Torque Control Tightening is the most commonly used tightening method as it is relatively simple and cost-effective to apply. The torque is usually applied with a calibrated torque wrench. Often an audible “click” signal is given when the required torque is reached.
The theoretical torque-preload relation is discussed in Section 6.3.
The main drawback of the torque-control method is that the scatter in achievable preload is rather significant as most of the applied torque is consumed by the various friction contributors and only 10% is converted into stretching the bolt (see Section 6.3.1). The pre-load for a torque controlled tightening can vary by more than 100 % between min Preload and max Preload, depending on the scatter in the friction coefficients (thread and underhead). Note that the up to 35% deviation shown in Table 6-1 (tooling uncertainty) applies to the preload between each tightening application for KNOWN friction figures.
Therefore, a good knowledge of the friction characteristics of a given joint assembly is essential to ensure that the resulting preload remains within the design range. It is important to determine these characteristics by torque-tension characterization testing as e.g. described in Ref. 12.16 or 12.17 or any equivalent method. Since friction characteristics are lot dependent a confirmation of derived characteristics by lot testing is recommended.