Vibration Analysis – Mechanical Looseness

Many texts and seminars today simply refer to a general term called Mechanical Looseness and that it either can be detected by high vibration at 2X RPM or multiple running speed harmonics.

However, a comprehensive study of a great amount of available literature has revealed that there are at least 3 different types of Mechanical Looseness, each of which has its own characteristic vibration spectra as well as vibration phase behavior.

1. Structural Frame/Base Looseness (Primarily 1X RPM);

2. Looseness Due To Rocking Motion or Cracked Structure/Bearing Pedestal (Primarily 2X RPM);

3. Loose Bearing in Housing or Improper Fit between Component Parts (Multiple Harmonics Due To Non-linearity Often Induced By Impulse Events): Bearing Loose in Housing, Excessive Internal Bearing Clearances, Bearing Liner Loose In Its Cap, Loose Rotor, Bearing Loose and Turning on Shaft.

One of the important facts about each type of mechanical looseness is that it alone is not a cause of vibration. Instead, looseness is a reaction to other problems which are present such as unbalance, misalignment, eccentricity, bearing problems, etc.

However, resolution of the other problems themselves will often remove most of the symptoms due to looseness. However, this is often virtually impossible in reality because such steps would require extraordinary levels of precision of alignment or balancing.

Therefore, in these cases, the looseness condition will first have to be resolved. Then, if remaining vibration is still high, other steps such as alignment and balancing can be accomplished with much greater ease than before the looseness conditions were resolved.

(Technical Associates Of Charlotte, P.C., Vibration Analysis – Technical Associates Level II)