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Flexural vibrations of a rotor system with transverse or slant crack are analyzed under torsional excitation by numerical simulation and experiment. Numerical results show that combination frequencies of the rotating speed and the torsional excitation frequency are prominent in the flexural responses of a slant-cracked rotor, but too weak to be identified for a transverse-cracked rotor. In order to verify the results, an experimental setup is installed to simulate the cracked rotor system and a special structure is designed to exert the torsional excitation on the cracked rotor. The experimental results present almost identical characteristic features about the combination frequencies to the numerical results. The numerical and experimental investigations demonstrate these features can be used to distinguish between the transverse crack and the slant crack on the shaft of a rotor system. [Copyright &y& Elsevier]

Abstract: External and internal bending–torsion coupling effects of a rotor system with comprehensive unbalances are studied by analytical analysis and numerical simulations. Based on Lagrangian approach, a full-degree-of-freedom dynamic model of a Jeffcott rotor is developed. The harmonic balance method and the Floquet theory are combined to analyze the stability of the system equations. Numerical simulations are conducted to observe the bending–torsion coupling effects. In the formulation of rotordynamic model, two bending–torsion coupling patterns, external coupling and internal coupling, are suggested. By analytical analysis, it is concluded that the periodic solution of the system is asymptotically stable. From numerical simulations, three bending–torsion coupling effects are observed in three cases. Under static unbalance, synchronous torsional response is observed, which is the result of external coupling under unbalanced force. Under dynamic unbalance, two-time synchronous frequency torsional response is observed, which is the result of internal coupling under unbalanced moment. Under comprehensive unbalance, synchronous and two-time synchronous frequency torsional components are observed, which are the results of both external and internal couplings under unbalanced force and moment. These observations agree with the analytical analysis. It is believed that these observed phenomena should make sense in the dynamical design and fault diagnostics of a rotor system. [Copyright &y& Elsevier]