What is a harmonic reducer, and why are robot joints full of them?
Three parts, no backlash and a very high reduction in one compact stage. The trade-off is a thin flexing part that dislikes shock.
Short answer: a harmonic reducer (also called strain wave gearing, or by the trade name Harmonic Drive) turns a fast, weak motor into a slow, strong joint using a flexible toothed cup. It is common in robot arms and humanoid upper bodies because it gives a large speed reduction with no backlash in a small, light package.
Three parts
- Wave generator: an elliptical hub with a thin ball bearing, driven by the motor. This is the input.
- Flexspline: a thin, flexible steel cup with teeth on the outside. This is the output.
- Circular spline: a rigid ring with teeth on the inside, fixed to the housing. It has two more teeth than the flexspline.
The elliptical wave generator pushes the flexspline into mesh with the circular spline at two opposite points. Because the tooth counts differ by two, each full turn of the input moves the flexspline by only two teeth. That small step per turn is where the large reduction comes from.
Why robots use them
- No backlash. Teeth stay engaged along the long axis of the ellipse, so there is no play when the joint changes direction. That matters for precise, repeatable positioning.
- High reduction in one stage. A single planetary gear stage reaches around 10:1; a strain wave gear gets far higher reduction in one stage. Fewer stages mean a shorter, lighter joint.
- Size does not grow with ratio. The gear's size and weight stay the same across reduction ratios, which simplifies joint design.
The trade-off
The flexspline bends twice per input revolution for the life of the gear. It handles steady loads well but is more vulnerable to impact than a rigid gear. That is one reason some humanoid makers use other transmissions, such as planetary gearboxes or roller-screw linear actuators, in the legs, where landing shocks are large.
Who makes them
Harmonic Drive Systems (Japan) is the best-known maker; Leaderdrive (绿的谐波) is a leading Chinese supplier. See how suppliers compare on our harmonic reducers ranking.