Harmonic Drives Explained: How They Work and Where They're Used

A strain-wave gearing trick from the 1950s is still doing most of the heavy lifting in humanoid robot joints.

A harmonic drive (also called a strain-wave gear) gets a very high reduction ratio — often around 100:1 — out of three concentric parts instead of a stack of gear pairs: a rigid circular spline, a flexible cup-shaped spline that deforms slightly as it turns, and an elliptical wave generator that drives that deformation.

Why robots use them

That gearing turns a small, fast, relatively weak motor into a slow, high-torque output with almost zero backlash — exactly what a joint needs to hold a heavy payload steady without slop. They're also compact and coaxial, which matters when you're trying to fit a joint inside a human-sized limb.

The tradeoff

High reduction ratios make a joint hard to backdrive — push it from the outside and you're fighting the gearing, which can damage the flexspline over time. That's a real problem when the outside force is the ground hitting a foot during a fall, which is part of why some robots use lower-ratio actuators at the ankles and knees instead.