5. The Technical Means of Diagnostics and Balancing.

Question 5.3.

For vibration signal analysis we use the instruments with FFT
for 400, 800, 1600, 3200, 6400 and more spectral lines. Your instruments have no
more than 1600 lines. The question is why?
( The question was asked by a representative of the company LeadFar Technologies,
Inc. from China.)

The question is answered by
Natalia A. Barkova:

verything 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.