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1、精选优质文档-倾情为你奉上液压传动第十讲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
2、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
3、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（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。
4、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
5、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
6、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
7、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
8、换向阀来返回线路或蓄水池。Fluid PowerNAVEDTRA 14105CHAPTER 10ACTUATORSOne 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 desi
9、red 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
10、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.CYLINDERSAn actuating cylinder is a device that converts fluid power to linear, or straight line, force and motio
11、n. 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 anc
12、hored 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
13、 and operation. Some of the variations of ram- and piston-type actuating cylinders are described in the following paragraphs.RAM-TYPE CYLINDERSThe 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 pist
14、on 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 t
15、o 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 satis
16、fy the requirements of different applications. Single-Acting RamThe 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
17、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
18、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 li
19、fts 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 RamA double-acting ram-type cylinder is illustrated in figure 10-2. In this cylinder, both strokes
20、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 th
21、e 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
22、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 differen
23、ce 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 t
24、he small area on the top surface of the ram lip provides the necessary force to retract the ram.Telescoping RamsFigure 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 a
25、s 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 t
26、he 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 a
27、nd 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 gre
28、ater 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 an
29、d 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 l
30、ift vehicles with low clearances to the required height. Other types of telescoping cylinders, like the one illustrated in figure 10-3, are of the double-acting type. In this type, fluid pressure is used for both the extension and retraction strokes. A four-way directional control valve is commonly
31、used to control the operation of the double-acting type. Note the small passages in the walls of rams 1 and 2. They provide a path for fluid to flow to and from the chambers above the lips of rams 2 and 3. During the extension stroke, return fluid flows through these passages and out of the cylinder
32、 through port B. It then flows through the directional control valve to the return line or reservoir. To retract the rams, fluid under pressure is directed into the cylinder through port B and acts against the top surface areas of all three ram lips. This forces the rams to the retracted position. The displaced fluid from the opposite side of the rams flows out of the cylinder through port A, through the directional control valve to the return line or reservoir.专心-专注-专业