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CIRCUIT DESING
Summary
The selection of hydraulic components for use in a given application is determined by their ability to meet the required specification within the desired cost framework. A variety of components can be arranged to fulfil a given function by using different circuit configurations as the fluid power system designer has the freedom, within the constraints set by the preferences of the machine builder and/or the user, to select components of his choice.
This freedom makes it difficult to summarise circuit on design however, the designer need to be able justify the circuit on the basis of technical considerations. This chapter therefore describes and, where applicable, evaluates variety of circuit options that can be used for the range of functions generally encountered in the application of fluid power systems.
1. Introduction
To a very large degree the main function of hydraulic circuits is to control the flow to one or several actuators as required by the application. There are, however, a variety of methods for controlling flow, some of which act indirectly by using pressure as the controlling parameter.
The circuits discussed in this chapter include:
· Directional control and valve configurations.
· Velocity controls with constant supply pressure.
· Velocity controls with load sensing.
· Variable displacement pump controls.
· Hydrostatic transmissions.
· Load control.
· Contamination control.
2. Pressure and Flow
Hydraulic systems provide flow from the pump that is directed to one or more actuators(motors) at a pressure level that satisfies the highest demand. Where a single output is being driven the pump pressure will float to the level demanded by the load. However, even for such simple systems the method that is employed to provide variable flow needs to be evaluated in order to ensure that best efficiency is obtained. In circuits with multiple outputs this aspect can be more difficult to evaluate.
For operation at pressures and flows that are lower than the required maximum values the efficiency of the system will depend on the type of pump being used (i.e. fixed or variable displacement). This can be represented diagrammatically as in Figure 1.
For fixed displacement pump system it is clear from Figure 1 that excess pump flow will have to be returned to the reservoir so that the power required by the pump is greater than that being supplied to the load. The level of inefficiency incurred is dependent on the ratio between the pressure required by the load and that at the pump outlet which can be controlled at the maximum level by the relief valve or at lower pressures by various types of bypass valves.
Figure1 Flow and pressure varlotion
For variable displacement pumps the generation of excess flow can be avoided. However, the lever of pump pressure will depend on the method that is used for controlling the displacement but clearly there is scope for achieving much higher efficiencies than with fixed displacement pumps.
Each of these control methods will require a particular circuit design employing components that have been described in the previous chapters.
3. Directional control
Valves used for controlling the direction of the flow can be put into fixed positions for this purpose but many types are frequently used in a continuously variable mode where they introduce a restriction into the flow path.
3.1 Two position valves
A four-way valve with two positions for changing direction of the flow to and from an actuator is shown in Figure 2. For supply flow, Q, the actuator velocities will be:
Extend UE=Q/Ap; Retract UR=Q/AA
Here, the actuator areas are Ap for the piston and AA for the annulus or rod end the actuator. Hence,
UR>UE as Ap>AR
Any external forces (F) that are acting on the actuator rod must be in opposition to the direction motion. For reversing force applications it will be necessary to apply restrictor control which will be discussed later in the chapter. These forces will create a supply pressure that is
= F/Ap or F/AA
Figure2 Two position four-way valve
Three-way valves are used in applications where only one side of the actuator needs a connection from the supply. A typical example for this is the operation of the lift mechanism on a fork lift truck, as shown in Figure 3 where the actuator is lowered under the action of the weight.
Figure3 Two position three-way
3.2 Three position valves
Three position valves have a third, central position that can be connected in different configurations. These variants are described.
Closed Centre Valves (Figure 4)
Closed centre valves block all of the four ports. This prevents the actuator from moving under the action of any forces on the actuator. The supply flow port is also blocked which may require some means of limiting the supply pressure supply pressure can be made by appropriate pump controls or by a relief valve.
Figure4
Tandem Centre valves (Figgure5)
Tandem centre valves block the actuator ports but the supply is returned to the tank at low pressure. If other valves are being supplied from the same source this type of valve may not be used-unless connected in series.
Figure 5
Open Centre Valves (Figure 6)
Open centre valves connect all of the four ports to the tank so that the supply and the actuator pressure are at low pressure. This allows the actuator to actuator to be free to be move under the action of any external forces.
Figure6
Where it is necessary to block the supply flow the configuration shown in Figure 7 can be used.
Figure 7
4. Load holding valves
The radial clearance between the valve and its housing of spool valves is carefully controlled in the manufacturing process to levels of around 2 micron. The leakage through this space, even at high pressures, is small but for applications where it is essential that the actuator remains in the selected position for long periods of time (e.g. crane jibs where any movement would be unacceptable) valves having metal-to-metal contact have to be used.
Check valves usually employ metal-to-metal contact but they are only open in one direction under the action of the flow into the valve. For their use in actuator circuits it is necessary that they are open in both directions as required by the DCV. This function can be obtained from a Pilot Operated Check Valve that uses a control pressure to open the valve against reverse flow.
Figure7 Pilot operated check valve
Figure 8 shows a typical pilot operated check valve (POCV) where by a pilot pressure is applied onto the piston to force open the ball check valve to allow flow to pass from port 1to 2 when the check valve would normally be closed. The ratio of the piston and valve sent areas has to be chosen so that the available pilot pressure can provide sufficient force to open the valve against the pressure on port 1.
The use of a POCV is shown in Figure 9 where the external force on the actuator is acting in the extend direction. With the DCV in the centre position the check valve will be closed because the pilot is connected to the tank return line that is at low pressure. Opening the DCV so as to extend the actuator causes the piston side pressure, now connected to the supply, to increase.
Figure9 Actuotor Circuit using o POCV
When this pressure reaches the level at which the check valve is opened against the pressure generated on the rod side of the actuator by the load force, the actuator will extend. The ratio of the pilot and ball seat diameters needs to be such that the pressure areas cause the POCV to be fully open against the annulus pressure. If the pilot pressure is insufficient to open the valve because of an intensified pressure at the check valve inlet from the actuator annulus and/or back pressure on the POCV outlet due to restriction in the DCV, oscillatory motion can result.
5. Velocity control
The velocity of actuators can be controlled by using a number of different methods. In principle the various methods can be employed for both linear and rotary actuators or motors but in some cases it may be necessary to refer to the manufacturer’s literature for guidance.
5.1 Meter-in control
Meter-in control refers to the use of a flow control at the inlet to an actuator for use with actuators against which the load is in opposition to the direction of movement.
For a meter-in circuit that uses a simple adjustable restrictor valve selection of the DCV to create extension of the actuator will cause flow to pass through the restrictor into the piston end of the actuator. The required piston pressure,, will depend on the opposing force on the actuator rod. With a fixed displacement pump delivering a constant flow, excess flow from the pump will be returned to tank by the relief valve at its set pressure. Consequently, the available pressure drop.
with this system the flow, and hence the actuator velocity variations are undesirable a pressure compensated flow control valve (PCFCV) can be used. This valve sill maintain a constant delivery flow providing that the pressure drop is greater than its minimum controlled level that is usually in the region of 10to 15 bar.
Figure10 Meter-in control Actuotor Extension
Figure 10 shows a typical system in which the flow control is bypassed with a check valve for reverse operation of actuator. If the load force varies considerably during operation, there will be transient changes in actuator velocity at a level that depends on the mass of the load.
For example, when the load force suddenly reduces, the piton pressure will reduce but at a rare that is dependent on the fluid volume and its compressibility and the mass of the load. During the period that the pressure is greater than the required new value, the actuator will accelerate and, as it does so the piston pressure will fall. The pressure can then fall below the new level and deceleration results and damped oscillations can occur.
In some situations the mass of the load can be such as to cause problems of cavitation and overrunning because the pressure falls transiently to a level at which absorbed air is released. If the pressure falls low enough the fluid will vaporize. Both of these phenomena are referred to as cavitation and noisy operation, and damage to the components can be the result.
A check valve having a spring cracking pressure that is high enough to suppress cavitation is sometimes used but this has the disadvantage if increasing the pump pressure and thus reducing the efficiency and increasing the heating effect on the fluid.
5.2 Meter-out control
Figure12 meter-out control
For overrunning load forces and/or those with a large mass, meter-out control is used where the actuator outlet flow during its extension passes through the restrictor or PCFCV as shown in the circuit of Figure 12.
The flow control operates by controlling the actuator outlet pressure at the level required to oppose the forces exerted on the actuator by the load and by the piston pressure which is the same as that of the pump. This prevents cavitation from occurring during transient changes arising from load force variations or due to forces that act in the same direction as the movement (i.e. pulling forces).
This system can, however, cause high annulus pressures to occur from the intensification of the piston pressure together with the pressure created by pulling forces. Further, when compared to meter-in, the rod and piston seals have to by capable of withstanding high pressures that may require a higher cost actuator to by used.
5.3 Bleed-control
For the fixed displacement pump system shown in Figure 13, excess flow is bled off from the supply so that the pump pressure is mow at the same level as that required at actuator piston.
Figure13 Bleed-off control
Bleed-off control is therefore more efficient than meter-in and meter-out because of the lower pump pressure. However, as for meter-in, it cannot be used with pulling loads and it can also only be used to control one actuator at a time from the pump. This is in contrast to meter-in and meter-out where several actuators can be supplied by a single pump as shown in Figure14.
Figure14 Multiple Actuotor Circuit with Meter-in control
Meter-in and meter-out controls can be supplied from a variable displacement pump that is operated with a constant pressure control (pressure compensated) which reduces the power wastage that is inherent with a fixed displacement pump. This is demonstrated by making a comparison of the efficiencies as follows:
For meter-in control the power efficiency,
For a pressure compensated pump the power efficiency,
as
Thus referring to Figure 1, the pump flow is always equal to that of the load, the pump is still capable of achieving the maximum demand, which is referred to as the ‘corner power’ of the pump. The fixed displacement pump operates at this rating continuously because of the use of the relief valve to control the flow to the actuator.
The flow control methods described in this section are usually preset in a system that is being used on a continuous basis such as for a production machine (e.g. injection moulding) where possibly the operations are being carried out sequentially. It would normally be expected that the duration of, say, actuator movement is small in relation to the overall cycle time so that the power losses are relatively small. Where a continuously variable flow control is required alternative components and need to be considered.
液壓回路設(shè)計(jì)
概要
具體應(yīng)用中選擇液壓元件的型號(hào)主要取決于滿足要求的性能和理想的價(jià)格。液壓系統(tǒng)設(shè)計(jì)者有一定的自由選用各種元件構(gòu)成不同的回路來實(shí)現(xiàn)制造商或者使用者所要求的特定功能。
這種自由使得概括回路設(shè)計(jì)有些困難,因此設(shè)計(jì)者必須能證明回路在已考慮的技術(shù)范圍內(nèi),本章描述了多種回路形式在一般液壓系統(tǒng)的應(yīng)用。
1.緒論
很大程度上,液壓回路的作用是控制流體按要求流向一個(gè)或幾個(gè)馬達(dá)。事實(shí)上有多種控制流體的方法,其中的一些直接以壓力作為控制參量.
本章討論的回路包括:
· 方向控制和控制閥的構(gòu)造
· 恒壓速率控制
· 負(fù)載速率控制
· 變量泵的控制
· 液壓傳動(dòng)
· 負(fù)載控制
· 綜合控制
2.壓力與流量
由泵向液壓系統(tǒng)提供滿足最大需求的壓力和流量，供給一個(gè)或幾個(gè)執(zhí)行元件。單個(gè)輸出時(shí)，泵的壓力根據(jù)負(fù)載調(diào)整。所以對(duì)一些簡(jiǎn)單系統(tǒng)按計(jì)算的需求提供流量的方法可以獲得最佳效率，多輸出時(shí)計(jì)算就較為困難。
系統(tǒng)壓力和流量低于最大需求量時(shí)，泵的類型（定量泵或變量泵）決定系統(tǒng)效率。這從下圖1可以看出
圖1 壓力與流量
圖中顯而易見，定量柱塞泵系統(tǒng)，因?yàn)槎嘤嗟牧髁勘仨毞祷氐接拖洌虼吮眯枰哪芰看笥诠┙o負(fù)載的能量，無用功的大小取決于負(fù)載所需的壓力和泵的出口壓力的比，泵的出口壓力可以用安全閥調(diào)制最大或用其他類型的旁通閥調(diào)到較低的壓力。
變量柱塞泵就可以避免產(chǎn)生多余的流量，它的壓力可通過控制排量的方式調(diào)整，顯然它有可能達(dá)到比定量柱賽泵更高的效率。
這些控制方法需要設(shè)計(jì)特殊的回路結(jié)構(gòu)，前面章節(jié)已經(jīng)講過。
3.方向控制
方向控制閥可以放在固定位置達(dá)到控制目的，但是多數(shù)類型經(jīng)常用在連續(xù)可變的模式，起到限流徑的作用。
3.1二位閥
二位四通閥控制流體進(jìn)出執(zhí)行元件的方向如圖2，進(jìn)入流量為Q時(shí)，活塞移動(dòng)的速度等于
UE=Q/Ap； 返回時(shí) UR=Q/AA
這里，Ap是無桿腔活塞面積，AA是有桿腔有效面積，所以
Ap>AA, AR>UE
任何作用在活塞桿上的外力都有阻止活塞運(yùn)動(dòng)的趨勢(shì)，為克服此力，應(yīng)進(jìn)行節(jié)流控制，在后面的章節(jié)將會(huì)介紹?？朔枇π枰膲毫Φ扔?
＝F/AA 或者 F/Ap
圖2 二位四通閥
當(dāng)執(zhí)行元件只有一端需要供壓時(shí)可以使用二位三通閥，典型的例子如起重機(jī)的升降機(jī)構(gòu)，如圖3所示，執(zhí)行元件在重力作用下下降。
圖3 二位三通閥
3.2 三位閥
三位閥第三個(gè)位置，中間位置有不同的構(gòu)造，下面介紹不同的中位機(jī)能。
中位關(guān)閉閥（圖4）
中位關(guān)閉閥關(guān)閉所有的四個(gè)端口，這樣就阻止執(zhí)行元件在任何力的作用下移動(dòng)，供壓端口也被關(guān)閉，因此需要對(duì)系統(tǒng)壓力進(jìn)行限制，可以通過對(duì)泵的適當(dāng)調(diào)整或通過安全閥控制。
圖4
中位卸載閥（圖5）
中位卸載閥關(guān)閉執(zhí)行元件端口，接通供壓端口和油箱端口，使供壓系統(tǒng)以較低壓力卸載，當(dāng)有其他壓力閥使用同一供壓源時(shí)，是不能使用中位卸載閥的，除非它們是串聯(lián)的。
圖5
中位互通閥（圖6）
中位互通閥的四個(gè)端口同時(shí)連通到回油箱，使得供壓系統(tǒng)和執(zhí)行元件都處在較低壓力下，讓執(zhí)行元件可以在任何外力作用下自由移動(dòng)。
圖6
當(dāng)需要關(guān)閉供壓端口時(shí)，結(jié)構(gòu)如圖7所示
圖7
4.單向閥
在制造過程中單向閥的閥體和閥芯的徑向間隙可以精確的控制在2微米的范圍，即使在高壓下，泄漏也很小，但卻是必要的，有時(shí)執(zhí)行元件要長(zhǎng)時(shí)間處在一個(gè)位置（例如起重機(jī)臂的移動(dòng)），金屬對(duì)金屬的接觸需要它潤(rùn)滑。
單向閥通常使用金屬對(duì)金屬接觸的結(jié)構(gòu)，在流體壓力作用下只在一個(gè)方向開啟，在液壓回路中使用，有時(shí)換向閥要求要在兩個(gè)方向都能開啟，液控單向閥可以實(shí)現(xiàn)這種功能，它有能逆流開啟的控制壓力。
圖8所示的時(shí)典型的液控單向閥（POCV），通過作用在活塞上的控制壓力打開球形閥，使流體從端口1流向端口2，普通的單向閥此時(shí)是關(guān)閉的?；钊烷y的作用面積比要通過計(jì)算選擇，使控制壓力能產(chǎn)生足夠的力克服端口1的壓力，打開球形閥。
圖8 液控單向閥
液控單向閥的用處見圖9所示，外力作用在液壓缸的拉伸方向，換向閥處在中位時(shí)，控制壓力與回油箱連通，壓力較低，單向閥關(guān)閉。打開換向閥，以使液壓缸伸長(zhǎng)，此時(shí)控制壓力接通到系統(tǒng)壓力，壓力升高。
圖9 液控單向閥的應(yīng)用
當(dāng)控制壓力達(dá)到一定水平克服有桿腔負(fù)載時(shí)，單向閥打開，液壓缸伸長(zhǎng)?？刂茐毫颓蛐伍y直徑的關(guān)系應(yīng)滿足：壓力作用有效面積能克服作用在有桿腔環(huán)狀面積上的壓力，使液控單向閥完全打開。如果從有桿腔進(jìn)入單向閥入口的壓力較大，或者換向閥限制了單向閥出口的壓力，使控制壓力不足以打開單向閥，可能會(huì)引起單向閥的震動(dòng)。
5. 速度控制
有多種方法控制執(zhí)行元件的速度，原則上這些方法既可以控制執(zhí)行元件的直線速度，又可以控制角速度，但是有些情況可能需要廠商的指導(dǎo)說明書。
5.1 入口節(jié)流調(diào)速
入口節(jié)流調(diào)速用在對(duì)執(zhí)行元件入口流量的控制，使執(zhí)行元件克服阻滯運(yùn)動(dòng)的負(fù)載。
構(gòu)建簡(jiǎn)單的入口節(jié)流控制虧回路，需要簡(jiǎn)單的可調(diào)式節(jié)流閥，通過換向閥的控制，讓流體經(jīng)過節(jié)流閥進(jìn)入液壓缸的活塞，液壓缸活塞需要的壓力Pp取決于作用在活塞桿上的負(fù)載。使用定量柱塞泵提供恒定的流量，多余的流量經(jīng)溢流閥調(diào)定的壓力回到油箱，從而，讓使用壓力下降。
這個(gè)系統(tǒng)中，流量和執(zhí)行元件的速度變化可通過帶有壓力補(bǔ)償?shù)墓?jié)流閥控制，當(dāng)系統(tǒng)壓降大于其最小控制壓力（10～15巴）時(shí)，這種節(jié)流閥能提供恒定的流量。
圖10所示的是一個(gè)單向調(diào)速的入口節(jié)流控制系統(tǒng)，如果運(yùn)行過程中負(fù)載變化頻繁，執(zhí)行元件的速度會(huì)隨著負(fù)載的變化而變化。
例如，當(dāng)負(fù)載突然變小時(shí)，作用在活塞上的壓力隨之減小，但是這要取決于流量，流體的可壓縮性和負(fù)載的慣性，這時(shí)系統(tǒng)壓力大于所需的壓力，液壓缸會(huì)加速運(yùn)動(dòng)，使壓力下降。形成新的系統(tǒng)壓力，最終減速并發(fā)生阻尼振動(dòng)。
圖10 入口節(jié)流調(diào)速系統(tǒng)
有些情況下負(fù)載的慣性可能會(huì)引發(fā)氣穴現(xiàn)象和超壓。當(dāng)壓力突然降到很低時(shí)，流體吸收的空氣被釋放出來，壓力足夠低時(shí)，還會(huì)引起流體蒸發(fā)，這就是氣穴現(xiàn)象，伴有噪聲，最終對(duì)系統(tǒng)造成損傷。
單向閥產(chǎn)生背壓足以抑制氣穴現(xiàn)象，但是同時(shí)會(huì)使泵的壓力升高，系統(tǒng)效率下降還會(huì)影響流體的溫度。
5.2出口節(jié)流調(diào)速
當(dāng)負(fù)載或負(fù)載慣性過大時(shí)出口節(jié)流調(diào)速用來控制執(zhí)行元件的出口流量，出口流體經(jīng)過節(jié)流閥或調(diào)速閥的回路如圖12
圖12 出口節(jié)流調(diào)速系統(tǒng)
這種流量控制方法通過控制執(zhí)行元件的出口壓力，使其符合：作用在活塞上的壓力（泵的壓力）能克服負(fù)載。由于負(fù)載變化時(shí)，總有壓力作用在與運(yùn)動(dòng)相同的方向上（如拉力），避免了氣穴現(xiàn)象的產(chǎn)生。
然而這種系統(tǒng)會(huì)引起有桿腔壓力和活塞桿拉力激烈的變化，與入口節(jié)流控制相比，活塞和活塞桿密封必須能承受高壓，可能導(dǎo)致執(zhí)行元件的使用成本升高。
5.3 旁路節(jié)流調(diào)速
如圖13所示的旁路節(jié)流調(diào)速系統(tǒng)，多余的流量直接從系統(tǒng)流回油箱，因此系統(tǒng)壓力總是等于執(zhí)行元件所需壓力。
圖13 旁路節(jié)流調(diào)速
因?yàn)楸脡毫^低，與入口節(jié)流調(diào)速和出口節(jié)流調(diào)速相比，旁路節(jié)流調(diào)速系統(tǒng)有較高的效率。但是它不能用在拉伸負(fù)載上，而且單泵供壓時(shí)，每次只能控制一個(gè)執(zhí)行元件，出口和入口節(jié)流調(diào)速可以同時(shí)控制多個(gè)執(zhí)行元件，如圖14所示
圖14 控制若干執(zhí)行元件的入口節(jié)流調(diào)速
變量柱塞泵向入口與出口節(jié)流調(diào)速系統(tǒng)供壓時(shí)，可以提供一個(gè)恒定的壓力（有壓力補(bǔ)償作用），使系統(tǒng)功耗降低，這正是比定量泵的優(yōu)越之處?？梢酝ㄟ^計(jì)算它們的效率來證明如下
入口節(jié)流調(diào)速系統(tǒng)效率，
具有壓力補(bǔ)償系統(tǒng)的效率
其中
參考圖1，泵的流量總是等于負(fù)載所需的流量，而且仍然能夠達(dá)到最大所需值，定量泵要想連續(xù)這樣的話，就需要溢流閥不斷控制進(jìn)入執(zhí)行元件的流量。
本節(jié)介紹的幾種流量控制方法都是基于連續(xù)工作的系統(tǒng)，如專用機(jī)床（如注塑機(jī)）上可能需要控制的地方預(yù)先布置好的，是點(diǎn)控。通常人們期望執(zhí)行元件的工作時(shí)間與整個(gè)運(yùn)行周期中的關(guān)聯(lián)較小，以降低功率損失，這就需要考慮調(diào)整系統(tǒng)結(jié)構(gòu)實(shí)現(xiàn)無極調(diào)速。