The purpose of bolted connection is to make the two connected parts closely fit, because the connecting parts to bear a certain dynamic load, so also need to be connected with enough pressing force, to ensure the reliable connection of the connected parts and normal work. In this way, it is required that the bolts used as connections should have sufficient axial preload after tightening. The application of axial preload is realized by “tightening”, and the tightening process is the process of applying torque.

In the process of tightening, the torque and preload change with the change of Angle, which can be roughly divided into three stages.

Stage 1: At the beginning of tightening, the bolt does not touch the connected part, so the pressing force is zero, but there is a certain friction force, so the torque T is maintained at a small value. Stage 2: When the bolt and the connected part begin to contact, the compression force F and torque T rise rapidly with the increase of the Angle A. Stage 3: When the yield point is reached, the bolt begins to undergo plastic deformation, and the Angle increases greatly while the compression force and torque increase little or even remain unchanged. As tightening continues, torque T and compression force F decrease and the bolt breaks.

According to the characteristics of the thread and the mechanical analysis in the tightening process, the relationship between T and F is: T=K F d, where: T is the tightening torque; F is the preload force; d is the nominal diameter, which is related to the type of bolt; K is the torque coefficient, which is related to the roughness and lubrication of the supporting surface, the precision and lubrication of the thread pair.