压力控制阀和流量控制阀外文翻译.doc

Pressure-Control Valves and Flow-Control ValvesPressure-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 level by(1)diverting higher-pressure fluid to a lower-pressure area，thereby limiting the pressure in the higher-pressure area, or(2)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 opened while performing their design function, A reducing valve is a normally open valve that restricts and finally blocks fluid flow into a secondary area. With either type of operation, the valve can be said to create automatically an orifice to provide the desired pressure control. An orifice is not always created when the valve is piloted from an external source. One valve of this type is the unloadingit is not self-operating；it depends on a signal from an external source. Relief, reducing counterbalance, and sequence valves can be fully automatic in operation, with the operating signal taken from within the envelope. 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 ValvesEight popular devices for pressure-control service are:Safety valve. Usually a poppet-type two-way valve intended to release 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 in one direction but permits free flow in the other direction.Sequence valve. Valve which directs flow to more than one portion of a fluid circuit, in sequence.Unloading valve. Valve 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. 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 a rise or fall in fluid pressure.Compound Relief ValveIn the study of ISO hydraulic symbols it was stated that simplified symbols 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 shown on the bias-spring of the pilot-relief valve in Fig.3-20a may be added to the bias-spring symbols of the simplified valve as shown in Fig.3-20b if the valve is adjustable, particularly if this information is significant to circuit operation. Figure 3-20a shows the complete symbols for a compound relief valve. All adjacent controls are shown, along with the main relief element. The envelope surrounding all the elements may have five connections. These are(1)pressure input, (2)tank connection, (3)remote-control-station connections, (4)test station, and (5)external drain for the pilot relief (not shown) that is provided only on special order. Fig.3-20 (a)Complete symbol for a compound relief valve (b)Simplified symbol for compound relief valve 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 connections for the respective functions. 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 3-21 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 Fig.3-20 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 each end of the main spool is the same. In certain poppet designs; the areas may not be exactly equal. One end may have a larger area to ensure certain functional actions. In operation, if fluid cannot escape through adjustment B, 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 path through the valve from pressure input to tank. When the pressure in the spring chamber above the main spool rises to a point where it can unseat the cone at adjustment B, a portion of the fluid will be passed to the tank port. When the volume of this flow exceeds that passing through 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 continues to rise at the lower end of the valve spool and the flow continues to increase through adjustment B 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 created in a directed in a direct spring-operated relief valve. Fig.3-21 Sliding-spool-type compound relief valve 1.pressure input 2.cavity supply line 3.non-adjustable orifice 4.auxiliary connection(vent) 5.control chamber 6.adjustment A 7.spring 8.adjustment B 9.cone 10.to tank 11.main spool Adjustment A would provide a specific maximum relieving pressure if adjustment B were completely relaxed. When adjustment B is in use, providing an additive pressure to the main-spool spring, the minimum relieving pressure 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 fluid flow created by adjustment B may be considered as a hydraulic additive to the valve of adjustment spring A. In many valves, the main-spool spring is not adjustable.The pocket containing the main-spool spring is called the control chamber. It will be to remember this term, as it is widely used in industrial hydraulics. Note the auxiliary vent connection in the upper left side of the valve in Fig.3-21. This port(which is connected to the control chamber)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 vent port, the maximum relieving pressure will be established by this additive at a remote point.Volume ControlsVolume or flow control valves are used to regulate speed. As 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 MethodsThere 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 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 speed in both directions, the flow control can be installed in the pump outlet line prior to the directional valve.The meter-in method is highly accurate. It is used in applications where the load continually resists movement of the actuator, such as raising a vertical cylinder 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 regular 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 directional 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 teed 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 loss of accuracy because the measured flow is to tank rather than 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 ControlsFlow control valves fall into two basic categories: pressure compensated and non-pressure compensated. The latter being used where load pressures retain relatively constant and feed rates are not too critical. They may be as simple as a fixed orifice or an adjustable needle 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(P) 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 (Fig.3-22). 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 hold it closed. Pressure in the chamber below the hydrostat increases due to the restriction of the throttle and causes it 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 excess 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. Figure 3-22 Flow Control and Relief Valve Meter In Figure 3-23 Pressure Compensated Restrictor Type Flow ControlThe Restrictor Type Flow Control-also maintains a constant 20 psi differential across its throttle by means of a hydrostat (Fig.3-23). In this valve, the hydrostat is normally open 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 the hydrostat tends to close it, permitting only that oil to enter the valve that 20 psi can force through the throttle.Because of their tendency to close off when flow tried 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 same pump since the excess pump delivery returns to tank through the relief valve.When placed in cylinder lines an integral check valve is optional to provide free flow for a rapid return stroke (Fig.3-24). 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 ValveFlow through a pressure compensated flow control valve is subject to change with variations in oil temperature. Later design Vickers valves incorporate a temperature compensating feature. 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 rod is installed between the throttle and its adjuster. This design also is available with a reverse free-flow check valve. Fig. 3-24 Flow Control and Check ValveRemote Flow Control Valves. Remote flow control valves permit adjustment of the throttle size by an electrical signal. The throttle spool is linked to the armature of a torque motor and moves in response to signals to the torque motor. Operation is otherwise the same as a pressure compensated flow control valve. 压力控制阀和流量控制阀在液压回路中压力控制阀用来控制回路各个不同部分要求的压力。压力控制阀通过转换高压液体到较低的压力区域来控制要求的压力，从而缩小了高压地区的压力或者是调节流量进入其它区域。改变流体方向的阀类可能是安全阀、溢流阀、背压阀、顺序阀和泄荷阀。调节流量进入其它区域的阀类可能是减压阀。压力阀也可定义为是常开式的或是常闭式的二通阀。溢流阀、顺序阀、泄荷阀和背压阀是常闭阀。二通阀依据其设计功能可能是部分或是完全常开的。调节和阻塞液体流入次要区域的减压阀是常开的。通过两者中的任一种操作方式，减压阀都能够自动的产生一个提供要求控制压力的节流口。在外力的作用下，减压阀的节流口是可以调节的。压力控制阀中的泄荷阀不是自动控制的。它依靠外力作用。溢流阀、减压阀、背压阀和顺序阀通过包络面内部的控制信号进行完全自动化操作。在这里我们将要研究不同形式的压力控制阀和学习它们在不同的液压系统中是如何应用的。压力控制阀的种类压力控制装置中八种常见元件：安全阀 通常当液体压力接近阀芯式二通阀的调整压力时，它将使液体流入其它区域。这种类型的阀可以使管道或设备避免过大的压力作用。溢流阀 溢流阀是回路中该阀所连接的区域内限制可以施加的最大压力的阀。背压阀 背压阀是维持抵抗流入一个部分但是允许液体自由流入其它部分的阀。顺序阀 顺序阀是引导液体按顺序流入液体回路各部分的阀。泄荷阀 泄荷阀允许压力升到某一固定值，然后只要控制油源在控制口处保持事先调定的压力，它就使液体流入旁路。减压阀 减压阀是不管入口压力的高低。但是能保持出口压力低压的阀。液压保险阀 液压保险阀是通过击穿目前压力阀在液压回路中产生的最大压力并没有法兰盘的元件。压力开关 压力开关是通过液体压力和对液体压力高低的反映来操作的。复合溢流阀国际标准组织对液压符号的研究表明，简化符号是被广泛应用的。因此，通常回路中的减压阀是用所有的辅助设备和连接表明的。相反，简化的符号仅仅表明的是基本的减压阀、压力输入、油箱连接、阀簧和表明阀体常关的偏移箭头。图3-20a中先导溢流阀的偏置弹簧上有一个箭头。如果该阀是可调的，特别是这个信息对回路的操作有重要意义时，则在阀的简化图3-20b中，也可在偏置弹簧符号上加一个同样的箭头。图3-20a表明的是复合溢流阀的全部符号。所有相邻的控制和主要减压部分都被表示出来。包围所有元素的外壳可能有五个连接。他们是压力输入、油箱连接、远程控制连接、检测装置和在特殊情况下先导阀上的全部排泄装置。输入的压力和油箱的连接提供了通过阀体的主要流量。流入试验站和遥控连接管的液体只要足够这两个部件的工作就可以了。检测装置通常被用于规管连接来检测流体的压力。远程控制连接通过的液体流量来自由主要压力元件的弹簧控制的流量的内部的节流口。通过先导阀的外部排泄装置的液体不比通过内部节流口的多。图3-20 （a）完整的溢流阀符号 （b）简单的溢流阀符号图3-21是一个 复合式溢流阀的剖面图。它表明主要的滑阀是阻塞输入压力到油箱通道的某一位置的弹簧来控制的，如图3-20表明的符号。输入压力被引入滑阀底部，若底没有节流口的弹簧内腔，弹簧内腔的进给管道是被管道中的节流口控制的。主要滑阀的末管地方是相同的。在某些提动阀芯的设计中，这些地方可能并不是相同的。某一末端有一大的空间来保证某种功能形式。在操作过程中，如果液体不能通过调节装置B流出，在滑阀任一末尾的平等区域将会提供一个平衡。这个弹簧维持滑阀阻塞阀的输入压力到油箱的通道的位置。当主滑阀弹簧工作腔内的压力上升到它能够移开在调节装置B中圆锥体的一点时，一部分液体就会流入油箱。当流过的体积超过通过节流口进入主要弹簧内腔的体积时，滑阀上部空间的主要压力将会低于下部区间压力，因此平衡就会被破坏。当滑阀底部压力继续上升和通过调节装置B流入液体继续增加时，主滑阀的不平衡程度会变得更加明显，这个压力就会强迫使主滑阀产生抵抗弹簧的作用。这会产生一个输入压力到油箱的通道，与直动弹簧工作或溢流阀形成的通道十分相似。如果调节装置B被完全放松，调节装置A将提供一个具体的最大的泄荷压力。当调节装置B作用时，它会对主滑阀弹簧产生一个附加力，最小泄荷压力在调节装置A的作用下被形成。泄荷压力不会低于阀中调节装置A产生的压力。如果通过其它通道的压力被放松，回路中的压力会更小。调节装置B产生的控制流体流量的阻止被认为是阀中调节弹簧A的附加液压力。在许多阀中，主滑阀弹簧是不可调节的。图 3-21 完整的先导式溢流阀 1.压力输入 2.内腔线 3不变油源 4.辅助连接 5.控制腔 6.装置A 7.弹簧 8.装置B 9.圆锥面10.油箱 11.主阀芯控制腔是包含主滑阀弹簧的罩。因为在工业液压系统中，它被广泛应用，所以记住它是很有用处的。图3-21中，辅助的排放口连接位于阀左边的上部。这个连接着控制腔的口允许液体直接进入油箱不需要通过节流口。因此，这不会对主滑阀弹簧产生一个附加液压力。 如果在回路当中有一个没有连接出口的小溢流阀，远程部分将会产生一个最大卸荷压力。容积控制 容积或流量控制阀用于调整速度。前面章节介绍过，油缸的速度依靠单位时间内进入的油液的多少。通过各种各样的活塞泵或柱塞泵，流量调整是可能的，但是在许多回路当中，最常用到的是活塞泵或柱塞泵的复合型或调整流量的容积控制阀。 流量控制方法有三种应用容积控制阀来控制油缸速度的基本方法。它们是：进口节流、出口节流