Analyzing spectra using the process of elimination
After collecting all the information needed, you can proceed to analyze an FFT spectrum. Analysis usually follows a process of elimination. Eliminate what is not on the spectrum and what is left is the problem(s).
1. Once running speed is determined, identify the spectrum’s frequency ranges
- Identify any harmonics of running speed (1x, 2x, 3x, etc.)
- Identify bearing fault frequencies
- Identify fan blade frequencies, if applicable
- Identify number of gear teeth, if applicable.
- Identify pump impeller frequencies, if applicable.
- Identify adjacent machinery vibration, if applicable.
- If monitoring an electric motor, identify peaks at line frequencies. Try to find out if they are electrical or mechanical
2. Verify suspected fault frequencies
The spectra may produce peaks at identified fault frequencies. These peaks may or may not represent the indicated fault. Look for harmonics to determine if the identified frequencies were generated from the indicated fault.
- If a peak appears at the fundamental fault frequency and another peak appears at two times the fundamental fault frequency, it is a very strong indication that the fault is real
- If no peak appears at the fundamental fault frequency, but peaks are present at two, three and maybe four times the fundamental fault frequency, then this also represents a strong indication that the indicated fault is valid
3. Determine the severity of the fault
- One way to determine the fault’s severity is to compare its amplitude with past readings taken under consistent conditions
Another way is to compare the amplitude to other readings obtained by similar machines running under the same conditions. A higher than normal reading indicates a problem