THE IMPACT OF AXIAL PISTON MACHINES MECHANICAL PARTS CONSTRAINT CONDITIONS ON THE THERMO-ELASTOHYDRODYNAMIC LUBRICATION ANALYSIS OF THE FLUID FILM INTERFACES

Abstract

The authors analyze the lubricating interfaces of axial piston machines considering thermo-elastohydrodynamic (TEHL) lubrication characteristics. The fluid film geometry in these conditions is strongly influenced by the surface elastic deformation of the solid boundaries. The surface elastic deformations derive from the high dynamic pressures developing in the fluid film, necessary to balance the external oscillating loads. Furthermore, elastic deflections of the fluid film develop from the thermal expansion of the solid bodies, caused by the heat generated due to viscous shear of the fluid film. The accurate determination of the solid boundaries elastic deformation is a key element to predict the fluid film geometry and consequently the lubricating interface performance. When solving for the static elastic deformation of a solid body, constraint conditions must be imposed to avoid rigid body motion. Constraint conditions strongly influence the elastic deformation analysis; therefore their definition must reflect and interpret the mechanical body real conditions. In an axial piston machine all the mechanical bodies defining the fluid film geometry are loosely constrained and significant linear displacements and rotations are intentionally allowed. Hence, the definition of proper constraint conditions for the solid bodies is not a trivial problem and advanced constraint conditions must be considered and implemented. In the fully-coupled numerical models of the lubricating interfaces developed by the research group of the authors, finite element analysis is used to determine the mechanical bodies’ elastic deformations. The finite element analysis is coupled with finite volume models of the fluid film, to study the impact of the surface elastic deformations on the interfaces behavior. In this paper, the authors present and discuss the implementation of the inertia relief method on the finite element elastic deformation analysis of the main mechanical parts of an axial piston machine. Inertia relief allows simulating unconstrained structures in a static analysis using their inertia to resist the applied loads. Typical applications of this method include modeling an aircraft in flight, a submarine under water or a satellite in space. The impact of this method on the elastic deformation of the fluid film solid boundary surfaces is shown and compared to standard constraint conditions. In addition, the influence of the inertia relief method on the piston/cylinder interface fluid film behavior is discussed, presenting numerical results for a fully-coupled TEHL simulation over one shaft revolution of a special test pump capable of measuring the piston/cylinder axial viscous friction force. The improved accuracy of the piston/ cylinder fully-coupled model including inertia relief effect is presented, comparing simulation results with friction force measurements.