液压系统简介外文文献翻译、中英文翻译、外文翻译
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附录一：液压系统简介任何可以自然流动或可以被强制流动的介质（液体或气体）都可以被用来在流体动力系统中传递能量。 所使用的最早的流体是水，因此最早的液压系统被称作液体系统。 在现代术语中，液压系统是使用矿物油的回路。 图1-1展示了一个液压系统基本动力单元。（需要注意的是，在90年代末水做的东西卷土重来，现代一些流体动力系统甚至在海水中运行）另一种在动力系统中常见中的流体是压缩空气。 正如图1-2，大气-被压缩了710倍-是现成的并且很容易流过管道，管件或软管去传输用来工作的动力。 其它气体，如氮气或氩气，可以使用，但使用它们的成本十分昂贵。图1-1：基本液压动力单元。图1-2：基本气动布局。机械、电气和流体者三种主要传递能量的方法中，流体是一般行业中被了解最少的。 通常情况下，一般力学认为最初是由流体动力分销商的销售人员设计的流体动力回路。 在大多数的设施中，流体动力系统的责任是机械工程师的描述部分。 随着流体动力培训和足够多的工作来处理，工程师往往取决于流体动力分配器的专业知识。 为了得到订单，经销商销售人员很乐意为设计回路， 并经常协助安装和启动。 这样的安排还算不错，但由于其他技术的进步，流体动力的很多功能被消减了。流体动力缸和电机结构紧凑，具有高能量的潜力。他们是适合在狭小不杂乱的空间使用的机器。这些设备可以被停止更长的时间内， 都是瞬间可逆的，具有无极变速，而且往往以低得多的成本取代机械联系。 有了良好的回路设计，电源，阀门和执行器将只需很少的维护，就可以延长时间运行。 主要缺点是缺少的设备，差的回路设计，这会导致过热和泄漏。 当机器使用较少的能量比功率单元提供发生过热。（过热通常是容易设计出一个回路。） 控制泄漏是采用直螺纹 O 型圈接头，使管道连接或更大尺寸的管道软管及 SAE 法兰接头的问题。 在设计回路的最小的冲击和低温运行也降低了泄漏。在液压或气动装置之间进行选择的钢瓶使用的一般规则是：如果指定的力需要4或5英寸或更大的气缸缸径，选择液压系统。 大多数气动回路是3马力，因为空气的压缩效率低。 这样的一个系统需要10马力的液压系统将使用约30至50空气压缩机马力。 空气回路来构建，因为不需要单独的原动机比较便宜，但操作费用也高得多，并且可以迅速地抵消低分量开支。 空气和液压回路能够与空气的逻辑控制或防爆电动控制使用。 有了一定的预防措施，气瓶和两种类型的电机可以在高湿度的环境中运行， 甚至在水中。一些应用需要液体的刚性，以便它似乎有必要使用在这些情况下，即使液压系统具有低功率需求。 对于这些系统，利用空气的组合为动力源和油作为工作液，以降低成本，仍然有弓步自由与精确的停止选项控制和持有。 空油箱系统， 串联气缸系统，气缸带一体式控制，并增强器有几个可用的组件。究其原因，液体可以传输能量的最佳方式从名为17世纪的帕斯卡。 帕斯卡定律是流体动力的基本规律之一。 此法说：在密闭容器内，施加于静止液体上的压强将以等值同时传到各点。如图1-3所示，该法的工作原理在气缸中的应用。从泵油流入被起吊的负载的圆柱体。 负载的电阻引起的压力建立在汽缸内，直到负载开始移动。 当负载运动时，在整个回路中的压力保持几乎恒定。 压力油正试图走出泵，管道和缸，但这些机制都强大到足以包含流体。 当对活塞面积的压力变得足够高，以克服负载电阻，油迫使负载向上移动。 了解帕斯卡定律可以很容易地看到所有的液压和气动回路的功能。图1-3：帕斯卡法如何影响一个缸请注意，在这个例子中两个重要的事情： 首先，泵没有做压力，它只是产生流量，泵从不主动压力，他们只给流量，泵流量造成压力。 这是流体动力即是最重要的，以排除液压回路的基本原则之一。 假设一台安装了泵的运行显示了它的压力表几乎为0磅，这是否意味着该泵是否损坏？ 如果没有一个流量计在泵出口处，技工可能会改变泵的，因为很多人认为泵产生压力。 这个回路的问题可能仅仅是一个开放的阀门，使所有泵的流量直接进入水箱。 由于泵的出口流量认为没有阻力，压力表显示很少或没有压力。图。 1-4：机械和液压杠杆率的比较显示帕斯卡定律的作用的另一个领域是制造液压和机械杠杆的比较。图1-4 显示了如何这两个系统的工作。 在这两种情况下，一个大的力通过一个小得多的力由于杠杆臂长度或活塞面积之差所抵消。注意到液压杠杆作用并不局限于一定的距离，高度或物理位置的类似的机械杠杆。 这是许多机制决定性的优势，因为使用的流体动力大多数设计占用的空间较小，并且不限制位置的考虑。 气缸，旋转驱动器，或液压马达具有几乎无限的力或力矩可以直接推或旋转机械构件。 这些操作只需要流线，并从所述致动器和反馈装置，用以显示位置。 联动动作的主要优点是精确定位和没有反馈控制的能力。第一眼看上去，它可能会出现机械或液压杠杆能够节约能源。 例如：40000 磅是到位10,000磅举行。 然而，请注意，杆臂与活塞面积之比是4:1。 这意味着通过增加额外的力量要说到10,000磅的一面，它降低和40,000磅的一面上升。当10,000磅的重量向下移动10。一个距离，40,000磅的体重只有向上移动2.5英寸工作是一种力量，通过一个穿越的距离测量（功=力距离）。 工作通常是体现在英尺 - 磅和，如公式状态，它是力量磅倍的距离以英尺为单位的产品。当缸升降机20,000磅载荷10英尺的距离，气缸执行200000英尺磅工作。 这个动作可能发生在三秒钟，三分钟内，三个小时，而不改变工作的量。当工作在一定的时间内完成，它被称为功率功率=（强制距离）/时间 功率的常用指标是马力-从早期的时候大多数人可能涉及到一匹马的力量取了一个术语。 这使得一般的人来评价对功率的新方法，如蒸汽发动机。 功率是做功的速率。 一马力被定义为磅的重量（力），马可在一秒内（时间）抬起一只脚（距离）。 对于一般的马这竟然是550磅。 一只脚在一秒钟。 变更时间为60 秒（一分钟），它通常表示为每分钟33000英尺磅。是必要的，在大多数的液压回路没有考虑对于可压缩性，因为油只能被压缩的一个非常小的量。 通常情况下，液体被认为是不可压缩的，但是几乎所有的液压系统具有一些空气捕集在其中。 气泡是如此之小，甚至人有良好的视力无法看到它们，但这些气泡允许每1000磅约0.5的可压缩性。 应用此少量的可压缩性也有不利影响包括：单冲程气油增压，这在非常高的循环速率运行系统，伺服系统，保持密切容忍定位或压力;以及包含大量流体回路。 在这本书中，呈现回路时，其中压缩是一个因素，它会指出，随着方法来减少或允许它。另一种情况，使得它显得有更多的压缩比前面提到的是，如果当压力管道， 软管和气缸管扩张。 这需要更多的液体量，以建立压力和执行所需的工作。 另外，当液压缸推压的载荷，机器部件抵抗该力可拉伸，再次使得需要更多的流体进入汽缸之前的周期可以完成。因为任何人都知道，气体都非常可压缩。 有些应用程序使用此功能。 在大多数流体动力回路，可压缩性是不利的，在许多，这是一个缺点。 这意味着它是最好的，以消除任何残留的空气中的液压回路，以允许更快的循环时间和使系统更硬。波义耳定律波义耳定律的气体状态： 它的原则是，相对较低的压力，理想气体的绝对压力保持在恒定的温度反比于气体的体积变化在向下的家庭语言，这意味着，如果一个10立方英尺的容积大气中的空气被挤压成一立方英尺的容器，压力增加了十倍。 （10 X 14.7 PSIA = 147 PSIA）。注意压力表示为 PSIA。图。 1-5：测量表压和绝对压力通常情况下，压力表阅读磅（没有额外的字母）。 通常被称为表压，磅忽略的14.7 PSIA 地球的大气压力，因为它没有影响正面或负面的流体动力回路。上 PSIA 末端的一个代表绝对的，并且将被显示在一个表，带有一个指针，从不变为零，除非它是测量真空度。 另一种类型的压力表，显示正反两方面的压力会与下面的英寸 - 的汞（英寸汞柱）规模零指针和 PSIG 规模以上为零。 这两个压力表可以读取压力或真空。（它们在制冷 repairperson 的工具包总是发现。制冷机组同时具有真空，并在系统的不同部分压力的同时。） 图1-5图片典型psig 的规格和一种类型的 PSIA 计。在上面的例子中，当10立方英尺的空气被挤压到一个1立方英尺的空间，这两个压力分别为 psia 给出。 看什么表压（psig）的是，从147-PSIA 读数减去一个大气压。 （147 PSIA 14.7 PSIA = 132.3 PSIG。）为了计算空气的压缩量在一个系统中，总是使用绝对压力，还是 PSIA，不 PSIG。 例如：在图1-6气缸包含八个立方英尺的空气在70 PSIG。 什么将压力增加时的外力推动活塞向后， 直到活塞后面的空间是2立方英尺？ 很明显的压力将上升4倍。起初它看起来容易采取70 PSIG4 = 280 psig 的，但是这个答案是错的。 为正确答案，表压必须改变，以绝对压力。 在这种情况下，通过增加一个大气压到70 psig 的读数。 （70 psig 的+ 14.7 PSIA = 84.7 PSIA）现在乘以4的84.7-PSIA 压力看的绝对压力是当气缸停止在1立方英尺的体积。 （84.74 = 338.8 PSIA）最后， 返回到表压，从绝对压力减去一个大气压。 （338.8 PSIA 14.7 PSIA = 324.1 PSIG。）注意正确的压力为44.1 psig 的比当表压为乘数较高。图。 1-6：压力变化的空气被压缩温度未在上述两种情况下考虑的，但是请注意，该法律规定，保持恒温。 压缩气体总是增加其温度，因为在较大体积的热量被立即打包到一个较小的空间。下一个法律说，随着温度的升高会增加压力，如果气体不能扩大。 这意味着， 给定压力的气体温度恢复到它最初是后计量。仪表读取 PSI， PSI 是压力公制或 SI 单位，约等于气压计的读数或一个大气压。 一个大气实际上是14.696磅，但在 SI 单位栏为14.5磅。查理定律加热气体或液体使其膨胀。 继续加热的液体将导致其转变为气态和可能发生自燃。 如果气体或液体不能扩大，因为它被限制在所包含的面积增大的压力。这是查理定律表示为：气体将固定质量的体积变化直接与绝对温度，所提供的压力保持恒定。 因为流体动力系统具有一些区域，其中流体被捕获，这是可能的， 加热该承压流体可能会导致部件损坏或发生爆炸。 如果一个回路必须在热的环境下操作，提供过压防护，例如卸压阀或热或压力敏感的破裂装置。 从来没有在任何流体动力组件，而该单位的正确方法加热或焊接。静压头压力在一个容器中的流体的重量施加在含有容器的侧面和底部的压力。 这就是所谓的静压头压力。 它是由地球的引力引起的。 头压力的一个很好的例子是一个社区水系统。 图1-7显示了一个水塔为80英尺的最顶层水位。 水一立方英寸重量0.0361磅。 因此，水的一平方英寸的列将发挥的0.0361 psi 的力量抬高的每一寸。 这个工程以每海拔英尺.433磅。 对于水塔在图1-7，在该基地的压力将是：80英尺 x 0.433磅/英尺= 34.6磅。 这种压力是始终可用，无泵在运行时也是如此。 当然，如果水位下降，静压头压力也将下降。图。 1-7：压力测量水塔液压油的比重约为0.9，所以乘以水的每英尺0.433磅0.9示出油施加每高程的脚0.39磅。 通常这部分是为了简单起见，取整到0.4。 如果水塔充满到80英尺，油，它会产生32磅地面的压力。 其它流体将根据其比重开发更高或更低的静压力。这种压力只有在实现了在塔底层。 网点在其他级别将根据流体表面以下的距离增加或减少。看到大多数水塔水箱只需存储容量。 压力不迅速下降，或者需要频繁水泵启动，保持液面。 该储箱的大小或形状不影响在基础压力。 压力在直80英尺管道的基础将是相同的，但有用的卷之前压降会发生急剧变化。 一定要记住：这不是流体的身体，决定压力，但有多深它是物理尺寸。头压力可以有一个液压系统产生不利影响，因为许多泵安装高于液体高度。这意味着该泵必须先建立足够的真空，以提高流体，然后创造出更高的真空加速和移动它。 因此，是有一定限度多远的泵可以位于上方的油位。 大多数泵指定的3 psi 的最大吸入压力。 在4 - 5 psi 的吸气压力，泵开始抽空。 。 。 造成内部损坏。 在6 - 7-PSI 的真空，汽蚀破坏是严重的噪音水平增加明显。 （空化的影响在第8章，流体动力泵和配套项目全面覆盖。）轴流式或在线活塞泵是特别容易受到高吸油真空度损伤，并应设置在液面以下，产生了积极的头部的压力。许多现代液压系统放在旁边的水库泵使流体水平总是高于泵的入口。 与这种类型的安装在泵总是有油在启动时，在其入口的正头压力。 一个更好的安排， 使泵上方的水箱采取更大的机头压力的优势。 应尽一切可能来完成，以保持压力降低，泵入口管线，因为可能的最高压降允许的是一个大气压（14.7磅海平面）。地球的大气层，我们呼吸的空气施加14.7磅在海平面平均每天出一份力。这种压力覆盖了整个地球表面，但在海拔低于海平面较高，它减少了每千英尺约0.5 磅。 地球的大气层，这种压力真空的动力之源。 尽可能高的真空度读数在任何位置是在它上面的空气的当时的重量。 可用最大真空度读数在本地天气预报的气压计的读数给出。 除以气压计的读数由两个获得在 PSI 的近似大气压。 这支部队可以直接测量，如果有可能，可以隔离空气中一个大气压高一平方英寸的柱在海平面位置。 因为这是不可能的，用于测量真空度的方法是体现在图1-8。图。 1-8：真空测量与汞淹没一个明确的管中汞的容器一端封闭，并允许它完全填满。 （管必须超过30英寸长，此示例才能正常工作时，汞是液体）后汞取代所有的空气在管，小心地抬起管的封闭端，保持开口端浸入所以水银不能一次用完，并通过空气所取代。 当管被垂直放置时，液体汞水平将降低，得到在英寸汞的大气压力读数（在海平面29.92 -英寸 H G）。 水银含量会波动从这点为高，低压天气系统移动过去。如果管子已经100项。 身高，汞水平仍然会下降到任何大气压力是在它的位置。究其原因，汞不会全部流出来的是大气压力保持它。使用另一种液体可以直接建立这个晴雨表但该管就必须较长，因为大多数其他液体具有低得多的比重比水银的13.546。 水，具有1.0的比重，将需要一个封闭端管的至少33.8英尺长，而油，具有约0.9的比重，就必须甚至更长的时间。真空泵可以在设计，空气压缩机相似。 有往复活塞式，隔膜，旋转螺杆式和裂转子设计。想象一下钩住一个空气压缩机的储气罐入口和离开出口与大气相通。 作为泵运行时，它抽空空气从接收器并且导致在其负压。图。 1-9：文丘里真空发生器的剖视图真空泵是一个额外的费用，通常在使用负压操作机器或使产品的稳定供应设施才发现。使用工厂的压缩空气作为动力源真空发生器也可提供。 这些部件没有移动部件，但是流过文丘里管，以产生一个小的供给负压利用植物空气， 图1-9示出了一个文丘里型真空发生器的简化剖视图。 该装置由本体 A 与压缩空气入口乙传递的空气的废气的空气流通过文丘里喷嘴 C.在通孔 D.更高的速度向大气中由于空气在上升速度流文丘里喷嘴附近的过去开口 E，它创建的负压和通过端口楼F 口吸入大气可以连接到需要一个真空源的任何外部设备。 真空计在端口 F 显示负压时将压缩空气供给到端口 B。真空发生器是便宜的，但可能是昂贵的操作。 每4立方英尺的空气供应给他们供电所需，他们用大约一个压缩机马力。 出于这个原因，文丘里型真空发生器通常是用一个控制阀安装仅在需要时将其打开。真空被限制为一个大气压时的最大值的任何位置，和标准的真空泵只有平均达到此约85（约12磅）。 因此，真空是没有强大到足以做很多的工作，除非它作用于大面积。图。 1-10：用真空起吊的简化表示法许多工业真空应用都与处理的部分。 大面积的吸盘可以举起一个大重部分自如， 如图1-10。 当升降机上升时，负压（真空）抽吸罩内导致在该部分的相反侧的大气压力来推动它。图。 1-11：工作真空控股简化表示法如玻璃和木材制造使用真空机加工或其它操作期间保持工件， 如图1-11所示。 这些作品被固定到位为下他们的负压引起的大气压力来推动反对他们。 一个弹性密封件放置在夹具的槽保持大气的空气进入模腔中的部分的下方。 这个槽可以切割的部分的轮廓相匹配。 在机加工操作中，密封件可以隔离内侧缺口， 允许同时紧握件的其余它们被除去。图。 1-12：用真空塑料片材成型简化表示法加热的塑料片可以是真空成型以低得多的成本比形成其他类型的塑料，使一些产品，如在图1-12提出了建议。 在一个腔或以上的形状成形加热塑料片是快速和积极的。 当大气压力下试图软化的片下填补负压区，片材被压入所需的形状。附录二：A Overview of Hydraulic SystemAny media (liquid or gas) that flows naturally or can be forced to flow could be used to transmit energy in a fluid power system. The earliest fluid used was water hence the name hydraulics was applied to systems using liquids. In modern terminology, hydraulics implies a circuit using mineral oil. Figure 1-1 shows a basic power unit for a hydraulic system. (Note that water is making something of a comeback in the late 90s; and some fluid power systems today even operate on seawater.) The other common fluid in fluid power circuits is compressed air. As indicated in Figure 1-2, atmospheric air - compressed 7 to 10 times - is readily available and flows easily through pipes, tubes, or hoses to transmit energy to do work. Other gasses, such as nitrogen or argon, could be used but they are expensive to produce and process.Fig. 1-1: Basic hydraulic power unit.Of the three main methods of transmitting energy mechanical, electrical, and fluid fluid power is least understood by industry in general. In most plants there arefew persons with direct responsibility for fluid power circuit design or maintenance. Often, general mechanics maintain fluid power circuits that originally were designed by a fluid-power-distributor salesperson. In most facilities, the responsibility for fluid power systems is part of the mechanical engineers job description. The problem is that mechanical engineers normally receive little if any fluid power training at college, so they are ill equipped to carry out this duty. With a modest amount of fluid power training and more than enough work to handle, the engineer often depends on a fluid power distributors expertise. To get an order, the distributor salesperson is happy to design the circuit and often assists in installation and startup. This arrangement works reasonably well, but as other technologies advance, fluid power is being turned down on many machine functions. There is always a tendency to use the equipment most understood by those involved.Fig. 1-2: Basic pneumatic power arrangement.Fluid power cylinders and motors are compact and have high energy potential. They fit in small spaces and do not clutter the machine. These devices can be stalled for extended time periods, are instantly reversible, have infinitely variable speed, and often replace mechanical linkages at a much lower cost. With good circuit design, the power source, valves, and actuators will run with little maintenance for extended times. The main disadvantages are lack of understanding of the equipment and poor circuit design, which can result in overheating and leaks. Overheating occurs whenthe machine uses less energy than the power unit provides. (Overheating usually is easy to design out of a circuit.) Controlling leaks is a matter of using straight-thread O-ring fittings to make tubing connections or hose and SAE flange fittings with larger pipe sizes. Designing the circuit for minimal shock and cool operation also reduces leaks.A general rule to use in choosing between hydraulics or pneumatics for cylinders is: if the specified force requires an air cylinder bore of 4 or 5 in. or larger, choose hydraulics. Most pneumatic circuits are under 3 hp because the efficiency of air compression is low. A system that requires 10 hp for hydraulics would use approximately 30 to 50 air-compressor horsepower. Air circuits are less expensive to build because a separate prime mover is not required, but operating costs are much higher and can quickly offset low component expenses. Situations where a 20-in. bore air cylinder could be economical would be if it cycled only a few times a day or was used to hold tension and never cycled. Both air and hydraulic circuits are capable of operating in hazardous areas when used with air logic controls or explosion-proof electric controls. With certain precautions, cylinders and motors of both types can operate in high-humidity atmospheres . . . or even under water.When using fluid power around food or medical supplies, it is best to pipe the air exhausts outside the clean area and to use a vegetable-based fluid for hydraulic circuits.Some applications need the rigidity of liquids so it might seem necessary to use hydraulics in these cases even with low power needs. For these systems, use a combination of air for the power source and oil as the working fluid to cut cost and still have lunge-free control with options for accurate stopping and holding as well. Air-oil tank systems, tandem cylinder systems, cylinders with integral controls, and intensifiers are a few of the available components.The reason fluids can transmit energy when contained is best stated by a manfrom the 17th century named Blaise Pascal. Pascals Law is one of the basic laws of fluid power. This law says: Pressure in a confined body of fluid acts equally in all directions and at right angles to the containing surfaces. Another way of saying this is: If I poke a hole in a pressurized container or line, I will get PSO. PSO stands for pressure squirting out and puncturing a pressurized liquid line will get you wet. Figure 1-3 shows how this law works in a cylinder application. Oil from a pump flows into a cylinder that is lifting a load. The resistance of the load causes pressure to build inside the cylinder until the load starts moving. While the load is in motion, pressure in the entire circuit stays nearly constant. The pressurized oil is trying to get out of the pump, pipe, and cylinder, but these mechanisms are strong enough to contain the fluid. When pressure against the piston area becomes high enough to overcome the load resistance, the oil forces the load to move upward. Understanding Pascals Law makes it easy to see how all hydraulic and pneumatic circuits function.Fig. 1-3: How Pascals Law affects a cylinderNotice two important things in this example. First, the pump did not make pressure; it only produced flow. Pumps never make pressure. They only give flow.Resistance to pump flow causes pressure. This is one of the basic principles of fluid power that is of prime importance to troubleshooting hydraulic circuits. Suppose a machine with the pump running shows almost 0 psi on its pressure gauge. Does this mean the pump is bad? Without a flow meter at the pump outlet, mechanics might change the pump, because many of them think pumps make pressure. The problem with this circuit could simply be an open valve that allows all pump flow to go directly to tank. Because the pump outlet flow sees no resistance, a pressure gauge shows little or no pressure. With a flow meter installed, it would be obvious that the pump was all right and other causes such as an open path to tank must be found and corrected.Fig. 1-4: Comparison of mechanical and hydraulic leverageAnother area that shows the effect of Pascals law is a comparison of hydraulic and mechanical leverage. Figure 1-4 shows how both of these systems work. In either case, a large force is offset by a much smaller force due to the difference in lever-arm length or piston area.Notice that hydraulic leverage is not restricted to a certain distance, height, or physical location like mechanical leverage is. This is a decided advantage for many mechanisms because most designs using fluid power take less space and are not restricted by position considerations. A cylinder, rotary actuator, or fluid motor with almost limitless force or torque can directly push or rotate the machine member. These actions only require flow lines to and from the actuator and feedback devices to indicate position. The main advantage of linkage actuation is precision positioning and the ability to control without feedback.At first look, it may appear that mechanical or hydraulic leverage is capable of saving energy. For example: 40,000 lb is held in place by 10,000 lb in Figure 1-4. However, notice that the ratio of the lever arms and the piston areas is 4:1. This means by adding extra force say to the 10,000-lb side, it lowers and the 40,000-lb side rises. When the 10,000-lb weight moves down a distance of 10 in., the 40,000-lb weight only moves up 2.5 in.Work is the measure of a force traversing through a distance. (Work = Force X Distance.). Work usually is expressed in foot-pounds and, as the formula states, it is the product of force in pounds times distance in feet. When a cylinder lifts a 20,000-lb load a distance of 10 ft, the cylinder performs 200,000 ft-lb of work. This action could happen in three seconds, three minutes, or three hours without changing the amount of work.When work is done in a certain time, it is called power. Power = (Force X Distance) / Time. A common measure of power is horsepower - a term taken from early days when most persons could relate to a horses strength. This allowed the average person to evaluate to new means of power, such as the steam engine. Power is the rate of doing work. One horsepower is defined as the weight in pounds (force) a horse could lift one foot (distance) in one second (time). For the average horse this turned out to be 550 lbs. one foot in one second. Changing the time to 60 seconds (one minute), it is normally stated as 33,000 ft-lb per minute.No consideration for compressibility is necessary in most hydraulic circuits because oil can only be compressed a very small amount. Normally, liquids are considered to be incompressible, but almost all hydraulic systems have some air trapped in them. The air bubbles are so small even persons with good eyesight cannot see them, but these bubbles allow for compressibility of approximately 0.5% per 1000 psi. Applications where this small amount of compressibility does have an adverse effect include: single-stroke air-oil intensifiers; systems that operate at very high cycle rates; servo systems that maintain close-tolerance positioning or pressures; and circuits that contain large volumes of fluid. In this book, when presenting circuits where compressibility is a factor, it will be pointed out along with ways to reduce or allow for it.Another situation that makes it appear there is more compressibility than stated previously is if pipes, hoses, and cylinder tubes expand when pressurized. This requires more fluid volume to build pressure and perform the desired work. In addition, when cylinders push against a load, the machine members resisting this force may stretch, again making it necessary for more fluid to enter the cylinder before the cycle can finish.As anyone knows, gasses are very compressible. Some applications use this feature. In most fluid power circuits, compressibility is not advantageous; in many, it is a disadvantage. This means it is best to eliminate any trapped air in a hydraulic circuit to allow faster cycle times and to make the system more rigid.Boyles LawBoyles Law for gasses states: It is the principle that, for relatively low pressures, the absolute pressure of an ideal gas kept at constant temperature varies inversely with the volume of the gas. In down-home language this means if a ten cubic foot volume of atmospheric air is squeezed into a one cubic foot container, pressure increases ten times. (10 X 14.7 psia = 147 psia.) Notice that pressure is statedas psia.Fig. 1-5: Measurement of gauge and absolute pressureNormally, pressure gauges read in psi (with no additional letter). Commonly called gauge pressure, psi disregards the earths atmospheric pressure of 14.7 psia, because it has no effect either negative or positive on a fluid power circuit. The a on the end of psia stands for absolute, and would be shown on a gauge with a pointer that never goes to zero unless it is measuring vacuum. Another type of gauge that shows both negative and positive pressures would have a pointer with an inches-of-mercury (in. Hg) scale below zero and a psig scale above zero. Both of these gauges could read pressure or vacuum. (They are always found in a refrigeration repairpersons tool kit. Refrigeration units have both vacuum and pressure in different sections of the system at the same time.) Figure 1-5 pictures a typical psig gauge and one type of psia gauge.In the example above, when ten cubic feet of air was squeezed into a one cubic-foot space, both pressures were given in psia. To see what gauge pressure (psig) would be, subtract one atmosphere from the 147-psia reading. (147 psia 14.7 psia =132.3 psig.) To calculate the amount of compression of air in a system, always use absolute pressure, or psia, not psig. For example: the cylinder in Figure 1-6 contains eight cubic feet of air at 70 psig. To what will pressure increase when an external force pushes the piston back until the space behind the piston is two cubic foot? It is obvious the pressure will rise four times. At first it might look easy to take 70 psig X4 = 280 psig, but this answer is wrong. For the correct answer, gauge pressure must be changed to absolute pressure. In this case by adding one atmosphere to the 70-psig reading. (70 psig + 14.7 psia = 84.7 psia.) Now multiply the 84.7-psia pressure by 4 to see what the absolute pressure is when the cylinder stops at one cubic foot volume. (84.7 X 4 = 338.8 psia.) Finally, to return to gauge pressure, subtract one atmosphere from the absolute pressure. (338.8 psia 14.7 psia = 324.1 psig.) Notice that the correct pressure is 44.1 psig higher than when gauge pressure is the multiplier.Fig. 1-6: Pressure change as air is compressedTemperature was not considered in both preceding cases, but notice that the law says kept at constant temperature. Compressing a gas always increases its temperature because the heat in the larger volume is now packed into a smaller space. The next law says that increasing temperature increases pressure if the gas cannot expand. This means the pressures given are measured after the gas temperature returns to what it was originally.Gauges today read in psi and bar. Bar is a metric or SI unit for pressure and is equal to approximately the barometer reading or one atmosphere. One atmosphere is actually 14.696 psi but the SI unit for bar is 14.5 psi.Charles LawHeating a gas or liquid causes it to expand. Continuing to heat a liquid will result in it changing to the gaseous state and perhaps spontaneous combustion. If the gas or liquid cannot expand because it is confined, pressure in the contained area increases. This is stated in Charles Law as: The volume of a fixed mass of gas varies directly with absolute temperature, provided the pressure remains constant. Because fluid power systems have some areas in which fluid is trapped, it is possible that heating this confined fluid could result in part damage or an explosion. If a circuit must operate in a hot atmosphere, provide over pressure protection such as a relief valve or a heat- or pressure-sensitive rupture device. Never heat or weld on any fluid power components without proper preparation of the unit.Static head pressureThe weight of a fluid in a container exerts pressure on the containing vessels sides and bottom. This is called static head pressure. It is caused by earths gravitational pull. A good example of head pressure is a community water system. Figure 1-7 shows a water tower with a topmost water level of 80 feet. A cubic inch of water weighs 0.0361 pounds. Therefore a one square-inch column of water will exert a force of 0.0361 psi for every inch of elevation. This works out to .433 psi per foot of elevation. For the water tower in Figure 1-7, the pressure at the base would be: 80 ft X 0.433 psi/ft = 34.6 psi. This pressure is always available, even when no pumps are running. Of course, if the water level drops, static head pressure also will drop.Fig. 1-7: Pressure measurement for water towerThe specific gravity of hydraulic oil is approximately 0.9, so multiplying waters0.433 psi per foot by 0.9 shows oil exerts 0.39 psi per foot of elevation. Usually this fraction is rounded to 0.4 for simplicity. If the water tower were filled to 80 ft with oil, it would exert a pressure of 32 psi at ground level. Other fluids would develop a higher or lower static pressure according to their specific gravities.This pressure is only realized at ground level at the tower. Outlets at other levels would be higher or lower according to their distance below the fluid surface.Tanks seen on most water towers simply store volume. Pressure does not drop rapidly or require frequent pump starts to maintain the fluid level. The size or shape of the tank does not affect pressure at the base. Pressure at the base of a straight 80-ft pipe would be the same, but useful volume before pressure drop would change drastically. Always remember: it is not the physical size of a body of fluid that determines pressure but how deep it is.Head pressure can have an adverse effect on a hydraulic system because many pumps are installed above the fluid level. This means the pump must first create enough vacuum to raise the fluid and then create even higher vacuum to accelerateand move it. Therefore there is a limit to how far a pump can be located above the oil level. Most pumps specify a maximum suction pressure of 3 psi. At 4- to 5-psi suction pressure, pumps start to cavitate . . . causing internal damage. At 6- to 7-psi vacuum, cavitation damage is severe and noise levels increase noticeably. (The effects of cavitation are covered fully in Chapter 8, Fluid power pumps and accessory items.) Axial- or in-line-piston pumps are especially vulnerable to high inlet vacuum damage and should be set up below the fluid level to produce a positive head pressure.Many modern hydraulic systems place the pump next to the reservoir so the fluid level is always above the pump inlet. With this type of installation the pump always has oil at startup and has a positive head pressure at its inlet. A better arrangement puts the tank above the pump to take advantage of even greater head pressure. Everything possible should be done to keep pressure drop low in the pump inlet line because the highest possible pressure drop allowable is one atmosphere (14.7 psi at sea level).The earths atmosphere the air we breathe exerts a force of 14.7 psi at sea level on an average day. This pressure covers the whole earths surface, but at elevations higher than sea level, it is reduced by approximately 0.5 psi per 1000 feet. This pressure of earths atmosphere is the source of the power of vacuum. The highest possible vacuum reading at any location is the weight of the air above it at that time. A reading of maximum vacuum available is given during the local weather forecast as the barometer reading. Divide the barometer reading by two to get the approximate atmospheric pressure in psi. This force could be directly measured if it were possible to isolate a one square-inch column of air one atmosphere tall at a sea level location. Because this is not possible, the method used to measure vacuum is demonstrated in Figure 1-8.Fig. 1-8: Vacuum measurement with mercurySubmerge a clear tube with one closed end in a container of mercury and allow it to fill completely. (The tube must be more than 30-in. long for this example to work when mercury is the liquid.) After the mercury displaces all the air in the tube, carefully raise the tubes closed end, keeping the open end submerged so the mercury cant run out and be replaced by air. When the tube is positioned vertically, the liquid mercury level will lower to give the atmospheric pressure reading in inches of mercury (29.92-in. Hg at sea level). The mercury level will fluctuate from this point as high and low-pressure weather systems move past. If the tube had been 100-in. tall, the mercury level would still have dropped to whatever the atmospheric pressure was at its location. The reason the mercury does not all flow out is that atmospheric pressure holds it in.This barometer could have been built using another liquid but the tube would have to be longer because most other liquids have a much lower specific gravity than mercurys 13.546. Water, with a specific gravity of 1.0, would require a closed-end tube at least 33.8 ft long, while oil, with a specific gravity of approximately 0.9, would have to be even longer.Vacuum pumps can be similar in design to air compressors. There are reciprocating-piston, diaphragm, rotary-screw, and lobed-rotor designs. (See air compressor types in Chapter 8, Fluid power pumps and accessory items.) Imagine hooking the inlet of an air compressor to a receiver tank and leaving the outlet open to atmosphere. As the pump runs, it evacuates air from the receiver and causes a negative press
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