Thread structure optimization and anti-loose application research
As the most widely used joint method, threaded joints are widely used in industrial production and people's daily life. It is especially important to have a high reliability in the connection. Because the standard ordinary threaded connection has self-locking property, when the normal temperature and load change are not large, it has high connection reliability and does not fall off by itself. However, under the conditions of shock, vibration or variable load, or when the working temperature changes greatly, the frictional resistance between the helical pairs is extremely unstable, and often decreases or even disappears at a certain moment, losing the self-locking ability, and the spiral pair Relative slip rotation occurs between them. Over time, the threaded coupling is loosened. This looseness can cause damage or disassembly of components or the entire equipment, and even cause major mechanical accidents or personal accidents.
Further tests have shown that the common thread will loosen to varying degrees because: the common threaded joint when the pre-tightening torque is small, the locking force is mainly concentrated on the thread surface of the first tooth of the screw, and then the thread The screwing is almost in the "floating" state, and as the tightening torque increases and the accompanying deformation of the first thread, the second tooth is subjected to a load and a locking force is generated. Ordinary thread connection, more than 80% of the load is always carried by the first and second teeth, and the other teeth are almost in the "floating" state. This state is easy to make the spiral pair appear under the action of vibration and impact. Relative slip rotation.
In order to prevent the looseness of the threaded coupling, a large number of experimental researches have been carried out by engineers of various countries, using double nuts, nylon inserts, pins, locking pieces, torque lock nuts, etc., anti-loose, mechanical anti-loose, and finally A method of preventing loosening of the relationship of spiral pair motion. Although these methods extend the threaded coupling to some extent and the common threaded fastener and the wedge-shaped threaded fastener are loosened, it can be improved that the threaded locking wedge-shaped locking thread technology can be improved by optimizing the thread structure. Time, but its different shortcomings are exposed, such as: need to add auxiliary materials, resulting in waste of resources, complex process, locking function is only effective once, can not be reused, even the torque lock nut can be reused, there are disassembly The number of times is limited, and the anti-loose performance is greatly reduced after repeated disassembly and assembly, especially in the high temperature or low temperature environment.
Finding a better way to prevent the loosening of the threaded coupling is a topic worthy of study in the current society.
The emergence of wedge-shaped self-locking threads gives this subject a clear direction. I can ask whether the shape of the thread can be changed to achieve the anti-loose effect of the coupling. 1 Research situation at home and abroad. A technique of changing the thread geometry to achieve thread locking. In 1995, the United States Shibie Company participated in the China International Machine Tool Exhibition and began to promote wedge-shaped anti-vibration thread technology in China. In 2000, American company products entered the Chinese railway industry. In the early 1990s, the track of the Shanghai Railway Institute began to study this technology and carry out large-scale production. It led the Chinese railway industry to start using wedge-shaped thread-proof technology. In 2002, the Ministry of Railways Beijing Changping Locomotive and Rolling Stock Co., Ltd. introduced the anti-loose thread technology from American Tweed Tools Co., Ltd., and also joined the production of wedge-shaped anti-vibration and anti-loose threads.
In the past two years, some automobile backbone manufacturers in China have begun trials of wedge-shaped lock threads, and the Aerospace and Aviation Group is also actively preparing for trial use of such threads.
2 Wedge thread anti-loose principle The wedge thread has a stronger anti-loose ability than the ordinary thread, thanks to the unique structure of the wedge thread.
As shown, the profile of the wedge thread is no longer a normal triangle, but has a 30-shaped bevel on the root of the internal thread. When the two threaded fasteners are mated, the tip of the external thread is tight. On the wedge-shaped bevel of the internal thread, the normal force of the contact point also changes due to the change of the wedge-shaped thread shape. It can be seen from the same axial load condition (F is the same): SP: the normal force of the tapered thread is greater than the ordinary thread normal force. Also by the friction formula: it can be seen that under the same conditions of u, the larger the Fn, the greater the friction.
Therefore, it can be concluded that the normal force of the tapered thread contact is different. According to the force of each tooth shape of the common thread and the wedge thread (as shown), the common thread is when the two threaded fasteners are connected to each other. The first tooth and the second tooth are subjected to 80% of the total stress, while the remaining teeth are always in a "floating" state, bearing little force. When the external force increases, the stress is transmitted one by one, causing the tooth The surface wears in turn, causing the sliding teeth.
If a strong vibration is added, the threaded fastener can easily overcome the locking force and cause self-relaxation. Because of its unique structure, the wedge-shaped thread has a relatively uniform load on each tooth. The same load can be distributed to each surface of each point to eliminate the stress concentration and prolong the life of the threaded fastener.
3 Comparison test of anti-loose performance The ordinary nut of model M10 and the “Spirit 2 type hexagonal self-locking anti-off nut†produced by Shanghai Deote Precision Fastener Co., Ltd. are matched with the same type of bolts in vibration test. The comparison test of the anti-loosening performance was carried out under the table. The results showed that when the vibration frequency f=30.5Hz and the pre-tightening force was 18Nm, the common nut loosened after 14min, and the joint fastening effect was lost. The type of hexagonal self-locking anti-off nut has not been detached until the 87th page, the driver, and the human-machine interface programs are all integrated and developed by a tool. As long as the programmer knows about this kind of tool, it can complete the programming of three different parts, which greatly simplifies the workload.
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