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Fan Bearing Failure

3000 rpm Fan

Abstract: A vibration increase was noted in a 3000 rpm fan. On shutdown, one bearing was found to have serious defects.

The following case study was taken from a regular survey of plant located at the site one of our customers.

Background: The aptGroup carries out routine surveys on cooling fans at a manufacturing plant in Sydney's west. These fans are critical to the operation of the plant. Due to their high operating speed, any failure tends to be catastrophic, so condition monitoring is vital.

Method: A portable data collector/analyser and accelerometer were used to take the vibration measurements. The accelerometer was attached to the machine using a magnet.

Measurements of velocity (0-600 Hz), acceleration (0-5000 Hz) and acceleration enveloping (5 - 40 kHz) were made.

Results: The fan showed an increase in vibration at a survey in August 1999. At this point an alert was raised, indicating that a problem was present.

The subsequent survey in September showed a dramatic increase in vibration (fig 1), with a very noisy spectrum (fig 2). The vibration spectrum does not show distinct bearing defect frequencies, but rather an increase in broadband noise is seen.

Shortly after this survey, the fan was shut down and the bearings replaced. It was found that one row of the twin-row ball bearing was completely missing.

Figure 1: Vibration Trend, showing levels before and after

Figure 2: Acceleration Spectrum from Damaged Bearing

Conclusion: Vibration monitoring prevented a catastrophic failure in this machine. There are two other observations to make:

The rapid rate of deterioration. The machine went from having slightly raised levels to dangerous levels in a few weeks, while a lower speed machine may have taken months.

The lack of bearing defect frequencies. The spectra never did show the classic bearing defect frequencies, even though the bearing was seriously damaged.