In some cases, weight added at one end of the rotor will compound vibration. Here, the unbalance mass and the added weight form a couple that deflects the rotor.
Figure 9 illustrates a rotor having dynamic unbalance. Weight added at the top of the rotor to counter ml will create a static unbalance on the m2 side of the rotor. To correct dynamic unbalance, weight must be added to both the top and the bottom of the rotor to produce a couple that will counter the effect of the couple causing the vibration. The difficult part of dynamic balancing is determining the correct combinations of weights and locations