液压系统及液压缸-外文翻译

液压传动液压传动 第十讲第十讲 制动器制动器 力流体动力系统的优秀的特性之一是由电源产生 通过适当的控制和指导 并 通过电线传输 就可以轻松转换到几乎任何类型的机械运动所需要用到的地方 使用一个合适的驱动装置 可以获得线性 直线 或者是旋转运动 驱动器是 一种转换流体动力机械力和运动的装置 缸 马达和涡轮机是最常见的将流体 动力系统应用于驱动设备的类型 这一章描述了各种类型的动作汽缸和他们的 应用程序 不同类型的流体汽车和使用流体动力系统的涡轮机 汽缸汽缸 制动汽缸是一种将流体动力转换成线性或直线 力和运动的装置 因为线性运 动是沿着一条直线前后移动的往复运动 这种类型的制动器有时被称为一个往 复 或线性 电动机 由 ram 或活塞组成的汽缸在一个圆柱孔内操作 制动汽 缸可以安装 以便汽缸被固定在一个固定的结构 ram 或活塞被连接到该机制 来操作 或者是活塞和 ram 可能被固定到固定结构 汽缸附加到机械装置来操 作 制动汽缸气动和液压系统的设计和操作是类似的 一些变化的 ram 和活塞 式制动汽缸的内容将在后面的段落中描述 冲压式缸冲压式缸 术语 ram 和活塞通常可以互换使用 然而 一个冲压式缸通常被认为是一个截 面积活塞杆超过一半的截面积活动元件 在大多数这种类型的制动汽缸中 杆 和活动元件各占一半 这种类型的活动元件经常被称为柱塞 冲压式缸主要是 用来推动而不是拉 一些应用程序需要 ram 的一部分在平坦的外部来推动或升 降单位操作 其他应用程序需要一些机械装置的附件 如一个 U 型夹或有眼螺 栓 冲压式缸的设计在很多其他方面不同 以满足不同应用程序的要求 单作用千斤顶单作用千斤顶 单作用千斤顶 如图 10 1 试用力只在一个方向 流体定向的汽缸取代 ram 和他外部的弹性元件 将物体举起放在上面 当流体压力释放后 由于没有规定由流体动力缩回 ram 物体的重量或者是一 些机械设备 比如一个弹簧 迫使 ram 回到汽缸 这种流体能量就倒回到容器 单作用冲压式汽缸通常用于液压千斤顶 用于移动飞机的飞行甲板和机库甲板 航空母舰的升降机也使用这种类型的汽缸 在这些升降机中 汽缸是水平安装 的 通过一系列的电缆和捆运行升降机 流体压力产生 ram 的外在能量使升降 机运行 当从 ram 中释放出流体压力 升降机的重量迫使 ram 回到汽缸 反 过来 这迫使流体回容器 双作用千斤顶双作用千斤顶 一个双作用冲压式缸如图 10 2 在这个汽缸中 两个中风的 ram 都是由加压 流体产生的 它有两个流体端口 每一个都在或是接近汽缸的两端 针对流体 压力的封闭端缸扩展 ram 和应用力 撤销 ram 和减少力 流体是指向截然相 反的汽缸 一个四通换向阀是通常用于控制双作用 ram 当阀门定位来扩展 ram 加压流体进入端口 A 作用于 ram 的地面 加强 ram 的外部力量 对于在液压系统中的回流管 或者气动系统中的大气 通过控制阀门 高于 ram 边缘的流体是免费流出端口 B 的 通常情况下 流体的压力和 ram 的中风 是相同的 记得第二章中 力等于压力乘以面积 F PA 注意到不同的领域 上的压力如图 10 2 所示 在扩展中风期间 在 ram 适用力的同时 对抗大的 表面积的压力是来自于 ram 的地下 在收缩行程的过程中 ram 不需要很大的 动力 作用于 ram 顶面的小面积的压力为收回 ram 提供了必要的动力 伸缩式千斤顶伸缩式千斤顶 图 10 3 显示了一个可伸缩的冲压式气缸 一系列的 ram 是嵌在可伸缩的装配 中 除了最小的 ram 每一个 ram 都是中空的 作为下一个较小的 ram 的油缸壳 ram 装配主要包括活塞部件 它还提供了流体端口 尽管这个装配需要一个小空 间的 ram 收回 但是当 ram 扩展的时候 伸缩式行动的组装提供了一个相对长 的冲程 这种型号的汽缸的一个很好的例子是应用在自动倾卸卡车上 它是用 来解除前端的卡车床和转储负载 在吊运作业 最大的力量是所需的初始提升的 负载 随着负载的提升并开始转储 所需的力量越来越少 直到负载是完全抛弃 在提 高循环时 加压流体通过端口 A 进入气缸 然后作用在 ram 的底部表面 Ram 1 有一个更大的表面积 因此提供了更大的力的初始负载 正如 Ram 1 完成它 的行程所需的力是减少的 当 Ram 2 移动时 提供较小的力需要继续提高负 载 当 ram 2 完成它的行程 一个更小的力量是必需的 是然后 Ram 3 往外移 动完成提高和倾销负载 一些可伸缩的冲压式的汽缸的单作用类型 像前面所 讨论的单作用千斤顶 这些可伸缩的冲压式汽缸通过重力或机械力收回 一些 液压千斤顶配备了可伸缩的 ram 这样的千斤顶是用来提升车辆通过更小的间 隙到达所需的高度 其他类型的可伸缩的汽缸 如图 10 3 所示 是双作用类 型 在这种类型中 流体压力用于扩展和收缩中风 一个四通换向阀是常用的控 制操作的双作用类型 注意在墙上的 ram1 和 2 的小通路 他们提供了一个路 径流体流向 在汽缸上面边缘的 ram2 和 3 之间 在附加行程中 返回流体通 过那些小通路和汽缸外面到达 B 端口 然后流经换向阀来返回线路或蓄水池 撤销 ram 流体压力是通过 B 端口直接进入气缸的 然后反作用于三个 ram 的边缘的表面区域 这迫使 ram 收回了位置 流离失所的流体从 ram 的另一 侧通过 A 端口流出汽缸 通过换向阀来返回线路或蓄水池 Fluid Power NAVEDTRA 14105 CHAPTER 10 ACTUATORS One of the outstanding features of fluid power systems is that force generated by the power supply controlled and directed by suitable valuing and transported by lines can be converted with ease to almost any kind of mechanical motion desired at the very place it is needed Either linear straight line or rotary motion can be obtained by using a suitable actuating device An actuator is a device that converts fluid power into mechanical force and motion Cylinders motors and turbines are the most common types of actuating devices used in fluid power systems This chapter describes various types of actuating cylinders and their applications different types of fluid motors and turbines used in fluid power systems CYLINDERS An actuating cylinder is a device that converts fluid power to linear or straight line force and motion Since linear motion is a back and forth motion along a straight line this type of actuator is sometimes referred to as a reciprocating or linear motor The cylinder consists of a ram or piston operating within a cylindrical bore Actuating cylinders may be installed so that the cylinder is anchored to a stationary structure and the ram or piston is attached to the mechanism to be operated or the piston or ram may be anchored to the stationary structure and the cylinder attached to the mechanism to be operated Actuating cylinders for pneumatic and hydraulic systems are similar in design and operation Some of the variations of ram and piston type actuating cylinders are described in the following paragraphs RAM TYPE CYLINDERS The terms ram and piston are often used interchangeably However a ram type cylinder is usually considered one in which the cross sectional area of the piston rod is more than one half the cross sectional area of the movable element In most actuating cylinders of this type the rod and the movable element have equal areas This type of movable element is frequently referred to as a plunger The ram type actuator is used primarily to push rather than to pull Some applications require simply a flat surface on the external part of the ram for pushing or lifting the unit to be operated Other applications require some mechanical means of attachment such as a clevis or eyebolt The design of ram type cylinders varies in many other respects to satisfy the requirements of different applications Single Acting Ram The single acting ram fig 10 1 applies force in only one direction The fluid that is directed into the cylinder displaces the ram and forces it outward lifting the object placed on it Since there is no provision for retracting the ram by fluid power when fluid pressure is released either the weight of the object or some mechanical means such as a spring forces the ram back into the cylinder This forces the fluid back to the reservoir The single acting ram type actuating cylinder is often used in the hydraulic jack The elevators used to move aircraft to and from the flight deck and hangar deck on aircraft carriers also use cylinders of this type In these elevators the cylinders are installed horizontally and operate the elevator through a series of cables and sheaves Fluid pressure forces the ram outward and lifts the elevator When fluid pressure is released from the ram the weight of the elevator forces the ram back into the cylinder This in turn forces the fluid back into the reservoir Double Acting Ram A double acting ram type cylinder is illustrated in figure 10 2 In this cylinder both strokes of the ram are produced by pressurized fluid There are two fluid ports one at or near each end of the cylinder Fluid under pressure is directed to the closed end of the cylinder to extend the ram and apply force To retract the ram and reduce the force fluid is directed to the opposite end of the cylinder A four way directional control valve is normally used to control the double acting ram When the valve is positioned to extend the ram pressurized fluid enters port A fig 10 2 acts on the bottom surface of the ram and forces the ram outward Fluid above the ram lip is free to flow out of port B through the control valve and to the return line in hydraulic systems or to the atmosphere in pneumatic systems Normally the pressure of the fluid is the same for either stroke of the ram Recall from chapter 2 that force is equal to pressure times area F PA Notice the difference of the areas upon which the pressure acts in figure 10 2 The pressure acts against the large surface area on the bottom of the ram during the extension stroke during which time the ram applies force Since the ram does not require a large force during the retraction stroke pressure acting on the small area on the top surface of the ram lip provides the necessary force to retract the ram Telescoping Rams Figure 10 3 shows a telescoping ram type actuating cylinder A series of rams is nested in the telescoping assembly With the exception of the smallest ram each ram is hollow and serves as the cylinder housing for the next smaller ram The ram assembly is contained in the main cylinder assembly which also provides the fluid ports Although the assembly requires a small space with all the rams retracted the telescoping action of the assembly provides a relatively long stroke when the rams are extended An excellent example of the application of this type of cylinder is in the dump truck It is used to lift the forward end of the truck bed and dump the load During the lifting operation the greatest force is required for the initial lifting of the load As the load is lifted and begins to dump the required force becomes less and less until the load is completely dumped During the raise cycle pressurized fluid enters the cylinder through port A fig 10 3 and acts on the bottom surface of all three rams Ram 1 has a larger surface area and therefore provides the greater force for the initial load As ram 1 reaches the end of its stroke and the required force is decreased ram 2 moves providing the smaller force needed to continue raising the load When ram 2 completes its stroke a still smaller force is required Ram 3 then moves outward to finish raising and dumping the load Some telescoping ram type cylinders are of the single acting type Like the single acting ram discussed previously these telescoping ram type cylinders are retracted by gravity or mechanical force Some hydraulic jacks are equipped with telescoping rams Such jacks are used to lift vehicles with low clearances to the required height Other types