液压专业英语教程2.docx

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