Flexible support and enhanced groove geometries for gas lubricated bearings

Gas lubricated Herringbone-Grooved Journal Bearings (HGJB) are a promising solution to support high-speed rotors in oil-free turbo-machinery due to their compactness, relatively low losses, no need for lubrication and low wear. Gas lubricated bearings, however, generally require very small clearance to diameter ratios to allow stable rotor operation. The consequences are high manufacturing cost, stringent misalignment tolerances and increased specific windage losses. Increasing the bearing clearance while maintaining the performance of the rotor-bearing-system allows to significantly reduce these shortcomings and would help to make the application of HGJBs in products less challenging. Two approaches have been investigated that allow an increase in bearing clearance while maintaining the rotordynamic stability of the system: (1) a flexible support structure for HGJB bushings and (2) enhanced groove geometries. Both approaches suggest a 50% increase in bearing clearance.

A dedicated test-rig was designed to allow the experimental characterization of stiffness and damping of various O-Ring geometries and installation specifications. In a first stage, a Design of Experiments approach enabled the identification of the governing variables on the stiffness and damping of O-Rings. The measurement results of 60 representative O-Ring geometries then allowed the development of data-driven, reduced-order models for the stiffness and damping of O-Rings as a function of the excitation frequency, rubber hardness, O-Ring geometry and squeeze.

A generic flexible bearing bushing support for HGJBs has been developed to address the shortcomings of the commonly used O-Rings to flexibly support the bearing bushings. The novel flexible support is based on membrane shaped disks featuring specific cut-outs, resulting in an independently tunable support stiffness. An alignment procedure has been presented that is able to perform the very challenging task of aligning two bearing bushings separated by a considerable distance. The novel flexible support has been successfully implemented in a base-line prototype and aligned with the presented alignment concept, which showed significantly improved repeatability, lower lift-off speeds and less wear compared to O-Ring supported bearing bushings.

An experimental campaign was conducted to verify the theoretically predicted potential of enhanced groove geometries to increase the bearing clearance of HGJBs. Good agreement between experimentally determined speed of instability onset and prediction was found for the investigated enhanced groove patterns. By using rotors with enhanced groove geometries, it was possible to increase the speed of instability onset by more than a factor three compared to a classically grooved shaft with the same bearing clearance, which confirmed the potential of enhanced groove geometries to stabilize HGJBs.