Hydrostatic vs. hydrodynamic components of fluid pressure in the tribological interfaces of axial piston machines

Abstract

Axial piston machines of swash plate design rely on three main lubricating interfaces to support the load on their slippers, pistons, and cylinder block. The respective loads on these parts are carried by the fluid pressure in these interfaces— pressure from hydrostatic sources (from the inlet and outlet ports of the pump), as well as hydrodynamic pressure, generated by the relative motion of the surfaces that border a fluid film. The present work investigates how the hydrostatic and hydrodynamic components of fluid pressure compare in terms of their importance to load support in the three interfaces. A methodology is proposed to calculate these two fluid pressure components, and their contributions to load support, for each interface in a given unit, at a given operating condition. Separating the interface's fluid pressure fields, and the forces they generate, into their hydrostatic and hydrodynamic components offers a novel view into the load-support mechanism of the interfaces, and how they compare to each other. Moreover, the proposed methodology can easily be adapted to bent axis machines. Through a case study involving a 75 cc commercial unit, the proposed methodology shows that the hydrodynamic contribution to load support in two of the interfaces of axial piston machines of swash plate design can significantly exceed the estimates typically provided in literature, and that in the third interface, the force from hydrostatic fluid pressure can be so large that less than 60% of it is required to balance the load, and over 40% of it is excess— i.e. it becomes the load force.