Gene Vogel
EASA Pump & Vibration Specialist 

There are many common causes of high vibration on rotating machinery; too many to list here. But often, what would otherwise be an acceptable level of vibration is amplified by resonance. All machines are susceptible to resonance. Resonance occurs when the natural frequency of some machine component coincides with an exciting force. When resonance occurs it is the combination of exciting force and a natural frequency that results in high vibration; both must be present at the same frequency for resonance to occur. When resonance does cause excessive vibration, it is important to identify the natural frequency and the mode shape of the vibration. A simple bump test, conducted with the machine not running, is a good first step in identifying the natural frequency (Figure 1).

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