In general, we analyze bolt breaks 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 the bolt
In fact, most of the bolt breaks are broken due to looseness and are broken due to looseness. Because the bolt loosening and the fatigue fracture are almost the same, in the end, we can always find the reason from the fatigue strength. In fact, the fatigue strength is so large that we can't imagine that the bolt will not use the fatigue strength during use.First, the bolt break is not due to the tensile strength of the bolt:
Take an M20×80 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. In general, we only use it tightly. The 20 kg part is only one thousandth of its maximum capacity. Even if other forces in the equipment act, it is impossible to break through a thousand times the weight of the component. Therefore, the tensile strength of the threaded fastener is sufficient, and it is impossible to damage the bolt because of insufficient strength.Second, the bolt break is not due to the fatigue strength of the bolt:
Threaded fasteners can be loosened only one hundred times in the transverse vibration test, and one million times in the fatigue strength test. In other words, the threaded fastener is loose when using one ten thousandth of its fatigue strength. We only use one ten thousandth of its large capacity, so the looseness of the threaded fastener is not due to the fatigue strength of the bolt. .Third, the real reason for the damage of threaded fasteners is loose:
When the threaded fastener is loose, it generates a huge kinetic energy mv2. This huge kinetic energy directly acts on the fasteners and equipment, causing the fastener to be damaged. After the fastener is damaged, the equipment cannot work under normal conditions, further causing Equipment damage.
Fasteners that are subjected to axial forces, the threads are broken and the bolts are broken.
For fasteners that are subjected to radial forces, the bolts are sheared and the bolt holes are made elliptical.Fourth, the use of thread anti-loose method with excellent anti-loose effect is the fundamental solution to the problem:
Take a hydraulic hammer as an example. The weight of the GT80 hydraulic hammer is 1.663 tons, and the side plate bolts are 7 sets of 10.9 M42 bolts. The tensile strength of each bolt is 110 tons. The pre-tightening force is calculated as half of the tensile force. The pre-tightening force is up to three or four hundred. Ton. But the bolts will break, and now the bolts are ready to be changed to M48. The root cause is that the bolts can't be solved.
Bolt breaks, the easiest conclusion is that the strength is not enough, so most of them use the method of increasing the bolt diameter strength level. This method can increase the pre-tightening force of the bolt, and the frictional force is also increased. Of course, the anti-loose effect can also be improved, but this method is actually a non-professional method, and its investment is too large, and the income is too small.
In short, the bolt is: "Not loose, and loose."
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