Hydraulic fluid is used in fluid power systems mainly because of its incompressibility or stiffness. This allows it to provide consistent and powerful transmission of energy throughout a hydraulic system, which allows the actuators to complete the work, whether they are linear or rotary motions. The advantage of a fluid’s compressibility ensures a hydraulic machine functions safely, repeatedly, reliably, and efficiently.

Secondly, hydraulic fluids help to cool a system by dispersing heat throughout the system, particularly by carrying it away from devices that create heat — such as those doing the work like actuators — through to the reservoir and other cooling devices, such as heat exchangers.

Hydraulic fluids also help to lubricate and seal internal components within a hydraulic system. This is especially true on components with tight clearances, such as valves, pumps and motors. All that is often required is a thin film of oil, which helps to reduce leakage over these clearances and reduce friction on sliding and rotating surfaces in a system. Preventing any metal-to-metal contact reduces wear and over-exposure to heat.

Additionally, hydraulic fluids minimize contamination in a hydraulic system. This function is very simple, as the fluid simply carries any particles and water away from critical components to the filters and reservoir, where they are captured or settled. Also, common additives help with a number of problems that can occur, such as anticorrosion agents, rust and oxidation inhibitors, fire-resistance, anti-foaming agents, and more.