High-strength bolts have the advantages of simple use, good load-bearing capacity, easy disassembly, good fatigue resistance, and not easy to loosen during operation. It is used more frequently on mechanical equipment. High-strength bolts need to be connected with corresponding nuts. After the nuts are tightened, the high-strength bolts will be subjected to a very strong preload, and at the same time, the surface of the connected parts will also generate a preload of the same magnitude. Under the action of this pressure, a large frictional force will be generated on the surface of the connecting parts. When the axial force is less than this frictional force, the connection will not be damaged, thus achieving a fastening effect.

High-strength bolt connections rely on the frictional force between the contact surfaces of the fasteners to prevent them from sliding against each other. To ensure that the contact surfaces have sufficient frictional force, it is necessary to increase the clamping force of the structural components and the friction coefficient of the contact surfaces of the structural components. The clamping force between structural components is achieved by increasing the preload on the bolts. Therefore, high-strength bolts need to be produced and manufactured using high-strength steel (such as 40r, SCM435, etc.). The grades of high-strength bolts are divided into 8.8 grade, 10.9 grade, 12.9 grade, and 14.9 grade.

In high-strength bolt connections, the level of the friction coefficient has a significant impact on the bearing capacity. After testing, it is concluded that the coefficient of friction is mainly affected by the way the contact surface is formed and the material of the structural components. To increase the friction coefficient of the contact surface, methods such as sandblasting and cleaning with wire brushes can be adopted to treat the contact surfaces of structural components within the connection range.

In fact, high-strength bolts are divided into two types: friction type and pressure-bearing type. The criterion for frictional high-strength bolts to withstand shear force is that the shear force caused by the design load does not exceed the frictional force. The design standard for pressure-bearing high-strength bolts is that the shank will not be sheared or the parts will not be crushed.