Chapter  3—ENGINE  MAINTENANCE SHAFTS  AND  BEARINGS The  principal  shafts  (crankshafts  and  cam- shafts)  and  associated  bearings  (journal  bearings and  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 all of these parts; others may be related to only one part. Causes of troubles common to all parts are metal  fatigue,  inadequate  lubrication,  and  opera- tion of the engine at critical speeds. Metal fatigue in crankshafts, camshafts, and bearings may lead to shaft breakage or bearing failure;  however,  you  must  keep  in  mind  that metal fatigue is only one of several possible causes which may lead to such troubles. Fatigue  failure  of  journal  bearings  in  internal combustion engines is usually caused by cyclic peak  loads.  Such  failures  are  accelerated  by improper or loose fit of the bearing shell in its housing, and by the lack of adequate priming of the lubricating oil system before the engine is started. Severe  overloading  or  overspeeding  of  an engine  increases  fatigue  failure.  Some  indication of the cause of the failure may be obtained by noting which half of a bearing failed. Overloading of the engine will cause failure of the lower halves of main journal bearings, while overspeeding may cause either the upper or the lower halves to fail. Crankshaft   or   camshaft   failure   does   not occur too often. When it does occur, it may be due to metal fatigue. Shaft fatigue failure may be  caused  by  improper  manufacturing  procedures, such as improper quenching or balancing, or by the  presence  of  torsional  vibration.  Shaft  fatigue failures generally develop over a long period of time. The  importance  of  lubrication  cannot  be overstressed. Much that has been stated previously about  proper  lubricants  and  adequate  supply  and pressure  of  lube  oils  is  also  applicable  to crankshafts, camshafts, and their associated bear- ings. Some of the troubles which may be caused by improper lubrication are damaged cams and camshaft  bearing  failure,  scored  or  out-of-round crankshaft journals, and journal bearing failure. Lubrication  difficulties  you  should  watch  for  are low lube oil pressure, high temperatures, and lube oil  contamination  by  water,  fuel,  and  foreign particles. Operation of an engine at critical torsional speeds and in excess of the rated speed will lead to  engine  shaft  and  bearing  difficulties.  Each multicylinder engine has one or several critical speeds which must be avoided in order to prevent possible breakage of the crankshaft, camshaft, and gear train. A critical speed of the first order exists when impulses due to combustion occur at the same rate as the natural rate of torsional vibration of the shaft. If the crankshaft receives an impulse from firing 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 may cause the shaft to break. If critical speeds are not avoided,  torsional  vibrations  may  not  only  cause shaft breakage but may also cause severe damage to the entire gear train assembly. In some engines, critical speeds fall within the normal operating range; the instruction manual for the specific engine will warn against engine operation for any length of time within the critical speed range. If the critical speed range falls within the  normal  operating  range,  it  must  be  con- spicuously  marked  upon  the  engine  tachometer, and  every  effort  should  be  made  to  keep  the engine from operating in the range. If this is not possible, the critical speed should be passed over as fast as possible. Overspeeding  of  an  engine  must  be  avoided. If the rated speed is exceeded for any extended period of time, the increase in inertia forces may cause excessive wear of the journal bearings and other  engine  parts,  and  in  uneven  wear  of  the journals. CRANKSHAFTS Scored crankshaft journals are caused not only by  lubrication  difficulties  but  also  by  journal bearing   failure   or   improper   and   careless handling during overhaul. Journal bearing failures may cause not only scoring but also broken or bent crankshafts and out-of-round  journals.  Journal  bearing  failures may be caused by several different factors and may lead to more than one trouble. The causes and the prevention of such failures are discussed in more detail later in this chapter. 3-31