In general, we analyzed the bolt breaking for the following four aspects:
First, the quality of bolt
second, the bolt tightening torque
third, strength of the bolt
fourth, bolt fatigue strength
In fact, the vast bolt breaking In most cases, it breaks because of looseness and is destroyed by looseness. Because the situation of loose bolt breakage is almost the same as fatigue fracture, in the end, we can always find the cause from the fatigue strength. In fact, the fatigue strength is so large that we cannot imagine that the bolt will not use the fatigue strength at all during use.
1. Bolt breakage 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, up to 100,000 of its own weight Times, in general, we will only use it to fasten 20 kg of parts, and only use one thousandth of its maximum capacity. Even the effect of other forces in the equipment cannot 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 breakage of the bolt is not due to the fatigue strength of the bolt:
The threaded fastener can be loosened only one hundred times in the lateral vibration test, and repeated vibration one million times in the fatigue strength test. In other words, threaded fasteners are loose when they use one ten thousandth of their fatigue strength, we only use one ten thousandth of its large capacity, so the looseness of threaded fasteners is not due to bolt fatigue strength .
3. The real reason for the damage of threaded fasteners is loosening:
After loosening of threaded fasteners, a huge kinetic energy mv2 is generated. This huge kinetic energy directly acts on the fasteners and equipment, resulting in damage to the fasteners and damage to the fasteners. After that, the device cannot work in a normal state, further causing damage to the device.
Fasteners subjected to axial forces break the threads and break the bolts.
For fasteners subjected to radial forces, the bolts are cut off and the bolt holes are punched into ellipses.
4. The use of thread anti-loosening method with excellent anti-loosening effect is the root of the problem:
Take 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. The pretension force is calculated by half of the tensile force. The pretension force is up to three or four hundred. Ton. But the bolts will break as well, and now the bolts are ready to be changed to M48. The fundamental reason is that the bolts cannot be loosened.
When the bolt is broken, the easiest conclusion that people can draw is that the strength is not enough, so most of them adopt the method of increasing the strength level of the bolt diameter. This method can increase the pre-tightening force of the bolts, and the frictional force has also been 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 benefits are too small.
In short, the bolt is: “If it doesn’t loose, it will break as soon as it looses.”
