Consider the turbo-machine rotor that was discussed earlier wherein each stage contains several blades around the circumference of a disk. Eventhough typically each stage is balanced in itself to the extent possible, it has a likely net unbalance. When the rotor is set to spin, it will cause dynamic forces and moments on the bearings that support the shaft. Therefore it is of interest to achieve “good balance” of this shaft so that the fluctuating forces on the bearings are reduced. Conceptually our strategy can be simply stated as follows:

Step 1: Consider the shaft supported on its bearings. For each unbalance mass, there will be a centrifugal force set-up when the rotor spins at some speed . This would cause some reactions at the supports. Estimate these support reactions that would come onto the bearings.

Step 2: Estimate the balancing mass that needs to be placed in the plane of bearings, to counter this reaction force due to unbalance mass.

Repeat steps 1 and 2 for each unbalance mass in the system and each time add the balancing masses obtained in step 2 vectorially to determine the resultant balancing mass required.