Total instability of turbocharger rotors—Physical explanation of the dynamic failure of rotors with full-floating ring bearings

Abstract

This article investigates the stability of turbocharger rotors with full-floating ring bearings. Rotors of turbochargers can show different bifurcations, when a control parameter—for instance rotor speed—is varied. Considering a rotor run-up, the rotor typically becomes unstable (first bifurcation) already at low rotor speeds and reaches a stable limit cycle ( 1. Subsynchronous). At higher rotor speeds, further bifurcations usually occur. For instance, the 1. Subsynchronous can become unstable and the system bifurcates into another limit cycle ( 2. Subsynchronous, 3. Subsynchronous). Also, limit cycles may collapse so that the rotor becomes stable again performing mere imbalance oscillations. Which bifurcations occur, depends on the rotor and bearing parameters. Since the limit cycle oscillations ( 1., 2. and 3. Subsynchronous) are normally stable, i.e., the amplitudes (bearing eccentricities and rotor amplitudes) of the limit cycles are moderate, they do not impair safe operation of the turbocharger. Although the Subsynchronous vibrations may not interfere with the proper operation of the turbocharger, they can be undesirable, since they may cause acoustic noise problems, for instance. Depending on the system parameters (rotor mass/inertia, shaft stiffness, bearing parameters, etc.) the rotor can, however, show a further kind of bifurcation and become totally unstable (dangerous high bearing eccentricities and rotor amplitudes), which in practice often leads to the destruction of the rotor. This phenomenon is called Total Instability in the present paper. The article at hand examines a medium-sized turbocharger supported in full-floating ring bearings and analyses the bifurcation into Total Instability. The dynamics of the rotor/bearing system is investigated in detail and a sound physical explanation of the Total Instability is given. For this purpose, transient multibody simulations and eigenvalue calculations of the rotor/bearing system are carried out. In addition, a run-up measurement, which exhibits Total Instability, is compared with simulation results. It is shown in this paper that bifurcation into Total Instability can physically be explained as synchronization of two limit cycles, namely as synchronization of the inner and outer oil whirl/whip of the floating ring bearings.