Article
Bearing with Cracked Inner Race

by Jim Crowe

ABSTRACT

This short case history does a good job of illustrating the power of enveloping and high frequency techniques at identifying bearing defects. The Peakvue brand of enveloping technique was used successfully by the author in detecting a cracked inner race on a fan bearing. This bearing defect was not seen in the normal velocity data but was easily seen in the Peakvue data.

PREVIEW

“A Twin City 150 HP belt-driven over-hung fan with SKF 22222 taper lock bearings on the fan shaft had been in operation for about 18 months and routinely monitored. Vibration readings showed that the #3 fan bearing (inboard fan bearing) had a defect in the Peakvue® spectrum and waveform.
Peakvue® utilizes a high-pass filter to focus on the high-frequency stress waves, removes higher amplitude low-frequency vibration, and stores the peak value over time. The spectrum indicates the averaged value. Readings from the previous month showed no defect frequencies (Figure 1). The current readings in Peakvue® indicated multiple harmonics of an inner race defect frequency with sidebands at operating speed (Figure 2).
The acceleration data showed no ab-normal amplitude or bearing defect (Figure 3). The Peakvue® spectrum in Figure 1 was 0.245 g peak at operating speed; the waveform was 6.2 g’s peak. Figure 2 shows 1.439 g’s peak at operating speed; the waveform was 35.5 g’s peak. The fan could not be shut down for repairs.”

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