Interface-DOF-Reduced Craig–Bampton Substructuring for Efficient Dynamic Characteristic Prediction of Shaft Generator Systems

This study applies a Craig–Bampton (CB) substructure reduction procedure to the prediction of natural frequency changes upon rotor replacement in a shaft generator shafting system and quantifies its accuracy and computational cost for that configuration. Because the shafting conditions are determined by the customer, performing full finite element analysis or experimental modal analysis (EMA) for every design change is impractical. The shafting system is divided into shaft and rotor substructures, and CB reduced-order models are constructed independently for each substructure. A three-stage verification framework—FE analysis versus EMA, the CB reduced-order model versus FE analysis, and the CB reduced-order model versus EMA—is employed to separate FE model error from CB reduction error. The CB pipeline reproduced the bending modes of the assembly with a maximum error of 0.26% against the parent finite element model, with a subspace MAC of 1.0000 for every mode group below 720 Hz, while reducing the model from 192,678 to 5379 degrees of freedom. With 20% of the interface degrees of freedom retained, the first three bending modes are predicted within 0.60%, and the reduced model comprises 1173 degrees of freedom, a reduction of 99.39%. The first bending mode error varies monotonically with the retention level, and the errors of all three bending modes remain bounded below 0.60% down to 20% retention. For the annular interface examined, the error-retention relation, therefore, provides a quantitative basis for selecting a retention level against a stated error tolerance, but its extension to other interface topologies, mesh densities, and mode ranges remains to be established.