What is a planetary roller screw, and why did Tesla bet on it?
A roller screw turns motor rotation into straight-line force through threaded rollers. It carries more load and lasts longer than a ball screw, at several times the price.
Short answer: a planetary roller screw is the transmission inside a linear actuator. Threaded rollers orbit the screw inside a nut, so turning the screw (or the nut) pushes a rod in a straight line. Humanoids use these actuators like muscles to swing a knee, ankle or elbow.
Roller screw versus ball screw
| Ball screw | Planetary roller screw | |
|---|---|---|
| Contact | Recirculating balls touching at points | Rollers touching along thread lines |
| Load in the same nut length | Lower | Higher |
| Life under the same load | Shorter | Several times longer |
| Duty cycle | Low duty, higher speed | Up to 100% duty at moderate speed |
| Efficiency (Tolomatic figures) | About 80% | About 75–80% |
| Indicative price | About 200–600 CNY | About 2,000 CNY |
Prices are supplier-reported and move quickly; treat them as orders of magnitude.
Why Tesla chose it
Tesla's 2022 Optimus design used 28 actuators, with linear units rated from 500 N to 8,000 N. Supplier reporting puts the current body at 14 rotary and 14 linear actuators, with the linear units built around planetary roller screws. Linear actuators suit joints such as knees and ankles, where large forces and impacts are common, and a roller screw spreads impact across many thread contacts.
The trade-offs
- Cost. The roller screw is about 64% of a linear actuator's cost in one supplier breakdown.
- Back-drivability. Fine-lead screws are hard to push backwards, so the actuator usually needs a separate force sensor to feel contact. A low-ratio rotary joint can sense contact from motor current.
- Torque across the range. A linear actuator's effective torque at the joint changes with angle and peaks mid-stroke.
- Manufacturing. Precision thread grinding makes roller screws hard to produce at volume.
Not every humanoid uses them: Unitree's (宇树) G1, for example, uses rotary actuators. See suppliers compared on our roller screws ranking.