Analysis and Design of Lubricating Interfaces in External Gear Machines for High and Low Viscous Working Fluids

Divya Thiagarajan, Purdue University


Lubricating interfaces represent a significant design constituent which contribute to the reliability and efficiency of many modern designs of gap compensated external gear machines (EGMs). However, the complex nature of the influence of fluid behavior in these interfaces on the structural and thermal effects of the solid components involved in the lubricating gap makes their design quite challenging. Moreover, the extensive range of applications currently available for EGMs warrant designs which can perform efficiently at extended range of operating conditions as well as with a comprehensive variety of working fluids. In order to improve the understanding of the physics in these lubricating gaps especially in gap compensated units, and thereby achieve virtual prototyping design tools for these conditions, the principal goal of this research is to extend the capabilities of the state of art numerical models for the lubricating interfaces in EGMs. The present research addresses the design and analysis of the lateral lubricating interface between the lateral bushing and the gears in EGMs for critical operating points as well as for reference working fluids with significant differences in their viscosities which challenge the lubricating ability of the thin film interface. A novel mixed film-TEHD (Thermo-Elastohydrodynamic) model for the lateral lubricating gap was developed as a part of this research to capture the effects of such widely varying design parameters. Two different instances of experimental validation of this mixed film lubrication model were carried out for the reference cases of conventional oil based EGMs, namely with measured torque losses and drain leakage measurements. Furthermore, the capabilities of the lateral gap model are utilized in studying the impact of the variations in surface finishes on the performance of a commercially available EGM chosen for this study, by considering lateral plate designs of varying surface roughness. Additional contributions have also been made to the modeling of lateral gaps in EGMs to extend their capabilities, which include consideration of frictional contact forces between the lateral bushing and the housing for the first time. This research demonstrates the significance of considering the effect of friction on the performance of the lubricating gaps in gear pumps by using a reference case of an asymmetrically balanced EGM used for aerospace fuel injection applications. In addition, a mass conserving cavitation algorithm to account for the cavitating conditions in the lubricating interface was also integrated with the mixed lubrication model to improve the stability of the numerical predictions of the pressures in the lubricating gap. Leveraging the design potential of the numerical tool developed in this research, designs to improve the lubrication performance of EGMs are presented in this work, which include surface shaping on the gears as well as achieving an optimal balance configuration in the lateral gap. These unique modifications along with the mixed lubrication model are then applied to a reference EGM case with water as its working fluid where the low viscosity of a fluid further adds to complexity in designing the lateral lubricating interface. A novel water based EGM prototype which can work at high pressures is thus proposed in this work, to implement and validate the proposed design approaches for the reference low viscous fluid. Furthermore, the present work also proposes certain unique variations to the common designs of oil-EGMs that need to be implemented in the prototype water hydraulic EGM to facilitate its practical implementation especially at high pressure operating conditions. The methodologies and models developed in this research along with their proven fidelity using experiments could potentially serve as a design tool to formulate new efficient EGM designs for an extensive range of applications and working fluids.




Vacca, Purdue University.

Subject Area

Mechanical engineering

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