Abstract：The operation of steam turbine generator shaft systems near critical speeds can induce resonant vibrations, compromising operational safety and stability. While existing studies have extensively investigated the lateral vibration critical speeds of individual rotors or the entire shaft system, the relationship between the critical speeds of constituent rotors and the integrated shaft system remains underexplored. Here, a 600 MW turbine-generator shaft system is examined by first establishing a transfer matrix model and a three-dimensional finite element model without support bearings to derive its inherent bending modes. Subsequently, finite element analysis is employed to elucidate the correlation between the critical speeds and modal shapes of the fully supported shaft system and those of its individual rotors. Furthermore, the influence of bearing oil film stiffness on the shaft system's critical speeds is systematically evaluated. Findings reveal that the first six modal shapes of the shaft system are predominantly governed by the modes of the individual rotor whose critical speed is closest to, yet lower than, the corresponding shaft system critical speed. Notably, the fifth and sixth modes exhibit coupled vibrations involving this rotor and its adjacent counterparts. Increasing the oil film stiffness of a rotor's bearings selectively elevates the critical speeds of shaft system modes associated with that rotor, while leaving other modes largely unaffected. These insights provide a foundational framework for optimizing the design and vibration monitoring of large-scale steam turbine generator shaft systems.
2026, Vol. 27 
