Modeling the lubricating interfaces of ultra-high pressure radial piston machines

Gautham Ramchandran, Purdue University

Abstract

A novel approach to modeling the lubricating interfaces of radial piston machines – operating at ultra-high pressures of 700 bar or higher – is presented in this study. The two types of lubricating interfaces present in rotating cam type radial piston machines are the Piston/Cylinder and Cam/Piston interfaces. Together, these two interfaces accounts for the power losses arising from shear stresses and leakages across the gaps. By formulating modeling approaches that accurately portray the physical behavior and characteristics of the two interfaces, a methodology for the virtual designing and prototyping of these machines can be established that allows for the exploration of new design features that can result in reduced power losses and larger lifetimes. The modeling of the cam/piston interface is complicated by the fact that the reference pump geometry to be modeled has rolling element bearings present around the eccentric shaft and a free-to-rotate outer race resting on these bearings that are in contact with all the pistons. In order to evaluate the friction at the cam/piston interface, an experiment is undertaken to characterize the motion of the outer race in the first stage of this analysis. The instantaneous angular velocity of the outer race as a function of the rotation of the eccentric shaft is found through the use of a camera. Once the kinematics of the outer race is captured, the instantaneous variation of the friction coefficient at the interface is evaluated through a previously developed friction model for the cam/piston contact interface at various operating conditions. This undertaking allows for an accurate prediction of the piston tilt within the lubricating gap as the magnitude and direction of the friction force at the cam/piston interface heavily influences the micro-motion of the piston. The use of the now accurate friction model also allows for the evaluation of the power losses due to viscous friction at the cam/piston interface. The second part of this study involves the exploration of circumferential grooved piston designs as a possibility of reducing the losses occurring at the piston/cylinder interface. Grooves located close to the displacement chamber ends (high pressure) of the pistons aid in the better balance and tilt of the pistons within the cylinder. It is observed that the full film assumption in the modeling of the piston/cylinder interface predicts regions of solid-solid contact during certain intervals of the pump cycle. In order to evaluate the effect of the surface roughness features and asperity loading at low gap heights observed in this interface, a Mixed Fluid Structure Interaction – Elastohydrodynamic (FSI-EHD) model is developed. It is seen that the full film assumption underestimates the losses due to leakages present in this gap at extreme operating conditions. The evaluation of the performance parameters to a greater degree of accuracy is now possible through the development of this model. An additional benefit is that it can predict the load supported by the fluid film as well as the asperities, and thus, allows for the evaluation of new designs where regions of mixed lubrication may be avoided.

Degree

M.S.M.E.

Advisors

Vacca, Purdue University.

Subject Area

Mechanical engineering

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