Evaluating Demodulated Bearing Spectra

When looking at demodulated spectra, it is important to keep in mind that they are not the same as normal vibration spectra. Spectral components in the demodulated spectrum of a bearing at the bearing forcing frequencies do not represent actual vibration at these frequencies. This is because the high-pass filter has filtered all the energy at these forcing frequencies out of the signal before the demodulation was performed. The spectrum of the demodulated vibration signal indicates the influence of the bearing faults on a high-frequency band of vibration that is not related to the forcing frequencies. Even though the vibration sensor is an accelerometer, the demodulated spectrum should not be scaled in acceleration units. This has led to confusion in the industry as to what is the proper amplitude unit to use in displaying demodulated spectra. It is the opinion of the author that a simple scaling in voltage decibels without reference to any physical vibration parameters is best. DLI Engineering Corp. has selected decibels related to 1 millivolt as the default scaling for demodulated spectra in the DC-7B data collector. This is abbreviated dBmV. The one-millivolt reference is not particularly significant, but it assures that all dB values likely to be encountered in practice will be positive, and the numbers will be in ranges that are familiar to persons used to working in dB Velocity.

The use of the decibel, which is a logarithmic ratio rather than a unit, is appropriate because the demodulated spectra are not evaluated in terms of absolute levels, but rather as signal to noise ratios, as will be described in the next section.