Two precision parts decide the bill of materials, and both are shifting from decade-life components to scheduled wear items.
A humanoid is best read as a set of load paths and joint-level actuator stacks. Each joint combines a motor, a reducer, an encoder or two, bearings and, for linear joints, a screw. The parts that are hardest to machine to tolerance are where the money and the supply risk concentrate.
Strain-wave (harmonic) reducers, typically at 50:1 to 100:1 ratios, are often the single most expensive component in a joint, frequently costing more than the motor and sensors combined. Screws are roughly 20% of today's bill of materials. Ball screws reach 90%+ efficiency and three suppliers (NSK, THK, SKF) hold about 70% of that market, while Chinese rolled ball screws from suppliers such as Shanghai Beite are an order of magnitude cheaper. Planetary roller screws last 10–15 times longer than ball screws but are supply-constrained and expensive, which makes them today's chokepoint. Schaeffler has consolidated much of that supply (GSA, Ewellix, ROLLVIS, INA) and MOOG acquired Italy's VCS.
The structural change is in lifetime expectations. OEMs increasingly treat reducers as wear items replaced every 12–18 months instead of parts that last a decade. That erodes the tribology-data moat of the Japanese incumbents and favors fast-iterating suppliers, and it changes what a customer is actually buying.
Two architectures are competing at the joint. High-reduction stacks suit static, precise, heavy tasks. Quasi-direct-drive (QDD, usually below about 20:1) suits dynamic, contact-rich, human-adjacent work and is friendlier to learned control. Cross-roller bearings becoming commodity parts was a quiet unlock: one bearing now carries radial, axial and moment loads.
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