Linear vs rotary actuators: why Tesla bet on the roller screw

Two ways to bend a knee

A rotary actuator is the familiar robot joint: a frameless torque motor drives a reducer (harmonic, planetary or cycloidal) whose output flange is bolted to the next limb segment. Torque is delivered directly on the joint axis, so the joint can rotate through a wide range and the torque curve is the same at every angle. Rotary joints dominate shoulders, hips and waists, and they dominate the Chinese supply chain, which has become very good at making them cheaply.

A linear actuator turns the same kind of motor, but instead of a reducer it spins a screw. A nut rides along the screw and pushes a rod, and the rod is pinned to the limb some distance from the joint pivot. The limb swings because the rod changes length, exactly the way a hydraulic cylinder swings an excavator arm, or a muscle-and-tendon pair swings your own shin. The screw does the gear reduction: one motor turn advances the nut by one lead, typically a few millimetres, so a small motor produces thousands of newtons of push.

Who uses what

The split in the industry is not academic. At AI Day 2022 Tesla showed an Optimus body with 28 structural actuators built from just six designs: three rotary units rated 20, 110 and 180 N·m, and three linear units rated 500, 3,900 and 8,000 N. One of the linear units was shown lifting a roughly 500 kg concert grand piano. Supplier reporting since then puts the count at 14 rotary and 14 linear actuators in the body, with the linear ones built around planetary roller screws that were initially sourced from the Swiss maker GSA (owner of Rollvis).

Apptronik went further. Apollo’s legs and arms use in-house linear actuators that the company says have been through more than 13 generations, chosen specifically to cut material cost and simplify field support. Unitree, at the other end of the price scale, builds its own rotary permanent-magnet actuators for the G1, with the knee rated at 90 N·m on the base model and 120 N·m on the EDU version. Schaeffler’s production-ready humanoid actuator, premiered at CES 2026, is also rotary: a two-stage planetary gearbox with motor, encoder and controller in one housing, covering 60 to 250 N·m. Boston Dynamics’ electric Atlas uses custom actuators that Hyundai Mobis will supply once the robot enters mass production.

Inside a planetary roller screw

A ball screw carries load on a train of recirculating balls, each touching the screw and nut at a single point. A planetary roller screw replaces the balls with a ring of threaded rollers held in a cage, orbiting the screw like planets around a sun. Each roller touches the screw along a line of thread contact rather than a point, so the same nut length carries far more load. Tolomatic, which sells both, quotes ball-screw actuators at around 80 percent efficiency and roller-screw actuators at 75 to 80 percent, with the roller screw tolerating 100 percent duty cycle and lasting several times longer under the same load because of the additional contact points.

Tesla’s twist is the inverted layout: the nut is the rotor, and the screw shaft slides in and out of the housing as the output. That keeps the whole unit short and lets the motor sit around the screw, but it makes the nut an awkward, high-precision part to machine. Industry commentary is consistent that this inverted configuration is one of the hardest parts of the design to manufacture at scale.

Why the screw dominates the bill of materials

A rotary joint’s cost spreads across three expensive parts. According to a cost breakdown published by the gear maker HONPINE, the harmonic reducer is about 36 percent of a rotary actuator, the torque sensor 30 percent and the frameless motor 13.5 percent. In a linear actuator, one component swallows the budget: the planetary roller screw is around 64 percent of the unit cost, with the force sensor at 16 percent and the motor at just 7 percent.

Absolute prices follow the same pattern. Chinese industry reporting in January 2026 put the roller screws in Optimus at roughly 2,000 yuan each, against 200 to 600 yuan for a ball screw, and bank estimates cited by Fast Company range from $1,350 to $2,700 per screw. Multiply by 14 screws in the body and the roller screws alone are a five-figure line item at current prices, which is why so much of the supply-chain effort in China (Hengli Hydraulic, Beite Technology, Wuzhou Xinchun) is aimed at driving that number down. Beite alone announced a 1.85 billion yuan factory dedicated to planetary roller screws in October 2024.

The trade-offs that actually decide it

Engineers usually frame the choice around four properties. The list below is where linear and rotary designs pull in different directions.

• Range of motion. A rotary joint can sweep 180 degrees or more with constant torque. A linear actuator’s usable arc is limited by its stroke and its lever geometry, and its effective torque changes with angle, peaking mid-stroke. That is why linear units end up on knees, elbows and ankles, which are high-load hinges with modest travel, while shoulders, hips and waists stay rotary.
• Shock and stiffness. A roller screw spreads impact across dozens of thread contacts and holds position with near-zero backlash; a harmonic drive’s thin flexspline is the part most likely to be damaged by a fall or a hard footstrike. Linear designs win on stiffness but, as HONPINE notes, show weaker dynamic response.
• Back-drivability. Low-ratio planetary rotary actuators, the quasi-direct-drive approach, can be pushed by the world and let the controller feel contact through motor current. Screws with fine leads are hard to back-drive, which is why force sensors sit in series with the rod on linear units. Schaeffler explicitly markets its rotary actuator’s low back-drivability as a safety feature that prevents unintended reverse rotation under load.
• Thermal headroom. Roller-screw units are rated for continuous, 100 percent duty at moderate speed, which suits joints that hold a standing load. Rotary joints carrying the same load depend on the motor’s continuous thermal rating, which is why continuous-duty and thermal-stability claims feature so heavily in current actuator launches such as Schaeffler’s.

The supply-chain question behind the engineering one

The roller screw is where East and West currently diverge. Swiss GSA and Rollvis, together with Ewellix (now part of Schaeffler), hold most of the global market for precision roller screws, and Chinese reporting concedes that the top four overseas makers still control more than 70 percent of China’s own demand, with domestic producers at roughly 8 percent share. Interesting Engineering’s November 2025 analysis reached the same conclusion: China leads on low-cost rotary actuators but trails on C3-grade planetary roller screws, the grade that limits lead error to about 3 microns per 300 mm.

Schaeffler’s position is worth watching for exactly this reason. It bought Ewellix in 2022, offers both ball and planetary roller screw assemblies for humanoids, and has now added an integrated rotary actuator with a two-stage planetary gearbox. A single Tier-1 supplier that can deliver either architecture, from bearings to controller, is new for this industry, and it is what the volume builders will want when they move past the current phase of in-house everything. Hyundai Mobis signing on as Atlas’s actuator supplier in January 2026 is the same story from the automotive side.

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