Axial Balance of External Gear Pumps and Motors: Modelling and Discussing the Influence of Elastohydrodynamic Lubrication in the Axial Gap

This paper focuses on the analysis of hydraulically balanced external gear pumps, in particular on the lubricating gap between the bushes and the gears. This topic is of key importance for the optimization of the machine efficiency because it both influences the mechanical-viscous friction and the volumetric losses. In this paper the intent is to investigate the role of the elastic deformation of the bushes surfaces, to compare the analysis done with rigid and elastic surfaces and finally to correlate the results with the design of the bushes balancing surfaces. A numerical procedure for the determination of the pressure distribution inside the gap bounded by gears sides and the bushes internal surfaces is presented and applied. With respect to past works of the authors, the procedure has been integrated taking into account the elastic deformation of the internal surfaces of the bushes and the variation of the dynamic viscosity of fluid, two well recognized phenomena that can play a key role on the determination of the bushes behaviour and lubricating gap pressure distribution. It is shown that, when the design of the bushes rear surfaces determines a strong balancing thrust, the bushes themselves need to tilt strongly with respect to the gears to generate an opportune widening thrust to avoiding contact with the gears. Useful suggestions for the bushes balancing surface design may be drawn from the analysis of the balancing maps reported in the paper, which illustrate the widening thrust magnitude and position in both the pure hydrodynamic and elasto-hydrodynamic cases, for different tilted positions and operating conditions.