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1、液壓傳動 第十講 制動器 力流體動力系統(tǒng)的優(yōu)秀的特性之一是由電源產(chǎn)生，通過適當(dāng)?shù)目刂坪椭笇?dǎo)，并通過電線傳輸，就可以輕松轉(zhuǎn)換到幾乎任何類型的機械運動所需要用到的地方。使用一個合適的驅(qū)動裝置，可以獲得線性（直線）或者是旋轉(zhuǎn)運動。驅(qū)動器是一種轉(zhuǎn)換流體動力機械力和運動的裝置。缸、馬達和渦輪機是最常見的將流體動力系統(tǒng)應(yīng)用于驅(qū)動設(shè)備的類型。這一章描述了各種類型的動作汽缸和他們的應(yīng)用程序、不同類型的流體汽車和使用流體動力系統(tǒng)的渦輪機。 汽缸 制動汽缸是一種將流體動力轉(zhuǎn)換成線性或直線、力和運動的裝置。因為線性運動是沿著一條直線前后移動的往復(fù)運動。這種類型的制動器有時被稱為一個往復(fù)、或線性、電動機。由

2、ram或活塞組成的汽缸在一個圓柱孔內(nèi)操作。制動汽缸可以安裝，以便汽缸被固定在一個固定的結(jié)構(gòu)，ram或活塞被連接到該機制來操作，或者是活塞和ram可能被固定到固定結(jié)構(gòu)，汽缸附加到機械裝置來操作。制動汽缸氣動和液壓系統(tǒng)的設(shè)計和操作是類似的。一些變化的ram和活塞式制動汽缸的內(nèi)容將在后面的段落中描述。 沖壓式缸 術(shù)語ram和活塞通?？梢曰Q使用。然而，一個沖壓式缸通常被認為是一個截面積活塞桿超過一半的截面積活動元件。在大多數(shù)這種類型的制動汽缸中，桿和活動元件各占一半。這種類型的活動元件經(jīng)常被稱為柱塞。沖壓式缸主要是用來推動而不是拉。一些應(yīng)用程序需要ram的一部分在平坦的外部來推動或升降單位操作。

3、其他應(yīng)用程序需要一些機械裝置的附件，如一個U型夾或有眼螺栓。沖壓式缸的設(shè)計在很多其他方面不同，以滿足不同應(yīng)用程序的要求。 單作用千斤頂 單作用千斤頂（如圖：10-1）試用力只在一個方向。流體定向的汽缸取代ram和他外部的彈性元件，將物體舉起放在上面。 當(dāng)流體壓力釋放后，由于沒有規(guī)定由流體動力縮回ram。物體的重量或者是一些機械設(shè)備，比如一個彈簧，迫使ram回到汽缸。這種流體能量就倒回到容器。單作用沖壓式汽缸通常用于液壓千斤頂。用于移動飛機的飛行甲板和機庫甲板航空母艦的升降機也使用這種類型的汽缸。在這些升降機中，汽缸是水平安裝的，通過一系列的電纜和捆運行升降機。流體壓力產(chǎn)生ram的外在

4、能量使升降機運行。當(dāng)從ram中釋放出流體壓力，升降機的重量迫使ram回到汽缸。反過來，這迫使流體回容器。 雙作用千斤頂 一個雙作用沖壓式缸如圖10-2。在這個汽缸中，兩個中風(fēng)的ram都是由加壓流體產(chǎn)生的。它有兩個流體端口，每一個都在或是接近汽缸的兩端。針對流體壓力的封閉端缸擴展ram和應(yīng)用力。撤銷ram和減少力、流體是指向截然相反的汽缸。一個四通換向閥是通常用于控制雙作用ram。當(dāng)閥門定位來擴展ram，加壓流體進入端口A，作用于ram的地面，加強ram的外部力量。 對于在液壓系統(tǒng)中的回流管，或者氣動系統(tǒng)中的大氣，通過控制閥門，高于ram邊緣的流體是免費流出端口B的。通常情況下，流體的

5、壓力和ram的中風(fēng)是相同的。記得第二章中，力等于壓力乘以面積（F=PA）。注意到不同的領(lǐng)域上的壓力如圖10-2所示。在擴展中風(fēng)期間，在ram適用力的同時，對抗大的表面積的壓力是來自于ram的地下。在收縮行程的過程中，ram不需要很大的動力。作用于ram頂面的小面積的壓力為收回ram提供了必要的動力。 伸縮式千斤頂 圖10-3顯示了一個可伸縮的沖壓式氣缸。一系列的ram是嵌在可伸縮的裝配中。除了最小的ram,每一個ram都是中空的,作為下一個較小的ram的油缸殼。ram裝配主要包括活塞部件,它還提供了流體端口。盡管這個裝配需要一個小空間的ram收回,但是當(dāng)ram擴展的時候，伸縮式行動的組裝提

6、供了一個相對長的沖程。這種型號的汽缸的一個很好的例子是應(yīng)用在自動傾卸卡車上。它是用來解除前端的卡車床和轉(zhuǎn)儲負載。在吊運作業(yè),最大的力量是所需的初始提升的負載。 隨著負載的提升并開始轉(zhuǎn)儲,所需的力量越來越少,直到負載是完全拋棄。在提高循環(huán)時,加壓流體通過端口A進入氣缸，然后作用在ram的底部表面。Ram 1有一個更大的表面積，因此提供了更大的力的初始負載，正如Ram 1完成它的行程所需的力是減少的，當(dāng)Ram 2移動時，提供較小的力需要繼續(xù)提高負載。當(dāng)ram 2完成它的行程,一個更小的力量是必需的。是然后Ram 3往外移動完成提高和傾銷負載。一些可伸縮的沖壓式的汽缸的單作用類型，像前面所討論

7、的單作用千斤頂，這些可伸縮的沖壓式汽缸通過重力或機械力收回。一些液壓千斤頂配備了可伸縮的ram。這樣的千斤頂是用來提升車輛通過更小的間隙到達所需的高度。其他類型的可伸縮的汽缸，如圖10-3所示，是雙作用類型。在這種類型中,流體壓力用于擴展和收縮中風(fēng)。一個四通換向閥是常用的控制操作的雙作用類型。注意在墻上的ram1和2的小通路。他們提供了一個路徑流體流向，在汽缸上面邊緣的ram2和3之間。在附加行程中，返回流體通過那些小通路和汽缸外面到達B端口。然后流經(jīng)換向閥來返回線路或蓄水池。撤銷ram，流體壓力是通過B端口直接進入氣缸的，然后反作用于三個ram 的邊緣的表面區(qū)域。這迫使ram收回了位置。流離

8、失所的流體從ram的另一側(cè)通過A端口流出汽缸，通過換向閥來返回線路或蓄水池。 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, ca

9、n 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,

10、 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 devi

11、ce 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 cylindric

12、al 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. Actu

13、ating 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 cyl

14、inder 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 t

15、o 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 eye

16、bolt. 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,

17、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

18、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 t

19、hrough 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-

20、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 forc

21、e. 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 s

22、urface 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. Re

23、call 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. Sinc

24、e 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 th

25、e 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

26、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 operat

27、ion, 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

28、 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 com

29、pletes 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

30、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 of telescoping cylinders, like the one illustrated in figure 10-3, are of the double-acting type. In this type, fluid pressure is us

31、ed for both the extension and retraction strokes. A four-way directional control valve is commonly 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 an

32、d 3. During the extension stroke, return fluid flows through these passages and out of the cylinder 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.