液压系统[外文翻译]

中文字数:6039,中文页数:4 英文字数:3949页数:5

 液压系统是利用液体为介质把能量从动力源传递到消耗位置的动力传递系统。所有液压系统原理都基于帕斯卡定律，是以发现这个定律的帕斯卡的名字而命名的。这个定律表明在一个密封的容器里，如缸体或管子，受压液体向容器表面所有方向施加相等的力。
 在实际液压系统中，帕斯卡定律解释从系统中得到的各种结果。泵使流体在系统里流动。泵的吸入口接到液压油容器，通常称为液压油箱。大气压压在油箱里液体上，使液体流入泵里。当液压泵工作时，它以适当的压力把液体从油箱压到管道里。
 泵排出的高压液体由阀控制。多数液压系统运用三种控制方式：（1）流体压力控制；（2）流体速度控制；（3）流体方向控制。
 在液压系统中由泵排出的液体经由控制阀到液压马达。液压马达利用受压的流体作为它的能量源产生旋转的力和运动。液压马达跟泵的结构类似，只不过它的工作原理是相反的。
 在要求直线运动代替旋转运动的地方是用液压缸，它由运动活塞和缸体构成。当活塞由高压流体驱动时，活塞杆传递力，推动负载移动一段设定的距离。
 当液压缸中的活塞运动受阻时，例如当活塞有负载，在液压缸中必然会产生一个相应的压力。活塞面积以平方英寸为单位，受压液体的压力乘以活塞面积，就等于在活塞杆的末端产生的一个以磅为单位的输出力。
 活塞杆的速度取决于受压液体进入液压缸里的多少。可以控制油液流如液压缸的其中一端，在活塞杆上产生一个拉力或产生一个推力。在活塞上有密封装置，防止流体的泄漏。
 活塞的方向控制取决于流体进入液压缸的哪一端，当高压流体进入液压缸的一端时，流体一定从另一端流出。排出的流体又回液压油箱。在利用空气的气动系统里，空气是由缸体排放入大气中的。
 方向控制阀也称作二通阀、三通阀和四通阀等等，是根据它们的基本功能命名的。压力控制阀和简单的节流阀，通常采用二通阀，他们仅有通和断功能。三通阀可以有几种功能,这些和三通阀的油口有关。例如，来自泵的高压流体可能传送到拖拉机的液压系统里驱动负载。在不需驱动时，三通阀将高压流体送到驱动输送机构的液压马达上。
 也可以用三通阀来控制单作用式液压缸（只能单方面施力）的运动。例如，当三通阀通时，它可以阻止高压流体流入液压缸。同时，该阀可以把液压缸中的液体导向液压油箱，因此，柱塞缸可以在重力或回程弹簧的作用下回到它的初始位置。
 四通阀有四个油口。压力油口控制流体连接到需要高压的地方。同时有一个油口从高压区排出油液。排出的液体流回液压油箱。

 Hydraulic systems are power-transmitting assemblies employing pressurized liquid to transmit energy from an energy-generating source to an energy-use area. All hydraulic systems depend on Pascal’s law, named after Blaise Pascal, who discovered the law. This law states that pressurized fluid within a closed container-such as cylinder or pipe-exerts equal force on all of the surfaces of the container.
 In actual hydraulic systems, Pascal’s law defines the basis of the results which are obtained from the system. Thus, a pump moves the liquid in the system. The intake of the pumps connected to a liquid source, usually called the tank or reservoir. Atmospheric pressure, pressing on the liquid in the reservoir, forces the liquid into the pump. When the pump operates, it forces liquid from the tank into the discharge pipe at a suitable pressure.
 The flow of the pressurized liquid discharged by the pump is controlled by valves. Three control functions are used in most hydraulic systems: (1) control of the liquid pressure, (2) control of the liquid flow rate, and (3) control of the direction of flow of the liquid.
 The liquid discharged by the pump in a fluid-power system is directed by valves to a hydraulic motor. A hydraulic motor develops rotary force and motion, using the pressurized liquid as its energy source. Many hydraulic motors are similar to pumps, except that the motor operates in a reverse manner from a pump.
 Where linear instead of rotary motion is desired, a cylindrical tube fitted with a movable piston, called a hydraulic cylinder, is often used. When the piston is moved by the pressurized fluid, the piston rod imparts a force or moves an object through a desired distance.
 Restricting the movement of the piston in a hydraulic cylinder, as when the piston carries a load, creates a specific pressure relationship within the cylinder. The surface area of the piston face is said to contain a specific number of square inches. The pressure of the pressurized liquid, multiplied by the piton area, produces an output force, measured in pound, at the end of the piston rod.
 The speed of movement of the piston rod depends on how fast the pressurized fluid enters the cylinder. Flow into the cylinder can be directed to either end, producing either a pushing or pulling force at the piston rod end. A seal around the rod where it passes through the cylinder end prevents leakage of the liquid.
 Directional control of the piston depends on which end of cylinder the liquid enters. As pressurized liquid enters one end of the cylinder, liquid must be drained from the other end. The drained liquid is led back to the reservoir. In a pneumatic system using air, the air in the exhausting end of the cylinder is vented to the atmosphere.
 Directional-control valves, also called two-way, three-way, four-way, etc. , are named in accordance with their basic function. Pressure-control and simple restrictor valves are usually two-way valves. They provide ON or OFF service. A three-way valves may perform several functions, all associated with the three-ports in the valve. For example, the power or pressurized liquid from a pump in a tractor may be sent to the hydraulic system serving the tractor’s front-end loader. Or the three-way valve may send the pressurized liquid to a hydraulic motor driving a feed conveyor while the front-end loader is not being used.
 Three-way valves may also be used to direct pressurized fluid to a single-acting hydraulic cylinder. As the three-way valve is actuated (operated) it can stop the pressurized flow to the cylinder. Further, the same valve can divert liquid from the cylinder to the reservoir, so the cylinder can retract by gravity or return springs and assume its original position.

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