Abstract: This paper pertains to case drain pressure limitation for axial piston swashplate pumps used in open-loop circuits. The critical case drain pressure for pumps of this type is considered from the oil film perspective of the slipper/swashplate pair: (1) height of the lubricating oil film, (2) supporting stiffness, and (3) location of the centroid of the equivalent hydrodynamic lifting force. A dynamic lubricating oil film simulation model is established to determine the critical case drain pressure for which the slipper cannot remain in a stable state. Based on the simulation results, the worst condition occurs at the point when the height of the lubricating oil film is the maximum, the supporting stiffness is the minimum, and the distance between the centroid of the equivalent hydrodynamic lifting force and the bottom center of the slipper is the maximum. The slipper is stable only when the difference between the case drain pressure and the suction pressure is within a reasonable range. Subsequently, a design criterion is put forward to specify the reasonable case drain pressure, and this is validated by experimental results.

Key words: Axial piston pump, Slipper/swashplate pair, Case drain pressure, Oil film

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