State of the art automatic condition diagnostic, monitoring, and balancing software

5. The Technical Means of Diagnostics and Balancing.

Question 5.4.

 The data collector DC-11 can measure the overall level of vibration but to do it the operator must set the lower and the upper frequency levels, viz. he must select the frequency band pass filter where the signal level will be measured. Why does the instrument have such a possibility and how to use it?
(The question was asked by the mechanic, responsible for machine diagnostics, Sergej Reshetkin from the joint-stock company "SeverStal")

The question is answered by Alexej V. Barkov:

Everything is defined by the accuracy needed to measure the harmonic vibration components' amplitudes. If the rotation frequency of the diagnosed equipment during the measurements is unstable then the components, that have to be in the spectrum harmonic ones (sharp), become to be dithered and the results of their amplitude measurements become much lower than their real value.

Lets make a quantitative evaluation of the necessary equipment rotation speed stability if the measured vibration component is located on the upper border of the spectrum frequency band represented by 1600 frequency lines. In this case the deviation of the rotation frequency has to be less than 1/3 of the frequency band of one line 1/3 x (1/1600) = 0.0002, viz. the deviation has to be less than 0.02% of the rotation frequency. The time of measurement equals usually the time of 50-100 rotor revolutions and to achieve such stability of rotation during such a relatively long time is possible only for very few mainly high speed machines. When the number of frequency lines is about 3200, 6400 and more the spectrum lines with frequencies higher than the rotor rotation frequency in 50-100 times will become "dithered" which leads to additional mistakes in diagnostics.

So for qualitative and certain diagnostics the number of frequency lines in the spectrum must be not more than 1000-2000 or the analysis program has to be provided with special means to restore the "dithered" spectra components including the groups with frequency lines located close to each others.