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