The pre-tightening force of the bolt is the pre-tightening force along the axis line of the bolt generated between the bolt and the connected member under the action of tightening torque during the process of bolt screwing. For a particular bolt, the magnitude of its preload is related to the tightening torque of the bolt, the friction force between the bolt and the nut, and the friction force between the nut and the connected part.
Pretightening can improve the reliability, anti-loosening ability and fatigue strength of the bolt connection, and enhance the tightness and rigidity of the connection. In fact, a large number of tests and experience have proved that a higher preload is beneficial to the reliability of the connection and the life of the connection, especially for the connection with sealing requirements. Of course, as the saying goes, "extreme", too high preload, if improper control or accidental overload, often lead to the failure of the connection. Therefore, it is very important to accurately determine the preload force of the bolt.
