动静态液压试验台液压系统设计-含PPT【含2张PDF图纸+CAD制图+文档】
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CIRCUIT DESINGSummaryThe 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. IntroductionTo 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 FlowHydraulic 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 varlotionFor 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 controlValves 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 valvesA 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/AAHere, the actuator areas are Ap for the piston and AA for the annulus or rod end the actuator. Hence, URUE as ApARAny 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/AAFigure2 Two position four-way valveThree-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-way3.2 Three position valvesThree 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 5Open 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.Figure6Where it is necessary to block the supply flow the configuration shown in Figure 7 can be used.Figure 74. Load holding valvesThe 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 valveFigure 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 POCVWhen 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 controlThe 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 manufacturers literature for guidance.5.1 Meter-in controlMeter-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 ExtensionFigure 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 controlFigure12 meter-out controlFor 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-controlFor 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 controlBleed-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 controlMeter-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.液压回路设计概要具体应用中选择液压元件的型号主要取决于满足要求的性能和理想的价格。液压系统设计者有一定的自由选用各种元件构成不同的回路来实现制造商或者使用者所要求的特定功能。这种自由使得概括回路设计有些困难,因此设计者必须能证明回路在已考虑的技术范围内,本章描述了多种回路形式在一般液压系统的应用。1.绪论很大程度上,液压回路的作用是控制流体按要求流向一个或几个马达。事实上有多种控制流体的方法,其中的一些直接以压力作为控制参量.本章讨论的回路包括: 方向控制和控制阀的构造 恒压速率控制 负载速率控制 变量泵的控制 液压传动 负载控制 综合控制2.压力与流量由泵向液压系统提供满足最大需求的压力和流量，供给一个或几个执行元件。单个输出时，泵的压力根据负载调整。所以对一些简单系统按计算的需求提供流量的方法可以获得最佳效率，多输出时计算就较为困难。系统压力和流量低于最大需求量时，泵的类型（定量泵或变量泵）决定系统效率。这从下图1可以看出图1 压力与流量图中显而易见，定量柱塞泵系统，因为多余的流量必须返回到油箱，因此泵需要的能量大于供给负载的能量，无用功的大小取决于负载所需的压力和泵的出口压力的比，泵的出口压力可以用安全阀调制最大或用其他类型的旁通阀调到较低的压力。变量柱塞泵就可以避免产生多余的流量，它的压力可通过控制排量的方式调整，显然它有可能达到比定量柱赛泵更高的效率。这些控制方法需要设计特殊的回路结构，前面章节已经讲过。3.方向控制方向控制阀可以放在固定位置达到控制目的，但是多数类型经常用在连续可变的模式，起到限流径的作用。3.1二位阀二位四通阀控制流体进出执行元件的方向如图2，进入流量为Q时，活塞移动的速度等于UE=Q/Ap； 返回时 UR=Q/AA这里，Ap是无杆腔活塞面积，AA是有杆腔有效面积，所以ApAA, ARUE任何作用在活塞杆上的外力都有阻止活塞运动的趋势，为克服此力，应进行节流控制，在后面的章节将会介绍。克服阻力需要的压力等于F/AA 或者 F/Ap图2 二位四通阀当执行元件只有一端需要供压时可以使用二位三通阀，典型的例子如起重机的升降机构，如图3所示，执行元件在重力作用下下降。图3 二位三通阀3.2 三位阀三位阀第三个位置，中间位置有不同的构造，下面介绍不同的中位机能。中位关闭阀（图4）中位关闭阀关闭所有的四个端口，这样就阻止执行元件在任何力的作用下移动，供压端口也被关闭，因此需要对系统压力进行限制，可以通过对泵的适当调整或通过安全阀控制。图4中位卸载阀（图5）中位卸载阀关闭执行元件端口，接通供压端口和油箱端口，使供压系统以较低压力卸载，当有其他压力阀使用同一供压源时，是不能使用中位卸载阀的，除非它们是串联的。图5中位互通阀（图6）中位互通阀的四个端口同时连通到回油箱，使得供压系统和执行元件都处在较低压力下，让执行元件可以在任何外力作用下自由移动。图6当需要关闭供压端口时，结构如图7所示图74.单向阀在制造过程中单向阀的阀体和阀芯的径向间隙可以精确的控制在2微米的范围，即使在高压下，泄漏也很小，但却是必要的，有时执行元件要长时间处在一个位置（例如起重机臂的移动），金属对金属的接触需要它润滑。单向阀通常使用金属对金属接触的结构，在流体压力作用下只在一个方向开启，在液压回路中使用，有时换向阀要求要在两个方向都能开启，液控单向阀可以实现这种功能，它有能逆流开启的控制压力。图8所示的时典型的液控单向阀（POCV），通过作用在活塞上的控制压力打开球形阀，使
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