Strength requirements of the screw material

Update:01 Sep 2020
Summary:

The material of ordinary screws is made of Q235 (ie A3) […]

The material of ordinary screws is made of Q235 (ie A3). The material of high-strength screws is 35# steel or other high-quality materials. After being made, heat treatment is carried out to improve the strength. High-strength screws can bear a greater load than ordinary screws of the same specification. The difference between the two is the difference in material strength.

   


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

From the perspective of strength level: high-strength screws 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 screws is lower, generally 4.4, 4.8, 5.6 and 8.8. 

From the point of view of force characteristics: high-strength screws exert pre-tension and transmit external force by friction. Ordinary screw connections rely on the shear resistance of the bolt rod 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 screws also impose a lot on the screw. The large pre-tension force produces an squeezing force between the connecting components, thereby causing a great friction force perpendicular to the screw direction. Moreover, the pre-tension force, the anti-slip coefficient and the type of steel directly affect the bearing capacity of the high-strength screw. According to the force characteristics, it is divided into pressure type and friction type. The calculation methods of the two are different. The minimum specification of high-strength screws is M12, and the commonly used are M16~M30. The performance of oversized screws is unstable and should be used carefully in the design.