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Fasteners are selected for connection to high strength bolts.
Sep 02, 2018

High-strength bolts are a new type of connection developed since the 1950s. They have the advantages of simple construction, good performance, removability, fatigue resistance and no looseness under dynamic load. Connection method.


       During assembly, the nut is tightened with a torque wrench to produce a large and controlled pre-tension Fp through the nut and the lining, which also produces a pre-stress Fp of the same size for the connected piece. Under the action of the pre-pressure Fp, a large frictional force is generated along the surface of the connected member. Obviously, as long as the sliding force F is smaller than the frictional force, the member will not slide and the connecting member will not be destroyed. This is a high-strength bolt connection. The principle.


       As mentioned above, the high-strength bolt connection is prevented from sliding against each other by the frictional force between the contact surfaces of the connecting members; in order to make the contact surface have sufficient friction, it is necessary to increase the clamping force of the members and increase the friction coefficient between the members. . The clamping force between the components is achieved by applying a pre-tightening force to the bolts. However, ordinary bolts made of low-carbon steel are limited by the strength of the material, and the pre-tightening force that can be applied is limited. The friction force is smaller than the shearing force of ordinary bolts. Therefore, if the friction force is to be transmitted by the friction force caused by the bolt pre-tension, the material strength of the bolt must be much greater than the strength of the component material, that is, the bolt must be used. Made of high-strength steel, which is why it is a high-strength bolt connection.


       The strength of the material used for high-strength bolts is 4 to 5 times that of ordinary bolts. The commonly used performance grades are 8.8 and 10.9. Grade 8.8 is made of high quality carbon steel No. 35 steel or No. 45 steel; Grade 10.9 is made of alloy structural steel 20MnTiB, 40B, 35VB. High-strength bolts are available in large hexagon bolts and torsion shears. The steel structure specification stipulates that the material of the high-strength bolts shall comply with the provisions of the current standards.


       The pre-tension of the high-strength bolt is determined by the yield of the material and the effective area of the bolt and taking into account a certain coefficient of friction. The pre-tension of the high-strength bolt is established by tightening the nut during construction. The methods for tightening (tightening) the nut are as follows:


      (a) Torque method


      According to the relationship between the torque M and the pre-tension, first tighten the nut with a normal wrench, and then use a special wrench that can display the torque value to screw to the specified torque value.


       (2) Corner method


      According to a method in which the angle of rotation of the nut is proportional to the pre-tension of the bolt after the close contact between the layers, a method is determined.


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


     (3) Twist the tail of the bolt


       For torsion-cut high-strength bolts, this bolt has a special tail. When tightening, use a special wrench to cover the bolt and the tail of the bolt. One sleeve rotates forward and the other sleeve reverses. When the nut is tightened to a certain extent, the tail of the bolt is broken. Since the notch depth of the bolt tail is determined by the relationship between the breaking torque and the pre-tensioning tension, the corresponding pre-tensioning value is reached when the wire is broken.


       In high-strength bolted joints, the coefficient of friction has a large influence on the 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 reverse force, which are 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 component within the connection range should be treated during construction. The method of treatment is sandblasting and wire brush cleaning. In the design, according to the engineering situation, the treatment method with large friction coefficient should be used as much as possible, and clearly marked on the construction drawing.


      In addition to the above treatment methods, there is a treatment method of grinding the contact surface with a portable electric grinding wheel, the grinding direction is perpendicular to the direction of the force, and the anti-slip coefficient is equivalent to sandblasting.


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