Synchronous vs. Non-Synchronous
Discrete Non-Synchronous Vibration Signatures:
· Oil Whirl
· Trapped Fluids in Rotor
· Rolling Element Bearing (REB) Faults
· Electrical Faults
· Belt Frequencies
· Another Rotating Shaft
Random Non-Synchronous Vibration Signatures:
· Turbulence / Turbulent Flow
· Rotor / Stator Rub
· Electro-Erosion / Fluting
· Late Stage Mechanical Looseness
· Paper Roll Barring / Corrugation
Discrete Non-Synchronous vibration produces a “steady” series of peaks in the spectrum. This peak series (by definition) does not occur at a multiple of 1x RPM. The list of possible sources can be pared down by inspection of the mechanical design of the machine. For example, direct coupled equipment will not generate belt frequencies. Vibration on pumps and fans will not produce electrical faults. Rolling Element Bearing machines will not have oil whirl. Solid shafts will not trap fluids; and so on.
The most common Non-Synchronous fault is attributed to REB defects. These peaks will occur at “expected” frequencies if the bearing vendor and model numbers are known.
Random Non-Synchronous vibration signatures can represent themselves as a series of discrete peaks at fractions of turning speed (rubs), as a “haystack” of equally spaced peaks or sidebands as in fluting, or as elevated random ranges of frequencies, or noise floor.
Like other vibration problems, the art of eliminating potential sources will help the analyst narrow down the root cause. Cavitation, Turbulence and Fluting problems tend to occur in different frequency ranges. Cavitation and flow related turbulence require a pump, fan, or compressor. Fluting issues develop in DC motors or AC motors equipped with Variable Frequency Drives (VFD’s). Rubs tend to be intermittent events. Inspection of the Time Waveform can be helpful in detecting these problems. Mechanical looseness starts out as a synchronous multiple type fault; but degrades into broad band noise floor. The same is true for late phase REB defects as wear increases.
Of course I have not mentioned measurement error. Even the most experienced analyst can collect “bad” measurements. Trended data is the key to documenting the progression of a machinery fault. A second opinion can always help as well.
Tip provided by Dan Ambre, P.E. [Modalguy]
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