液压专业英语教程2.docx
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液压专业英语教程2.docx
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液压专业英语教程2
Unit2Pressure,Work,andPower
Basictermthatarecommonlyusedinthefieldofhydraulicsandpneumaticsmustbediscussedandunderstood.
PRESSURE
Thewordpressureisdefinedasforceperunitarea.Althoughotherunitsmaybeused,pressureiscommonlyexpressedinsuchunitsapoundspersquareinch.Theabbreviationpsiisusuallyemployedtoindicatepoundspersquareinch.
Fig.1showsanarrangementoftwocylindersthatareconnectedbyapipeortube.Aclose-fittingpistonisplacedineachcylinder.Ineachcylinder(underthepiston),theliquidandtheconnectingtubeareshown,Ifitisassumedthatthereisnomovementofeachpistonandthatthereisnoleakagepasteachpiston,theliquidandallthepartsareatrest-astaticcondition.ItisalsoassumedthataforceF1of100poundsactsonpistonactsonpistonNo.qandthatthereisnofrictionbetweeneachpistonanditscylinderwall.IfpistonNo.1hadaflatorfaceareaof2squareinchesthatisindirectcontactwiththeliquid,thepressureintheliquidunderpistonNo.1isequaltotheforcedividedbythearea(100dividedby2),or50poundspersquareinch.Thus,theliquidpressureatthefaceofpistonNo.1is50psi.
AssumingthatpistonNo.2isessentiallyatthesamelevelaspistonNo.1,theliquidbetweenthepistonsservesasamediumtotransmitthepressurefromonepistonfacetotheotherpistonface.Thus,theliquidpressureatthefaceofpistonNo.2is50psi,IftheareaofpistonNo.2is6squareinches,theforceF2onthefaceofpistonNo.2is(6×50),or300pounds,Thus,aforceof100poundsatpistonNo.1developsaforceof300poundsatpistonNo.2;thisisaccomplishedbymakingtheareaofpistonNo.2equaltothreetimestheareaofpistonNo.1.Inasense,thearrangement(seeFig.1)isafluidlever,similartoamechanicalleverusingametalbarandpivot.
Equalpressureateverypointandineverydirectioninthebodyofastaticliquid(aliquidatrest)ischaracteristicofallstaticfluids,liquids,orgases.ThisiscalledPascal’slaw,afteranearlyexperimenterinthisfieldofstudy.Thislawofpressureisveryuseful,andcanbesuedtoadvantageincountlessapplications.
AtmosphericPressure
Ablanketofairsurroundstheearth;thisiscalledtheatmosphere.Atthesurfaceoftheearth,atmosphericpressure,whichisduetotheweightoftheairabovethesurfaceoftheearth,canbemeasured.Atmosphericpressureiscommonlymeasuredwithamercurybarometer.Thus,atmosphericpressureisoftencalledbarometricpressure.
Fig.2illustratesthebasicprincipleofamercurybarometer.Theglasstubeisopenatthelowerendandclosedtattheupperend.Initially,thetubeiscompletelyfilledwithpuremercury;thenitisinverted,withtheopenendsubmerged,inasmallvesselorcisterncontainingmercury.Theheightofthecolumnofmercurygivesthedirectreadingofthebarometer;theweightoftheairabovethebarometerbalancestheweightofthemercurycolumn.Barometricpressureisusuallyexpressedininchesofmercury.Abarometricheightof29.32inchesofmercurycorrespondstoanatmosphericpressureofabout14.7poundspersquareinch.
PressureMeasurement
ManyinstrumentsorgaugesthatareusedformeasuringpressureemployaBourdontubeisahollowmetaltubethatismadeofbrassorasimilarmaterial;itisovalorellipticalincrosssection,andisbentintheformofacircle.OneendofthebourdontubeisfixedtotheframeatpointA(wherethefluidenters)theotherendB(closed)isfreetomove.ThefreeendBactuatesapointerthroughalinkagesystem.Asfluidpressureinsidethetubechanges,theellipticalcrosssectionchanges,andthefreeendBoftheBourdontubemovesinwardoroutward,dependingonthecharacterofthechange.Aconvenientpressurescaleordialcanbearrangedfromacalibrationofthegauge.
ThepositionofthefreeendBoftheBourdontubedependsonthedifferenceinfluidpressurebetweentheinsideandtheoutsideofthetube.IftheoutsideoftheBourdontubeisexposedtoatmosphericairpressure,theinstrumentreadingisameasureofso-calledgaugepressure.Forexample,ifthepressurereadingattheoutletofanaircompressoris100psigauge,thisindicatesthattheoutletairpressureis100psiaboveatmos0phericpressure.Inthisbook,allpressuresreferredtoaregaugepressures.
Insomeinstances,thepressureinapieceofequipmentmaybebelowatmosphericpressure;thisconditionisdesignatedasvacuum.Forexample,iftheairinatankisatapressurethatisbelowatmosphericpressure,thepressuregaugeindicatesacertainvacuum,ornegativegaugepressure,
DEFINITIONOFWORK,ENERGY,ANDPOWER
AsshowninFig.4,bodyweighing20poundsatagivenlevelisindicatedinpositionNo.1.Ifthebodyismovedverticallythroughadistanceordisplacementof9feet,theactioninvolveswork.Thetechnicaltermworkisdefinedastheproductofforcetimesdisplacement,withtheforceinthedirectionofthedisplacement.AsthebodymovesfrompositionNo.1topositionNo.2,aforceof20poundsmovesthe20-Ib.bodythroughadisplacementof9feet.Thisequals(9×20),or180foot-poundsofwork.
Energyisdefinedasthecapacitytodowork.Energyreferstoapossibility.AbodyrestingatpositionNo.2hascertaincapacity,oracertainenergy.Ifthebodyismovedtothelevel9feetbelow,(9×20),or180foot-poundsisavailabletodowork.
Thetermworkinitselfdoesnotinvolveatimeelement.Rateofmovement,orspeed,isoftenimportant.Powerisdefinedasthetimerateofdoingwork.Ifthebodyweighing20poundsweremovedataconstantspeedandinaverticaldirectionthroughaverticaldistanceof9feetinatimeof2seconds,the“power”canbecalculatedasfollows:
Power=
Thus,thepowerrequiredis90foot-poundspersecond.Onehorsepowerhasbeenarbitrarilydefinedasequalto550foot-poundspersecond.
FORCEANDWORKINAFLUIDDEVICE
Fig.5isanillustrationofapumporcompressordeliveringfluid(eitheroilcompressedair)totheleft-handsideofapistoninacylinder.LetPrepresentthefluidpressure,inpsi,andArepresentthepistonareainsq.in.(abbreviationforsquareinches).
ThentheforceFactingontheleft-handfaceofthepistonisPA.ForapressurePof50psiandanarea.Aof2sq.in.,theforceactingontheleft-handfaceofthepistonisequalto(50×2),or100pounds.Assumingnofrictionduetothecylinderwall,theforceFatthepistonrodisequalto100pounds.
Fig.6isanotherillustrationofapumporcompressordeliveringfluidtotheleft-handsideofapistonisacylinder.Asinthepreviousexample,letPrepresentthefluidpressure(psi)andletArepresenttheleft-handpistonarea(sq.in.).ThenthefluidforceFactingontheleft-handpistonfaceisF=PA.IfthisforceremainsconstantwhilepushingthepistonthroughadisplacementofdistanceL(inches),theworkdonebythefluidontheleft-handfaceofthepistonisequaltotheforceFtimesthedisplacementLortheworkW=PAL=PL.Forafluidpressureof50psi,apistonareaof2sq.in.,anddisplacementof3inches,theworkWconbedeterminedasfollows.
W=(50×2×3)=300inch-pounds
DISPLACEMENTACTION
Fig.7isadiagramofapumpdeliveringhydraulicfluidthroughapipeortubeintoacylinder,ThepistonisshownatpositionNo.1atagiventimeorpartofthestroke.Ifthepumpcontinuestodeliverfluidintothecylinder,thefluidpushesthepistontotheright-handsideadistanceof3inches(positionNo.2).Thelineardisplacementormovementofthepistontotherightisequalto3inches.Forapistonareaof2squareinchesandapistondisplacementof3inches,thevolumetricdisplacementofthepistonisequal20(2spin×3in).or6cubicinches.Assumingnoleakageoffluidacrossthepistonfromtheleft-handsideofthepistontotheright-handsideofthepiston,thetotalamountoffluidaddedtothecylinderisequalto6cubicinches;inotherwords,6cubicinchesoffluidwasadmittedtothecylinderandpushedthepiston,foravolumetricdisplacementequalto6cubicinches.Withnoleakage,thisactionisfrequentlycalledapositive-displacementaction.Theamountoffluidenteringthecylinderisequaltothevolumetricdisplacementofthepiston.
InreferencetothesystemillustratedinFig.1(twocylindersandtwopistons),theareaofpistonNo.1is2squareinchesandtheareaofpistonNo.2is6squareinches(3timesthatofpistonNo.1).Ifaforceof100poundsisappliedatpistonNo.1,thereiscorrespondingforceof300poundsatpistonNo.2.Foragivenfluidpressure,thereisaforcemultiplicationbecauseofthedifferenceinpistonareas,assumingnoleakage.IfitisassumedthatpistonNo.1movesdownwardadistanceof0.03inchforapositive-displacementactionoftheincompressiblehydraulicfluid,thevolumeoffluiddisplacedbypistonNo.1isequaltothevolumeoffluiddisplacedbypistonNo.2.Forapositive-displacementaction,pistonNo.2thenmovesupward0.01inch(pistonareatimesdisplacementisequaltothedisplacedvolume).ForpistonNo.1,thetotalworkis(100×0.03),orinch-pounds,ForpistonNo.2,thetotalworkis(300×0.01),or3inch-pounds.
RateofFlowandPistonTravel
Thevolumerateoffluidflowthroughadevicecanbeexpressedinvarious.Foxexample,itiscommontoexpressvolumerateofflowforliquidsingallonsperminute(gpm).Onegallonisequalto231cubicinches.Forapracticalexample,anoilpumpmaybesaidtodeliveraflowof10gpm;thiscorrespondstoarateof
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