C r a n k s h a f t 1.3
DETROIT DIESEL 53
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.
Fig. 7 Dimensions of Crankshaft Journals .
In-Line Engine
Failures resulting from bending start at the pin fillet
and progress throughout the crank cheek, sometimes
small areas, designated as critical areas, sustain most
extending into the journal fillet. If main bearings are
of the load (Fig. 5).
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
Bending fatigue failures result from bending of the
most likely to occur on either side of the damaged
crankshaft which takes place once per revolution.
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 45o to the
axis of the shaft.
A loose, damaged or defective vibration damper, a
loose flywheel or the introduction of improper or
type of failure. Also, overspeeding of the engine or
resetting the governor at a different speed than
intended for the engine application may be contribu-
tory 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 45o
SEC. 1.3 Page 5
November, 1973
