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1. General aspects of monitoring and diagnostics.

Question 1.8.

Are there any significant differences between the well known methods of resonance demodulation and envelope detection that are widely used in rolling element bearing diagnostics?
(The question was asked by the participants of "Vibration diagnostics of rotating machine and equipment" seminar)

The question is answered by Alexej Barkov:

These two methods have practically the same technical and hardware realization but have very different physical basics in diagnostics. The method of resonance demodulation investigates the natural oscillations of the bearing elements (fixed race, rolling elements and other) that are excited by separate shock pulses which appear as a result of faulted rolling surfaces contacts. The algorithm of investigation includes the following operations:

  •  natural bearing element oscillation frequency search;

  • extraction of the natural oscillation from the vibration signal, for example by a frequency band filter;

  • enveloping of the extracted part of the signal, for example by detection with the sequential filtration of the low frequency part of the detected signal;

  • spectral analysis of the envelope;

  • the analysis of the frequencies and amplitudes in the spectra of the envelope.

 

The envelope method used in our systems investigates the oscillations excited by both the friction forces and shock pulses. The investigation algorithm differs practically only in the first and the last operations and includes:

  • search of a signal frequency band where there are no resonances and harmonic components (to tune away from natural and harmonic oscillations);

  • extracting high frequency random vibration components with a band pass filter;

  • enveloping of the extracted part of the signal, for example by detection with the sequential filtration of the low frequency part of the detected signal;

  • spectral analysis of the envelope;

  • calculation of the modulation amplitude of the harmonic components of the envelope spectra and analysis of the frequencies and amplitudes of the random vibration modulation.


The difference in these two methods is in that in the first case the natural oscillations are investigated, whereas in the second case the forced oscillations of the bearing elements are investigated. The natural oscillations give maximum information about the features of the oscillating system, for example, about the rigidity, mass, losses in the oscillation system, and only limited information about the oscillating forces that define the initial deflection of the oscillation system from the equilibrium point whereas the forced oscillations give maximum information about the oscillating forces but minimum information about the oscillating system. The rolling element bearing vibration diagnostics is, first of all, the analysis of the features of the oscillating forces but not of the oscillation system that has many individual peculiarities for each bearing unit in each certain machine. From the above, it follows that the envelope method has certain advantages for the rolling element bearing diagnostics but often investigators try to use the resonance demodulation method instead of the envelope method. The question is why? There are technical and physical reasons. The technical reason lies in the fact that the resonance increases the contribution of the investigated components in the measured signal by dozens of times and allows extraction of them from the total signal without alteration, even with instruments made many years ago that have very narrow dynamic range and bad linearity. Modern digital analyzing instruments have both the dynamic range and the linearity sufficient to avoid the use the mechanical resonance to gain the signal and not loose the major part of the diagnostic information. The physical reason lies in the fact that, if there are harmonic components in the part of the signal that is extracted by the filter for further enveloping, then it corrupts the diagnostic information received from the envelope spectrum. The resonance of the bearing elements gain the random vibration components and thus decrease the negative influence of the harmonic components that coincide with the filter frequency band in the features of the measured envelope spectrum. But, if we consider this fact and take into account the capabilities of modern instruments for signal analysis, then we must say that the harmonic components can be extracted from the signal before enveloping using for example adaptive filters. So, modern measuring and signal analyzing techniques enable using completely the advantages of the envelope method to analyze the constrain forces in the friction units not only of mechanical nature but also of air and hydrodynamic origin. This enables solving the friction units diagnostics in more detail than the methods of investigation the natural oscillations excited by the shock pulses. And the last consideration. The method of resonance demodulation is patented (the USA patent of 1974). The envelope method is patented in the USSR in 1978. At least the second patent is already expired. But one should take into account that in the USA there is a patent # 5477730 issued 12.26.95 that concerns the method of separating the harmonic and random vibration components before enveloping that allows the use of the envelope method without the need for concern for the presence of harmonic components in the measured data. We have employed this last technique in data acquisition drivers for hardware from VSC and Data Physics for our DOS DREAM and have similar drivers planned for our DREAM for Windows software for the same manufacturer's current hardware. More information on this technique can be found in our on-line library of technical papers in the Bearing, Data Acquisition section.

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