In general, we analyze bolt breakage from the following four aspects:

First, the quality of the bolts

Second, the bolt-tightening moment

Third, bolt strength

Fourth, the fatigue strength of bolts

In fact, the vast majority of bolt breakage is due to loose and broken, is due to loose and broken. Because the situation of bolt loosening and breaking is roughly the same as that of fatigue fracture, finally, we can always find the reason for the fatigue strength. In fact, the fatigue strength is so large that we can't imagine that the fatigue strength of bolts is not needed at all in the use process.

First, bolt fracture is not due to the tensile strength of the bolt:

Take an M20×80 class 8.8 high-strength bolt for example, it weighs only 0.2 kg, and its minimum tensile load is 20 tons, up to 100,000 times its own weight, under normal conditions, we will only use it to tighten 20 kg parts, and only use one-thousandth of its maximum capacity. Even other forces in the equipment cannot break through thousands of times the weight of the component, so the tensile strength of threaded fasteners is sufficient and it is impossible to damage due to insufficient bolt strength.

Two, the fracture of the bolt is not due to the fatigue strength of the bolt:

The threaded fasteners can be loosened only 100 times in the transverse vibration test, while they need to vibrate one million times in the fatigue strength test. In other words, threaded fasteners loosen when they use 1/10,000 of their fatigue strength. We only use 1/10,000 of their large capacity, so the loosening of threaded fasteners is not because of bolt fatigue strength.

Three, the real cause of screw fastener damage is loose:

After threaded fasteners are loose, huge kinetic energy mv2 is generated. This huge kinetic energy directly acts on fasteners and equipment, resulting in fastener damage. After fasteners are damaged, the equipment cannot work in a normal state, which further leads to equipment damage.

For fasteners subjected to axial force, the thread is broken and the bolt is pulled off.

For fasteners subjected to radial forces, the bolts are cut and the bolt holes are beaten into ellipses.

Four, the selection of thread anti-loosening excellent effect is the fundamental solution to the problem:

Take a hydraulic hammer, for example. The weight of the GT80 hydraulic hammer is 1.663 tons, and its side plate bolts are 7 sets of 10.9 M42 bolts. The tensile force of each bolt is 110 tons. The preload is calculated by taking half of the tensile force, and the preload is as high as 3 or 400 tons. However, the bolts will also break. Now we plan to change the bolts to M48. The root cause is that the bolts cannot be loosened.

In bolt fracture, people most easily draw the conclusion that the strength is not enough, so most of the use of increasing the bolt diameter strength grade method. This method can increase the preload of bolts, its friction has been increased, and of course, the effect of anti-loosening can also be improved, but this method is actually a non-professional method, its investment is too big, and the benefit is too small.

In short, the bolt is: "not loose, loose will break."