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11
Fast Fourier Transform (FFT)
• Looseness is evidenced by a marked increase in the number of harmonics.
If there is looseness in the main support of the asset, an increase in the
fundamental frequency amplitude will also be noticed.
Figure 11.1 Graph on left shows a normal system; Centre graph shows a balance issue;
Graph on right shows a misalignment issue.
Figure 11.2 Graph on left shows a loose equipment issue (additional frequencies near the
fundamental); Graph on right shows an impulse vibration (between 3rd and 4th harmonic) .
Frequency of the impulse will vary from machine to machine (speed, size, etc.).
#NAS100051; r. AC/76460/77189; en-GB
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