When carrying out vibration measurements on bearings, it is necessary to take into account which will be the most suitable transducer. Investigations carried out on the reliability of vibration measurements in bearings with hydrodynamic lubrication have led to the conclusion that in these bearings there is an attenuation of the vibration signal, losing part of the information, when taking measurements with contact sensors attached to the bearing housing. As we already saw in chapter 3, there are non-contact sensors on the market called proximeters, whose operation is based on the emission of radiofrequency signals whose contact with the shaft induces eddy currents that depend on the gap between the shaft and the bearing.
This type of non-contact sensors are the most recommended for vibration measurements in plain bearings, and the most convenient way of mounting these sensors is by placing on each bearing two radial transducers 90° apart. This will allow the possibility of obtaining shaft orbits using a multichannel analyzer.
At present, the application of these sensors is limited to online monitoring systems, through which we obtain continuously on the screen an overall vibration reading. In advance, alarm values are established according to API, ISO, VDI, etc. or following the manufacturer's recommendations, so that if these vibration levels are exceeded at some point, the machine will automatically stop its operation. In modern online monitoring systems, more information can be obtained: frequency spectra, waveforms, trend plots, orbits, DC gap, transients, etc. that in addition to protecting the machine help us to determine and locate the existing problems to act only on the rootcause of the problem. The frequency bands that we recommend for the monitoring of plain bearings are those that appear in the following table.
|Oil whirl||0.2x to 0.8x RPM|
|Imbalance, clearance and misalignment||0.8x to 1.5x RPM|
|Misalignment and clearance||2.5x to 15x RPM|
|Lubrication issues||1 kHz to 20 kHz|