How to improve the strength of bolt connection? We can achieve it from the following five points.
01 Improve the load distribution between threads
When the common nut is used, the distribution of axial load is not uniform
As shown in Fig. 1 (a), the load of the first circle is the largest from the bearing surface of the nut, and then decreases.
Theoretical analysis and experiments show that the more the number of turns is, the more obvious the uneven load distribution is. After the 8th to 10th turns, the thread is almost free of load.
Therefore, the thick nut with more turns can not improve the connection strength.
If the mounting (tension) nut in Fig. 1 (b) is used, the conical mounting section of the nut and the bolt rod are tensile deformation, which helps to reduce the screw moment difference between the nut and the bolt rod, so as to make the load distribution more uniform.
Fig. 1 (c) shows a ring groove nut, whose function is similar to that of a mounting nut.
02 Avoid or reduce additional stress
Due to negligence in design, manufacture or installation, the bolt may be subjected to additional bending stress (Fig. 2), which has a great influence on the fatigue strength of the bolt and should be avoided.
For example, when installing bolts on rough surfaces such as castings or forgings, structures such as boss or countersunk seat are often used, and flat bearing surfaces can be obtained after cutting (Fig. 3).
03 Reduce stress concentration
The root of thread, the joint of bolt head and bolt rod have stress concentration, which is the dangerous part of fracture.
The stress concentration of thread root has great influence on the fatigue strength of bolt.
The stress concentration can be reduced by increasing the fillet radius of the thread root, increasing the fillet in the transition part of the bolt head (Fig. 4 (a)) or cutting the unloading groove (Fig. 4 (b), 4 (c)).
04 Reduce stress amplitude
When the maximum stress of bolt is constant, the smaller the stress amplitude is, the higher the fatigue strength is.
Under the same working load and residual preload, reducing the bolt stiffness or increasing the stiffness of the connected parts can achieve the purpose of reducing the stress amplitude (see Figure 5), but the preload should be increased.
The measures to reduce the rigidity of the bolt are: increasing the length of the bolt appropriately; reducing the diameter of the bolt rod partially or forming a hollow structure, i.e. flexible bolt.
The elastic element (Fig. 6) installed under the nut can also play the role of flexible bolt.
The flexible bolt has large deformation, strong energy absorption and is also suitable for bearing impact and vibration.
In order to increase the stiffness of the connected system, it is not suitable to use the gasket with small stiffness. It is better to use a sealing ring for the sealing connection shown in Fig. 7.
05 Improve manufacturing process
The manufacturing process has a great influence on the fatigue strength of bolts. For high strength steel bolts, it is more significant.
When the thread is rolled, due to the effect of cold work hardening, there is residual compressive stress in the surface layer, the metal streamline is reasonable, and the fatigue strength of the bolt is higher than that of turning.
Carbonitriding, nitriding and shot peening can improve the fatigue strength of bolts.
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