外文翻譯--液壓管路和管接頭.doc

徐州工程學(xué)院畢業(yè)設(shè)計1附錄附錄一英文原文HydraulicConductorsandFittingsEricSandgren*,T.M.CamerontoaccountforuncertaintyaMechanicalEngineering,VirginiaCommonwealthUniversity,601WestMainStreet,P.O.Box843015,Richmond,VA23284-3015,USAReceived19October2001；accepted5June20021.1INTRODUCTIONInahydraulicsystem,thefluidflowsthroughadistributionsystemconsistingofconductorsandfittings,whichcarrythefluidfromthereservoirthroughoperatingcomponentsandbacktothereservoir.Sincepoweristransmittedthroughoutthesystembymeansoftheseconductinglines(conductorsandfittingsusedtoconnectsystemcomponents),itfollowsthattheymustbeproperlydesignedinorderforthetotalsystemtofunctionproperly.Hydraulicsystemsuseprimarilyfourtypesofconductors:1.Steelpipes2.Steeltubing3.Plastictubing4.FlexiblehosesThechoiceofwhichtypeofconductortousedependsprimarilyonthesystemsoperatingpressuresandflowrates.Inaddition,theselectiondependsonenvironmentalconditionssuchasthetypeoffluid,operatingtemperatures,vibration,andwhetherornotthereisrelativemotionbetweenconnectedcomponents.Conductinglinesareavailableforhandlingworkpressuresupto10,000Paorgreater.Ingeneral,steeltubingprovidesgreaterplumbingflexibilityandneaterappearanceandrequiresfewerfittingsthanpiping.However,pipingislessexpensivethansteeltubing.Plastictubingisfindingincreasedindustrialusagebecauseitisnotcostlyandcircuitscanbeveryeasily徐州工程學(xué)院畢業(yè)設(shè)計2hookedupduetoitsflexibility.Flexiblehosesareusedprimarilytoconnectcomponentsthatexperiencerelativemotion.Theyaremadefromalargenumberofelastomeric(rubberlike)compoundsandarecapableofhandlingpressuresexceeding10,000Pa.Stainlesssteelconductorsandfittingsareusedifextremelycorrosiveenvironmentsareexpected.However,theyareveryexpensiveandshouldbeusedonlyifnecessary.Copperconductorsshouldnotbeusedinhydraulicsystemsbecausethecopperpromotestheoxidationofpetroleumoils.Zinc,magnesium,andcadmiumconductorsshouldnotbeusedeitherbecausetheyarerapidlycorrodedbywater-glycolfluids.Galvanizedconductorsshouldalsobeavoidedbecausethegalvanizedsurfacehasatendencytoflakeoffintothehydraulicfluid.Whenusingsteelpipeorsteeltubing,hydraulicfittingsshouldbemadeofsteelexceptforinlet,return,anddrainlines,wheremalleableironmaybeused.Conductorsandfittingsmustbedesignedwithhumansafetyinmind.Theymustbestrongenoughnotonlytowithstandthesteady-statesystempressuresbutalsotheinstantaneouspressurespikesresultingfromhydraulicshock.Whenevercontrolvalvesareclosedsuddenly,thisstopsthefluid,whichpossesseslargeamountsofkineticenergy.Thisproducesshockwaveswhosepressurelevelscanbetwoorfourtimesthesteady-statesystemdesignvalues.Pressurespikescanalsobecausedbysuddenstoppingorstartingofheavyloads.Thesehigh-pressurepulsesaretakenintoaccountbytheapplicationofanappropriatefactorofsafety.1.2CONDUCTORSIZINGFORFLOW-RATEREQUIREMENTSAconductormusthavealargeenoughcross-sectionalareatohandletheflow-raterequirementswithoutproducingexcessivefluidvelocity.Wheneverwespeakoffluidvelocityinaconductorsuchasapipe,wearereferringtotheaveragevelocity.Theconceptofaveragevelocityisimportantsinceweknowthatthevelocityprofileisnotconstant.AsshowninChapter5thevelocityiszeroatthepipewallandreachesamaximumvalueatthecenterlineofthepipe.Theaveragevelocityisdefinedasthevolumeflowratedividedbythepipecross-sectionalarea:AQavgvvInotherwords,theaveragevelocityisthatvelocitywhichwhenmultipliedbythepipeareaequalsthevolumeflowrate.Itisalsounderstoodthatthetermdiameterbyitselfalwaysmeansinsidediameterandthatthepipeareaisthatareathatcorrespondstothepipeinsidediameter.Themaximumrecommendedvelocityforpumpsuctionlinesis4ft/s(1.2m/s)inordertopreventexcessivelylowsuctionpressuresandresultingpumpcavitation.The徐州工程學(xué)院畢業(yè)設(shè)計3maximumrecommendedvelocityforpressurelinesis20ft/s(6.1m/s)inordertopreventturbulentflowandthecorrespondingexcessiveheadlossesandelevatedfluidtemperatures.Notethatthesemaximumrecommendedvaluesareaveragevelocities.EXAMPLE1-1Apipehandlesaflowrateof30gprn.Findtheminimuminsidediameterthatwillpro-videanaveragefluidvelocitynottoexceed20ft/s.SolutionRewriteEq.(3-26),solvingforD:invQD782.0612.02030408.0408.0EXAMPLE1-2Apipehandlesaflowrateof0.002sm/3.Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed6.1m/s.SolutionPerEq.3-35)wesolvefortheminimumrequiredpipeflowarea:232000328.01.6002.0msmvsmQmATheminimuminsidediametercannowbefound,because42DASolvingforDwehavemmmAD4.200204.0000328.0441.3PRESSURERATINGOFCONDUCTORSTensileStressAconductormustbestrongenoughtopreventburstingduetoexcessivetensilestress(calledhoopstress)inthewalloftheconductorunderoperatingfluidpressure.Themagnitudeofthistensilestress,whichmustbesustainedbytheconductormaterial,canbedeterminedbyreferringtoFigure4-1.InFig.4-1(a),weseethefluidpressure(P)actingnormaltotheinsidesurfaceofacircularpipehavingalength(L).ThepipehasoutsidediameterD0,insidediameterDi,andwallthicknesst.Becausethefluidpressureactsnormaltothepipesinsidesurface,apressureforceiscreatedthatattemptstoseparateonehalfofthepipefromtheotherhalf.徐州工程學(xué)院畢業(yè)設(shè)計4Figure4-1(b)showsthispressureforcePFpushingdownwardonthebottomhalfofthepipe.Topreventthebottomhalfofthepipefromseparatingfromtheupperhalf,theupperhalfpullsupwardwithatotaltensileforceF.One-halfofthisforce(orF/2)actsonthecross-sectionalarea(tL)ofeachwall,asshown.Sincethepressureforceandthetotaltensileforcemustbeequalinmagnitude,wehavewhereAistheprojectedareaofthelowerhalf-pipecurved-wallsurfaceontoahor-izontalplane.Thus,AequalstheareaofarectangleofwidthDiandlengthL,asshowninFigure4-1(b).Hence,Thetensilestressinthepipematerialequalsthetensileforcedividedbythewallcross-sectionalareawithstandingthetensileforce.Thisstressiscalledatensilestressbecausetheforce(F)isatensileforce(pullsontheareaoverwhichitacts).SubstitutingvariableswehavetPDtLLDPtLPAtLFii22)(22where=Greeksymbol(sigma)=tensilestress.AscanbeseenfromEq.(4-2),thetensilestressincreasesasthefluidpressureincreasesandalsoasthepipeinsidediameterincreases.Inaddition,asexpected,thete