A torque wrench is a device used to precisely set the torque of a fastening such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms.
Beam Type
The simplest form of torque wrench consists of a long lever arm between the handle and the wrench head, made of a material which will bend elastically a little under the applied torque. A second smaller bar carrying an indicator is connected back from the head in parallel to the lever arm. This second arm is under no strain at all, and remains straight. A calibrated scale is fitted to the handle, and the bending of the main lever causes the scale to move under the indicator. When the desired indicated torque is reached, the operator stops applying force. This type of wrench is simple but not very precise.
Click Type
A more sophisticated method of presetting torque is using a calibrated clutch mechanism. At the point where the desired torque is reached, the clutch slips, preventing overtightening. The commonest form uses a ball detent and spring, with the spring preloaded by an adjustable screw thread, calibrated in torque units. The ball detent transmits force until the preset torque is reached, at which point the force exerted by the spring is overcome and the ball "clicks" out of its socket. The advantage of this design is greater precision and a positive action at the set point. A number of variations of this design exist for different applications and different torque ranges.
Differences between types
Click type torque wrenches are more precise when properly calibrated - however the more complex mechanism can result in them losing calibration far quicker than the beam type, where there is little to malfunction.
Beam type torque wrenches are impossible to use in situations where the scale cannot be read - and these 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. There is also the issue of increased user error with the beam type - the torque has to be read off each and every use.
