Lines of resolution, F-max, bandwidth
After calculation of the FFT on the digital signal, the frequency domain of the signal can be displayed on the collector/analyzer screen. The FFT is a spectrum of amplitude vs frequency. The resolution is the number of lines (or bins) that are used to display the frequency spectrum. The number of lines could be 200, 400, 800, 1600, 3200, 6400 and 12 800. F-max is the maximum frequency selected on the analyzer by the user when the data are collected. Bandwidth is calculated by dividing the F-max by resolution.
It can now be deduced that when resolution is high there is a better distinction between frequency peaks. Selection of F-max upon collecting data requires experience. If F-max is set too high, the bandwidth gets larger and resolution is affected. On the other hand, if the F-max is set too low, valuable high-frequency vibration data could be lost.
Furthermore, some may find it amusing to know that the time required for collecting the data varies inversely with F-max. The higher F-max, the quicker the FFT can be displayed. This is due to a fixed mathematical relationship between sampling rate and the number of bins in the FFT. As a general guideline, the following advice is provided to
select F-max values:
• For general rotating machinery likes pumps, fans, blowers and motors, set the F-max to 20× or 40×, where × is the running speed.
• When measuring vibrations on gearboxes, the F-max setting should be at least three times higher than the gear mesh frequency, where the gear mesh frequency is the number of teeth of pinion and gear times their respective running speeds.
• However, if an analysis on a machine is conducted for the first time, it is advisable to begin by taking two spectra, one at 10× the running speed and another at 100× running speed. This is to ensure that no important frequencies are lost in the high- or low-frequency zone. Once the range of the suspicious frequencies is noted, the F-max setting should be selected accordingly.
After calculation of the FFT on the digital signal, the frequency domain of the signal can be displayed on the collector/analyzer screen. The FFT is a spectrum of amplitude vs frequency. The resolution is the number of lines (or bins) that are used to display the frequency spectrum. The number of lines could be 200, 400, 800, 1600, 3200, 6400 and 12 800. F-max is the maximum frequency selected on the analyzer by the user when the data are collected. Bandwidth is calculated by dividing the F-max by resolution.
It can now be deduced that when resolution is high there is a better distinction between frequency peaks. Selection of F-max upon collecting data requires experience. If F-max is set too high, the bandwidth gets larger and resolution is affected. On the other hand, if the F-max is set too low, valuable high-frequency vibration data could be lost.
Furthermore, some may find it amusing to know that the time required for collecting the data varies inversely with F-max. The higher F-max, the quicker the FFT can be displayed. This is due to a fixed mathematical relationship between sampling rate and the number of bins in the FFT. As a general guideline, the following advice is provided to
• For general rotating machinery likes pumps, fans, blowers and motors, set the F-max to 20× or 40×, where × is the running speed.
• When measuring vibrations on gearboxes, the F-max setting should be at least three times higher than the gear mesh frequency, where the gear mesh frequency is the number of teeth of pinion and gear times their respective running speeds.
• However, if an analysis on a machine is conducted for the first time, it is advisable to begin by taking two spectra, one at 10× the running speed and another at 100× running speed. This is to ensure that no important frequencies are lost in the high- or low-frequency zone. Once the range of the suspicious frequencies is noted, the F-max setting should be selected accordingly.
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