起重机-英语教材-完整版

目录1.StressandStrain2.LimitsandTolerances3.Cams4.Springs5.HeatTreatmentofMetals6.Dynamic-ForceAnalysis7.MechanismsandTheirComponents8.ApplicationsandClassificationofOptimization9.HI-TECTRopes(I)10.MobileCranes11.Thecranes12.MechanicalRequirements(I)13.MechanicalRequirements(III)14.TheBeltConveyorandPneumaticconveyor15.IntroductiontoFluidPower16.Ship-Unloader17.InternalCombustion18.ForkLiftTruck(I)19.ForkLiftTruck(II)20.OverviewofConnectionBehaviour(I)Text1stressandStrainwhenanexternalforceisappliedtoabody,itcausesthebodytodeformorchangeslightlyinshape.Italsocausesinternalforces(stresses)toactwithinthebody.Mechanicsofmaterialsisthesciencethatanalyzesthestressesandstrains(deformations)causedbytheapplicationofexternalforces.Theanalysisanddesignofaxiallyloadedmembersarecoveredinthischapter.Axialloadingisencounteredinawidevarietyofpracticalapplicationsinallengineeringdisciplines.Althoughtheapplicationsareofsomeinterest,theconcepts,definitions,andproceduresareofparticularsignificance.Theyformthefoundationoffuturework,andareappliedandextendedtodevelopthetheoryandpracticeforothertypesofloadingsituations.Inthesolutionofallproblemsconcerningthemechanicsofmaterials,weshouldunderstandthephysicalactionstakingplacewithinthemember.Therefore,itisimportanttobeableto“visualize”thestressandstrainoccurringinabody.Verylittleformulamemorizationisnecessaryinthesolutionoftheseproblems.However,thehabitofmakingcomplete,carefullydrawndiagramsofthemembersunderloadwillaidtremendouslyinunderstandingthesubject.Stressisafunctionoftheinternalforcesinabodythatarecausedbytheapplicationofexternalloads.Mechanicsofmaterialsisastudyofthemagnitudeanddistributionoftheseinternalforces.Togetanunderstandingofthecompositionanddistributionoftheinternalforces,considerasimplebarsubjectedtoanaxialforcePateachend,asshowninFig.1-1.Assumethatthisbarismadeupofalargenumberoffibersinparallelalignment.Ifasectionispassedthroughthebar,afreebodysimilartothatshowninFig.1-2willbeobtained.Abasicprincipleofstaticsisthatifastructureisinequilibrium,anyportionofthestructureisinequilibrium.InthefreebodydiagramofFig.1-2theappliedexternalforceistotheright.Sincethebodyisinequilibrium,theremustalsobeforcesactingtowardtheleft.Thereforces,whichresisttheappliedload,aretransmittedbythefibersofthebar.Thisisanalogoustothecaseofthestrandsofarope.Whenaropecarriesaload,eachindividualstrandsupportsafractionofthatload.Inasimilar,thoughnotasevidentmanner,eachfiberofthebartransmitsaportionoftheload.Thesumoftheloadscarriedbyallfibersisequaltothetotalappliedload.Thetotalinternalforceintheresultantofalltheforcesinthefibers,andisequaltoPN.Itismotcommon,however,tospeakofthetotalforceinthebar,butratheroftheintensityofforceinthefibers.Thisintensityiscalledthestress,orunitstress.Unitstressisdefinedastheforceperunitofarea.Writteninalgebraicterms,σ=P/AWhereσ=unitstressinN/m2,P=appliedloadinN,A=areaoverwhichtheloadactsinm2.Inengineeringpractice,theterm“stress”issometimeslooselyusedtomeaneitherthetotalinternalforceorunitstress.Thecontextofthediscussionusuallyindicatesthepropermeaningoftheterm.Inthistext,however,theterm“stress”willalwaysmeanunitstress.IntheEnglishsystemthedistinctionbetweenapound-forceandapound-massisoftenambiguoussincetheword“pound”isusedforboth.SImoreclearlydistinguishesbetweenforceandmass.ThebaseSIunitsforsolidmechanicsarethemeter(length),kilogram(mass),andsecond(time).Thekilogramisameasureofmass,notforce.TheNewtonisthemeasureofforce.StressinSIunitsismeasuredinNewtonspersquaremeter(N/M2).ThisisdesignatedaPascal(Pa)andsuchsmallunitsthatmultiplesofthisunitaremoreconvenienttouse.Prefixes,symbolizingMultiplesof10×10×10,areusedasfollows:1kN=1×103N1MN=1×106N1kPa=1×103Pa=1×103N/M21Mpa=1×106Pa=1×106N/M2Theprocedureforcalculatingunitstressis,ofcourse,thesameregardlessofthesystemofmeasurement.Thedefinitionσ=P/Aisveryimportantandusefulinengineeringmechanics.Twothingsshouldbenotedinitsuse.Thefirstisthatitappliestomembersthatareloadedeitherintensionorincompression.Thesecond,andmostimportant,isthattheloadsmustbeappliedthroughthecentroidofthecrosssecond,andmostimportant,isthattheloadsmustbeappliedthroughthecentroidofthecrosssectionandcoincidentwiththeaxisofthemember.Ifanaxialloaddoesnotpassthroughthecentroidofthecrosssectionofthemember,σ=P/Adoesnotapplydirectly.ConsiderabarsubjectedtoanaxialtensileforceP,asshowninFig.1-3.Whentheforceisapplied,aunitstressisdevelopedinthebarthatisequaltoσ=P/A.Inaddition,thebarelongatesslightlyduetotheapplicationoftheload.Inmechanicsofmaterials,thesechangesinlength(alsoreferredtoasdeformations,elongations,orconstructions)areknownasstrains.Astrainis,therefore,thechangeinlengthofamember.Thedefinitionsoftotalstrainandunitstrainarenecessaryinthesolutionofmanyproblems.Thetotalstrainisthetotalchangeoflengthofthemember.Itisthedimensionδ(Greekletterdelta)showninFig.1-4.Amethodofcomputingtotalstrainwillbegiveninnextsection.Unitstrainisdefinedasthechangeinlengthperunitoflength,andissymbolizedbytheGreekletterε(epsilon).Expressedalgebraically,theunitstrainis,ε=δ/LWhereε=unitstrai