In physics, torque (or often called a moment) is a measure of how much a force acting on an object causes that object to rotate. The SI unit for torque is the newton metre (N m). In U.S. customary units, it is measured in pounds-feet (lb-ft) (a.k.a. "foot-pounds"). The symbol for torque is τ, the Greek letter tau.
Torque is defined as :
- r is the particle's position vector
- F is the force acting on the particle
Torque is part of the basic specification of an engine: the power output of an engine is expressed as its torque multiplied by its rotational speed. Internal-combustion engines produce useful torque only over a limited range of rotational speeds (typically from around 1,000–6,000 rpm for a small car). The varying torque output over that range can be measured with a dynamometer, and shown as a torque curve.
A dynamometer, or "dyno" for short, is a machine used for measuring mechanical force, or power, transmitted by a rotating shaft; measure torque and rotational speed (rpm) from which power produced by an engine, motor or other rotating prime mover can be calculated.
A torque wrench is a tool used to precisely set the force of a fastening such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms. A torque wrench is used where the tightness of screws and bolts is crucial. It allows the operator to measure the torque applied to the bolt so it can be matched to the specifications.
A torque wrench is probably be one of the most expensive hand tools in your collection. There are three common types of torque wrenches for home shop use; the "beam" type, the dial torque wrench type and the "clicker" type.