Outdoor Conveyer Belt system
The following example is from an outdoor heavy-duty conveyor belt used to transport earth. The belt runs uphill and is about ½ mile long and is four feet wide. The drive is through a large gearbox at the upper turn-around roller turning at about 70 RPM. The data shown in the example was measured at the far end of the belt at the lower turn-around roller bearing. The bearings in these rollers are spherical roller bearings with two sets of rollers each. These bearings will tolerate a significant amount of angular misalignment due to the spherical shape of the outer race. They will not, however, tolerate very much thrust loading.
The conventional vibration spectrum, shown below, was measured in the radial direction on the bearing housing:
The cursor is set at the turn speed of 71 RPM, with harmonics cursors activated. The frequency scaling is in orders of run speed. The large peaks at about 12X with a harmonic and at14X with 2 harmonics could be due to bearings in the small support rollers under the belt, and not from the bearing being monitored. The spectrum is complex, with a high noise floor due to the several hundred other rollers that support the belt. It is difficult to make a definite diagnosis from spectrum since the machine is so complex.
The following figure is the demodulated spectrum measured from the same location. Its frequency span is 35 orders of the roller turn speed.
Note here strong peak at 14.9 orders with 1X sidebands around it. The noise floor is quite flat and uniform compared to the conventional spectrum. The small flag indicates the turn speed component at 1 order.
14.9X is recognized as a bearing tone since it is not synchronous with the roller turn speed and has 1X sidebands around it. The sidebands indicate that it represents an inner race fault in the bearing. This is because the fault goes in and out of the bearing load zone at once per revolution, modulating the amplitude of the bearing tones at the turn speed.
The bearing was replaced, and the next figure is a photograph of the inner race of the old bearing:
It is seen that the fault is localized and only occurs on one side of the race. This indicates that the bearing experienced thrust loading, relieving the force on the other half of the race.
The following figure shows the outer race of the same bearing:
The outer race damage is localized in the load zone of the bearing.
This is a classic case of the demodulated spectrum showing an inner race fault and also eliminating almost all of the contaminating noise that was so evident in the vibration base band spectrum.
The defective bearing was replaced, and the following figure shows the conventional vibration spectrum form the same measurement location:
Note that this spectrum resembles the first conventional spectrum, shown earlier. There are still plenty of run speed harmonics and noise components. This illustrates that the bearing fault did not present a very effective signature in the original spectrum. This, of course, is due to the excessive complexity and noisiness of the spectrum.
Now, look at the demodulated spectrum, taken at the same time and location as the previous conventional spectrum, shown next:
In this spectrum, we find no bearing tones, and the smooth, uniform noise floor that is characteristic of a classic demodulated spectrum. This illustrates the effectiveness of the demodulation process in zeroing in on localized faults and eliminating noise components that come from more distant parts of the machine.
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