1、 Problems existing in the overhaul of metro train frame in China
With the continuous development of social science and technology in China, people's way of life is more and more convenient, especially the emergence of the subway solves many people's travel problems, but also eases the problem of urban traffic congestion. But there will be some problems in the operation of the subway, so the subway overhaul is particularly important. As one of the important means of transportation in the city, it is related to the safety of urban traffic residents. Under normal circumstances, the subway company is through overhaul to maintain, repair way to eliminate hidden dangers, to ensure the normal operation of the subway. But there are some problems in subway maintenance, such as the high cost of subway vehicle maintenance is a major reason. Moreover, with the establishment of urban rail transit network operation mode and the continuous growth of passenger flow in China, metro companies are facing the operation tension and the maintenance problems such as overhaul and UN wheeling repair become more and more prominent because more and more vehicles are put into use. Not only the process scope of maintenance is not perfect, for example, there is no clear requirement for fastener torque, and the staff do not understand it in place, which indicates that the technical management is not in place. In addition, in the scope of maintenance, there are missed inspection and repair. First of all, there are problems in the process implementation, such as not following the process for maintenance, simply relying on past experience for maintenance, and the cleanliness of flaw detection is not enough, and the flaw detection requirements are not strictly implemented. Finally, there are still some problems such as the fuzzy function of maintenance management and the imperfection of maintenance technology and equipment.
2、 How to judge whether the fastener torque is correct
（1） Is the tightening torque correct
In the assembly process, the tightening torque is affected by many factors, so according to the standard value to judge the tightening torque is a certain deviation. First of all, there is the possibility of secondary tightening, because the torque wrench can not completely maintain a stable force, it may produce secondary tightening. Secondly, the fastener bolt is not easy to rotate at the beginning. There are certain objective factors that cannot be changed, so there will be deviation. And when the bolt turns is difficult to determine, which has certain subjective factors. There is also the fastener bolt rotation, to overcome greater friction, but in fact the actual torque value is certainly greater than the installation torque value, 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 looseness
At present, fastener looseness often exists in overhaul. Many people think that fastener looseness is due to overuse, in short, fatigue strength. However, experiments show that few fasteners are loosed due to overuse. The main reason is that the huge power of subway directly acts on the fasteners and equipment, which directly leads to the fastener damage. Once the fasteners are damaged, the equipment can not work normally. In the working process of fastener, the thread of fastener under the action of axial force will be damaged, and then the bolt may be damaged. Therefore, in the assembly process, we should pay attention to the use of some high-quality anti loosening effect of good materials, is to solve the main determinant of fastener loosening.
3、 How to determine and check the torque of Metro fastener
（1） How to determine the pre tightening force of fastener
In the rail transit overhaul, the pre tightening force must be determined before the fasteners are installed. The most important target value of no fastener in the connection process is the pre tightening force. As we all know, it is impossible to measure or estimate the pre tightening force absolutely accurately. No matter how accurate the calculation is, there will be more or less deviation due to external factors. On this basis, in the fastening of metro vehicles, torque is a characteristic value that can consider the quality. In order to ensure the safety of metro, it is necessary to check the torque and fastening quality of fasteners, and transfer the torque into the system for evaluation. 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 science and technology can not solve it, even if there is a way to solve it, the cost is quite high, and it is not suitable for the subway. But with the development and progress of science and technology, this problem will be solved one day. Due to the problem of friction coefficient, it is impossible to draw a correct conclusion about how much preload to produce when assembling bolts according to torque.
（2） Tightening method and inspection method of fasteners
According to the current development of science and technology in China, a large number of fasteners are used in rail transit operation equipment, so it is particularly important to tighten the fasteners in Metro overhaul. In the process of fastening, most of them are bolts. Bolts and nuts are the most commonly used fasteners in the industry, and 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, unloaded and reused. At present, the main tightening method of fasteners in Metro overhaul is torque method. As the name suggests, the torque method is to use the corresponding bolt tightening value according to the first designed pre tightening force, and finally use the wrench to tighten it. The detection method is increasing torque method, which means to use the wrench to increase the torque steadily, observe the tiny gap when the bolt rotates and the torque value of instantaneous rotation is the largest, do not stop to continue to rotate, and see the torque return to the transient stable state. At this time, it is the torque obtained by inspection. This method is relatively simple, but there are also some deviations, For reference only. In addition, this method also needs to have very skilled experience to be able to complete.
With the continuous development of China's society and science and technology, people's way of life is more and more convenient, especially the emergence of the subway has solved many people's travel problems, but also eased the problem of urban traffic congestion. But there will be some problems in the operation of the subway, so the subway overhaul is particularly important. As one of the important means of transportation in the city, it is related to the safety of urban traffic residents. A large number of fasteners are used in rail transit operation equipment, so it is particularly important to tighten the fasteners in Metro overhaul. The purpose of fasteners is to achieve the preload, through the torque to achieve the preload, but there are many factors need to be controlled, and the torque value also has a certain gap. Therefore, it is necessary to find the key factors that affect the torque value through the combination of experiments and theoretical analysis. After finding these influencing factors, control in the process, especially control some key factors, so that the range of torque can be applied in practice.
