How Harmonic (Strain Wave) Gears Work

The three core parts — wave generator, flexspline and circular spline — and how elastic deformation produces precise, backlash-free motion.

The three core components

A harmonic reducer is built from just three parts. The circular spline is a rigid outer ring with internal teeth. Inside it sits the flexspline, a thin-walled, elastically deformable cup or sleeve with slightly fewer external teeth. At the center is the wave generator, an elliptical cam with a thin-race bearing that presses the flexspline outward into an ellipse.

Motion through elastic deformation

As the wave generator rotates, the long axis of the ellipse forces the flexspline teeth to mesh fully with the circular spline at two opposite points, while the short-axis teeth disengage. Because the flexspline has fewer teeth, each full rotation of the wave generator advances the flexspline by the tooth difference — this is the reduction. The teeth continuously cycle through engage-in, full mesh, engage-out and disengage.

Why it is so precise

Because many teeth mesh simultaneously at two symmetric positions, tooth-pitch errors average out, giving very high positional and rotational accuracy. With teeth in continuous contact, there is effectively no mechanical play — hence "zero backlash" — which is why harmonic drives dominate robot joints and precision positioning axes.