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We have found that most commercially available biodegradable hydraulic oils show low static friction coefficients not depending on the kinds of wet friction materials. Since hydraulic motors of a hydraulic excavator contain wet parking brakes, the most commercially available biodegradable hydraulic oils can not provide enough brake torque capacity to park safely. Furthermore, all kinds of commercially available biodegradable hydraulic oils do not have adequate oil performance for a hydraulic system of construction eq ipment. Thus we have developed a n high performance biodegradable hydraulic oil. To obtain a high static friction coefficient and compatibility with elastomers, a new synthetic ester base fluid has been developed. To improve oxidation stability and to attain compatibility with mineral oil based hydraulic oils,. an additive formulation has also been developed.

Recently high copper concentration of used oils has been frequently detected in our oil analysis system, although any excessive wear of the copper parts in the engines and the power-train was not detected. We have found the copper comes from copper solder in the oil cooler. Thus we have investigated the copper corrosion characteristics of commercially available engine oils and power-train oils in a laboratory test. Most power-train oils did not show copper corrosion, however some engine oils showed large weight loss of a copper test piece. In a field test of off-road dump trucks, we have found that the copper, dissolved in the oil, caused glazing and friction reduction problem of the wet brake paper disk. To clarify the glazing mechanism of the disk, simulated bench tests and used oil analysis have been conducted.

To adapt biodegradable hydraulic oils to heavy-duty construction machines, we have investigated eight commercially available vegetable-based biodegradable hydraulic oils for an oxidation stability and corrosion characteristics by laboratory tests and an axial piston pump rig test. From the laboratory tests, it became clear that all vegetable hydraulic oils show poor oxidation stability and two vegetable hydraulic oils show strong corrosion to bronze materials. Three vegetable hydraulic oils, which contain 0.7 %, 0.25 % and 0% sulfur, have been tested by a high pressure axial piston pump. The 0.7 %S vegetable hydraulic oil resulted in rapid viscosity increase and serious bronze corrosion in the 32 MPa x 95°C pump test. In the 32 MPa x 80°C pump test, the oxidation and corrosion of the 0.7 %S vegetable hydraulic oil were much lower. To compare the vegetable hydraulic oils, 42 MPa x 80°C pump tests were conducted.