What Is a Frameless Torque Motor?

A frameless torque motor is delivered as a stator and rotor, without its own housing, output shaft, or bearings. Both parts integrate directly into the joint structure, reducing redundant packaging and shortening the drivetrain. It is common in integrated joint actuators and humanoid robot joints.

Why Joint Modules Use Frameless Motors

• Structural fusion: the stator is heat-fitted into the joint housing and the rotor couples straight to the planetary gearbox sun gear — the shortest possible drivetrain, shrinking joint volume by roughly 30%;
• Large-diameter pancake form: torque motors use high pole counts at a large air-gap radius for low-speed, high-torque output — exactly a joint's duty profile, and the source of high torque density;
• Coaxial hollow bore: the annular stator/rotor geometry naturally supports hollow-shaft cable routing through the joint's center.

Inrunner vs. Outrunner

Frameless torque motors come as inrunners (rotor inside — better cooling, lower inertia) and outrunners (rotor outside — more torque per volume, flatter). Dynamic legged robots often pick outrunners for torque density. The BXI 85/70/50-series joint actuators build on a frameless torque motor with a 19.5:1 planetary reduction in a quasi-direct-drive architecture, completed by dual absolute encoders.