The definition of high-strength bolts and their applications

The definition of high-strength bolts and their applications

Bolts made of high-strength steel or requiring significant preload can be referred to as high-strength bolts. High strength bolts are commonly used in bridges, steel rails, high-pressure and ultra-high-pressure equipment.

The fracture of this type of bolt is mostly brittle. It is generally suitable for high-strength bolts on ultra-high pressure equipment. In order to ensure the sealing of the container, it is required to apply a large amount of pre-stress.

Several concepts about high-strength bolts: 1. According to regulations, bolts with a strength grade of 8.8 or higher are called high-strength bolts. Currently, the national standard only lists M39. Regarding large-sized specifications, especially high-strength bolts with a length greater than 10-15 times, domestic production is still in the short term.

Differences between high-strength bolts and ordinary bolts

Selection of high-strength bolts:

Length=total thickness of the connecting plate layer+tightening length extension value+nominal thickness of the nut+number of washers * washer thickness+3 * thread pitch

High strength bolts have an acceptable load greater than ordinary bolts of the same specification. The production material of ordinary bolts is Q235 (i.e. A3).

The production material of high-strength bolts is 35 # steel or other high-quality materials. After production, heat treatment is stopped to further improve strength.

The difference between the two is the strength of the material.

2、 From the strength level of high-strength bolts:

High strength bolts are becoming increasingly common in use. Two strength levels, 8.8s and 10.9s, are commonly used, with 10.9 being the most common. The strength level of ordinary bolts should be lower, with grades 4.4, 4.8, 5.6, and 8.8 being common.

1、 From the perspective of raw materials, high-strength bolts:

High strength bolts are made of high-strength steel. The screws, nuts, and washers of high-strength bolts are all made of high-strength steel, commonly used in 45 # steel, 40 boron steel, and 20 manganese titanium boron steel. Ordinary bolts are commonly made of Q235 steel.

3、 From the stress characteristics of high-strength bolts:

High strength bolts apply pre tension and transmit external forces through friction. The connection of ordinary bolts relies on the shear resistance of the bolt rod and the bearing pressure of the hole wall to transmit shear force. When tightening the nut, the pre tension force is very small, and its impact can be ignored. In addition to its high data strength, high-strength bolts also apply a large pre tension force to the bolts, causing squeezing pressure between the connecting components, and thus having a great friction force perpendicular to the screw direction. Moreover, the pre tension force, anti slip coefficient, and steel variety all directly affect the bearing capacity of high-strength bolts.

According to the stress characteristics, it can be divided into pressure bearing type and friction type. The calculation methods for the two are different. The minimum specification of high-strength bolts is M12, commonly used is M16-M30. The function of oversized bolts does not fluctuate, and should be carefully used in design.

The difference between high-strength bolt friction type and pressure bearing type connection:

High strength bolt connection refers to the clamping of the connecting plate components by a large tightening pre tension force inside the bolt rod, which is sufficient to generate significant friction force, thereby improving the integrity and stiffness of the connection. When subjected to shear force, according to different design and force requirements, it can be divided into two types: high strength bolt friction type connection and high strength bolt pressure bearing type connection. The essential difference between the two is that the ultimate form is different. Although they are the same type of bolt, in terms of calculation methods, requirements There are significant differences in the scope of application and other aspects. In shear design, the high-strength bolt friction type connection is the maximum frictional force provided by the bolt tightening force when the external shear reaches the contact surface of the plate, which is the ultimate form, ensuring that the internal and external shear forces of the connection do not exceed the maximum frictional force throughout the entire service period. The plate will not experience absolute slip deformation (the original gap between the screw and the hole wall is always maintained), and the connected plate will be subjected to all elastic forces. In the shear design, the external shear force is allowed to exceed the maximum friction force in the pressure bearing connection of high-strength bolts. At this time, the absolute slip deformation occurs between the connected plates until the bolt rod contacts the hole wall, and then the connection is transmitted by the bolt body shear and the pressure bearing of the hole wall as well as the friction force between the contact surfaces of the plates. At first, the shear of the rod body or the pressure bearing destruction of the hole wall is taken as the ultimate form of continuous shear. In short, friction type high-strength bolts and pressure type high-strength bolts are actually the same type of bolt, but it is only a matter of whether the design can consider slip. Friction type high-strength bolts are relatively non sliding and do not accept shear force. Once sliding occurs, the design assumes that they have reached a failure state and are relatively mature in technology; Pressure bearing high-strength bolts can slide, and the bolts also accept shear force. The ultimate failure is equivalent to the failure of ordinary bolts (bolt shear failure or steel plate compression failure).

From the perspective of application, high-strength bolts:

The bolt connection of the main components of the construction structure is usually connected with high-strength bolts. Ordinary bolts can be reused, while high-strength bolts cannot be reused. High strength bolts are commonly used for eternal connection. High strength bolts are pre-stressed bolts. Friction type bolts use a torque wrench to apply regular pre-stress, while pressure type bolts unscrew the plum blossom head. Ordinary bolts have poor shear resistance and can be used in major structural parts. Ordinary bolts only need to be tightened.

Ordinary bolts are generally classified as Grade 4.4, 4.8, 5.6, and 8.8. High strength bolts are generally

8.8 level and 10.9 level, with 10.9 level being the majority.

8.8 and 8.8S are opposite levels. The stress function and calculation method of ordinary bolts and high-strength bolts are different. The force on high-strength bolts is first subjected to a pre tension force applied externally, and then frictional resistance occurs on the contact surface between the connected parts to accept the external load, while ordinary bolts directly accept the external load.

More specifically, high-strength bolts:

High strength bolt connection has the advantages of complex construction, good load-bearing function, detachable replacement, fatigue resistance, and non loosening under dynamic loads, making it a promising connection method. High strength bolts use a special wrench to tighten the nut, causing a large and controlled pre tension on the bolt. After passing through the nut and backing plate, there is also an abnormal amount of pre pressure on the connected parts. Under the action of preload, a significant frictional force will occur along the surface of the connected component. Obviously, as long as the axial force is less than this frictional force, the component will not slip and the connection will not be damaged. This is the principle of high-strength bolt connection.

High strength bolt connection relies on the frictional force between the contact surfaces of the connecting components to prevent their mutual sliding. In order to have sufficient frictional force on the contact surfaces, it is necessary to improve the clamping force of the components and increase the friction coefficient of the contact surfaces of the components. The clamping force between components is achieved by applying pre tension to the bolts, so the bolts must be made of high-strength steel, which is the reason for the connection of high-strength bolts.

The size of the friction coefficient has a significant impact on the bearing capacity in high-strength bolt connections. Experiments have shown that the friction coefficient is mainly influenced by the way the contact surface and the material of the component. In order to increase the friction coefficient of the contact surface, construction often uses methods such as sandblasting and wire brushing to clean the contact surface of components within the connection range.

Summary of high-strength bolts:

1. There are actually two types of high-strength bolts: friction type and pressure type.

2. The principle of accepting shear force for friction type high-strength bolts is that the shear force caused by the design load does not exceed the frictional force.

3. Pressure bearing high-strength bolts are designed based on the principle that the rod body is not sheared or the plate is not crushed.

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