Static characteristics of aerostatic porous journal bearings with a surface-restricted layer

Abstract

Graphite porous materials have been successfully used in an aerostatic bearing. This is because graphite porous materials have a lower permeability than porous metal, which makes it possible to achieve a higher stiffness in aerostatic bearings. However, aerostatic porous bearings have a disadvantage in that they are prone to cause pneumatic hammer instability. Therefore, a restricted layer which has a permeability lower than the bulk of the porous material is usually formed on the bearing surface to avoid this instability. In this paper, the static characteristics of aerostatic porous journal bearings with a surface-restricted layer are investigated theoretically and experimentally. In addition, the effects of a surface-restricted layer on the static characteristics are discussed for two types of loads, symmetrical and coupled. NOTATIONe c shaft displacement at the bearing end for coupled loads (m) e s shaft displacement for symmetrical loads (m) dr s thickness of a surface-restricted layer (m) ds thickness of end seal (m) D diameter of the shaft (m) h local bearing clearance (m) h 0 average bearing clearance (m) k permeability of the bulk of porous material (m 2 ) k 0 permeability of a restricted layer (m 2 ) L bearing length (m) L 1 width of circumferential air supply groove (m) m tilt moment (N m) M dimensionless tilt moment h 0 m= …p s ¡ p a †DL 2 £ ¤ p a ambient pressure (Pa) p s supply pressure (Pa) p 1 , p 2 pressures in porous material and bearing clearance (Pa) P dimensionless pressureˆ…p ¡ p a †=p a q mass¯owrate (kg/s) q r , q z , q y mass¯owrates in the r, z, y directions (kg/s) q v volume¯owrate (m 3 /s) Q mass¯ow reduction ratio r, z, y coordinates in the r, z, y directions r s radial thickness of porous material (m) r 0 radius of the bearing (m) r 1 outer radius of porous material (m) < gas constant T temperature (K) w load-carrying capacity (N) W dimensionless load-carrying capacitŷ w= …p s ¡ p a †DL ‰ Š Z dimensionless coordinate in the z directionˆz=L a parameter pertaining to a surface restrictorˆ…k 0 /k †/…dr s /r s † e c eccentricity ratio at the bearing end for coupled loadsˆe c L/…2h 0 † e s eccentricity ratio for symmetrical loadsˆe s /h 0 r 1 , r 2 densities of air in porous material and bearing clearance (kg/m 3 )

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