Harmonic drive

A harmonic drive is a compact zero-backlash gear transmission using a flexible cup-shaped spline deformed by an elliptical wave generator. Achieves 50:1 to 320:1 gear ratios in a lightweight form factor. Standard in humanoid joints where high torque and precision matter — shoulders, hips, knees.

Two suppliers — Harmonic Drive SE and Nabtesco — own essentially the entire global market. This is a meaningful cost driver in humanoid bills-of-materials; published humanoid cost-down stories often hinge on substituting cycloidal drives or planetary architectures. Tesla's Optimus team has reportedly been working on vertically integrating this — the only credible threat to the duopoly. Form factor and ratio range make full substitution non-trivial, which is why the cost reduction has been incremental rather than step-change so far.