The Viscosity-Temperature-Pressure Relationship of Lubricating Oils and Its Correlation With Chemical Constitution

Abstract

Viscosities of twenty well-defined, representative mineral-oil fractions have been determined at temperatures from 25 to 90 deg C (77 to 194 deg F) and at pressures up to about 1000 atmospheres (15,000 psi) with the aid of a falling-needle viscometer. An analysis has been made of both the present measurements and reliable data from literature, which chiefly concern mineral oils and pure hydrocarbons, but also include some silicones, fatty oils, and alcohols. Many literature data cover ranges of viscosity, temperature, and pressure that are more extensive than those of the authors. Newly developed empirical formulas are presented for the isobaric viscosity-temperature relationship, the isothermal viscosity-pressure relationship, and the complete viscosity-temperature-pressure relationship. The formulas have been found to be satisfactorily applicable to all the aforementioned liquids in a wide range, that is, generally, from about 20 to 150 deg C (68 to 302 deg F) and up to pressures of at least 3000 atmospheres (44,000 psi). Diagrams derived from these formulas have proved particularly suitable for a systematic study of the correlation between, on the one hand, the temperature and pressure variation of viscosity of the liquids concerned and, on the other hand, their chemical constitution. This is exemplified by the results for the mineral oils investigated. In fact, it proved possible, presumably for the first time, to establish for mineral oils a really satisfactory quantitative correlation between their viscosity-temperature-pressure dependence and their chemical constitution; the latter has been characterized by the carbon distribution according to the “Waterman analysis” in the form of the so-called n-d-M method.