Page 284 - Gear Technology Solutions

19.6 Maximal Ratios and Self-Locking

High reduction hypoids, such as HRH and SRH have a tapered worm as pinion
(Figure 9). The common ratios of these transmissions are between 7 and 50,
however, larger ratios are possible. The lowest number of pinion teeth is one,
which will result in a very low pinion lead angle.

Figure 9: Super reduction hypoid (SRH) used as a speed increaser

Gleason developed the calculation of a back-driving coefficient CBD = Tbr/Tdr
especially for SRH gearsets. This calculation is based on a friction factor of m =
0.08 and computes the torque which tries to back-drive the pinion versus the
breaking torque. If CBD > 1, then back-driving is not possible. The back-driving
ability of SRH gearsets is important for all applications which require a smooth
coasting if the driving torque from the prime mover is reduced or turned off.

The lead angle of a 5-tooth and a 2-tooth pinion are compared in Figure 10. A
worm with a lead angle less than 10° is considered self-locking [8]. Self-locking
means the gear cannot drive the pinion. This is a general statement and must
be verified, considering the type of lubrication, the lead curvature of the gear
teeth, and if the back-driving occurs on the drive side (if the gear in Figure 9
rotates counterclockwise) or if the back driving occurs on the coast side.

It is advisable not to use high reduction bevel worm gear drives as speed in-
creasers. The efficiency of such a transmission when used as speed increaser
is considerably lower than in the case when it is used as a speed reducer.

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