Selection Principle Of Bolts Of Different Grades

Many people who are just engaged in mechanical design may not understand what kind of bolt and bolt grade should be used in the specific design. As a professional fastener manufacturer, Xinchi Fastener Company has classified and summarized the methods of selecting bolts for common structures in as much detail as possible, and what kind of structure should use what kind of bolt grade, hoping to be helpful to you.

Let’s first introduce high-strength bolts and general bolts.

The tensile strength of high-strength bolts is relatively high, so it is mainly used in some projects such as outdoor, wind power or dynamic machinery. High-strength bolts are used more and more widely nowadays. Two strength grades of 8.8s and 10.9s are commonly used, among which 10.9 is the majority.

General bolts are generally used in some small machinery, such as turbines, mainly depending on the application environment and operation mode. The strength levels of ordinary bolts are lower than those of high-strength bolts, generally rated 4.4, 4.8, 5.6 and 8.8.

From the raw materials of high-strength bolts and ordinary bolts:

High-strength bolts are made of high-strength materials. The screw, nut and washer of the high-strength bolt are made of high-strength steel, commonly used 45 steel, 40 boron steel, and 20 manganese titanium boron steel. Ordinary bolts are commonly made of Q235 steel.

From the strength level of high-strength bolts and ordinary bolts:

High-strength bolts are increasingly used. Two strength grades of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. Common bolt strength levels are lower, generally 4.4, 4.8, 5.6 and 8.8.

From the characteristic of the force of high-strength bolts and ordinary bolts:

High-strength bolts apply pretension and transfer external forces by friction. The connection of an ordinary bolt relies on the shear resistance of the bolt rod and the pressure of the hole wall to transfer the shear force. When tightening the nut, the pre-tightening force of the ordinary bolt is small and its influence can be ignored. In addition to the high strength of the material, the high-strength bolt also exerts a lot of pre-tension, so that the connecting members produce extrusion pressure so that there is a lot of friction perpendicular to the direction of the screw. And the pre-tension, anti-slip coefficient and type of steel directly affect the bearing capacity of the high-strength bolt.

According to the stress characteristics, it can be divided into friction type high strength bolt and pressure type high strength bolt. The minimum specification of high-strength bolts is M12, usually M16~M30. The performance of oversized bolts is unstable, so they should be used carefully in the design.

Friction type high strength bolts and pressure type high strength bolts are actually the same kinds of bolts, the difference is only whether the design takes to slip into account. The friction-type high-strength bolt must not slide, the bolt does not bear the shear force, once the slip, the design is considered to reach the failure state. The friction-type high-strength bolt is relatively mature in technology. Pressure-bearing high-strength bolts can slide, bolts also bear shear force, and the final failure is equivalent to the failure of ordinary bolts (bolt shear or steel plate compression).

From the use of high-strength bolts and ordinary bolts:

The bolt connection of the main component of the building structure generally adopts high strength bolt connection. Common bolts can be reused, but high-strength bolts cannot be reused. High-strength bolts are generally used for permanent connections.

High strength bolt is a prestressed bolt, a friction-type high-strength bolt adopts a torque wrench to apply the specified prestress, while a pressure-type high-strength bolt is screwed off the plum head. Ordinary bolts have poor shear performance and can be used in secondary structural parts. Ordinary bolts only need to be tightened.

Anji Xinchi Fastening Piece Co., Ltd is a one-stop solution company for fasteners, which can offer a variety of high-quality fasteners, including t-nuts, screws, washers, sleeves, bolts, rivets, and so on. We are the designated fastener supplier to “STAPLES”. Our fasteners have many advantages, such as large output, fast delivery, high quality, zero complaints, low price, and high fit. If you need to buy fasteners or ask for fastener knowledge, please contact us.