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The difference between high-strength bolts and ordinary bolts

Update:30-01-2021
Summary:

High-strength bolts can bear greater load than ordinary […]

High-strength bolts can bear greater load than ordinary bolts of the same specification.
The material of ordinary bolts is made of Q235 (ie A3).
The material of high-strength bolts is 35# steel or other high-quality materials. After being made, heat treatment is performed to improve the strength.

The difference between the two is the difference in material strength.
From the perspective of raw materials:
High-strength bolts are made of high-strength materials. The screws, nuts and washers of high-strength bolts are all made of high-strength steel, commonly used 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, etc. Ordinary bolts are usually made of Q235 (equivalent to A3 in the past) steel.

From the perspective of strength level: high-strength bolts are increasingly used. Two strength levels of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6 and 8.8.

From the point of view of the force characteristics: high-strength bolts exert pretension and transmit external force by friction. Ordinary bolted connections rely on the shear resistance of the bolt and the pressure of the hole wall to transmit the shear force. The pretension generated when the nut is tightened is very small, and its effect can be ignored. In addition to its high material strength, high-strength bolts also impose a large amount on the bolt The pre-tension force produces an squeezing force between the connecting components, so that there is a great friction force perpendicular to the screw direction, and the pre-tension force, the anti-slip coefficient and the type of steel directly affect the bearing capacity of the high-strength bolt.

According to the characteristics of the force, it is divided into pressure type and friction type. The calculation methods of the two are different. The minimum specification of high-strength bolts is M12, and commonly used is M16~M30. The performance of oversized bolts is unstable and should be used carefully in the design.
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