Harmonic Drive is the brand name of strain wave gear trademarked by the Harmonic Drive company, and invented in 1957 by C.W. Musser.

Harmonic Drive AG strain wave gear set consisting of wave generator bearing (top left), flexspline cup (top right) and circular spline ring (bottom).

It is very commonly implemented in robotics today[1] and used in aerospace as well,[2] for gear reduction but may also be used to increase rotational speed, or for differential gearing.

The advantages include: no backlash, high compactness and light weight, high gear ratios, reconfigurable ratios within a standard housing, good resolution and excellent repeatability (linear representation) when repositioning inertial loads,[7] high torque capability, and coaxial input and output shafts.[6] 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).

Harmonic Drive, mechanical speed-changing device, invented in the 1950s, that operates on a different principle from, and has capabilities beyond the scope of, conventional speed changers. It consists of a thin ring that deflects elastically as it rolls on the inside of a slightly larger rigid circular ring.

The three elements of the basic Harmonic Drive are shown in the Figure. The circular spline has internal teeth that mesh with external teeth on the flexspline. The flexspline has fewer teeth and consequently a smaller effective diameter than the circular spline. The wave generator is a link with two rollers that rotates within the flexspline, causing it to mesh with the circular spline progressively at diametrically opposite points. If the wave generator (the input) rotates clockwise while the circular spline is fixed, the flexspline (the output) will rotate or roll around on the inside of the circular spline at a much slower rate in a counterclockwise direction.

The ratio of the input speed to the output speed depends on the difference in the number of teeth in the circular spline and in the flexspline. Speed ratios as high as 320 to 1 can be produced in single-reduction Harmonic Drives that are lighter, smaller, and more efficient than conventional high-ratio drives. Compound drives can produce ratios as high as 1,000,000 to 1. Either the circular spline, the flexspline, or the wave generator may be fixed while the other two elements serve as input and output.

LCD-14-C-I  LCD-14-XXX-C-I-ST High Precision Harmonic Drive Speed Reducer for human robots