Design, analyses and experimental study of a foil gas bearing with compression springs as a compliance support

A new foil bearing with compression springs is designed, built, analyzed, and tested. This foil gas bearing uses a series of compression springs as a compliant structure instead of corrugated bump foils. A spring model to estimate the stiffness of compression springs was developed and showed a good level of agreement with the experimental results. The spring dynamics model was combined with a non-linear orbit simulation to investigate the non-linear behavior of foil gas bearings. The approach could also predict the structural loss factor given the geometry of the underlying springs. A series of rotor-bearing orbit simulations using the compression spring with stiffness of the free-free case, predicted the critical speed and the onset speed of instability at around 7500 rpm and 14,500 rpm with a WFR ~ 0.5. The low critical speed was due to the relatively soft support. The hydrodynamic rotor instability was predicted under the equivalent viscous damping extracted from the spring dynamics, implying the viscous damping alone within the spring cannot suppress hydrodynamic instability of the foil gas bearings. The load capacity of the compression spring foil gas bearing was measured at 20,000 rpm with and without air cooling, to demonstrate the feasibility of the new foil bearing. The constructed bearing with rather soft springs showed a small load capacity of 96N at 20,000 rpm under no cooling. The developed cooling method using direct air supply holes machined on the bearing sleeve, proved to be very effective in cooling the test bearing. The measured level of structural stiffness and damping evidenced the existence of a necessary level of damping for stable bearing operation. The structural stiffness was highly nonlinear and showed different behavior for static loading and the sinusoidal dynamic loading. The measured equivalent viscous damping coefficients increased with the applied load amplitude. A series of parametric design studies were performed to investigate the effects of various design parameters on the bearing stiffness and overall rotordynamic performance. Rotor-bearing orbit simulations showed there is a range of spring stiffness for high onset speeds of instability. Increasing the pitch of the spring while maintaining the same stiffness increased the structural loss factor slightly, manifesting a smaller number of coils is better in terms of damping. The onset speed of instability increases slightly with the rotor mass due to increased static eccentricity and presumably smaller cross-coupled stiffness. However, increasing the rotor mass in order to render a high eccentricity was not effective in increasing the onset speed of instability because of reduced natural frequency and increased inertia. Instead, orbit simulations confirmed that small rotor mass with external loading is the most effective way to increase the bearing stability