Prof. dr. Darko Lovrec, univ. dipl. inž., Univerza v Mariboru, Fakulteta za strojništvo

Abstract:
The viscosity of a hydraulic fluid is one of its most important material properties. The choice of the appropriate viscosity of the fluid for a particular application is one of the most important tasks of a hydraulic engineer. But the viscosity value itself is not the only important parameter when choosing the fluid. It is extremely important to know how the viscosity changes with temperature – viscosity-temperature behaviour. The latter is given by the VI – the viscosity index.
The paper presents the influence of the viscosity index of different hydraulic fluids on the width of the temperature operating range of a hydraulic device. The significance and method of determining the viscosity index is explained in the introduction, while the actual values of the viscosity index for common mineral hydraulic oils of ISO HLP quality and various ISO viscosity classes are shown. In the continuation of the paper, the typical values of the viscosity index of other types of hydraulic fluids are discussed for comparison: for mineral oil type HV with improved temperature-viscosity behaviour, and for HE and HF types of hydraulic fluid. The last part of the paper presents the advantages of using ionic liquids, which are newer, environmentally friendly and fire-resistant liquid lubricants, which are characterized by a high viscosity index and consequently a wide temperature operating range and a very low solidification point.

Keywords:
hydraulic fluids, viscosity index, determining, operating temperature range, mineral oils, ionic liquids