The directional valve shown in Fig. a can be employed either as a normally closed or normally open valve, which is mechanically operated from one side. When used as a normally closed valve, compressed air is supplied from the main to port 1, and port 2, the outlet port, is connected through a channel in valve member 5 to port 3, which is connected to the atmosphere. The valve is operated by pressing palm button d of plunger 6. As the plunger is shifted downward, first rubber insert 7 shuts off the channel in valve member 5, disconnecting port 2 from the atmosphere. Upon further motion, rubber insert 4 of valve member 5 is pushed off its seat. At this, air from port 1 is admitted into port 2. When used as a normally open valved, compressed air is supplied from the main to port 3, and port 1 is used for connection to the atmosphere. The given design has a palm button for manual operation. Other versions can be operated by levers or other mechanisms. The principle of the valve when used as normally closed is shown schematically in Pigs. b and c.
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