Harmonic Drive

Harmonic Drive, mechanical speed-changing device, invented in the 1950s, that reduces the gear ratio of a rotary machine to increase torque. It operates on a principle different from that of conventional speed changers. The device consists of a thin ring that deflects elastically as it rolls inside a slightly larger rigid circular ring.

There are three elements in a basic Harmonic Drive: a circular spline, a flexspline, and a wave generator. 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 elliptical in shape and acts as 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 inside 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 on the flexspline. Speed ratios as high as 320 to 1 can be produced in single-reduction Harmonic Drives, which 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.

A unique and useful characteristic of the Harmonic Drive is its ability to transmit motion through sealed walls. The flexspline teeth can be placed near the centre of a long hermetically sealed flexible cylindrical tube. The wave generator can be inside the tube, and, by its rotation, it can deflect the flexspline and produce a slow rotation of the encircling circular spline. A rotary-to-linear version of the Harmonic Drive uses a screw and moves the control rod in a nuclear reactor head without mechanical contact through a sealed tube. Harmonic Drive systems are useful in increasing the output torque in gears used in milling, in manufacturing, and in machines that use robotic arms. They have been employed in a variety of applications, ranging from such low-cost consumer items as vending machines to sophisticated systems for military and aerospace use.