Five parts, one motion

A planetary roller screw has five main parts: a screw, a nut, a set of threaded rollers between them, two ring gears and two carriers. The screw and nut carry multi-start triangular threads. The rollers carry a single-start thread with a rounded profile, so they touch the screw and the nut along many small contact zones instead of on a few balls. Spur teeth at each end of every roller mesh with ring gears fixed in the nut. Those teeth keep the rollers parallel to the screw and stop them drifting along it, while the carriers hold the rollers at an even spacing.

Turn the screw and the rollers roll around it like planets around a sun, spinning on their own axes as they orbit. In the standard design the nut moves forward by one lead for each turn of the screw. Because the load passes through many rolling contacts spread over all the rollers, a roller screw carries more load in a given envelope than a ball screw. Tolomatic describes its roller screws’ dynamic load rating as “very high” against “medium” for ball screws, and its life as many times longer.

Standard, inverted and recirculating

Catalogues sort roller screws into three families. In the standard planetary type the screw is driven and the nut travels. In the inverted type the roles swap: the nut is driven and the screw becomes the output, advancing one lead per nut turn, and the rollers only need to be as long as the threaded section of the screw they engage. That layout lets a hollow motor rotor spin the nut directly while the screw slides out as a push rod, which is why humanoid linear actuators use it. 36Kr reports that Optimus carries about 14 “reverse” (inverted) planetary roller screws in its hips, knees and ankles.

The recirculating type uses circumferentially grooved rollers instead of threaded ones. After each turn around the nut, cams and an axial groove return each roller to its start position. That allows very fine leads, down to 1 mm in the Ewellix range, for high resolution and high stiffness at low speed, but it gives up speed.

Lead: the screw’s gear ratio

The lead is the distance the output travels per turn. It plays the role a gear ratio plays in a rotary joint. Ignoring friction, the motor torque needed for a given thrust is the thrust times the lead divided by 2π, and the efficiency sits under that line. Ewellix says the overall mechanical efficiency of its roller screws is usually above 80 percent, and recommends calculating with a friction coefficient about 30 percent above the reference value, which costs roughly 5 points against the theoretical figure.

A worked example shows the scale. Tesla rated its largest Optimus linear actuator at 8,000 N at AI Day 2022. Through a 5 mm lead at 80 percent efficiency, 8,000 N needs about 8.0 N·m at the nut. Through a 2 mm lead it needs about 3.2 N·m. Those are the torques of a small frameless motor, not of a geared joint, which is the whole point of the screw. The price is speed: at a 2 mm lead the nut has to turn 3,000 rpm to move the rod 100 mm/s. Most roller screws can also be back-driven, so Ewellix tells designers to add a brake or gear stage where back-driving must be prevented.

Load ratings and the cube law

Every roller screw catalogue quotes a dynamic load rating, Ca. Ewellix defines it as the constant, centred axial load under which the calculated life under ISO 3408-5 reaches one million revolutions. Life then scales with the cube of Ca divided by the real load, so a screw run at half its rating lasts eight times longer, and one run at a quarter lasts 64 times longer. The smallest standard planetary screws in the Ewellix range already sit far above a humanoid’s peak forces.

Applied to the smallest screw in the chart, the cube law turns ratings into distance. A 15 × 5 mm planetary screw with Ca = 26 kN travels 5 mm per revolution, so its rated life at a given load can be expressed in kilometres of rod travel. The figures below are the basic calculation only. Real duty cycles mix loads, and the life of the mixed cycle follows a cube-weighted mean load, so occasional impact loads from walking and falling count far more than their share of time.

Precision classes and the grinding problem

Lead accuracy is graded under ISO 3408-3. Ewellix ships G5 as standard, with G3 and G1 on request; the allowed travel variation over 300 mm is 23, 12 and 6 µm respectively. Chinese suppliers usually quote the equivalent C-classes, and Chinese and Western commentators alike describe high-precision grades as the area where domestic makers still trail.

The hard part is the inverted nut. Its multi-start thread is internal, long and hardened. EMAG, which sells the machining line, describes typical robotics nuts of about 150 mm length and 20 mm bore, length-to-diameter ratios of 5:1 to 8:1 for inverse designs, hardening to 60 HRC, Ra 0.4 surface finish and single-digit-micrometre tolerances, with pitch errors under 2 µm between thread turns. Its process chain runs from soft turning through heat treatment and hard turning to finish grinding of the internal thread. Long tool overhangs in a narrow bore cause vibration and chatter, which forces lower speeds and drives up cost.

The price is collapsing in China

For years the market belonged to Europe and the US: GSA and Rollvis in Switzerland, Ewellix (now Schaeffler) and SKF. Humanoid demand has pulled in Chinese automotive and bearing suppliers. China Daily reported in January 2026 that Ningbo’s Shuanglin Group had started mass production of a planetary roller screw priced at 499 yuan, which industry insiders compared with more than 5,000 yuan for international equivalents, and that more than 20 joints in XPeng’s IRON humanoid use Shuanglin parts. 36Kr names Wuzhou Xinchun as the main supplier of inverted screws, priced at about half of Rollvis, and reports that Xinjian Transmission is building a line for one million humanoid roller screws a year.

What to ask a roller screw supplier

• Ca and C0a for the exact size and lead. Then compute life against your own cube-weighted load, including impacts, not against the average load.
• Precision class and how it is measured. Ask for the ISO 3408-3 class and the travel variation over 300 mm, and whether it is measured on every part or sampled.
• Preload and backlash. Backlash-free versions (BR, IBR, BV in the Ewellix codes) cost more and add friction.
• Back-drivability. Whether the screw back-drives at your lead decides if the joint needs a brake to hold a pose with the power off.
• Thread process and capacity. Ground or rolled internal threads, the grinding machines behind them, and committed annual volume.

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