Module 2: Excitation reduction at source and factors affecting vibration level
  Lecture 5: Balancing of Rigid Rotors
 

Balancing of Rigid Rotors

When the operating speed of a rotor coincides with any of the natural frequencies of its transverse vibration, then the rotor undergoes a significant transverse deflection.This speed is referred to as critical speed.

For operating speed far below the first critical speed, the rotor deflection is negligible. In such cases, the rotor is assumed to be rigid and the complete balancing of its inertia (centrifugal) forces can be achieved by attaching two masses at any two arbitrarily chosen axial planes called the balancing planes.

If, for practical reasons, the masses cannot be attached, the balancing can still be done by removing the rotor material from the positions diametrically opposite to those positions, the balancing masses would have occupied.


Basic Principles of Balancing Machines

  1. total unbalance of a rigid rotor can be completely expressed in terms of the unbalances in any two conveniently chosen balancing planes.
  2. a rigid rotor balanced at one speed can be considered as balanced for any other speed well below the first critical speed.
  3. the inertia force of the rotor depends on the product of the mass and eccentricity. The amount of a balancing mass may be adjusted depending on its convenient radial position to result in the requisite value of the product.