Torque is a rotational force. It is what makes a robot joint turn. Apply force at a distance from the axis and you create torque. The unit is the newton-meter in metric, the foot-pound in imperial. A motor produces torque. A gearbox multiplies it. A load resists it.

Torque is to rotation what force is to linear motion. A robot arm needs torque to lift a payload. The further the payload is from the joint, the more torque is needed. That is why a robot arm can lift more close to its base than at full extension. The torque requirement grows with the length of the arm and the weight of the load.

Torque in robot joints

• Static torque: holds a position against gravity.
• Dynamic torque: accelerates and decelerates the load.
• Peak torque: maximum momentary output.
• Continuous torque: sustainable without overheating.
• Torque density: torque per unit of motor weight.

Motor torque is proportional to current. That makes torque control straightforward: measure the current, and you know the torque. A force sensor at the wrist measures the force, and the controller computes the joint torques needed. Direct-drive motors produce high torque at low speed without a gearbox, but they are heavy and expensive. Geared motors produce high torque from small motors, but gears introduce backlash and friction. Harmonic drives reduce backlash to near zero. Cycloidal drives handle high shock loads. The choice of motor and gearbox determines the robot's torque capability. Torque is the raw physical power of the robot. Without enough of it, the robot cannot do the work. With too much, it becomes dangerous and inefficient.