Planetary roller screw

What it means

A planetary roller screw is a mechanical transmission that converts rotation into linear motion. Threaded rollers engage the screw and nut, distributing load across several contacts. An electric actuator can use the screw assembly to turn motor rotation into extension or retraction. Tolomatic describes the matching precision-ground screw and roller threads and their load-carrying role. [1]

The screw is a transmission component, not the complete actuator. Motor, bearings, housing and feedback devices belong to the larger assembly. Tolomatic's comparison explains that either the screw or nut may rotate while the other element is constrained to produce translation. [2]

How it applies to humanoids

A linear actuator can move a rotating robot joint through the surrounding mechanism. Kepler describes K2 as combining roller-screw linear actuators with rotary actuators in a serial-parallel architecture. Its release identifies planetary roller screws as the rotary-to-linear component rather than describing every K2 joint as the same actuator. [3]

This example also explains a terminology trap: Kepler calls the architecture hybrid because it combines mechanical arrangements. That wording does not imply a mixture of electric and hydraulic energy sources. Read the manufacturer's description of the actual components before interpreting the adjective. [3]

Performance and terminology traps

A roller screw uses rollers, while a ball screw uses balls as its load-transfer elements. Tolomatic's comparison associates the roller design with more contact area and high-force applications. That component comparison is not a guarantee that an arbitrary humanoid will lift more weight than another design. [1, 2]

Screw performance depends on the selected design and operating cycle. Tolomatic discusses force, speed, duty cycle, service life and maintenance as selection factors. Roller-screw mechanisms still have lubrication requirements in demanding use; an all-electric actuator is not automatically maintenance-free. [2]

Actuator thrust and joint torque are different quantities. A screw's linear output force acts through the robot's geometry, while the site records joint torque in newton-metres. Do not enter a force rating into a torque field or infer payload from the transmission name alone. [2, 3]

How to read it on humanoidversus.com

Check actuation for the sourced energy category and the overview for disclosed transmission details. K2 is linked because its record cites the official release describing this architecture. The broad electric label does not preserve every mechanical distinction. [3]

There is no roller-screw ranking here. Compare payload_kg and max_joint_torque_nm only under their own stated conditions. A missing torque, thrust or screw specification should remain unknown, even when a manufacturer publishes a detailed explanation of its overall design.

Further reading

Tolomatic's product page explains the contact mechanism, and its white paper compares screw technologies and application requirements. Kepler's K2 release provides a humanoid-specific implementation description. [1–3]