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附件
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.
液壓回路中的壓力控制閥是用來確保回路中不同部分的壓力達到預(yù)期值。壓力控制閥通過以下幾種方式實現(xiàn)預(yù)期值：（1）把高壓回路中的流體通過低壓區(qū)，來限制高壓區(qū)的壓力：（2）分流到其他區(qū)域。這類閥可以分為安全閥、溢流閥、平衡閥、順序閥和卸荷閥。分流型閥是可以減壓的。
壓力閥也可以稱為常閉式或者常開式的兩通閥，溢流閥、順序閥、卸荷閥、和平衡閥是常閉式的，兩通閥處于常開或半開狀態(tài)以完成其工作任務(wù)。減壓閥是一種常開閥，可以控制進入后續(xù)回路的油液壓力。任何一種類型閥，都能自動調(diào)節(jié)阻尼孔的大小進行壓力控制。當(dāng)閥靠外控先導(dǎo)油液進行控制時，并不需要阻尼孔調(diào)節(jié)。卸荷閥沒有自我調(diào)節(jié)的能力，主要依靠外部油源的信號進行調(diào)節(jié)。而節(jié)流閥、減壓閥、平衡閥和順序閥是完全自動進行調(diào)節(jié)的。
下面是幾種常用的動力控制元件：
安全閥：通常為壓控式，當(dāng)流體的壓力達到設(shè)定值時，安全閥泄荷，這種閥保護系統(tǒng)以免受到過載、壓力變化劇烈等做造成的破壞。
溢流閥：這種閥在回路中可以在所連接的部分限制其最大壓力。
單向閥：這種閥只保證油液一個方向的流動阻力，而另一個方向自由流動。
順序閥：引導(dǎo)油液順次流向回路的各個部分。
卸荷閥：該閥允許壓力升到某一調(diào)定值，然后只要控制油源在控制口處保持事先調(diào)定的壓力值，它就使液流旁路通過。
減壓閥 ：此閥不管進油口的壓力值有多大，都能保持其出口壓力值的降低。
圖1-1 順序閥的工作原理圖
先導(dǎo)式溢流閥已經(jīng)廣泛的應(yīng)用了。因此，用在普通液壓回路中的溢流閥和其輔助裝置的一些連接件，在液壓圖中很少表達出來。因為這些簡單的液壓符號表達只是閥的基本元件，比如壓力輸入，液壓油箱連接，閥芯彈簧等，偏置箭頭表明此種閥屬于常閉閥。溢流閥的先導(dǎo)閥彈簧上標(biāo)有一個斜箭頭。如果 該閥是可調(diào)的，特別是該元件對回路的控制影響很大時，則在閥的簡化圖中，也可以在閥芯彈簧加一個同樣的斜箭頭。先導(dǎo)式溢流閥的符號，包括基本部分和相關(guān)控制。如點劃線框所包含的有5個接口，分別是進油口、接油箱口、遠程控制口、測試口，以及先導(dǎo)閥的在特殊情況中才使用的外泄口。進油口和油箱接口是主油道，流入測試口和遠程控制口的流量，只需要供這兩個部分工作就夠了。測試口的作用是測量管中油液的壓力，并不一定要有油液流過。遠程控制回路油液需要通過其閥芯的固定阻尼孔，流量取決于主閥芯彈簧?；芈分械囊缌鏖y，通過其阻尼孔的油液并不比通過內(nèi)部固定阻尼孔的多。進油口與位于彈簧下面的主閥芯底部直接相通，控制油與彈簧腔之間的連接管路有阻尼孔的限制。主閥芯上下端面的面積是相同的。在某些設(shè)計上，也有可能不完全相等。某一端面積稍大，可以完成特定功能。工作時如果先導(dǎo)油液無法從調(diào)節(jié)口通過，主閥由于面積相同處于平衡狀態(tài)，閥芯在彈簧的作用下，處于封閉進油口和出油口的位置。如果主閥芯上端的彈簧腔的壓力升高，作用在錐閥上的作用力超過彈簧力時，部分先導(dǎo)壓力油流回油箱。當(dāng)這部分壓力油的流量大于阻尼孔的最大可通流量時，作用于閥芯上端的力就會小于下端的力，此時，閥芯的上下端面所受到的壓力不等，處于不平衡狀態(tài)。當(dāng)作用于閥芯下端壓力繼續(xù)上升時，通過調(diào)節(jié)口的先導(dǎo)流量增大，主閥上下腔壓力不平衡程度加大，形成的壓力差超過彈簧的預(yù)緊力。這就產(chǎn)生了一個從輸入壓力油口到回油箱口的通道，這與直動式彈簧溢流閥的通道形成過程極其相似。
調(diào)壓彈簧A在彈簧B處于非工作狀態(tài)時，會提供一個最大的預(yù)緊力。當(dāng)調(diào)節(jié)彈簧B處于工作狀態(tài)時，主閥芯又會受到一個附加的作用力，最小開啟壓力取決于彈簧力A。此壓力不會低于彈簧A的力。如果其他支路的阻力比較小，則回路中的壓力比較低。由彈簧B對先導(dǎo)閥所產(chǎn)生力，一般被認(rèn)為是對調(diào)壓彈簧A所產(chǎn)生的附加力。在很多閥中，主閥芯彈簧是不可調(diào)節(jié)的。
主閥芯和彈簧所在的腔叫做控制腔，我們應(yīng)該熟記這個術(shù)語，因為它廣泛的應(yīng)用于工業(yè)液壓系統(tǒng)中。注意閥體左側(cè)上部的一個連接口，連接到控制腔可允許其中的油液自由的流回油箱。因此，對主閥芯彈簧不會產(chǎn)生任何附加力。如果在遠程控制口布置一個小溢流閥，則最大的壓力值由此閥來決定。
流量控制閥
容積或流量控制閥常用來調(diào)節(jié)速度。由前述已經(jīng)知道，油缸的速度取決于單位時間內(nèi)泵輸入的油量。可以用一個變量泵調(diào)節(jié)流量，而在許多回路中是用定量泵調(diào)節(jié)流量的，所以常用流量控制閥調(diào)節(jié)流量。
流量調(diào)節(jié)方法：實現(xiàn)用流量調(diào)節(jié)閥來控制油缸速度的方法有三種，分別是：進油口節(jié)流、出油口節(jié)流和旁路節(jié)流。
進油口節(jié)流回路 在回路中，節(jié)流閥串聯(lián)在泵和液壓缸之間，用這種方式，可以控制流入液壓缸的油量。泵輸送的多余的油通過溢流閥回油箱。由于節(jié)流閥安裝在液壓缸油路上，油液只能朝一個方向流動，所以在節(jié)流閥內(nèi)或并聯(lián)安裝了單向閥，使倒流油液通過。如果希望油液雙向控制速度，節(jié)流閥必須安裝在泵出口方向閥的前面。節(jié)流控制的控制精度比較高，常用在液壓缸有常負(fù)載的回路中，例如，在負(fù)載作用下做垂直上升運動的液壓缸，或以某控制速度推動液壓缸。
出油口節(jié)流回路 出口節(jié)流回路多用在負(fù)載元件可能會出現(xiàn)速度失控的情況。節(jié)流閥在此處的作用就是限制液壓缸的油液流盡。為了能雙向調(diào)速，節(jié)流閥多被安裝在方向閥和油箱之間。更常見的是只需要一個方向控制，控制閥安裝在方向閥與液壓缸之間。
旁路節(jié)流調(diào)速回路 在旁路節(jié)流回路中，節(jié)流閥安裝在泵的出油口處的旁路，調(diào)速閥從旁路把油分流回油箱的流量，從而調(diào)節(jié)液壓缸的速度。其優(yōu)點在于泵是在工作壓力下進行調(diào)節(jié)，多余的油是通過節(jié)流閥而不是溢流閥流回油箱的。缺點是調(diào)節(jié)精度比較差，原因在于精確分流的油液是回到油箱，而未流經(jīng)液壓缸，這樣由于工作負(fù)載的變化，使泵的輸出不穩(wěn)定。旁路節(jié)流調(diào)速回路在負(fù)載失控的情況下不能應(yīng)用。
節(jié)流調(diào)節(jié)的類型
節(jié)流閥閥有兩種基本類型：壓力補償型和非壓力補償型。后者常用在負(fù)載壓力相對穩(wěn)定，運動速度要求不太嚴(yán)格的地方。盡管其構(gòu)件比較復(fù)雜，甚至包含有油液反向自由流動的單向閥，這種方法就像一個固定節(jié)流口或可調(diào)節(jié)的針閥一樣，仍然是非常簡單的。非壓力補償型應(yīng)用受到了一定的限制，這是因為通過阻尼孔的流量與流經(jīng)它所產(chǎn)的壓力差平方根成比例。這表明工作負(fù)載的巨大變化會影響到運動速度。
壓力補償型節(jié)流閥可進一步劃分為調(diào)速閥型和溢流節(jié)流型。兩者都利用一個補償器或穩(wěn)定器來穩(wěn)定通過可調(diào)節(jié)流口而產(chǎn)生的壓力差。
溢流節(jié)流型節(jié)流閥是帶過載保護裝置的壓力補償控制閥，它有一個常閉的節(jié)流口，當(dāng)打開它時，可以使油液流到油箱。工作負(fù)載所需的壓力值由位于上腔內(nèi)的傳感器探測，該壓力與一根輕彈簧共同使閥口處于常閉狀態(tài)。節(jié)流口下端的腔內(nèi)壓力由于受到節(jié)流閥的阻力而不斷增加，當(dāng)其上下端受到的壓力差達到足以克服彈簧力時，多余的油液就會流回油箱。這個壓力差，通常為20psi，不管工作負(fù)載有多大，都會由于確定量的油液經(jīng)過節(jié)流口而形成穩(wěn)定值。
過載保護裝置由一個可調(diào)節(jié)的彈簧提供，以限制節(jié)流口上腔的最大壓力值，只要工作負(fù)載超過設(shè)定值，它就相當(dāng)于一個溢流閥。溢流節(jié)流型控制閥只用在進口節(jié)流路。如果用在出口節(jié)流調(diào)速回路中，回油不能通過節(jié)流閥流回油箱，而無法抑制引起的負(fù)載失控。
調(diào)速閥型流量控制閥依靠的是穩(wěn)定器使油液通過節(jié)流閥維持2MP的壓力差。這種閥的閥芯處于常開狀態(tài)，可以自動關(guān)小節(jié)流口以阻止過多的油液流過。在閥中，工作負(fù)載和閥芯上端的一根彈簧共同使閥芯處于開啟狀態(tài)。節(jié)流口進口和閥芯下端的壓力欲使其關(guān)閉，僅允許油液進入閥內(nèi)，維持通過節(jié)流閥2MPa的壓力差。 當(dāng)油液達到節(jié)流閥的設(shè)定值時，閥口有關(guān)閉的趨勢。調(diào)速閥型節(jié)流閥可以安置在進油口，出油口和旁路節(jié)流調(diào)速回路上。與溢流節(jié)流型節(jié)流調(diào)速不同，調(diào)速閥型可以在一個泵源中使用兩個或多個節(jié)流閥，泵輸出的多余油液通過溢流閥返回油箱。 當(dāng)設(shè)置節(jié)流閥在液壓缸回路上時，為能迅速反向回流，應(yīng)該選擇整體式單向閥。而單向閥不允許設(shè)置在主供油管路，方向閥的回油箱管路以及旁路節(jié)流調(diào)速中。
溫度補償型節(jié)流閥：流經(jīng)壓力補償節(jié)流控制閥的流量易于受溫度變化的影響。油液溫度升高時，油液流動阻力小。當(dāng)溫度升高時，通過減小節(jié)流閥的開口尺寸仍能維持流量的穩(wěn)定，這可以通過一個能熱脹冷縮的補償桿來實現(xiàn)。節(jié)流閥芯是一個能來回進出的柱塞。溫度補償桿就安裝在節(jié)流閥和調(diào)節(jié)器之間，這種設(shè)計也可以設(shè)置一個可反向自由流動的單向閥來實現(xiàn)。
遠程流量控制閥 遠程流量控制閥允許通過電信號調(diào)節(jié)節(jié)流閥的開口大小，節(jié)流閥芯和扭距馬達的轉(zhuǎn)子相連，并且能對扭距馬達的電信號做出反應(yīng)，其操作原理與壓力補償調(diào)速閥是一樣的。
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