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Couple Unbalance

Couple unbalance exists when two unbalances exist 180 degrees apart, but in different planes.  This condition of unbalance has a central principal mass axis intersecting the rotating centerline. Unlike static unbalance, couple unbalance cannot be detected by allowing the rotor to spin freely.  In fact, during the process of static balancing, one can add the weight in the wrong plane, as seen in the Methods of Fixing Static Unbalance, C Figure.  When the weight is added disproportionately, a coupled unbalance is created.  Couple unbalance can only be detected when the part is rotating and can be identified by comparing the bearing or shaft vibration amplitude and phase readings at each end of the rotor.

Readings from a rotor experiencing couple unbalance will reveal equal amplitudes of vibration with phase readings which differ by 180 degrees.  Again, this method of detecting the type of unbalance does not apply to overhung rotors.

Correction of couple unbalance requires that weight be added or subtracted within two correction planes.  In only very few cases will a rotor have true static or true couple unbalance.  Normally, an unbalanced rotor will have some of each type.  Combinations of static and couple unbalance are further classified as quasi-static or dynamic unbalance.

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