The Joints — Precision Gearing & Reducers
Three companies in Japan make every robot joint that matters. There are no second sources.
A robot arm needs a gear-reducer at every joint — a small, very precise mechanical device that takes a fast-spinning motor and turns it into the slow, powerful, jitter-free motion needed to put a screw in or pick up an egg without cracking it. Two flavors dominate: "harmonic drives" (compact, zero-backlash — used in robot wrists and elbows) and "cycloidal reducers" (heavier, more torque — used in shoulders and hips). Tesla's Optimus uses both. Figure's robot uses both. Every Boston Dynamics product uses both. Harmonic Drive Systems in Japan invented and still dominates the harmonic flavor; Nabtesco in Japan dominates the cycloidal flavor; Leaderdrive in China is the only credible non-Japanese alternative.
If you can't get a harmonic drive, your humanoid does not move. Tesla's Optimus production is rumored to be gated on harmonic drive supply already. Lead times stretched to 12+ months in 2025.
3 names on the watchlist
Inventor & ~70% global share of harmonic drives — the joint-of-record for every modern robot wrist
A Japanese mid-cap that invented the harmonic drive in the 1960s and still owns roughly 70% of the world market . Every Tesla Optimus prototype, every Figure 02, every Boston Dynamics Atlas, every Agility Digit uses harmonic drives — and most of them ship from this one factory in Shizuoka. Customer list is essentially a who's-who of humanoid + cobot makers globally.
~60% global share of cycloidal RV reducers — the high-torque joint at the shoulder and hip
Japanese industrial that makes "RV reducers" — cycloidal gearboxes that handle the heaviest joints in industrial robots and humanoid shoulders/hips . ABB, Fanuc, Yaskawa, and Kuka all buy from Nabtesco. Also makes aircraft components and railway brakes — robotics is the growth engine.
China's only credible harmonic-drive challenger — state-backed, fast-improving
The only Chinese harmonic-drive maker with credible scale — closing the precision gap with Harmonic Drive Systems each year. Won meaningful share at UBTech, Xiaomi humanoid, Unitree. Hard to own from US accounts (Shanghai STAR Market). Geopolitical risk in both directions: if China decouples, this stock benefits; if US restricts, sentiment hits.