In addition to the common "Bump Test" method to determine a system's natural frequency, an analytical technique called "Coastdown" is typically included in most data collector and analyzer systems for Predictive Maintenance (PdM). This method involves the continuous monitoring of the vibration level using a peak hold function while shutting down the equipment. Typically, one would expect the vibration level to drop at a steady rate. Hence, if the vibration levels start rising at any time while the equipment is being shut down, the speed at which the amplitudes increase is a possible natural frequency. Sometimes, a waterfall plot of the spectral data is used to identify a peak vibration level at a certain speed during the coastdown. To more accurately pinpoint the resonance condition at the speed where the vibration amplitude increases, it is recommended that the phase of the vibration also be captured while the equipment is being shut down. By adding a photo tachometer and a piece of reflective tape on the rotating shaft of the equipment along with the standard vibration transducer being in place, one can monitor both the vibration amplitude and its phase. This will allow the analyst to observe the amplitude and phase shift at all running speeds of the equipment. If there is no resonance excited by the turning speed, the vibration levels should drop at a steady rate. If the vibration peaks occur at a certain speed and the phase shifts by 180 degrees, this indicates a natural frequency of the equipment or structure. The actual natural frequency is the frequency situated in the middle of the phase shift (90 degrees). Using this technique, a Bode plot is sometimes displayed where vibration amplitude and phase are plotted in a stacked format against machinery running speed.
Tip provided by: Dennis Shreve, Channel Support Engineer Commtest, Inc. Tel: +1 865.588.2946 or +1 877.582.2946