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.
... patented the world's first click-type torque wrench ... In 1939, the first torque screwdriver was patented ... In 1987, The entirely electronic torque wrench, the DAZ-E, was developed which was able to display, save, analyse and document measurements for the first time ...
... Click type torque wrenches are precise when properly calibrated—however the more complex mechanism can result in loss of calibration sooner than the beam type, where ... Beam type torque wrenches are impossible to use in situations where the scale cannot be directly read—and these situations are common in automotive applications ... The scale on a beam type wrench is prone to parallax error, as a result of the large distance between indicator arm and scale (on some older designs) ...
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