1. School of Mechanical Engineering and State Key Laboratory for Strength and Vibration of Mechanical Structure, Xi’an Jiaotong University, Xi’an, Shanxi 710049, P. R. China
2. Key Laboratory of Education Ministry for Modem Design and Rotor-Bearing System, Xi’an Jiaotong University, Xi’an 710049, P. R. China
3. School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China