How Gear Ratios Shape a Robot Joint's Torque and Speed

The single number that decides whether a joint is strong and slow, or fast and weak.

Gearing trades speed for torque in a fixed ratio: a 100:1 reduction multiplies the motor's output torque by roughly 100 (minus mechanical losses) while dividing its speed by the same factor. There's no way around that tradeoff — it's a direct consequence of conservation of energy, not an engineering limitation someone hasn't solved yet.

Picking the right ratio

A hip or shoulder joint that needs to hold a heavy load steady for long periods usually gets a high ratio, prioritizing torque and efficiency at low speed. A joint that needs to move quickly and absorb impacts — often an ankle or a wrist — gets a lower ratio, prioritizing speed and backdrivability over raw holding torque.

Efficiency losses

Every stage of gearing loses some energy to friction, typically single-digit percentages per stage but compounding across multi-stage gearboxes. That's part of why torque density and drivetrain efficiency, not just peak torque, are the numbers serious engineers scrutinize on a datasheet.