There are several important clues to diagnosing a natural frequency resonant amplification problem.

  1. Natural Frequencies are Non-Synchronous
  2. Natural Frequencies do NOT have Harmonics (unless we are talking musical instruments)
  3. Resonant Response if Very Directional (A, H, V)
  4. A Phase Shift of 90 degrees ALWAYS accompanies a Natural Frequency.
  5. Natural Frequencies MUST be excited by another frequency to become resonant.
  6. Resonance problems tend to confuse Balance Programs (phase shift).
  7. Lots of Damping creates a blunt-wide resonance response in the spectrum (less amplification).
  8. Very Little Damping creates a sharp resonance response in the spectrum (more amplification).
  9. A ring-down in the Time Waveform indicates a natural Frequency.
  10. A successful Bump Test on a Large Structure requires a large softer impact source (hammer).
  11. A successful Bump Test on a Small Structure requires a small harder impact source (hammer).
  12. For Fluid Film Bearings, elongated elliptical Prox Probe Orbits indicates a potential resonance.
  13. Roll Barring issues usually have a resonance contribution to the overall problem.

As with any sure-fire clear-cut guidelines - variations do exist and will occur. Testing is the key to resonance detection.

Tip by Dan Ambre, P.E. [Modalguy]
Full Spectrum Diagnostics, PLLC

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