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.