TM 5-3810-300-24&P-3
1.3 Crankshaft
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.
Fig. 5. Crankshaft Fatigue Cracks
The crankshaft is supported between each of the
cylinders by a main bearing and the load imposed by the
Magnetic Particle Method: The part is magnetized and
gas pressure on top of the piston is divided between the
then covered with a fine magnetic powder or solution.
adjacent bearings. An abnormal bending stress in the
Flaws, such as cracks, form a small local magnet which
crankshaft, particularly in the crank fillet, may be a result
causes the magnetic particles in the powder or solution
of misalignment of the main bearing bores, improperly
to gather there, effectively marking the crack. The
fitted bearings, bearing failures, a loose or broken
crankshaft must be de-magnetized after the test.
bearing cap, or unbalanced pulleys. Also, drive belts
which are too tight may impose a bending load upon the
Fluorescent Magnetic Particle Method: This method
crankshaft.
is similar to the magnetic particle method, but is more
sensitive since it employs magnetic particles which are
Failures resulting from bending start at the pin fillet and
fluorescent and glow under "black light". Very fine
progress throughout the crank cheek, sometimes
cracks that may be missed under the first method,
extending into the journal fillet. If main bearings are
especially on discolored or dark surfaces, will be
replaced due to one or more badly damaged bearings, a
disclosed under the "black light".
careful inspection must be made to determine if any
cracks have started in the crankshaft. These cracks are
Fluorescent Penetrant Method: This is a method
most likely to occur on either side of the damaged
which may be used on non-magnetic materials such as
bearing.
stainless steel, aluminum and plastics.
A highly
fluorescent liquid penetrant is applied to the part. Then
Torsional fatigue failures result from torsional vibration
the excess penetrant is wiped off and the part is dried. A
which takes place at high frequency.
developing powder is then applied which helps to draw
the penetrant out of the flaws by capillary action.
A combination of abnormal speed and load conditions
Inspection is carried out under "black light".
may cause the twisting forces to set up a vibration,
referred to as torsional vibration, which imposes high
A majority of indications revealed by the above
stresses at the locations shown in Fig. 4.
inspection methods are normal and harmless and only in
a small percentage of cases is reliability of the part
Torsional stresses may produce a fracture in either the
impaired when indications are found. Since inspection
connecting rod journal or the crank cheek. Connecting
reveals the harmless indications with the same intensity
rod journal failures are usually at the fillet at 45 to the
as the harmful ones, detection of the indications is but a
axis of the shaft.
first step in the procedure.
Interpretation of the
indications is the most important step.
A loose, damaged or defective vibration damper, a loose
flywheel or the introduction of improper or additional
All Detroit Diesel crankshafts are magnetic particle
inspected after manufacture to ensure against any shafts
failure. Also, overspeeding of the engine or resetting the
with harmful indications getting into the original
governor at a different speed than intended for the
equipment or factory parts stock.
engine application may be contributory factors.
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