Unbalance

Unbalance is a condition where shaft geometric centerline and mass centerline do not coincide or the center of mass does not lie on the axis of rotation.

Vibration spectrum will be dominated at 1x running speed and the time waveform will sinusoidal for pure unbalance.

There are three types of unbalance condition :

1. Static unbalance : In this type of unbalance we have a heavy spot at a single point in the rotor, it will shows up even when the rotor is not running or if you put it on the frictionless bearings the rotor will turn so the heavy spot is at the lowest position.

The vibration signal at each end of the machine in the same direction will be in phase. There will be 90 ^o + 30 ^o phase different between horizontal and vertical direction.

Figure 1. Static unbalance

2. Couple Unbalance : A rotor with couple imbalance seems well balanced in static condition (the rotor not turn when placed on frictionless bearings). But it going to produce centrifugal force on the bearings when the rotor is rotating.

Vibration signal at each end of the machine taken at the same direction will be 180 ^o out of phase. There will be 90 ^o + 30 ^o phase different between horizontal and vertical direction.

figure 2. Couple unbalance

3. Dynamic Unbalance : is a combination of static and couple unbalance. Spectrum at 1x running speed will dominated overall vibration. The highest vibration usually at horizontal direction where the machine can move more freely. Vibration signal at each end of the machine taken at the same direction will be 0 ^o to 180 ^o. There will be 90 ^o + 40 ^o phase different between horizontal and vertical direction.

figure 3. Dynamic unbalance