High-strength Bolts

2026-09-20 16:37:48

A high-strength bolt is a type of connector capable of bearing heavier loads. Compared with ordinary bolts, it shares basically the same shape and structure, yet it can generate much higher pre-tension. It delivers greater friction between connected components and stronger extrusion force, thus providing higher shear resistance to withstand larger loads. Bolts manufactured from high-strength steel or those requiring substantial pre-tightening force are all referred to as high-strength bolts.

High-strength bolts are widely used for connections in bridges, steel rails, high-pressure and ultra-high-pressure equipment. Fracture of such bolts is mostly brittle fracture. For high-strength bolts applied to ultra-high-pressure equipment, large pre-stress must be applied to guarantee sealing of pressure vessels.

Strength Grades

High-strength bolts are seeing growing application. The commonly adopted grades are 8.8S and 10.9S, with Grade 10.9 being the more prevalent. Ordinary bolts have lower strength grades, typically 4.4, 4.8, 5.6 and 8.8.

Force-bearing Characteristics

High-strength bolts transfer external force by applying pre-tension and relying on friction. Connections with ordinary bolts transfer shear force through bolt shank shear and hole wall bearing. The pre-tension produced when tightening nuts is negligible. In contrast, besides high material strength, high-strength bolts are subjected to large pre-tension, which creates compressive force between connected members. This yields considerable friction perpendicular to the bolt shank. The bearing capacity of high-strength bolts is directly affected by pre-tension, slip coefficient and steel type.

High-strength bolts are classified into bearing-type and slip-critical types according to force transfer mechanism, with different calculation methods for the two types. The minimum size of high-strength bolts is M12; sizes from M16 to M30 are commonly used. Extra-large bolts exhibit unstable performance and shall be used cautiously in design.