Analysis of bolt fracture causes


Analysis of bolt fracture causes:

Generally, we analyze bolt fracture from the following four aspects:

First, the quality of bolts

Second, the pre tightening torque of bolts

Third, the strength of the bolt

Fourth, the fatigue strength of bolts

In fact, most of the broken bolts are broken because of looseness. Because the situation of bolt loosening and breaking is basically the same as that of fatigue fracture. Finally, we can always find the reason from the fatigue strength. In fact, the fatigue strength is too high for us to imagine, and the bolt can not use the fatigue strength in the process of use.

1、 Bolt fracture is not due to tensile strength of bolt:

Taking an M20 × 80 grade 8.8 high strength bolt as an example, its weight is only 0.2kg, and its minimum tensile load is 20t, which is up to 100000 times of its own weight. Under normal circumstances, we only use it to fasten 20kg parts, and only use one thousandth of its maximum capacity.

Even if it is the effect of other forces in the equipment, it is impossible to exceed a thousand times the weight of the parts. Therefore, the tensile strength of the threaded fasteners is sufficient, and it is impossible to damage the threaded fasteners because of the insufficient strength of the bolts.

2、 The fracture of the bolt is not due to the fatigue strength of the bolt

In the transverse vibration loosening test, the threaded fastener can be loosened only 100 times, while in the fatigue strength test, it needs repeated vibration for one million times. In other words, the threaded fastener is loose when it uses one tenth of its fatigue strength. We only use one tenth of its large capacity. Therefore, the looseness of the threaded fastener is not due to the fatigue strength of the bolt.

3、 The real reason for the damage of threaded fasteners is looseness

After the screw fastener is loose, huge kinetic energy MV2 will be produced. This huge kinetic energy directly acts on the fasteners and equipment, resulting in the fastener damage. After the fastener is damaged, the equipment can not work in the normal state, which further leads to the equipment damage.

For fasteners under axial force, the thread is damaged and the bolt is broken.

For fasteners subjected to radial force, the bolt is sheared and the bolt hole is made into ellipse.

4、 The key to solve the problem is to select the thread anti loosing method with excellent anti loosing effect

Take the hydraulic hammer as an example. The weight of gt80 hydraulic hammer is 1.663 tons. Its side plate bolts are 7 sets of 10.9 grade M42 bolts. The tensile strength of each bolt is 110 tons. The pre tightening force is calculated as half of the tensile force, and the pre tightening force is as high as three or four hundred tons. But the bolt will also be broken. Now we are going to change it to M48 bolt. The fundamental reason is that the bolt locking cannot be solved.

The most easily concluded conclusion of bolt fracture is that the strength is not enough, so the method of increasing the bolt diameter strength grade is mostly adopted. This method can increase the pre tightening force of bolts, and the friction force is also increased. Of course, the anti loosing effect can be improved. However, this method is actually a non professional method, and its investment is too large and the income is too small.

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