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1、Lesson3DieselEngineConstruction（１）TheBedplateThebedplate(Fig.7a)isthefoundationonwhichthe2-strokeengineisbuilt.Itmustberigidenoughtosupporttheweightoftherestoftheengine,andmaintainthecrankshaft,whichsitsinthebearinghousingsinthetransversegirders,inalignment.A
2、tthesametimeitmustbeflexibleenoughtohogandsagwiththefoundationplatetowhichitisattachedandwhichformspartoftheship’sstructure.Ifthebedplatewastoorigid,thenasthehullflexed,theholdingdownbolts,whichsecuretheengineintotheshipwouldbelikelytobreak,andtherewouldbeada
3、ngerofthebedplatecracking.Basicallythebedplateconsistsoftwolongitudinalgirders,whichrunthelengthoftheengine.Connectingtheselongitudinalgirdersarethetransversegirders,whicharepositionedbetweeneachcrankshaftthrow,andeithersideofthethrustcollar.Builtintothetrans
4、versegirdersarethemainbearingpocketsforthecrankshafttorunin.(1)(a)(b)Fig.7TheA-FramesTheA-Frames(Fig.7b)carrythecrossheadguidesandsupporttheengineentablature(thecylinderblock).Onlargeengines,theA-framesareindividuallyerectedonthebedplatedirectlyabovethetransv
5、ersegirders.Whenboxedinwithplatingtheyformthecrankcase.Thetrendnowadaysistobuildtheframeboxasaseparatefabricatedconstructionandthen,afterstressrelievingandmachiningthematingsurfaces,tomountitonthebedplate.Thishastheadvantageofsavingweight.(2)Whentheframesarea
6、lignedonthebed-platetheyaresecuredtogetherbydrillingandreamingandusingfittedbolts.ThetierodsToholdthebed-plate,framesandentablaturefirmlytogetherincompression,andtotransmitthefiringforcesbacktothebedplate,longtiebolts(Fig.8a)arefittedthroughthesethreecomponen
7、tsandthentightenedhydraulically.Topreventexcessivebendingmomentsinthetransversegirders,thetieboltsarepositionedasclosetothecenterofthecrankshaftaspossible.Becausethetieboltsaresoclosetothecrankshaft,someenginesemployjackboltstoholdthecrankshaftmainbearingcapi
8、npositioninsteadofconventionalstudsandnuts.(3)25(a)(b)Fig.8ThecrankshaftThecrankshaftsonthelargemodern2-strokecrossheadenginescanweighover300tons.Theyaretoobigtomakeasasingleunitandsoarec
