Vibration Analysis Case Study - Fan Drive Motor Bearing Defect

Applied Diagnostics Ltd - Precision with Pride

Introduction

This industrial fan comprises of a 260kW motor driving an overhung fan via 5 V - belts. The fan is supported by two spherical roller bearings situated at each end of the fan shaft. Vibration data is taken from this unit on a monthly basis, as part of a Condition Based Monitoring regime. Due to its high criticality in the plant process, an unplanned failure could severely reduce production.

Motor Trend Monitoring

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Frequency Spectrum Analysis

In both instances prior to motor bearing wear occurring, the waveform peak-to-peak trend was seen to increase, indicating a potential problem. An inspection of the frequency spectrum was made 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 was seen to return to more acceptable amplitudes. The old bearings were cut in half to reveal their internal condition and analyse the Root Cause of the wear. In both cases “Electrical Fluting” was evident. This is caused by stray electrical currents which penetrate the bearing raceways and cause the horizontal ridges to develop. An examination of the first bearing replacement revealed secondary wear in the form of spalling to the raceway and ball bearings. Had this been left the bearing would have failed, causing costly production losses and potentially secondary damage to the motor. It was recommended that insulated bearings were fitted to isolate the stray currents which were causing this problem and alleviate this problem from occurring again.

Motor Bearing Damage Photos

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