Figure 19-21. Desired Tooth Contact Under Light Load.
Figure 19-20. Checking Backlash Between
Drive Gear and Pinion.
19-5. Gear Tooth Contact Pattern.
a. Apply a thin coat of hydrated ferric oxide, commonly
known as yellow oxide of iron, or equivalent, to both the
drive and coast side of the drive gear teeth. Rotate the
drive gear one complete revolution in both directions while
the load is being applied. Apply the load with a round bar
or screwdriver between the carrier casting and the differen-
tial case flange. This action will leave a distinct contact
pattern on both the drive and coast side of the drive gear
teeth. The gear tooth contact pattern will disclose whether
the correct rear pinion bearing mounting shim has been
Figure 19-22. Incorrect Tooth Contact Pattern (Increase
installed and the drive gear backlash set properly. Backlash
Shim Pack Thickness).
between the drive gear and pinion must be maintained
within the specified limits until the correct tooth contact
pattern is obtained.
b. Observe the contact pattern on the drive gear teeth
19-23, to determine if the pattern is properly located. With
the pinion depth of mesh and gear backlash set properly,
Notice that the correct contact pattern is well centered on
both drive and coast sides of the teeth. When tooth contact
patterns are obtained by hand, they are apt to be rather
small. Under the actual operating load, however, the
contact area increases.
c. If after observing the contact pattern you find it
Figure 19-23. Effect on Tooth Contact Pattern as Shim Pack
away from the centerline of the ring gear. The contact
Thickness is Increased.
pattern will appear high on the heel on the drive side and
high on the toe on the coast side. To correct this type of
tooth contact pattern, increase the thickness of the shim
pack located behind the rear pinion bearing cup (figure
19-23). This will cause
the high heel
contact on the drive
side to lower and move toward the toe; high toe contact
on the coast side will lower and move toward the heel.