Why is high-strength screw called high strength? Is it because it is earthquake resistant? In fact, high-strength screws are called high-strength because of the different materials. The high-strength screws are made of high-strength steel or bolts that require a large preload. The high-strength screws are tightened with a special wrench to create a large and controlled pre-tension of the bolt. Through the nut and the backing plate, the same amount of pre-stress is also applied to the connected parts. High-strength screws are often used for the connection of bridges, rails, high-pressure and ultra-high pressure equipment.
Several concepts about high strength screws:
According to the specified performance level of bolts above 8.8, it is called high-strength bolts. The current national standard is only listed to M39. For large-size specifications, especially high-strength bolts with lengths greater than 10~15 times, domestic production is still short-term. .
From the perspective of strength grade: high-strength screws are increasingly used. Commonly used 8.8s and 10.9s two intensity levels, of which 10.9 level is mostly. Ordinary bolt strength grades are low, generally 4.4, 4.8, 5.6 and 8.8.
From the point of view of the force characteristics: the high-strength bolt exerts a pre-tension and transmits the external force by friction. Ordinary bolt connection relies on the shearing of the bolt and the pressure of the hole wall to transmit the shear force. When the nut is tightened, the pre-tension is small, and the influence is negligible. However, in addition to the high material strength, the high-strength bolt also exerts a large bolt. The pre-tensioning force causes a pressing force between the connecting members, so that a large frictional force is perpendicular to the screw direction, and the pre-tensioning force, the anti-slip coefficient and the type of the steel directly affect the bearing capacity of the high-strength bolt. According to the characteristics of the force, it is divided into bearing type and friction type. The calculation methods are different. High-strength bolts have a minimum specification of M12, and M16~M30 are commonly used. The performance of bolts with large specifications is unstable, and should be used with caution in the design.
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