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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.

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.