In the past three decades an increasing amount of research has been performed in the field of tribodynamics of fluid power pumps and motors. The main incentives for this research are optimization of reliability and efficiency through the study of loss and wear mechanisms. These mechanisms are very difficult to study experimentally, whereby modeling and simulation are necessary. The modeling of tribodynamics is a multiphysics problem involving multibody dynamics, fluid mechanics, thermodynamics and solid mechanics. Consequently, the simulation durations can easily become impractical for parametric analysis or optimization. The coupling between multibody dynamics and fluid mechanics depend on the formulation of the solid body motion equations, where two approaches have historically been used. One approach is where the external forces on any lubricated joint are balanced by the fluid forces, such that solid body inertia is neglected. The other approach includes the inertia terms in the calculation of microdynamics. The inclusion of inertia terms entails a need for smaller time steps in comparison to the force balance approach, wherefore it is of interest to analyze the influence of the inertia term. In this paper the influence of the inertia term on the lubrication gaps of a radial piston motor are studied by a parametric analysis of the piston and cylinder density in a multibody tribodynamic simulation model. The motor is modeled as a digital fluid power displacement machine and a series of full-stroke displacement simulations are used as basis for the parametric analysis. From the parametric analysis a change, in the minimum film thickness as function of piston and cylinder density, is shown for certain operating modes of the digital fluid power displacement motor. This indicate a need for careful assessment of the applicability, of the force balance condition, if it is used in multibody tribodynamic simulations of radial piston digital fluid power displacement motors.

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