A Glossary of Actuator Specs You'll See on a Datasheet

Decoding the numbers manufacturers put on an actuator spec sheet, in plain language.

The core terms

Continuous torque: the torque an actuator can sustain indefinitely without overheating. Peak (stall) torque: the maximum torque it can produce briefly, usually for a few seconds. Reduction ratio: how much the gearbox multiplies motor torque (and divides speed) — e.g. 100:1.

Backdrivability: how easily an external force can move the joint by pushing on its output, rather than the motor driving it. Reflected inertia: how much of the motor and gearbox's own rotational inertia the joint's output 'feels,' which affects how responsive and backdrivable it is.

More terms worth knowing

Torque density: torque output per unit mass or volume — the key figure for comparing actuators fairly across sizes. Backlash: the free play in a gear train before output actually responds to input motion. Duty cycle: what fraction of time an actuator can run at a given torque level before it needs to cool down.