Hypoid Drive

a particular type of helical-gear drive that uses conical gears with axes that cross. In hypoid drives the axis of the small gear is offset relative to the axis of the large gear. With gear ratios of i = 1-2.5, the offset is E ≦ (0.33-0.23) D_k, where D_k is the diameter of the gear and E is the perpendicular distance between the lines of the two axes; when i > 2.5, the offset is E ≤ 0.20 D_k.

Gears of hypoid drives may have beveled or curvilinear teeth, with the angle between the two axes usually being 90°. The gear ratios usually do not exceed ten, though in some cases the ratios rise to 30 and higher. The load-carrying capacity of the hypoid drive is higher than that of other transmissions with crossing axes because of the greater linear contact and the larger number of tooth pairs in contact at one time. The good lapping of working tooth surfaces ensured by the hypoid drive produces smooth and quiet operation. At the same values of D_k and i, the hypoid drive gear is larger in size than the average bevel gear, permitting use of a shaft of larger diameter and consequently greater stiffness. Furthermore, larger and more durable bearings may be used.

A disadvantage of the hypoid drive is that it has a higher tendency to seize because of the sliding motion along the length of the tooth contact. This condition is accompanied by a reduction of the load-carrying capacity of the oil wedge. However, the seizing possibility is eliminated by the application of antiseizing oil (hypoid oil) and heat treatment of the gear teeth to give high surface hardness.

The hypoid drive is used in the power train of the driver wheels of motor vehicles, tractors, and diesel locomotives. It is also used in textile machines to transmit rotary motion from one shaft to dozens of spindles and in machine tools for high accuracy at high gear ratios; in precision machine tools it takes the place of worm gearing.