Linear Actuators vs. Rotary Actuators in Humanoid Design

Most robot joints rotate, but a surprising number of designs convert rotary motion into a straight-line push instead.

Most humanoid joints are rotary — a motor and gearbox turn a shaft directly, mimicking how a human joint hinges. A linear actuator instead converts rotary motor output into straight-line motion, usually via a ball screw or lead screw, and pushes or pulls a joint rather than spinning it directly.

Why some designs go linear

A linear actuator can be mounted away from the joint it drives, connected by a lever arm — useful for keeping mass closer to the body's center and reducing a limb's swing inertia. Some knee and hip designs use this to get high torque without putting a heavy gearbox right at the joint itself.

The tradeoffs

Ball screws add friction and wear points, and the lever-arm geometry means the actual torque delivered to the joint varies nonlinearly with joint angle, which complicates control. Most designers still default to rotary actuation unless there's a specific packaging or mass-distribution reason to go linear.