Seen from raw materials, high strength bolts use high strength materials. High strength screw bolts, nuts and washers are produced by high-strength steel, most commonly used steel 45, 40 boron steel, 20 Mn-Ti-B steel. Ordinary bolt is made of Q235 steel. Judged from the intensity level, high-strength bolts are used widely. Two common strength grades are 8.8s and 10.9s, which the majority is 10.9s. Ordinary bolt strength grade are usual 4.4s, 4.8s, 5.6s and 8.8s. Viewed from the loading characteristics, bolts transmiss external force by imposing pre-tension and friction. Ordinary bolts connection transfer shear force by rod bolts' shearing and hole-walls' bearing pressure. For the preload produced by tightening the nut is small, its impact can be negligible. The high-strength bolts has not only high strength of material, but also impose large pre-tension in order to produce toextrusion pressure between components connections. And then it makes much friction along the direction perpendicular to the screw. The pre-pull, anti-slip coefficient and high-strength steel types have a direct impact of the bearing capacity of bolts. According to the mechanical characteristics, it can be divided sub-pressure type and friction type. There are two different calculation methods. The minimum specification of high-strength bolts is M12, and the most commonly used ones are M16-M30. If the specification of the bolt is too large, it cann't stable performance very well. It should be used cautiously in design. Source: Goodbolts. Com
Source:
http://www.goodbolts.com/read.asp?id=989
