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ASSEMBLING BEVEL GEARS Tooth bearings of hypoid gearsHypoid gears have curved teeth and the statements made concerning spiral gears also apply to these gears. In the case of hypoid gears, a cross bearing indicates that the shafts are not spaced at the correct offset.
Tooth bearing of zerol gearsThe tooth bearing of zerol gears is comparable to that for straight bevel gears having localized tooth bearing. Tooth bearing of straight bevel gearsIn the case of straight bevel gears cut on some of the latest generators, the tooth bearing is localized, as shown in Figure 9. The ideal tooth bearing is slightly nearer the toe than the heel, and the bearing of the pinion teeth is somewhat higher than the bearing on the gear. Its length under light load should be about one-half the tooth length, except for instrument type gears where the bearing should extend three-quarters the length of the tooth. Thus, these gears have the same advantages of slight adjustability as spiral bevels. However, because of their overlapping tooth action, spiral bevels are smoother and quieter. When the straight bevel gear cutting machine does not possess a means of tooth bearing localization, the tooth bearing will be substantially full-length, as shown in Figure 11. The gears are cut so that the area of contact begins at the small end of the tooth and extend about three-quarters of the length of the tooth. Thus, when the bearing shifts towards the heel under the heavy load, it will extend the entire length of the tooth, but will not be concentrated on the heel of the tooth. Here again, the bearing should be somewhat higher on the pinion than on the gear teeth. The incorrect tooth bearings for straight bevel gears are shown on the next page. The corrections are the same as those given under section "Checking the Installation" and those given for spiral bevel gears on the "Tooth Bearing Chart". Moreover, the effect is the same whether or not the tooth bearing is localized.
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