What Is Torque Density?

Torque density is the torque an actuator can deliver per kilogram of its own mass, expressed in N·m/kg, quoted either at peak or at rated torque. It is an important robot joint motor metric because actuators mounted in a limb become payload for every joint upstream.

How to Compute and Compare It

Torque density = output torque ÷ total actuator mass (gearbox and encoders included). Always align the basis before comparing:

• Peak vs. rated: marketing figures are usually peak-basis, often 3× or more above the rated-basis number;
• Driver included or not: integrated actuator modules include the drive electronics — bare-motor figures are not comparable;
• Current integrated joint actuators typically reach 30–70 N·m/kg peak; top products exceed 100 N·m/kg.

For the BXI 85/70/50 series (peak basis): the BXI8515-19 delivers 150 N·m ÷ 1.4 kg ≈ 107 N·m/kg, the BXI7010-19 50 N·m ÷ 0.8 kg ≈ 63 N·m/kg, the BXI5018-19 ≈ 64 N·m/kg, and the BXI5014-19 50 N·m/kg.

Where High Torque Density Comes From

A large-diameter, high-pole-count outrunner torque motor provides the base torque; a planetary gearbox multiplies it at low added mass. The quasi-direct-drive (QDD) approach uses a low-to-medium reduction ratio to balance torque density against backdrivability, while harmonic drives can quote higher density figures at the cost of impact tolerance and backdrivability.