Failure analysis and countermeasures of oil lubricated bearings in locomotive traction motors

With the construction of electrified railways in China, locomotives are developing towards high-speed and heavy-duty direction, and the lubrication method of locomotive traction motor bearings is also shifting from grease lubrication to oil lubrication. The adaptability study of oil lubricated bearings in complex environments is a new topic for the safe operation of heavy-duty AC locomotives. Since the oil lubricated bearing traction motor was put into operation, many bearing failure problems have occurred. Bearing failure may cause the operation of the traction motor to fail, posing a great hidden danger to the safety of locomotive operation. Therefore, analyzing the main causes of bearing failure and taking corresponding measures is extremely important for ensuring the reliable operation of bearings and driving safety.

1. Bearing lubrication mode

The locomotive traction motor is mainly composed of stator, rotor, end cover, bearings, speed measuring device, and driving gear. A bearing is a component that connects the stator and rotor, achieving a mechanical connection for stable rotation of the stator and rotor. The locomotive traction motor has a large load and high speed at the drive end, and usually uses N332 cylindrical roller bearings. The bearings adopt the method of thin oil splash lubrication, and the traction motor drive end bearings share lubricating oil with the locomotive gearbox. The lubricating oil in the gearbox is thrown to the top baffle of the gearbox by the centrifugal force of the gear rotation, converges and flows into the oil inlet on the upper part of the bearing cover, enters the bearing through the oil inlet channel for lubrication, and then flows back to the gearbox from the lower oil return port of the oil seal on the other side of the bearing. To prevent lubricating oil from entering the stator of the electric motor, a labyrinth chamber and a pressure balance channel are provided inside the motor.

2. Typical characteristics of failed bearings

The typical failure characteristics of the failed bearing during disassembly inspection are as follows: (1) There are wear marks on the surface of the roller, with a bright color in the middle and dark gray at both ends. After measurement, the roller is dumbbell shaped, showing an abnormal wear state with a small diameter in the middle and a large diameter at both ends (Figure 1);

Figure 1 Abnormal wear of rollers

(2) Both ends of the roller, inner and outer raceway have peeled off; There is a foreign object embedded in the pocket hole of the cage, showing typical failure characteristics of severe abrasion, as shown in Figure 2.

Figure 2 Characteristics of Bearing Wear and Peeling

3. Analysis of Failure Causes

There are no structural defects in the manufacturing process and bearing design of locomotive traction motors. The wear and jamming of bearings during locomotive operation are directly related to the abnormal lubrication environment of the bearings.

3.1 Lubrication Environment Analysis

Multiple motor lubricating oil samples were extracted from multiple locomotive depots and sent to professional testing institutions and bearing companies for testing and analysis. The results indicate that the main cause of bearing wear is due to the presence of a large amount of mineral silicon and metal particles in the lubricating oil. The pollution inside the bearing is shown in Figure 3.

Figure 3 Pollution situation inside the bearing chamber

In the early stage, the bearing lubricating oil was tested and statistically analyzed. The average silicon content in the samples from each locomotive depot is shown in Table 1.

Table 1: Average Silicon Content in Lubricating Oil

Composition testing was conducted on the foreign object embedded in the failed retaining frame, and it was found that the foreign object contained SiO2, and the main component of sand particles was SiO2, as shown in Figure 4.

Figure 4: Detection results of foreign objects embedded in the holder and their components

3.2 Bearing Failure Analysis

When the bearing operates in contaminated lubricating oil, the sand and dust in the lubricating oil quickly rub and cut the surface of the bearing roller, producing a large amount of metal wear particles. Dust particles and metal wear particles continuously enter the cage pocket hole (Figure 5) to form a high wear rate wear chain, causing wear on the middle of the roller (Figure 6a), while the cage overtravel groove reduces the wear on the end of the roller, resulting in continuous wear between the roller and the cage pocket hole surface, causing the roller to gradually wear into a dumbbell shape (Figure 6b).

Figure 5 Schematic diagram of abrasive wear

Figure 6 Roller Wear Profile Curve

Under normal wear conditions, the roller surface is uniformly worn. Under abnormal wear conditions, the surface of the roller shows uneven wear, with dumbbell shapes with high ends and low middle. The load-bearing part moves to the end, resulting in increased stress at the roller end, as shown in Figure 7.

Figure 7 Schematic diagram of force on dumbbell shaped rollers

After testing, the high point length L of the dumbbell shaped roller end of N332 bearings is usually 2-5 mm. As shown in Figure 8, through finite element analysis, the maximum stress at the end of the roller can reach 2353 MPa when L=5 mm; When L=2 mm, the maximum stress at the end of the roller can reach 4723 MPa; Both of them far exceed the yield stress of 1690 MPa of the roller and raceway surface materials, inevitably leading to fatigue spalling. The ends of the roller, inner ring, and outer raceway surface first peel off, and later the peeling area expands from the end to the middle, as shown in Figure 9.

Figure 8 Stress analysis of dumbbell shaped bearing rollers

Figure 9 Physical picture of bearing peeling off

In summary, it can be concluded that the contamination of the gearbox lubricating oil has deteriorated the lubrication conditions of the bearings, resulting in abnormal wear of the bearings and ultimately leading to bearing peeling and jamming.

4. Improvement measures

Based on the causes of bearing failure, corresponding improvement measures are proposed as follows: (1) change the direction of the airflow inlet inside the traction motor to the outside atmosphere (Figure 10), to prevent external dust from entering the traction motor oil lubrication system through the pressure balance channel;

Figure 10 Schematic diagram of changing the direction of airflow inlet for ventilation duct

(3) Optimize the sealing structure of the traction motor, change the gap between the sealing device and the shaft, and prevent external dust from entering the traction motor oil lubrication system through the labyrinth sealing device;

(3) Use a filtering device to filter the lubricating oil inside the gearbox, reducing the harmful content of silicon, iron, aluminum, etc. in the lubricating oil to the bearings. By taking the above measures, the lubrication and wear conditions of the bearings have been effectively improved. The lubrication status of the bearings after 50000 kilometers of operation is shown in Figure 11, with no wear and good performance.

Figure 11 Improved bearing condition

5. Conclusion

By analyzing the reasons for the failure of oil lubricated bearings in locomotive traction motors, corresponding improvement measures have been proposed, effectively improving the lubrication environment and bearing wear of oil lubricated bearings, reducing machine damage and maintenance caused by bearing failure, and ensuring the safety of locomotive operation. At the same time, it also provides reference for the design, manufacturing, and maintenance of traction motors, and is of great significance for the development of heavy-duty traction technology.

2024 September 4th Week KYOCM Product Recommendation:

Slewing Bearing ：

Rotary bearings consist of an inner ring and an outer ring, one of which usually contains a gear. Together with the connecting holes in the two rings, they enable optimized power transmission through simple and fast connections between adjacent machine parts. Bearing raceways are designed with rolling elements, cages or gaskets to accommodate loads acting individually or in combination in any direction.

Features and advantages:

High carrying capacity

High stiffness for rigid bearing applications

Low friction

Long service life

Surface protection and corrosion resistance

Integrate other features including:

Driving mechanism

Control device

Lubrication system

Monitoring system

Sealed cassette tape

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