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外文翻译--浅谈油管液压管路系统设计.doc

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外文翻译--浅谈油管液压管路系统设计.doc

TalkingAboutTheDesignofHydraulicConductorsEricSandgrenThispaperisaccountforuncertaintyMechanicalEngineering,UniversityofCalifornia,SanFrancisco,avialon503WestMainStreet,P.O.Box9625311.1INTRODUCTIONInahydraulicsystem,thefluidflowsthroughadistributionsystemconsistingofconductorsandfittings,whichcarrythefluidfromthereservoirthroughoperatingcomponentsandbacktothereservoir.Sincepoweristransmittedthroughoutthesystembymeansoftheseconductinglinesconductorsandfittingsusedtoconnectsystemcomponents,itfollowsthattheymustbeproperlydesignedinorderforthetotalsystemtofunctionproperly.Thechoiceofwhichtypeofconductortousedependsprimarilyonthesystemsoperatingpressuresandflowrates.Inaddition,theselectiondependsonenvironmentalconditionssuchasthetypeoffluid,operatingtemperatures,vibration,andwhetherornotthereisrelativemotionbetweenconnectedcomponents.Conductinglinesareavailableforhandlingworkpressuresupto10,000Paorgreater.Ingeneral,steeltubingprovidesgreaterplumbingflexibilityandneaterappearanceandrequiresfewerfittingsthanpiping.However,pipingislessexpensivethansteeltubing.Plastictubingisfindingincreasedindustrialusagebecauseitisnotcostlyandcircuitscanbeveryeasilyhookedupduetoitsflexibility.Flexiblehosesareusedprimarilytoconnectcomponentsthatexperiencerelativemotion.Theyaremadefromalargenumberofelastomericrubberlikecompoundsandarecapableofhandlingpressuresexceeding10,000Pa.Stainlesssteelconductorsandfittingsareusedifextremelycorrosiveenvironmentsareexpected.However,theyareveryexpensiveandshouldbeusedonlyifnecessary.Copperconductorsshouldnotbeusedinhydraulicsystemsbecausethecopperpromotestheoxidationofpetroleumoils.Zinc,magnesium,andcadmiumconductorsshouldnotbeusedeitherbecausetheyarerapidlycorrodedbywaterglycolfluids.Galvanizedconductorsshouldalsobeavoidedbecausethegalvanizedsurfacehasatendencytoflakeoffintothehydraulicfluid.Whenusingsteelpipeorsteeltubing,hydraulicfittingsshouldbemadeofsteelexceptforinlet,return,anddrainlines,wheremalleableironmaybeused.Conductorsandfittingsmustbedesignedwithhumansafetyinmind.Theymustbestrongenoughnotonlytowithstandthesteadystatesystempressuresbutalsotheinstantaneouspressurespikesresultingfromhydraulicshock.Whenevercontrolvalvesareclosedsuddenly,thisstopsthefluid,whichpossesseslargeamountsofkineticenergy.Thisproducesshockwaveswhosepressurelevelscanbetwoorfourtimesthesteadystatesystemdesignvalues.Pressurespikescanalsobecausedbysuddenstoppingorstartingofheavyloads.Thesehighpressurepulsesaretakenintoaccountbytheapplicationofanappropriatefactorofsafety.1.2CONDUCTORSIZINGFORFLOWRATEREQUIREMENTSAconductormusthavealargeenoughcrosssectionalareatohandletheflowraterequirementswithoutproducingexcessivefluidvelocity.Wheneverwespeakoffluidvelocityinaconductorsuchasapipe,wearereferringtotheaveragevelocity.Theconceptofaveragevelocityisimportantsinceweknowthatthevelocityprofileisnotconstant.AsshowninChapter5thevelocityiszeroatthepipewallandreachesamaximumvalueatthecenterlineofthepipe.TheaveragevelocityisdefinedasthevolumeflowratedividedbythepipecrosssectionalareaAQvInotherwords,theaveragevelocityisthatvelocitywhichwhenmultipliedbythepipeareaequalsthevolumeflowrate.Itisalsounderstoodthatthetermdiameterbyitselfalwaysmeansinsidediameterandthatthepipeareaisthatareathatcorrespondstothepipeinsidediameter.Themaximumrecommendedvelocityforpumpsuctionlinesis4ft/s1.2m/sinordertopreventexcessivelylowsuctionpressuresandresultingpumpcavitation.Themaximumrecommendedvelocityforpressurelinesis20ft/s6.1m/sinordertopreventturbulentflowandthecorrespondingexcessiveheadlossesandelevatedfluidtemperatures.Notethatthesemaximumrecommendedvaluesareaveragevelocities.EXAMPLE11Apipehandlesaflowrateof30gprn.Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed20ft/s.SolutionRewriteEq.326,solvingforDinvQD782.0612.02030408.0408.0EXAMPLE12Apipehandlesaflowrateof0.002sm/3.Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed6.1m/s.SolutionPerEq.335wesolvefortheminimumrequiredpipeflowarea232000328.01.6002.0msmvsmQmATheminimuminsidediametercannowbefound,because42DASolvingforDwehavemmmAD4.200204.0000328.0441.3PRESSURERATINGOFCONDUCTORSAconductormustbestrongenoughtopreventburstingduetoexcessivetensilestresscalledhoopstressinthewalloftheconductorunderoperatingfluidpressure.Themagnitudeofthistensilestress,whichmustbesustainedbytheconductormaterial.weseethefluidpressurePactingnormaltotheinsidesurfaceofacircularpipehavingalengthL.ThepipehasoutsidediameterD0,insidediameterDi,andwallthicknesst.Becausethefluidpressureactsnormaltothepipesinsidesurface,apressureforceiscreatedthatattemptstoseparateonehalfofthepipefromtheotherhalf.FigureshowsthispressureforcePFpushingdownwardonthebottomhalfofthepipe.Topreventthebottomhalfofthepipefromseparatingfromtheupperhalf,theupperhalfpullsupwardwithatotaltensileforceF.OnehalfofthisforceorF/2actsonthecrosssectionalareatLofeachwall,asshown.Sincethepressureforceandthetotaltensileforcemustbeequalinmagnitude,wehavewhereAistheprojectedareaofthelowerhalfpipecurvedwallsurfaceontoahorizontalplane.Thus,AequalstheareaofarectangleofwidthDiandlengthL,asshowninFigure41b.Hence,Thetensilestressinthepipematerialequalsthetensileforcedividedbythewallcrosssectionalareawithstandingthetensileforce.ThisstressiscalledatensilestressbecausetheforceFisatensileforcepullsontheareaoverwhichitacts.SubstitutingvariableswehavewhereGreeksymbolsigmatensilestress.AscanbeseenfromEq.thetensilestressincreasesasthefluidpressureincreasesandalsoasthepipeinsidediameterincreases.Inaddition,asexpected,thetensilestressincreasesasthewallthicknessdecreases,andthelengthofthepipedoesnothaveanyeffectonthetensilestress.BurstPressureandWorkingPressureTheburstpressureBPisthefluidpressurethatwillcausethepipetoburst.ThishappenswhenthetensilestressequalsthetensilestrengthSofthepipematerial.Thetensilestrengthofamaterialequalsthetensilestressatwhichthematerialruptures.NoticethatanaxialscribelineisshownonthepipeouterwallsurfaceinFig.41a.Thisscribelineshowswherethepipewouldstarttocrackandthusruptureifthetensilestressreachedthetensilestrengthofthepipematerial.ThisrupturewilloccurwhenthefluidpressurePreachesBRThus,fromEq.42theburstpressureisTheworkingpressureWPisthemaximumsafeoperatingfluidpressureandisdefinedastheburstpressuredividedbyanappropriatefactorofsafetyFS.Afactorofsafetyensurestheintegrityoftheconductorbydeterminingthemaximumsafelevelofworkingpressure.IndustrystandardsrecommendthefollowingfactorsofsafetybasedoncorrespondingoperatingpressuresFS8forpressuresfrom0to1000PaFS6forpressuresfrom1000to2500PaFS4forpressuresabove2500Pa

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