What are high strength friction bolts

Turned and fitted bolts are similar to 'ordinary' grade 4.6 and 8.8 bolts, but are machined on their shanks and installed in reamed holes. This create a very tighly fitting bolt, slip is negligible, and higher stresses can be used. Due to high cost of the bolts themselves and the costs of installation, these bolts are not often used.

High Strength DFriction Grip bolts (HSFG bolts) work in a different way. They are made from a material very similar to grade 8.8 bolts, but have different shaped heads and nuts. The nut is tightened in a carefully controlled manner to put a large pretension into the bolt shank. This has the effect of clamping together the plates between the bolt head and the nut. The mating surfaces are called 'faying' surfaces, and they must be flat and unpainted. Because the tightening requires careful supervision, this connection is relatively expensive and generally ued only where slip of a connection must be avoided, for example to avoid a connection working loose due to vibration, or where the direction of loading can be reversed due to normal imposed loads. They are not normally essential for reversal due to wind loads only.