外文翻译--液压系统和气压系统

中文3467字

附录:

Hydraulic system and Peumatic System

Hui-xiong wan1,Jun Fan2

The history of hydraulic power is a long one, dating from man’s prehistoric efforts to harness the energy in the world around him. The only source readily available were the water and the wind—two free and moving streams.

The watermill, the first hydraulic motor, was an early invention. One is pictured on a mosatic at the Great Palace in Byzantium, dating from the early fifth century. The mill had been built by the Romans. But the first record of a watermill goes back even further, to around 100BC, and the origins may indeed have been much earlier. The domestication of grain began some 5000 years before and some enterprising farmer is bound to have become tired of pounding or grinding the grain by hand. Perhaps, in fact, the inventor were some farmer’s wives. Since the often drew the heavy jobs.

Fluid is a substance which may flow; that is, its constituent particles may continuously change their positions relative to one another. Moreover, it offers no lasting resistance to the displacement, however great, of one layer over another. This means that, if the fluid is at rest, no shear force (that is a force tangential to the surface on which it acts) can exist in it.

Fluid may be classified as Newtonian or non--Newtonian. In Newtonian fluid there is a linear relation between the magnitude of applied shear stresses and the resulting rate of angular deformation. In non—Newtonian fluid there is a nonlinear relation between the magnitude of applied shear stress and the rate of angular deformation.

The flow of fluids may be classified in many ways, such as steady or non steady, rotational or irrotational, compressible or incompressible, and viscous or no viscous.

All hydraulic systems depend on Pascal’s law, such as steady or pipeexerts equal force on all of the surfaces of the container.

In actual hydraulic systems, Pascal’s law defines the basis of results which are obtained from the system. Thus, a pump moves the liquid in the system. The intake of the pump is connected to a liquid source, usually called the tank or reservoir. Atmospheric pressure, pressing on the liquid in the reservoir, forces the liquid into the pump. When the pump operates, it forces liquid from the tank into the discharge pipe

at a suitable pressure.

The flow of the pressurized liquid discharged by the pump is controlled by valves. Three control functions are used in most hydraulic systems: (1) control of the liquid pressure, （2）control of the liquid flow rate, and (3) control of the direction of flow of the liquid.

Hydraulic drives are used in preference to mechanical systems when(1) powers is to be transmitted between point too far apart for chains or belts; (2) high torque at low speed in required; (3) a very compact unit is needed; (4) a smooth transmission, free of vibration, is required;(5) easy control of speed and direction is necessary; and (6) output speed is varied steplessly.

Fig. 1 gives a diagrammatic presentation of the components of a hydraulic installation. Electrically driven oil pressure pumps establish an oil flow for energy transmission, which is fed to hydraulic motors or hydraulic cylinders, converting it into mechanical energy. The control of the oil flow is by means of valves. The pressurized oil flow produces linear or rotary mechanical motion. The kinetic energy of the oil flow is comparatively low, and therefore the term hydrostatic driver is sometimes used. There is little constructional difference between hydraulic motors and pumps. Any pump may be used as a motor. The quantity of oil flowing at any given time may be varied by means of regulating valves( as shown in Fig.7.1) or the use of variable-delivery pumps.

The application of hydraulic power to the operation of machine tools is by no means new, though its adoption on such a wide scale as exists at present is comparatively recent. It was in fact in development of the modern self-contained pump unit that stimulated the growth of this form of machine tool operation.

Hydraulic machine tool drive offers a great many advantages. One of them is that it can give infinitely-variable speed control over wide ranges. In addition, they can change the direction of drive as easily as they can vary the speed. As in many other types of machine, many complex mechanical linkages can be simplified or even wholly eliminated by the use of hydraulics.

The flexibility and resilience of hydraulic power is another great virtue of this form of drive. Apart from the smoothness of operation thus obtained, a great improvement is usually found in the surface finish on the work and the tool can make heavier cuts without detriment and will last considerably longer without regrinding.

Hydraulic and pneumatic system

There are only three basic methods of transmitting

power:electrical,mechanical,and fluid power.Most applications actually use a combination of the three methods to obtain the most efficient overall system. To properly determine which principle method to use,it is important to know the salient features of each type. For example, fluid systems can transmit power more economically over greater distances than can mechanical types. However, fluid systems are restricted to shorter distances than are electrical systems.

Hydraulic power transmission system are concerned with the generation, modelation, and control of pressure and flow，and in general such systems include:

1.Pumps which convert available power from the prime mover to hydraulic power at the actuator.

2.Valves which control the direction of pump-flow, the level of power produced, and the amount of fluid-flow to the actuators. The power level is determined by controlling both the flow and pressure level.

3.Actcators which convert hydtaulic power to usable mechanical power output at the point required.

4.The medium, which is a liquid, provides rigid transmission and control as well as lubrication of componts, sealing in valves, and cooling of the system.

5.Conncetots which link the various system components, provide power conductors for the fluid under pressure, and fluid flow return to tank(reservoir).

6.Fluid storage and conditioning equipment which ensure sufficient quality and quantity as well as cooling of the fluid.

Hydraulic systems are used in industrial applications such as stamping presses, steel mills, and general manufacturing, agricultural machines, mining industry, aviation, space technology, deep-sea exploration, transportion, marine technology, and offshore gas and petroleum exploration. In short, very few people get through a day of their lives without somehow benefiting from the technology of hydraulicks.

The secret of hydraulic system’s success and widespread use is its versatility and manageability. Fluid power is not hindered by the geometry of the machine as is the case in mechanical systems. Also, power can be transmitted in almost limitless quantities because fluid systems are not so limited by the physical limitations of materials as are the electrical systems. For example, the performance of an electromangnet is limited by the saturation limit of steel. On the other hand, the power limit of fluid systems is limited only by the strength capacity of the material.

Industry is going to depend more and more on automation in order to increase productivity. This includes remote and direct control of production operations,

manufacturing processes, and materials handling. Fluid power is the muscle of automation because of advantages in the following four major categories.

1.Ease and accuracy of control. By the use of simple levers and push buttons, the operator of a fluid power system can readily start, stop, speed up or slow down, and position forces which provide any desired horsepower with tolerances as precise as one ten-thousandth of an inch.

2.Multiplication of force. A fluid power system(without using cumbersome gears, pulleys, and levers) can multiply forces simply and efficiently from a fraction of an ounce to several hundred tons of output.

3.Constant force or torque. Only fluid power systems are capable of providing contant force or torque regardless of speed changes. This is accomplished whether the work output moves a few inches per hour, several hundred inches per minute, a few revolutions per hour, or thousands of revolutions per minute.

4.Simplicity, safely, economy. In general, fluid power systems use fewer moving parts than comparable mechanical or electrical systems. Thus, they are simpler to maintain and operate. This, in turn, maximizes safety, companctness, and reliability. For example, a new power steering control designed has made all other kinds of power systems obsolete on many off-highway vehicles. The steering unit consists of a manually operated directional control valve and meter in a single body. Because the steering unit is fully fluid-linked, mechanical linkages, universal joints, bearings, reduction gears, etc, are eliminated. This provides a simple, compact system. In addition, very little input torque is required to produce the control needed for the toughest applications. This is important where limitations of control space require a small steering wheel and it becomes necessary to reduce operatot\r fatique.

Additonal benefits of fluid power systems include instantly reversible motion, automatic protection against overloads, and infinitely variable speed control. Fluid power systems also have the highest horsepower per weight ratio of any known power source. In spite of all these highly desirable features of fluid power, it is not a panacea for all power transmission problems. Hydraulic systems also have some drawbacks. Hydraulic oils are messy, and leakage is impossible to completely eliminate. Also, most hydraulic oils can cause fires if an oils occurs in an area of hot equipment.

Peumatic System

Pneumatic systems use pressurized gases to tansmit and control power. A s the name implies, pneumatic systems typically use air(rather than some other gas) as the fluid medium because air is a safe, low-cost, and readily available fluid. It is

particularly safe in environments where an electrical spark could ignite leaks from system components.

In pneumatic systems ,compressors are used to compress and supply the necessary quantities of air. Compressors are typically of the piston, vane or screw type. Basically a compressor increases the pressure of a gas by reducing its volume as described by the perfect gas laws.Pneumatic systems normally use a large centralized air compressor which is considered to be an infinite air source similar to an electrical system where you merely plug into an electrical outlut for electricity. In this way, pressurized air can be piped from one source to various locations throughout an entire industrial plant. The air then flows through a pressue regulator which redeces the pressure to the desired level for the particular circuit application. Because air is not a good lubircant（contains about 20% oxygen）, pneumatics systems required a lubricator to inject a very fine mist of oil into the air discharging from the pressure regulator. This prevents wear of the closely fitting moving parts of pneumatic components.

Free air from the atmosphere contains varying amounts of moisure. This moisure can be harmful in that it can wash away lubricants and thus cause excessive wear and corrosion. Hence ,in some applications ,air driers are needed to remove this undesirable moisture. Since pneumatics systems exhaust directly into the atmosphere, they are capable of generating excessive noise. Therefore, mufflers are mounted on exhaust ports of air valves and actuators to reduce noise and prevent operating personnel from injury resulting not only from exposure to noise but also from high-speed airborne particles.

There are several reasons for considering the use of pneumatic systems instead of hydraulic systems. Liquids exhibit greater inertia than do gases. Therefore, in hydraulic systems the weight of oil is a potential problem when accelerating and decelerating actuators and when suddenly opening and closing valves. Due to Newton’s law of motion(force equals mass multiplied by acceleration), the force required to accelerate oil is many times greater than that required to accelerate an equal volume of air. Liquids also exhibit greater viscosity than do gases. This results in larger frictional pressure and power losses. Also ,since hydraulic systems use a fluid foreign to the atmosphere, they require special reservoirs and noleak system designs. Pneumatic system use air which is exhausted directly back into the surrounding environment. Generally speaking, pneumatic systems are less expensive than hydraulic systems.

However, because of the compressibility of air, it is impossible to obtain precise controlled actuator velocities with pneumatic systems. Also, precise positioning control is not obtainable. While pneumatics pressures are quite low due to compressor design limitations(less than 250 psi), hydraulic pressures can be as high as 10000 psi. Thus, hydraulics can be high-power systems, whereas pneumatics are confined to low-power applications. Industrial applications of pneumatics systems are growing at a rapid pace. Typical examples include stamping, drilling, hoist, punching, clamping, assembling, riveting, materials handling, and logic controlling operations.

液压系统和气压系统

万辉雄1，范军2

流体和液压系统

水力的历史由来已久，始于人类为利用它周围的能源而做出的努力。容易利用的能源就是水和风——两种自由的流动流体。

第一台液力装置水车是最早的发明。从15世纪早期，水车图画就出现在大宫殿的马赛克上。磨粉机由罗马人发明，而水磨机的历史更早，可以追溯到大约公元前100年。当一些上进的农场主厌恶由手工冲击、研磨谷物时，谷物的家庭养殖已开始5000多年。也许，真正的发明家是那些农场主的妻子，因为她们经常要干重的农活。

流体是可以流动的物体，与就是说，构成物质的粒子可以连续地改变它们之间的相对位置，而且，它提供流体层间流动非连续的阻力。这意味着流体在静止时，在其内部没有剪切力（作用表面切向方向的受力）存在。

流体可以分为牛顿流体或非牛顿流体。在牛顿流体中，流体层间作用的剪切力和角度变形总量的大小成线性关系。在非牛顿流体中，流体层间作用的剪切力和角度变形总量的大小成非线性关系。

流体的流动可按多种方式分类，如定常或非定常流、有旋流或无旋流、可压缩或不可压缩流以及黏性流或无黏性流。

所有的液压系统遵守与帕斯卡定律，命名是由帕斯卡而来的，是他发明了此定律。这条定律指出在密封容积内压缩的液体——例如圆柱筒或管子——在容积的各个不同面上作用着相等的力。

在实际液压系统中，帕斯卡定律是解释从系统中获得的各种结果的基础。因此，泵使液体在系统中流动，泵的进口连接液流源，经常叫油槽或油箱。作用在油箱液面上的气压使流体进入油泵。当油泵工作是，在适当的压力作用下，油泵迫使流体从油箱流动到出口。

由油泵泵出的压缩液体通过各种阀门来控制。在大多数液压系统中采用3种控制功能：（1）液体压力的控制（2）液体流速的控制（3）液体流动方向的控制

当处于下列几种情况时，液压驱动被优先使用，（1）对于链传动和皮带传动来说，功率的传递位置太远：（2）低速高转矩的场合（3）很紧凑的结构（4）要求传动平稳、避免振动的场合（5）速度和方向容易调节的场合（6）输出速度无级可调的情况。

由电气驱动的油泵供有传递能量的油量，并可传递给液压电动机或油缸，从而将液压能转换成机械能。通过阀门控制油的流动，压力油流产生线性的或旋转

的机械运动。油流的动能相对比较低，因此有时采用静压传动。液压电动机和液压油缸之间几乎不存在构造上的不同。任一油泵可以被用作液压电动机。在任一时间里的油流量可以通过调节阀门或采用变量泵来改变。

液压传动运用到机床的运行中绝不是新的，虽说现在的大规模采用出现不久。现代油泵的发展促进了这类机床的增多。

机床的液压驱动具有许多优点。其中一个是液压驱动在广泛的范围内提供无限变化的速度。另外，它们能像改变速度一样容易来改变驱动的方向。像许多其他类型的机床一样，许多复杂的机械装置能够被简单化或者由于液压驱动的使用完全取消。

液压驱动的另一个优点是它的柔性和缓冲性。除了运行平稳外，还发现了许多改进，如工件表面光洁度的改善，在不损坏刀具的前提下能加大刀具的负荷，并能在刃磨刀具的情况下工作更长时间。

液压与气压系统

仅有以下三种基本方法传递动力：电气、机械和物流。大多数应用系统实际上是将三种方法组合起来而得到最有效的最全面的系统。为了合理地确定采取哪些方法，重要的是了解各种方法的显著特征。例如液压系统在长距离上比机械系统更能经济地传递动力。然而液压系统与电气相比，传递动力的距离较短。

液压动力传递系统涉及电动机、调节装置和压力和流量控制，总的来说，该系统包括：泵：将原动机的能力转换成作用在执行部件上的液压能。阀：控制泵产生流体的运动方向、产生的功率的大小，以及到达执行部件液体的流量。功率大小取决于对流量和压力大小的控制。

执行部件：将液压能转换成可用的机械能。

介质即油液：可进行无压缩传递和控制，同时可以润滑部件，使阀体密封和系统冷却。

联接件：联接各个系统部件，为压力流体提供功率传输通路，将液体返回油箱。油液储存和调节装置：用来确保提供足够质量和数量并冷却的液体。

液压系统在工业中应用广泛，例如冲压、钢类工件的磨削及一般加工业、农业、矿业、航天技术、深海勘探、运输、海洋技术，近海天然气和石油勘探等行业，简而言之，在日常生活中很少有人不从液压技术得到某些益处。

液压系统成功而又广泛使用的秘密在于它的通用性和易操作性。液压动力传递不会像机械系统那样受到机器几何形体的制约，另外，液压系统不会像电气系统那样受到材料物理性能的制约，它对传递功率几乎没有量的限制。例如，一个电磁体的性能受到钢的磁炮和极限的限制，相反，液压系统的功率仅仅受材料强度的限制。

企业为了提高生产率将越来越依靠自动化，这包括远程和直接控制生产操作、

加工过程和材料处理等。液压动力之所以成为自动化的重要组成部分，是因为有如下主要的四种优点：

1.控制方便精确通过操作一个简单的操纵杆和按钮，液压系统的操作者便

能立即起动、停止、加减速和能提供任意功率、位置精确为万分之一英寸的位置控制力。

2.増力一个液压系统（没有使用笨重的齿轮、滑轮和杠杆）能简单有效地将

不到一盎司的力放到长生几百吨力的输出。

3.恒力或横扭矩只有液压系统能提供不到速度变化而变化的恒力或横扭矩，

它可以驱动对象从每小时移动几英寸到每分钟几百英寸，从每小时几转到每分钟几千转。

4.简单、安全、经济总的来说，液压系统比机械或电气系统使用更少的运动

部件，因此，它们运行与维护简单。它使得系统结构紧凑，安全可靠。例如一种用于车辆上的新型动力转向控制装置已淘汰其他类型的转向动力装置，该转向部件中包含有人力操纵方向控制阀和分配器。因为转向部件是全液压的，没有万向节、轴承、减速齿轮等机械连接，这使得系统简单紧凑。

另外，只需输入很小的扭矩就能产生满足及恶劣工作条件所需的控制力，这对于因操作空间限制而需要很小方向盘的场合很重要，这也是减轻司机疲劳度所必需的。

液压系统的其他优点包括双向运动、过载保护和无级变速控制，在已有的任何动力系统中液压系统具有最大的单位质量功率比。

尽管液压系统具有如此高性能，但它不是可以解决所有动力传递问题的灵丹妙药。液压系统也有些缺点，液压油又污染，比且泄流不可能完全避免，另外如果油液渗漏发生在灼热设备附近，大多数液压油能引起火灾。

气压系统

气压系统是用压力气体传递和控制动力，正如名称所表明的那样，气压系统通常用空气（不用其他气体）作为楼梯介质，因为空气是安全、成本低而又随处可得的流体，在系统部件中产生电弧又可能点燃泄漏物的场合下（使用空气作为介质）尤其安全。

在气压系统中，压缩机用来压缩并提供所需的孔子。压缩机一般又活塞式、叶片式和螺旋式等类型。压缩机基本上是根据理想气体法则，通过减小气体体积来增加气体压力的。气压系统通常考虑采用大的中央空气压缩机作为一个无限量的气源，这类似于电力系统中只要将插头插入插座便可或得电能。用这种方法，压力气体可以从气源输送到整个工厂的各个角落，压力气体可通过空气滤清器出去污物，这些污物可能会损坏气动组件的精密配合部件如阀和气缸等，随后输送

到各个回路中，接着空气流经减压阀以减少气压值适合某一回路使用。因为空气不是好的润滑剂（包括20%的氧气），气压系统需要一个油雾器将细小的油雾注射到经过减压阀减压的空气中，这有助于减少气动组件精密配合运动件的磨损。

由于来自大气的空气含有不同数量的水分，这些水分是有害的，它可以带走润滑剂引起过分磨损和腐蚀，因此，在一些试用场合中，要用空气干燥器来除去这些有害的水分。由于气压系统直接向大气排气，会产生大噪声，因此可在气阀和执行组件排气口安装消声器来降低噪声，以防止操作人员因接触噪声及高速空气粒子又可能引发的伤害。

用气动系统代替液压系统有以下几条理由：液体的惯性远比气体大，因此，在液压系统中，当执行组件加速减速和阀突然开启关闭时，油液的质量便是一个潜在的问题，根据牛顿运动定律（力等于质量乘以加速度），产生加速运动油液所需的力要比加速同等体积空气所需的力高出许多倍。液体比气体具有更大的粘性，这会因为内摩擦而引起更大的压力和功率损失：另外，由于液压系统使用的液体要与大气隔绝，故它们需要特殊的油箱和无泄露系统设计。气压系统使用可以直接排到周围环境中的空气，一般来说气压系统没有液体系统昂贵。

然而，由于空气的可压缩性，使得气压系统执行组件不可能得到精确的速度控制和位置控制。气压系统由于压缩机局限，其系统压力相当低（低于250psi），而液压力可达10000psi之高，因此液压系统可以是大功率系统，而气动系统仅用于小功率系统，典型例子有冲压、钻孔、提升、冲孔、夹紧、组装、铆接、材料处理和逻辑控制操作等。