ENVIRONMENTALLY ADAPTED HYDRAULIC OILS

A comparison has been carried out between one mineral based hydraulic oil and three environmentally adapted hydraulic oils. Two of these are semi-synthetic oils, i.e. mixtures of vegetable base oil and syntheti c esters, and the last is based on synthetic esters only. Technical pr operties such as viscosity, pour point etc., and chemical properties s uch as phosphorus and sulphur content etc. were documented using stand ard test methods. The lubricant capability properties were determined through measurements of the capability of each oil to build a film in an elastohydrodynamic contact. The oil film thickness is important to avoid wear and failure and to guarantee separation of surfaces. The re sults show that at 40 degrees C the environmentally adapted oils give a thicker film than the mineral oil. At 80 degrees C there is no signi ficant difference between the different types of oil in their capabili ty to build a lubricating film. The maximum shear strength was measure d and these results are presented with the constant of proportionality gamma, from the theoretical model tau approximate to gamma p. which i s valid at very high pressures. The measurements were carried out at c ontact pressures in the range 5-7 GPa. The results show that all envir onmentally adapted oils give a lower gamma value than the mineral oil, which is an advantage since a low shear strength results in lower fri ction in highly loaded contacts. Measurements of toxicity to aquatic o rganisms (Daphnia magna) show very large differences between the teste d oils, and this implies that a change from a mineral oil to an enviro nmentally adapted oil can give important environmental advantages. (C) 1997 Elsevier Science S.A.