| Oil-based hydraulic medium has made hydraulics an efficient, high energy density, advanced power and motion control technology. But with the development of society and science, sustainable development catches more and more attention, traditional hydraulics is losing its application field because of its disadvantages: Firstly, it is not environmental friendly, the medium pollutes nature resources such as soil, water, and products, they irritate eyes and skin. Using water as working fluid is an attractive alternative to oil based system, the working fluid is free, clean, non-toxic, nonflammable, and having no adverse effects on the environment. It has been proved successfully in entertainment facilities, food industry, paper machinery, under water tools, fire fighting. But water is an aggressive medium, a poor lubricant. In order to meet the extended life and increased operating pressure which means high PV levels, screening out suitable combinations for friction pairs in water hydraulic pump or motor becomes the most demanding task.This paper documents the material investigation under water lubrication, the mathematic model of the friction pairs of an axial-piston swash-plate pump, and the development of water hydraulic relief valve. Such research works were performed: (1) Tribological properties of ceramic-ceramic, ceramic-polymer, stainless steel-polymer combinations were investigated. The tested ceramic combinations are not suitable to be friction pairs in water hydraulic piston pump because of their high friction coefficient, and brittle characteristic. The polymers used are modified PEEK composites, the composites were filled with graphite and PTFE, reinforced with carbon fiber. It was found that the friction and wear behaviors of PEEK composites were closely dependent on the mass fraction of the additives, the load applied, and the material of the counterpart. Sharply reduced friction coefficient and wear rate indicate that the tribological properties is better when sliding against plasma sprayed ceramics than that sliding against stainless steel. Friction coefficient and wear rate of the best combination is 0.05, 1.5 X 10"7 mm3/Nm irrespectively under 900 N. Experiments of water hydraulic pump demonstrated that the combination can satisfy the requirement. (2) Mathematic model of the key friction pairs of water hydraulic piston pump were established. Semi-balanced slipper bearing was demonstrated the better structure. Hydrodynamic force and lateral force were considered in the piston model.Calculation results shown that the force on the piston is 1.8 smaller when the two forces were neglected. This is important when the polymer and ceramic are used. (3) New structures of the key friction pairs were designed according to the characteristic of polymer and ceramic, the key friction pairs endured 100 hours test. The pump's mechanical efficiency wass around 84%, volumetric was 76%, and the total efficiency was 64% under 9 Mpa, 1450 r/min. (4) One directed operated and one pilot operated relief valve were developed. Interfering jet, special passage, and two-stage serial throttle were adopted to reduce cavitation problem. The over shoot was smaller than 30%, adjusting time was shorter than 500 ms, the control accuracy was 90.7% under 14MPa. Dynamic and static experiments show that the performance of the valves accomplished the expectation. (5) Parallel valve ports, a special structure, was patented to reduced cavitation. Its characteristic is that two valve ports are formed between one spool and one valve bore, then the velocity around the valve ports will be reduced under same flow rate. The mathematic model of a single stage relief valve with parallel valve ports was researched. Three advantages, higher control accuracy, smaller operating force, and small size, are supported by results of the mathematic model.Significance is described in chapter 1. Characteristic of different hydraulic medium, oil-base medium, water medium, etc were compared. Examples of water hydraulics were illustrated. Key problems of water hydrau...
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