Balancing is a process used to correct the unequal distribution of geometric mass (see Unbalance). In the case of centrifugal pumps, this is performed in such a manner as to render the rotation axis of the pump's rotor a free axis (no free inertia effects during rotation). A distinction is made between static and dynamic balancing.
In static balancing, the pump rotor's centre of mass is placed as accurately as possible in the rotation axis of the pump shaft, and, consequently, the resultant of the centrifugal forces (not to be confused with the resultant moment) is eliminated. This is also called "single-plane balancing" and is usually applied when the axial dimension of the rotating mass is substantially smaller than its diameter.
In dynamic balancing on balancing machines, the pump rotor's rotation axis becomes the principal axis of inertia. As a consequence, not only the resultant force is eliminated, but also the resultant static moment of the centrifugal forces. This is referred to as two-plane balancing i(see also Hydraulic unbalance of centrifugal pumps).