Detailed explanation of the classification of bolt connections for the selection of fasteners

There are five classification methods of bolt connection, and the use rate of bolts for this kind of connection fasteners is quite high. Nuts account for 60% to 70% of fasteners. This type of connection is the most representative of fasteners and is also the leading product of fasteners. It can be divided into:

(1) Classified by force form

1、Tensile bolt connection

Tensile connection is a connection in which the bolt transmits force along the axis. The load acts along the axis of the bolt, and the force is transmitted by the tension of the bolt.

2. Shear bolt connection

Shear bolt connection is that the transmitted load is perpendicular to the bolt axis, and the force is transmitted by the bolt rod shear and extrusion.

3, Tensile and Shear Connection

The transmitted load is a combination of tensile and shear

(2) Classification according to installation status

1. Bolt connection with pre-tightening force

The bolts and nuts should be tightened during installation, and the force should be pre-applied to ensure that they are tight. Most connections belong to this type of connection.

2. Bolt connection without pre-tightening force

Nuts are not tightened during installation, and the bolts are stressed only when they are under working load. The application range is small, such as lifting hooks and suspension bolts.

(3) Classified according to the thread of the product used

Generally, ordinary threads are used, which are divided into coarse and fine threads. Coarse thread has a high degree of tooth contact, strong abrasion resistance, and is not easy to slip. The fine thread has good self-locking performance, and the screw rod has high strength, which is beneficial to withstand shock, vibration and variable load.

(4) Classified by material and performance level

The most widely used bolt materials are low-carbon steel and carbon steel. For bolts subjected to impact, vibration, and variable load, alloy steel is used; when there are special requirements, stainless steel, aluminum alloy, copper alloy and titanium alloy are used.

The national standard stipulates that carbon steel and alloy steel bolts are divided into 10 strength grades of 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9 according to their mechanical properties. Bolts below grade 6.8 are made of low carbon steel or medium carbon steel without heat treatment; grade 8.8~10.9 bolts are made of medium carbon steel or low carbon alloy steel and have been heat treated; grade 12.9 bolts are made of alloy steel and have been heat treated.

(5) Simple classification of high-strength bolts

High-strength connection is a steel structure connection method developed after riveting and welding. It has the advantages of simple construction, detachability, replacement, good stress, fatigue resistance, no looseness, and safety. High-strength bolt connections can be divided into three types: friction type connection, tension type connection and pressure type connection according to their stress conditions.

1. Friction type connection

This kind of connection is fastened with high-strength bolts at the joints of the components, so that the connecting plates are tightly attached, and the friction force generated between the contact surfaces between the connecting pieces is used to transmit external force. The role of the bolt in the connection is to apply contact pressure between the connecting parts, thereby causing friction on the contact surface. When this kind of connection is under force, the bolt is only stretched and not sheared. Even under repeated loads, it will not cause the bolt axial force to drop. Therefore, in the friction joint, the fatigue strength is between the base material and the porous member. This kind of connection stress transfers smoothly and the joint has good rigidity. Generally, it is not necessary to check the bearing pressure of the board. The high-strength bolt connection usually refers to this friction type connection

2, Tension type connection

This is a connection method that transmits the axial force acting on the bolt. It is the same as the friction type connection. It also uses the pressure generated between the components when the bolt is tightened for stress transmission. However, the form of stress transmission is fundamentally different from friction type connection. The characteristic of the tension connection is that under the action of external force, the additional axial force of the bolt changes very little, and the joint obtains greater rigidity.

3, pressure-bearing connection

This is a connection method that uses high-strength bolts to connect and fasten joint components, so that the friction generated between the components and the shear force of the bolt central axis and the bearing pressure of the components act simultaneously, thereby transferring stress. This connection method has the combination of high-strength bolt friction connection and riveted shear connection. It belongs to the shear type connection, so it is restricted by some conditions in the use occasion, but in general it can be as friction connection. To use. According to the shape and construction method of pressure-bearing connection, it can be divided into three connection methods, namely high-strength bolt pressure-bearing connection, grinding high-strength bolt pressure-bearing connection and driving-in high-strength bolt pressure-bearing connection.

With the increasing development of high-strength bolts, there are more and more types of high-strength bolts. In addition to ordinary large hexagon head high-strength bolts, torsion shear type high-strength bolts, high-strength bolts with crown nuts, Notch high-strength bolts, drive-in high-strength bolts, refined high-strength bolts and channel bolts. The head type of the drive-in high-strength bolts includes large hexagon head, round head, plate head, etc. The outer diameter of the thread is slightly smaller than that of the screw. The screw has ribs. The bolt hole is slightly smaller than the outer diameter of the bolt. The bolt is driven in and bolted during construction. There is no gap between the hole and the screw. Refined high-strength bolts are used for pressure-bearing connection. The screw has been processed. The gap between the screw hole and the screw is 0.2mm. The screw of the grooved bolt is provided with longitudinal and circumferential notches, which can engage each other in the bolt holes to prevent Excessive sliding between the connecting parts during the force process.