When both gear and pinion are fastened in place, the tooth bearing should be checked, as already mentioned. The bearing on the teeth of correctly mounted straight bevel gears should be very similar to that shown in figure 9 and Figure 11. Spiral bevel, hypoid and Zerol gears should have tooth bearings similar to those shown in figures 3 and 7.
Inaccuracies in mounting or assembling will change the location of the tooth bearing on properly cut bevel gears. The deviation from the proper tooth bearing indicates the nature of the error. The effect is similar for both straight and spiral bevel gears and the following remarks apply to both.
When the bearing is too high on the pinion and too low on the gear, the pinion is to near the center of the gear. This is corrected by increasing the pinion mounting distance. In the opposite case, with the bearing low on the pinion and high on the gear, the pinion mounting distance should be decreased.
If the shafts are not accurately located in the same plane, a cross bearing will result – the contact will be concentrated at one end of the tooth on one side and the opposite end on the other side. The shafts must be lined up to correct this.
If the shaft angle is not correct, the tooth bearing will be concentrated at the same end of the teeth on both sides. If the bearing area is concentrated at the toe or inner end, the shaft angle is too great. If the bearing is at the heel or large end, the angle is too small, It will be necessary to correct the shaft angle to obtain the proper tooth bearing.
Figure 6. Meaning of terms
Spiral bevel gears are produced to run with localised tooth bearing, as shown in Figure 3. Since the area of contact does not cover the whole tooth, a slight tolerance in positioning the gears in assembly and some displacement under operating loads is possible, without concentration of the load on the ends of the teeth.
The ideal tooth bearing is slightly high on the pinion and low on the gear (Figure 7). Its length under light load is usually about one-half of the tooth length (except in the case of instrument type gears, where a three-quarter length bearing is preferred), so as to provide sufficient justability consistent with smoothness. Moreover, since the tooth bearing under heavy load will generally shift toward the heel (outer end of tooth), the gears are cut so as to place the bearing, with the gears mounted in correct position, slightly nearer the toe (inner end of the tooth). The resulting bearing is called a central toe bearing.
Figure 7. Ideal tooth bearings for spiral bevel gears
The tooth bearing chart on the next page indicates the corrections to be made, if the proper bearing is not obtained the first time.
It should be noted that average conditions are assumed and special applications may require special consideration.