VIBRATION - SPECTRUM ANALYSIS

When excessive vibration is detected, it is usually measured with a modern digital instrument and an accelerometer. The accelerometer is placed on the machine bearing (four noted below), and a very small signal from the piezo-electric crystals are amplified and sent to the data-collector/analyzer. This signal is processed and can be displayed as;
MILS ---------- displacement    (0.001”)
IN/SEC -------- velocity (severity noted below)
IN/SEC/SEC -- acceleration
SKETCH of MACHINE

< 0.10 Acceptable
0.10-0.30 Fair
0.30-0.60 Rough
0.60-1.00 Very Rough
> 1.00 Extreme
SEVERITY CHART
The instrument also converts the wave-form (raw data) to a graph, commonly known as a “spectrum”. The plot has amplitude (in/sec) on the vertical axis verses frequency along the base line. The frequency is measured in cpm (cycles per minute), or hertz (cycles per second). The frequency is the key to analyzing the vibration, and determining the source of vibration. This allows the Technician to provide recommendation to reduce the vibration or perform dynamic field balancing. Before attempting a “field balance”, the vibration must contain a strong one times frequency (1x) or fundamental frequency in the spectrum. For instance if the fan noted above was running at 1000 rpm, the spectrum must contain over 75% of the energy at 1000 cpm, preferably in a radial direction (horizontal or axial).