High strength bolts are bolts made of high-strength steel or requiring a large preload, which can be referred to as high strength bolts. High strength bolts are mostly used for connecting bridges, steel rails, high-voltage and ultra-high-voltage equipment.

From the perspective of raw materials: high-strength bolts are made of high-strength materials. The screw, nut, and washer of high-strength bolts are made of high-strength steel, commonly used as 45 # steel, 40 boron steel, 20 manganese titanium boron steel, and 35CrMoA. Ordinary bolts are commonly made of Q235 (equivalent to A3 in the past) steel.

From the strength level, high-strength bolts are increasingly widely used. Two strength levels, 8.8s and 10.9s, are commonly used, with 10.9 being the most common. The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6, and 8.8.

From the perspective of stress characteristics: high-strength bolts exert pre tension and transmit external forces through friction. Ordinary bolt connections rely 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 pressure generated is very small, and its impact can be ignored. In addition to its high material strength, high-strength bolts also apply a large pre pressure to the bolt, causing a squeeze force between the connecting members, resulting in a large friction force perpendicular to the screw direction. Moreover, the pre pressure, anti slip coefficient, and steel type all directly affect the bearing capacity of high-strength bolts. According to the stress characteristics, it is divided into pressure bearing type and friction type. The calculation methods for the two types are different. The minimum specification of high-strength bolts is M12, and commonly used bolts are M16 to M30. The performance of oversized bolts is unstable, and they should be used with caution in design.

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