spool assembly consists of a valve spool (10) anda spring (4).Fluid under main pressure enters the inlet port(11) and under all conditions is free to flowthrough the valve and the outlet port (5). (Eitherport 5 or port 11 maybe used as the high-pressureport.)Figure 6-19, view A, shows the valve in theopen position. In this position, the pressure in thereduced-pressure outlet port (6) has not reachedthe preset operating pressure of the valve. Thefluid also flows through passage 8, through smallerpassage 9 in the center of the valve spool, and intochamber 12. The fluid pressure at outlet port 6is therefore distributed to both ends of the spool.When these pressures are equal the spool is hydrau-lically balanced. Spring 4 is a low-tension springand applies only a slight downward force on thespool. Its main purpose is to position the spooland to maintain opening 7 at its maximum size.As the pressure increases in outlet port 6 (fig.16, view B), this pressure is transmitted throughpassages 8 and 9 to chamber 12. This pressure alsoacts on the pilot valve poppet (1). When thispressure increases above the preset operatingpressure of the valve, it overcomes the force ofpilot valve spring 2 and unseats the poppet. Thisallows fluid to flow through the drain port (15).Because the small passage (9) restricts flow intochamber 12, the fluid pressure in the chamberdrops. This causes a momentary difference inpressure across the valve spool (10) which allowsfluid pressure acting against the bottom area ofthe valve spool to overcome the downward forceof spring 4. The spool is then forced upward untilthe pressures across its ends are equalized. As thespool moves upward, it restricts the flow throughopening 7 and causes the pressure to decrease inthe reduced pressure outlet port 6. If the pressurein the outlet port continues to increase to a valueabove the preset pressure, the pilot valve will openagain and the cycle will repeat. This allows thespool valve to move up higher into chamber 12;thus further reducing the size of opening 7.These cycles repeat until the desired pressure ismaintained in outlet 6.When the pressure in outlet 6 decreases to avalue below the preset pressure, spring 4 forcesthe spool downward, allowing more fluid to flowthrough opening 7.COUNTERBALANCE VALVEThe counterbalance valve is normally locatedin the line between a directional control valve andthe outlet of a vertically mounted actuatingcylinder which supports weight or must be held6-14in position for a period of time. This valve servesas a hydraulic resistance to the actuating cylinder.For example, counterbalance valves are used insome hydraulically operated forklifts. The valveoffers a resistance to the flow from the actuatingcylinder when the fork is lowered. It also helpsto support the fork in the UP position.Counterbalance valves are also used in air-launched weapons loaders. In this case the valveis located in the top of the lift cylinder. The valverequires a specific pressure to lower the load. Ifadequate pressure is not available, the load cannotbe lowered. This prevents collapse of the load dueto any malfunction of the hydraulic system.One type of counterbalance valve is illustratedin figure 6-20. The valve element is a balancedspool (4). The spool consists of two pistonspermanently fixed on either end of a shaft. Theinner surface areas of the pistons are equal;therefore, pressure acts equally on both areasregardless of the position of the valve and has noeffect on the movement of the valve—hence, theterm balanced. The shaft area between the twopistons provides the area for the fluid to flowFigure 6-20.—Counterbalance valve.
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