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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, Higher Frequencies

Defective or worn gear teeth   Gearmesh of number of teeth x rpm and/or harmonics of that frequency. Often there is no defect or excessive wear, but instead vibration at that frequency could be grossly magnified by resonance. Best seen via time waveform.
Gear eccentricity and gear teeth are OK   High gearmesh amplitude and its harmonics. Centerline has large orbit due to unbalance, misalignment or other lower frequency-related error, causing teeth to mesh improperly. Also check for resonance.
Defective rolling element bearings  
Symptoms are in stages depending on how close to impending failure. First stage is at relatively high frequencies, non-synchronous. Frequencies spread out and create more peaks as bearing deteriorates. Later stage shows frequencies as calculated. Number of sidebands of 1 x rpm increases. In all situations, amplitudes may or may not increase.
Turbine blades   Frequency of number of turbine blades x rpm. This one usually requires a turbine specialist. Sometimes due to other turbine problems not readily put into simple chart forms. If in doubt, contact turbine manufacturer.
Rub   Similar to previous rub symptoms, but producing higher frequencies. See Rub (with no resonance) under "Less Than 1 x rpm."
Motor; with loose or otherwise defective rotor bars   Sometimes the symptom for this defect produces excessive vibration at relatively high frequency. Instead of a sharp peak at rpm x number of rotor bars, there are many sideband peaks, producing what is usually referred to as a "haystack." The difference frequency between sidebands is usually equal to the electrical hum frequency. Also check for primary frequency that is often at 1 x rpm. (With a sideband on each side of it.) The difference frequency between a sideband and the primary frequency is equal to the rotor slip times the number of poles. Normally, sideband amplitudes are proportional to load.

 

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