Bearing Inner Race Damage
Case History Number 2

Vibration data collected from P272 motor over the last 32 months, had exhibited low and acceptable values, typically around 1.5 mm/s rms throughout. Vibration data collected in January 1997 indicated an upward trend at the pump drive end, indicating a probable fault within the pump, see Figure 1 below.

Figure 1 - Trend plot indicating continued increase in overall vibration levels.

Vibration spectra collected from the pump drive end bearing on the 13th January 1997 did not exhibit any of the calculated fundamental defect frequencies for an SKF 6311C3 bearing, although the vibration spectrum did indicate baseline lifting between 24000 cpm and 72000 cpm indicating a 'noisy' bearing, see Figure 2 below.

Figure 2 - Spectral plot indicating base line lifting.

Based on the overall vibration levels and the early signs of bearing defect in the spectrum it was decided to continue running the machine and review the next set of data. 35 days later the machine was re-tested and exhibited a further increase in overall vibration levels along with an increase in broad band activity in the vibration spectrum, see the spectrum plot below. Based on this information it was planned to replace the pump bearings as soon as the production process allowed.

Figure 3 - Vibration spectrum indicating increased baseline lifting .

Examination of the pump bearings highlighted an area of flaking on the inner race of the ball bearing and subsequent damage of the outer race and several balls. Figure 4 below is photographic evidence indicating the damaged area on the ball bearing inner race.

Figure 4 - Photograph of damaged bearing inner race.

Failure of this machine would not have resulted in serious production losses as a spare unit was readily available. However, if a mechanical seal had failed as a result of high vibration or bearing failure, this would have had serious environmental implications due to the nature of the product.

The bearings were replaced and the machine was brought back into service with minimum disturbance to the process. The parts and labour cost saving due to early fault detection was estimated to be £4,000.

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