Two unbalances (shown here as arrows) can have the same size and angular position and can be the same distance from the centre of gravity. The same condition results for an individual, twice as large, unbalance that acts at the centre of gravity, i.e. in this case in the middle of the rotor. If such a rotor is supported on two knife edges, it would swing until the "heavy point" was facing downwards. This means this unbalance acts even without rotation; it is therefore called "static unbalance". It causes the centre of mass to shift away from the geometric centre, which in turn causes the rotor to swing oscillate parallel to its rotational axis when it is running.
Static unbalance should be corrected in the centre of gravity plane. This is achieved by removing material at the "heavy point" or attaching material at the opposite side. Correction of static unbalance in one correction plane occurs particularly frequently in disc-shaped rotors. Therefore, vertical balancing machines are most suitable for balancing these.