The hydraulic components and gradually realize the standardization and serialization of the varieties, its specifications, quality and performance have been greatly improved, especially the new technology such as electronic technology, servo technology and so on, the quality of the hydraulic system has been greatly improved, which has played an important role in the national economy and military industry. From different points of view, the hydraulic system can be divided into different forms.
1.According to the circulation of oil, hydraulic system can be divided into open system and closed system. Open system refers to the hydraulic pump drain oil from the tank, the oil through a variety of electric hydraulic control valves, to drive hydraulic components, then return to the tank through the hydraulics directional control valve. The structure of the system is relatively simple, and can play the role of heat dissipation and precipitation of the fuel tank, but because the oil is often in contact with the air, the air is easy to penetrate into the system, resulting in the unstable motion of the mechanism. The oil tank of open system is big, and the oil pump self-priming performance is good. In the closed system, the oil inlet pipe of the hydraulic pump is directly connected with the oil return pipe of the actuating element, the working fluid in the system for closed loop pipeline.Its structure is compact, less contact with air, and air is not easy to penetrate the system, so the transmission is more stable. The speed and direction of the working mechanism are realized by adjusting the variable mechanism of the pump or the motor, so as to avoid the hydraulic shock and energy loss in the process of the opening system. However, the closed system is more complex than the open system, because there is no oil tank, the oil cooling and filtration conditions are poor. In order to compensate for the leakage in the system, a small flow oil pump and oil tank is needed. As the single pole double acting cylinder large cavity flow range, in the work process will make use of power down, so the implementation of a closed system components for hydraulic motor.
2.According to the number of hydraulic pumps in the system, can be divided into single pump system, double pump system and multi pump system.
3.According to the different forms of hydraulic pump, can be divided into quantitative pump system and variable pump system. The advantages of variable pump is that it can make full use of the power of the engine within the scope of the regulation, but its structure and manufacturing process is complex and high cost, which can be divided into manual variables, as far as possible control variables, servo variable, pressure compensating variable and constant pressure variable, hydraulic variable etc..
4.According to the different ways of oil supply, the system can be divided into series system and parallel system. In the series system, the oil return of the last executive element which is the oil inlet of the next executive element, and the pressure of each component is reduced by one time. In the series system, when the main pump supply oil to each of the actuators controlled by the multi way valve, as long as the outlet pressure of the hydraulic pump is sufficient, the movement of the actuating elements can be realized. However, due to the pressure of the components are superimposed, so the external load capacity will be reduced with the increase of the number of components. In parallel systems, when a hydraulic pump supply oil to a set of actuators, the flow into each of the actuators is only part of the output flow of the hydraulic pump. Flow distribution varies with the load of different parts, first into the small load actuator, the simultaneous action can be achieved only when the load of the actuators are equal. The advantages and disadvantages of the mechanical performance of the full hydraulic transmission mainly depends on the performance of the hydraulic system, including the quality of the components, the basic circuit is appropriate or not, etc.. The performance of the system, in addition to meet the functional requirements, should be evalutated in installation and debugging efficiency, power from hydraulic system use, speed range and fine tuning characteristics, vibration and noise and the system is convenient and reliable etc.. Modern engineering machinery almost all use the hydraulic system, and combine with the electronic system and computer control technology, become an important part of modern construction machinery.