Torque

Torque is the vector product of position vector and force:

Its magnitude is T=rF\sin\theta, where \theta is the angle between lever arm and force. Torque has units of newton metre, but it is not the same physical quantity as energy; torque is a rotational moment, while energy is force integrated through displacement.

Engineering use

In rotational dynamics, net torque produces angular acceleration:

where I is moment of inertia and \alpha is angular acceleration. In power transmission:

where P is power and \omega is angular speed. This relation explains why gear reduction can increase output torque while reducing speed, neglecting losses.

Torque also creates shear stress in shafts and preload in threaded fasteners. For fasteners, applied torque is only an indirect control of clamp load because friction under the head and in the threads absorbs much of the input energy. For shafts, torque must be evaluated with stress concentration, fatigue, keyways, bearing loads, and dynamic transients.

Specification

Torque values need a clear reference location and operating condition. Motor datasheets may list stall torque, rated continuous torque, peak torque, holding torque, or torque after gearbox losses. Fastener specifications may define tightening torque with lubrication assumptions, thread condition, tightening sequence, and allowable scatter. Without this context, the same numerical torque can imply very different stresses, heat generation, fatigue duty, or preload reliability.

Common mistakes

A common mistake is treating torque wrench setting as a precise preload measurement. Another is comparing torque values without speed, duty cycle, gear ratio, measurement location, and sign convention. A strong torque review states axis, direction, lever arm, operating speed, transient or steady condition, measurement method, losses, and whether the value is input, output, peak, continuous, stall, or tightening torque.