What Is a Planetary Gearbox?

A planetary gearbox is a gear train built from a central sun gear (input), several planet gears, and an outer ring gear. The planets spin on their own axes while orbiting the sun — like planets around a star, hence the name. It converts a motor's high speed into low-speed, high-torque output.

Structural Advantages

• Load sharing: torque is split across multiple planet gears, giving high torque density and impact tolerance.
• Coaxial input/output: the compact cylindrical form factor suits integrated joint motors naturally.
• High efficiency: single-stage efficiency typically exceeds 95%, higher than harmonic drives.
• Hollow-friendly: a through-bore at the center enables hollow-shaft cable routing.

Planetary vs. Harmonic

Harmonic drives offer large ratios (50–160) and zero backlash but lower efficiency, limited stiffness, and higher cost. Planetary gearboxes have smaller ratios (roughly 3–10 per stage, ~20 with compounding) and slight backlash, but win on efficiency, shock tolerance, and cost — decisive advantages for high-dynamic humanoid leg joints.

The BXI 85/70/50-series joint motors all use a 19.5-ratio planetary design with a uniform 100 rpm rated output speed, so sizing reduces to picking the right torque tier.