Crankshaft failures are rare and when one cracks or breaks completely, it is very important to make a thorough
inspection for contributory factors. Unless abnormal conditions are discovered and corrected, there will be a
repetition of the failure.
There are two types of loads imposed on a crankshaft in service: a bending force and a twisting force. The
design of the shaft is such that these forces produce practically no stress over most of the surface. Certain
small areas, designated as critical areas, sustain most of the load (Fig. 4).
Bending fatigue failures result from bending of the crankshaft which takes place once per revolution.
The crankshaft is supported between each of the cylinders by a main bearing and the load imposed by the gas
pressure on top of the piston is divided between the adjacent bearings. An abnormal bending stress in the
crankshaft, particularly in the crank fillet, may be a result of misalignment of the main bearing bores, improperly
fitted bearings, bearing failures, a loose or broken bearing cap, or unbalanced pulleys. Also, drive belts which
are too tight may impose a bending load upon the crankshaft.
Failures resulting from bending start at the pin fillet and progress throughout the crank cheek, sometimes
extending into the journal fillet. If main bearings are replaced due to one or more badly damaged bearings, a
careful inspection must be made to determine if any cracks have started in the crankshaft. These cracks are
most likely to occur on either side of the damaged bearing.
Torsional fatigue failures result from torsional vibration which takes place
at high frequency.
Figure 4. Critical Crankshaft Loading Zones
A combination of abnormal speed and load conditions may cause the twisting forces to set up a vibration,
referred to as torsional vibration, which imposes high stresses at the locations shown in Fig. 4.
Torsional stresses may produce a fracture in either the connecting rod journal or the crank cheek. Connecting
rod journal failures are usually at the fillet at 45to the axis of the shaft.
A loose, damaged, or defective vibration damper, a loose flywheel, or the introduction of improper or additional
pulleys or couplings are usual causes of this type of failure. Also, overspeeding of the engine or resetting the
governor at a different speed than intended for the engine application may be contributory factors.
As previously mentioned, most of the indications found during inspection of the crankshaft are harmless. The
two types of indications to look for are circumferential fillet cracks at the critical areas and 45cracks (45with