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
Fig. 4. Critical Crankshaft Loading Zones
small areas, designated as critical areas, sustain most of
Inspection for Cracks
Carefully check the crankshaft for cracks which start at
Bending fatigue failures result from bending of the
an oil hole and follow the journal surface at an angle of
crankshaft which takes place once per revolution.
45 to the axis. Any crankshaft with such cracks must
Several methods of determining the
The crankshaft is supported between each of the
presence of minute cracks not visible to the eye are
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
Magnetic Particle Method: The part is magnetized and
crankshaft, particularly in the crank fillet, may be a
then covered with a fine magnetic powder or solution.
result of misalignment of the main bearing bores,
Flaws, such as cracks, form a small local magnet which
improperly fitted bearings, bearing failures, a loose or
causes the magnetic particles in the powder or solution
broken bearing cap, or unbalanced pulleys. Also, drive
to gather there, effectively marking the crack. The
belts which are too tight may impose a bending load
crankshaft must be de-magnetized after the test.
upon the crankshaft.
Fluorescent Magnetic Particle Method: This method
Failure resulting from bending start at the pin fillet and
is similar to the magnetic particle method, but is more
progress throughout the crank cheek, sometimes
sensitive since it employees magnetic particles which
are fluorescent and glow under "black light, ". Very fine
cracks that may be missed under the first method,
especially on discolored or dark surfaces, will be
disclosed under the "black light".
Fluorescent Penetrant Method: This is a method
which may be used on both non-magnetic and magnetic
materials. A highly fluorescent liquid penetrant is
applied to the part. Then the excess penetrant is
removed from the surface and the part is dried. A
developing powder is then applied which helps to draw
the penetrant out of the flaws by capillary action.
Inspection is carried out under "black light ".
A majority of indications revealed by the above
inspection methods are normal and harmless and only in
impaired when indications are found. Since inspection
Fig. 5. Crankshaft Fatigue Cracks.
reveals the harmless indications with the same intensity