Rationale of the dielectrometric method of definition of lubricant oils' tribological characteristics' changes

Abstract

The article demonstrates the contradiction of generally accepted methods of research of lubricants tribological properties on the example of spent oil studies and critically considers the possibility of using the obtained results as criteria for assessing the serviceability of lubricating oils after a certain service life. In particular, regulatory parameters such as viscosity, flash point, acid value and insoluble sludge content were considered. Viscosity, in particular, was assessed by the Heppler’s and the Ostwald’s methods. As a result, it was found that the first of these methods shows lower values. Similar ambiguity is demonstrated by normative methods for determining the flash point in open and closed crucibles, determining the acid number by potentiometric or calorimetric titration of an oil sample dissolved in various solvents and determining the sludge content by extraction with n-pentane, n-heptane and extraction benzene. As it turned out, the results obtained by different methods are impossible to compare. The analysis of the obtained data also confirms the opinion that the indicators determined in the laboratory have no practical application, because they do not correlate with the real operating conditions of the oil. Such laboratory studies can only be used to compare two or more samples of lubricants. As an objective criterion that would reflect the interaction of the components of lubricating oils and friction units during operation and could serve as an indicator of the degree of aging of the oil, the author proposed to use the dielectric constant in an alternating electric field. These first research results show that changes in dielectric constant as a function of service life can be considered an indicator of the degree of aging of the oil, and the results allow us to estimate the concentration of sludge and mechanical contaminants in oils. However, given the multicomponent nature of modern oils and the influence of other factors on the dielectric penetration of the material, the results obtained should be considered only preliminary and those that require further refinement and processing.