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Using high water-based fluids in a hydraulic system requires the application of other materials than in oil hydraulic facilities. High water-based fluids have a content of water not under 80%. The rest is a HWBF concentrate. The content of water defines the features of the liquid. Because of the low viscosity of a high water-based fluid, the loading of the surfaces in water hydraulic systems is much higher than in a machine operating with oil. The low viscosity produces smaller gaps in tribological contacts. It is possible that the surfaces of a tribological contact will not be separated. The problem of adhesive and abrasive damage is increased. The selection of a suitable material pair for a tribological contact is especially important. To substitute a material in a pump or in a motor directly by another material requires great efforts and may become very lengthy and expensive.
IFAS is concerned with the development of a hydrostatic transmission operating with high water-based fluids. The goal of the project is to design and construct a constant displacement motor and a pump with a variable displacement volume based upon the swash plate principle for pressures up to 315 bar. In order to achieve the goal specially coated material pairs and their surface finish have to be tested with the objective of decreasing friction and wear. To reduce the expense of testing, the selection of materials is conducted with a special model. Two ring-shaped test bodies are pressed against each other by a hydraulic cylinder. One of the test bodies is rotated. Both test bodies are flushed by a high water-based fluid. Under different loadings and with different velocities of rotation the friction coefficient and the wear of different material combinations were established. Conventional materials, plastics, PVD coatings, and ceramics were tested. To verify the positive results obtained in the model, one of the materials which was classified as suitable is tested in a swashplate unit.
hydraulics, high water-based fluids, tribology, ceramics, coatings, plastics, surfaces, wear, friction
Dipl.-Ing., IFAS, Aachen,
Professor, IFAS, Aachen,