The physical, chemical and operational properties of modern hydraulic oils are significantly improved by the introduction of functional additives — anti-oxidation, anti-corrosion, anti-wear, anti-foam, etc.Most mass grades of hydraulic oils are produced on the basis of well-purified base oils obtained from ordinary oil fractions using modern technological processes of extraction and hydro-catalytic purification.be compatible with hydraulic system materials.to protect parts of the hydraulic system from wear;have the necessary deaerating, demulsifying and anti-foam properties;have good filterability;protect hydraulic drive parts from corrosion;

it has a high antioxidant potential, as well as thermal and chemical stability, which ensures long-term permanent operation of the fluid in the hydraulic system;have an optimal viscosity level and good viscosity-temperature properties over a wide temperature range, i.e. a high viscosity index;In order to meet the requirements dictated by these trends in the development of hydraulic drives, modern working fluids (hydraulic oils) for them must have certain characteristics:reducing the working gaps between the parts of the working body (output and receiving cavities of the hydraulic system), which tightens the requirements for the cleanliness of the working fluid (or its filterability in the presence of filters in hydraulic systems).reducing the total weight of the drive or increasing the ratio of transmitted power to weight, which leads to more intensive operation of the working fluid;increased operating pressures and the associated expansion of the upper temperature limits of operating fluids;The hydraulic drive cannot operate without a liquid working medium, which is a necessary structural element of any hydraulic system.

The following trends are observed in the continuous improvement of hydraulic drive designs:The main function of working fluids for hydraulic systems is to transfer mechanical energy from its source to the place of use with a change in the value or direction of the applied force.This Chapter discusses working fluids for hydraulic systems of mobile equipment, designated GOST 17479.3-85 as hydraulic oils, as well as some of the most common hydraulic brake and shock-absorbing fluids on petroleum and synthetic bases.for hydraulic drives, hydraulic transmissions and circulating oil systems of various units, machines and mechanisms that make up the equipment of industrial enterprises.for hydraulic brakes and shock absorbers of various machines;for aircraft, mobile land, river and marine equipment;According to their purpose, they are divided according to the scope of application:Hydraulic oils (working fluids for hydraulic systems) are divided into petroleum, synthetic and water-glycol.General requirements and properties

Viscosity and low-temperature properties determine the operating temperature range of hydraulic systems and have a decisive influence on the output characteristics of the hydraulic drive. When choosing the viscosity of hydraulic oil, it is important to know the type of pump. Pump manufacturers usually recommend viscosity limits for it: maximum, minimum, and optimal. Maximum is the highest viscosity at which the pump is able to pump oil. It depends on the pump capacity, diameter and length of the pipeline. The minimum is the viscosity at the operating temperature at which the hydraulic system works reliably enough. If the viscosity decreases below the permissible limit, volume losses (leaks) in the pump and valves increase, respectively, the power decreases and the lubrication conditions deteriorate. The reduced viscosity of hydraulic oil causes the most intensive manifestation of fatigue types of wear of the contacting parts of the hydraulic system. The increased viscosity significantly increases the mechanical losses of the drive, complicates the relative movement of pump parts and valves, and makes it impossible for hydraulic systems to operate at low temperatures. The oil viscosity is directly related to the boiling point of the oil fraction, its average molecular weight, and the group chemical composition and structure of hydrocarbons. These factors determine the absolute viscosity of the oil, as well as its viscosity-temperature properties, i.e. the change in viscosity with temperature changes. The latter is characterized by an oil viscosity index.

The hydraulic systems of machines and mechanisms contain parts made of different metals: different grades of steel, aluminum, bronze, which can be subjected to corrosion and chemical wear. Metal corrosion can be electrochemical, usually occurring in the presence of water, and chemical, occurring under the influence of chemically aggressive media (acidic compounds formed during the oxidation of oil) and under the influence of chemically active products of splitting additives at elevated contact temperatures of the friction surfaces. Additives introduced into the oil — oxidation inhibitors that prevent the formation of acidic compounds, and special anti-corrosion additives help to eliminate 
Increasing the anti-oxidative properties of hydraulic oils is achieved by introducing anti-oxidizing additives, usually phenolic and amine types.Anti-oxidative and chemical stability characterize the oil's resistance to oxidation during operation under the influence of temperature, enhanced oil bubbling by air during pump operation. Oxidation of the oil leads to changes in its viscosity (usually to an increase) and to the accumulation of oxidation products in it, which form precipitation and varnish deposits on the surfaces of hydraulic system parts, which makes it difficult to work.
To improve the viscosity-temperature properties, viscous (thickening) additives — polymer compounds are used. In the composition of commercial hydraulic oils, polymetacrylates, polyisobutylenes and polymerization products of vinyl-butyl ether (vinipol) are used as thickening additives.