Abstract
Having come of age, gas film bearings enable high-speed oil-free (micro) rotating machinery with gains in efficiency and reliability, longer maintenance intervals, and a reduction in contaminants released to the atmosphere. Among gas bearing types, porous surface gas bearings (PGBs) have proven successful for 50+ years and presently are off-the-shelf mechanical elements. This paper reviews the literature on PGBs since the 1970s and reproduces an exact solution for the performance of cylindrical PGBs. Both the analytical model and an accompanying finite-element (FE) model predict the performance for two PGBs, a commercially available 76 mm diameter bearing and a smaller 25 mm diameter laboratory unit whose experimental performance is available. As expected, the FE model results reproduce the analytical predictions obtained in a minuscule computing time. For a set external supply pressure, as the radial clearance increases, the flow rate through the bearing grows until reaching a peak magnitude. The PGB load capacity is a fraction of the product of the set pressure difference (pS − pa) and the bearing projected area with a significantly large centering static stiffness evolving over a narrow region of clearances. Operation with shaft speed enhances the bearing load capacity; however, at sufficiently high speeds, significant magnitude cross-coupled forces limit the stable operation of a PGB. At constant operating shaft speed, as the whirl frequency grows, the bearing effective stiffness (Keff) increases, while the effective damping (Ceff) becomes positive for whirl frequencies greater than 50% shaft speed. Similar to a plain hydrodynamic journal bearing, the PGB is prone to a half-frequency whirl, albeit the system natural frequency can be high, mainly depending on the external supply pressure. In essence, for the cases considered, PGBs are linear mechanical elements whose load capacity is proportional to the journal eccentricity.
Highlights
Bearings constructed with a layer of porous material offer an alternative to other bearing types, such as orifice compensated bearings and foil bearings [2]
I.e., operation with shaft speed (ΛΩ > 0), the analysis reports cross coupled stiffness and damping coefficients, growing in proportion to the shaft speed until ΛΩ → 10, to drop for operation at higher shaft speeds
From dynamic load experiments conducted with shaft speeds at 6 and 9 krpm (32 and 64 m/s surface speed), the identified Porous surface gas bearings (PGBs)’s direct stiffness and damping coefficients are practically invariant with excitation frequency, The pads’ pivot compliance largely determines the bearing direct stiffness coefficients
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. L, and diameter D = 2R), the gas physical properties (density ρ and viscosity μ), the magnitudes of supply pressure (pS) and ambient pressure (pa), the porous material permeability coefficient (κ) and the liner radial thickness (tp), and the operating conditions of shaft angular speed (Ω and precessional frequency (ω). PGB with journal spinning is displays where the vector gradient operator, the gas viscosity, andthe pS is the pressure of with speed. The PGB geometry, porous material properties, andmaterial operating conditions to produce three fundamental (dimensionless) parameters,. The PGB geometry, porous material properties, and operating conditions combine to produce three fundamental (dimensionless) parameters, 12κ. PS > pa for the pressurized gas to flow through both the porous layer and through the film clearance to exit at the bearing sides. The porosity parameter Λκ relates the flow conductance across the porous layer (κ/tp ) to the conductance through the film, which is proportional to (c3/ R2 )
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