Optimisation of bearing diagnostic techniques using simulated and actual bearing fault signals

In this paper, bearing fault vibrations are modelled as a series of impulse responses of a single-degree-of-freedom system. The model incorporates sli ght random variations in the time between pulses so as to resemble actual v ibration signals. Although the bearing fault harmonics in the raw spectrum are caused by the random fluctuations to smear over one another, they remai n quite clear in the spectrum of the envelope. However, the envelope spectr um is still prone to masking by discrete and random noise. Therefore, the s imulated bearing fault signals were used to investigate the efficient appli cation of self-adaptive noise cancellation (SANC) in conjunction with envel ope analysis in order to remove discrete frequency masking signals. Two way s of combining these techniques have been suggested, both of which require the original signal lo be band-pass filtered and frequency-shifted in order to reduce the number of samples to be processed by SANG. The subsequent en velope analysis can then be performed by using the Hilbert transform techni que or band-pass rectification. Band-pass rectification is simpler but requ ires extra zero padding above and below the demodulation band, making the l ength of the signal processed by SANG twice as long as with the former meth od, but still only a fraction of the length of the original signal. On the other hand, the Hilbert technique requires an extra forward and inverse dis crete Fourier transform operation compared with band-pass rectification. Th ese two methods reduce the masking effects in the envelope spectrum by remo ving pseudo-sum frequencies or placing them outside the frequency range of interest. This is illustrated with examples of simulated and actual vibrati on signals. The removal of discrete frequency noise using SANG is also demo nstrated for actual vibration signals. The threshold for which analysing th e squared envelope or its higher powers gives an improvement in the envelop e spectrum has also been defined using simulated and actual vibration signa ls. The treatment in the paper is qualitative and non-mathematical for purp oses of clarity, but reference is made to a quantitative treatment of the e ffects of masking. (C) 2000 Academic Press.