液压外文翻译2.doc

大学毕业设计（外文翻译）Power modulation and controlValves are essential elements for controlling fluid system performance.Virtually every fluid-power system requires some type of valve. In a hydraulic system, valves may control pressure, flow to an actuator , or quantity of flow permitted past a given point. Pneumatic valves are similar in design and operation to their hydraulic counterparts, although they may differ in construction.Trends in the valve industry today include miniaturization of traditional designs and space-saving stackable valves. Compatibility with electronic controls means enhanced valve performance .manufacturers are also turning to plastics to cut weight while improving lubricity and corrosion resistance .And advanced ceramics are being used for longer life and contamination resistance .Pressure valvesThe primary concern in fluid power circuits is to control either pressure level or the rate of flow. In theory, most flow-control valves could be used control pressure .If orifice size, supply flow, and fluid viscosity are fixed, pressure remains constant; if any one of the three varies, pressure varies. Typically, however, such valves produce only the crudest kind of pressure control. For more accurate control, several types of pressure-control valves have been developed; they are categorized by the function to be performed.Counterbalance valves resist moment or balance the load being held by a cylinder or motor. These valves, by controlling pressure, provide excellent dynamic control. If they must hold the load for long periods, experts recommend that they be supplemented with a pilot check valve, which has better static-holding capabilities.Diverting valves (known in the mobile industry as sequence valves) establish flow priorities within a circuit by using a pressure-actuated three-way valve with controls. The valve assembly directs pressurized fluid to a primary port until a predetermined pressure level is reached. At this valve, flow is diverted from the primary outlet to a secondary as long as the pressure in the primary outlet is maintained. Pressures can be cascaded though several steps if needed.Sequence valves are used to determine the sequence of machine operations by sensing pressures other than maximum. These normally closed valves permit flow between inlet and output ports when the pressure reaches preset levels, and can be fitted with free-flow checks to permit flow in the opposite direction.Many sequence valves have two or more spools or poppets that must be actuated before flow can pass though the valve. Typically, a signal shifts the control spool, ensuring that a certain minimum pressure has been developed in one part of the circuit before fluid can pass through another part.Reducing valves can limit pressure levels by restricting flow though a portion of a hydraulic system in a normally open two-port unit, the reducing valve receives the signal from its low- pressure outlet. The valve is often biased by a spring or weight that may be supplemented with a pilot mechanism.Reducing valves are used with suitable orifices to provide uniform pressure drop in flow-control valves. A check valve can be include for uncontrolled return flow through the valve, or pilot pressure can be used to hold the spool in the open positive to permit free return flow through the valve.Unloading valves provide free passage through a low-pressure area when a signal is applied to a pilot connection. In a typical application, unloading valves can be arranged to accept a signal from an accumulator. At a predetermined pressure, when the accumulator is charged to the specified level, the pump unloads to tank. The unloading pressure of this type of valve is commonly determined by a spring-loaded spool; the spring can be adjusted to vary unloading pressure. Alternatively, the valves can be controlled by application of a pilot pressure to hold the valve closed at pressures higher than that provided for by the spring.Safety valves pop open to avoid or eliminate abnormally high pressure peaks. They are designed strictly for fast action rather than pressure modulation, and they may well be subject to such problems as noise and chatter. They are typically nonadjustable, or have the pressure setting protected from tampering.Essentially, safety valves perform the function of fuses in the system. In lieu of one, devices called hydraulic fuses also can be used. These quasi-valves use a disc or similar device that fractures at a preset pressure. They do not reset automatically and must be manually repaired after fracturing.Relief valves do the same job as safety valves, but they also smoothly and continuously modulate flow to keep pressure from exceeding a preset level. A relief valve is normally closed until the pressure level approaches a preset value. As system pressure rises, relief flow through a properly sized valve increase until the entire pump output passes through the valve. When system pressure drops, the valve closes smoothly and quietly.Relief valves are available with simple direct actuation or with piloted operation. In addition, some electrically modulated relief valves perform an almost servo function to instantly modulate system pressure over a wide range of electrically signaled values.Flow valvesFlow is controlled by either throttling or diverting it. Throttling involves reducing orifice size until all of the flow cannot pass through the orifice; by passing involves routing part of the flow around the circuit so that the actuator receives only the portion needed to perform its task. If the flow inlet to an actuator is controlled, the circuit is said to be a “meter-in” system. If actuator outlet is controlled, it is called a “meter-out” circuit. When that part of the fluid being diverted to the reservoir or another part of the circuit is controlled, it is said to be a “bleed-off” system.Noncompensated flow controls are simple valves that meter flow by restricting or throttling. The amount of flow that passes through an orifice and the pressure drop across it are directly related. As pressure increases, valve flow increases.Common non-compensated valves are adjustable needle valve; flow through them varies with fluid viscosity and pressure across the valves. Usually, a needle valve is paired with a check valve that offers resistance to flow in one direction only. The combination permits flow to be adjusted in one direction, with free flow upon reverse. This type of two-valve combination is typically called an adjustable restrictor valve. For some tasks, adjustability is either unimportant or potentially harmful. For such tasks, a fixed resistor valve can be used. Basically, it consists of a check valve with an orifice embodied in the valve. Some fixed restrictors make provision for disassembling the valve and changing the orifice; others have no such provision. In either type, the orifice is not changed during circuit operation so the valve is considered non adjustable. Both fixed and variable restrictor valves are simple, reliable, and inexpensive. They can be used in any circuit , using any metering method. Experts recommend these non-compensated valves when accuracy is not important, when heat generation through power loss can be tolerated, and in such circuits as gravity lowering, where they can be used efficiently.Pressure-compensated flow controls maintain nearly constant flow despite variations in circuit pressure. Like the non-compensated units, they incorporate a metering orifice. Flow pressure drop across this orifice is used to shift a balanced spool against a control spring. This spool movement is used to maintain a constant pressure drop across the orifice, which in turn, produces a constant flow. Pressure drop across the orifice is relatively low.Check valves use a ball or poppet to prevent flow in one or more directions. In two-port valves, the ball or poppet is usually lightly spring loaded against one of the ports. In three-port valves, or shuttle valves, internal ridges guide the ball between ports.Restrictive flow regulators work like an automatic variable orifice to control flow by throttling or restricting. Compensator spool movement blocks fluid flow through the valve. Flow passing through the metering orifice is accompanied by a pressure drop that is applied to each end of a balanced spool .The resulting force imbalance moves the spool against the control spring. Spool movement progressively blocks off flow areas restricting or throttling flow through the valve.Restrictive flow regulators are ideally suited to constant-pressure closed center circuits and meter-out situations. Experts say they are the only pressure-compensatend flow control that can be used in these applications .They are also recommended for gravity lowering devices where uniform lowering speed is required regardless of the load.Bypass flow regulators control flow by diverting excess pump output to the reservoir .The same basic control orifice and compensator spool are used as in the restrictive flow regulator. But instead of restricting flow through the valve, spool movement diverts or bypasses excess flow to the reservoir. These regulators are used exclusively in variable-pressure open-center circuits, and only as a meter-in device. The resulting pump or supply pressure is slightly higher than required to do the work, and automatically changes with load .Bypass flow regulators cannot be used as meter-out devices in any circuit or as meter-in devices in constant-pressure circuits. The ability of these valves to accept all flow supplied to them excludes them from these applications.Combination bypass and restrictive flow regulators are a combination of the first two mentioned. They control flow by both restricting and bypassing, Permitting full use of both regulated and bypass flow. Flow through a controlled orifice produces a pressure that shift a compensating spool. Movement of the spool first uncovers the bypass-flow area. If bypass-circuit pressure is greater than regulated-circuit pressure, the spool moves farther to restrict or throttle the controlled flow. Regardeless of pressure in either circuit, the combination flow regulator maintains a constant controlled flow .Full pump flow is at the higher of the two pressure.Combination flow regulators are sometimes called “priority” valves. They establish priority flow to the control circuit and bypass to the secondary circuit only when the flow demands of the primary circuit are met. If pump supply is less than required at the regulated port , all flow goes to the regulated port and none is diverted .This type of combination valve is ideally suited for meter-in speed control in open-center fixed-displacement pump circuit.功率的调节和控制阀是用于流体系统性能控制的主要元件事实上，凡是流体动力系统都需要一些阀。在一个液压系统中,阀可以控制系统压力、液体流向以及通过的流量。气动阀在结构上与液压阀不同，但在设计与操作方面相似。在现代工业发展中，阀有由传统设计向小型化和集约化发展的趋势。在电子控制装置的应用下，阀的性能和其互换性得到了提高。生产商在注重减少塑料重量的同时，也重视提高其产品的润滑性和耐腐蚀性。现代陶瓷使用寿命久，抗污染能力强。被广泛地应用于阀体产品中，用于延长产品的使用寿命，改善其抗污染性，降低其阻力。压力阀压力阀应用于液压回路中，主要用来控制回路的压力和流量。从理论上来讲，大部分流量控制阀都能用来控制液压系统的压力。给定阀的节流面积，流量和流体粘度，假设压力保持不变，则其中三个量有一个变化都会引起其它量的变化。然而，这种阀一般只能产生有限的压力控制。为了更准确的进行压力控制，按其所起作用的不同，已经研究出不同类型的压力阀。背压阀是使系统有一定的抵抗力矩或平衡负荷的阀，目的是用来保持液压缸或马达的正常运行。背压阀能够控制系统压力，实现良好的的动态控制。如果背压阀必须长期保持一定负荷，建议在系统中增加一个液控单向阀，从而达到较好的保持静态的作用。分流阀（在液压系统中也可当做顺序阀）是通过控制压力来确定回路内流量的优先权的一种三通阀。分流阀受到流体压力的作用，直到系统压力达到调定压力为止，阀口开启。在分流阀中，多余流量从其他分流出口流出，从而使压力和开口量保持不变。如果需要减压，压力可以分段地减小。顺序阀是通过对外在压力信号的控制来限定机器的操作顺序的一种阀。这些常闭阀当阀的压力到达预定值时，允许流体在阀的进出口之间流动，并且通过检验流出的流量，使其阀芯换向相反的方向。顺序阀有多个阀芯，阀芯开启液流才得以流通。一般顺序阀，要在最小压力已达到回路压力，流体可以流向下一部分时，控制信号才控制滑阀移动。减压阀是用来调定液压系统压力极限的阀。减压阀是常开的，它的开口量是通过出口压力来调定的。减压阀常常是通过弹簧或阀芯自重作为导向补充机构。减压阀通过设置合理的开口度，以提供统一的压力降。单向阀也可以与减压阀联合使用以保证不受控制的回流穿过该阀，否则导向压力就用来使阀芯保持在开口的位置使得回流穿过该阀。卸荷阀是用来为油液通过低压区提供自由的通道，当控制信号作用到一个试点上，一般，卸荷阀是通过蓄能器的压力信号来控制卸荷的。当蓄能器压力达到预定压力时，泵把流体卸荷到储罐。卸荷阀的阀芯是通过弹簧限定其位置的，通过调节弹簧可以改变卸荷压力。另外，控制该阀开启所施加的力，要比弹簧所提供的力大。安全阀是用来避免和消除系统异常的高压。安全阀的设计是用来实现快速响应而不是压力调节，同时，它们也被很好地用来避免诸如噪声和冲击这样的问题。一般安全阀是不可调节的，或者设有压力保护，目的是为了避免干扰。实质上，安全阀在系统中所起的作用相当于“保险丝”。被称作液压保险丝的装置也可以被采用。泄压阀具有安全阀所起的作用，然而它们能平稳地，连续地调整流体使其压力不超过预定值。泄压阀是常闭阀，当压力没有达到预定值，阀是关闭的。随着系统压力的上升，通过阀的卸荷流量逐渐增加直到泵的全部输出流量通过该阀卸荷。当系统压力下降时，该阀平稳而缓慢地关闭。泄压阀以直动式或者先导式的形式被应运。此外，一些电子控制运行的卸荷阀和伺服系统控制系统压力相似，能在很宽的电子信号范围内调节系统的压力。流量阀流量阀通过节流和旁流来控制流体流量。节流就是控制节流面积直到所有的流量都不能通过该节流阀；旁流就是控制支路流