Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 6, Guide for Possible Sources of Vibration Based on Frequency
and Amplitude
Section 2, Less than 1 x rpm
Looseness, such as bearing "wiggle" loose
in housing; other loose parts |
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Most often at exactly ½ x rpm. To be stimulated,
it usually has to have enough vibratory force from another source
such as unbalance or misalignment. Sometimes accompanied by "strange"
harmonics above 1 x rpm such as 1 ½ x, 2 x, 2 ½ x,
etc. Can also be seen as higher than usual harmonic amplitudes up
to and beyond 5, 6 and 7 x rpm. |
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Rub (with no resonance) |
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Exact fractions of 1 x rpm. Usually exactly ½ x rpm. Often
accompanied by harmonics above 1 x rpm such as 1 ½ x, 2 x,
2 ½ x, etc. Often shows larger than usual harmonics at 3
x, 4 x, 5 x -- up to approximately 6 to 8 x rpm (depending on the
intensity of the rub). |
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Oil whirl (journal bearing) |
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Slightly less than ½ x rpm such as
43 percent to 46 percent of 1 x rpm.
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Rub, exciting a sub-harmonic resonance of shaft or rotor |
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Exact fractions of 1 x rpm, such as 1/4 x rpm, 1/3 x rpm, ½
x rpm, 2/3 x rpm, 3/4 x rpm, etc. Seems to show up only when rotor
resonance is near an exact fraction. |
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Rolling element bearing cage defect |
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Sometimes exhibited at less than 1 x rpm. Varies with the bearing
design. Most cage defect calculated frequencies in range of 35 to
46 percent of rpm. This non-synchronous vibration can easily be
confused with oil whirl frequencies (journal bearings). Often does
not show at calculated frequency but instead at harmonics of that
frequency. |
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Defective belts |
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If unbalanced belt, frequency will be at rotational frequency
of belt. |
Textbook
Index
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