In China, rail transit is developing faster and faster, making people's lives more and more convenient. A large number of fasteners are used in rail transit equipment. These fasteners have many applications in the panel products and electrical products of electrical equipment companies. In the application of fasteners, there are requirements for torque values. It is the need to determine the torque to ensure the quality of the fastening. Bolt tightening methods have been used in many companies, mainly using the pre-tightening force required in the original design to obtain the bolt tightening torque corresponding to it, and then using a torque wrench to fix the twist. Then this method is desirable in the subway overhaul, this article will be the key analysis. It can be seen that the rigor of subway overhaul is very important, and the fasteners are directly related to the safety and service life of the subway.
I. Problems existing in the overhaul of subway trains in China
With the continuous development of China's social science and technology, people's lifestyles are becoming more and more convenient. Especially the emergence of the subway has solved many people's travel problems and eased the problem of urban traffic congestion. However, there will be certain certainties in the operation of the subway, so the subway overhaul is particularly important. As one of the city's important means of transportation, it is related to the safety of urban traffic residents. Under normal circumstances, the subway company is to repair and repair the way through overhaul to eliminate hidden dangers and ensure the normal operation of the subway. However, there are certain problems in the maintenance of subways. For example, the high maintenance cost of subway vehicles is a big reason. Moreover, with the establishment of China's urban rail transit network operation mode, passenger traffic continues to grow, the subway company faces operational stress and the maintenance of overhaul and repairs becomes more and more due to the increasing use of vehicles. The more prominent it is. Not only the process scope of the overhaul is not well established, such as the fact that there is no clear requirement for the fastener torque, and the staff understands that the problem is not in place, which indicates that the technical management is not in place. In addition, there are cases of missed inspection and missed repair in the scope of inspection. Firstly, there is no maintenance according to the process in the execution of the process. The maintenance is based solely on past experience, and the cleanliness of the flaw detection work is not enough, and the requirements for flaw detection work are not strictly implemented. Finally, there are still problems in the ambiguity of the maintenance management work function and the imperfect maintenance technical equipment.
Second, how to judge whether the fastener torque is correct
(1) Is the tightening torque correct?
During the assembly process, the tightening torque is affected by many factors, so there is a certain deviation in judging the tightening torque according to the standard value. First of all, there is a possibility of secondary fastening, because the torque wrench cannot completely maintain a smooth force, and it is possible to generate a secondary fastening. The second is that the fastener bolts are not easy to rotate at the beginning. There are certain objective factors, which are immutable, so there will be deviations. As well as when the bolts are rotated, it is difficult to determine, and there are certain subjective factors. There is also a large friction force when the fastener bolt is rotated, but in fact the actual torque value is definitely greater than the torque value during installation, so the test value is generally too large. It can be seen that although the operation is simple, there are many problems that have not been solved.
(2) Is there any looseness?
At present, there are often loose fasteners in overhaul, and there are loose fasteners. Many people think that loose fasteners are due to excessive use, in short, fatigue strength, but the test proves fasteners. Rarely loosened because of overwork. Mainly because the huge power of the subway directly acts on the fasteners and equipment, it directly causes the fastener to be damaged. Once the fastener is damaged, the equipment can not work normally. During the operation of the fastener, the fastener subjected to the axial force may be damaged, and the bolt may be pulled. Therefore, in the assembly process, it should be noted that the use of some high-quality materials with good anti-loose effect is the main determinant of the loosening of fasteners.
Third, how to determine and verify the torque of subway fasteners
(1) How to determine the pre-tightening force of the fastener
In the rail transit overhaul, the pre-tightening force is determined before the upper fasteners are attached. The most important target value of the fasteners in the connection process is the pre-tightening force. It is known that it is impossible to measure absolutely accurately or Estimating the pre-tightening force, no matter how accurate the calculation, will be more or less biased due to external factors. On this basis, in the fastening of metro vehicles, torque is a characteristic value that can be considered for quality. In order to ensure the safety of the subway, the torque and fastening quality of the fastener must be checked, and the torque is transferred to the system. In the assessment. In practice, there is a certain relationship in the calculation. The value of this relationship depends on the geometry of the fastener and the friction coefficient, but the friction coefficient is an unavoidable problem, and the current level of technology cannot be solved, even if there is a way. Solved, the cost is also quite high, not suitable for trial use in the subway. But with the development and progress of technology, this problem will be solved one day. Since the problem of the friction coefficient exists, it is impossible to draw a correct conclusion when it is necessary to generate a preload force when assembling the bolt according to the torque.
(2) Fastening method and inspection method of fasteners
According to the current development of science and technology in China, the use of fasteners in rail transit equipment is so important that the tightening of fasteners during subway overhaul is particularly important. Most of the fastening process is mainly bolts. Bolts and nuts are the most commonly used fasteners in the industry. They are also the most suitable fastening elements in the industrial field. They can not only withstand high-strength work but also can be loaded and unloaded. And reuse. At present, the main tightening method for fasteners in subway overhaul is the torque method. As the name suggests, the torque method refers to the tightening force according to the first design, and the corresponding bolt is used to tighten the value. Finally, it can be screwed with a wrench. The method of detection is to increase the screwing method, which means to increase the torque with the wrench and use the force to increase the torque. Observe the slight difference between the rotation of the bolt and the torque value of the momentary rotation. Do not stop the rotation and watch the torque return to the short-term stable state. The time is to check the torque obtained. This method is relatively simple, but it also has a certain deviation, which is only for reference. In addition, this method requires a very skilled experience to complete.
Conclusion
With the continuous development of China's society and science and technology, people's lifestyles are becoming more and more convenient. In particular, the emergence of the land F has solved many people's travel problems and eased the problem of urban traffic congestion. However, there are certain problems in the operation of the subway, so the subway overhaul is particularly important. As one of the city's important means of transportation, it is related to the safety of urban traffic residents. The use of fasteners in rail transit equipment is so important that tightening fasteners during subway overhauls is particularly important. The purpose of the fastener is to be able to achieve the pre-tightening force, and the pre-tightening force is achieved by the torque, but there are many factors that need to be controlled in the middle, and there is also a certain gap in the torque value. Therefore, it is necessary to find the key factors affecting the value of the torque through a combination of experimental and theoretical analysis. After finding these influencing factors, control in the process, especially controlling some key factors, can be applied to the actual range of torque.
references
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[2] Rong Fei. Torque Coefficient and Standard Deviation of Bolt Fastening [J]. China Equipment Engineering, 2016(2): 42-44.
[3] Guo Xinming. Comparison and Analysis of Torque Coefficient of Threaded Fasteners[J].Machinery Industry Standardization & Quality,2013(4):27-29.
[4]CHEN Tingzhi. Research on Determination Method of Tightening Torque of Thread Fasteners[J]. Journal of Wuhan Polytechnic, 2013(1): 102-105.
[5]TAN Mingqiang, YIN Jun, SUN Zhengshi. Research on Torque Checking of Overhaul Fasteners in Guangzhou Metro Vehicles[J]. Rail Transit Equipment and Technology, 2016(4): 23-25.
[6]Yu Lili, Chai Zhilong, Li Wei, et al. Study on the influence of surface treatment process on the assembly performance of threaded fasteners [J]. Automotive Technology and Materials, 2016(1): 17-20.
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