1. Abnormal motor bearing noise was observed. A feed pump high-pressure (6kV) motor model YKK400-2, with a power rating of 450kW and a speed of 2975 rpm, was examined. The shaft extension end is equipped with a deep groove cylindrical roller bearing (NU3E222), while the non-load end uses a deep groove ball bearing (6222). During operation, the sound from the shaft extension was sharp and unpleasant, different from typical electromagnetic noise or dry grinding sounds. The noise occurred in cycles—lasting about 2 minutes, followed by a 2-minute interval. Vibration measurements were taken using a vibrometer (VA-80A). At a vibration amplitude of 0.021 mm, the vibration velocity reached 53.6 m/s, sometimes even up to 97 m/s, which far exceeded the standard limit of 28 m/s. Additionally, the current fluctuated significantly.
During the maintenance process, it was found that the inner oil cover showed uneven wear marks, and two of the deep groove cylindrical rollers were damaged. Measurements of the bearing positioning dimensions, end cover, and oil retaining caps revealed that the allowable clearance was 0.7 mm. However, when the motor's bearing temperature reached 100°C, the expansion value was approximately 0.9 mm, which was insufficient for normal operation. After several replacements of the deep groove cylindrical bearings, the abnormal noise disappeared, and the noise cycle extended to 4 minutes.
2. Failure analysis and resolution
The original design used NU-type deep groove cylindrical roller bearings, which allow axial movement. These bearings have ribs on both sides of the inner ring but none on the outer ring, enabling the shaft to move in two directions and accommodate thermal expansion. Compared to deep groove ball bearings, these have a larger clearance and provide linear contact rather than point contact, resulting in an elliptical motion path due to the gap between rolling elements and raceways. The force is primarily concentrated at the bottom, and during high-speed operation, the force path becomes parabolic due to the bearing clearance.
In the case of the feed pump motor, the main load is axial, and the driven load is stable. Therefore, the need for radial play provided by the cylindrical roller bearing was reduced. As a result, the front bearing was replaced with a deep groove ball bearing with a C3 clearance of approximately 0.04 mm, which met operational requirements. To address thermal expansion, a spacer of about 0.8 mm was added to the oil retaining ring’s small cover.
Microscopic irregularities in the curvature of the rolling elements and raceways can cause impact during operation, generating high-frequency pulses. This leads to higher high-frequency interference readings on the vibrometer. Deep groove cylindrical roller bearings have more contact area with the raceway, resulting in greater high-frequency impacts compared to the point contact of deep groove ball bearings. Replacing the cylindrical roller bearing with a deep groove ball bearing effectively eliminated the abnormal noise.
After the replacement, the bearing noise disappeared completely. Following a period of operation, no noise reappeared. The vibration amplitude dropped to 0.013 mm, and the acceleration was 2.8 m/s². The motor operated smoothly, with stable load performance and minimal current fluctuations.
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