Torquetoweight

Torquetoweight is a metric used in engineering to describe how much torque a mechanism can produce relative to its weight. It is commonly defined as TTW = τ / W, where τ is the output torque and W is the weight of the device or assembly that delivers the torque. In practice, TTW is used to compare actuators, motors, gearboxes, and robotic joints, particularly in mobile or weight-sensitive applications such as drones, portable tools, and exoskeleton components. A higher TTW value indicates more torque obtainable per unit of weight, which can translate to improved payload capacity or maneuverability.

Variants exist in how TTW is defined. Some publications use mass instead of weight, defining TTW = τ / m, which changes the units to N·m/kg. Others treat weight as a force (newtons) and acknowledge that τ/W yields a derived unit that is conceptually a length, reflecting a design leverage rather than a pure dimensionless efficiency. Because of these differences, practitioners often specify the exact definitions and units when reporting TTW and may relate TTW to related concepts such as specific torque.

Limitations of TTW include its failure to capture speed, efficiency, control performance, or duty cycle. It should be considered alongside other metrics such as power-to-weight ratio, peak torque, continuous torque, and overall system efficiency to provide a fuller picture of performance.

See also: torque, weight, specific torque, power-to-weight ratio, actuator efficiency.