Detecting Imbalance, Misalignment and Looseness

Imbalance is the condition of a rotating component where the center of rotation is not the same as the center of mass.

Looseness is characterised by harmonics of TS with raised noise floor. Waveform displays random impacting which may be in a non-repetitive irregular pattern. Highest amplitudes typically occur in the radial direction, specifically the vertical plane for horizontally mounted equipment.

The number of turning speed harmonics and their amplitudes increase with the severity of the problem.

Unstable phase characterises looseness.

 Fractional harmonics may develop due to looseness.
 Common causes of imbalance:
  • Material buildup
  • Wear
  • Broken or missing parts
  • Improper assembly or poor design
  • Thermal distortion
Imbalance vibration is always at 1x turning speed (TS).
  • Turning speed peak amplitude increases with icrease in speed
  • Little axial energy except with overhung machines
  • Little or no turning speed harmonics.
  • Simple, sinusoidal, periodic waveform
  • One event per shaft rotation
  • Little or no impacting

Misalignment is when rotational center lines of coupled shafts are not colinear.

High, 2 x Turning Speed (TS), peak characterises offset misalignment; a high 1 x signifies angular misalignment, though both frequently combine to cause 1 x TS and 2 x TS in the spectrum. Highest amplitudes typically occur in the radial direction for horizontally mounted equipment; over hung rotors may exhibit higher amplitudes in the axial plane.

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