Imbalance is the most common source of vibration in rotary machinery. Imbalance occurs when the axis of rotation of the rotating part does not coincide with the axis of the same.
Several types of imbalance have been defined:
Static imbalance occurs when the center of mass is displaced from the axis of the rotating piece. Another way of understanding this is to say that the axis defined by the mass distribution and the axis of rotation are offset from each other, both being parallel. The phenomena described correspond to an eccentricity of the mass center, which is the effect of the existence of a single unbalanced force. To correct this type of imbalance, a single balancing plane is sufficient.
The axis of rotation and the axis representing the mass distribution of the unit are not coincident but intersect at one point. The masses responsible for the imbalance are distributed in such a way that they are equivalent to a couple of forces of equal magnitude and direction but of opposite direction. To correct this type of imbalance it is necessary to have two balancing planes.