High tensile bolts
High tensile bolts made of high-strength steel, or bolts that require greater pre-tightening force, can be called high-strength bolts. High-strength bolts are mostly used for the connection of bridges, rails, high-pressure and ultra-high pressure equipment. The fracture of this kind of bolt is mostly brittle fracture. High-strength bolts applied to ultra-high pressure equipment need to be prestressed in order to ensure the sealing of the container. Several concepts about high-strength bolts 1. According to the requirements of the performance level of bolts above 8.8, it is called high-strength bolts.
The difference between High tensile bolts and ordinary bolts:
High strength bolts can bear a larger load than ordinary bolts of the same specification.
The material of ordinary bolts is made of Q235 (namely A3). The material of high-strength bolts is 35# steel or other high-quality materials. After being made, heat treatment is performed to improve the strength.
The difference between the two is the difference in material strength.
From the perspective of High tensile bolts 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 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, etc. Ordinary bolts are usually made of Q235 (equivalent to A3 in the past) steel.
From the perspective of High tensile bolts strength level:
high strength bolts are increasingly used. Two strength levels of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6 and 8.8.
From the point of view of High tensile bolts the force characteristics:
high-strength bolts exert pretension and transfer external force by friction. Ordinary bolted connections rely on the shear resistance of the bolt rod and the pressure of the hole wall to transmit the shear force. The pre-pressure generated when the nut is tightened is small, and its effect can be ignored. In addition to the high material strength of the high-strength bolt, it also applies a lot to the bolt. The pre-compression force produces an squeezing force between the connecting components, so that there is a great friction force perpendicular to the screw direction, and the pre-compression, anti-slip coefficient and steel type directly affect the bearing capacity of the high-strength bolt. According to the characteristics of force, it is divided into pressure type and friction type. The calculation methods of the two are different. The minimum specification of high-strength bolts is M12, and commonly used are M16~M30. The performance of oversized bolts is unstable and should be used with caution in design.
The difference between High tensile bolts friction type and pressure type connection:
The High tensile bolts connection is to clamp the plates of the connecting plate through the large tightening pre-pressure in the bolt rod, which is sufficient to generate a large friction force, thereby improving the integrity and rigidity of the connection. When subjected to shear force, according to the design and The different force requirements 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 bolts, they are calculated in terms of calculation methods, requirements, and scope of application. All are very different. In the shear design, the high-strength bolt friction type connection is the limit state when the external shear force reaches the maximum possible friction force provided by the bolt tightening force between the contact surfaces of the plate, which is to ensure that the internal and external shear force of the connection does not exceed during the whole use Maximum friction.
The plates will not undergo relative sliding deformation (the original gap between the screw and the hole wall is always maintained), and the connected plates will be stressed as a whole by elasticity. In the shear design, the external shear force is allowed to exceed the maximum friction force in the high-strength bolt pressure-bearing connection. At this time, relative sliding deformation occurs between the connected plates until the bolt rod contacts the hole wall, and then the connection depends on the bolt rod Shaft shear and hole wall pressure and the friction between the contact surfaces of the plate jointly transmit force, and finally the shaft shear or hole wall pressure failure is regarded as the limit state of connection shear. In short, friction-type high-strength bolts and pressure-bearing high-strength bolts are actually the same kind of bolts, it is just whether slippage is considered in the design. Friction-type high-strength bolts must not slide, and the bolts do not bear shearing force. Once slipped, the design is considered to have reached a failure state, which is technically mature; pressure-bearing high-strength bolts can slide, and the bolts also bear shearing force. The final damage is equivalent to ordinary Bolt damage (bolt cut or steel plate crushed).
From the perspective of use:
High strength bolts are generally used for the bolt connection of the main components of the building structure. Ordinary bolts can be reused, but high strength bolts cannot be reused. High-strength bolts are generally used for permanent connections.
High-strength bolts are pre-stressed bolts. The friction type uses a torque wrench to apply the specified pre-stress, and the pressure-bearing type unscrews the plum head. Ordinary bolts have poor shear resistance and can be used in secondary structural parts. Ordinary bolts only need to be tightened.
Ordinary bolts are generally 4.4, 4.8, 5.6 and 8.8. High-strength bolts are generally 8.8 and 10.9, of which 10.9 is the majority.
Level 8.8 is the same level as 8.8S. The force performance and calculation methods of ordinary bolts and high-strength bolts are different. The force of high-strength bolts is first applied to the interior of the pretension P, and then friction resistance is generated on the contact surface between the connected parts to withstand the external load, while the ordinary bolts directly bear the external load.
High-strength bolt connection has the advantages of simple construction, good mechanical performance, detachability, fatigue resistance, and no loosening under dynamic load, and is a promising connection method.
High-strength bolts use a special wrench to tighten the nut, so that the bolt generates a huge and controlled pretension. Through the nut and the backing plate, the same amount of prestress is also generated on the connected parts. Under the action of pre-pressure, a relatively large friction force will be generated along the surface of the connected part. Obviously, as long as the axial force is less than this friction force, the component will not slip and the connection will not be damaged. This is a high-strength bolt connection The principle.
High-strength bolt connections rely on the friction between the contact surfaces of the connectors to prevent them from sliding. In order to make the contact surfaces have sufficient friction, it is necessary to increase the clamping force of the component and increase the friction coefficient of the component contact surface. The clamping force between the components is achieved by applying pre-tension to the bolts, so the bolts must be made of high-strength steel, which is why it is called high-strength bolt connection.
In the high-strength bolt connection, the friction coefficient has a great influence on the bearing capacity. Experiments show that the friction coefficient is mainly affected by the form of the contact surface and the material of the component. In order to increase the coefficient of friction of the contact surface, methods such as sandblasting and wire brush cleaning are often used to treat the contact surface of the components within the connection range.
Today’s advanced manufacturing represented by large aircraft, large power generation equipment, automobiles, high-speed trains, large ships, and large complete sets of equipment has entered an important development direction. As a result, fasteners will enter an important stage of development. High-strength bolts are used for the connection of important machinery. Repeated disassembly or various installation torque methods require high-strength bolts. Therefore, the surface condition and thread accuracy will directly affect the service life and safety of the host. In order to improve the coefficient of friction and avoid rust, seizure or seizure during use, the technical requirements stipulate that the surface should be treated with nickel-phosphorus plating. The thickness of the coating is guaranteed to be within the range of 0.02～0.03mm, and the coating is uniform, dense and without pinholes.
18Cr2Ni4W, 25Cr2MoV steel; bolt specification: M27～M48. Since this type of steel is easy to form a passivation film on the surface, and this passivation film will prevent the bolt from obtaining a chemical nickel-phosphorus layer with good adhesion, special pretreatment measures must be taken to remove the film first, and measures should be taken To prevent it from regenerating, it is possible to ensure a good bonding force between the plated layer and the substrate after plating. At the same time, due to the large geometric size of the bolt, the quality inspection of the nickel-phosphorus plating treatment and process has increased difficulty.
Friction-type high-strength bolts: suitable for steel frame structure beam and column connection, solid web beam connection, heavy crane beam connection in industrial plant, brake system and connection of important structures under dynamic load.
Pressure-bearing high strength bolts: can be used for shear connection in static load structures that allow a small amount of sliding or indirect dynamic load components.
Tensile high-strength bolts: When the bolts are under tension, their fatigue strength is low. Under dynamic load, their bearing capacity cannot easily exceed 0.6P (P is the allowable axial force of the bolt). Therefore, it is only suitable for use under static load. Such as flange butt joints and T-joints of pressure rods.
(1) Floating rust, oil stains, burrs on the bolt hole wall, welding bead, etc. should be cleaned up.
(2) After the contact friction surface is processed, the specified anti-scratch coefficient requirements must be met. The high-strength bolts used should have matching nuts and washers. They should be used as a set when in use and cannot be interchanged.
(3) When installing the friction surface of the processed component, it is not allowed to be stained with oil, dirt, etc.
(4) The friction surface of the module should be kept dry during installation and should not be operated in the rain.
(5) Strictly check and correct the deformation of the connected steel plates before installation.
(6) It is forbidden to hammer into the bolt during installation to prevent damage to the bolt thread.
(7) The electric wrench that is tested regularly when in use, to ensure the accuracy of the torque, and operate in the correct tightening sequence.
Main safety technical measures
(1) The size of the wrench opening of the adjustable wrench should be consistent with the size of the nut, and a small wrench should not be used to add casing. A dead wrench should be used for high-altitude operations. For example, a rope should be used to fasten the adjustable wrench, and the safety belt should be fastened.
(2) When assembling the connecting bolts of steel components, it is strictly forbidden to insert the connecting surface or probe the screw holes by hand. When picking and placing the shim plate, your fingers should be placed on both sides of the shim plate.