The output piston can be raised higher andmaintained at this height if additional componentsare installed as shown in figure 2-20. In thisillustration the jack is designed so that it can beraised, lowered, or held at a constant height.These results are attained by introducing a numberof valves and also a reserve supply of fluid to beused in the system.Notice that this system contains the five basiccomponents—the reservoir; cylinder 1, whichserves as a pump; valve 3, which serves as adirectional control valve; cylinder 2, which servesas the actuating device; and lines to transmit thefluid to and from the different components. Inaddition, this system contains two valves, 1 and2, whose functions are explained in the followingdiscussion.As the input piston is raised (fig. 2-20, viewA), valve 1 is closed by the back pressure fromthe weight of the output piston. At the same time,valve 2 is opened by the head of the fluid in thereservoir. This forces fluid into cylinder 1. Whenthe input piston is lowered (fig. 2-20, view B), apressure is developed in cylinder 1. When thispressure exceeds the head in the reservoir, it closesvalve 2. When it exceeds the back pressure fromthe output piston, it opens valve 1, forcing fluidinto the pipeline. The pressure from cylinder 1 isFigure 2-20.—Hydraulic jack; (A) up stroke; (B) downstroke.thus transmitted into cylinder 2, where it acts toraise the output piston with its attached liftplatform. When the input piston is again raised,the pressure in cylinder 1 drops below that incylinder 2, causing valve 1 to close. This preventsthe return of fluid and holds the output pistonwith its attached lift platform at its new level.During this stroke, valve 2 opens again allowinga new supply of fluid into cylinder 1 for the nextpower (downward) stroke of the input piston.Thus, by repeated strokes of the input piston, thelift platform can be progressively raised. To lowerthe lift platform, valve 3 is opened, and the fluidfrom cylinder 2 is returned to the reservoir.HYDRAULIC BRAKESThe hydraulic brake system used in theautomobile is a multiple piston system. A multiplepiston system allows forces to be transmitted totwo or more pistons in the manner indicated infigure 2-21. Note that the pressure set up by theforce applied to the input piston (1) is transmittedundiminished to both output pistons (2 and 3),and that the resultant force on each piston isproportional to its area. The multiplication offorces from the input piston to each output pistonis the same as that explained earlier.The hydraulic brake system from the mastercylinders to the wheel cylinders on mostFigure 2-21.—Multiple piston system.2-16
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