Axial Piston Machine Cylinder Block Bore Surface Profile for High-Pressure Operating Conditions with Water as Working Fluid

Abstract

Construction, agriculture, forestry, aerospace equip- ment: axial piston machines of swash plate design (APMSPD) are the positive displacement machines of choice in a wide variety of hydraulic systems. The performance of these highly efficient units is delicately hinged on the rigorous design of three major lubricating interfaces, each striving to keep the machine’s moving components a hair’s width apart in order to avert poten- tially catastrophic metal-to-metal contact, whilst simultaneously limiting the leakage of high-pressure fluid into the unit’s low- pressure case. Of the three, the most difficult in its conception for these duties is the piston-cylinder interface, owing to the fact that especially during high-pressure operation, the pistons of APMSPD must bear a considerable side load. The measure of challenge this side load presents is heavily modulated by the choice of lubricant (i.e., the hydraulic system’s working fluid). While the use of oil still dominates the hydraulics industry, the past few decades have seen the re-emergence of water hydraulics. In its non-toxicity, its inflammability, its availability, its low cost and green footprint, water embodies an almost ideal hydraulic fluid; however, the illusion unravels in giving consideration to its viscosity, which is low enough to raise serious load-support concerns for the aforementioned interface, therewith barricading the design of marketable APMSPD for high-pressure operation with water. In aiming to enable such operation, micro surface shaping on the bores in the cylinder block through which the pistons in APMSPD move has been examined as an effective means of enhancing load support. The focus of the present work is a surface profile that has the walls of these bores curving inwards. A past exploration of this profile defined its shape via a radius and a shift; the present investigation refines that definition to two radii and a shift, thereby significantly opening the design space. In a simulation study spanning several different operating conditions, the effect of dimensional variations of this design on load support and power loss is captured with a non-isothermal fluid-structure interaction model developed by the Maha Fluid Power Research Center. The resulting design trends reveal the potential of this surface profile to handle these operating conditions.