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Three-way Valves
Figure 6-30.Cutaway view of poppet-type, four-way directional control valve.

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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port (4). In hydraulic systems, the return port is connected by a line to the reservoir. In pneumatic systems,  the  return  port  is  usually  open  to  the atmosphere. Pilot-Operated  Three-Way  Valves A   pilot-operated,   poppet-type,   three-way directional control valve is shown in figure 6-29. Valves of this design are often used in pneumatic systems.  This  valve  is  normally  closed  and  is forced   open   by   fluid   pressure   entering   the pilot  chamber.  The  valve  contains  two  poppets connected to each other by a common stem. The poppets are connected to diaphragms which hold them  in  a  centered  position. The movement of the poppet is controlled by the  pressure  in  the  pilot  port  and  the  chamber above  the  upper  diaphragm.  When  the  pilot chamber  is  not  pressurized,  the  lower  poppet  is seated against the lower valve seat. Fluid can flow from  the  supply  line  through  the  inlet  port  and through the holes in the lower diaphragm to fill the  bottom  chamber.  This  pressure  holds  the lower  poppet  tightly  against  its  seat  and  blocks flow from the inlet port through the valve. At the same  time,  due  to  the  common  stem,  the  upper poppet  is  forced  off  of  its  seat.  Fluid  from  the actuating  unit  flows  through  the  open  passage, around  the  stem,  and  through  the  exhaust  port to  the  atmosphere. When  the  pilot  chamber  is  pressurized,  the force  acting  against  the  diaphragm  forces  the poppet down. The upper poppet closes against its seat, blocking the flow of fluid from the cylinder to the exhaust port. The lower poppet opens, and the  passage  from  the  supply  inlet  port  to  the cylinder  port  is  open  so  that  the  fluid  can  flow to  the  actuating  unit. The valve in figure 6-29 is a normally closed valve.  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 the lower poppet is closed. Fluid is free to flow from the inlet port through the cylinder to the actuating unit. When pilot pressure is applied, the poppets are  forced  downward,  closing  the  upper  poppet and  opening  the  lower  poppet.  Fluid  can  now  flow from the cylinder through the valve and out the exhaust  port  to  the  atmosphere. FOUR-WAY   VALVES Most actuating devices require system pressure for  operation  in  either  direction.  The  four-way directional   control   valve,   which   contains   four ports,  is  used  to  control  the  operation  of  such devices. The four-way valve is also used in some systems to control the operation of other valves. It  is  one  of  the  most  widely  used  directional control  valves  in  fluid  power  systems. The typical four-way directional control valve has four ports: a pressure port, a return or exhaust port,  and  two  cylinder  or  working  ports.  The pressure  port  is  connected  to  the  main  system pressure line and the return line is connected to the reservoir in hydraulic systems. In pneumatic systems the return port is usually vented to the atmosphere.  The  two  cylinder  ports  are  connected by  lines  to  the  actuating  units. Poppet-Type Four-Way Valves Figure 6-30 shows atypical four-way, poppet- type directional control valve. This is a manually operated  valve  and  consists  of  a  group  of conventional spring-loaded poppets. The poppets are  enclosed  in  a  common  housing  and  are interconnected by ducts to direct the flow of fluid in the desired direction. Figure 6-29.—Three-way, poppet-type, normally closed directional control valve (pilot-operated). 6-20







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