when the valve is open. A small piston (9) isattached to the bottom of the spool valve.When the valve is in the closed position, thetop piston of the spool valve blocks the dischargeport (8). With the valve in this position, fluidflowing from the actuating unit enters the inletport (5). The fluid cannot flow through the valvebecause discharge port 8 is blocked. However,fluid will flow through the pilot passage (6) to thesmall pilot piston. As the pressure increases, it actson the pilot piston until it overcomes the presetpressure of spring 3. This forces the valve spool(4) up and allows the fluid to flow around theshaft of the valve spool and out discharge port8. Figure 6-20 shows the valve in this position.During reverse flow, the fluid enters port 8. Thespring (3) forces valve spool 4 to the closedposition. The fluid pressure overcomes the springtension of the check valve (7). The check valveopens and allows free flow around the shaft ofthe valve spool and out through port 5.The operating pressure of the valve can beadjusted by turning the adjustment screw (1),which increases or decreases the tension of thespring. This adjustment depends on the weightthat the valve must support.It is normal for a small amount of fluid to leakaround the top piston of the spool valve and intothe area around the spring. An accumulationwould cause additional pressure on top of thespool valve. This would require additionalpressure to open the valve. The drain (2) providesa passage for this fluid to flow to port 8.DIRECTIONAL CONTROL VALVESDirectional control valves are designed todirect the flow of fluid, at the desired time, to thepoint in a fluid power system where it will dowork. The driving of a ram back and forth in itscylinder is an example of when a directionalcontrol valve is used. Various other terms are usedto identify directional valves, such as selectorvalve, transfer valve, and control valve. Thismanual will use the term directional control valveto identify these valves.Directional control valves for hydraulicand pneumatic systems are similar in designand operation. However, there is one majordifference. The return port of a hydraulic valveis ported through a return line to the reservoir,while the similar port of a pneumatic valve,commonly referred to as the exhaust port, isusually vented to the atmosphere. Any otherdifferences are pointed out in the discussion ofthe valves.Directional control valves may be operated bydifferences in pressure acting on opposite sidesof the valving element, or they maybe positionedmanually, mechanically, or electrically. Often twoor more methods of operating the same valve willbe used in different phases of its action.CLASSIFICATIONDirectional control valves may be classified inseveral ways. Some of the different ways are bythe type of control, the number of ports in thevalve housing, and the specific function of thevalve. The most common method is by the typeof valving element used in the construction of thevalve. The most common types of valvingelements are the ball, cone or sleeve, poppet,rotary spool, and sliding spool. The basicoperating principles of the poppet, rotary spool,and sliding spool valving elements are discussedin this text.PoppetThe poppet fits into the center bore of the seat(fig. 6-21). The seating surfaces of the poppet andthe seat are lapped or closely machined so thatthe center bore will be sealed when the poppet isFigure 6-21.—Operation of a simple poppet valve.6-15
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