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1、Unit2Pressure,Work,andPowerBasictermthatarecommonlyusedinthefieldofhydraulicsandpneumaticsmustbediscussedandunderstood.PRESSUREThewordpressureisdefinedasforceperunitarea・Althoughotherunitsmaybeused,pressureiscommonlyexpressedinsuchunitsapoundspersquareinch.Theabbreviationpsiisusuallyemployedt
2、oindicatepoundspersquareinch.Fig.lshowsanarrangementoftwocylindersthatareconnectedbyapipeortube.Aclose-fittingpistonisplacedineachcylinder.Ineachcylinder(underthepiston),theliquidandtheconnectingtubeareshown,Ifitisassumedthatthereisnomovementofeachpistonandthatthereisnoleakagepasteachpiston,t
3、heliquidandallthepartsareatrest-astaticcondition.ItisalsoassumedthataforceF)of100poundsactsonpistonactsonpistonNo.qandthatthereisnofrictionbetweeneachpistonanditscylinderwall.IfpistonNo.1hadaflatorfaceareaof2squareinchesthatisindirectcontactwiththeliquid,thepressureintheliquidunderpistonNo.1i
4、sequaltotheforcedividedbythearea(100dividedby2),or50poundspersquareinch.Thus,theliquidpressureatthefaceofpistonNo.1is50psi.PISTONAREA100POUNDS100POUNDSPISTONAREA•2SQUAREINCHES•2SQUAREINCHESPISTONNOJPISTONNOJPRESSURE-50PSICYLINDERN0.1PRESSURE-50PSICYLINDERNO.l“•300POUNDSPISTONNO.2TUBINGORPIPEC
5、YLINDERNO.2PISTONAREA•6SQUAREINCHESFig.1.Illustratingahydraulicjysfemhavingtwocylindersandtwopistonsofdifferentsizes.AssumingthatpistonNo.2isessentiallyatthesamelevelaspistonNo.l,theliquidbetweenthepistonsservesasamediumtotransmitthepressurefromonepistonfacetotheotherpistonface.Thus,theliquid
6、pressureatthefaceofpistonNo.2is50psi,IftheareaofpistonNo.2is6squareinches,theforceF2onthefaceofpistonNo.2is(6X50),or300pounds,Thus,aforceof100poundsatpistonNo.ldevelopsaforceof300poundsatpistonNo.2;thisisaccomplishedbymakingtheareaofpistonNo.2equaltothreetimestheareaofpistonNo.l.Inasense,thea
7、rrangement(seeFig.1)isafluidlever,similartoamechanicalleverusingametalbarandpivot.Equalpressureateverypointandineverydirectioninthebodyofastaticliquid(aliquidatrest)ischaracteristicofallstaticfluids,liquids,orgases.ThisiscalledPascal'slaw,aft
