Experimental Response of a Rotor Supported on Rayleigh Step Gas Bearings

Abstract

Reliable gas bearings will enable the rapid deployment of high speed oil-free micro-turbomachinery. This paper presents analysis and experiments of the dynamic performance of a small rotor supported on Rayleigh step gas bearings. Comprehensive tests demonstrate that Rayleigh step hybrid gas bearings exhibit adequate stiffness and damping capability in a narrow range of shaft speeds, up to ∼ 20 krpm. Rotor coast down responses were performed with two test bearing sets with nominal radial clearance of 25 μm and 38 μm. A near-frictionless carbon (NFC) coating was applied on the rotor to reduce friction at liftoff and touchdown. However, the rotor could not lift easily and severe rubbing occurred at shaft speeds below ∼ 4,000 rpm. The tests show that the supply pressure raises the rotor critical speed and decreases the system damping ratio, while only affecting slightly the rotor-bearing system onset speed of instability. Whirl frequencies are nearly fixed at the system natural frequency (∼ 120 Hz) with subsynchronous amplitude motions of very large magnitude that prevented rotor operation above ∼ 20 krpm. The geometry of the Rayleigh steps distributed on the rotor surface generates a time varying pressure field, resulting in a sizable 4X super synchronous component of bearing transmitted load. Predictions show the synchronous stiffness and damping coefficients decrease with shaft speed. Predicted threshold speeds of instability are much lower than measured values due to the analytical model limitations assuming a grooved stator. The predicted synchronous responses to imbalance correlate well with the measurements. The Rayleigh step gas bearings are the most unreliable rigid bearing configuration tested to date.

Analytical analysis and optimisation of the Rayleigh step slider bearing

“…Tello [12] has theoretically studied the regularity of the solution to the Reynolds equation in Rayleigh step type bearings for both compressible and incompressible fluids by employing a rigorous mathematical approach. Zhu [13] has investigated both numerically and experimentally the response of a rotor supported on Rayleigh step gas bearing using Galerkin finite element method. Thermohydrodynamic lubrication (THL) analysis method has also been applied to the study of (Rayleigh) step bearing by Hideki [14].…”

“…In a most recent work, Naduvinamani and Siddangouda [16] have also studied the effect of surface roughness on the hydrodynamic lubrication of porous step-slider bearings with couple stress fluids theoretically. Rayleigh step bearings have been of interest in industry from shaft-bearing systems such as (high-speed) turbomachinery to microelectromechanical systems (MEMS) as well as the computer industry in devices such as magnetic head recording or hard/floppy disc drives [12][13][14]. Their higher load capacity and cheap/easy manufacturing has made them more favourable to be used especially in situations wherever the air or gases are used as a lubricant in which because of their lower viscosity application of a hybrid hydrostatic/hydrodynamic mode is inevitable.…”

Experimental Analysis of the Dynamic Characteristics of a Hybrid Aerostatic Bearing

“…The most recent and complete work dealing with the experimental analysis of aerostatic bearings are those of San Andres et al [5][6][7][8][9] and of the research team at Politécnico di Torino [10][11][12]. The experimental results of San Andres and co-workers were obtained on a test rig consisting of a light rotor supported by two identical bearings.…”

Recirculation Flow and Pressure Distributions in a Rayleigh Step Bearing

“…Thus, the solutions of Reynolds lubrication equation for the pressure distribution, load capacity, and frictional coefficient were obtained through a relatively simple calculation and some detailed flow information on Rayleigh step bearing has been neglected [20]. Only a few investigations used the calculation of the Navier-Stokes equations with finite volume method [11] or infinite element method [12,14]. In recent years, the full Navier-Stokes equations have been increasingly applied to solve some lubrication problems [21][22][23].…”

“…Zhu [12] investigated both numerically and experimentally the response of a rotor supported on Rayleigh step gas bearing using Galerkin finite element method. Constantinescu et al [13] analyzed the pressure variation due to fluid inertia effects in Rayleigh step bearings.…”