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Static Unbalance

Static unbalance is a condition of unbalance where the central principal axis is displaced parallel to the rotating centerline.  It can be detected by placing the rotor at its point of rotation on each end.  The heavy side of the rotor will swing to the bottom.  A part is considered statically balanced when it does not rotate regardless of the position in which it is placed.

When testing a rotor that is symmetrically supported between identical bearings, identical vibration amplitude and phase readings will be measured at the bearings / end of shaft if the unbalance is truly static.  This does not apply for rotors which are mounted in an overhung configuration.

Static unbalance can be corrected by adding or removing weight in only one correction plane.  In the figure to the right, the balance correction weight in scenario A is added as one singular weight addition in the same plane as the unbalance.  This will result in a well balanced rotor.  In scenario B, the static unbalance is corrected by placing the correction weights in-line at opposite ends of the rotor.  This method is typically used when it is not possible to add a single correction weight at the center portion of the rotor.  This results in a statically balanced rotor; however, during faster rotations, there is an increased chance of bending moments.

Scenario C shows an unacceptable attempt of balancing a rotor.  The correction weight was added in a different plane than the one containing the rotor center of gravity.  The rotor may be considered statically balanced, due to the fact that no heavy spot would swing to the bottom if the rotor were suspended and allowed to spin freely; however, when the rotor is rotated, the original heavy spot and correction weight, being located in different planes, produce moments of inertia which cause the central principal axis to intersect the rotating centerline, thus creating another type of unbalance.

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