DETROIT DIESEL 53
Inspection for Cracks
the maximum runout on the intermediate journals
must not exceed .002 " total indicator reading.
Carefully check the crankshaft for cracks which start
at an oil hole and follow the journal surface at an
angle of 45o to the axis. Any crankshaft with such
cracks must be rejected. Several methods of determin-
ing the presence of minute cracks not visible to the eye
are outlined below.
Magnetic Particle Method: The part is magnetized
and then covered with a fine magnetic powder or
solution. Flaws, such as cracks, form a small local
magnet which causes the magnetic particles in the
powder or solution to gather there, effectively marking
the crack. The crankshaft must be de-magnetized after
Fluorescent Magnetic Particle Method: This method is
similar to the magnetic particle method, but is more
sensitive since it employs magnetic particles which are
fluorescent and glow under "black light". Very fine
cracks that may be missed under the first method,
Measure all of the main and connecting rod bearing
especially on discolored or dark surfaces, will be
journals (Fig. 7).
Measure the journals at
disclosed under the "black light".
several places on the circumference so that taper, out-
of-round and bearing clearances can be determined. If
Fluorescent Penetrant Method: This is a method which
the crankshaft is worn so that the maximum
connecting rod journal-to-bearing shell clearance (with
stainless steel, aluminum and plastics. A highly
new shells) exceeds .0045 " (In-line engine)
fluorescent liquid penetrant is applied to the part.
or the main bearing journal-to-
Then the excess penetrant is wiped off and the part is
bearing shell clearance (with new shells) exceeds
dried. A developing powder is then applied which
.0040 " (In-line
type engines), the crankshaft
helps to draw the penetrant out of the flaws by
must be reground. Measurements of the crankshaft
capillary action. Inspection is carried out under "black
should be accurate to the nearest .0002 ". Also, if the
journal taper or out-of-round is greater than .003 ", the
A majority of indications revealed by the above
crankshaft must be reground.
inspection methods are normal and harmless and only
in a small percentage of cases is reliability of the part
Also measure the crankshaft thrust surfaces (Fig. 10).
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
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
Fig. 6 Crankshaft Fatigue Cracks
practically no stress over most of the surface. Certain