Performance of Environmentally Adapted Hydraulic Fluids at Boundary Lubrication

Abstract

The lubricating performance of environmentally adapted hydraulic fluids in boundary lubrication situations has been examined and compared to that of a conventional mineral oil based hydraulic fluid. The oils tested were all fully formulated hydraulic oils with the same ISO viscosity grade. Oil A was a conventional mineral oil whilst oil's B and C were mixtures of rape seed and synthetic ester oils; oil C containing a significantly higher amount of synthetic ester than oil B. Oil D was a pure synthetic ester based hydraulic fluid. The ability of the test oils to reduce friction and wear was investigated in a Plint and Partner High Frequency Friction Machine (TE 77B). In all the tests the contact geometry was of the cylinder-plate type and the contact surfaces were of roller bearing steel. The sliding friction force was measured at different oscillating sliding speeds, contact pressures and temperatures. The wear tests were performed at constant load, speed and temperature. Multifactor analysis showed that the main factors affecting the coefficient of friction at the contact were oil type, temperature, sliding speed and load. The coefficient of friction decreased with increasing sliding speed and load and increased with increasing temperature. The environmentally adapted hydraulic fluids were seen to give lower friction compared to the conventional mineral oil based hydraulic fluid. Further analysis of variance showed that combined interaction effects exists, but at much lower order of magnitude than the main effects. A comparison between friction measurements in elasto-hydrodynamic lubrication and boundary lubrication showed that the environmentally adapted hydraulic fluids gave lower friction under both lubricating conditions. This implies low shear stress and minimal temperature rise in concentrated contacts, which increase life of both hydraulic equipment and fluid. The wear tests indicated that the environmentally adapted hydraulic fluids resulted in higher wear rate than the mineral oil based hydraulic fluid. This is thought to be a result of the competition between the polar molecules of the environmentally adapted base oils and the anti-wear (AW) and extreme pressure (EP) additives, in covering metal surfaces. The superior lubricity of the environmentally adapted hydraulic fluids seems not to be sufficient to replace the use of AW and/or EP additives in a fully formulated product. When choosing EP or AW additives for a non mineral oil based product, the interference between base oil molecules and additive molecules must be considered.