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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 8, Harmonics that are Measured in Velocity Terms (not Displacement or Acceleration) as Units

The peaks for the "lower harmonics," at frequencies above 1 x rpm but below the "higher frequencies" range, most often originate from shaft misalignment. They can also originate from other sources such as at vane or blade pass frequencies. More rarely, they may also originate from loose bearings or other mechanical looseness, excessive clearance between the inner and outer bearing races and so on. So far, there are no clear guidelines in numerical form that have emerged to help determine whether the amplitude at each lower harmonic is acceptable or not. Therefore, the following approach is substituted for definite numbers.

Unless there is a resonance to magnify the amplitude at a specific harmonic frequency, the usual lower harmonics that originate from one source, such as shaft misalignment, decrease in amplitude as their harmonic frequencies increase. For example, the second harmonic (2 x rpm) is usually of lower amplitude than the amplitude at 1 x rpm. The harmonic at 3 x rpm is usually a much smaller amplitude than that of the 2nd harmonic. The 4th harmonic amplitude would be smaller yet. As the vibration frequencies of the harmonics get greater and greater, their amplitudes usually get so small that for the 5th harmonic on up, the harmonic amplitudes are usually negligible or not visible. However, please note that this last statement is based primarily on a series of harmonics caused by a vibration source, such as shaft misalignment. If the source for a specific harmonic is, instead, caused by a machine problem that resulted in a frequency of rpm x number of pump vanes, that harmonic can be relatively large.

For all harmonics, absolute numbers are not as good a guide for analysis as an evaluation as to whether the harmonic amplitudes are "higher than usual." Although "higher than usual" is more valid than absolute numbers, those new in vibration probably don't know what higher than usual is as they have not yet experienced enough case histories. Therefore, guidance will be given based on velocity units, but necessarily only on a relative basis.

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