Paper 19: Wear-Reducing Properties of Mineral Oils and Their Fractions

Abstract

Refined lubricating oils from several crudes have been separated chromatographically and by thermal diffusion into fractions of different chemical and physical nature. Four-ball and pin-on-disc machines have been used to investigate the effects of the chemical nature of the oils and their fractions on wear-reducing properties. In the four-ball machine the ‘aromatic’ fractions (aromatic compounds plus sulphur compounds eluted as one fraction from silica gel) were found to have better wear-reducing properties than the corresponding naphthene–paraffin fractions and the original oils from which they were separated. The ‘aromatic’ fractions from bright-stocks were found to have the best wear-reducing properties of all the components tested. The studies of wear in the pin-on-disc machine have demonstrated the superior wear-reducing properties of the naphthene– paraffin fractions. Viscosity has been shown to have some effect in the wear tests, but chemical nature appears to have more influence on anti-wear performance than changes in viscosity within the normal range of conventional oils. The type of ‘aromatic’ and saturated hydrocarbons that are most effective in wear reduction is not known at present. It is likely that mineral oils contain compounds which are even more active in wear reduction than the ‘aromatic’ and naphthene–paraffin fractions which are, of course, complex mixtures of many different types of hydrocarbons and sulphur compounds. There is little doubt that it is possible to isolate, from various crudes, groups of compounds possessing better wear-reducing properties and load-carrying capacity than the conventionally produced base oils