Hydraulics | Kinematic Viscosity of Hydraulic Fluids

Basically, a hydraulic system consists of a hydraulic fluid reservoir, pumps, valves, pistons and lines. Hydraulic fluids are means of power transmission in systems operated by liquid pressure. Therefore, they must not trap gas, as that would change the compressibility of the fluid. Such fluids are widely used, e. g. within construction equipment, aircraft, road vehicles, or industrial, agricultural and forestry machinery. Key features are low compressibility, capacity for heat transfer, lubrication of system components, and keeping contaminants in suspension. Further requirements are low temperature sensitivity of viscosity, thermal and chemical stability, a low pour point, providing protection against corrosion, low foaming tendency and compatibility with system components. Recently, also the sustainability of these products has gained in importance.
Due to their wide field of use, fluids with different chemical bases and properties are available: Mineral oil / petroleum based, phosphate ester based synthetic liquids, polyol ester based, glycol-water based, vegetable hydraulic fluids.

Depending on the intended use of the hydraulic fluid, viscosity is specified respectively stated at different temperatures. Many hydraulic fluids are industrial oils and therefore
their viscosity is specified according to ISO 3448. Hydraulic fluids must be selected according to the requirements of the equipment. Kinematic viscosity is an important and safety relevant parameter. It influences the function of the entire system. Too low viscosity can lead to reduced volumetric efficiency, to overheating and increased component wear. A lack of lubrication can cause mechanical damage. Too high viscosity can cause sluggish response and even system failure. In-service monitoring is important to ensure that the system runs safely and reliably as these fluids age over time and due to contaminants, “worn” additives and degradation products, they change their viscosity.