DETROIT DIESEL 53
Fig. 7 - Dimensions of Crankshaft Journals - In-Line Engine
small areas, designated as critical areas, sustain most of the load (Fig. 5).
Bending fatigue failures result from bending of the crankshaft which takes place once per revolution.
Fig. 9 - Crankshaft Journal Fillets
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.
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. 5.
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 45° to 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 45°