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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 3, Lower Harmonics

Up to approximately 10 x rpm or number of fan blades x rpm (whichever is greater). Does not include "Higher Frequencies" described later. Some frequencies in this range may look like harmonics but are not true harmonics as they are non-synchronous with 1 x rpm (such as electrical hum and bearing frequency. Amplitudes are usually not high in absolute terms. Review the section “Evaluating Harmonics Due to Shaft-to-Shaft/Coupling Misalignment.”

Shaft/Coupling Misalignment   Primary vibration is still most often (but not always) 1 x rpm. Amplitude at 2 x rpm is larger than usual. Misalignment suspected when 2 x rpm is above 1/3 the 1 x rpm amplitude. When higher 2 x rpm amplitude exceeds ½ the amplitude of 1 x rpm, the symptom is much stronger. Symptoms are very strong when other lower harmonic amplitudes are also "higher than usual." If coupling has segment amplitudes at rpm x # of segments in one coupling, half would also be much larger than usual. Always compare with phase symptoms.
Coupling Binding   Misalignment symptoms increase even though the misalignment itself may not have changed. Instead, the so called flexible coupling becomes less effective due to excessive wear or improper lubrication.
Number of pump vanes x rpm  
Very often accompanied by resonance. If not resonance, check for pump with too low a flow rate. Sometime new product design problem.
Number of blades x rpm   Most often a resonance problem.
Loose base, "wiggle" loose bearings, loose part   2 x rpm, but sometimes shows up at ½ x rpm instead. See Looseness under "Less Than 1 x rpm."
Rolling element bearing with too much clearance between inner and outer races   Symptoms similar to that of looseness. Most often exact harmonic frequencies. Harmonics larger than usual up to and beyond 5 x, 6 x or 7 x rpm. Rollers sliding too much. See chapter "Vibration Due To Rolling Element Bearings."
Eccentric armature or "running eccentrically"   Either machined eccentrically or made to "run eccentrically" due to shaft centerline orbits originating from unbalance or misalignment.
Unbalanced reciprocating parts, such as pistons, connecting rods, vibrating screens   Replacement parts should be weight balanced to previous weight. All like parts are to weigh the same. Reciprocating screens require proper counter weighting.
Electrical hum   Often looks like a harmonic but is non-synchronous and, therefore, not a true harmonic. For 60 Hz current, frequency is 120 Hz (7200 cpm). For 50 Hz current, frequency is 100 Hz (6000 cpm).
Rub or looseness   Could produce higher amplitudes than usual for several lower harmonics up to and beyond 5, 6, 7 x rpm. Sometimes reported at frequencies of 1 ½ x, 2 ½ x; 3 ½ x rpm; etc. Sometimes in the series, a harmonic or two are skipped. Orbit of displacement or time waveform analysis very helpful to distinguish between rub and other sources.
Misaligned rolls   Papermachine rolls may be aligned within tolerance and yet exhibit high 2 x rpm amplitude due to the magnification of resonance. Compare with time waveform.

 

 

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