holdtorque

Hold torque, sometimes written as holdtorque, is the torque a motor can apply to resist movement and hold a load in a fixed position when the rotor is at rest or moving very slowly, with power applied. It represents the static torque available to counteract external disturbances such as gravity, friction, or inertia. Hold torque is a common specification for motors used in position-control applications, including servo motors and stepper motors, and it is typically specified at the rated current or voltage.

Measurement and relation to other torques: Hold torque is measured with the rotor stationary and current applied, at the motor’s rated operating conditions. It is generally higher than the running or continuous torque available during motion, due to dynamic effects, friction, and motor losses when the rotor is moving. In stepper motors, hold torque is the torque produced when the rotor is stalled under full current, while running torque refers to torque delivered while the rotor turns at a given speed and current.

Factors and design considerations: Hold torque depends on electrical current, motor design (magnet strength, winding geometry), and thermal conditions. As temperature rises, resistance increases and magnet strength can degrade, reducing hold torque. Designers must ensure that hold torque exceeds the maximum expected external load by a safety margin; otherwise, the system may drift or lose position. Overuse or insufficient cooling can lead to torque degradation and positional errors.

Applications and methods: Hold torque is crucial in robotics, CNC machines, and automation where precise positioning must be maintained when not actively moving. If hold torque is insufficient, designers may employ mechanical brakes, locking devices, or gearing to increase holding capability, or use feedback control to compensate.

See also: stall torque, running torque, servo, stepper motor, brake mechanisms.