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.