汽车起重机液压系统的设计

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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 Ap>AA, AR>UE 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7 4.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，使流体从端口1流向端口2，普通的单向阀此时是关闭的。活塞和阀的作用面积比要通过计算选择，使控制压力能产生足够的力克服端口1的压力，打开球形阀。 图8 液控单向阀 液控单向阀的用处见图9所示，外力作用在液压缸的拉伸方向，换向阀处在中位时，控制压力与回油箱连通，压力较低，单向阀关闭。打开换向阀，以使液压缸伸长，此时控制压力接通到系统压力，压力升高。 图9 液控单向阀的应用 当控制压力达到一定水平克服有杆腔负载时，单向阀打开，液压缸伸长。控制压力和球形阀直径的关系应满足：压力作用有效面积能克服作用在有杆腔环状面积上的压力，使液控单向阀完全打开。如果从有杆腔进入单向阀入口的压力较大，或者换向阀限制了单向阀出口的压力，使控制压力不足以打开单向阀，可能会引起单向阀的震动。 5. 速度控制 有多种方法控制执行元件的速度，原则上这些方法既可以控制执行元件的直线速度，又可以控制角速度，但是有些情况可能需要厂商的指导说明书。 5.1 入口节流调速 入口节流调速用在对执行元件入口流量的控制，使执行元件克服阻滞运动的负载。 构建简单的入口节流控制亏回路，需要简单的可调式节流阀，通过换向阀的控制，让流体经过节流阀进入液压缸的活塞，液压缸活塞需要的压力Pp取决于作用在活塞杆上的负载。使用定量柱塞泵提供恒定的流量，多余的流量经溢流阀调定的压力回到油箱，从而，让使用压力下降。 这个系统中，流量和执行元件的速度变化可通过带有压力补偿的节流阀控制，当系统压降大于其最小控制压力（10～15巴）时，这种节流阀能提供恒定的流量。 图10所示的是一个单向调速的入口节流控制系统，如果运行过程中负载变化频繁，执行元件的速度会随着负载的变化而变化。 例如，当负载突然变小时，作用在活塞上的压力随之减小，但是这要取决于流量，流体的可压缩性和负载的惯性，这时系统压力大于所需的压力，液压缸会加速运动，使压力下降。形成新的系统压力，最终减速并发生阻尼振动。 图10 入口节流调速系统 有些情况下负载的惯性可能会引发气穴现象和超压。当压力突然降到很低时，流体吸收的空气被释放出来，压力足够低时，还会引起流体蒸发，这就是气穴现象，伴有噪声，最终对系统造成损伤。 单向阀产生背压足以抑制气穴现象，但是同时会使泵的压力升高，系统效率下降还会影响流体的温度。 5.2出口节流调速 当负载或负载惯性过大时出口节流调速用来控制执行元件的出口流量，出口流体经过节流阀或调速阀的回路如图12 图12 出口节流调速系统 这种流量控制方法通过控制执行元件的出口压力，使其符合：作用在活塞上的压力（泵的压力）能克服负载。由于负载变化时，总有压力作用在与运动相同的方向上（如拉力），避免了气穴现象的产生。 然而这种系统会引起有杆腔压力和活塞杆拉力激烈的变化，与入口节流控制相比，活塞和活塞杆密封必须能承受高压，可能导致执行元件的使用成本升高。 5.3 旁路节流调速 如图13所示的旁路节流调速系统，多余的流量直接从系统流回油箱，因此系统压力总是等于执行元件所需压力。 图13 旁路节流调速 因为泵压力较低，与入口节流调速和出口节流调速相比，旁路节流调速系统有较高的效率。但是它不能用在拉伸负载上，而且单泵供压时，每次只能控制一个执行元件，出口和入口节流调速可以同时控制多个执行元件，如图14所示 图14 控制若干执行元件的入口节流调速 变量柱塞泵向入口与出口节流调速系统供压时，可以提供一个恒定的压力（有压力补偿作用），使系统功耗降低，这正是比定量泵的优越之处。可以通过计算它们的效率来证明如下 入口节流调速系统效率， 具有压力补偿系统的效率 其中 参考图1，泵的流量总是等于负载所需的流量，而且仍然能够达到最大所需值，定量泵要想连续这样的话，就需要溢流阀不断控制进入执行元件的流量。 本节介绍的几种流量控制方法都是基于连续工作的系统，如专用机床（如注塑机）上可能需要控制的地方预先布置好的，是点控。通常人们期望执行元件的工作时间与整个运行周期中的关联较小，以降低功率损失，这就需要考虑调整系统结构实现无极调速。