What is a harmonic drive?

A cool kind of gear. It was invented in 1957 by C.W. Musser.

The advantages include: no backlash, compactness and light weight, high gear ratios, reconfigurable ratios within a standard housing, good resolution and excellent repeatability (linear representation) when repositioning inertial loads, high torque capability, and coaxial input and output shafts.High gear reduction ratios are possible in a small volume (a ratio from 30:1 up to 320:1 is possible in the same space in which planetary gears typically only produce a 10:1 ratio).

Disadvantages include a tendency for ‘wind-up’ (a torsional spring rate) and potential degradation over time from mechanical shocks and environment.

Cross-section of a strain wave gearing mechanism.A: circular spline (fixed)

B: flex spline (attached to output shaft, not shown)

C: wave generator (attached to input shaft, not shown)

For a strain wave gearing mechanism, the gearing reduction ratio can be calculated from the number of teeth on each gear:

For example, if there are 202 teeth on the circular spline and 200 on the flex spline, the reduction ratio is (200 − 202)/200 = −0.01

Thus the flex spline spins at 1/100 the speed of the wave generator plug and in the opposite direction. This allows different reduction ratios to be set without changing the mechanism’s shape, increasing its weight, or adding stages.

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