A torque wrench is used in conjunction with a threaded fastener for the single purpose of controlling the clamping ability of the fastener. The stresses induced in the body of the bolt or screw by tightening provide the force which does the clamping.
Since the torque applied to the head of the fastener is directly proportional, or nearly so, to the load applied, it is possible to measure bolt stress by means of a torque wrench.
The first requirement in determining the amount of torque to apply is a knowledge of the desired bolt stress. It this stress is not dictated by the function of the assembly it is common practice to base the limit on the yield strength of the bolt material.
Theoretically, screws and bolts may be tightened to the yield point. However, in order to avoid fastener failure during the assembly process it is recommended that the induced stress not be allowed to exceed 80% of the yield strength.
In the original design of a fastener which will be subject to external loading, whether static or dynamic, it will be necessary to establish bolt size and allowable stress in accordance with current engineering practice.
The mathematical relationship between torque applied and the resulting tension force in the bolt has been determined to be as follows: