GAP GEOMETRY VARIATIONS IN DISPLACEMENT MACHINES AND THEIR EFFECT ON THE ENERGY DISSIPATION

摘要

The energy dissipation of displacement machines is mainly influenced by the design of individual lubricating gaps between parts, having relative motion to each other. The gap serves as a hydrostatic or hydrodynamic bearing respectively for the transmission of forces inside the machine. In addition the gap provides a sealing function between spaces of different pressure levels. The functioning as well as the efficiency of the displacement machine depend on the design of its lubricating gaps, thus an optimization of the machine means essentially optimization of the gaps. This paper concentrates on the gap between piston and cylinder of a swash plate type axial piston machine. In this machine type the piston is loaded with a very high lateral force, which represents the limiting factor for enlargement of the swash plate angle. Still the obtained results can be applied to other gaps as well. The paper explains by means of simulation, how a specific choice of the gap macro geometry can improve the pressure distribution and thus the load capacity of the piston/cylinder assembly. This leads to an increase of the gap heights in order to avoid mixed friction. However, in case the external forces predominate the fluid forces inside the gap, mixed friction occurs, which depends, beside others, on the micro geometry of the sliding surfaces, which are investigated experimentally. The investigation is done by means of simulation, using the CAE design tool for axial piston machines CASPAR and by means of experiment, using the TRIBO TEST RIG, both developed at the Institute for Aircraft Systems Engineering.