Design and manufacture of vibration resistance of bolted joints
Engineering practice has proved that bolts that work for a long time in a vibrating environment will have a fracture phenomenon when their working stress is much smaller than the strength limit, and there is no obvious residual deformation at the time of fracture, even if the bolt is made of a plastic material, it is broken. It is also almost the same as the brittle material fracture, so it is difficult to find the problem during maintenance. In this paper, combined with the actual work experience, from the aspects of fastener processing, assembly process and structural shape, it is helpful to improve the anti-vibration of fasteners. Sexual method i affects the vibration resistance of the bolted joint. Through the analysis of the bolt after fatigue and vibration, it is known that the fracture source first produces a fatigue source on the surface, the root of the bolt round, the mechanical scratch, the grain boundary, and the inclusion. The interface between the object and the base metal, the micro cracks and the corrosion points of the processing may become the fatigue source. After the fatigue source is generated, the crack gradually expands due to the alternating stress of the vibration. When the pressure is separated, the smooth surface is formed on the crack surface near the fatigue source, and the fatigue spokes and the fatigue arc appear, such as vibration. Under the action of cyclic stress, the crack will be further expanded. When a certain amount is reached, the static strength will be insufficient and the bolt will be suddenly broken. 2 The solution is to improve the anti-vibration performance of the bolt, in addition to considering the selection of materials and appropriate In addition to factors such as heat treatment and machining process, it is necessary to take appropriate measures from the aspects of bolt structure, strength and rigidity, and take appropriate measures in combination with the actual project. 21 Use the rolling method to squeeze the fastening bolts for important parts of the equipment. Hydraulic pressure can be used. The method compresses the transition fillet of the bolt head and the rod portion to improve the vibration fatigue strength of the joint. The specific measure is to roll the transition fillet of the bolt head and the rod portion, so that the compressive stress is generated at the round corner, and the cold stamping process is adopted on the head of the bolt, so that the fiber structure of the material exhibits a continuous streamline type. To improve the vibration resistance of the place.
22 The appropriate structural form is the structural form of the anti-vibration bolt. It is verified that the transition fillet r should have a value of 0. 2d., and the tapered surface transition can also be used to reduce the rigidity of the part, which is beneficial to improve its anti-vibration performance. Has been recognized by the designer, so in the design, the diameter d0 of the bolt polished rod can be reduced, the value is 0.8, A is the diameter of the root of the thread to take the rounded corner of the thread (0.1 å±® 0.22) t, t is the pitch In the same strength design, the stress concentration at the root of the thread is reduced, and the tensile strength of the bolt is not affected. From the theory of fatigue strength, it is known that the fatigue resistance is related to the stiffness as well as the strength. Under the condition of the bolt, the flexibility and deformability of the bolt are large, and the impact energy can be withstood under the same allowable stress to improve the flexibility of the bolt. The length of the bolt can be appropriately increased, and the cushion has a larger elastic modulus. Gaskets to meet their assembly requirements. If the diameter of the polished rod must be equal to or greater than the outer diameter of the threaded section due to some special requirements, it can be drilled in the center of the polished rod (as shown), which can also reduce the overall rigidity of the bolt. If the bolt exceeds a certain length, in order to avoid additional bending stress caused by the error caused by machining or assembly, the nut and the gasket in contact with it can be processed into a spherical structure (as shown) to have an automatic positioning function. In addition, the assembly should try to make the bolt only bear the axial force, avoiding the additional stress. The design of the nut is because the bolt and the nut are used together, so the structural form of the nut has a great influence on the anti-vibration performance of the bolt. . The distribution of the load along the height of the nut shows that the distribution of the load F along the height s of the nut is not uniform, and the threads of the first ring and the second ring of the nut bearing surface are the most stressed, and the force of the threads of each ring is gradually reduced. The thread bearing capacity of the 1st to 3rd turns can reach 75% of the entire bolt bearing capacity, while the thread after 8 turns is almost unstressed. Therefore, the height of the nut cannot increase the joint strength of the bolt. It is important to make the nut evenly to solve the problem. The nut can be designed as shown, so that the thread with the largest force can be cut off and the ring can be guaranteed. The threads in the slots are subjected to tensile stress while the remaining threads are subjected to compressive stress. In order to make the bolt load even after the bolt is tightened, it can also be started from the bolt. As shown, machining a tapered hole at the end of the bolt and combining the corresponding nut can promote the load of the threaded joint to a certain extent to avoid Vibration fatigue first damages the thread that has the most force on the thread.
Anti-vibration bolt structure form load distribution 24 coating adhesive in the middle thread segment (as shown in paragraph B) is coated with adhesive, the outer casing of the plastic sleeve packaging adhesive is epoxy resin, assembled in the melt state, The plastic sleeve is damaged, and the adhesive flows out into the assembly gap. After a certain period of time, the adhesive is solidified. At this time, the bolt is solidly integrated with the fitting through the adhesive. The purpose is to use a non-metallic material to have a certain vibration absorbing capability. In order to reduce the impact of vibration on the bolts, different grades and types of adhesives can be selected according to the requirements of use.
The conclusion of coating adhesive 3 is to improve the anti-vibration performance of the bolt and avoid the early fracture under the action of low stress. It is necessary to analyze and design from the aspects of material selection, structural arrangement processing and load distribution, as long as possible. The bolts only bear axial forces and avoid other additional stresses. In particular, bolts used in some important applications can be used in a variety of ways to reduce the stress concentration at the threads and to evenly distribute the load they are subjected to.
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