Measures to improve the strength of bolt connection

Measures to improve the strength of bolt connection

The strength of bolted connection mainly depends on the strength of bolts. There are many factors that affect the strength of bolts, such as structure, size parameters, assembly process, materials, manufacturing accuracy grade, etc.

1. Improve the fatigue strength of bolts

Theory and practice have proved that under the condition of constant working load and residual preload, reducing the bolt stiffness or increasing the stiffness of the connected part can improve the fatigue strength of the bolt, but the preload should be appropriately increased to ensure the tightness of the connection. The common measures to reduce the bolt stiffness are: appropriately increasing the bolt length, reducing the bolt rod diameter or making a hollow structure - Flexible bolt. The flexible bolt has large deformation and strong energy absorption when under stress, and is also suitable for bearing impact and vibration. The elastic element is installed under the nut. When the working load is transmitted from the connected part, it can also play the effect of flexible bolt due to the large deformation of the elastic element. In order to increase the stiffness of the connected parts, it is not suitable to use gaskets with small stiffness. Sealing ring is preferred for tight connection.

2. Improving load distribution between threads

When an ordinary nut is used, the axial load is unevenly distributed among the turns of the screw thread, as shown in figure 9-39 of the textbook. From the bearing surface of the nut, the load on the first turn is the largest, and then each turn decreases. Theoretical analysis and experiments show that the more the number of turns, the more significant the uneven load distribution, and the threads after the 8th to 10th turns are almost free of load. Therefore, the use of thick nuts with a large number of turns can not improve the connection strength. If the mounting (tension) nut as shown in figure 9-41 is used, both the conical mounting section of the nut and the bolt rod are subject to tensile deformation, which helps to reduce the difference in pitch between the nut and the bolt rod, so as to make the load distribution more uniform.

3. Reduce stress concentration

There is stress concentration at the root and end of the thread, and at the joint between the bolt head and the bolt rod, which is a dangerous position for fracture: especially at the root of the screw thread, the tooth is subject to bending shear due to the stretching of the bolt rod, and the stress is uneven, which is more serious. The fatigue strength of bolts can be increased by 20%~40% by properly increasing the root fillet radius to reduce stress concentration; Good results have been achieved by using a tool withdrawal groove at the end of the thread and by retaining threads in the bolt rod within the pressure bearing surface of the nut. The newly developed MJ bolts are used for aviation and spacecraft bolts. Its main structural feature is that the root fillet radius is increased.

High strength steel bolts are sensitive to stress concentration, but the results are favorable because they can be tightened with greater preload and higher ultimate strength.

4. Adopt reasonable manufacturing process

The manufacturing process has a great influence on the fatigue strength of bolts. When the milled thread is used, due to the effect of cold work hardening, there is residual compressive stress on the surface, the metal streamline is reasonable, and the fatigue strength of the bolt can be 30%~40% higher than that of the turned thread; The effect of rolling after heat treatment is better. In addition, carbonitriding, nitriding and shot peening can improve the fatigue strength of bolts.