Using vibration to diagnose looseness

Mechanical looseness, or the improper fit between component parts, is generally characterized by a long string of rotating frequency harmonics or 1/2 rotating frequency harmonics at abnormally high amplitudes.

It’s important to note that these harmonics may be sporadic. For example, looseness may display peaks at 2x, 3x, 4x, 5x, 6x, etc., or at 3x, 3,5x, 4x, 5,5x, 6x, etc.

What causes looseness?

Possible causes of wear/looseness are:

  • The machine has come loose from its mounting
  • A machine component has come loose
  • The bearing has developed a fault that has worn down the bearing elements or the bearing seat

The effects of looseness

  • If the looseness is bearing related, the effects are the same as imbalance, only more severe
  • If looseness is generated from a component (for example, a fan blade), there is a possibility the part will become detached, causing secondary damage.

Diagnosing the problem

Use FFT spectra and phase to diagnose looseness.

Spectrum analysis

Typically, looseness is identified by an abnormally high running speed amplitude followed by multiples or 1/2 multiples. Harmonic peaks may decrease in amplitude as they increase in frequency (except at 2x, which, when measured in the vertical position, can be higher in amplitude).

Chart 1 - Running speed 0901d19680559c7e


Chart 2 - Running speed 0901d19680559c7f

Summing it up

  • If there are a series of three or more synchronous or 1/2 synchronous multiples of running speed (range 2x to 10x), and their magnitudes are greater than 20% of the 1x, then there may be mechanical looseness
  • If the machine is rigidly connected (no coupling or belt), and the radial 2x is high, then there may be mechanical looseness