液压阀-外文-翻译

毕业设计（论文）外文翻译（2012届）外文题目： Valve 译文题目： 阀 学生姓名： 学 号： 专业班级： 机械设计制造及其自动化0806班 指导教师： 所在学院： 机械工程学院 Valves 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 valve maintains the desired pressure levels by diverting higher-pressure fluid to a lower-pressure area. Thereby limiting the pressure in the higher-pressure area by restricting flow into another area. Valves 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 value which limits the maximum pressure that can be applied in that portion of the circuit to which it is connected. Counterbalance value which maintains resistance against flow to one direction but permits free flow in the other direction. Sequence value which directs flow to more than one portion of a fluid circuit in sequence. Unloading valve Value which allows pressure to build up to an adjustable setting, then bypasses the flow as long as a remote source maintains the preset pressure on the pilot port. Pressure-reducing 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 operated by fluid pressure and responsive to raise 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 f 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 pilot-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 the entire 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 needs flow to the test-station and remote-control connection for the respective function. The test stations 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 provide 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 minimum 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 pilot-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 spring. 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. A 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. Meter-In Circuit In meter-in operation, the flow control valve is placed between the pump and actuator. In this way, it controls the amount of fluid going into the actuator. Pump delivery in excess of the Metered amount I 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 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 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. Here too a bypass 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 workloads. 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 pressures 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 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 rise 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 maintains 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 rises. 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.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预期值。压力控制阀通过以下几种方式实现预期值：（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。这类阀可以分为安全阀、溢流阀、平衡阀、顺序阀和卸荷阀。分流型阀是可以减压的。 压力阀也可以称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，可以控制进入后续回路的油液压力。任何一种类型阀，都能自动调节阻尼孔的大小进行压力控制。当阀靠外控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。 下面是几种常用的动力控制元件： 安全阀：通常为压控式，当流体的压力达到设定值时，安全阀泄荷，这种阀保护系统以免受到过载、压力变化剧烈等做造成的破坏。 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。 单向阀：这种阀只保证油液一个方向的流动阻力，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，都能保持其出口压力值的降低。 图1-1 顺序阀的工作原理图先导式溢流阀已经广泛的应用了。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影响很大时，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，流入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，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。回路中的溢流阀，通过其阻尼孔的油液并不比通过内部固定阻尼孔的多。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处于非工作状态时，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。如果在远程控制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