Two unbalances (shown here as arrows) can have the same size, but their angular position is offset by precisely 180° to each other. This unbalance distribution cannot be detected by knife edges as with static unbalance, because the rotor does not take up a unique position at rest. The rotating rotor executes a wobbling movement about its vertical axis (perpendicular to the axis of rotation), because the two unbalances exert a moment. Ergo, this type of unbalance distribution is called couple (moment) unbalance.
An opposing moment is required to correct the couple unbalance, i.e. two equi-sized correcting unbalances, which are arranged in the two balancing planes at an offset of 180°, corresponding to the original unbalance. It is particularly important to take couple unbalances into account in elongated cylindrical rotors. Therefore, horizontal balancing machines are particularly suitable for correcting this type of unbalance