When evaluating any machinerys health, the analyst should:

  • Visually inspect each machine for overall condition of the machine, such as leaking seals, other damaged or incomplete components, cracked welds, deterioration in the grouting, missing clamps and hold downs while visiting site to collect vibration data.
  • Ensure that you can determine the turning speed (TS) at the measurement point location before any critical measurement is taken.
  • Once TS is identified, determine TS harmonic relationships
  • When harmonics of TS are determined, other vibration peaks of interest and other harmonic relationships may be identified for analysis


Every machine will vibrate when excited by a forcing function. EWach machine has one or more natural or resonance frequencies. When any forcing function is near the natural frequency, the resulting vibration will be significantly amplified and could cause premature failure.

A critical speed is when the rotating element is turning at a speed which excites resonance in the machine. Many times a forcing function (such as from a rotating shaft) excites a resonant frequency in another part of the same or nearby machine. This resonant frequency may be identified by an impact test, typically when the machine is off; the machine structure may be “rung” like a bell. Changing a structures stiffness, mass, damping, operating speed and/or reduction of the forcing function will affect the resonance issue and may help sort the problem.

Resonance is an increasing problem in industry because:

  • New equipment is often built lighter
  • More variable speed machinery is being used in industry
  • Machines are often now run at a higher speed