The Influence of Temperature and Shear Rate on the Viscosity of Selected Motor Oils

Abstract

One of the most important physical quantities which has an influence on bearings and micro-bearings functioning is the viscosity of a lubricant. The data about a viscosity value dependence on temperature and shear rate are essential for designing sliding friction pairs. In design calculations usually there is assumed that a lubricant is a Newtonian fluid, therefore viscosity is constant over the full range of shear rates. During operation of friction pairs the contamination particles get into a lubricant and this causes that the lubricant becomes a non-Newtonian pseudoplastic or viscoelastic fluid. A similar effect on lubricating oil properties have combustion products or special performance additives. Furthermore, a lubricating oil ages and wears out, i.e. during its operating the initial structure of a particles is destroyed which can cause the change of the oil viscosity and lubricity values. The aim of this work is to determine the dynamic viscosity values in dependence on temperature and shear rate for selected new and used lubricating oils. In this research motor oils for passenger vehicles and tractors were investigated. This paper presents the results of measurements of the viscosity changes, in dependence on shear rate and temperature, made for the new and used oils. One of the investigated oils is Superol CC-40, which was used in four-stroke 4562 cm3 diesel engine for twenty months, which corresponds to 250 hours of operating. The second of investigated oils is Shell Helix Ultra AV-L which was used in four-stroke 2000 cm3 diesel engine for ten months at a distance of 15 000 kilometers. The viscosity measurements for the new and used lubricating oils were made with the Thermo Scientific Haake Mars III rheometer, in the range of temperatures from 10°C to 120°C and of shear rates to 51000 1/s. Moreover, the analysis of wear products, contaminants and additives in the investigated new and used lubricating oils was made with the rotating disc electrode atomic emission spectrometer Spectro Incorporated Spectroil Q100, which gives possibility to determine 22 most common elements which occur in motor, turbine and gear oils. The obtained information will be used in future studies related to hydrodynamic lubrication of slide bearings and micro-bearings. It also may be useful for designing bearings and sliding friction pairs.