What is a harmonic drive (strain wave gear)?
A harmonic drive is a compact, zero-backlash gear that gives very high reduction ratios in one stage, widely used in robot arm and humanoid joints.
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Definition
A harmonic drive, or strain wave gear, is a speed reducer with three parts: a wave generator (elliptical bearing), a flexspline (thin flexible toothed cup) and a circular spline (rigid ring with a couple more teeth).
How it works
As the wave generator spins, it deforms the flexspline so its teeth engage the circular spline at two points. Because of the tooth-count difference, the flexspline slowly rotates backwards, producing a high gear ratio with near-zero backlash.
Why it matters
Precision reducers are one of the costliest parts of an actuator, and a humanoid needs dozens of them. Supply of harmonic and planetary reducers and screws is a key cost driver as humanoid robots move toward mass production; designs built for sim-to-real sometimes favor more backdrivable alternatives.
Frequently asked questions
How does a harmonic drive work?
An elliptical wave generator flexes a thin toothed flexspline inside a rigid circular spline that has slightly more teeth; each rotation shifts the mesh by a few teeth, giving ratios like 50:1 to 160:1.
Why do robots use harmonic drives?
They are small, light, precise and have almost no backlash, ideal for robot joints that must position accurately.
What are the downsides?
They are expensive, have limited torsional stiffness and shock tolerance, and are hard to backdrive — which is why some designs, like Boston Dynamics' 2026 Atlas hand, use direct drive instead.
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