In the hydraulic system, the working fluid not only transmits the liquid power as the working medium, but also lubricates the relatively moving parts in the hydraulic component, and also has the functions of preventing corrosion, flushing the components and contaminants in the pipeline, and taking away heat for cooling. The oil should have the following basic properties.

(1) Viscosity requirements

Viscosity determines the quality of lubrication. Different thickness of liquid, the thickness of the formed lubricating oil film is different, the viscosity of water is very low, it is difficult to form a lubricating oil film, and its lubricating performance is very poor. Therefore, the viscosity is the most basic and most suitable for working liquid. Important requirements. Viscosity indicates the magnitude of the intermolecular frictional resistance when the oil flows. When the viscosity is large, the fluid flow resistance is increased, so that the energy loss during the work process increases and the temperature rises. The suction performance of the hydraulic pump is poor, and cavitation may occur. If it is too small, the leakage will increase and the volumetric efficiency will decrease. It is possible that the lubricating oil film between the relatively moving parts is cut, resulting in poor lubrication performance and increased wear, resulting in increased leakage in the system, and even sintering due to oil-free lubrication. Therefore, the requirements for viscosity also include requirements for lubricity and abrasion resistance.

(2) viscosity temperature characteristics requirements

The relationship between viscosity and temperature is: temperature rises, viscosity decreases; temperature decreases, viscosity increases.

The relationship between viscosity and pressure is: pressure increases, viscosity increases; pressure decreases, viscosity decreases, and the effect is high at high pressure. Viscosity and temperature characteristics: The rate of change of viscosity with temperature is called viscosity temperature characteristic and is expressed by viscosity index. The size of the viscosity index directly affects the performance of the working medium, which is as important as the viscosity itself. The greater the viscosity index, the smaller the viscosity of the working fluid in the hydraulic system decreases with temperature, so that the internal leakage is not too large and the lubrication performance will not decrease much. The viscosity index should generally not be lower than 90.

(3) Anti-wear and lubricity requirements

Has good abrasion resistance and lubricity, in order to reduce mechanical friction and wear. With the development of hydraulic technology in the direction of high pressure, high speed and high performance, higher requirements are put forward for reducing the wear and tear caused by friction between the moving parts of hydraulic components, so the lubrication and wear resistance of working liquids are proposed. More and more high requirements.

(4) Antioxidant stability requirements

Should have good antioxidant stability. The anti-hydrogenation carrier refers to the ability to resist chemical reactions with oxygenates when the oil temperature rises. Generally, for every 10 °C increase in oil temperature, the chemical reaction rate is approximately doubled. Hydraulic oil with good oxidation stability is not prone to oxidative deterioration after long-term use.

(5) Anti-emulsification and anti-foaming requirements of gear pumps

Demulsibility and anti-foaming properties are better. Anti-emulsification refers to the ability of oil and water to be mixed into a white emulsion after being stirred. After standing, the water is immediately separated from the oil to restore the true color of the oil. Anti-foaming means that air is mixed into the oil and is stirred. Emulsion, the ability of bubbles to separate from oil after standing. The working fluid with poor anti-emulsification and anti-foaming property will reduce the volume modulus of the oil, increase the compressibility, reduce the rigidity of the system and generate vibration and abnormal noise.

(6) Anti-shear stability requirements

The shear stability is better. In order to improve the viscosity of the oil, high molecular weight polymers such as polymethacrylate and polyisobutylene are often added to the oil, and the molecular chain is long. When the oil flows through the small holes and slits of the hydraulic component, it will be sheared. The molecular chain is sheared and the viscosity and viscosity index are changed (decreased).

(7) Pour point

The lowest temperature at which the oil can continue to flow when the oil is cooled under the conditions specified by the standard is called the pour point. The selection of hydraulic oil usually takes into account the pour point of the hydraulic oil. The pour point of the lubricating oil should be 5 to 10 ° C lower than the lowest temperature of the environment.

(8) flash point

Under the specified conditions, the lubricating oil is heated. When the oil temperature reaches a certain temperature, the vapor of the lubricating oil is mixed with the surrounding air, and when it comes into contact with the flame, a flashing phenomenon occurs, and the lowest flashing temperature is called a lubricating oil. The flash point. When selecting lubricating oil, the flash point of the lubricating oil should be considered according to the operating temperature. The general flash point should be 20~30 °C higher than the operating temperature to ensure safe use and reduce volatilization loss. The flash point of lubricating oil (hydraulic oil) should be Something is required.

(9) Acid value

The acid value refers to the number of milligrams of potassium hydroxide required to neutralize all of the acidic substances in the lg hydraulic oil.

The acid value is an important indicator for measuring the degree of oxidation of hydraulic oil and is one of the important parameters for the performance of hydraulic oil. During use, the oil with a large acid value is likely to cause corrosion of the machine parts, and also promotes deterioration of the oil and increases mechanical wear. When the acid value exceeds the specified value, it is necessary to replace the new oil.

(10) Corrosion

Corrosion is the corrosive effect of hydraulic fluid on specified metal specimens under specified conditions. In addition, the working fluid should also have the following requirements. ; not compressible under pressure. This is to transfer energy and ensure the quality of energy transfer. For example, the pressure can be transmitted quickly, and the hydraulic system can be made rigid to absorb vibration caused by pressure fluctuations. In order to dissipate heat, the specific heat capacity and heat transfer coefficient of the working fluid used should be large, and the coefficient of thermal expansion should be small. In order to prevent rust, the working fluid used should have anti-corrosion and anti-rust properties. In order to prevent the water from entering, the hydrolysis of the oil, the deterioration of the working fluid and the corrosion of the metal components, the work requires good hydrolysis stability and anti-emulsification ability. 'In order to prevent the lubrication condition from deteriorating due to the air bubbles mixed in the oil, the lubrication system is deteriorated, the system stiffness is reduced, and abnormal noise is generated. The working fluid should have good anti-foaming property and air release property. Good compatibility with rubber seals and coatings. It has good filterability, so that it can easily filter impurities in the oil when passing through the oil filter to ensure the oil is clean. It does not produce odor and toxicity, is conducive to environmental protection, and is also convenient for recycling of waste oil. Meet the requirements of use under other special conditions, such as high temperature, high cold, submarine operations and other harsh conditions.