We have said that the balancing achieved by our method was independent of speed. This is true only up to a certain limit, as we have implicitly assumed the shaft to be perfectly rigid. As the speed increases, the unbalance forces increase and the shaft tends to deform elastically under the action of these forces. Also, as the speed becomes closer to the fundamental transverse bending natural frequency of the shaft in its bearings, chances of resonance are also there. Thus the assumption of shaft being perfectly rigid is no longer valid. A rotor can be assumed to be “rigid” if the speed of rotation is less than about 1/3 rd the fundamental natural frequency. Two plane balancing technique is essentially a rigid rotor balancing method and is useful only within the speed limit mentioned above.