ABSTRACT
The selection of the hydraulic lubricant depends on the hydraulic system operating conditions. Mineral lube oil base stocks provide good lubrication as long as the lubricant film is maintained between moving parts. This kind of hydrodynamic lubrication is mainly dependent on the oil viscosity. When the lubrication film is broken due to high temperature, pressure, and velocity, a chemical compound is needed to react with the metal surface and form a protective coating. This type of lubrication is called “boundary lubrication” (Raab and Hamid, 2003). The literature reports on the influence of lubricant viscosity on the interfacial friction and the surface quality of Pb-coated steel (Deng and Lovell, 2000). Lube oil base stocks need to have good low temperature fluidity and oxidation stability but also good interfacial properties at the oil/air interface, such as resistance to foaming and air entrainment, at the oil/water interface, such as demulsibility, and at the oil/metal interface, such as rust inhibition, corrosion inhibition, and wear prevention. The literature reports that the viscosity and the VI of base stocks affect low temperature fluidity, energy losses, cooling efficiency, and wear protection. Volatility affects oil consumption, oil thickening, and deposit formation. Oxidation stability affects acid formation, metal corrosion, oil thickening, and deposit formation. Solvency affects seal compatibility, engine cleanliness, and the formulation stability (Shell, 2002).
