We all know that at important bolt connections, bolts need to be tightened, and this tightening has quantitative evaluation indicators, that is, tightening torque (or rotation angle, etc.). Different bolts have different tightening torque in different applications. Even for bolts of the same specification, different manufacturers, accuracy, lubrication coatings, the tightening torque of bolts is different, because when designing bolts, we want to get the pre tightening force of bolts, Tightening bolts is only one of the methods to realize bolt pre tightening. An important purpose of applying pre tightening force is to improve the fatigue life of bolts. Why does the fatigue life of bolts increase after preloading?
First of all, what are the factors that affect the fatigue life of bolts? Fatigue strength of materials, amplitude of alternating stress on bolts, average stress, frequency of stress cycle. For the same specific application, the only way to prolong the fatigue life of bolts without changing the bolt specification is to reduce the amplitude of alternating stress or reduce the average stress.
Let's take a look at the bolt force under the same external load when the bolt is tightened and non tightened respectively.
1. When the bolt does not apply pre tightening force (only each connecting surface is pasted)
Under this working condition, because the pre tightening force of the bolt is very small, when the workpiece is under tension, the connecting surfaces of the workpiece are separated from each other. The tension, bolt force and prying force form a lever balance force system, and the bolt force increases proportionally with the increase of external force. In this case, the corresponding relationship between tensile force and bolt force is shown in figure 2-curve 2
The bolt force increases obviously with the increase of external force.
2. When pre tightening force is applied to the bolt
Under this working condition, because the pre tightening force of the bolt is large enough, when the connecting surfaces are under pressure, when the workpiece is under tension, the pressure between the connecting surfaces of the workpiece begins to decrease. When the tension reaches a certain degree, the pressure between the connecting surfaces is zero and begins to separate from each other. Before that, the bolt force changes slowly, and then the tension, bolt force and prying force form a lever balance force system, and the bolt force increases proportionally with the increase of external force. In this case, the corresponding relationship between tensile force and bolt force is shown in figure 2-curve 1
It can be seen from the figure that when the bolt connection bears the alternating load from 0 to t, if the bolt is not tightened, the variation range of the bolt force will be large. When the bolt is tightened according to the design torque, and the bolt connection is under the same alternating load from 0 to t, the variation range of the bolt force is much smaller. The variation range of the bolt force has a great impact on the fatigue life of the bolt. The smaller the variation range of the bolt stress is, the longer the fatigue life, In order to reduce the variation range of bolt force, it is often necessary to ensure that the bolt connection will not separate when bearing external load in the design. At the same time, in order to ensure that the bolt will not be broken under the limit load, the pre tightening force of the bolt cannot be greater than a specific value. A reasonable design of the pre tightening force of the bolt can maximize the material utilization of the bolt.
