Order Normalization

Order normalization is performed by most condition monitoring software, and under certain conditions it is possible for the software to select the wrong peak as the 1X component. For this reason it is important for the analyst to verify that the normalization was correctly done if a spectrum looks vastly different than other spectra taken from the same machine. In such a case, the analyst must re-normalize the spectrum.

Following is a non-order normalized spectrum, scaled from zero to 30,000 RPM.

Conventional Vibration Spectrum

Note that many peaks appear to be equally spaced, but it may be difficult to tell which one near 20,000 CPM is a shaft harmonic.

The next figure is a normalized spectrum scaled from 0 to 10 orders. Note that the harmonics of turn speed are integers on the frequency scale, and that the peak below 7X is immediately seen as a non-synchronous component.

Order Normalized Spectrum

Order normalization of spectra has the following advantages:

The fundamental turn speed is instantly recognizable at 1.0 order.
Harmonics of the turn speed will be integers
A second shaft in a gear-driven machine will have an order equal to the gear ratio
Excitation frequencies, such as gear mesh and pump vane pass, are readily recognized because their order is equal to the number of elements
Bearing tones will be non-integer, often the only major non-integer components
Sidebands around bearing tones will be easily recognized because they will be at the tone order ± 1, ±2, etc.
Most important: Because machine speed is almost never exactly the same from test to test, the peaks in the spectrum will not be at the same frequencies, and the spectra cannot be averaged. Normalized spectra have the peaks at the same orders from test to test, and they can be averaged without smearing.

DLI Engineering provides products and services for Predictive Maintenance including vibration analysis instruments, monitoring and diagnostic software, and consulting for CBM programs.

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