译文：
换向阀是借助于滑阀和阀体之间的相对运动，使与阀体相连的各油路实现液压油流的接通、切断和换向。换向阀的中位机能是指换向阀里的滑阀处在中间位置或原始位置时阀中各油口的连通形式，体现了换向阀的控制机能。采用不同形式的滑阀会直接影响执行元件的工作状况。因此，在进行工程机械液压系统设计时，必须根据该机械的工作特点选取合适的中位机能的换向阀。中位机能有Ｏ型、Ｈ型、Ｘ型、Ｍ型、Ｙ型、Ｐ型、Ｊ型、Ｃ型、Ｋ型，等多种形式。 一、Ｏ型符号为 其中Ｐ表示进油口，Ｔ表示回油口，Ａ、Ｂ表示工作油口。结构特点：在中位时，各油口全封闭，油不流通。机能特点：１、工作装置的进、回油口都封闭，工作机构可以固定在任何位置静止不动，即使有外力作用也不能使工作机构移动或转动，因而不能用于带手摇的机构。２、从停止到启动比较平稳，因为工作机构回油腔中充满油液，可以起缓冲作用，当压力油推动工作机构开始运动时，因油阻力的影响而使其速度不会太快，制动时运动惯性引起液压冲击较大。３、油泵不能卸载。４、换向位置精度高。 二、Ｈ型符号为 结构特点：在中位时，各油口全开，系统没有油压。机能特点：１、进油口Ｐ、回油口Ｔ与工作油口Ａ、Ｂ全部连通，使工作机构成浮动状态，可在外力作用下运动，能用于带手摇的机构。２、液压泵可以卸荷。３、从停止到启动有冲击。因为工作机构停止时回油腔的油液已流回油箱，没有油液起缓冲作用。制动时油口互通，故制动较Ｏ型平稳。４、对于单杆双作用油缸，由于活塞两边有效作用面积不等，因而用这种机能的滑阀不能xx保证活塞处于停止状态。 三、Ｍ型符号为 结构特点：在中位时，工作油口Ａ、Ｂ关闭，进油口Ｐ、回油口Ｔ直接相连。机能特点：１、由于工作油口Ａ、Ｂ封闭，工作机构可以保持静止。２、液压泵可以卸荷。３、不能用于带手摇装置的机构。４、从停止到启动比较平稳。５、制动时运动惯性引起液压冲击较大。６、可用于油泵卸荷而液压缸锁紧的液压回路中。 四、Ｙ型符号为 结构特点：在中位时，进油口Ｐ关闭，工作油口Ａ、Ｂ与回油口Ｔ相通。机能特点：１、因为工作油口Ａ、Ｂ与回油口Ｔ相通，工作机构处于浮动状态，可随外力的作用而运动，能用于带手摇的机构。２、从停止到启动有些冲击，从静止到启动时的冲击、制动性能０型与Ｈ型之间。３、油泵不能卸荷。 五、Ｐ型符号为 结构特点：在中位时，回油口Ｔ关闭，进油口Ｐ与工作油口Ａ、Ｂ相通。机能特点：１、对于直径相等的双杆双作用油缸，活塞两端所受的液压力彼此平衡，工作机构可以停止不动。也可以用于带手摇装置的机构。但是对于单杆或直径不等的双杆双作用油缸，工作机构不能处于静止状态而组成差动回路。２、从停止到启动比较平稳，制动时缸两腔均通压力油故制动平稳。３、油泵不能卸荷。４、换向位置变动比Ｈ型的小，应用广泛。 六、Ｘ型符号为 结构特点：在中位时，Ａ、Ｂ、Ｐ油口都与Ｔ回油口相通。机能特点：１、各油口与回油口Ｔ连通， 处于半开启状态，因节流口的存在，Ｐ油口还保持一定的压力。２、在滑阀移动到中位的瞬间使Ｐ、Ａ、Ｂ与Ｔ油口半开启的接通，这样可以避免在换向过程中由于压力油口Ｐ突然封堵而引起的换向冲击。３油泵不能卸荷。４、换向性能介于０型和Ｈ型之间。 七、Ｕ型符号为 结构特点：Ａ、Ｂ工作油口接通，进油口Ｐ、回油口Ｔ封闭。机能特点：１、由于工作油口Ａ、Ｂ连通，工作装置处于浮动状态，可在外力作用下运动，可用于带手摇装置的机构。２、从停止到启动比较平稳。３、制动时也比较平稳。４、油泵不能卸荷。 八、Ｋ型符号为 结构特点：在中位时，进油口Ｐ与工作油口Ａ与回油口Ｔ连通，而另一工作油口Ｂ封闭。机能特点：１、油泵可以卸荷。２、两个方向换向时性能不同。 九、Ｊ型符号为 结构特点：进油口Ｐ和工作油口Ａ封闭，另一工作油口Ｂ与回油口Ｔ相连。机能特点：１、油泵不能卸荷。２、两个方向换向时性能不同。 十、Ｃ型符号为 结构特点：进油口Ｐ与工作油口Ａ连通，而另一工作油口Ｂ与回油口Ｔ连通。机能特点：油泵不能卸荷；从停止到启动比较平稳，制动时有较大冲击。 举例分析 １、利用滑阀的中位机能设计成卸荷回路，实现节能。当滑阀中位机能为Ｈ、Ｋ或Ｍ型的三位换向阀处于中位时，泵输出的油液直接回油箱，构成卸荷回路，可使泵在空载或者输出功率很小的工况下运动，从而实现节能，如图１所示。这种方法比较简单，但是不适用于一泵驱动两个或两个以上执行元件的系统。 ２、利用滑阀的中位机能设计成制动回路或锁紧回路。为了使运动着的工作机构在任意需要的位置上停下来，并防止其停止后因外界影响而发生移动，可以采用制动回路。最简单的方法是利用换向阀进行制动例如滑阀机能为Ｍ型或Ｏ型的换向阀，在它恢复中位时，可切断它的进回油路，使执行元件迅速停止运动。如图２所示：装载机动臂液压缸采用Ｍ型中位机能的换向阀构成的制动油路，动臂在将铲斗举升到{zg}位置和下降至{zd1}放平位置时能自行限位制动，图中的回位限位阀（即Ｍ型和Ｈ型四位四通换向阀）是靠钢球定位的，当铲斗移至限位点时碰触开关，二位电磁阀换向，接入压缩空气，将定位钢球压回槽内，回位限位阀便在弹簧作用下恢复中位，切断动臂油缸的进、回油油路，于是动臂连同铲斗一起被限位制动。 ３、利用Ｈ型、Ｙ型换向实现浮动。例如液压起重机的回转机构在负载下回转时，如果制动过急，惯性力将产生很大的液压冲击，因此，常常采用滑阀机能为Ｈ型或Ｙ型的换向阀，如图３所示，当换向阀回中位时，回转马达处于浮动状态，然后再用脚制动使它平稳的停止转动。图２所示的装载机动臂液压缸。当Ｍ型和Ｈ型四位四通换向阀处于Ｈ位，即浮动位置，这时可以下铲取物料或者平整场地，铲斗可随地面的高低而升降，即实现浮动；另外这种回路在遇到系统突然停止工作时，仍能顺利放下铲斗。在履带挖掘机行走马达的换向阀上采用Ｙ型滑阀机能的换向阀，它可以使挖掘作业时行走马达处于浮动状态不承受制动载荷。 ４、换向阀滑阀中位机能的选用对压路机开式振动液压系统的换向速度，对压路机的振动工作性能有着十分重要的影响。利用Ｈ型三位四通换向阀， 当滑阀处于中位时，Ｐ、Ｔ、Ａ、Ｂ四个油口相通而构成连通同路。由于激振器旋转惯性的作用，会使振动轮产生余振，从而造成被压实的铺层表面产生压痕，但这对于压路基的振动压路机来说，给基础层压实效果产生的影响不大，反而还减少了系统的液压冲击力。在图４中，单频双幅振动开式液压系统中。对于压路面的振动压路机，则要求在压实作业过程中需停振或或变幅时，激振器能在1.5－1.7s的时间内，迅速的停止旋转以避免瞬间的余振使压实表面出现压痕，而影响压实质量。常采用Ｍ型三位四通换向阀，当滑阀处于中位时，Ａ、Ｂ两个工作油口截止，能产生很大的背压，促使马达克服激振器的惯性力矩而急速停止旋转，这样就避免了在路面压实时产生压痕，但是会在马达回路中造成很高的瞬时压力峰值，提高马达及其他有关元件损坏率。因此通常在换向阀的Ａ、Ｂ油口设置两个溢流阀对系统进行保护。 总之，在进行换向阀的选用时，一定要根据工作机构的工作特点选用适当的中位机能。

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原文：
Valve is a slide valve and the valve body by means of the relative movement so that the oil line connected with the body of hydraulic oil flow in turn, cut off and commutation. The median valve function is the valve inside the slide valve in the middle or the original position in the oil port valve connected forms, reflects the control of valve function. Different forms of slide valve actuator will directly affect working conditions. Accordingly, when hydraulic system design, must be based on the work of the mechanical characteristics of selecting the appropriate function of the valve in place. The median function with O-, H-type, X type, M type, Y-, P-type, J type, C type, K type, and other forms. 1, O-symbol Which said inlet P, T, said return oil port, A, B, said work port. Structural characteristics: in the medium, the oil port fully closed, oil does not flow. Functional Features: 1, the progress of the work unit, return oil port is closed, the working body can be fixed in any position stationary, even if external force is not working organizations can move or rotate, and thus can not be used with a hand in the body. 2, from stop to start relatively stable, because the working body cavity full of oil back to oil, you can have a dampening effect, when the pressure oil promotion work bodies begin to exercise, due to the impact of the oil resistance to the speed is not too fast, the system When moving large inertia caused by fluid percussion. 3, pump can not uninstall. 4, for the location accuracy. 2, H-type symbol Structural characteristics: in the medium, the oil port open, there is no hydraulic system. Functional Features: 1, inlet P, T and work back to the oil export oil port A, B all connected, so that the work of agencies into a floating state, the next movement in the external force, can be used with a hand in the body. 2, hydraulic pump can be unloaded. 3, an impact from stop to start. Because the work to stop the oil chamber when the oil return flow back into the fuel tank has no oil from the buffer. Braking oil port each other, so brake smoothly than the O-type. 4, for the single rod double acting cylinder, due to effective area of the piston on both sides ranging from, so use this function can not assure the slide valve piston is stopped. 3, M-type symbol Structural characteristics: the median, the working oil port A, B closed, inlet P, return oil port T connected directly. Functional features: 1, as a result the oil port A, B closed, the working body can remain stationary. 2, hydraulic pump can be unloaded. 3, can not be used with hand-cranked device body. 4, from stop to start relatively stable. 5, hydraulic braking caused by the impact of large inertia. 6, can be used to pump hydraulic cylinder unloading and locking of the hydraulic circuit. 4, Y-type symbol Structural characteristics: the median time, P inlet closed, the working oil port A, B and return oil port T connected. Functional features: 1, because the work oil port A, B and return oil port T connected, working bodies floating, and can exercise with the role of external forces, can be used with a hand in the body. 2, from stop to start a blow when starting from a static to impact, braking performance and the H-type between 0. 3, pump can not be unloaded. 5, P-type symbol Structural characteristics: the median, the return oil port T close to the inlet port P and the working oil A, B connected. Functional Features: 1, equal to the diameter double acting double rod cylinder, piston hydraulic pressure at both ends of each balance suffer, work organizations can not stop moving. Can also be used with hand-cranked device body. But for single pole or double pole double diameter ranging from the role of tank, working body at rest can not be formed differential circuit. 2, from stop to start relatively stable, brake cylinder pressure of the oil through the two cavities are so smooth braking. 3, pump can not be unloaded. 4, for changes to the location of H-type than the small, wide range of applications. 6, X-type symbol Structural characteristics: the median time, A, B, P and T oil port are connected back to the oil port. Functional Features: 1, the oil port and oil return port T connected In a semi-open state, due to the presence of the orifice, P oil port also maintains a certain pressure. 2, the slide valve to move to the position of the moment to P, A, B and T half-open mouth connected to the oil, it can avoid the process of change due to the pressure P oil port closure caused by a sudden change to the impact. 3 oil pump can not be unloaded. 4, for the performance between 0 and H-type between. 7, U-type symbol Structural characteristics: A, B port connected to the work of oil, inlet P, T return oil port closed. Functional features: 1, as a result the oil port A, B connectivity, equipment floating, and can exercise under loads that can be used with a hand device body. 2, relatively stable from stop to start. 3, when more stable braking. 4, pump can not be unloaded. 8, K-type symbol Structural characteristics: in the medium, to the inlet port P and the working oil A and oil return port T connected, while the other work of the oil port B closed. Functional Features: 1, oil pump can be unloaded. 2, when the two directions for the performance of different. 9, J-type symbol Structural features: P and work inlet oil port A closed, another oil port B and back to work the oil port T is connected. Functional Features: 1, oil pump can not be unloaded. 2, when the two directions for the performance of different. 10, C-type symbol Structural features: P inlet oil port A connected with the work, while the other work of the oil port B and return oil port T connected. Functional Features: Pump can not unloading; from stop to start relatively stable, have greater impact braking. Example of 1, using the median function slide valve unloading circuit designed to achieve energy efficiency. When the slide valve in the position function for the H, K or M type 3 valve in the medium, the output of the oil pump directly back to the tank to form unloading loop, can pump in a small load or output power Condition movement in order to achieve energy efficiency, shown in Figure 1. This method is simple, but does not apply to a pump-driven implementation of two or more components of the system. 2, using the median function slide valve designed to brake or lock loop circuit. In order to make a moving work of agencies in the position of any need to stop and prevent the cessation occurs due to external influence Yidong, you can use brake circuit. The simplest method is to use brake valve spool functions such as M-type or O-type valve, in which restoration of median cut off its oil line into the back, so that the implementation of components quickly to stop movement. Figure 2: Loader Boom Cylinder with M-function valve in place constitute the brake circuit, the boom that will give rise to the highest position and the bucket fell to the lowest level position, placed limits on their own Bit brake, diagram valve back to position limit (ie, M-and H-4 four-way valve) is by positioning the ball, when the bucket moved to touch the switch when the limit point, two electromagnetic reversing valve, access to compressed air, the positioning ball pressure back to tank, back-bit limit valve will resume in the spring under the action of the medium, cut into the boom cylinder, back to greasy road, so with the shovel boom bucket along with braking limit. 3 using H-, Y-type for the realization of floating. Institutions such as the rotary hydraulic crane load in a turn, if you brake too fast, will have a great inertia force hydraulic shock, therefore, often used for the H-type slide valve function, or Y-type valve, shown in Figure 3 said that when the valve Huizhong bit, the rotary motor in a floating state, and then foot brake to stop it turning smoothly. Figure 2 shows the loader boom cylinder. When the M-and H-4 four-way valve in the H-bit, or floating position, then you can take the next shovel material or leveling the ground with the bucket to the ground level of the lift, that is, to achieve floating; addition of this circuit in the face of the system suddenly stops working, managed to put down the bucket. In the crawler excavator motor using Y-valve on the valve spool valve function, it can make the excavation work in a floating state when the motor is not running braking load to bear. 4, valve position slide valve function in the selection of vibration of the roller open the hydraulic system for the speed, the vibration of the roller has a great influence on performance. Use of H-3 four-way valve, When the slide valve is in bits, P, T, A, B, constitute the four oil port connected connectivity go the same way. As the exciter rotor inertia effect, will generate Yu Zhen wheel vibration, resulting in compaction of the overlay surface indentation, but the vibratory roller for the pressure Foundation for the base layer to the impact of the compaction effect small, but also reduce the impact of hydraulic system. In Figure 4, the single-frequency two-amplitude vibration of open hydraulic system. The pressure vibration road roller, then the request need to stop the process of compaction or vibration, or amplitude, the exciter in 1.5-1.7s of the time, quickly stop the rotation in order to avoid the instant Yu Zhen to compaction the surface indentation and impact of compaction quality. Regular use of M-3 four-way valve, when the slide valve is in bits, A, B two working oil export cut-off, can have a great back pressure, to promote the motor moment of inertia to overcome the exciter and rapid stop rotation, thus avoiding the immediate effect of indentation in the surface pressure, but will result in the motor circuit in the high instantaneous peak pressure, motor and other components to improve the failure rate. So often in the valve of the A, B Ports to set the system to protect the two relief valves. In short, when making the selection valve, must be based on characteristics of the work of agencies use the median function properly.