In general, we analyze bolt fracture from the following four aspects:
First, the quality of the bolt
Second, the pre-tightening torque of the bolt
Third, the strength of the bolt
Fourth, the fatigue strength of bolts
In fact, most cases of bolt breakage are due to loosening and are broken due to loosening. Because the situation of loose and broken bolts is roughly the same as that of fatigue fractures, we can always find the reason from the fatigue strength.
1. Bolt fracture is not due to the tensile strength of the bolt:
Take an M20×80 grade 8.8 high-strength bolt as an example, its weight is only 0.2 kg, and its minimum tensile load is 20 tons, which is as high as 100,000 times its own weight. It can only use one-thousandth of its maximum capacity even for a 20kg component. Even with other forces in the equipment, it is impossible to exceed a thousand times the weight of the component, so the tensile strength of the threaded fastener is sufficient, and it is impossible to damage the bolt due to insufficient strength.
2. The fracture of the bolt is not due to the fatigue strength of the bolt:
Threaded fasteners can be loosened only 100 times in the lateral vibration loosening test, while they need to be repeatedly vibrated one million times in the fatigue strength test. In other words, threaded fasteners are loosened when they use 1/10,000 of their fatigue strength, and we only use 1/10,000 of their maximum capacity, so the loosening of threaded fasteners is not due to the fatigue strength of bolts.
3. The real cause of damage to threaded fasteners is loosening:
After the threaded fasteners are loosened, huge kinetic energy mv2 is generated. This huge kinetic energy directly acts on the fasteners and equipment, causing damage to the fasteners. After the fasteners are damaged, the equipment cannot work in a normal state, which further leads to Equipment damage.
Fasteners are subjected to axial force, the threads are damaged, and the bolts are pulled off.
Fasteners are subjected to radial force, the bolts are sheared, and the bolt holes are punched into an ellipse.
4. The choice of thread anti-loosening method with excellent anti-loosening effect is the root of the problem:
Take the hydraulic hammer as an example. The weight of the GT80 hydraulic hammer is 1.663 tons. The side plate bolts are 7 sets of 10.9 grade M42 bolts. The tensile force of each bolt is 110 tons. Ton. But the bolts will also break, and now I am going to change them to M48 bolts. The root cause is that the bolts cannot be prevented from loosening.
When the bolt is broken, the easiest conclusion that people can draw is that the strength is not enough, so most of them use the method of increasing the strength level of the diameter of the bolt. This method can increase the pre-tightening force of the bolt, and its friction force can also be increased. Of course, the anti-loosening effect can also be improved, but this method is actually a non-professional method. Its investment is too large and the benefit is too small.
In short, the bolt is: “If it is not loose, it will break.”
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