Hydraulic components have gradually achieved standardization, serialization and generalization, with their specifications, varieties, quality and performance greatly improved. In particular, after the adoption of new technologies and processes such as electronic technology and servo technology, the quality of hydraulic systems has been significantly improved, and they have played a major role in the national economy and military industry.

　　From different perspectives, hydraulic systems can be divided into different forms.

　　(1) According to the oil circulation mode, hydraulic systems can be divided into open systems and closed systems. An open system means that the hydraulic pump draws oil from the tank; after passing through various control valves, the oil drives the hydraulic actuator, and the return oil then returns to the tank through the directional valve. This system has a relatively simple structure and can make use of the tank's functions of heat dissipation and impurity settling. However, because the oil is often in contact with air, air easily penetrates into the system, resulting in consequences such as unstable mechanism movement. The open system has a large tank and good pump self-priming performance. In a closed system, the oil inlet pipe of the hydraulic pump is directly connected to the return pipe of the actuator, and the working fluid circulates in a closed loop within the system's pipelines. Its structure is compact, there is little opportunity for contact with air, and air does not easily penetrate into the system, so transmission is relatively stable. The speed change and reversing of the working mechanism are achieved by adjusting the variable mechanism of the pump or motor, avoiding the hydraulic shock and energy loss that occur during the reversing process of an open system. However, a closed system is more complex than an open system. Because there is no tank, the heat dissipation and filtration conditions of the oil are poorer. To compensate for leakage in the system, a small-flow charge pump and tank are usually required. Since the flow rates of the large and small chambers of a single-rod double-acting cylinder are unequal, power utilization will decrease during operation, so the actuators in closed systems are generally hydraulic motors.

　　(2) According to the number of hydraulic pumps in the system, they can be divided into single-pump systems, dual-pump systems and multi-pump systems.

　　(3) According to the different forms of hydraulic pumps used, they can be divided into fixed-displacement pump systems and variable-displacement pump systems. The advantage of a variable-displacement pump is that within the adjustment range it can make full use of the engine's power, but its structure and manufacturing process are complex and its cost is high. It can be divided into various modes such as manual variable, as far as possible control variable, servo variable, pressure-compensated variable, constant-pressure variable and hydraulic variable.

　　(4) According to the different ways of supplying oil to the actuators, they can be divided into series systems and parallel systems. In a series system, the return oil of the previous actuator is the inlet oil of the next actuator, and the pressure drops once each time the oil passes through an actuator. In a series system, when the main pump supplies oil to the actuators controlled by the multi-way valve, as long as the outlet pressure of the hydraulic pump is sufficient, the compound movement of the actuators can be realized. However, because the pressures of the actuators are additive, the ability to overcome external loads will decrease as the number of actuators increases.

　　In a parallel system, when one hydraulic pump supplies oil to a group of actuators, the flow entering each actuator is only part of the hydraulic pump's output flow. The distribution of flow varies with the external load on each component; the actuator with the smaller external load is supplied first, and simultaneous action can be achieved only when the external loads on all actuators are equal.

　　The performance of fully hydraulic transmission machinery mainly depends on the quality of the hydraulic system performance, including the quality of the components used and whether the basic circuits are appropriate. In addition to meeting the functional requirements of use, the quality of system performance should be evaluated from aspects such as the efficiency of the hydraulic system, power utilization, speed regulation range and fine-tuning characteristics, vibration and noise, and whether the installation and commissioning of the system are convenient and reliable.

　　Modern construction machinery almost all adopts hydraulic systems, and in combination with electronic systems and computer control technology, they have become an important part of modern construction machinery.