A torque wrench is a wrench 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. 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. This permits proper tension and loading of all parts. A torque wrench indirectly measures bolt tension. The technique suffers from inaccuracy due to inconsistent friction between the fastener and its mating hole. Measuring bolt tension (bolt stretch) is more accurate but most often torque is the only means of measurement possible.
Beam type torque wrench. The indicator bar remains straight while the main shaft bends proportionally to the pressure applied at the handle.
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.
Close up of beam type torque wrench showing detail of the torque display scale. This shows a torque of about 160 inch pounds or 17 newton meters.
Click-type torque wrench, adjusted by turning the knurled handle
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 most common 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. A modification of this design is used in some drills to prevent gouging the heads of screws while tightening them.
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 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. There is also the issue of increased user error with the beam type—the torque has to be read off each and every use.
For the click type, when not in use, the force acting on the spring should be removed by setting the scale to zero in order to maintain the spring's strength. In the case of the beam type, there is no strain on the component that provides the reference force except when it is in use.
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