Single-shaft machine
Mark the harmonics of 1X on the spectra. This is simplified if you use a ten-point divider.
Identify the fan blade pass frequency and mark it on the spectra. This is the number of blades multiplied by the RPM. Note the harmonics of blade pass frequency if they are prominent.
Look for bearing tones, which are between the harmonics of the 1X run speed and not synchronous with it. Mark them on the spectra. There are other machine components besides bearings that generate non-synchronous tones. Probably the most common one is belt drives.
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