What Are High Strength Friction Grip Bolts – Difference Between HSFG and Normal Bolts

1) Raw materials

HSFG bolts are made of high-strength materials. The screws, nuts, and washers of HSFG are made of high-strength steel, commonly used as 45# steel, 40# boron steel, and 20# manganese titanium boron steel. Ordinary bolts are usually made of Q235 steel.

2) Strength grade

HSFG bolts usually have two strength grades of 8.8s and 10.9s, most of which are 10.9. The strength grade of ordinary bolts shall be low, generally 4.4, 4.8, 5.6, and 8.8.

3) Stress characteristics

HSFG bolts exert pre-tension and transmit external force by friction. Ordinary bolt connection transmits shear force by means of bolt rod shear resistance and hole wall pressure bearing. The pre-tension generated when tightening the nut is very small, and its impact can be ignored. In addition to its high material strength, HSFG bolts also exert a large pre-tension on the bolt, so that there is a large friction force perpendicular to the screw direction, and the pre-tension, anti-slip coefficient, and steel type directly affect the bearing capacity of HSFG bolts. The minimum specification of HSFG bolts is M12, and M16 – M30 is commonly used. The performance of oversized bolts is unstable, so they should be used with caution in design.

4) Usage

The bolt connection of the main components of the building structure is generally connected with HSFG bolts. Ordinary bolts can be reused, while HSFG bolts cannot be reused. HSFG bolts are generally used for permanent connection. HSFG bolts are pre-stressed bolts. The torque wrench is used to apply the specified pre-stressed force for friction-type bolts, and the box head is screwed off for pressure-type bolts. Common bolts have poor shear resistance and can be used in secondary structures. Ordinary bolts only need to be tightened.

5) Durability

HSFG bolts can bear more load and have a longer service life, but at the same time, the use cost will be higher.

The core difference between high-strength friction grip bolts and ordinary bolts is not the strength of the materials used, but the form of stress. The essence is whether to apply preload and use static friction to resist shearing. Under the same grade, the design values of tensile strength and shear strength of ordinary bolts are higher than those of high strength friction grip bolts. The design strength of tensile and shear resistance of high-strength friction grip bolts is lower than that of ordinary bolts. The essence of its high strength is that no relative slip is allowed in the joints during normal operation, that is, small elastic-plastic deformation and large joint stiffness.