In swash plate type axial piston pumps/motors a hydrostatic spherical bearing is mounted as a ball joint connecting the piston and the slipper. The motion and lubrication of the piston/slipper assembly which plays most important role of the pumps/motors are strongly influenced by the tribological characteristics of the spherical bearings. Hydrostatic spherical bearings [1] as machine elements are usually designed and used in fluid film lubrication, while the bearings for hydraulic equipment are enforced to operate in mixed lubrication because the diameter of the sphere is smaller than that of the piston, resulting in that the hydrostatic load-carrying capacity is insufficient to support the load. This paper deals with the theory and experiment of hydrostatic spherical bearings including interaction and contact of surface roughness and discusses the optimum design of the bearing-seat geometry.
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