Strength of high strength bolts

High strength bolts are mainly used to connect steel plates in steel structure engineering. High strength bolts are divided into torsion shear type high strength bolts and large hexagon high strength bolts. The large hexagon high strength bolt belongs to the high strength grade of ordinary bolt, while the torsion shear type high strength bolt is an improved type of large hexagon high strength bolt with better structure. The structure of high strength bolt must be tightened first and then tightened. The initial tightening of high strength bolts requires the use of impact type electric wrench or torque adjustable electric wrench. However, there are strict requirements for the final tightening of high strength bolts. The torque type electric wrench must be used for the final tightening of high-strength bolts, and the torque type electric wrench must be used for the final tightening of high-strength bolts.

Why is high strength bolt called high strength? Is it because it is earthquake resistant? In fact, high strength screws are called high strength bolts because they are made of different materials. High strength screws are made of high-strength steel or bolts that require greater preload. High strength screw is to use a special wrench to tighten the nut, so as to make the bolt produce controllable huge pre tightening force. Through the nut and the base plate, the same pre pressure is generated on the connected parts. High strength screw is mainly used for the connection of bridge, rail, high pressure and ultra-high pressure equipment. According to the specified performance grade of bolts above grade 8.8, they are called high strength bolts. At present, only M39 is listed in the national standard. For large size specifications, especially high strength bolts with length greater than 10-15%, domestic production is still very short. In terms of strength grade, high strength screws are widely used. The commonly used strength grades are 8.8s and 10.9s, of which the highest is 10.9s. The strength grade of ordinary bolts is low, usually 4.4, 4.8, 5.6 and 8.8.

From the point of view of stress characteristics: high strength bolts exert prestress and transmit external force through friction. The shear force of ordinary bolt connection is transmitted by the shear resistance of bolt rod and the pressure of hole wall. The pre tightening force produced by tightening nut is very small, and its influence can be ignored. In addition to the high material strength of high-strength bolts, a large amount of prestressing force is applied on the bolts, resulting in the extrusion pressure between the connecting members, thus producing great friction force in the direction perpendicular to the screw direction, and the preload, anti-skid coefficient and various materials of steel will 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 minimum specification of high-strength bolts is M12, usually m16-m30. The performance of super large bolt is unstable, so it should be used carefully in design.

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