Chapter 3—ENGINE MAINTENANCESHAFTS AND BEARINGSThe principal shafts (crankshafts and cam-shafts) and associated bearings (journal bearingsand antifriction bearings) of an internal combus-tion engine are all subject to several types of trou-ble. Some of the troubles may be common to allof these parts; others may be related to only onepart. Causes of troubles common to all parts aremetal fatigue, inadequate lubrication, and opera-tion of the engine at critical speeds.Metal fatigue in crankshafts, camshafts, andbearings may lead to shaft breakage or bearingfailure; however, you must keep in mind thatmetal fatigue is only one of several possible causeswhich may lead to such troubles.Fatigue failure of journal bearings in internalcombustion engines is usually caused by cyclicpeak loads. Such failures are accelerated byimproper or loose fit of the bearing shell in itshousing, and by the lack of adequate priming ofthe lubricating oil system before the engine isstarted.Severe overloading or overspeeding of anengine increases fatigue failure. Some indicationof the cause of the failure may be obtained bynoting which half of a bearing failed. Overloadingof the engine will cause failure of the lower halvesof main journal bearings, while overspeeding maycause either the upper or the lower halves to fail.Crankshaft or camshaft failure does notoccur too often. When it does occur, it may bedue to metal fatigue. Shaft fatigue failure maybe caused by improper manufacturing procedures,such as improper quenching or balancing, or bythe presence of torsional vibration. Shaft fatiguefailures generally develop over a long period oftime.The importance of lubrication cannot beoverstressed. Much that has been stated previouslyabout proper lubricants and adequate supply andpressure of lube oils is also applicable tocrankshafts, camshafts, and their associated bear-ings. Some of the troubles which may be causedby improper lubrication are damaged cams andcamshaft bearing failure, scored or out-of-roundcrankshaft journals, and journal bearing failure.Lubrication difficulties you should watch for arelow lube oil pressure, high temperatures, and lubeoil contamination by water, fuel, and foreignparticles.Operation of an engine at critical torsionalspeeds and in excess of the rated speed will leadto engine shaft and bearing difficulties. Eachmulticylinder engine has one or several criticalspeeds which must be avoided in order to preventpossible breakage of the crankshaft, camshaft,and gear train.A critical speed of the first order exists whenimpulses due to combustion occur at the same rateas the natural rate of torsional vibration of theshaft. If the crankshaft receives an impulse fromfiring at every other natural vibration of the shaft,a critical speed of the second order occurs. Opera-tion at these speeds for any length of time maycause the shaft to break. If critical speeds are notavoided, torsional vibrations may not only causeshaft breakage but may also cause severe damageto the entire gear train assembly.In some engines, critical speeds fall within thenormal operating range; the instruction manualfor the specific engine will warn against engineoperation for any length of time within the criticalspeed range. If the critical speed range falls withinthe normal operating range, it must be con-spicuously marked upon the engine tachometer,and every effort should be made to keep theengine from operating in the range. If this is notpossible, the critical speed should be passed overas fast as possible.Overspeeding of an engine must be avoided.If the rated speed is exceeded for any extendedperiod of time, the increase in inertia forces maycause excessive wear of the journal bearings andother engine parts, and in uneven wear of thejournals.CRANKSHAFTSScored crankshaft journals are caused not onlyby lubrication difficulties but also by journalbearing failure or improper and carelesshandling during overhaul.Journal bearing failures may cause not onlyscoring but also broken or bent crankshafts andout-of-round journals. Journal bearing failuresmay be caused by several different factors andmay lead to more than one trouble. The causesand the prevention of such failures are discussedin more detail later in this chapter.3-31
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