In the industrial field, the application of bolted joints is quite extensive, and the use of bolted joints is most common. In general, the strength of such bolted joints depends on the strength of the bolts. There are many factors affecting the strength of the bolt, such as the material size parameter structure manufacturing and assembly process. As far as its impact is concerned, it involves the stress distribution of the thread teeth, the stress stress concentration stress, the mechanical properties of the material manufacturing process and so on. The damage of the tension bolt is mostly the fatigue fracture of the bolt part. According to statistics, it takes about 65 at the first turn of the thread from the force transmission, and about 20 at the intersection of the polished rod and the threaded part. The bolt head and the rod About 15% of the junction. Due to manufacturing and installation errors, other parts of the bolt may also break, but rarely occur. Therefore, the second circle is the weak link of the bolt by the force transmission, and increasing its strength is the key to improving the bolt joint strength. In the following, we will explore measures to improve the strength of bolted joints by reducing the burden and improving the ability or at the same time.
1 Adjusting the force distribution of the threaded teeth Even if the bolts and nuts are manufactured and assembled very accurately, the force of the threaded teeth of the coils is not uniform when the force is transmitted. The bolt is pulled. The pitch increases; the nut is pressed. The pitch is reduced. Since the bolt stiffness is much smaller than the nut stiffness, the bolt pitch is also increased much more than the nut pitch reduction. This difference in pitch variation is mainly compensated by the deformation of the thread and the deformation of the thread. That is to say, the deformation of each thread thread is different. From the moment of the force transmission, the thread is opened to the maximum, and thus the force is also the largest, and then decreased. The more the number of thread turns, the more. The degree of unevenness of the force is also more significant. 1. The experimental research shows that the axial direction shared by each circle is proportional to 1.
It can be seen from the above. The bolt load is mainly carried by the second thread, after the 810th turn. The thread is almost unstressed. therefore. The heightening nut is used to increase the number of turns, and there is no obvious a1 thread for improving the load capacity of the thread. Under the force of action. In view of the above analysis. In the actual project, the following methods can be used to make the thread teeth relatively evenly 1 suspension nut. The deformation of the mother body and the bolt is reduced to reduce the difference in pitch variation, and the fatigue strength of the bolt can be increased to 4, 2 inner nut, which can reduce the rigidity of the thread with large force and transfer the force to the tooth with small force. The high-bolt fatigue tolerance ring groove snail can be promoted, and the countermeasures for improving the bolt joint strength under the nut (1) nut Peng Pingping Fuling Teachers College Higher Vocational College. Chongqing Fuling 408003 teeth force distribution; reduce stress amplitude; reduce additional stress; reduce stress concentration; improve manufacturing process.
The tension and elasticity of the part can increase the fatigue strength of the bolt up to 30; 4 The nut with low soft modulus of elasticity of the material, such as steel bolts with non-ferrous metal nuts, can improve the fatigue strength of the bolt by increasing the force distribution of the thread. ; 5 in the threaded hole or nut, loaded with a diamond-shaped section of steel wire wound into a high bolt fatigue strength of 30; 6 pairs of special large-scale screw inspection joints. When the machining process, the pitch of the bolt is obviously smaller than the pitch of the nut, which can greatly improve the uneven force between the threads of each circle and improve the fatigue strength.
2 The main factor that reduces the stress amplitude affecting the fatigue strength of the variable load parts is the stress amplitude. When the working load is in, with a change between 0. bolt tension in the mortal preload and the stiffness of the piece, then the strength condition of the threaded portion is the midday stress amplitude of the snail d screw 1 snail; the maximum diameter of the bolt The stress timing, the smaller the stress amplitude. The higher the fatigue strength, the working load, and the remaining preload force are unchanged. To reduce the bolt stiffness or the large joint stiffness can achieve the purpose of reducing the stress amplitude 2, but the pre-tightening force F is correspondingly large. When the bolt stiffness is reduced to half of the stiffness of the joint, the bolt tension is only the width of the working load, and is divided.
Measures to reduce the bolt stiffness are to increase the bolt length appropriately; partially reduce the bolt diameter or make a hollow flexible bolt; install elastic members under the nut. In order to achieve the rigidity of the large connected piece. A rigid ring should be used as much as possible. It is worth noting that if the stiffness of the bolt and the connected part does not change. As long as the pre-tightening force is appropriately increased, the fatigue strength of the bolt can also be increased.
4 increase the stiffness of the joint 3 reduce the additional stress The additional stress is mainly the bending stress, because the thread root is sensitive to bending. therefore. Bending stress often plays a key role in bolt breakage. If the joint is not stiff enough, the bolt is subjected to an eccentric load such as a hook bolt. Will cause bending stress. Also. The support surface of the coupling bolt head nut or the like is inclined. The threaded hole is not correct. It also causes bending stress. Measures to reduce or avoid bending stress are to ensure that the coupling member has sufficient rigidity, and the hook bolts are used as little as possible. The process ensures that the axis of the threaded holes is perpendicular to the support surfaces in the joint, and a spherical washer is used in the structure. Diagonal washer. Ring waist boss. The form of the sinker seat.
4 Reduce the stress concentration at the intersection of the bolt head and the bolt at the root and end of the thread. There is stress concentration, which is a dangerous part of fatigue fracture; especially at the root of the screw thread, the teeth are bent due to the tension of the bolt. Moreover, the situation is more uneven and the situation is more serious. In the processing, the root radius of the root can be appropriately increased to reduce the stress concentration, and the fatigue strength of the bolt can be increased to 2040; the bolt with the undercut in the nut support surface can retain the thread, etc., to varying degrees. Reducing stress concentration 5 Improving manufacturing processes Manufacturing processes also have an important impact on bolt fatigue strength. If twisted threads are used. There is residual compressive stress due to the cold work hardening layer. The metal flow line is reasonable, and the fatigue strength of the bolt can be 3040 higher than that of the vehicle thread; the rolling is performed after the heat treatment, and the effect is better. Pitch error should be reasonably controlled during manufacturing to avoid uneven force on the thread. In addition, cyanide nitriding treatment can effectively improve the fatigue strength of bolts.
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