The requirements for fasteners should have sufficient yield strength, moderate stiffness and high toughness. The material used for higher strength bolts is usually made of medium carbon steel No. 35 and is quenched and tempered into a metallographic structure of sorbite. Therefore, the comprehensive mechanical properties such as strength, plasticity and impact toughness are obtained. The clamping force of the coupled part is achieved by the elastic deformation of the bolt and the connected part. The rigidity of the bolt should be moderate, not too large, the bolt is not as hard as possible, and the bolt must meet the clamping force. Higher toughness and high toughness metal under the action of variable load, the ability to absorb energy is increased due to intragranular friction, which can quickly alleviate and eliminate vibration, moderate or low stiffness and high toughness. It can improve the anti-fatigue performance of bolts, so the selection of bolts should be based on the state and nature of the force, rather than simply the higher the performance level of the bolts. The higher the strength, stiffness and hardness, the more plastic. Reduced toughness and weakened fatigue resistance. The fastening mechanism of the bolt must have sufficient pre-tightening bolts. The pre-tightening force of the bolt can be obtained by converting the torque applied to the bolt. The pre-tightening force causes the bolt to generate tensile stress, and at the same time, the coupled member generates compressive stress for fastening purposes. The bolt stiffness is C1, the tensile deformation is δ1, the stiffness of the joint is C2, the compression deformation is δ2, the tensile force and compression force of the bolt are equal to F1, and the bolt is combined with the force deformation diagram of the coupled part. Together, F1 is the preload.
The pre-tightening force is set by the fastener to set the pre-tightening force to ensure that the screen box and the steel frame are always in a safely closed state under any working condition. The preload force is determined by the force state described in the previous section. It should allow the fastener to reliably withstand the maximum inertia pulling force and reliably transmit the lateral maximum inertial friction shear force by means of the tightening friction. And leave enough safety margins. In the case of flat screen work, the maximum inertia pulling force of the joint bolt to withstand the screen box is equal to the maximum inertia friction shear force. However, for the bolt force, the latter is much larger than the former, because the friction shear force is tangent to the pre-tightening force, and it is necessary to find the equivalent pre-tightening force that matches the friction shear force. Let f be the friction coefficient of the joint surface, R be the frictional force opposite to the maximum friction shear force on the joint surface, and T be the normal force of the joint surface (ie, the preload force). According to Coulomb's law, f=R/T, can be written as T=R/f, f value is much less than 1, then T is much larger than R, the absolute value of T. This connection extends the length of the screw and improves the fatigue resistance. It should be mentioned that if the nut block is machined into a full thread, or the nut block is placed in the reverse position, the end of the thread is in the shear plane, which is incorrectly designed. If the screw and the nut block are not concentric, When equipped with interference, it is easy to cause fatigue fracture of the root of the thread (in the shear plane) under the action of alternating shear. If the above method cannot be adopted due to structural restrictions, the pad can be thickened on the side of the bolt head to extend the length of the bolt, and the bolt hole of the screen box flange can be appropriately enlarged to fit the bolt freely, and the hole facing the thick pad and the bolt Normally fit the machining, the bolts can be freely assembled in place without interference, and after the clearance is confirmed by hand, the thick pads are spot welded, the fasteners are tightened, and the thick pads are welded firmly. As shown. For the same reason, it is also OK to place a thick pad on the side of the steel frame. In this way, concentric assembly is achieved for the coupled members of different sizes and sizes, and the fatigue resistance is also improved. In addition, in order to increase the friction between the flanges, a long strip of steel can be added to change the friction surface from 1 to 2. The surface quality requires that the fastener surface is not allowed to have scratches, bumps and other hard scratches. There is anti-corrosion treatment, and there must be a performance level mark. The tanning process is used to make the surface exhibit compressive stress, which can offset part of the tensile stress during work to improve the fatigue resistance. If shot peening, the effect is better. Fasteners shall comply with fastener assembly requirements. For fasteners that only function as clamping, it is required to be freely fitted into the bolt holes and fastened. However, for a porous joint with a large area and a long hole spacing, although the diameter of the bolt and the bolt hole is 1 to 2 mm, it is difficult to ensure free assembly under normal processing conditions, and the final assembly often occurs. One or two bolts cannot be freely loaded. There is also a case where the flat-shaped steel pipe structure is flat, it is difficult to realize the reaming hole positioning bolt, but after assembling with many bolts, there will always be several tightly fitting bolts, which directly bear the transverse shear force and play a similar The function of the reaming bolts is obviously random, disordered and not very reliable.
The assembly process should include the grinding operation. When there is interference, it can be manually expanded or used to meet the fastener assembly requirements. Reaming Hole Locating Bolt Problem For vibrating machinery, the frictional force generated by the bolt pre-tightening force alone is not very reliable. It is necessary to directly bear the transverse shear force with the reaming bolt. When the flat screen works, the reaming bolts are subjected to both the longitudinal pulsating tensile force and the transverse alternating shearing force. The bolt strength is calculated according to the fourth strength theory. It is worth mentioning that the tensile elastic modulus of the steel is The amount E and the tangential elastic modulus G represent the rigidity of the steel. If the material has a Poisson's ratio of about 0.2, E=2.5G, indicating that the tangential stiffness of the bolt is much smaller than the axial stiffness. Therefore, the shearing force easily causes the bolt to be plastically deformed, so that a fatigue source is formed. Therefore, the shear plane of the hinged hole bolt subjected to the composite load is a dangerous section, and attention should be paid to the selection of materials and processes. Spring washer problems Spring washers are widely used on domestic flat screens. The function of the spring washer is to increase the friction of the thread by its own elastic force to prevent the nut from loosening. This simple anti-loose device is effective under static load, but it is not as reliable under static load as under static load. When the flat screen is working, although the spring washer is fastened by the nut, the maximum pulsating tensile force of 4 times/s and the maximum frictional shear force of 8 times/s drive the pre-tightening force to reciprocate within a certain amplitude. The tightening force is transmitted through the spring washer. Although the vibration frequency of the flat screen is not too high, the amplitude λ is large, so the vibration intensity λω2/g is still not small (g is the gravitational acceleration). This kind of large and small impact pre-tightening force acts on the thread friction surface. After a long time, there may be slight creeping, which gradually destroys the self-locking state and leads to failure. Therefore, the spring washer should be used with caution, and it should not be installed in the same part of the bolt axial direction as the vibration direction, otherwise it will leave hidden dangers. Conclusion From the flat screen transmission system, the screen box-steel frame fastener plays an important role in ensuring the reliable operation of the flat screen. In the long-term production technology and after-sales service practice, this is gradually recognized. To solve the problem of reliability and durability of flat screen, it is necessary to study and improve the fatigue problem of important fasteners, and take measures to try to reduce the stress level of the fastener below the fatigue limit of the material. The second is to ensure that there is no interference assembly, and forcible assembly is prohibited, because the ability of metal materials to accept tangential stress is much weaker than the ability to accept positive stress. Under unfavorable conditions, 10 ~ 20MPa may cause plastic deformation. , causing serious consequences. It is very important for the operator to recognize this.
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