A wrench that is used to set the torque of a fastening such as a nut or bolt to a great degree of precision is called a torque wrench. It is actually a socket wrench that comes with special internal mechanisms and is of great use where you need to be absolutely sure about the tightness of the screws and the bolts. You can measure how much torque is applied so that you are sure that the tightness is up to the specifications. So, you can ensure that there is proper loading and tension of all the parts. So indirectly, it is measuring bolt tension. However, thanks to the friction between the mating hole and the fastener, this technique does suffer from inaccuracy sometimes. So it is better to measure the bolt tension or the bolt stretch and very often torque is the only measurement that is possible and accurate.
Let's take a look at what it looks like. The simplest one is a long lever arm that runs between the handle and the wrench head and is made of a material which bends elastically under the applied torque. There is a second arm which is smaller and it carries an indicator which is connected from the head and is parallel to the lever arm. This remains straight and there is no strain that it suffers from at all. The handle has a calibrated scale fitted to it and as the main lever bends, the scale moves. As soon as the desired torque is achieved, the operator stops applying pressure. This is an extremely simple device but also very precise.
You do get more sophisticated versions where a calibrated clutch mechanism is used. As soon as you get the desired torque, a clutch slips and you prevent over tightening. The most common one uses a ball detent and spring and the spring comes preloaded with an adjustable screw thread and this is calibrated in torque units. This ball detent uses force till you get the desired torque and then the spring is released and the balls clicks out of the socket. There are quite a few variations to this design and you have a whole range of them for various applications and sizes. This design is, in fact, modified and used in drills to prevent the gouging-out of the heads of screws while tightening them.
What are the differences between these different types of torque wrenches? You do get better precision when they have been calibrated well but when the mechanism is complex, they can end up losing their calibration pretty quickly. The beam type on the other hand does not have too many parts that can malfunction as it is a more mechanical operation. However, in some situations, the beam type torque just cannot be read and therefore is not used. Further, the scale is sometimes prone to parallax error because there is such a long distance between the indicator arm and scale. There could also be increased user error with this type as you have to read it off each time. When it comes to the click type, remember to remove the force from the spring and set the scale to zero when it is not in use.
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