Why can the bolt fatigue life be improved after the bolt is tightened?

We all know the important bolt joints, the bolts need to be tightened, and this tightening is a quantitative evaluation index, that is, tightening torque (or corner, etc.), different bolts are used in different applications, the tightening torque is not the same, even Bolts of the same specification, different manufacturers, precision, and lubricated coatings, the bolt tightening torque that needs to be applied is also different, because in the design of the bolt, we want the pre-tightening force of the bolt, and tightening the bolt is only to achieve the pre-tightening of the bolt. One of the methods, an important purpose of applying the preload is to increase the fatigue life of the bolt. Why does the fatigue life of the bolt increase after the preload?

First, what are the factors that affect the fatigue life of bolts? The fatigue strength of the material, the magnitude of the alternating stress applied to the bolt, the average stress, and the frequency of the stress cycle. For the same specific application, if you want to extend the fatigue life of the bolt without changing the bolt specifications, you can only reduce the amplitude of the alternating stress or reduce the average stress.

Then look at the bolt force when the bolt is subjected to the same external load under tightening and non-tightening conditions.

1. When the bolts are not preloaded (only the joint faces are sturdy)

Under this condition, because the bolt pre-tightening force is very small, when the workpiece is subjected to tensile force, the workpiece connecting faces are separated from each other, and the pulling force, the bolt force and the squeezing force form a lever balance force system, and the bolt force is proportional to the increase of the external force. increase. In this case, the corresponding relationship between the pull-down force and the bolt force is as shown in Fig. 2 - curve 2.

The bolt force increases significantly as the external force increases.

1. When the bolt exerts a preload

Under this condition, since the bolt pre-tightening force is large enough, when the joint faces are subjected to pressure, when the workpiece is subjected to tensile force, the pressure between the joint surfaces of the workpiece begins to decrease. When the tensile force reaches a certain level, the pressure between the joint surfaces is zero. And began to separate from each other. Before this, the bolt force changes slowly. After that, the pulling force, the bolt force and the squeezing force form a lever balance force system, and the bolt force increases proportionally with the increase of the external force. In this case, the corresponding relationship between the pull-down force and the bolt force is as shown in Fig. 2 - curve 1.

As can be seen from the figure, when the bolt connection is subjected to an alternating load of 0 to T, if the bolt is not tightened, the bolt force varies greatly, and when the bolt is tightened according to the design torque, the bolt is connected at the same 0 to T. When the alternating load is applied, the range of bolt force variation is relatively small. The range of bolt force variation has a great influence on the fatigue life of the bolt. The smaller the range of bolt stress variation, the longer the fatigue life, and the bolt force change is well reduced. Scope, in the design often ensure that the bolt joints will not separate when subjected to external loads. At the same time, in order to ensure that the bolt is not pulled under the limit load condition, the bolt pre-tightening force can not be greater than a certain value, and the pre-tightening force of the reasonable design bolt can maximize the material availability of the bolt.