Continuous, peak and stall torque.

Three labels answer three different questions

Continuous torque asks what output can be sustained while temperatures remain within defined limits. A credible value needs voltage, speed or speed range, cooling, ambient, mounting, winding or housing limit and a steady-state criterion. A single headline number without those conditions is still useful as a manufacturer claim, but it is not a portable design guarantee.

Peak torque describes an intermittent limit. It is only actionable when duration, starting temperature, repetition, current limit and speed are known. Stall torque is the zero-speed endpoint of a motor or servo curve. At stall there is no mechanical output power, current can be high and heat accumulates quickly.

The first RCI sample shows why one torque column fails

HEBI publishes separate continuous and peak torque for the R8-9+, together with a warning in its hardware documentation that torque and maximum speed are not available simultaneously. CubeMars publishes rated and peak torque for the AK80-9 V3.0, while the visible table does not define the complete thermal test behind the rated value. ROBOTIS publishes a stall torque at a specified voltage and current for the XH540-W270-R and warns users through its performance guidance not to treat the stall point as continuous output.

All three values use N·m, yet their labels and conditions differ. Unit normalization alone would erase the most important information.

Torque must be read with speed, current and heat

Mechanical power is torque multiplied by angular speed. A joint cannot simultaneously deliver the maximum torque and maximum no-load speed printed at opposite ends of a speed–torque curve. Electrical current creates copper loss, while speed-dependent losses, drive switching, gearing and bearings add heat elsewhere in the module.

For legged robots, the engineer also needs the event sequence: stance load, impact, acceleration, regenerative braking and recovery. Two actuators with the same peak torque can behave very differently if one tolerates the event for a fraction of a second and another for several seconds with a defined cool-down.

Questions to ask before using a torque value

A procurement or design record should answer the following before a torque comparison is promoted to a selection decision.

• Is the value motor-side, gearbox-output or complete-joint torque?
• Is it continuous, rated, intermittent, peak, stall or a controller command limit?
• At what voltage, current, speed, ambient and starting temperature?
• How long can it be held, how often can it repeat and what cooling is assumed?
• Does the limit come from motor temperature, drive current, transmission strength, bearings or firmware?
• Is the value independently measured or only manufacturer reported?