According to ISO 11342: 1998 [1], balance quality of a flexible rotor can be evaluated based on its residual modal unbalance (RMU). Calculation of equivalent RMU is based on a magnitude of the influence coefficient (IC) in the most sensitive plane. IC is a calculated value from two vibration readings and a trial weight. It is known that even in the same balance plane on the same rotor different ICs are often obtained. This has no effect on rotor's balance quality when it is evaluated by residual vibration levels but it has profound effect when RMU is employed. Because values of ICs and RMU are directly related, deviations in ICs lead to deviations in corresponding values of RMU, and in turn, to errors in balance quality evaluation of a flexible rotor. Results of three special experiments are presented to illustrate such deviations. They show mat when the balance quality of a rotor is evaluated based on its RMU a realistic error of 20-30% should be considered. Test data from a turbine rotor demonstrates that if improper balance planes (such as couplings) are selected for RMU calculation and evaluation, the results lead to destructive vibration levels. Test data demonstrates that quality of ICs and proper balance plane selection are extremely important factors for accurate calculations of RMU. Anybody evaluating the quality of high speed balance for a flexible rotor based on its RMU needs to take these factors into consideration.
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