Impact of thrust bearing pad design and allocation on automotive turbocharger rotordynamics

Abstract

The journal and thrust bearing design process remains a key aspect while developing an automotive turbocharger center section. The rotor assembly has to be supported in both the radial and axial direction assuring a safe operation with increased life time span. Both bearing types have to be developed sufficing multiple restrictions. Journal bearings are designed with focus on shifting undesired critical frequencies outside the range of operation while supporting various types of nonlinear vibrations. Thrust bearings are configured aiming at a balanced ratio of load capacity and friction losses. In general, both bearing types are treated as two distinct entities during development even when coping with the same center section. This paper investigates the journal and thrust bearing cross-influence effects during the design process of thrust bearing pads. It is conducted by accounting for the thrust- and journal bearing interaction during transient nonlinear run-up simulations. Focus is set on pad designs with different geometrical dimensions, which based on stationary calculations have nearly identical load capacity and friction loss properties. It is shown that thrust bearings consisting of such pads do no necessarily have the same impact on the rotor assembly's nonlinear vibrations during run-ups. In addition, the inherited thrust- and journal bearing interaction ends up with transient hydrodynamic pad properties that do no longer match stationary predictions. Furthermore, it is demonstrated that thrust bearings equipped with geometrically identical pads, but differently allocated along the circumference, interact with the rotor assembly in a different manner depending on the nature of the radial bearing. A semi- or a fully-floating journal bearing combined with a thrust bearing having a symmetric or an asymmetric pad allocation can have either positive or negative influence on shaft motion and load capacity. As a result, the associated friction losses obtained by stationary calculations are either over- or under predicted.