When forces are large or rigid connections are required, a special type of bolt called HSFG bolt is often used. What are HSFG bolts? What is the difference between high strength friction grip bolts and normal bolts, let’s learn about those in this article.
HSFG (High Strength Friction Grip) bolts are usually made from medium carbon steel and have a higher tensile strength than the ordinary black bolt, as well as improved vibration and impact resistance. Bolts of class 8.8 and 10.9 are commonly used. HSFG bolts with induced initial tension are called high-strength friction grip bolts or HSFG bolts.
There are two different types of high strength friction grip bolts, they are parallel shank and waisted shank type bolts. A parallel shank bolt is designed not to slip at service load but slip into bearing at ultimate load. While a waisted shank bolt is designed not to slip both at service load and ultimate load. Waisted shank bolts are more grid and parallel shank bolts are more widely used.
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.
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.
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.