液压系统及液压缸

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。GLND NUTPACKED LANDCTLlNCEflHOUSINGFigiim 10-1,Si g|p-ar ting ram-(ypb acl mling(ylimpr.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。Figure 10-2.Dumbte-acling rain-iy|xhcylinder.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二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。LOWESFigtir? 1Q-XTpjipsrnping rajn-hpp artiiatingcylinclpri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rami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 PowerNAVEDTRA 14105CHAPTER 10ACTUATORSOne of the outstanding features of fluid power systems is that force, gen erated by the power supply, con trolled and directed by suitable val uing, and tra nsported by lin es, can be conv erted 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 commortypes of actuating devices used in fluid power systems. This chapter describes various types of actuati ng cyli nders and their applications,differenttypes of fluid motors, and turbines used in fluidpower systems.CY LINDERSAn actuating cylinder is a device that converts fluid power to linear, or straight line, force and motion. Since linear motion is a back-a nd-forth moti on along a straight line, this type of actuator is sometimes referred to as a reciprocating,or linear, motor. The cylinderconsists of a ram or piston operating within a cylindrical bore. Actuating cyli nders may be in stalled so that the cyli nder is an chored to a stati onary structure and the ram or pist on is attached to the mecha nism to be operated, or the pist on or ram may be an chored to the stati onary structure and the cyli nder attached to the mecha nism 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 actuati ng cyli nders are described in the follow ing paragraphs.RAM-T YPE CY LINDERSThe terms ram and piston are often used interchangeably. However, a ram-type cyli nder is usually con sidered one in which thecross-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 eleme nt is freque ntly referred to as a plun ger. The ram-type actuator is used primarily to push rather than to pull. Someapplications 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 mecha ni cal means of attachme nt, such as a clevis or eyebolt. The desig n of ram-type cyli nders varies in many other respects to satisfy the requireme nts of differe nt applicati ons.Si ngle-Acti ng RamThe sin gle-act ing ram (fig. 10-1) applies force in only one directi on. The fluid that is directed into the cylinder displaces the ram and forces it outward, lifting the object placed on it.GLMD UTFLUIIO SUPPLYPWKED GLANDCYLINDERHQUSiNiGFigure IOL1Singlp-acling ram-type actimlbg nlinder.Since there is no provision for retracting the ram by fluid power, when fluidpressure is released, either the weight of the object or somemechanical means, such as a spring, forces the ram back into the cylinder. This forces the fluid back to the reservoir. The sin gle-act ing ram-type actuating cylinder is often used in the hydraulic jack. The elevators used to move aircraft to and from the flight deck and han gar deck on aircraft carriers also use cyli nders of this type. In these elevators, the cyli nders are in stalled horiz on tally 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-Acti ng RamA double-acting ram-type cylinder is illustrated in figure10-2. Inthis 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.Fluidun der pressure is directed to the closed end of the cyli nder to exte nd 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 con trol valve is no rmally used to con trol the double-act ing ram. Whe n 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.I- igurc 10-2,-Doubk acting ram typo Mluklin胃 cy linder.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 pn eumatic 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 duri ng the retractio n stroke, pressure acting on the small area on the top surface of the ram lip provides the n ecessary force to retract the ram.Telescop ing 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 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 whe n 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.Fipun：110-1Ti*Ips oping riia-lyp1 ailyating nliTidrr.As the load is lifted and beg ins 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 ram1 reaches the end of its stroke and the required force is decreased, ram2 moves, providing the smaller force needed to continue raising the load. Whe n ram 2 completes its stroke, a still smaller force is required. Ram3 the n moves outward to finish rais ing and dump ing the load. Some telescop ing ram-type cyli nders are of the sin gle-act ing type. Like the sin gle-act ing ram discussed previously, these telescop ing ram-type cyli nders are retracted by gravity or mecha ni cal force. Some hydraulic jacks are equipped with telescoping rams. Such jacks are used to lift vehicles with low cleara nces to the required height. Other types of telescoping cylinders, like the one illustrated in figure 10-3, are ofthe double-acting type. In this type, fluid pressure is used for both the extension and retraction strokes. A four-way directional controlvalve is commonlyused 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 and3. During the extension stroke, return fluid flows through these passages and out of the cylinder through port B. It then flows through the directi onal con trol valve to the retur n line or reservoir. To retractthe 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 oppositeside of the rams flows out of the cyli nder through port A, through the directi onal con trol valve to the retur n line or reservoir.