Argumentation of filter width selection with the use of envelope spectrum in vibration diagnostics of rotary machine defects

The paper shows certain ambiguity of the narrow-band boundary of random processes obtained by different authors. The use of filters of different widths in obtaining the envelope spectrum makes it difficult to compare the obtained results by estimating the depth of amplitude modulation in vibration diagnostics of defects in rotary machines. We studied the results of bandpass filtering of the noise process only, as well as of amplitude-modulated noise. The analysis of the results of filtering of a broadband normal random process with constant spectral density and the representation of the spectrum width of the resulting oscillations through spectral moments showed that processes selected by a filter of no more than 1/3 octave bandwidth should be considered narrowband. Simulation of the amplitude of a broadband noise by the harmonic process and estimating the dependence of the modulating harmonic amplitude, the depth of amplitude modulation, skewness and kurtosis, as well as envelope characteristics on the width of the selecting filter allowed us to establish that a relative filter width of 30% should be applied when the envelope spectrum is used.