There are five types of bolting connections, which are used at a high rate. Bolts and nuts account for 60% to 70% of fasteners. This type of connection is the most representative of fasteners and is the leading product of fasteners. It can be divided into:
(1) Classification by force form
1, tensile bolt connection
A 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 bolt. The thread that is screwed with the nut participates in the force.
2, shear bolt connection
The shear bolt connection is that the transmitted load is perpendicular to the axis of the bolt and is sheared and squeezed by the bolt rod.
3, tensile and shear connection
The transmitted load is a combination of tensile and shear resistance
(2) Classified according to the installation status
1, with pre-tightening bolt connection
When installing, the bolts and nuts should be tightened. It is necessary to pre-add the force of loosening to ensure the fastening. Most connections belong to this type of connection.
2, no pre-tightening bolt connection
The nut is not tightened during installation. The bolt is stressed when subjected to the working load, and the application range is small, such as lifting hooks and suspension bolts.
(3) According to the thread classification of the products used
Generally, ordinary threads are used, which are divided into coarse teeth and fine teeth. The tooth contact of the coarse thread has a high degree of wear resistance and is not easy to slide. The self-locking performance of the fine thread is good, and the strength of the screw rod is high, which is good for impact, vibration and load change.
(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 load change, alloy steel is used; when special requirements are required, 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 three strength grades according to mechanical properties: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9. Bolts below 6.8 are made of low carbon steel or medium carbon steel without heat treatment; 8.8~10.9 bolts are medium carbon steel or low carbon alloy steel and heat treated; 12.9 grade bolts are alloy steel and heat treated.
(5) Simple classification of high-strength bolts
The high-strength connection is a steel structure connection method developed after riveting and welding. It has the advantages of simple construction, detachable, changeable, good force, fatigue resistance, no looseness and safety. High-strength bolted joints 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 connection is fastened with high-strength bolts at the joint of the member, so that the layer of the connecting plate is in close contact, and the external force is transmitted by the frictional force generated between the contact faces of the connecting plate layers. The role of the bolt in the connection is to apply a contact pressure between the connectors, so that the contact surface produces friction. When the connection is stressed, the bolt is only pulled and not sheared, and even under repeated loads, the bolt axial force does not decrease. Therefore, in the friction joint, the fatigue strength is between the base material and the perforated member. This connection stress transfer is smooth and the joint is rigid. Generally, it is not necessary to check the bearing pressure of the board. The high-strength bolt connection usually referred to is the friction type connection.
2, tension type connection
This is a connection method for transmitting the axial force acting on the bolt, and is the same as the friction type connection, and is also subjected to stress transmission between the members when the bolt is tightened. However, the form of stress transfer and the friction type connection are essentially different. The characteristic of the tension type connection is that under the action of external force, the additional axial force of the bolt changes little, and the joint obtains greater rigidity.
3, pressure type connection
This is a connection method in which the joint member is fastened by high-strength bolts, and the frictional force generated between the members and the shearing force of the bolt center shaft simultaneously with the bearing pressure of the member, thereby transmitting stress. This connection method has the property of a combination of a high-strength bolt friction connection and a riveted shear type connection, and belongs to a shear type connection, so it is limited by some conditions in use, but in general, it can be connected by friction. To use. According to the shape and construction method of the pressure-type connection, it can be divided into three connection methods, namely high-strength bolt pressure connection, grinding high-strength bolt pressure connection and driving high-strength bolt pressure connection.
With the development of high-strength bolts, the form of high-strength bolts is also increasing. In addition to ordinary large hexagonal high-strength bolts, torsion-shear high-strength bolts, high-strength bolts with crown nuts, and high scores Strength bolts, drive-in high-strength bolts, refined high-strength bolts and channel bolts. The head type of the driven high-strength bolt has a large hexagonal head, a round head, a head, etc. The outer diameter of the thread is slightly thinner than the screw, the screw has a rib, the bolt hole is slightly smaller than the outer diameter of the bolt, and the bolt is driven in during construction. There is no gap between the hole and the screw. Refined high-strength bolts for pressure connection. The screw is machined. The screw hole and screw clearance are 0.2mm. The screw of the channel bolt has longitudinal and circumferential slots, which can be engaged with each other in the bolt hole to prevent Excessive sliding between the connectors during the force.
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