Bearing condition monitoring - Part 6
November 2006
Maintenance, Test & Measurement, Calibration
The last episode described the successful application of the shock pulse method on ultra slow rotating machines. We are now going to look at how the shock pulse method can help one to not only keep a handle on fan bearing operating and lubrication condition but also with some help protecting the fan from failure due to imbalance.
Most of the currently sold fan protection systems use a combination of overall vibration severity and in some cases temperature. Now if we look at the trend graphs (see Figure 1) we can see that shock pulse is used together with overall vibration severity in the horizontal and axial directions. Cooling water pressure and bearing temperature is also measured. If we look at the trend graphs from left to right we see that there was quite a peak in the horizontal vibration with corresponding smaller increases in temperature, axial vibration and both dBc and dBm. This is a typical case of fan imbalance. Due to the increased load of the imbalance on the bearing, the bearing temperature increased. The increased load (and temperature) also had an effect on the dBc, indicating reduced lubrication efficiency. After the fan was cleaned and balanced, everything returned to normal for a while. The next event was when the axial vibration increased. This was due to a problem on the suction side of the fan caused by an improperly adjusted draught valve. One notices a temperature increase but no change in the shock pulse level. This can be explained by the fact that the axial load carrying capability of the bearing was not reached.
Figure 1. Trend graphs
The next event shows a sharp increase in shock pulse level. This is typical of a failing bearing. If one looks at the change in vibration and temperature after replacement of the bearing one can notice a small change, however the method that showed the problem most prominently was SPM. One can thus see that for imbalance problems overall vibration is the best method to use, but for detecting bearing failures there is nothing that can touch SPM.
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