液压元件逐步实现了标准化、系列化，其规格、品种、质量、性能都有了很大提高，尤其是采用电子技术、伺服技术等新技术新工艺后，液压系统的质量得到了显著的提高，其在国民经济及军事工业中发挥了重大作用。从不同的角度出发，可以把液压系统分成不同的形式。 1）按油液的循环方式，液压系统可分为开式系统和闭式系统。开式系统是指液压泵从油箱吸油，油经各种控制阀后，驱动液压执行元件，回油再经过换向阀回油箱。这种系统结构较为简单，可以发挥油箱的散热、沉淀杂质作用，但因油液常与空气接触，使空气易于渗入系统，导致机构运动不平稳等后果。开式系统油箱大，油泵自吸性能好。闭式系统中，液压泵的进油管直接与执行元件的回油管相连，工作液体在系统的管路中进行封闭循环。其结构紧凑，与空气接触机会少，空气不易渗入系统，故传动较平稳。工作机构的变速和换向靠调节泵或马达的变量机构实现，避免了开式系统换向过程中所出现的液压冲击和能量损失。但闭式系统较开式系统复杂，因无油箱，油液的散热和过滤条件较差。为补偿系统中的泄漏，通常需要一个小流量的补油泵和油箱。由于单杆双作用油缸大小腔流量不等，在工作过程中会使功率利用下降，所以闭式系统中的执行元件一般为液压马达。 2）按系统中液压泵的数目，可分为单泵系统，双泵系统和多泵系统。 3）按所用液压泵形式的不同，可分为定量泵系统和变量泵系统。变量泵的优点是在调节范围之内，可以充分利用发动机的功率，但其结构和制造工艺复杂，成本高，可分为手动变量、尽可能控变量、伺服变量、压力补偿变量、恒压变量、液压变量等多种方式。 4）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。并联系统中，当一台液压泵向一组执行元件供油时，进入各执行元件的流量只是液压泵输出流量的一部分。流量的分配随各件上外载荷的不同而变化，首先进入外载荷较小的执行元件，只有当各执行元件上外载荷相等时，才能实现同时动作。 全液压传动机械性能的优劣，主要取决于液压系统性能的好坏，包括所用元件质量优劣，基本回路是否恰当等。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。 现代工程机械几乎都采用了液压系统，并且与电子系统、计算机控制技术结合，成为现代工程机械的重要组成部分。
The hydraulic presses system of the form and the evaluation
The hydraulic presses the component to carried out to standardize gradually, the series turn, its specification, species, quantity, functionses all had to raise very greatly, particularly is to adopt the new craft of new technique of electronics technique, servo . after, the hydraulic presses the exaltation that the quantity of the system gets to show the highest , it developped the important function in national economy and the military industries.
Set out from the different angle, can press the system to the liquid to be divided into the different form.
1）Press the circulating way of the oil liquid, the liquid press the system and can is divided into the open type system and shut type open type system mean that the liquid presses the pump to absorb the oil from the fuel tank, the oil is through various control valve, driving the liquid to press to carry out the component, returning to oil to has been changed to return to fuel tank toward valve kind of system structure is more simple, can develop the fuel tank to spread hot, precipitate the miscellaneous quality function, but often get in touch with with air because of the oil liquid, make air be easy to seep into the system, causing the organization exercise the gravamen steady etc. open type system fuel tank is big, oil pump from absorb the function and the shut type system, the liquid presses the pump of into pipeline directly with performance the component return to pipeline connect with each other, work the liquid carries on closing the circulation in the tube road of the structure tightly packed, with the air opportunity to get in touch with little, not easy infiltration system of air, so spread to move more work organization become soon and change to depend to regulate the pump or motors to change to measure the organization realization, avoided the open type system change toward process appear of liquid press the impact and energy the shut type system more the open type system complications, because of having no fuel tank, the oil liquid spread hot and filter the condition is order to compensate the leakiness in the system, usually need a small discharge to repair the oil pump and fuel use the oil a size of a function of high bar discharge not etc., will make power make use of descend in work process, so the performance component within the shut type system presses the motor for the liquid generally.
2）According to system the liquid presses the number of the pump, can is divided into the list pump system, double pump system and pump system more.
3）Press the dissimilarity of pump the form with the liquid by, can is divided into the fixed amount pump system and change the quantity pump advantage that changes to measure the pump is to regulate the scope inside, can make use of the power of launch the machine well, but its structure and the manufacturing craft complications, the cost is high, can is divided in to move to change the quantity, control to change the quantity possibly, servo change the quantity, pressure to compensate to change the quantity, press to change the quantity, liquid to press to change to measure various ways of etc..
4)Press toward performance the component to provide the dissimilarity of the oil method, can is divided in to establish the system and merge the blish in the system, it is next performance component that previous performance component return to oil of into oil, pass a performance component pressure and will lower once each establish system, the each performance component that be the lord pump toward many roads valve control provides the oil, as long as the liquid presses the exit pressure of the pump enough, the compound of the sport that can carry out each performance component the pressure of the performance component is to fold to add of, so overcome the outside carry ability will with carry out the increment of the component quantity but e in the system, be a the pedestal liquid to press the pump toward a performance component to provide the oil, discharge that enters each performance component be just the liquid press the pump exportation discharge of a allotment of the discharge with each last the outside carries the lotus of dissimilarity but variety, enter first outside carry the smaller performance component of lotus, only be each performance component up outside carry the lotus equality, then can carry out to act at the same time.
The whole liquid presses to spread the good and bad of move the machine function, mainly being decided by the quality that the liquid presses the system function, including the component quantity good and bad use, basic back track whether fitting quality of the system function, in addition to satisfying to use the function to request, should make use of, adjust from the efficiency, power that the liquid press the system soon scope and tiny adjust characteristic, vibration and voice of noises and the gearing of the systems and adjust to try whether convenient credibility etc. carry on.
The modern engineering machine almost adopts the liquid to press the system, and combine with electronics system, the calculator control technique, the importance that become the modern engineering machine constitutes the part.
