The bearings inside humanoid joints: how crossed rollers carry the robot

Three loads, one ring

A humanoid joint is a hostile place for a bearing. Speeds are low, typically 10 to 50 rpm at the output, but torque is high, the motion reverses constantly, and the load arrives from every direction at once. Bearing maker CCTY sorts what the joint must carry into three families: radial loads perpendicular to the axis, axial loads along it in both directions, and moment loads – the twisting that appears whenever the robot holds a mass out at arm’s length. An ordinary deep-groove ball bearing handles the first well and the third barely at all.

The crossed-roller bearing is the industry’s answer. Cylindrical rollers sit between a single inner and outer ring, each roller turned 90 degrees to its neighbour, so alternating rollers carry load in alternating directions. One compact ring takes radial load, axial load both ways and tilting moment simultaneously, with high rigidity, minimal clearance and low friction. That is why crossed rollers sit at the output of shoulder and hip actuators, typically at P4 precision, and why industry teardowns of Tesla’s Optimus describe its rotary joints – shoulders, wrists, hips, trunk – as angular-contact ball bearings paired with crossed roller bearings. Harmonic Drive’s CSF and CSG gear units go one step further and build the crossed-roller ring into the reducer itself, so the joint needs no separate output support.

A bearing map of the humanoid body

No single bearing type covers the whole robot. The shoulders and hips take the biggest moments, so they get crossed rollers. Elbows and wrists care more about grams and section height, so they run thin-section four-point contact ball bearings, whose split-groove geometry takes axial load in both directions in one narrow row. Linear actuators load their screws in pure thrust and lean on paired angular-contact bearings. And the hands are their own discipline: deep-groove micro-bearings with 2 to 4 mm bores, plus plain bushings where even a micro-bearing will not fit.

Customisation is the norm at the load-bearing joints. CCTY’s published shoulder-joint case used a crossed-roller ring with reworked internal geometry to raise moment capacity, and its standard menu of tricks reads like a weight-saving negotiation: thicken the bearing wall where stiffness is short, cut threads directly into the inner ring, integrate the mounting flange into the bearing so a separate housing part disappears. Schaeffler went the other way and designed a new bearing class outright – the XZU, a double-row angular-contact needle bearing developed for humanoid joints in lightweight and compact variants.

The flexible bearing is the clock inside the joint

One bearing in the humanoid never gets to run round. The wave generator of a harmonic reducer is an ellipse, and the flexible ball bearing pressed over it must take that shape, flexing through two full deformation cycles per input revolution while transmitting the load that strains the flexspline. Fatigue, not wear, sets its lifetime. Industry targets sit at 8 to 15 million cycles, raceway roundness is held to 0.1 µm, and when a harmonic joint dies early, this bearing is the usual suspect.

It is also the sharpest measure of how fast China’s bearing industry is closing on Japan’s. In 2023, domestic flexible bearings were reported below 3 million cycles – a different generation from what Harmonic Drive Systems and NSK could deliver. By 2026, C&U Group and Zhejiang XCC were reporting roughly 8 million, turning a generational gap into a multiples gap. The remaining distance to the 15-million ceiling is where the reducer makers’ loyalty will be decided.

Who makes the rings

The supplier map splits three ways. Japan holds the precision high ground: NSK, JTEKT and Nachi-Fujikoshi on bearings, Harmonic Drive Systems on gear units with the crossed-roller ring already inside. Europe answers through SKF and, above all, Schaeffler, which has moved past selling rings toward selling whole joints – bearings, screws, motors and now complete actuators – and in May 2026 landed a component deal with the UK startup Humanoid covering its first thousand-plus robots. China’s specialists attack segment by segment: C&U and XCC on flexible bearings, CW Group on micro deep-groove bearings for hands, WANDA on thin-section rings.

The bearing makers are not waiting to be commoditised. NSK stood up a dedicated Robotics Business Department in March 2026, signed an MOU with Delta Electronics that May to co-develop rotary and linear joint modules, and is reported to be targeting 2028 for its own humanoid actuator line. It is the roller-screw story repeating one component further down the stack: a part that was a line item becomes a module, and the module maker captures the margin.

Three things worth watching from here.

• Integration. Every merger of bearing and housing – Harmonic Drive’s integrated output ring, CCTY’s flanged custom rings, Schaeffler’s all-in-one actuator – deletes a part, a seal and an alignment step. Expect the standalone joint bearing to become rarer as modules win.
• The flexible-bearing race. If Chinese flexible bearings reach the 15-million-cycle ceiling, the last technical argument for imported harmonic reducers in mid-range humanoids goes with it.
• Hands. Micro-bearings with 2–4 mm bores and millinewton-metre starting torque are watchmaking economics at robot volumes. Whoever industrialises them cheaply owns a chokepoint nobody is pricing yet.

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