Static Structural Characterization of Multilayer Gas Foil Journal Bearings

The aim of this study was to evaluate the structural performance of multilayer gas foil bearings (GFBs). Three test GFBs and a dummy rotor with a diameter of 28 mm were manufactured for application to a series of structural tests. The test bearings were designed with an identical axial length of 22.5 mm, a radial clearance of 0.170 mm, and a mechanical preload of 0.050 mm at a 50% offset within the (machined) lobe bearing housings. All the bearings had multilayer (doublelayer) thin foils supported by an elastic bump strip layer, i.e., a configuration with a top foil, shim foil, and bump strip layer. Note that the shim foil is installed between the top foil and bump strip layer to enhance the dry-friction damping effect. The first bearing (Type A) had a single top foil, single shim foil, and single bump strip layer. All had an arc length of 360°. The second version (Type B) had a single top foil with an arc length of 360°, three shim foils with an arc length of 120°, and three bump strip layers with an arc length of 120°. The third version (Type C) had three top foils, three shim foils, and three bump strip layers. All had an arc length of 120°. The top foil is spot-welded to a key that is inserted into an axial key slot in the bearing housing. The shim foil and bump strip layer are inserted into an axial foil slot in the bearing housing. A series of static load-deflection tests were conducted on the test GFBs floating on the fixed, non-rotating test rotor. The measured results for the bearing deflection and structural stiffness were found to be in very good agreement with the model predictions for a GFB with a mechanical preload. In general, the test results were found to exhibit a similar radial sway space (or assembly radial clearance) and structural stiffness for all three test GFBs. Small local hysteresis loops appeared as the magnitudes of the load increased, thus determining the local structural stiffness and structural loss factor versus the displacement. The local stiffness was found to increase while the loss factor decreased as the magnitude of the displacement increased. The estimated structural loss factor can be as large as 0.9 within the radial sway space under low-load conditions.