DETROIT DIESEL 53
Torsional stresses may produce a fracture in either the
in a small percentage of cases is reliability of the part
connecting rod journal or the crank cheek. Connecting
impaired when indications are found. Since inspection
reveals the harmless indications with the same intensity
as the harmful ones, detection of the indications is but a
first step in the procedure.
Interpretation of the
indications is the most important step.
All Detroit Diesel crankshafts are magnetic particle
inspected after manufacture to ensure against any
shafts with harmful indications getting into the original
equipment or factory parts stock.
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, however,
Fig. 6. Dimensions of Crankshaft Journals -
sustain most of the load (Fig.-4).
Bending fatigue failures result from bending of the
rod journal failures are usually at the fillet at 45 to the
crankshaft which takes place once per revolution.
axis of the shaft.
The crankshaft is supported between each of the
A loose, damaged or defective vibration damper, a
cylinders by a main bearing, and the load imposed by
loose flywheel or the introduction of improper or
the gas pressure on top of the piston is divided between
the adjacent bearings. An abnormal bending stress in
type of failure. Also, overspeeding of the engine, or
the crankshaft, particularly in the crank fillet, may be a
resetting the governor at a different speed than intended
result of misalignment of the main bearing bores,
for the engine may be contributory factors.
improperly fitted bearings, bearing failures, a loose or
broken bearing cap, or unbalanced pulleys. Also, drive
As previously mentioned, most of the indications found
belts which are too tight may impose a bending load
during inspection of the crankshaft are harmless. The
upon the crankshaft.
two types of indications to look for are circumferential
fillet cracks at the critical areas, and 45 cracks (45 with
Failures resulting from bending start at the pin fillet and
the axis of the shaft) starting from either the critical fillet
progress throughout the crank cheek, sometimes
locations or the connecting rod
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
Torsional fatigue failures result from torsional vibration
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
Fig. 7. Crankshaft Journal Fillets
stresses at the locations shown in Fig. 4.
1972 General Motors Corp.
SEC. 1.3 Page 5