外文翻譯--液壓控制閥

1 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, 2 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. 3 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. 4 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 5 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 6 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. 7 液 壓 回路中的 壓 力控制 閥 是用 來 確?；芈分胁煌糠值?壓 力 達 到 預 期 值 。 壓 力控制閥 通 過 以下幾 種 方式 實現(xiàn)預 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 。 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。 當閥 靠外控先 導 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 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 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 式溢流 閥 已 經(jīng) 廣泛的 應 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 8 流 閥 的先 導閥彈 簧上 標 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 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?閥 ，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 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 導壓 力油流回油箱。 當這 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 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 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 預緊 力。 當調(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 導閥 所 產(chǎn) 生力，一般被 認為 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 們應該 熟 記這個術(shù)語 ，因 為它 廣泛的 應 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?？梢杂靡?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 9 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 負載 的回路中，例如，在 負載 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