Technical Reading.docx
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Technical Reading.docx
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TechnicalReading
TechnicalReading
课外阅读资料
Unit3Simplemachines
TaskReadthefollowingtextandtrytofindoutthe‘mechanicaladvantage’ofeachsimplemachineinfigure3-1andexplainhowyoucalculateit.Note:
youdon’thavetogiveouttheaccurateanswer,justevaluateitaccordingtotheproportionofthefigure.
Mechanicaladvantage
Accordingtoengineeringandscience,workistheenergyittakestomoveanobject.Associatedwiththisdefinitionisamathematicalconceptwhichwillbeusedhere:
Work=[ForceApplied]x[DistancetheObjectMoved]
Whatdrivespeopletoinventmachines?
Itispeoplealwayswanttofindwaystomakeworkeasier.Machinesallowustodomanythingsquickerorwithlesseffort.Theyalsoenableustodothingsthatweotherwisewouldnotbeabletodo.
Justasbricksareanessentialpartofabrickhouse,therearefundamentalpartsofmachinesaswell.Therearesixsimplemachinesthatcanbefoundinmanyeverydayitems:
∙InclinedPlane-Aninclinedplaneisarampthatreducestheforceneededtomoveanobject.Atthesametime,theobjectmusttravelalongerdistance.InclinedplaneswereusedbytheEgyptianstobuildthepyramids.
∙Screw-Ascrewisaninclinedplanethatiswrappedaroundacylinder.Examplesofscrewsinclude:
fastenersthatareusedtoattachwoodormetal;liftingscrewsthatareusedtoliftheavyobjectsanddigholes;andboltsthatareusedwithnutstokeepthingstogether.
∙Wedge-Thewedgeistwoinclinedplanesputtogether.Itcanbeusedtosplitthingsapart,suchasanax,ortoholdthings,suchasadoorstop.
∙Lever-Aleverconsistsofabarthatrotatesaroundafulcrum.Leverslessenworkbyapplyingforceoveralongerdistance.Examplesofleversincludedaseesawandthehumanarm.
∙Wheel-and-Axel-Awheel-and-axelisawheelattachedtoarodorstick.Itworkssimilarlytoalever,inthat,consideringmovementaboutthecircumference,thedistancethewheel-circlemovesismuchgreaterthenthedistancethesmalleraxel-circlemoves.
∙Pulley-Apulleyisawheelwithagrooveforarope.Whensomethingisattachedtotherope,itcanbemovedbypullingontheotherendthathasloopedaroundthepulley.
Keepinmindthattheamountofworkneededtomoveanobjectacertaindistanceisalwaysthesame.Basically,thatmeansthatitwillalwaystakethesameamountofworktomoveanobjectfrompointAtoPointBnomatterhowyougetitthere.Thesimplemachinesdonotchangethetotalamountofworkthatyouhavetodo,buttheychangehowitfeelstodothatwork.
Butexactlyhowsimplemachinesmakeworkmucheasier?
Theanswertothisquestionisknownasmechanicaladvantage,whichisdefinedtocharacterizeamachine'sabilitytolessentheburdenofwork.
Mechanicaladvantage=outputforce/inputforce=inputdistance/outputdistance
Figure3-5Themechanicaladvantageofaninclinedplane
Forexample,themechanicaladvantageoftheinclinedplaneis(RefertoFigure3-5):
Mechanicaladvantage=inputdistance/outputdistance=Slopelength/Height
Themechanicaladvantageofthescrewis(RefertoFigure3-6):
mechanicaladvantage=inputdistance/outputdistance
=π×Diameter/Pitch
Figure3-6Themechanicaladvantageofascrew.
Unit4MoreMechanisms
MechanicalToys
Therearemanytoysbasedonmechanicalmechanisms.Let'stakeacloselookofthesetoys.TheyalltakeadvantagesofCAMmechanisms.
Toy1
Asthemechanicalduckispulledforwardthewingsflapupwardsanddownwards.Thisisachievedthroughtheuseofaeccentriccamfixedtoeachaxle.Figure4-12showshowthemechanismworks.
Asthewheelsandaxlerotatetheeccentriccamsalsorotate.Astheeccentriccamsmoveupwardstheypushthewingsupwards.Astheeccentriccamsrotatedownwardsthewingsdropdown.Thefasterthetoyispulledforwardthefasterthewingsflap.
Figure4-12Atoyduckwithswingwings
Toy2
ThemechanicaltoyinFigure4-13isbasedonacammechanismtoo.Asthetoyispulledalongbyachild,theeccentriccamrotatesasitisfixedtotheaxleandthewheels.Theheadsandconnectingrods,whichactasfollowerhere,moveupanddownastheaxleandeccentriccamrevolve.
Figure4-13Atoyduckwithamovinghead.
Toy3
ThetoyinFigure4-14(a)reliesonan‘eccentric’cam,asimplefollowerandaslide.
CAMmechanismsworkwelliftheyaremadeaccurately.However,anyinaccuracyinmakingthedevicecanleadtothemechanism‘jamming’whentheprofilerotates.Also,inaccuratemakingcanleadtothemovementofthefollowerbeinglessthansmoothwhentheprofilerotates.
Figure4-14(b)showsometypicalCAMfaults.Thefollowerhasjammedastheprofilerotatesinaclockwisedirection..
Therearetwopossiblereasonsthatleadtothisproblem:
1.Theslidesaretoofarapart,allowingthefollowertojamastheprofilerotates.
2.Theshapeofthefollowermeansthatthemovementofthemechanismislikelytojamoratbest,moveroughlyratherthansmoothly.
Thetwofaultscanbecorrectedquiteeasilybyaltering:
1.Theshapeofthefollower.A‘flat’followerisused.Thismeansthatthemovementoftheprofileandfollowerismorelikelytobesmoothandefficient.
2.Theslideshavebeenmovedclosetogethersothatthefollowerisforcedtomovevertically,without‘jamming’.RefertoFigure4-14(c).
(a)(b)(c)
Figure4-14(a)Atoykittenbehindtrees.
(b)Acamsystemwithtypicalfaults.
(c)Thefaultshavebeencorrected.
Unit6Engineeringmaterials
Thefunnystoriesoftechnician
——Propertiesofmaterials
Itisimportanttounderstandkeypropertieswhenstudyingmaterialsandespeciallywhenselectingmaterialsforaprojectordesign.Themostimportantpropertiesareoutlinedbelow.
STRENGTH
Theabilityofamaterialtostanduptoforcesbeingappliedwithoutbending,breaking,shatteringordeforminginanyway.
Figure6-3Thetechnologytechniciandemonstratesthe‘strength’ofamaterialbyperformingahandstandonastrongpieceoftimber(wood).Itdoesnotbendevenunderhisweight.Hehaseatenpiesanddrunkalargeamountofbeerfortwentyyearsandyetthestrongmaterialdoesnotbend,flexordeform(changeshape)inanyway.
ELASTICITY
Thepropertyofamaterialtoreturntoaninitialformorstatefollowingdeformation.
Figure6-4Thetechnologytechniciandemonstratesthe‘elasticity’ofamaterialbyspringingupanddownonapieceofsteelrod.Donottrythisathomeasanaccidentmayresult.
PLASTICITY
Theabilityofasolidbodytopermanentlychangeshape(deform)inresponsetomechanicalloadsorforces.
Figure6-5TheFigurerepresentsashearstresscomponentwithrespecttoashearstraincomponent,underincreasingstrainloading.Plasticregionisthepermanentlychangestagebeforefracture.
TENSILESTRENGTH
Theabilityofamaterialtostretchwithoutbreakingorsnapping.
Figure6-6Thetechniciandemonstrates‘tensilestrength’bystretchingapieceofsteeluntilitsnaps.Hethinksheisincrediblystrong.
DUCTILITY
Theabilityofamaterialtochangeshape(deform)usuallybystretchingalongitslength.
Figure6-7Ourtechnicianstretchestheleadabovehishead.Asitstretchesitdeforms(changesshape).Hethinksheisastrongman,littledoesherealizethatleadisaverysoftmetalandstretchesveryeasily.
MALLEABILITY
Theabilityofamaterialtobereshapedinalldirectionswithoutcracking
Figure6-8Thetechnologytechniciandemonstratesthe‘malleability’ofamaterialbyheatingapieceofmildsteeluntilitisredhot.Hethenbeatsitwithalargeforginghammertoreshapeit.Becauseofthehightemperatureitreacheswhileheatingthesteelbecomesmalleable,itcanbereshapedpermanently.
TOUGHNESS
Acharacteristicofamaterialthatdoesnotbreakorshatterwhenreceivingabloworunderasuddenshock.
Figure6-9Thetechnologytechniciandemonstratesthe‘toughness’ofamaterialbyhittingapieceormaterialtoseeifitwillbreakorshatter.
HehasbeenknowntotestauthenticChineseMingDynastypotterywiththesametechnique.ThisiswhyheisoftenarrestedinMuseumsandhasbeenbannedfromthelocalAntiquedealers.
HARDNESS
Theabilityofamaterialtoresistscratching,wearandtearandindentation.
Figure6-10Thetechnologytechnicianslidesalongthefloortoseeifitwillscratch.Itwillbeconsideredtohardwearingifitresistsscratching.
CONDUCTIVITY
Theabilityofamaterialtoconductelectricity.
Figure6-11Thetechnologytechniciandemonstrates‘conductivity’bypressinglivewiresagainsteithersideofhishead(PLEASENOTE-THISWILLKILLNORMALPEOPLE).Thetechniciansurvivesbecausehisskullisempty.
Unit7ForcesinEngineering
TaskStudythediagramandtextandthendrawadiagramtorepresenteachoftheseforces.
Differenttypesofforces
Therearedifferenttypesofforcesthatactindifferentwaysonstructuressuchasbridges,chairs,buildingsandanystructure.Themainexamplesofforcesareshownbelow.
Figure7-8STATICLOAD(standingstill)
AStaticLoad:
Agoodexampleofthisisapersonseenontheleft.Heisholdingastackofbooksonhishandsbutheisnotmoving.TheforcedownwardsisSTATIC.
ADynamicLoad:
Agoodexampleofadynamicloadisthepersonontheright.Heiscarryingaweightofbooksbutwalking,sometimesslowlyandsometimesfast.TheforceismovingorDYNAMIC.
Figure7-9DYNAMICLOAD(moving)
Figure7-10COMPRESSION
Compression:
Theweightlifterfindsthathisbodyiscompressedbytheweightsheisholdingabovehishead.
ShearForce:
Agoodexampleofshearforceisseenwithasimplescissors.The
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