T-222
the central part of the tooth width.
Even when a gear is ideally manufactured, it may reveal poor surface contact due to lack of
precision in housing or improper mounting position, or both. Usual major faults are:
1. Shafts are not intersecting, but are skew (offset error).
2. Shaft angle error of gear box.
3. Mounting distance error.
Errors 1 and 2 can be corrected only by reprocessing the housing/mounting. Error 3
can be corrected by adjusting the gears in an axial direction. All three errors may be the
c a u s e
o f
i m p r o p e r
backlash.
19.2.1 The Offset Error
of Shaft
Alignment
If a gear box has an
offset error, then it will
produce
crossed
end
contact, as shown in Figure
19-2. This error often
appears as if error is in the
gear tooth orientation.
19.2.2 The Shaft Angle
Error of Gear
Box
A s
Figure
19-3
shows, the contact trace
will move toward the toe
end if the shaft angle error
is positive; the contact
trace will move toward the
heel end if the shaft angle
error is negative.
19.2.3 Mounting Distance
Error
When the mounting
distance of the pinion is a
positive error, the contact of
the pinion will move towards
the tooth root, while the
contact of the mating gear
will move toward the top of
the tooth. This is the same situation as if the pressure angle of the pinion is smaller than that of
the gear. On the other hand, if the mounting distance of the pinion has a negative error, the
contact of the pinion will move toward the top and that of the gear will move toward the root.
This is similar to the pressure angle of the pinion being larger than that of the gear. These
errors may be diminished by axial adjustment with a backing shim. The various contact patterns
due to mounting distance errors are shown in Figure 19-4.
Fig. 19-2 Poor Contact Due to Offset Error of Shafts
Error
Error
Fig. 19-3 Poor Contact Due to Shaft Angle Error
(+) Shaft Angle Error
(–) Shaft
Angle Error
