Hydraulic Fluids - A Design Element - Preconditions and Possibilities for Long-Life Lubrication

Apart from engine oils, hydraulic fluids are the second important group of lubricants. They make up about 15 % of the total lubricant consumption. Mineral oil-based hydraulic fluids account for about 85-90 % of all hydraulic fluids, fire-resistant fluids have a market share of about 5 %, rapidly biodegradable fluids and specialities of about 5 % [1, 2].

The increase in effective life of lubricants combined with smaller volumes, extended oil change intervals and lower lubricant consumption have the combined effect of increasing the stress on lubricants. Higher temperatures, smaller oil volumes and greater performance requirements all have the effect of increasing performance and quality demands made on hydraulic fluids. In this paper we would like to present results of thermal and oxidation stability tests of different hydraulic fluids, as well as results of friction coefficient measurements of different kinds of hydraulic fluids. We will show how we can influence evaporating losses by the careful selection of the base oil and we will describe the filtration behaviour of different additive packages under the influence of water in a dynamic filtration test rig. The hydraulic fluid as an important part of compatible fluid family systems in machine tools will be described.