Evaluating Machine Vibration Spectra
Most machines have a relatively simple set of vibration forcing frequencies, determined by the geometry of the machine and its speed. The existence of other frequencies than the forcing frequencies, such as harmonics of 1X, in the vibration signature of the machine indicates non-linearities, and the combined magnitude of these new frequencies is a good indicator of the overall health of the machine. As a machine wears, its clearances typically become greater and its vibration signature becomes more complex due to generation of harmonics and sidebands.
In trending the vibration level of a machine over time, a rise in the level of a forcing frequency indicates a change in the mechanism causing that particular forcing frequency, but does not necessarily indicate any damage to the machine. For instance, an increase in 1X at a motor bearing indicates an increasing imbalance condition, but if harmonics of 1X begin to appear, this indicates damage, such as bearing clearance increases, looseness, or cracking of the structure. Therefore, a strong 1X vibration means the rotor should be balanced, but the appearance of harmonics of 1X means the bearing and surrounding structure should also be inspected for damage.
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