The original motor used NU-type deep-groove cylindrical roller bearings, which allow axial movement of the motor. The inner ring of the bearing has retaining ribs on both sides, while the outer ring has none, permitting bidirectional displacement of the shaft relative to the bearing and accommodating elongation caused by thermal expansion. Additionally, the bearing clearance is relatively larger compared to deep-groove ball bearings, but the load is distributed linearly, which is better than the point load of deep-groove ball bearings.  

1. Abnormal Noise in Motor Bearings  

A high-pressure (6kV) feedwater pump motor (model YKK400-2, 450 kW, 2975 rpm) had a deep-groove cylindrical roller bearing (NU3E222) at the shaft extension end and a deep-groove ball bearing (6222) at the non-load end. During operation, the shaft extension end produced a sharp, piercing noise—unlike electromagnetic noise or the sound of dry friction due to insufficient lubrication. The noise lasted about 2 minutes, by a 2-minute pause. A vibration analyzer (VA-80A) measured a bearing vibration amplitude of 0.021 mm. During abnormal noise, the vibration velocity reached 53.6 m/s, sometimes even 97 m/s—far exceeding the standard value of 28 m/s—while the current fluctuated significantly.  

Since the shaft extension end used a clearance fit, adjusting the axial positioning of the bearing was impossible. During inspection, uneven wear marks were found on the inner oil cover, and two cylindrical rollers in the bearing were damaged. Measurements of the bearing, end cover, and inner/outer oil seal covers, along with calculations, showed that the allowable bearing clearance was 0.7 mm. However, when the bearing temperature reached 100°C, the thermal expansion value was about 0.9 mm, which did not meet normal operating requirements. After multiple replacements of the cylindrical roller bearing, the noise not only persisted but the abnormal sound cycle extended to 4 minutes.  

2. Fault Analysis and Resolution  

Analysis based on bearing characteristics: The original NU-type deep-groove cylindrical roller bearing allowed slight axial movement of the motor shaft. The inner ring had retaining ribs on both sides, while the outer ring had none, permitting bidirectional displacement to accommodate shaft elongation from thermal expansion. Additionally, the bearing clearance was relatively larger than that of deep-groove ball bearings, but the load was distributed linearly, which is better than the point load of ball bearings. The bearing's motion trajectory was not circular but elliptical due to the clearance between the rollers (or balls) and the raceway. Under high-speed operation, the clearance caused the load-bearing point to shift, turning the motion trajectory into a parabolic curve.