到目前为止，液压元件逐渐实现了系列化标准化，它的质量，品种，规格，甚至是性能都有了比较稳定的提升，尤其目前采用电子技术伺服技术等技术工艺之后，液压系统的能量更是得到了明显的提升。目前液压元件在军事工业和经济方面都发挥出了起到了重大的作用，从不同的方面出发，我们可以把液压元件分为不同的形式，今天就由小编来为大家简单的介绍一番。
So far, hydraulic components have gradually achieved serialization and standardization. Their quality, varieties, specifications and performance have seen steady improvements. In particular, the adoption of electronic and servo technologies has greatly enhanced the energy efficiency of hydraulic systems. Nowadays, hydraulic components play an important role in national defense industry and economic development. Hydraulic components can be classified into different types according to various criteria. Here is a brief introduction.
按照油液的循环方式，液压系统可以分为闭式系统以及开式系统，其中开式系统指的是液压泵，可以从油箱中吸收油。油在经过多个控制阀之后用来驱动液压执行元件回油之后再经过换向阀回到油箱中，这种开式系统结构较为简单，能够很好的发挥出油箱的沉淀杂质和散热的能力，但是由于油液经常会跟空气所接触，使得外界的空气更容易渗入液压系统之中，导致液压机构运动过程中不平稳。开式系统的油箱比较大，油泵的自吸性能比较好，而在闭式系统之中，液压泵的进油管直接是跟实行元件的回油管相互连接，工作的液体在系统中的管路能够达到封闭循环的作用。
Classified by fluid circulation mode, hydraulic systems are divided into open circuit and closed circuit.
In an open circuit, the hydraulic pump suctions fluid from the reservoir. The fluid passes through multiple control valves to drive hydraulic actuators, and the return fluid flows back to the reservoir via directional valves. This circuit features a simple structure. The reservoir can effectively settle contaminants and dissipate heat. Nevertheless, frequent contact between fluid and ambient air leads to easy air ingress, which causes unstable motion of hydraulic mechanisms. Open circuits adopt large-capacity reservoirs, and the hydraulic pump delivers good self-priming performance.
In a closed circuit, the suction line of the hydraulic pump is directly connected to the return line of the actuator, forming a closed circulation loop for the working fluid inside the pipeline.
闭式结构的结比较紧凑跟空气所接触的面积小空气不容易渗入液压系统之中，所以在传动过程中非常的平稳，工作机构的变向以及换向是依靠马达的变量机构来实现的，避开了开式系统在换向过程中可能导致的液压冲击和能量的损耗。不过闭式系统其比较复杂，由于没有油箱，油液的散热和过滤条件都比较差，为了弥补系统中可能会出现泄漏，通常需要安装一个小流量的补油泵和油箱。
The closed circuit has a compact structure and a small contact area with ambient air, which prevents air from entering the hydraulic system and ensures smooth transmission. Speed regulation and direction change of the working mechanism are realized by the variable displacement mechanism of the hydraulic motor, eliminating hydraulic shock and energy loss during commutation that occur in open circuits.However, the structure of closed circuits is relatively complicated. Without a dedicated reservoir, the system suffers poor heat dissipation and filtration performance. To compensate for system leakage, a low-flow charge pump and an auxiliary tank are usually installed.
按向执行元件供油方式的不同，可分为串联系统和并联系统。串联系统中，上一个执行元件的回油即为下一个执行元件的进油，每通过一个执行元件压力就要降低一次。在串联系统中，当主泵向多路阀控制的各执行元件供油时，只要液压泵的出口压力足够，便可以实现各执行元件的运动的复合。但由于执行元件的压力是叠加的，所以克服外载能力将随执行元件数量的增加而降低。
Hydraulic systems can be classified into series systems and parallel systems according to different oil supply modes for actuators.
In a series system, the return oil of the preceding actuator serves as the inlet oil of the next one, and the pressure drops each time the fluid passes through an actuator. When the main pump supplies oil to multiple actuators controlled by multi-way valves, combined motion of all actuators can be achieved provided the outlet pressure of the hydraulic pump is sufficient. However, as the pressures of actuators are cumulative, the load-carrying capacity decreases with the increase in the number of actuators.
液压系统性能的好坏，除满足使用功能要求外，应从液压系统的效率、功率利用、调速范围和微调特性、振动和噪声以及系统的安装和调试是否方便可靠等方面进行。
Apart from meeting functional requirements, the performance of a hydraulic system shall be evaluated in terms of efficiency, power utilization, speed regulation range, fine tuning characteristics, vibration, noise, as well as the convenience and reliability of system installation and commissioning.
