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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   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.
Rub (with no resonance)   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).
Oil whirl (journal bearing)  
Slightly less than ½ x rpm such as 43 percent to 46 percent of 1 x rpm.
Rub, exciting a sub-harmonic resonance of shaft or rotor   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.
Rolling element bearing cage defect   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.
Defective belts   If unbalanced belt, frequency will be at rotational frequency of belt.



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