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## 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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