Design solutions for piston machines with high operating pressures and water as a working fluid
Water hydraulics: the idea of using a nonflammable, environmentally friendly, cheap, nontoxic fluid in hydraulic systems has a lot of appeal. Water's renaissance as a working fluid, however, is crowned for only half the kingdom--- it has found application in many low and medium pressure systems, but in high-pressure systems, axial piston units of swashplate type and other piston machines subjected to high side loads in the presence of high pressure gradients cannot survive running on water. This is because water is around 30 times less viscous than mineral oil, which gives it an extremely low load-carrying capacity that is especially detrimental for the piston-cylinder tribological interface of the piston machines described. Increasing the load-carrying capacity of this interface would enable such piston machines to operate in systems with a 300 to 420 bar pressure gradient from the low to the high pressure line, e.g. construction machinery, forest machinery, aircraft machinery. A series of simulation studies have been conducted using FSTI, the fluid structure thermal interaction model developed at the Maha Fluid Power Research Laboratory, in order to investigate the effect of minimum clearance, speed, four micro surface shaping forms, and four different material properties on the load-carrying capacity of the piston-cylinder interface of an existing 75cc axial piston unit of swashplate type. The axial piston unit was chosen on account of the fact that axial piston units of swashplate type suffer from a particularly high side load, turning this into a type of worst-case scenario. The four surface shapes studied are the piston profile developed by Lasaar, the axial sine wave piston profile invented by Ivantysynova, Garrett, and Frederickson, a new version of this profile consisting of axial sine waves that switch between two amplitudes at their inflection points, and wide circumferential grooves in the bushing. It was determined that the last surface profile in that list can effect the greatest change in load-carrying capacity with the least amount of unwanted side-effects out of all profiles examined. The four material properties examined are piston and bushing density, modulus of elasticity, coefficient of thermal expansion, and thermal conductivity. Here it was determined that the modulus of elasticity of the piston, in particular, had the highest level of influence on the pressure distribution of the interface, and thus its load carrying capacity, out of all material properties investigated in this work. Finally, from the simulation studies conducted, it is apparent that a significant improvement in the load-carrying capacity of water-lubricated piston-cylinder interfaces with high side loads is possible.
Ivantysynova, Purdue University.
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