Gear Ratio

How many turns a motor makes for one turn of the output shaft.

Gear ratio is the ratio of input speed to output speed through a reducer (a gear mechanism that converts a motor's high speed into lower speed with more torque). A 10:1 ratio means the motor turns 10 times for every 1 turn of the output, ideally multiplying torque about tenfold while cutting speed to a tenth. Robot motors spin fast but produce little torque on their own, so a reducer is needed to drive the joint. Gear ratio is a core design trade-off: a high ratio gives strong, precise joints but makes them “stiff” and hard to backdrive (push from the outside), because the motor rotor's inertia is amplified at the output by the square of the ratio (reflected inertia); a low ratio has the opposite effect. Industrial arms typically use harmonic or RV reducers with high ratios, while legged robots often use quasi-direct-drive joints with low gear ratios.

ExampleThe joint actuators on MIT's Mini Cheetah use only a 6:1 planetary reduction, trading torque for good backdrivability and impact resistance.

Also called

Transmission Ratio, Reduction Ratio

Related

Speed Reducer / Gearbox · Strain Wave Gear (Harmonic Drive) · Planetary Gearbox · Backdrivability · Reflected Inertia · Quasi-Direct Drive

Sources

Gear train - Wikipedia