机电工程学院 班级： 学号： 职称： 职称：
专业： 机械设计制造及其自动化 学生姓名： 指导教师姓名： 评阅教师姓名：
2008 年 6 月
液压回路中的压力控制阀是用来确保回路中不同部分的压力达到我们的预期值。压 力控制阀通过以下几种方式实现我们的预期值： （1）把高压回路中的流体通过低压区， 来限制高压区的压力： （2）分流到其他区域。这类阀可分为安全阀、溢流阀、平衡阀、 顺序阀和卸荷阀。分流型阀是可以减压的。 压力阀也可以定义为常闭式或者常开式的两通阀，溢流阀、顺序阀、卸荷阀、和平 衡阀是常闭式的， 两通阀处于常开或半开状态以完成其工作任务。 减压阀是一种常开阀， 可以限止最终阻止油液进入出口回路。任何一种类型阀工作，都能自动调节阻尼孔的大 小进行压力控制。当阀依靠外线先导油液进行控制时，并不需要阻尼孔调节。卸荷阀没 有自我调节的能力，主要依靠外部油源的信号进行调节。而节流阀、减压阀、平衡阀和 顺序阀是完全自动进行调节的。本章我们将研究不同种类的压力阀，学习这些阀如何应 用于不同的回路中。 压力阀的类型 下面是几种常用的动力控制元件： 安全阀 通常用提动型两通阀，当流体的压力达到设定值时，释放流体进入出口，这 种阀保护管路和设备，以免由于压力过大而损坏。 溢流阀 这种阀在回路中可以在所连接的部分限制其最大压力。 单向平衡阀 这种阀维持油液一个方向的流体阻力，而另一个方向自由流动。 顺序阀 引导油液顺次流向回路的各个部分。 卸荷阀 该阀允许压力升到某一调定值， 然后只要控制油源在控制口处保持事先调定 的压力值，它就使液流旁路通过。 减压阀 此阀不管进油口的压力值有多大，都能保持其出口降低压力值。 液压保险 此阀中安装了易碎的薄片，当系统中的压力达到预设值时，薄片破碎。 压力开关 由油液的压力进行控制， 同时也能对油液的压力的变化做出相应的反应。 先导式溢流阀 国际标准的简化液压符号，已经广泛的应用了。因此，用在普通液压回路中的溢流 阀和其辅助装置的一些连接件，在液压图中很少表达出来。因为这些简单的液压符号表 达只是阀的基本元件，比如压力输入，液压油箱连接，阀芯弹簧等，偏置箭头表明此种 阀属于常闭阀。溢流阀的先导阀弹簧上标有一个斜箭头。如果该阀是可调的，特别是该 元件对回路的控制很大影响时，则在阀的简化图中，也可以在阀芯弹簧加一个同样的斜 箭头。先导式溢流阀的完全符号，包括基本部分和相关控制。如点划线框所包含的有 5 个接口，分别是进油口、接油箱口、远程控制口、测试口，以及先导阀的在特殊情况中 才使用的外泄口。 进油口和油箱接口是主油道，流人测试口和远程控制口的流量，只需要供这两个部 分工作就够了。测试口的作用是测量管中油液的压力，其中并不一定要有油液流过。远 程控制回路油液需要通过其阀芯的固定阻尼孔，流量取决于主阀芯弹簧。如果是有外泄
回路的溢流阀，通过其阻尼孔的油液并不比通过内部固定阻尼孔的多。 图示的是复合(先导式)溢流阀的剖面图。主阀芯与上面的弹簧相连，处于一定的位 置，阻止来自进油口的油液流回油箱。 进油口与位于弹簧下面的主阀芯底部直接相通， 控制油与弹簧腔之间的连接管路上 有阻尼孔的限制。主阀芯上下端面的面积是相同的。在某些设计上，也有可能不完全相 等。某一端面积稍大，可以完成特定功能。工作时如果先导油液无法从调节口通过，主 阀由于面积相同处于平衡状态， 阀芯在弹簧的作用下， 处于封闭进油口和出油口的位置。 如果主阀芯上端的弹簧腔的压力升高，作用在锥阀上的作用力超过弹簧力时，部分先导 压力油流回油箱。当这部分压力油的流量大于阻尼孔的最大可通流量时，作用于阀芯上 端的力就会小于下端的力， 此时， 阀芯的上下端面所受到的压力不等， 处于不平衡状态。 当作用于阀芯下端压力继续上升时，通过调节口的先导流量增大，主阀上下腔压力不平 衡程度加大，形成的压力差超过弹簧的预紧力。这就产生了一个从输入压力油口到回油 箱口的通道，这与直动式弹簧溢流阀的通道形成过程极其相似。 调压弹簧 A 在弹簧 B 处于不工作状态时，会提供一个最大的预紧力。当调节弹簧 B 处于工作状态时，主阀芯又会受到一个附加的作用力，最小开启压力取决于弹簧力 A。 此压力不会低于弹簧 A 的力。如果其他支路的阻力比较小，则回路中的压力比较低。由 弹簧 B 对先导阀所产生力，一般被认为是对调压弹簧 A 所产生的附加力。在很多阀中， 主阀芯弹簧是不可调节的。 主阀芯和弹簧所在的腔叫做控制腔，我们应该熟记这个术语，因为它广泛的应用于 工业液压系统中。注意阀体左侧上部的一个连接口，连接到控制腔可允许其中的油液自 由的流回油箱。因此，对主阀芯弹簧不会产生任何附加力。 如果在远控口布置一个小溢流阀，则最大的压力值由此阀来决定。 流量控制阀 容积或流量控制阀常用来调节速度。由前述已经知道，油缸的速度取决于单位时间 内泵输入的油量。 可以用一个变量泵调节流量， 而在许多回路中是用定量泵调节流量的， 所以常用流量控制阀调节流量。 流量调节方法：实现用流量调节阀来控制油缸速度的方法有三种，分别是：进口节 流、出口节流和旁路节流。 进口节流回路 在回路中，节流阀串联在泵和液压缸之间，用这种方式，可以控制 流人液压缸的油量。泵输送的多余的油通过溢流阀回油箱。由于节流阀安装在液压缸油 路上，油液只能朝一个方向流动，所以在节流阀内或并联安装了单向阀，使倒流油液通 过。如果希望油液双向控制速度，节流阀必须安装在泵出口方向阀的前面。节流控制的 控制精度比较高，常用在液压缸有常负载的回路中，例如，在负载作用下做垂直上升运 动的液压缸，或以某控制速度推动液压缸。 出口节流回路 出口节流回路多用在负载元件可能会出现速度失控的情况。 节流阀 在此处的作用就是限制液压缸的油液流尽。为了能双向调速，节流阀多被安装在方向阀 和油箱之间。 更常见的是只需要一个方向需要控制， 控制阀安装在方向阀与液压缸之间。
为了能使液压缸能快速回程，此处也可以设置一个旁路单向阀。 旁路节流调速回路 在旁路节流回路中，节流阀安装在泵的出油口处的旁路，调节 从旁路把油分流回油箱的流量，从而调节液压缸的速度。其优点在于泵是在工作压力下 进行调节，多余的油是通过节流阀而不是溢流阀流回油箱的。缺点是调节精度比较差， 原因在于精确分流的油液是回到油箱，而未流经液压缸，这样由于工作负载的变化，使 泵的输出不稳定。旁路节流调速回路在负载失控的情况下不能应用。 节流调节的类型 节流阀阀有两种基本类型：压力补偿型和非压力补偿型。后者常用在负载压力相对 稳定，运动速度要求不太严格的地方。尽管其构件比较复杂，甚至包含有为油液反向自 由流动的单向阀，这种方法就像一个固定节流口或可调节的针阀一样，仍然是非常简单 的。非压力补偿型应用受到了一定的限制，这是因为通过阻尼孔的流量与流经它所产生 的压力差平方根成比例。这表明工作负载的巨大变化会影响到运动速度。 压力补偿型节流阀可进一步划分为调速阀型和溢流节流型。 两者都利用一个补偿器 或稳定器来稳定通过可调节流口而产生的压力差。 溢流节流型节流阀是带过载保护装置的压力补偿控制阀，它有一个常闭的节流口， 当打开它时， 可以使油液流到油箱。 工作负载所需的压力值由位于上腔内的传感器探测， 该压力与一根轻弹簧共同使阀口处于常闭状态。 节流口下端的腔内压力由于受到节流阀 的阻力而不断增加，当其上下端受到的压力差达到足以克服弹簧力时，多余的油液就会 流回油箱。这个压力差，通常为 20psi，不管工作负载有多大，都会由于确定量的油液 经过节流口而形成稳定值。 过载保护装置由一个可调节的弹簧提供，以限制节流口上腔的最大压力值，只要工 作负载超过设定值，它就相当于一个溢流阀。溢流节流型控制阀只用在进口节流调速回 路。如果用在出口节流调速回路中，回油不能通过节流阀流回油箱，而无法抑制引起的 负载失控。 调速阀型流量控制阀依靠的是稳定器使油液通过节流阀维持 20psi 的压力差。 这种 阀的阀芯处于常开状态，可以自动关小节流口以阻止过多的油液流过。在阀中，工作负 载和阀芯上端的一根弹簧共同使阀芯处于开启状态。 节流口进口和阀芯下端的压力欲使 其关闭，仅允许油液进入阀内，维持通过节流阀 20psi 的压力差。 当油液达到节流阀的设定值时，阀口有关闭的趋势。调速阀型节流阀可以安置在进 油口，出油口和旁路节流调速回路上。与溢流节流型节流调速不同，调速阀型可以在一 个泵源中使用两个或多个节流阀，泵输出的多余油液通过溢流阀返回油箱。 当设置节流阀在液压缸回路上时，为能迅速反向回流，应该选择整体式单向阀。而 单向阀不允许设置在主供油管路，方向阀的回油箱管路以及旁路节流调速中。 温度补偿型节流阀 流经压力补偿节流控制阀的流量易于受温度变化的影响。威格士公司后期设计的 阀，采用了温度补偿型节流阀的结构。油液温度升高时，油液流动阻力小。当温度升高
时，通过减小节流阀的开口尺寸仍能维持流量的稳定，这可以通过一个能热胀冷缩的补 偿杆来实现。节流阀芯是一个能来回进出的柱塞。温度补偿杆就安装在节流阀和调节器 之间，这种设计也可以设置一个可反向自由流动的单向阀来实现。 远程流量控制阀 远程流量控制阀允许通过电信号调节节流阀的开口大小，节流 阀芯和扭距马达的转子相连，并且能对扭距马达的电信号做出反应，其操作原理与压力 补偿调速阀是一样的。 本文引自《机械给工程专业英语》216～220 页，贺自强主编 北京理工大学出版社 1989.8
Pressure-Control Valves Pressure-control valves are used in hydraulic circuits to maintain desired pressure levels in various parts of the circuits. A pressure-control valves maintains the desired pressure levels by diverting higher-pressure fluid to a lower-pressure area . Thereby limiting the pressure in the higher-pressure area. restricting flow into another area. Valve that divert fluid can be safety, relief, counterbalance, sequence,
and unloading types. Valves that restrict flow into another area can be of the reducing type. A pressure-control valve may also be defined as either a normally closed or normally open two-way valve. Relief , sequence , unloading and counterbalance valves are normally closed , two-way valves that are partially or fully open valve that restricts and finally blocks fluid flow a secondary. With either type of operation, the valve can be said to create automatically an orifice to provide the desired pressure control An orifice is nit always created unloading valve. It is piloted from an external source. One valve of this type is the unloading valve. Relief, reducing, counterbalance, and sequence valves can be fully automatic in operation. With the operating signal taken from within the envelop. In this chapter, we shall study the different types of pressure-control valves and learn how they are used in various hydraulic circuits. Types of Pressure-Control Valves Eight popular devices for pressure-control are: Safety valve Usually a poppet-type two-way valve intended to release fluid to a secondary area when the fluid pressure approaches the set opening pressure of the valve. This type of valve protects piping and equipment from excessive pressure. Relief valve Valve which limits the maximum pressure that can be applied in that portion of the circuit to which it is connected. Counterbalance valve Valve which maintains resistance against flow to one direction but permits free flow in the other direction. Sequence valve sequence. Unloading valve Valve which allows pressure to build up to an adjustable setting, then bypasses Valve which directs flow to more than one portion of a fluid circuit, in
the flow as long as a remote source maintains the preset pressure on the pilot port. Pressure-reducing valve Valve which maintains a reduced pressure at its outlet regardless of the
higher inlet pressure. Hydraulic fuse Device equipped with a frangible disk which establishes the maximum pressure
in a hydraulic circuit by rupturing at a preset pressure valve. Pressure switch Switch operated by fluid pressure and responsive to arise or fall in fluid pressure.
Compound Relief Valves In the study of ISO hydraulic symbol, it was stated that simplified symbol are widely used. Because of this , pressure-relief valves used in common hydraulic circuits are rarely shown complete with all auxiliary devices and connections. Instead, the simplified symbol shows only the basic relief valve, pressure input tank connection, valve spring , and the offset arrow indicating that the valve is normally closed. A slash arrow as shows on the bias spring of the pliot-relief valve if the valve is adjustable , particularly if this information is significant to circuit operation. Figure shows the complete symbol for a compound relief valve. All adjacent controls are shown, along with the main relief element . The envelope surrounding all the element may have five connections . These are pressure input , tank connection , remote-control-station connections, test station , and external drain for the pilot relief that is provided only on special order. The input pressure and tank connection provide the major flow through the valve. Only enough fluid need flow to the test-station and remote-control connection for the respective function. The test station is generally used for a gauge connection to check fluid pressure. This does not require a flow of fluid. The remote-control connection passes the quantity of fluid coming through the fixed internal orifice at the rate established by the spring in the main relief element. An external drain from the pilot-relief valve, if fitted, will not pass more fluid than passes through the fixed internal orifice. Figure shows a cutaway view of compound relief valve. Note that the main spool is held by the spring in a position that blocks the passage from the pressure input port to the tank port, just as the symbol in Figure shows. Input pressure is directed to the bottom of the spool below the spring cavity without restriction. The supply line to the spring cavity is restricted by an orifice in the line . The area of the main spool is she same. In certain poppet designs; The areas may not be exactly equal. One end may have a larger area to ensure certain function actions. In operation, if fluid cannot escape through adjustment port, a balance is provided
by the equal areas at each end of the spool. The spring then maintains the spool in the position where it blocks the valve from pressure input to tank. When the pressure in the spring chamber above the spool rises to a point where it can unseat the cine at adjustment port, a portion of the fluid will be pass to the orifice into the main spring chamber, the pressure that is effective on the upper spool area may be less than that on the lower area so that balance is no longer maintained. As the pressure continue to rise at the lower end of the valve spool and the flow continues to increase through adjustment port while the degree of unbalance of the main spool becomes more pronounced, the pressure will force the main spool against the spring. This creates a path from the pressure input to the tank much like that much like that created in directed spring-operated relief valve. Adjustment A would provided a specific maximum relieving pressure if adjustment B were completely relaxed. Where adjustment B is in use, providing an additive pressure to the main-spool spring, the minmum relieving providing will be fixed by adjustment A . The relieving pressure can never be less than that established by adjustment A in the valve. Pressure in the circuit could be less if there were relaxation through some other path. Resistance to pliot-flow created by adjustment B may be considered as a hydraulic additive to the value of adjustment spring A. In many valves, the main-spool is not adjustable. The pocket containing the main-spool spring is called the control chamber. It will be well to remember this term, as it widely used in industrial hydraulics. Note the auxiliary vent connection in the upper left side of the valve in Figure. This port permits the escape of fluid directly to the tank without restriction. Thus, there can be no hydraulic additive pressure to the main spool spnng. If a small relief valve is placed in the circuit with a connection to the reliving pressure will be established by this additive at a remote point. Volume Control Volume or flow control valves are used to regulate speed. An was developed in earlier chapters, the speed of an actuator depends on how much oil is pumped into it per unit of time. It is possible to regulate flow with a variable displacement pump, but in many circuits it is more practical to use a fixed displacement pump and regulate flow with a volume control valve. Flow Control Methods There are three basic methods of applying volume control actuator speeds. They are meter-in, meter-out and bleed-off.
In meter-in operation, the flow control valve is placed between the pump and
actuator. In this way, it control the amount of fluid going into the actuator. Pump delivery in excess of the metered amount is diverted to tank over the relief valve. With the flow control valve installed in the cylinder line as shown, flow is controlled in one direction. A check valve must be included in the flow control or placed in parallel with it for return flow. If it is desired to control directional valve. The meterqn method is highly accurate. It is used in applications where the load continually resists movement of the actuator, such as raising a vertical cylinder under load or pushing a load at a controlled speed. Meter-Out Circuit Meter-out control control is used where the load might tend to "run away". The flow control is located where it will restrict exhaust flow from the actuator. To regulate speed in both directions, the valve is installed in the tank line from the directional valve. More often control is needed in only one direction and it is placed in the line between the actuator and direction valve. check valve would be required for a rapid return stroke. Bleed-Off Circuit In a bleed-off arrangement, the flow control is bleed off the supply line from the pump and determines the actuator speed by metering a portion of the pump delivery to tank. The advantage is that the pump operates at the pressure required by the work, since excess fluid returns to tank through the flow control instead of through the relief valve. Its disadvantage is some less of accuracy because the measured flow is to tank' rather into the cylinder, making the latter subject to variations in the pump delivery due to changing work loads. Bleed-off circuits should not be used in applications where there is a possibility of the load running away. Types of Flow Controls Flow control valves fall into two basic categories: pressure compensated and non-pressure compensated. The latter being used where load pressure remain relatively constant and feed rates are not too critical. They may be as simple as a fixed orifice or an adjustable needle for free valve, although more sophisticated units may even include a check valve for free flow in the reverse direction. Use of Here too a bypass
non-pressure compensated valves is somewhat limited, since flow through an orifice is essentially proportional to the square root of the pressure drop across it. This means that any appreciable change in the work load would affect the feed rate. Pressure compensated flow controls are further classified as restrictor and by-pass types. Both utilize a compensator or hydrostat to maintain a constant pressure drop across an adjustable throttle.
The By-Pass Type-combines overload protection with pressure compensated control of flow. It has a normally closed hydrostat which opens to divert fluid, in excess of the throttle setting, to the tank. Pressure required by the work load is sensed in the chamber above the hydrostat and together with a light spring tends to hole it closed. Pressure in the chamber below the hydrostat increase duo to restriction of the throttle and cause is to raise diverting any excess flow to tank when the difference in pressure is sufficient to overcome the spring. This difference, usually 20 psi, is maintained across the throttle providing a constant flow regardless of the work load. Some horsepower saving is accomplished in that the pump need operate at only 20 psi above work load pressure. Overload protection is provided by an adjustable spring loaded poppet which limits the maximum pressure above the hydrostat, causing it to function as a compound relief valve whenever work load requirement exceed its setting. The by-pass flow control can only be used in a meter-in circuit. If used for metering out, exhaust oil which could not get through the throttle would be diverted to tank permitting the load to run away. The Restrictor Type Flow Control-also maintain a constant 20 psi differential across its throttle by means of a hydrostat. In this valve, the hydrostat is normally epen and tends to close off blocking all flow in excess of the throttle setting. In these units the work load pressure acts with a light spring above the hydrostat to hold it open. Pressure at the throttle inlet and under the hydrostat tend to close it, permitting only that oil to enter the valve that 20 psi can force through the throttle. Because of their tendency close off when flow tales to exceed the throttle setting, restrictor type valves may be used in meter-in, meter-out and bleed-off circuits. Unlike the by-pass type , two or more restrictor valves may be used with the same pump since the excess pump delivery returns to tank through the relief valves. When placed in cylinder lines an integral check valve is optional to provide free flow for a rapid return stroke. One would not be required for valves placed in the main supply line, the tank line of a directional valve or when they are used in bleed-off circuits. Temperature Compensated Flow Control Valve Flow through a pressure compensated flow control valve is subject to change with variations in oil temperature. Later design Vickers valves incorporate a temperature. Although oil flows more freely when it is hot, constant flow can be maintained by decreasing the size of the throttle opening as the temperature
This is accomplished through a compensating rod which lengthens with heat and contracts when
cold. The throttle is a simple plunger that is moved in and out of the control port. The compensating is installed between the throttle and its adjuster. This design also is available with a reverse free-flow check valve. Remote Flow Control Valves Remote flow control valves permit adjustment of the throttle size by an electrical signal. The throttle spool is linked to armature of a torque motor and moves in response to signal to the torque motor. Operation is otherwise the same as a pressure compensated flow control valve.