文献翻译-液压传动基础

1附录 A液压传动基础1. 流体静力学大多数的液压系统在其性质上都可以看作是流体静力学的简单应用，利用压力能来传递能量。所以这样系统的性能就不会受到液体特征太大的影响，除了液体的压缩特性。一旦在系统中引入一定数量的运动液体，液体摩擦力、液体惯性和其它的一些速度元件就会变的非常的重要在一个正在运行的系统中。有一些是受到了流体静力，而其它又受到流体动力学的作用。该系统的性能就会受到其它的一些特征的影响，较高的液体流速是控制系统压力损失的最主要的因素。大体上，高流速系统之中，也可能根本就是一个流体动力学状态，尽管基于液体静力学的原则。在这样的情况下，基于流体静力学的对于该系统的合理的分析可能会高估可能实现的工作性能。流体静力学最基本的一个基本的法则是液体之中的压力随着液体的距液体表面的深度的增加会增加，对于一个恒定的质量的液体。 Whp式中 W 为液体的重量，H 为该液体的深度。如果对于一个特殊的位置来说，由位置定义的头叫做位置水头，也叫比位能，所谓的比位能叫做比压能，又叫压力水头。对于所有的静止的液体来说，比压能是一个恒定的量。这是伯努力方程在速度趋于零时的特殊情况。事实上流体动力学的方程应用于液体力学时，将所有涉及到液体速度的项的值带入零。相反的在应用液体传递力的过程中应用到流体力学的一些原则时，必须考虑到流体的速度，尽管在实际中这一点小到可以忽略不考虑，这种情况大多数是在流速很小的时候、在压力传递中液体是一个被动的因素，或是在液体的流动对于该系统的性能产生的影响并不是很大的时候。液体中的静力中液体中的任何一点任何一个方向都是相等的。作用在任何一个浸在水中的表面上的力都是在大气压力和液体压力的共同作用的合力。同理，作用于一个封闭的系统当中的液体上的力会等值的传递到该系统当中的任何一处，而垂直作用于与该液体相互接触的所有的表面。对于实际中的液体静力学应用中，液体的重量可以被忽略，比位能与工作的压力来说是非常小的。这些方程定义了液压增力系统的最基本的性能，2在此液体的可压缩性能可以忽略不计。并不是所有的这样的系统都必须是对力进行放大。它可同样的应用于进行高效的力的传递。比较典型的就是汽车的制动系统中的应用，这里就采用了从动缸的直径小于主动缸的直径。2. 液压泵的选择作为液压伺服系统的液压泵的选择通常是建立在一定的惯例实践基础之上的，主要是因为某一类型的泵以经逐渐的发展成为具有其自身专门的用途。无论是哪一种类型，影响其选择的主要的参数有：工作压力、需要的能力（流速和功率），效率、控制、速度、液体、噪音、成本、维护、配件和检修等。一般情况下，高压可以通过安装有固定阀的泵获得，尽管在设计的细节上的改进可以在某种程度上克服回转阀和液口的固有的限制。在这里，在最大流量下的可以获得的额定功率是给定的。可以预料到重压下和相似的应用中所需要的非常高的压力似乎只有带有固定阀的多活塞的泵才合适当需要提供持续的高压时也是如此。在低压的系统中泵的选择的范围扩大了。在一般的工业液压系统之中，系统的压力可以在 30bar 至 50bar 到 100bar 范围之内变动。事实上可以选择任何一种正排量的泵，因此泵类型的选择就取决于其它的一些规格要求了，或者是功率。然而，高压系统越来越常见，因此 140bar 也可以被认为是工业的级别。这已经超越了一般的叶片泵的极限了，除非它们相互配合成一个两级的单元。这已经成为了将来一种很明显的趋势了，这种总的趋势，逐渐的提高系统的压力为实现更调质工作压力，使需要的性能已经超越了基本简单型式的泵的能力了。例如拿正排量泵来说，所产生的压力完全取决于所承受的负载，并且任何这样一种型式的泵在稳定运行的条件下也不会产生高于它所接入的系统所给予的阻力。压力和流量也与泵功率的决定有直接的关系。正排量泵的流量能力的大小直接与该泵的工作速度和泵的排量是成比例关系的。每转的排量乘以其转速就等于理论流量 。对于一个给定大小的泵来说，最大流量由其泵所能达到的最大转速来决定的。而这又要从机械方面来考虑。流量在控制系统之中的执行机构的工作速度是相当重要。输出轴功率就会随着负载和速度而产生。尽管其会方便的以力和流量的形式来提供或以当量功率提供。如此建立起来的功率给予了泵工作能力，关于总效率，则是决定所需要的输入功率。从机械方面来考虑也限制了其某种类型泵所具有的最大实际功率。这显示了常见的一些类型泵的典型最大功率。3一个正排量泵的理论功率曲线如图所示。压力的大小由外负载来决定，最大值由泵自身的机械强度来设定。最大流量也是建立在最大允许转速时。最大功率于是也就在最大压力和流量时而产生。相应的点在曲线上就叫做转角功率。事实上轮廓并不是直角的，由于在随着负载的增加，其泄漏也会增加。从而使轮廓形成如图的形状。泄漏值的大小在很大程度上取决于泵加工的精度可能低于 3%或 4%在最大压力的作用下，泄漏的影响当然就是减少总效率。泵在最大流量和最大压力作用下的持续工作的能力完全可以由轴功率曲线的形状来确定。它们可以封闭线内任何一处运行，直接以转角轴功率运行，前提就是负载是稳定的。任何的工作循环之中都有可能出现瞬间的过载现象，而这也是不可忽略的。否则，短暂的过载会使处于其轴功率超出其额定许用功率和最大功率的范围。通常在这种情况下的就会把这个封装曲线 画成一条恒定的直线。这允许瞬间的过载因为其在转角轴功率以下，也就确定了过载安全极限的限制了。在恒定轴功率曲线上的瞬时流量相应的被决定了，驱动的过载也就被决定了。3. 液压马达液压马达和液压泵在本质上具有相似的元件，而且大多数的现代泵都可以很好的作为马达来使用。只是那些具有较低的效率或者具有球或与球相似的阀的泵不可以当作马达来使用。液压马达所具有的一些具体的特点及优点：（a） 结构紧凑与同样轴功率的电动机相比其尺寸要小的多。（b） 低的惯性具有准确和快速速度控制和变速。（c） 较宽的速度范围和简单正速度控制。（d） 全功率情况下停转，而不造成马达的损坏（e） 半弹性的联接而不是刚性的联接，并有一定的泄漏量。（f） 在需要的时候能以高轴功率输出。（g） 具有较高的能量转换效率。（h） 简化和正的控制。（i） 较低的磨损，因为通常马达是在全润滑的条件下运行，润滑充分。在很多的情况下液压马达都可以代替电动机直接地做为简单的动力驱动源，并且具有通过阀来很简单的实现对速度的控制。同样的也可以实现无级变速液压驱动。在此将泵和马达连在同一个回路中，这样就能实现泵的输出循环，泵是一个可变流量的，因此4其流量就可以从 0 至最大 通过杠杆的运动。马达的速度与泵流量是成正比的。于是马达自身也就是无级变速从零至最大，同时在两个方向都可以实现，如果需要的话，这样马达的速度也就直接与杠杆的运动成比例了，也就是一种真正的单杠杆的控制了。在此变速过程中没有应用到离合器和齿轮，如果马达转速过快，它功能转化为与泵相似，任何过多的压力都是无效的在溢流阀的存在条件下.相反的，如果由于过载而停转，马达会保持无限期的停车而不造成任何的伤害，泵也相应的处于泄荷的状态，以避免液体出现过热的危害。液压马达可以大致的分为以下的几类：（a） 大扭矩马达（b） 大力矩马达（c） 中扭矩及中速马达（d） 高速马达大扭矩马达和大力矩马达有些相似，在设计上都从起动开始就提供实际最大扭矩。然而，大气知马达通常是低速马达，有时候只有一转，或者更少的转数。大力矩马达通常被设计为具有较高转速的马达。高速马达，另一方面来说，通常是低扭矩的马达。实际的特性在很大程度上取决于马达的设计和工作准则。齿轮马达可以是外齿也可以是内齿的，外齿马达比较简单，并且使用很广泛以完成一些简单的工作。在一个壳体内包括两个相互啮合的齿轮，输出轴从其中一个齿轮延伸出来。齿轮形式的马达与齿轮泵是完全相同的，通常是正变位齿轮。马达较高的容积效率的获得就使得要额外的考虑到马达的设计和结构如何来减少内部的泄漏，有些时候为了获得较高的容积效率而失去一些机械效率，但是这在齿轮泵的设计中比齿轮马达的设计中更合适。理想的状态下为了马达启动更容易一些，在运转中的摩擦 更小一些这通常需要齿轮所受的压力是平衡的程度。值得注意的是在传统的应用在齿轮的压力平衡的方法上，来减少内部的泄漏，通常是单向的运转，可能并不适用于有固定侧向间隙的马达身上，而这又会增加加工的而带来的费用，主要是因为需要较大的精度。较便宜的齿轮马达，因此，可能在总效率，最大流量的压力上会有一定的限制，在没有产生过度的载荷和摩擦状态下。另外一个设计良好，精确的齿轮马达的转速可以达到 5000rpm 至 10000rpm.可以在 210bar 的压力下工作。内齿轮马达通常仅限应用于低功率的情况下，但可以提供一些独特的优点。内齿轮驱动外齿轮 和轴的旋转，典型的结构包括了一般比外齿轮少一个齿的具有平等中心的两5个齿轮。结果就使轴所产生的转速高于两个齿轮的转速，所以两个齿轮的相对转速较低的情况下，可以获得轴的较高的转速。另外一个可供选择的结构就是叶片转子形式，内外齿特别的加工成特殊的形状来提供一个密封的空间以与叶片转子相同的形式。这压力油直接作用在马达上进入到马达中通过适当的液口和换向阀内齿轮因此就会相对于静止的外齿旋转，各式各样的形状，轮廓都是可能的 configuration，derives 机械输出可以由相互啮合的齿轮通常被 布置来提供驱动力的减小，这促内齿轮 reduction 通常被用于低速运动。4. 液压缸，千斤顶和柱塞液压缸，千斤顶和柱塞这三个术语可以被认为是相似的，第一个可以可以是一个一般性的描述。用千斤顶来描述一般是在被用来举物品的时候，同样在工业中的一个特别领域最通用的用途就是做为一个线性的执行机构来实现千斤顶类似的动作。用柱塞来描述则表示是应用在重型设备或者是输出的力非常的大其它的一些机构也指定柱塞是一种活塞与活塞杆的直径一样的地方就这样称，尽管更正确的叫法应该是柱塞式油缸容积式缸，这样一种通常是单作用的并有一定的使用限制。液压缸可以是单缸也可以是双缸的，在单作用液压缸时，缩回动作可以在减少缸内的压力，并在弹簧或是外力的作用下来实现。在弹簧的情况下，应该考虑到液压的阻力再加上弹簧的阻力。双作用液压缸是目前在一般的场合应用较普遍的。进出液口被安装在缸体的两端，轮流的作为进液口和排液口，通过一个换向阀的转换来实现。最大输出压力要比同样的单作用的液压缸小一些，当缸体伸出的时候，液体全部作用在整个活塞的表面上，在出液口的一端就会产生一定的力，密封圈需要来防止泄漏当活塞在压力作用下向相反的方向运动，相应就带来的了一定的摩擦阻力。在向相反方向运动的时候 也就是回缩的时候可以得到的力要小一些，由于作用的面积只是活塞也杆的截面积的差值，反力作用同样存在，这样的性能损失可能是很小的，但是对实际中的理论上的性能更改很大，通常在理论性能之后同时也标注上所允许的摩擦损失。多数的液压缸都是单作用的。如果需要额外的刚度，通学采用双头活塞杆，在双作用的液压缸中，推力和缩回的力是相等的。在相同的直径条件下，双作用的液压缸能够提供的推力要明显小于单作用的液压缸，由于活塞杆的面积的损失，密封圈的摩擦损失也要高，因为在其两端都有，而单作用的液压缸只是一端有。6液压缸被广泛的应用于工业的液压系统中，这些液压缸又通常被称为直线型或是互换型的马达。液压缸的组有一个圆形的管子，在两端的密封作用的装置，在管子中活塞和杆可以来回的运动。活塞被设计在缸的两端或在缸的一端。液体在缸内沿活塞的泄漏通过一个设计合适的密封圈来解决。液压缸将压力油传输到推或拉的活塞杆腔内。液压缸被设计用来多种用途，在这里要学习的是各种液压缸的类型及他们的工作原理，液压缸的一些知识在学习工业液压的用途中很有帮助的。7附录 BThe basic of hydraulic power transmission1. HydrostaticsThe majority of simple hydraulic systems are hydrostatic in character, transmitting power by energy. The performance of such systems is thus largely unaffected by fluid characteristics other than compressibility. Once apprecialbe fluid motion is introduced into the system ,fluid frictin ,inertia and other velocity components become increasingly significant as the system is operating , partly as a hydrostatic one and partly as a hydrodynamic one . Performance is then affected by other fluid characteristics ,notably fluid velocity which is a primary factor governing the loss of pressure energy .In general,high flow rate systems may be essentially hydrodynamic in character,although operating on sydrostatic principles. In such caaes suuitable analysis on a hydrostatic basis will overestimate performance likely to be achieved.A basic law of hydrostatics is that pressure (p)in a fluid at rest will increase with increasing depth below the surface ,or ,for a fluid of constant specific weight(W)WhpWhere h depth of “head” of fluid If referred to a specific datum Fig.3-1 the head defined by position is called the potential head and the total head (repersented by h+z)as the piezometric head .Thus ，the piezometric head is a constant for all fluids at rest . this is special case of the bemoulli equation where the velocity terms are zero . In fact ,hydrodynamic equations are directly applicable to fluid statics by rendering all the terms involving fluid must take velocity into account if fluid movement is involved ,although in practice this may be small enough to be negligible. This applies generally when flow rates are small ,fluid performs a “passive”role in the transmissio of pressure and the small amount of fluid movement involved has no significant effect on the performance of the system. 8The pressure at any point in a static fluid is the same in every direction. The pressure exerted on any surface immersed on a fluid in a closed system is transmitted equally to all parts of the system and acts perpendicularly to all surfaces in contact with the fluid. For practical hydrostatic application the weight of fluid can be ignored since the potential head involved is very small compared with the applied pressure .These equations define the performance of a basic hydrostatic system of force multiplication over pressure ranges where the compressibility of the fluid is negligible. Not all such systems are necessarily forece multipliers,however. They are equally suitable for force transfer with high efficiency. Thus the typical autombile brake system may,in fact ,employ slave cylinders of smaller diameter than the master cylinder.2. Pump selection Choice of pump type for hydraulic servicesd is lften based on traditional practice ,largely because certain types of pump have been developed for specific duties. Regardless of type ,however, the main parameters affecting pump selection are :working pressure required ,capacity required(flow rate and power )efficiency ,control,speed,flued ,noise ,cost maintenance ,spares,and service .In general ,high pressures can only be achieved by pumps having seated valves ,although the inherent limitations of rotary valves or ports can be overcome to some extent by refinements in design detail. Here ,a somewhat nomial rating of pump types against the maximum pressuers they can be expected to deliver is given. It will be seen that for the very high pressures needed for heavy persses and similar applications,only multi-piston pumps with seated valves are likely to be suitable ,when pressuers of the order of 700bar can be achieved continuously.For lower system pressure the choice of pump type becomes much wider. Thus ,for general industrial hydraulics, where system pressuers may range from 30-35bar up to about 100bar ,virtually any type of positive sisplacement pump can be used ,selection of type then being based on other specific requirements ,or power rating. However ,higher system pressuers are becoming commonplace ,so that can also be considered an industrial rating;this is beyond the limit of simple vane pumps unless they are coupled as two-stageunits. This ,in fact, has been a noticeable trend for some time. The general tendency to uprate system pressures for higher working effciency has extended the performance requiements beyond the capabilities of certain 9basic (and simpler)types of pump,calling either for the use of alternative pumps or further development of such basic types to meet new pressure requiements. Either solution has generally resulted in a more effcient pump, although a more costly one .In the case of positive displacement pumps,the pressure developed is entirely dependent on the load and any such pump operating under steady conditions can not develop a pressure greater than the resistance offered by the system to which it is connected. Pressure ,and delivery are also directly related in determining the power of the pump .The capacity of delivery of a positive displacement pump is directly proportional to the speed and its displacement. Displacement per revolution multiplied by thespeed in rpm gives the theoretical delivery. For a given size of pump,therefore ,maximum delivery is governed by the maximum speed at which the pump can be run ,this being governed by mechanical consideration.The delivery is mainly significantin governing the speed of separation of the actuators in the system. Power output then follows as the product of the load and speed ,although it is more conveniently rendered in terms of pressure and flow or equivalent hydraulic horsepower.The power rating so established gives the “work capacity” of the pump and ,with reference to the over all efficiency,determines the input power required. Mechanical considerations limit the maximum power rating practical with certain types of pump. This shows typical maximum power rating for the more common types of pump .A theoretical power cureve for a positive displacement pump is shown in figure. The pressure level is determined by the external load ,the maximum value being set by the mechanical strength of the pump itself. Maximum flow is established by the maximum permissible running speed. Maximum power is thus developed at maximum pressure and delivery ,the corresponding point on the curve being called the corner horsepower.In practice the envelope is not quite rectangular,since increasing load will produce increasing slip ,modifying the envelope to the form shown in figure. The value of slip depends to a large extent on the precision with wich the pump is manufactured and may be as low as 3%-5% at maximum pressure. The effect of slip is ,of course,to reduce the overall efficiency achieved.Pumps capable of operating continuously at maximum pressure and maximum delivery can be fully defined for power rating by the form of curve shown in figure. They can obviously be operated anywhere within the envelope ,or right up to the corner horsepower ,provided the 10loading is steady.Many duty cycles will,however,involve transient loads ,in which case these must be taken into account ,otherwise an immediate load may place a demand on the pump beyond either its available horsepower line ,as in figure. This now allows for transient loads to be accepted within the corner horsepower,and also defines the safe limit for transient loads. The overload on the driver can also be determined,relative to any momentary displacements above the constant horsepower line. 3. Hydraulic motorsHydraulic pumps and motors sre essentially similar components and the majority of modern pumps will act equally well as motors. Only those with low mechanical efficiency,or with ball or similar valves, will not act as motors. Specific advantages offered by hydraulic motors are:(a) Compactness -very much smaller than an electric motor of the same power.(b) Low inertia capable of providing precise and rapid speed control and reversibility.(c) Wide ranges of speeds with simple positive speed control .(d) The capability of being stalled under full power without damage.(e) Semi-elastic rather than rigid coupling ,with a “slip” capacity.(f) High energy conversion efficiency.(g) High power outputs if required.(h) Simple and positive control.(i) Low wear since the notor always operates under fully lubricated conditions.In many applications the hydraulic notor can offer a direct alternative to an electric motor for simple power drives,with the simple speed control through valves. Equally ,the infinitely variable speed hydrostatic drive . Here pump and motor are connected in the same circuit so that the pump output is recirculated. The pump used is of the varialbe delivery type, so that its outputcan be varied from zero to maximum, by the movement of a lever. Motor speed is proportional to pump delivery, and so itself is infinitely variable from zero to maximum, in both directions if required. Motor speed is therefore directly proportional to lever movement ,giving true single lever control. No clutch or gear changing is involved ,and even a clutch is unnecessary as , if the motor over runs ,it acts as a pump and any excess pressure is dealt harm, 11provided the pump is suitably relieved to avoid overheating the fluid .hydraulic motors may be broadly classified under:(a) High torque motors.(b) High moment motors (c) Medium torque /meduim speed motors (d) High speed motors High torque and high moment motors are similar in that both are designed to provide virtually maximum torque from starting up. However, high torque motors are usually low speed types ,sometimes operating down to 1rev/min or less. High moment motors sre generally designed for much higher working speeds. High speed motors, on the other hand, are usually low torque motors. Actual characteristics are largely governed by the design and working principles of the motor.Gear motors can be of either external or internal configuration. The external gear motor is the simpler, and the most widely used for simple duties, comprising two intermeshing gears contained within a suitable housing, the output shaft extending from one of the gears. Gear form is identical with that used for gear pumps.ie usually spur gears.The achievement of high volumetric efficiencies places a premium on both desigh and construction, to minimise internal leakage. Sometimes some mechanical efficiency is sacrificed in order to improve volumetric efficiency, but this is more applicable to a gear pump design than a gear motor design. Ideally ,for easy starting up and smooth running friction should be kept to a minimum with a gear motor and this normally calls for some degree of pressure balanching of the gears. It should be noted that the conventional method of pressure balanching applied to gear pumps to minimise internal leakage normally favours uni-directional running and may not always be applicable in the case of a motor with fixed side clearances involves a manufacturing cost penalty because of the greater percision requried. Low cost gear motors, therefore, may have certain limitations as regards overall efficiency and the maximum fluid pressures they can accept withou generating excessive bearing loads and friction. On the other hand a well designed and precisely made gear motor can run at speeds of 5000 to 10000rev/min and be capable of working at pressuers up to 210bar(3000lb/in2).Internal gear motors are usually limited to liw power applications,but can offer specific 12advantages. The internal gear drives round the external gear and shaft, typically comprising one tooth less than the external gear, with offset centres. The efect is to produce a shaft speed higher than the rotational speeds of the two gears ,so that a relatively high shaft speed can be achieved with relatively low gear to gear sliding speeds.An alternative construction is the lobe-motor form, where internal and external gear “teeth” are specially shaped to provide sealed “pockets” of fluid in the same manner as a lobe-rotor pump. The pressurised fluid fed to the motor acts directly on the exposed internal gear “teeth”via appropriate porting or a distributor valve. The inner gear is thus caused to rotate relative to the stationary outer gear. Various configuration are possible and the mechanical output can be derived through intermediate gearing, usually arranged to provide a drive reducton. Internal gears of this type are normally used for low speed motors . 4. Cylinders,jacks and ramsThe terms cylinder,jack and ram can be considered synonymous,the first being the general description. The description “jack”is commonly used for cylinders employed as lifting devices, and also in specific industries where the most common application of a linear actuator is to provide a “jacking” action. The description “ram” as a cylinder in which the piston and rod are of the same diameter,although this is more correctly called a plunger-type cylinder,or displacement cylinder. Such types are usually single acting and have relatively limited applicati