateisessentiallyconstantovertheirdesignspeedrages.

Hydraulicdevicescangeneratemuchmorepowerinthesamevolumethancanelectricaldevices.Thereasonforthisliesinthecharacteristicsofthematerialsusedinelectricalequipment.Theverybestarmaturesteelscandeveloponlytheepuivalentofabout200psiintractivepowerbeforetheybecomesaturated.Bycomparision,3000psihydraulicequipmentisverycommonandpressuresupto5000psiarenotrare.

Becauseofthegreaterpressuredensity,hydraulicequipmentinpowersizesgreaterthan1hp.tendstobemuchsmallerthanelectricalmotorsandsomewhatsmallerthanpneumaticmotors.

Hydraulicmotorssupportheavyorspringyloadsmuchbetterthandoelectricalorpneumaticmotors.Lookingfromtheloadbackintothemotors,electricalfieldsorcompressedgassesarequitesoft.Therelativelyincompressibleoilis,however,stiff.

Hydraulictransmissionsystemsefficientlytransmitpowerfartherthandomechanicalsystems,butnotasfaraselectricalsystems.

Hydraulictransmissionsystemsmaybelesshazardousthanelectricalsystemsandarefarsaferthanmechanicalpowertransmissionsystems.

Hydraulicoilisagoodheatconductorsothattheheavilyloadedworkingareastaysrelativelycool.Heatgeneratedattheworkinginferfaceisconductedawayrapidly.Theworkingareascanthereforebemadesmallbecauseheatabsorbingmassisnotneeded.Becauseofthisandbecauseofthehighpressuredensity,hydraulicmotorsweighttatiosofupto2.5arenotuncommon.

Hydraulicpowersystemsalsohavesomedrawbacks.Hydraulicoilsaremessy.Leaksareinevitable.Thegreatlubricityofmosthydraulicoilsmakesleaksandspillageveryhazardous.Certainoilsareexcellentremoversofpaintandelectricalinsulation.

Hydraulicsystems,particularlythehighperformanceservosystemswhichusehydraulicpowersoefficiently,areextremelyvulnerabletodirtandcontamination.Theymustconstantlybewatchedtopreventtheadditionorgenerationofcontaminationandhydraulicsystemsneedexpensivefiltrationtokeepthemclean.

Aruptureorburstofanelementofahighpressurehydraulicsystemcanbeverydangerous.Thehighvelocitystreamofoilwhichresultscandamageeyes,pierceskin,andharmotherequipment.Theauthorhasseenapieceoftubingthreefeetlongand1.5inchesindiameterthrownwhirlingformorethanfiftyfeetwhenaruptureoccurredina3000psiline.

Unlessfire-resistantfluidsareused,hydraulicleakscreatveryrealfirehazards.Mostpetroleumbasedfluidsarehighlyflammable.Hightemperaturescanbegeneratedwhereahighvelocitystreamofoilfromaruptureimpactsonaflatsurface.

Finally,hydraulicpowerissocompactthatdevelopingandcontrollingfractionalhorsepowersisdifficultunlessthepressuredensityisreducedtolevelswhereelectricalpowerisextremelycompetitive.

POWERSYSTEMANALOGIES

Hydraulicsystems-withinthelaminarflowregimeatleast-functioninwayswhicharequiteanalogoustomechanicalandelecteicalsystems.Laterinthesediscussionstheepuationofanalogywillbedeveloped.Atthispoint,illustrationsshowtheanalogiesbetweensystemsandtranslatoryandrotarymechanicalsystems.Similaranalogiescouldbedevelopedforpneumatic,thermalorotherenergysystems.Theseelementscanbecombinedinwayswhich-atleastinelectricalsystems-aretermed“passive”networks.Theyarepassivebecausenoenergyisaddedtothesysteminthenetworksthoughenergyislostthroughconversiontoheatintheresistanceelements.Thesenetworksaffectthestabilitysuchassinusoidalcycling.

Becauseoftheincreasingimportanceofthedynamicsofthehydraulicsystemindesigningforhighperformance,thisbookwillrelate-inapraticalway-thosefactorswhichaffectthedynamicaloperationofthesystemtothedesignofcomponentsandtothedispositionoftheseveralcomponentswithinthesystem.Noattemptwillbemadetodevelopdesigntechniquesotherthantoshowprevioussolutionstotypicalproblems.Instead,thisbookwillrelatethedisscussionoftheseveralpartsofthehydraulicsystemtobasicengineeringprinciples.Asincereattemptwillbemadetoshowhowthesebasicprinciplescanbebroughttobearonthesolvingoftypicalcomponentandsystemdesignproblems.

Whererelevant,recoursewillbemadetothecalculustosupportthedevelopmentofequationsbut,inmostcases,aknowledgeofthecalculuswillnotbenecessaryinordertosolveproblems.Certainbooksmaybeofvalueingainingadditionaldetailedunderstandingofspecificsubjects.Thisbook,however,isintendedtostandonitsownandallreasonableeffortshavebeenmadetoobviatetheneedforadditionalresources.

Someofthepracticeswhichhavedevelopedinthehydraulicsindustrymakeproblem-solvingdifficult.AmongthesearetheuseoffeetheadofwaterasameasureofpressureandunitshavebeenconvertedintotheirEnglishequivalents.Throughpersonalpreference,theauthoravoidsconfusioninconversionbyputtingeverythingintounitsofpound-force,inchesandseconds.Whereverthismightconflictwithestablishedpractice-suchasusinguitsofNewtsforkinematicviscosity-conversionformulaeareprovided.

Wherethetextmaterialisappropriate,problemsaregiventoallowthestudenttopracticetheachievingofsolutions.Theseproblemsnearlyalwaysrepresentformsofdifficultywhichhavefacedtheauthoratvarioustimes.Theyarepracticalproblemsinadvancedhydraulics.Inmanycases,asolutionispossibleonlybecausesimplifyingassumptionsweremade.Becauseoftheseassumptions,thehydraulicsengineershouldbewareofblindapplicationoftheformulae.Carefulconsiderationofeffectofpracticaldeviationsformtheassumedconditionsisnecessaryifsuccessinovercomingrealdifficultiesistobeachieved.

HYDRAULICFLUIDS

Clearly,thesinglemostimportantmaterialinthehydraulicsystemistheworkingfluid,Modernhydraulicfluidsareoftencomplexformulationscarefullypreparedtomeettheirexactingtasks.Inadditiontoabasefluidtheymaycontainadditivestoimproveviscositycharacteristics,tominimizesystemcorrosion,tomakethefluidsmorestabletotheirenvironmentsandtoreducefoamingandflammability.Dr.RogerHatton,inhisexcellentbook“IntroductiontoHydraulicFluids”,givesthefollowingascharacteristicsofimportancetoahydraulicfluid:

Goodlubricity

Chemicallyandenvironmentallystable

Possessesfavorableviscosity

Compatiblewithothersystemmaterials

Heattransfercapability

Highbulkmodulus

Lowvolatility

Lowfoamingtendencies

Lowweigh