Clé dynamométrique
Many nuts have to be tightened to a specific torque setting given in Nm (Newton metres), kg/m or lb/ft (imperial). If a nut is over-tightened, the bolt may be damaged or shear off; if it is not tightened enough, the nut may work loose. Cylinder-head nuts, for example, need tightening in sequence to equal pressures to avoid head distortion.
The torque, or turning power, can be measured on a torque wrench, which has a handle with a square drive that fits into a socket spanner, and a measuring or setting scale, sometimes up to 150 lb/ft (210 Nm). The commonest type has a scale and pointer. More expensive wrenches have a dial indicator or a micrometer scale that can be preset to the required torque, and give an audible click when it is reached.
A torque wrench is a tool used to precisely apply a specific torque to a fastener such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms. It was invented by Conrad Bahr in 1918 while working for the New York City Water Department. It was designed to prevent overtightening bolts on water main and steam pipe repairs underground.
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 fastener so it can be matched to the specifications for a particular application. This permits proper tension and loading of all parts. A torque wrench measures torque as a proxy for bolt tension. The technique suffers from inaccuracy due to inconsistent or uncalibrated friction between the fastener and its mating hole. Measuring bolt tension (bolt stretch) is more accurate but often torque is the only practical means of measurement.
Torque screwdrivers and torque wrenches have similar purposes and mechanisms.
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