Hydraulic components have gradually achieved standardization, serialization, and transformation, and their specifications, varieties, quality, and performance have been greatly improved. Especially after the use of new technologies and new processes such as electronic technology and servo technology, the quality of the hydraulic system has been significantly improved. It has played an important role in the national economy and military industry.

From different perspectives, the hydraulic system can be divided into different forms.

(1) According to the oil circulation mode, the hydraulic system can be divided into an open system and a closed system. Open system means that the hydraulic pump sucks oil from the oil tank. After the oil passes through various control valves, it drives the hydraulic actuators, and returns the oil to the oil tank through the reversing valve. This kind of system has a relatively simple structure and can play the role of heat dissipation of the fuel tank and precipitation of impurities, but because the oil is often in contact with air, it is easy for air to penetrate the system, resulting in unsteady movement of the mechanism. The open system has a large oil tank and the oil pump has good self-priming performance. In a closed system, the oil inlet pipe of the hydraulic pump is directly connected with the oil return pipe of the actuator, and the working fluid is closed in the system pipeline. Its structure is compact, there is less chance of contact with air, and air is not easy to penetrate the system, so the transmission is relatively stable. The speed change and reversal of the working mechanism are realized by adjusting the variable mechanism of the pump or the motor, which avoids the hydraulic shock and energy loss in the reversing process of the open system.