Hardware

Planetary Gear

A planetary gear is a gear train in which several planet gears rotate around a central sun gear while meshing with an outer ring gear, with a carrier linking the planets. The arrangement delivers high torque density, coaxial input and output, and load sharing across multiple teeth. Single-stage reductions of roughly 3:1 to 10:1 with modest backlash make planetary stages common in robot joints and quasi-direct-drive actuators.

Why it matters for physical AI

Low-ratio planetary reductions enable the quasi-direct-drive actuation behind modern dynamic legged robots, preserving backdrivability and torque transparency that high-ratio harmonic drives sacrifice.