A Path Toward an Effective Scaling Approach for Axial Piston Machines

Lizhi Shang, Purdue University

Abstract

Lacking reliable scaling rules, hydraulic pump and motor manufactures pay a high monetary and temporal price for attempting to expand their production lines by scaling their existing swashplate type axial piston machines to other sizes. The challenge is that the lubricating interfaces, which are the key elements in determining the performance of a positive displacement machine, namely the piston/cylinder interface, the cylinder block/valve plate interface, and the slipper/swashplate interface, are not easily scalable. The aim of this work is not to find an effective scaling rule, but rather to propose a path toward an effective scaling approach that allows axial piston machine scaling research to advance; the establishment of this new scaling approach entails developing a multi-physics and multi-domain simulation model for evaluating the performance of the virtually scaled pumps and motors, analyzing the size-dependence of the fundamental physics of the elastohydrodynamic lubricating interfaces, and providing a guide to scaling an axial piston machine to another size with minimum redesign effort while upholding the same energy efficiency and life expectancy. This work includes a novel swashplate type axial piston machine performance prediction model. This simulation tool is the first of its kind to allow for the virtual scaling of an axial piston pump or motor to a different size, and an examination of its performance that compares the scaled unit against the pre-scaled baseline using a semi-empirical temperature prediction model that negates the need for measured thermal boundaries. The accuracy of the simulation proposed in this work is vastly improved thanks to the next-generation piston/cylinder interface model. The challenge of scaling an axial piston machine while upholding its efficiency is the size-dependence of the machine performance with regards to significant physical phenomena that describe the behavior of its three most critical lubricating interfaces. The phenomena, including non-isothermal elasto-hydrodynamic effects in the fluid domain, and heat transfer and thermal elastic deflection in the solid domain, is analyzed and explained through fundamental physics in this work. Based on the findings, a guide to scaling swashplate type axial piston machines such as to uphold their efficiency is proposed. The proposed guideline is applied to a design study focusing on the effective scaling rules for the lubricating interfaces in swashplate type axial piston machine. Different clearances, piston guide lengths, and materials of the piston/cylinder interface are studied for different unit sizes, and a nonlinear clearance scaling method with a groove profile is found effective in maintaining the efficiency of the piston/cylinder interface while scaling the baseline a size eight time larger and a size eight time smaller. The same scaling range is applied to the cylinder block/valve plate interface study and slipper/swashplate interface study. For the cylinder block/valve plate interface, a nonlinear scaling rule for the sealing land dimensions is found to improve the energy efficiency of the scaled interface. Furthermore, it is found that the balance factor in the slipper/swashplate interface, which determines the ratio between the hydrostatic pressure force and the external load, should be scaled nonlinearly in order to achieve the same energy efficiency as the pre-scaled baseline.

Degree

Ph.D.

Advisors

Vacca, Purdue University.

Subject Area

Physics|Energy|Marketing|Mechanics|Thermodynamics

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