Speed Reducer / Gearbox

A geared mechanism mounted behind a motor that trades high speed and low torque for low speed and high torque.

A speed reducer (or gearbox) sits between a motor and its load, using gears to convert the motor's high rotational speed and low torque into low speed and high torque. A motor by itself spins fast but doesn't produce much force, while a robot joint needs to move slowly with a lot of force, so almost every joint actuator module is built as “motor + gearbox + encoder + driver.” A larger gear ratio (input speed divided by output speed) gives more output torque, but backdrivability gets worse, and the effects of backlash and friction become more noticeable. Robots commonly use three kinds: harmonic drives (compact, nearly zero backlash, mostly used in arms and wrists), RV reducers (rigid and impact-resistant, mostly used in the large joints of industrial robot arms), and planetary gearboxes (low ratio, good backdrivability, mostly used in the quasi-direct-drive joints of quadrupeds and humanoids).

ExampleOn an industrial 6-axis arm, load-bearing joints like the base and upper arm commonly use an RV reducer, while lighter joints like the wrist commonly use a harmonic drive.

Also called

Gear Reducer, Precision Reducer

Related

Gear Ratio · Strain Wave Gear (Harmonic Drive) · Planetary Gearbox · Rotate Vector (RV) Reducer · Joint Actuator Module · Backlash

Sources

Wikipedia: Gear train
Wikipedia: Strain wave gearing