A novel CFD – Axial motion coupled model for the axial balance of lateral bushings in external gear machines

This paper presents a novel modeling approach for studying the lateral lubricating gap between sliding lateral bushes and spur gears in external gear machines. Pressure compensated lateral bushings are important design elements for efficient operation of an external gear pump or motor, being responsible for functions such as sealing the displacement chambers, guaranteeing a proper timing for the connections with outlet and inlet port limiting pressure peaks and cavitation. As regards the sealing function, they must be designed with the two main goals of reducing power losses due to leakages and of maintaining full film lubrication in the gap, to minimize shear stress and prevent wear in the elements. Because of the complexity of the simultaneous processes that characterize the operation of this kind of units, only limited research has been performed on modeling the lubricating gap in the past. The model presented in this paper is the first tool that can predict the lateral lubricating gap features, accounting for the main features of machine operation. A Computational Fluid Dynamics solver that solves for the flow field in the lubricating gap is coupled with a model of the axial motion of the lateral bushes to determine the lubricating gap heights. The model also interacts closely with a lumped parameter model as well as a geometric model of gear teeth control volumes, and therefore provides a tool for a “complete simulation” of the unit. The forces acting on the lateral bushing are seen to lead to an axially balanced condition. An original method of decomposition of forces was developed to perform an analysis of the effect of the hydrodynamic as well as hydrostatic forces acting on the bushing. It was found that the hydrodynamic generation of pressure from the “wedge” mechanism determines the orientation of the lateral bushing at axial balance. Gap heights and the resulting power losses from the lubricating gap are also calculated for a range of operating conditions. Using the proposed methodology, competing designs of lateral bushes can be evaluated and optimized for low chances of wear, as well as low losses. The tool developed has the potential to be used to drive design of gear machines with greater efficiency and reliability.