Vibration and bifurcation analysis of rotor systems with air ring bearings including ring tilting

Abstract

Air ring bearings may be considered as a further development of classical air bearings. A drawback of classical rigid air bearings is their reduced damping behavior, which usually prevents a secure operation above the linear threshold speed of instability. Considering air ring bearings, the ring-shaped bearing bushing is visco-elastically mounted in the housing so that external damping is provided. Alternatively, the bushing ring may also be mounted in the housing by a foil structure (beam or bump foil design) so that dissipation is generated by dry friction. Due to the external damping/friction provided by the ring mounting, rotor systems with air ring bearings can be operated above the threshold speed of instability so that stable self-excited vibrations with moderate amplitudes are observed. For certain system parameters, however, the rotor system can bifurcate into a rather dangerous vibration mode, where bearing ring modes are excited by the whirl/whip frequency of the air films. To accurately predict the bifurcation into the problematic ring modes, ring tilting has to be taken into account.Here, a detailed analysis for rotor systems with air ring bearings is presented, which fully incorporates ring tilting effects. The influence of ring tilting on the stability and bifurcation behavior is discussed in detail with the help of nonlinear rotor/bearing models. In connection with the bifurcation into the dangerous ring mode, an interesting nonlinear mode coupling effect (1:2 mode synchronization) is detected. A detailed analysis and a clear physical explanation of this mode coupling phenomenon is given.