Analysis: Fastener selection series of high-strength bolt connection

High-strength bolts are a new connection form developed since the 1950s. It has the advantages of simple construction, good mechanical performance, removable, fatigue resistance, and no loosening under dynamic loads. It has a promising future. Connection method.

When assembling, use a torque wrench to tighten the nut to make the bolt generate a huge and controlled pretension Fp. Through the nut and the liner, the same pretension Fp is generated on the connected parts. Under the action of pre-pressure Fp, a relatively large friction force will be generated along the surface of the connected part. Obviously, as long as the sliding force F is less than the friction force, the component will not slide and the connection will not be damaged. This is a high-strength bolt connection The principle.

As mentioned above, high-strength bolt connections rely on the friction between the contact surfaces of the connectors to prevent them from sliding; in order to make the contact surfaces have sufficient friction, it is necessary to increase the clamping force of the components and increase the inter-component The coefficient of friction. The clamping force between the components is achieved by applying pre-tightening force to the bolts, but ordinary bolts made of low-carbon steel, due to the limitation of material strength, the pre-tightening force that can be applied is limited. The friction force is smaller than the shear resistance of ordinary bolts, so if the friction force caused by the bolt pretension is to be used to transmit the force, the material strength of the bolt must be much greater than the strength of the component material, that is, the bolt must be used It is made of high-strength steel, which is why it becomes a high-strength bolted connection.

The strength of the material used for high-strength bolts is 4 to 5 times that of ordinary bolts, and the commonly used performance levels are 8.8 and 10.9. Grade 8.8 uses high-quality carbon steel No. 35 or No. 45 steel; Grade 10.9 uses alloy structural steel 20MnTiB, 40B, 35VB. There are two types of high-strength bolts: large hexagon head bolts and torsion shear type. The steel structure specification stipulates that the material of high-strength bolts should meet the requirements of the current standards.

The pretension of a high-strength bolt is determined by the yield of the material and the effective area of u200bu200bthe bolt, taking into account a certain coefficient of friction. The pre-tension of high-strength bolts is established by tightening the nut during construction. There are several ways to tighten (tighten) the nut:

(1) Torque method

According to the relationship between the torque M and the pretension force, the nut is preliminarily tightened with an ordinary wrench, and then a special wrench that can display the torque value is used to tighten to the specified torque value.

(2) Corner Method

A method that is determined according to the relationship between the rotation angle of the nut and the pretension of the bolt after the close contact between the plates.

When tightening, first use a short wrench to screw the nut to the non-rotating position, and then use a long wrench to screw the nut to the specified position to achieve the pre-tension.

(3) Unscrew the tail of the bolt

For torsion shear type high-strength bolts, this bolt has a special tail. When tightening, use a special wrench to cover the bolt and the end of the bolt. One sleeve rotates forward and the other sleeve reverses. When the nut is tightened to a certain extent, the end of the bolt is broken. Since the depth of the notch at the end of the bolt is determined by the relationship between the twisting torque and the pre-tightening force, the corresponding pre-tension value is reached when it is twisted off.

In the high-strength bolt connection, the friction coefficient has a great influence on the load-bearing capacity. Tests have shown that the friction coefficient is directly related to the material of the component, the roughness of the contact surface, and the magnitude of the opposing force, mainly the form of the contact surface and the material of the component. In order to increase the friction coefficient of the contact surface, the contact surface of the components within the connection range should be treated during construction. The treatment methods include sandblasting and cleaning with wire brushes. In the design, according to the engineering situation, try to use a treatment method with a larger friction coefficient, and clearly indicate on the construction drawing.

In addition to the above treatment methods, there is also a treatment method for polishing the contact surface with a portable electric grinding wheel. The polishing direction should be perpendicular to the force direction, and its anti-slip coefficient is equivalent to sandblasting.

It should be pointed out that high-strength bolts are actually divided into friction type and pressure-bearing type. The criterion for friction-type high-strength bolts to bear the shear force is to make the shear force caused by the design load not exceed the friction force. The pressure-bearing high-strength bolts are designed based on the fact that the shaft is not damaged or the plate is not crushed. Its force characteristics and calculation methods are basically the same as ordinary bolts, but because the bolts are made of high-strength steel, Has a high carrying capacity.