port (4). In hydraulic systems, the return port isconnected by a line to the reservoir. In pneumaticsystems, the return port is usually open to theatmosphere.Pilot-Operated Three-Way ValvesA pilot-operated, poppet-type, three-waydirectional control valve is shown in figure 6-29.Valves of this design are often used in pneumaticsystems. This valve is normally closed and isforced open by fluid pressure entering thepilot chamber. The valve contains two poppetsconnected to each other by a common stem. Thepoppets are connected to diaphragms which holdthem in a centered position.The movement of the poppet is controlled bythe pressure in the pilot port and the chamberabove the upper diaphragm. When the pilotchamber is not pressurized, the lower poppet isseated against the lower valve seat. Fluid can flowfrom the supply line through the inlet port andthrough the holes in the lower diaphragm to fillthe bottom chamber. This pressure holds thelower poppet tightly against its seat and blocksflow from the inlet port through the valve. At thesame time, due to the common stem, the upperpoppet is forced off of its seat. Fluid from theactuating unit flows through the open passage,around the stem, and through the exhaust portto the atmosphere.When the pilot chamber is pressurized, theforce acting against the diaphragm forces thepoppet down. The upper poppet closes against itsseat, blocking the flow of fluid from the cylinderto the exhaust port. The lower poppet opens, andthe passage from the supply inlet port to thecylinder port is open so that the fluid can flowto the actuating unit.The valve in figure 6-29 is a normally closedvalve. Normally open valves are similar in design.When no pressure is applied to the pilot chamber,the upper poppet is forced off of its seat and thelower poppet is closed. Fluid is free to flow fromthe inlet port through the cylinder to the actuatingunit. When pilot pressure is applied, the poppetsare forced downward, closing the upper poppetand opening the lower poppet. Fluid can now flowfrom the cylinder through the valve and out theexhaust port to the atmosphere.FOUR-WAY VALVESMost actuating devices require system pressurefor operation in either direction. The four-waydirectional control valve, which contains fourports, is used to control the operation of suchdevices. The four-way valve is also used in somesystems to control the operation of other valves.It is one of the most widely used directionalcontrol valves in fluid power systems.The typical four-way directional control valvehas four ports: a pressure port, a return or exhaustport, and two cylinder or working ports. Thepressure port is connected to the main systempressure line and the return line is connected tothe reservoir in hydraulic systems. In pneumaticsystems the return port is usually vented to theatmosphere. The two cylinder ports are connectedby lines to the actuating units.Poppet-Type Four-Way ValvesFigure 6-30 shows atypical four-way, poppet-type directional control valve. This is a manuallyoperated valve and consists of a group ofconventional spring-loaded poppets. The poppetsare enclosed in a common housing and areinterconnected by ducts to direct the flow of fluidin the desired direction.Figure 6-29.—Three-way, poppet-type, normally closed directional control valve (pilot-operated).6-20
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