Spectrum Plots

Datat Interpretation Spectrum Plots

Spectrum Plots

Spectral Tools

Vibration spectra provides important clues to machine problems. There are tools provided in all software packages that help with this analysis. The most important are:

  1. Moveable cursor - A "base" cursor that can be moved to any frequency and identifies the amplitude at that frequency.
  2. Harmonic cursors - Activating this tool creates additional cursors (as many as are required) that appear at integer multiples of the base cursor. If the base cursor is located at 1x rpm, harmonics will appear at 2x, 3x, 4x, etc. This is the most important analysis tool available. All harmonic cursors are at higher frequencies than the base cursor
  3. Sideband cursors - Activating this tool creates additional cursors at frequencies to either side of the base cursor. If the 1st (closest) sideband cursors are located 50 cpm to either side of the base cursor, additional sideband cursors (as many as required) will each be located an additional 50 cpm away. For instance, the 2nd sideband cursors will be 100 cpm away from the base cursor, the 3rd will be 150 cpm away, etc.

These three tools are sufficient for the beginner and for 99% of most analyses. There are, of course, other useful tools such as "labels" and frequency groups but that is more vendor specific and should be learned in a software class and through experience.

Remember, ALL spectral analysis is limited in its use and accuracy by the spectrum resolution.

Spectrum Terminology

Commonly used terms include:

How is a "Spectrum" plot created?

What the transducer actually 'senses' is an analog signal - one that mirrors the actual movement of the bearing at the location of the transducer.

The signal processing that follows the analog signal collection consists of a couple of mathematical processes:

The FFT process is extremely complex when being applied to mechanical vibrations. Although a fairly reliable and useful tool, it MUST be understood that a spectrum is always suspect because these mathematical processes (A/D and FFT) often cause either or both of the following to happen:

For these reasons, it is strongly recommended that at the very least the time domain be used where it is most useful and the spectrum is the weakest:

The reasons for this lie in what the FFT process actually does and what factors influence its output (the spectrum).

What is a "Spectrum" actually made up of ?

What is a 'Spectrum' actually made up of?

The spectrum resolution (width of each frequency range) equals the Fmax divided by the Lines of Resolution.

Further Information

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