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This blog post concerns rolling element bearings and not journal bearings.

When a rolling element bearing begins to deteriorate the damage usually manifests itself in one of the races (either inner or outer) followed by the rolling element, and finally the cage.  When the races begin to have defects these tend to excite the natural frequencies of the race which typically show up beyond the maximum frequency that most analyzers collect data to.

The specific defect frequencies are determined by the bearing geometry. One would normally start seeing peaks in the FFT spectrum in the 5× to 7× range and sideband peaks spaced at 1× rotational speed. As the defects progress, harmonics of the component defect frequency will move lower in the FFT with more harmonics showing, while the number and amplitude of the sidebands increases as well.

When you begin to see the defect frequencies of multiple components, then this indicates that the damage is progressing. In the time-waveform’s early stages you will see an increase in the amplitude of the peaks, indicating impacting; as damage increases the amplitude of the impacts will increase and for a time the pattern will resemble what is known as the “angle fish” pattern. This pattern will not last and may not even be seen depending on the frequency of the data collection. The pattern tends to go away because of continued deterioration of the bearing components.