Hydraulic oil is the working medium for transmitting energy in hydraulic systems. In a sealed hydraulic system connected by a hydraulic system (hydraulic motor), hydraulic control valve, hydraulic cylinder and oil pipe, the transmission of energy is achieved through the changes in pressure and flow rate during the flow of hydraulic oil. Hydraulic oil flows through all pipes, small holes, gaps and openings in the hydraulic system, so the quality of hydraulic oil and whether it is selected correctly play a very important role in the hydraulic system.

In addition to transmitting energy in the hydraulic system, hydraulic oil should also have other functions, such as lubrication, sealing, cooling and rust prevention. In hydraulic components, there are various kinematic pairs due to the interaction of components. They are either rotating or reciprocating, and there is friction, so proper lubrication is required. This lubrication does not require the addition of lubricants, but is achieved by the hydraulic oil in the hydraulic system. There is inevitably a gap between the mating surfaces of hydraulic components, which has a greater impact on the pressure of the hydraulic system. In addition to lubrication, the oil film between the gaps also plays a sealing role, which can prevent large leakage of hydraulic parts and maintain the pressure in the system. The oil in the hydraulic system is circulating. The heated oil flows through the pipes and oil tanks during the circulation process. The exposed pipes and oil tanks of a certain volume can dissipate the heat of the oil and get proper cooling, which plays the role of cooling the system. Hydraulic oil can also prevent rust on metal parts in the system, so that metal parts are often under the rust protection of oil.

The development of science and technology and the practical application of engineering have put forward higher and higher requirements for hydraulic systems, especially requiring hydraulic systems to develop in the direction of high pressure and large flow. The working conditions of hydraulic systems are constantly changing with the pressure and flow of oil. At the same time, the properties of hydraulic oil are required to be relatively stable and have a long service life. Therefore, the hydraulic system has clear requirements for the performance of oil.

1. Viscosity of hydraulic oil

The hydraulic oil has a certain viscosity during the flow process. This viscosity ensures the continuous flow of the oil, the pressure and flow of the oil, and at the same time, it also enables the hydraulic oil to play the role of lubrication, sealing, cooling, and rust prevention. However, different hydraulic systems and different working conditions have different requirements for the viscosity of the oil. The viscosity of the hydraulic oil is expressed by viscosity. Generally speaking, suitable oil viscosity can make the hydraulic pump have good oil absorption capacity, reduce the pressure loss of the hydraulic system, and make the friction pair have good lubrication. Therefore, it is required that the viscosity of the hydraulic oil changes less under the action of pressure, temperature and shear force.

2. Compressibility of hydraulic oil

When studying the static characteristics of the hydraulic system, the hydraulic oil can usually be considered as an incompressible liquid. However, when considering the dynamic characteristics of the hydraulic system, such as the instantaneous dynamic response of the servo system, the compressibility of the hydraulic oil is one of the important factors affecting its dynamic characteristics. Therefore, the hydraulic system requires the compressibility of the hydraulic oil to be as small as possible.

3. Lubricity of hydraulic oil

In order to reduce power loss and improve the performance of the hydraulic system, it is very necessary to make the friction pair of the hydraulic component have good lubrication performance. The lubricity of hydraulic oil is a necessary condition to prevent the hydraulic component from getting stuck and dry friction. In addition, when the hydraulic pump and hydraulic motor are started, whether the lubrication of the hydraulic oil is sufficient has a great impact on its starting performance characteristics.

4. Stability of hydraulic oil

Since hydraulic oil contains a small amount of water and is inevitably in contact with air, the hydraulic oil will be oxidized and hydrolyzed due to the increase in humidity under high pressure and high temperature working conditions, causing corrosive substances to be generated in the oil. Such harmful substances will clog the filter device and reduce the filtering performance of the system. Therefore, the hydraulic system hopes that its oil has the stability of anti-oxidation and anti-hydrolysis, so that the amount of harmful corrosive substances generated in the system is as small as possible to extend the service life of the hydraulic oil.

5. Rust resistance and corrosion resistance of hydraulic oil

Under the action of water and air, the metal surface of hydraulic components will rust and corrode. The particles generated by rust and corrosion will circulate in the hydraulic system, causing wear of the moving surface and causing the hydraulic system to malfunction. Therefore, the hydraulic oil is required to have strong rust resistance and corrosion resistance to prevent the metal surface from rusting and corroding.

6. Defoaming property of hydraulic oil

The hydraulic oil contains a small amount of air, which will cause bubbles and foam in the oil. Such bubbles and foams will cause vibration and noise in the hydraulic system, causing the performance of the system to drop sharply. Therefore, the hydraulic system requires that the bubbles and foams generated by the hydraulic oil should be as few as possible and disappear quickly.

7. Anti-emulsification of hydraulic oil

Since a small amount of water will be mixed into the hydraulic oil during operation, the hydraulic oil containing water will produce emulsions under high-speed and intense stirring. The presence of emulsions in the hydraulic oil will produce precipitation or other sediments, which is very harmful to the hydraulic system. Therefore, the hydraulic oil should have good anti-emulsification properties and make it easy to separate the oil and water.

8. Compatibility of hydraulic oil

Hydraulic oil is in contact with seals or hoses for a long time, and seals and hoses are mostly made of rubber materials. Therefore, it is hoped that the hydraulic oil has good compatibility with rubber materials. That is, the hydraulic oil does not have a deteriorating effect on the seals and hoses, so that the seals and hoses are as little deformed and deteriorated as possible.

9. Flame resistance of hydraulic oil

Hydraulic oil should have high ignition point, low volatility, good flame resistance, and ensure the safety of the hydraulic system under high temperature conditions.