Torque

The rotational force (moment of force) that tends to cause rotation about an axis, measured in Newton-meters (Nm) or pound-feet (lb-ft). Torque is the rotational equivalent of linear force and is the parameter that determines the capacity of power transmission components: shafts (shear stress τ = 16T/πd³), couplings, gearboxes, and belt/chain drives. Relationships: T = P / ω, where P is power (watts) and ω is angular velocity (rad/s). Practical formulas: T(Nm) = 9,549 × P(kW) / n(RPM), or T(lb-ft) = 5,252 × P(HP) / n(RPM). In speed reduction: a 3:1 gear or belt ratio reduces output speed to 1/3 of input while (ideally) tripling output torque, minus efficiency losses. Starting torque is critical for belt drive design: motor starting torque can be 150-300% of rated torque (NEMA Design B), and the drive must handle these peaks without belt slip. Per ISO 1081 and ARPM for belt drive torque calculations. For bolt tightening: torque wrenches apply specified torque to achieve target bolt preload. For coupling selection: service torque = rated torque × service factor. Unit conversion: 1 Nm = 0.738 lb-ft, 1 lb-ft = 1.356 Nm.