Hydraulic fluids have the task of transporting hydraulic energy from the location of generation (pump) to the location of conversion (cylinder, motor) with the minimum possible losses. They are also used to reduce the wear of parts that slide against each other by providing lubrication and to protect the moving parts such as piston and spool areas as well as the surfaces against corrosion. As it flows through the system, the hydraulic fluid conducts away the locally occurring heat and takes with it contamination, which is caused by debris, for example.
For this task, mineral oils and flame-resistant fluids are available. In addition, in environments that are critical for biology (construction machines in water protection zones, machines for forestry purposes in woodland, snow grooming equipment in the mountains), readily biodegradable hydraulic fluids are used. These fluids can be produced from mineral oil or using renewable materials. They are also referred to as bio-based hydraulic fluids.