Simplified prediction of stability limit for oil film bearing rotor systems

Abstract

Oil film bearings are often used in large rotating machinery because they have large load capacity and damping coefficients. They, however, generate fluid forces and cause unstable vibrations such as oil whirl and oil whip at high rotational speed. Because the amplitudes of such self-excited vibrations are severe, it is important to predict the stability limit of rotational speed to avoid the unstable vibration problems in rotor systems supported by oil film bearings. This paper describes the vibration analyses of a flexible rotor system supported by oil film bearings and the simplified prediction method for the rotational speed limits of the self-excited vibration induced by the fluid force in oil film bearings. In the vibration analyses, the equations of motion for the rotor system are simplified and modeled as a two-degree-of-freedom system with parallel and flexural modes of vibration. The stability analyses are conducted by introducing the approximation equations instead of complex eigenvalue calculations. The rotational speed limits for the stability of the system obtained by the present method are compared with the calculation results in the references to discuss the validity of the present method. This paper also shows the importance of anisotropic properties of oil film bearings to increase the accuracy of stability prediction when the clearance of the oil film bearing is large.

Keywords

• flexible rotor
• oil film bearing
• oil whirl
• oil whip
• instability prediction
• rotational speed limit