A check of contact is, typically, only done to verify the accuracy of the installation, rather than the individual gears. The usual method is to blue dye the gear teeth and operate for a short time. This reveals the contact area for inspection and evaluation.
19.2 Surface Contact Of A Bevel Gear
It is important to check the surface contact of a bevel gear both during manufacturing and again in final assembly. The method is to apply a colored dye and observe the contact area after running. Usually some load is applied, either the actual or applied braking, to realize a realistic contact condition. Ideal contact favors the toe end under no or light load, as shown in Figure 19-1; and, as load is increased to full load, contact shifts to 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 cause of improper 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.
As 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 Df 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.
Mounting distance error will cause a change of backlash;
positive error will increase backlash; and negative, decrease. Since the
mounting distance error of the pinion affects the surface contact greatly,
it is customary to adjust the gear rather than the pinion in its axial
direction.
