Study on the performance of gas foil journal bearings with bump-type shim foil

Abstract

The upscaling of turbomachinery using gas foil journal bearings (GFJBs) is limited because of their limited load capacity and dynamic stability. The improvement potential of shim foil inserted under the bump foil of such bearings is investigated in terms of better bearing performance. The arch height difference Δ hb between the shim foil and bump foil can be zero or not to attain the different effect. By considering the local hardening structural stiffness and an Initial installation clearance due to the shim foil, the static and dynamic characteristics of the novel bearing were calculated through the finite difference method (FDM) and perturbation method, respectively. In the analysis, a modified bump stiffness model considering the variable foil thickness was established, and a 2 D thick plate model was adopted for the top foil. The characteristics of novel GFJB with and without preload were compared with the traditional bearing. The results indicate that the load capacity and direct stiffness of the novel GFJB with shim foil can be increased largely, especially when there is a preload (Δ hb≠0). And the improvement is reinforced as the increment of Δ hb. Moreover, the stability threshold speed ( STS) of rotor supported by the novel GFJBs is enhanced by the preload, which means better stability. In addition, an air compressor test has also been conducted to verify the improved supporting performance of novel bearings. Based on this study it is convinced that the addition of shim foil under a GFJB's bump foil can be of practical interest in the quest of enhanced load capacity and dynamic stability. Moreover, the installation of shim foil is not affected by the working environment and could even be retrofited on the existing GFJBs.