Cooling Tower Drive Shaft Coupling Failure
Case History Number 1

A vibration survey was requested on cell number 1 of a 6 cell cooling tower. This request was due to high vibration of the motor, reported by operational personnel.
Historically the overall vibration levels recorded at the motor had been between 0.8 mm/s rms and 2 mm/s rms, see Figure 1 below.

Figure 1 - Trend plot indicating healthy overall vibration levels.

Vibration data collected on the 7th-Jan-1997 indicated a significant step change in the overall levels recorded at the motor. The worst indicated area was at the drive end location in the vertical direction with a recorded overall value of 19.8 mm/s rms, see Figure 2 below.

Figure 2 - Trend plot indicating high step change in overall vibration levels.

The vibration spectrum recorded from this position indicated the dominant component to be that of 25Hz (Motor rotational frequency) see Figure 3. A recommendation to examine the coupling condition between the carbon fibre drive shaft and the fan gearbox was made.

Figure 3 - Vibration spectrum indicating a high imbalance condition.

On examination of the coupling, it was evident that one half of the coupling rubber had detached see Figure 4.

Figure 4 - Photograph of damaged coupling.

After the installation of a new coupling, the machine was brought back on line and a further set of vibration data collected. The source of the problem had been detected and rectified. Due to prompt action by all parties secondary damage had been prevented.

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