Planetary Roller Screws

What a planetary roller screw does

A planetary roller screw converts rotation into straight-line motion. Where a ball screw uses recirculating balls, a roller screw uses several threaded rollers that sit between a threaded shaft and a threaded nut and orbit the shaft like planets around a sun. According to an SKF Motion Technologies engineer writing in Machine Design, the shaft has at least three thread starts, and most designs use five or six starts with 9 to 13 rollers.

The rollers give more and larger contact points than balls, so a roller screw carries more load for its size. The same article puts the dynamic load capacity at about three times that of a typical ball screw, and because fatigue life scales with the cube of the load rating, estimates life at 25 to 30 times that of a comparable ball screw. Tolomatic, which builds roller screw actuators, makes the same point: more contact points and a larger contact radius mean less stress per contact. These are generic engineering figures; real ratings depend on size, lead and load.

The inverted design used in humanoid robots

In an inverted roller screw the long threaded part is the nut, and the rollers ride on a screw shaft that is threaded only where they sit. As Linear Motion Tips explains, this makes it relatively easy to attach magnets to the nut and use it as the rotor of an integrated motor, so motor and screw become one compact push-rod actuator. The cost is manufacturing difficulty: the thread has to be machined inside a long nut.

That is the layout Tesla described for its Optimus humanoid. At Tesla AI Day 2022, the company said the robot's 28 actuators had been reduced to six designs, three rotary and three linear, and that the linear actuator uses planetary rollers and an inverted planetary screw as its gear train. Tesla demonstrated one lifting a half-ton concert grand piano. Optimus hardware has been revised since, so treat this as the 2022 design.

Where China stands

The established makers are specialists in Europe and the US. Rollvis, in Geneva, has made roller screws since 1970, including inverted types; US actuator makers such as Tolomatic build roller screw actuators.

Chinese entrants are mostly auto-parts manufacturers moving into precision screws from adjacent machining work. Their own 2026 interim reports are the best guide to where they really are:

• Shanghai Beite Technology (北特科技, 603009.SH) signed an agreement in October 2024 to invest ¥1.85bn in a planetary roller screw R&D and production base in Kunshan. By mid-2026 the main plant structure was finished and its Shanghai production line had moved there; phase one of a second screw base, in Thailand, is scheduled for June 2027. The company states that its screw products have not yet had a noticeable effect on its results (2026 interim report, pp. 9, 13, 27).
• Zhejiang XCC Group (五洲新春, 603667.SH), a bearing maker, sells inverted planetary roller screws alongside ball screws, miniature screws and robot bearings. Several products are in customer validation or small-batch delivery, and its robot components business made about ¥20m of revenue in the first half of 2026 and is not yet profitable (2026 interim report, p. 12).
• Leaderdrive (绿的谐波, 688017.SH), China's leading harmonic reducer maker, also develops planetary roller screws and says they now rest on a complete in-house technology base (2026 interim report, p. 12).
• Wuxi Best Precision Machinery (贝斯特, 300580.SZ) produced its first in-house planetary roller screw samples in 2023 through its subsidiary Wuxi Yuhua Precision Machinery, and is co-developing domestically made equipment for the key processes (2026 interim report, p. 13).

XCC's report also gives a frank industry view: in the first half of 2026, humanoid robot makers were moving from prototypes to small-batch trial production, roller screws were still going through whole-robot validation, and large-scale commercialisation had not happened.

What to watch

• Revenue, not capacity. Announced plants are easy to find; the useful signal is when a supplier starts reporting roller screw revenue that moves its results. As of mid-2026, none of the companies above did.
• Process equipment. Grinding long internal threads to tight tolerances is the hard part of making inverted screws. Leaderdrive states that the high-precision thread grinding machines this requires depend heavily on imports, are subject to export controls, and come with high prices and long lead times that slow line construction (2026 interim report, p. 10). Best Precision is co-developing domestic equipment for its key processes. Tooling, not just demand, sets the pace of localisation.
• Which design wins. Our view: roller screws compete with ball screws for humanoid linear joints, and the balance will depend on cost at volume. A shift by major robot makers in either direction would change the size of this market quickly.