Vibration Analysis Case Study - Pump Drive Motor Bearing Defect

Applied Diagnostics Ltd - Precision with Pride

Introduction

This unit pumps river water as part of a continuous process within a chemical plant. It comprises of a 280kW motor driving a pump through a direct coupling. The motor is supported by two rolling element bearings at each end. The pump impellor is centrally supported on two sleeve bearings. Due to its high criticality in the plant process, an unplanned failure could severely reduce production. Vibration data is taken from this unit on a monthly basis, as part of a Condition Based Monitoring regime.

Motor Trend Monitoring

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Frequency Spectrum Analysis

The initial baseline surveys of this unit showed that the waveform peak-to-peak trend fluctuated around the fault alarm indicating a potential problem. The frequency spectrum was analysed to diagnose the cause of the increase:

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Follow Up Actions / Conclusions

It was recommended that the motor bearings were replaced, following this action the trend returned to more acceptable amplitudes. When the bearing was cut in half to examine the Root Cause of the problem, ‘Electrical Fluting’ was noted in the outer raceway. Stray electrical currents inherent within this motor type penetrate the bearing raceways and cause horizontal ridges to develop. Had this motor been left to run, surface spalling of the raceway would eventually occur and cause the bearing to fail. It was recommended that insulated bearings are fitted in the motors to reduce the effects of electrical erosion. Longer intervals between motor overhauls have now been achieved.

Bearing Damage Photo

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