The process of rolling thread for high-strength bolts

The process of rolling thread for high-strength bolts

Generally speaking, high-strength bolts are formed through cold working. Rolling thread is a processing method that utilizes the principle of plastic deformation to form spiral teeth into high-strength bolts. High strength bolts (also known as thread rolling plates) are a type of high-strength bolt formed by combining plastic deformation principles with high-strength bolts.

This process uses a rolling die with the same pitch and shape as the machined high-strength bolt threads (i.e. backing plates), which extrudes cylindrical billets during the construction process to continuously rotate the billets. The ultimate goal is to transfer the contour above the rolling die to the small billet of high-strength bolts, thereby forming threads.

It should be noted that the rolling speed of thread processing should not be too high, as excessive speed may reduce efficiency and cause separation or derailment on the surface of high-strength bolt threads. If the number of rolls is too small, which is not conducive to the formation of high-strength bolt threads, the thread diameter is prone to loss of roundness. The initial rolling pressure is very high, and in this state, the life of the mold will be greatly shortened.

From the perspective of raw materials, high-strength bolts are made of high-strength materials. The screws, nuts, and washers of high-strength bolts are all made of high-strength steel, commonly used in 45 # steel, 40 boron steel, and 20 manganese titanium boron steel. Ordinary bolts are commonly made of Q235 steel.

From the perspective of strength level, high-strength bolts are increasingly widely used. Two strength levels, 8.8s and 10.9s, are commonly used, with 10.9 being the most common. The strength level of ordinary bolts is generally lower, including 4.4, 4.8, 5.6, and 8.8 levels.

3. From the perspective of stress characteristics, ordinary bolt connections rely on the shear resistance of the bolt rod and the bearing pressure of the hole wall to transmit shear force, while high-strength bolts not only have high material strength, but also apply a large pre tension force to the bolts, causing squeezing pressure between the connecting components, and thus creating a large frictional force perpendicular to the screw direction.

High strength bolt connection is a method of clamping the plates of the connecting plate through a large tightening pre tension force inside the bolt rod, which is sufficient to generate a large amount of friction force, thereby improving the integrity and stiffness of the connection. When subjected to shear force, according to different design and force requirements, it can be divided into two types: high strength bolt friction type connection and high strength bolt pressure bearing type connection. The essential difference between the two is that the limit state is different. Although they are the same type of bolt, in terms of calculation method There are significant differences in requirements, scope of application, and other aspects.

In shear design, the high-strength bolt friction type connection is the ultimate state where the external shear force reaches the maximum possible frictional force provided by the bolt tightening force between the contact surfaces of the plate, which ensures that the internal and external shear forces of the connection do not exceed the maximum frictional force throughout its entire service life. The plate will not experience relative slip deformation (the original gap between the screw and the hole wall is always maintained), and the connected plate will be subjected to elastic overall force.