Mass Unbalance
Force Unbalance will be in-phase and steady. Amplitude due to unbalance will increase by the square of speed (3x speed increase = 9x higher vibration. 1x RPM always present and normally dominates the spectrum. Can be corrected by placement of only one balance weight in one plane at rotor centre of gravity (CG).
Couple Unbalance tends toward 180° Out-of-phase on same shaft. 1x always present and normally dominates the spectrum. Amplitude varies with square of increasing speed. May cause high axial vibrations as well as radial. Correction requires placement of balance weights in at least 2 planes. Note that approx. 180° phase difference should exist between Outboard and Inboard horizontals as well as Outboard and Inboard verticals.
Overhung Rotor Unbalance causes high 1x vibration in both Axial and Radial directions. Axial readings might be unsteady. Overhung rotors often have both force and couple unbalance, each of which will likely require correction.
Eccentricity occurs when the centre of rotation is offset from the geometric centreline of a sheave, gear, bearing, motor armature, etc. The largest vibration occurs at 1x RPM of eccentric component in a direction through the centres of the two rotors. Comparative horizontal and vertical phase readings usually differ either by 0° or by 180° (each of which indicate straight line motion). Attempts to balance an eccentric rotor often results in reducing the vibration in one direction, but increasing it in the other radial direction (depending on the amount of eccentricity).
Bent Shaft problems cause high axial vibration with axial phase differences tending toward 180° on the same machine component. The dominant vibration ins normally at 1x if bent near the shaft centre, but at 2x if bent near the coupling. (Be careful to account for the transducer orientation for each axial measurement if you reverse probe direction).
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