Strain Wave Gear (Harmonic Drive)

A precision reducer that gets its ratio from a flexible gear ring bending elastically — compact with almost no backlash

A strain wave gear is built from three parts: a wave generator (an elliptical cam wrapped in a flexible bearing), a flexspline (a thin-walled, elastically flexible gear ring with external teeth), and a circular spline (a rigid ring gear with internal teeth). As the wave generator turns, it flexes the flexspline into an ellipse, so it only meshes with the circular spline at the two ends of the long axis; because the flexspline typically has two fewer teeth than the circular spline, one full turn of the wave generator advances the flexspline by only two teeth relative to the circular spline, giving a very high reduction ratio from a single stage — roughly 30:1 to 320:1. The principle was invented by the American engineer C. Walton Musser, who filed the patent in 1955 (it was granted in 1959); Japan's Harmonic Drive Systems is the leading manufacturer, alongside Chinese makers such as Leaderdrive (绿的谐波) and Laifu Harmonic. It's compact, lightweight, and has near-zero backlash, making it widely used in collaborative robot arms and the arm and wrist joints of humanoid robots; its downsides are that the flexspline can fatigue over time, and its stiffness and impact resistance are lower than an RV reducer's.

ExampleJoint modules on collaborative robot arms often use a hollow “frameless torque motor plus harmonic drive” design, with cables routed straight through the center.

Also called

Harmonic Drive, Strain Wave Gearing

Related

Speed Reducer / Gearbox · Wave Generator · Flexspline · Circular Spline · Backlash · Leaderdrive

Sources

Wikipedia: Strain wave gearing
Harmonic Drive: Strain Wave Gear Technology
US Patent 2,906,143: Strain Wave Gearing（C. W. Musser，1955 年申请） (Chinese)