Considerations on the Field Balancing of the Overhung Rigid Rotors

Any unbalance distribution in a rigid rotor assembly can be completely balanced by fixing at a determined radius two correction weights (equal and opposite to the equivalent unbalance weights), each one located in any two planes perpendicular to the rotation axis. This paper presents a tutorial that develops, in a simple, clear, concise, and precise form, the general equations that relate the unbalance weights in the correction planes with the vibrations in the supporting bearings, based on the concept of the mechanical impedance, for the particular case of an overhung rigid rotor. Important insights are provided to understand thoroughly the inherent additional cross-effect that is produced by the application of each of the existing exact methods of influence coefficients to the balancing of the overhung rigid rotors. Also discussed is a recent advance that consists of a new solution for the field balancing of the overhung rigid rotors. This alternative solution again combines the least cross-effect with the minimum number for the trial weights, and gives the correction weights directly for both balancing planes. The advantage of this new technique, with respect to the previous variation of the method of the influence coefficients, is that its implementation in any industrial balancer would require only minor changes in the programming. An explanation of the conversion that is necessary for different phase conventions within a computer program and examples of calculations with experimental data are included for completeness.