Abstract: Guangzhou Metro Line 1 vehicles have been in use for many years. With the increase of operating mileage, the failure rate of axle boxes with grounding devices in vehicles has significantly increased. Therefore, the reasons for the failure of axle box grounding devices in Line 1 vehicles are analyzed, and corresponding solutions and countermeasures are proposed to reduce losses.
Keywords: subway vehicles; Axle box; Grounding device; Brittle fracture
Guangzhou Metro Corporation (hereinafter referred to as Guangzhou Metro) opened and operated Metro Line 1 in 1997. As of now, the vehicles on this line have been in use for 14 years, with a cumulative mileage of over 1.5 million kilometers. As the usage time increases, the failure rate of the axle box with grounding device in the vehicle significantly increases, mainly manifested as mechanical fracture of the internal structure. A specific analysis of this type of fault is beneficial for improving the standard of maintenance operations and improving the safety of vehicle operation on the main line.
1. Grounding device structure
The vehicles of Guangzhou Metro Line 1 are equipped with grounding devices on all wheel sets (4 pieces) of the high-speed train and the axle boxes of the second and third wheel sets of the trailer. The total number of trains is 20. The entire grounding device consists of an outer end cover, a copper mounting bracket (with grounding carbon brush), a grounding device shell (with mounting bracket fixed seat), and a conductive copper rod (see Figure 1). One end of the conductive copper rod exposed to the axle box is fastened to the flexible vehicle grounding wire (copper braided wire), and the other end is fixed to the copper mounting bracket. The mounting bracket has a through hole with internal threads, and one end of the conductive copper rod is fastened to the mounting bracket by a fixing bolt. The installation bracket is also fastened with bolts and grounding carbon brush wires, and the clamp type constant pressure spring keeps the grounding carbon brush tightly against the copper sliding disc (which is fixed on the wheel set). When receiving current, the current flows through the conductive copper rod to the installation bracket, and then flows into the wheel set through the grounding carbon brush.
Figure 1 Whole grounding device
2. Fault analysis
Since 2006, Guangzhou Metro Line 1 vehicles have experienced complete grounding device failures, characterized by loose and falling fixing bolts of conductive copper bars inside the entire piece, as well as broken fixing seats of installation brackets. Due to both types of faults, the components of the vehicle's current collection circuit are damaged, making it difficult for the current to flow back well. When it is necessary to carry a large current, there is a certain safety hazard. Upon inspection of all grounding devices, it was found that this fault is not related to the vehicle number and the occurrence cycle is irregular. However, when the fixing seat of the installation bracket breaks, there must be a phenomenon of the conductive copper rod fixing bolt falling off at the same time. The opposite is not necessarily true, indicating that the fixing seat fracture may be a secondary fault caused by the falling of the conductive copper rod fixing bolt, and the probability of occurrence is high.
2.1 Analysis of the drop of conductive copper rod fixing bolts
The conductive copper rod is a cylindrical copper rod material that has been machined and made of T2 red copper. There is a section of area at both ends of the copper rod that needs to be milled, and there are through holes on the plane at the milling point, as shown in Figure 2.
(a) front (b)side
Figure 2 Schematic diagram of copper rod
The length of the internal thread on the installation bracket is only about 5 mm, resulting in limited pre tightening force generated by the fastening bolts. If the conductive copper rod is affected by external forces, the tensile force transmitted along the copper rod acts on the fastening bolt, which can easily cause bolt fastening failure. The other end of the conductive copper rod is connected to the flexible vehicle body grounding wire. After analysis, external forces can be generated from the connection between the grounding wire and the conductive copper rod.
During vehicle operation, although constrained by a series of herringbone springs, there is still relative motion between the axle box and the frame, which can be roughly decoupled into axial and vertical motion. The flexible vehicle body grounding wire is partially fastened to the bogie by a cable clamp, as shown in Figure 3. If the vehicle body grounding wire between the fixed position of the cable clamp and the copper wire connection is too short, during the axial movement of the axle box, the flexible vehicle body grounding wire will be stretched with the inner and outer movement of the axle box, resulting in a tensile force at one end of the copper wire connection, in the direction of pointing towards the outer side of the copper rod along the instantaneous vehicle body grounding wire direction. Long term exposure to this tensile force is equivalent to applying alternating loads at the fastening point between the copper rod and the installation bracket, which will cause the bolt to loosen, resulting in an increase in local stress at the connection point between the conductive copper rod and the installation bracket when the vehicle is in motion. Subsequently, there may be two possible failure phenomena: firstly, the fastening bolt may detach from the installation bracket and fall into the grounding device; Either before the fastening bolts detach from the installation bracket, the material at the through-hole of the copper rod at this end is broken, and the bolts are still fixed on the installation bracket.
Figure 3 Schematic diagram of axle box installation
2.2 Fracture analysis of the fixed seat of the installation bracket
There are a total of 3 fixed seats for copper installation brackets, which are evenly distributed on the inner side of the grounding device shell with a circumference of 120 ° apart. They are cast together with the grounding device shell, and their local side schematic diagram is shown in Figure 4. The fixed seat can be seen as a cantilever beam connected to the shell. When the bolt falls into the grounding device, the fixed seat may break, and the number of fractures may vary. According to the statistics of all fracture fixed seats, it was found that the sides of the fracture surface are basically along the dashed ED direction in Figure 4.
Figure 4 Partial Side View of Fixed Seat
According to the theory of Damage mechanics, the smooth area is the crack propagation area, and the rough part is the transient fracture area (1). Taking the physical object shown in Figure 5 as an example, there are two relatively smooth small areas irregularly distributed within the fracture surface, with a total visual area of about 10%, while the rest are rough surfaces. There is a smooth area distributed as an island within the fault plane, which does not extend to the edge. Another smooth area overlaps with the contour of the edge. The condition that causes this crack should be a casting defect, or the crack nucleus generated after long-term use expands into a shorter crack. If this phenomenon is the cause of fracture, it should be a common phenomenon in the same batch of axle boxes, which is inconsistent with the fact.
Figure 5 Physical image of fixed seat fracture
In the grounding device where the fracture occurred, there were severe scratches on the back of the fixed seat, some with regular toothed indentations, located in the CD section, with a depth of approximately 2-3 mm and varying widths. The copper sliding disc also had similar scratches at this corresponding position. Based on the phenomenon that the area of the instantaneous fracture zone is much larger than the crack propagation zone, it is inferred that the fixed seat should be damaged first, and then suddenly subjected to external forces exceeding the bearing strength, leading to fracture. Due to the fact that the operating track conditions, load conditions, and service life of all vehicles are basically the same, that is, the bearing conditions of the fixed seat are basically the same. However, fracture failures do not occur in batches at the same time, and the fracture only occurs after the fixed bolt falls. Therefore, the possibility of fixed seat fracture under normal operating conditions can be ruled out.
The fastening bolt is a flange anti loosening bolt made of steel, with a surface plated with yellow inscription and a surface hardness of 800-1000 HV. The flange surface and thread of the dropped bolt have severe wear, but the surface of other components of the grounding device is basically undamaged.
In summary, there are two possible reasons for the fracture of the fixed seat caused by the analysis of falling bolts (2):
A. The bolt is stuck between the gap between the back of the fixed seat and the copper sliding disc, which is below the CD segment. Due to the small space, during the rotation of the wheel set, the copper sliding disc compresses the bolts stuck in the gap and generates external stress on the fixed seat, which cannot withstand this stress and breaks;
B. The bolt was fixed on the rotating copper sliding disc at the end of the shaft and repeatedly collided with the fixed seat, causing cracks to first appear on the fixed seat and ultimately fracture when subjected to a large load.
According to the installation position of the fixed seat, the possibility of being hit on the front is very low, and only the side is likely to be hit and cause the fixed seat to break. However, after on-site inspection, there are no obvious scars on the side of the broken fixed seat, and the contour line of the side fracture basically matches the edge of the falling position. According to previous analysis, the fixed seats must be damaged first and obvious impact points should be left on the side. However, no obvious impact points were found on all fixed seats upon inspection. Moreover, bolts are prone to hitting the side of the fixed seat inside the axle box. If this causes fracture, this fault will inevitably occur frequently, which is inconsistent with the fact that the fracture of the fixed seat is only found to have a certain failure rate.
So there is only one possibility, which is that when the bolt is clamped on the back of the fixed seat, it will cause damage and ultimately cause the fixed seat to fracture. The DE section is the main load-bearing part of the fixed seat. Once scratched, it is equivalent to a geometric defect in the material. Based on the sensitivity of the material to defects, this part is prone to fatigue and fracture under stress.
3. Response measures
Due to the fact that the fixed seat is cast as a whole with the grounding device casing, and the space position is narrow, it is difficult to carry out welding work. Therefore, the faulty component is generally scrapped as the entire grounding device, resulting in significant cost losses. Improvements can be made in these areas, with reasonable application to reduce the occurrence rate of this fault:
1)Adjust the length of the connection between the grounding wire and the conductive copper rod to ensure that the grounding copper rod vehicle is not affected by external forces during operation;
2)Improve the spare parts for grounding copper rods. To create material defects, a V-shaped groove is created near the fastening bolt between the grounding copper rod and the copper installation bracket. By utilizing the sensitivity of the material's defects, the copper rod breaks when subjected to external forces, so that the fixing bolt remains fixed on the copper installation bracket and will not fall, thus avoiding the occurrence of secondary faults caused by the breakage of the fixing seat;
3)Increase the number of cover opening inspections and promptly identify faults.
4. Summary
The grounding device problem of Guangzhou Metro Line 1 can be divided into a bolt drop fault and a secondary fixed seat fracture fault, among which the secondary fault is more harmful. From various aspects, it can be inferred that the reason for the fracture of the fixed seat is caused by the compression stress of the falling bolt during vehicle operation. Reasonable improvement measures have been taken to reduce the failure rate in response to this fault.
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2023-05-18