Check Gear Tooth Contact
The final check of differential assembly adjustment is
checking the gear tooth contact by the paint impression
Apply oiled red lead lightly to the hypoid gear teeth.
When the pinion is rotated, the red lead is squeezed
away by the contact of the teeth, leaving bare areas the
exact size, shape and location of the contacts (Fig. 36).
Sharper impressions may be obtained by applying a
small amount of resistance to the gear with a flat steel
bar and using a wrench to rotate the pinion. When
making adjustments, check the drive side of the gear
teeth. Coast side should be correct when drive side is
correct. Generally, coating approximately twelve teeth is
sufficient for checking purposes.
With adjustments properly made, a correct tooth
contact similar to that shown in Fig. 36 will be secured.
The area of contact starts near the toe of the gear and
extends about 62 1/2 percent of the tooth length. This
adjustment results in a quiet running gear and pinion set
which, because the load is distributed over the teeth
within the proper area, will deliver all the long service
built into it.
When checking paint impressions on gear teeth of
an axle under heavy load, the impressions usually
spread out somewhat longer than the patterns obtained
from a bench test. This can be considered as normal.
Ring gears when mounted should show a bearing toward
the toe or small end of the tooth, but never at the heel or
large end. The reason being that it is practically
impossible to make gears and gear mounting so rigid
that no deflection will occur when full torque is applied.
This deflection causes the bearing to approach the heel
of the tooth. And when gears are adjusted so that the
bearing is toward the heel of the tooth, it results in a
concentration of load on the top corner of the heel and
breakage will follow.
Fig. 36. Tooth Contact Impressions
Bearing Too High
Bearing Too Low
Too Much Heel Bearing
Too Much Toe Bearing
Proper Tooth Contact
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