Appearance of Demodulated Spectra

The typical demodulated spectrum from an accelerometer connected to a rolling element bearing will usually have a fairly uniform and level “noise floor” with discrete peaks rising above it, as shown in the following figure. If the loading on the machine increases, the entire noise floor and peaks will rise in level, but, and this is the crucial part, the relative height of the peaks above the noise floor will remain almost exactly the same. This means the loading of the machine is not nearly as important as it is in measuring vibration directly, and the demodulated spectra are more consistent in their appearance.

The noise floor in demodulated spectra is generally quite smooth and uniform in level, in contrast to the random nature of the noise in conventional spectra. This is because the initial high-pass filter filters out almost all the random noise in the vibration signal.

Typical Appearance of a Demodulated Acceleration Spectrum

The following hypothetical spectra represent the progression of damage in a rolling element bearing. It must be borne in mind that this is meant to be a guide only, and must not be taken as an absolute standard that applies to all machines. The data presented here are a distillation of the analysis and verification of many hundreds of demodulated machine spectra collected over a period of about 10 years on a variety of industrial machines. However, there is no substitute for knowing the characteristics of your particular machine and especially trending the rate of increase in bearing tones in demodulated spectra.

Stage 1

The figure shown above is a conventional vibration spectrum in velocity dB alongside a demodulated spectrum of the same measurement scaled in dB volts. The VdB spectrum shows a few run speed harmonics and a normal noise floor at a low level. The demodulated spectrum shows a smooth noise floor at an arbitrary level we can use as a reference.

Stage 2

The next figure above shows the first stage of bearing degradation due to a tiny flaw in the inner race. The conventional spectrum shows very little if any bearing tones and the same residual looseness indicated by the harmonics of run speed. The demodulated spectrum, however, shows the bearing tone at 2 - 3 dB above the reference noise floor, and also some run speed harmonics. Run speed harmonics in the demodulated spectrum indicate a small increase in looseness due to bearing clearance increasing. They may or may not be apparent at this stage.

At this stage, the bearing need not be replaced, but its condition should be monitored closely.

Stage 3

The next stage of degradation is shown above. The conventional spectrum still does not show any bearing tones at significant levels. The demodulated spectrum has bearing tones at 5 to 10 dB above the reference noise floor. The bearing is in poor condition, but still may have significant service life left.

Stage 4

Here, the bearing has degraded to unacceptable condition. Bearing tones appear in the velocity spectrum, and also appear with run speed sidebands in the demodulated spectrum. Note that the entire demodulated spectrum has risen in level about 10 dB, and the bearing tones are 10 dB or more above the floor.

Stage 5

In this stage, the bearing needs to be replaced immediately. Bearing tones with 1X sidebands appear in both spectra, along with run speed harmonics in the demodulated spectrum. Note that the noise floor in the demodulated spectrum has risen by nearly another 10 dB.

Stage 6

In the figure above, the spectra indicate total failure of the bearing is imminent. Bearing tones are missing in both spectra because the fault has become distributed over the race, rather than being localized. The increased harmonic content in the conventional spectrum is due to increased clearance between the balls and the races.