Hydraulic systems are used in many technical applications, e.g. in agricultural and forestry machines. Hydrostatic pumps and engines as well as hydraulic fluids are essential components of these systems. Thereof, the hydraulic fluids are of great importance since not only technical but also ecological requirements have to be fulfilled. Apart from fluid power transmission the most important technical function of hydraulic fluids is lubrication and the resulting reduction of friction and wear. In times when saving energy and resources have become environmental matters, lubricants are increasingly attracting public awareness. Important ecological aspects which need to be considered since they have a major impact on the application of technical products and decide about their success are biodegradability, water pollution and ecological toxicity. This is why ester based hydraulic fluids have started to substitute mineral oil based fluids. Additionally, ester based fluids have excellent tribological features due to the polarity of the ester molecules. However, ester based fluids change their physical and chemical properties due to usage. This paper will show that both the total acid number and the viscosity is increased by ageing while the level of additives and the viscosity index is decreased. Therefore, the impact of ageing on the tribological performance of an ester based hydraulic fluid will be analysed by friction and wear investigations. Exemplary, two tribological contacts are chosen: a mixed friction and an elasto-hydrodynamic contact. These are representative conditions for hydrostatic pumps and engines. Analysing the fluid’s behaviour being stressed by mixed friction is performed by using a tribometer. Investigating the ageing impact on elasto-hydrodynamic behaviour of the fluid, a bearing test facility is used. Finally, after having been dealing with tribological contacts used in hydrostatic machines the performance of a hydrostatic machine itself will be analysed. The impact of increased viscosity on friction which is generated within a hydrostatic gear unit at low speed will be discussed.

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