Cooling Tower Fan Gearbox Damage
Case History Number 8
Gearmesh vibration frequencies are
typically easy to recognise, but not easy to interpret. This is due to two reasons:
1) It is not normally possible to place the transducer close to the problem gears.
2) The number of vibration sources in a multi-gear drive unit result in a complex array of
gear mesh, modulation and running speed frequencies.
For the analysis of suspected gear problems a high resolution spectrum analyser is
required to enable a high frequency range spectrum to be taken without loss of sideband
data. Sidebands are very important and in most cases enable the analyst to determine which
of the two meshing gears is at fault.
The following case history shows how spectral analysis proved invaluable in helping
identify a problem gear in a cooling tower fan gearbox.
Vibration data is collected from the electric motor by means of a vibration spectrum
analyser and a magnetically mounted accelerometer.
Due to the inaccessibility of the fan gearbox, permanently mounted accelerometers are
fitted to the input and output shaft bearing housings and cabled out to a safe location.
Figure 1 below shows the spectrum collected from the gearbox output shaft.
Figure
1 - Vibration spectrum indicating gear damage.
From the
above spectrum, and knowing the fan was running at low speed, we could identify the input
bevel gear meshing frequency. Harmonic activity at the input shaft frequency of 16.5 Hz
are evident at either side of the input gearmesh frequency. From this information, and
technical data from the gearbox (Figure 2) it was concluded that the input bevel gear had
sustained some degree of gear tooth damage.
Figure 2 - Layout of
gearbox internals.
Examination
of the gearbox proved the analysis to be accurate, with damage to several of the bevel
gear teeth evident. Figure 3 below is photographic evidence of the damaged gear teeth.
Figure 3 - Damaged input bevel
gear teeth.
It is
estimated that a saving of £4500 was made due to early fault detection,
however, had this unit catastrophically failed then secondary damage could have run into many
THOUSANDS of pounds more as the potential for damage to blades and structure is high.
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MIET FIDiagE
MICML
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