From the theoretical point of view, a turbine rotor is a balanced body but in actual practice, errors of balance are introduced by various causes such as:

Therefore, it is essential to test balance of a complete turbine rotor and make any adjustments necessary to ensure that the balance is as good as possible. The purpose of balancing of rotors is to reduce the amplitude of vibration on a tolerable level which can be taken to be about 0.0254mm at the bearing pedestrals of a 300 rpm machine. There are two types of balancing- Static and dynamic.

It means that the weight of the rotor is evenly disposed around the axis of the shaft. It is checked by rolling the rotor on horizontal knife edge supports.

It means that the moments of the out-of-balance weights along the axis about either bearing add upto 0. It is checked by spinning the rotor on resilient bearings detecting the vibration and adding or subtracting weights untill the vibration is negligible.

Normally, rotors are balanced at 400 rpm. The adjustment in weight is made in two planes, one at each end of the rotor by varying screwed plugs in tapped holes, or by removing metal from portion of a rim added for this purpose, or by fixing weights in a groove by means of screws. Preference is given to subtraction of weights instead of addition, since there is a chance of coming the loose weights drift.