Usually dynamic unbalance is characteristic of long rotors, such as shafts, and static unbalance — of narrow ones. However, if a narrow rotor is mounted with a skew relative to the axis of rotation, or is deformed (a "figure-eight" wobble), a hard-to-remove dynamic unbalance appears.
The masses in the upper and lower parts of such a rotor create forces F1 and F2, directed in opposite directions and not lying on one line. These forces do not compensate each other, and in rotation they form a moment applied to the rotor.
Since the rotor is narrow, the distance between the correction planes is small, and the arm available for creating the compensating moment is short. Correction weights of considerable mass may therefore be required. This in turn produces an additional effect — the so-called induced unbalance, caused by deformation of the narrow rotor under the centrifugal forces of the correction masses themselves (see ISO 21940-11, the standard on balancing procedures and tolerances for rotors with rigid behaviour).
This is noticeable on narrow fan impellers, where aerodynamic unbalance acts in addition to the mass unbalance. The aerodynamic force is directly proportional to the angular speed of the rotor, while the centrifugal force of a correction mass is proportional to the square of the angular speed. Therefore the compensation is exact only at the specific speed at which the rotor was balanced; at other speeds an additional error appears. The same can be said about electromagnetic forces in an electric motor, which are also proportional to the angular speed.
It follows that it is impossible to eliminate all causes of machine vibration by balancing alone. Balancing removes mass unbalance; the vibration that remains points to other sources — this is what the FFT spectrum in Balanset-1A is for.
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