[Abstract] This paper compares the structure and function of couplings and gearboxes in Shanghai Metro Line 1 and Line 2, and proposes rational suggestions for the localization design of couplings and gearboxes in future urban rail transit vehicles in China. [Keywords ] Coupling, Gearbox Domestic railway vehicle bogies are powered by diesel locomotives, and the carriages are all trailer cars. Therefore, domestic designers have not considered the issue of couplings when designing bogies. However, with the rapid development of urban construction in China, many cities have proposed to build subways to improve the increasingly serious urban traffic problems. Relying on imported foreign subway vehicles is very expensive. Improving the localization of urban rail is our inevitable path. First, let's understand the arrangement of couplings and gearboxes on subway vehicle bogies. Currently, Shanghai Metro vehicles are composed of 6 cars. The first and last trailer cars do not provide power; the power to the train comes from the 4 middle motor cars. The bogie arrangement of these 4 motor cars is shown in Figure 1 (the figure shows a top view of half a bogie of a Shanghai Line 2 motor car). The transmission principle is that when the motor (1) is energized, the motor rotates, which drives the coupling to rotate. The coupling then drives the gear in the gearbox (2) to rotate, and the gear drives the wheelset to rotate, thus completing the power transmission. [align=center] Figure 1 Bogie of a 4-car train[/align] Next, let's understand what a coupling is. The English word for coupling is coupling, which means connection, engagement, or coupling. So, as the name suggests, the coupling is the connection mechanism between the traction motor and the gearbox. Perhaps some people will ask if it is possible to connect the traction motor and the gearbox directly without a coupling? The answer is absolutely not. Because when the AW3 state (i.e., the vehicle is fully loaded with passengers) is lower than the empty state, the center of the gearbox, the center of the coupling, and the center of the traction motor are not on the same straight line. When the vehicle is running, the coupling, the traction motor, and the gearbox gears all need to rotate at high speed. It is conceivable that if there is no coupling connection, the motor and the gearbox will directly hinge and rotate on different planes, and the consequences will be terrible. Let's take a look at the actual situation of couplings and gearboxes in Shanghai Metro vehicles. 1 Introduction to Line 1 Coupling and Gearbox Line 1 coupling is a rubber element (as shown in Figure 2). It consists of three parts: two rubber elements (1) and (2) and a flange for connecting the traction motor. It is hollow inside, and the two rubber elements are fixed by two screws. One is fixed to the traction motor, and the other is fixed to the gearbox. The two ends of the coupling are connected to the motor and the gearbox by splines. [align=center] Figure 2 Rubber Element[/align] 1.1 Design Intent of Line 1 Coupling and Gearbox The installation structure of Line 1 gearbox is shown in Figure 3. Figure 3 Gearbox Installation Structure Since Line 1 traction motor uses DC motor (direct current motor), which weighs 1045kg and has a relatively large volume, the distance between the motor and the axle box is very small. Therefore, a rubber coupling suitable for narrow spaces is adopted. At present, this coupling is rarely used in countries around the world. It can be said that it is almost never used. 1.2 Advantages of Line 1 Coupling and Axle Box Suitable for situations where the distance between the gearbox and the traction motor is small. There is no oil leakage because no internal lubrication is required. 1.3 Disadvantages of Line 1 Couplings and Gearboxes: Monthly inspections require checking the height of the two mating surfaces of the coupling—2.3mm (to avoid excessive height difference under AW0 (empty) and AW3 operating conditions, a certain height difference is adjusted under AW0). They are easily damaged and have poor rigidity. Special tools are required for measurement, and pry bars are needed when adjusting the dimensions. The inspection environment is very poor. Two tie rods are required for fixation, which is not only wasteful of materials but also brings many troubles to maintenance and disassembly. 2 Introduction to Line 2 Couplings and Gearboxes The Line 2 coupling is a rigid structure (as shown in Figure 4). Two circular metal covers cover the connection points of the two parts of the coupling, and the covers are sealed with sealing rubber rings to prevent oil leakage. It uses an interference fit connection with the gearbox and motor, which differs from the spline connection of the Shanghai Metro Line 1 coupling. The structure of the Line 2 gearbox is shown in Figure 5. Its structure is simpler than that of the Line 1 gearbox, so it is cheaper. [align=center]Figure 4 Line 2 Coupling[/align] [align=center]Figure 5 Line 2 Gearbox Structure[/align] 2.1 Design Intent of Line 2 Coupling and Gearbox Since the traction motors for Line 2 use AC motors, which are half the weight of the traction motors for Line 1 and much smaller in size, rigid gear-hinged couplings were chosen for the designers. 2.2 Advantages of Line 2 Coupling and Gearbox High rigidity, not easily worn, and high safety factor. 2.3 Disadvantages of Line 2 Coupling and Gearbox "Breathing" causes oil mist problems. (When the wheelset rotates, it generates a lot of heat, so the oil in the gearbox is agitated into oil mist by the gears and expands to be discharged through the small holes. After the train enters the depot at night, the gearbox cools down, the air inside the gearbox contracts, and cold air and dust are drawn in, causing a mixture of oil mist and dust to fill the area around the small holes.) Because the coupling uses oil lubrication, oil leakage sometimes occurs. If the gearbox oil level is insufficient, the gears inside will rotate without oil, potentially leading to gearbox failure or even derailment. Maintenance is also inconvenient. Although the design cost of this gearbox is relatively cheaper than that of Line 1, disassembling it is extremely difficult. Removing the entire gearbox and coupling requires a puller and a hydraulic pump. Typically, disassembling a single gearbox and coupling takes 3 hours. Therefore, the design of the gearbox on Line 2 has certain problems. As a maintenance worker, I hope that domestic designers will consider labor costs in the design scope and give it due attention. According to Siemens' calculation method, the company needs to pay 35 RMB per worker per hour for repair work. Let's assume that a certain project needs to be repaired on the couplings of Line 2: Currently, Line 2 has 24 trains, each with 16 couplings, totaling 16 x 24 = 384 couplings. Each coupling takes 6 hours to install and remove, plus 1 hour for repair, so the repair project for one coupling would cost the company 384 x 7 x 35 = 94,080 RMB. These figures do not include the intermediate tools, parts, and resource consumption. If the installation and removal of the coupling only takes 2 hours, then nearly 2/3 of the cost can be saved. It can be seen from the above that the ease of product installation and removal can directly generate economic benefits for the user. 3. Comparison of Shanghai Metro Lines 1 and 2 Based on the comparison of the advantages and disadvantages of Shanghai Metro Lines 1 and 2, we can pay attention to the following aspects when designing similar products: Use rigid couplings and avoid using rubber components, which is safer and more reliable. Adopt the gearbox structure of Line 1 for easy installation. Install a small mirror on the surface of the gearbox at the dangerous fluid level position, so that workers can easily observe the fluid level during inspection and add oil in time. 4. Conclusion The above is an overview of the couplings and gearboxes of Shanghai Metro vehicles. It is hoped that this information will provide some help to those engaged in vehicle technology, expanding their thinking and striving to achieve the localization of metro coupling components as soon as possible, filling the current gap in this field in China.