Guide

Torque and tightening values

Torque is force times lever arm and therefore always contains a length unit. N·m and lb·ft sit numerically close together but differ by about 36 % — a difference that matters with bolted joints.

What torque is physically

Torque arises when a force acts at some distance from an axis of rotation. It is the product of force and lever arm, which is why every torque unit consists of a force unit and a length unit. The SI unit is the newton metre, written N·m.

N·m and lb·ft

One pound-force is 4.4482216152605 newtons, one foot 0.3048 metres. That gives 1 lb·ft = 1.3558179483314003 N·m. Conversely 100 N·m is about 73.76 lb·ft. The difference of about 36 % is large enough to overload a joint or leave it too loose.

Tightening values belong to the joint

A tightening torque is not a property of a tool but of a specific bolted joint. It depends on the thread, material, strength class, lubrication and substrate. Figures from tables or forums therefore cannot be transferred without checking. Unit arithmetic only converts between scales; it does not recommend a tightening value.

Torque is not power

Torque and power are often quoted together for engines but are different. Torque describes force at a lever arm, power additionally involves rotational speed. An engine can deliver high torque at low speed and still produce little power. Power is torque times angular velocity.

Matching conversions

Matching tables

Unit family: Torque

Sources