What Is a Hollow Shaft Motor?

A hollow shaft motor is a motor or actuator whose output shaft has a through-bore along its rotation axis. Unlike solid-shaft designs, cables, hydraulic lines, and sensor harnesses can pass straight through the joint's center of rotation instead of looping around the outside.

Why Robots Need Hollow Shafts

High-DOF robots — such as a 31-DOF humanoid — chain many joints in series. If every joint routes its cables externally:

• harnesses fatigue and fail where joints flex repeatedly;
• exposed cables limit range of motion and snag on the environment;
• wiring complexity and assembly time grow rapidly with joint count.

Routing through the center bore solves all three at once, which is why hollow-shaft designs are central to modern integrated joint motors.

Engineering Trade-offs

A larger bore passes more wiring but complicates the layout of the rotor, encoders, and gearbox. The BXI 85/70/50-series joint motors provide 6–10 mm hollow bores, using a planetary gearbox to balance torque density against routing space.