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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, 1 x rpm (Rotor Speed)

Rotor unbalance   Large 1 x rpm peak with relatively low 2 x rpm and negligible other harmonic amplitudes.
Shaft/coupling misalignment   Check for other frequency symptoms, such as higher than usual peaks at 2 x rpm and other lower harmonics. Check phase symptoms as well, to more surely distinguish from unbalance.
Sheave misalignment  
Often looks like unbalance symptoms of either the sheave or the rotor to which it is attached, except the axial amplitudes are usually much larger than for unbalance. Sometimes accompanied by larger than usual amplitude at 2 x rpm. May give similar symptoms as for shaft/coupling misalignment.
Mismatched belts   Often accompanied by higher than usual axial vibration.
Eccentric sheave   Large amplitude is usually unidirectional in line connecting center with other sheave. Approximately zero or 180° out-of-phase relationship between vertical and horizonal readings at the same bearing.
Bent shaft   Similar to static unbalance except for 180° out-of-phase axial readings. Could be resonant rotor (resonant whirl).
Eccentric armature or "running eccentrically"   Either machined eccentrically or made to "run eccentrically" due to shaft centerline orbits originating from unbalance or misalignment.
Eccentric gear (very rare) or gear "running eccentrically"   Rarely due to machining errors. Most often made to "run eccentrically" due to centerline orbits originating from unbalance or misalignment. Could be caused by assembly error (see section "Unbalance Due To Assembly Errors").
Motor; with loose or otherwise defective rotor bars   Primary frequency is 1 x rpm. (With one or more sidebands on each side.) The difference frequency between a sideband and the primary frequency is equal to the rotor slip times the number of poles. Normally, its amplitude and amplitudes of sidebands are proportional to load.

The symptom for this defect at times 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.

 

 

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