When pilot air is admitted into port 4, spool 6 is shuttled to the position shown in Fig. a, and port 1, supplied by compressed air from the main, is connected to port 2. Port 3, connected to the atmosphere, is closed off. When pilot air is admitted to port 5 without cutting off air admission to port 4, spool 6 moves to the left because the right end of spool 6 is of greater area than its left end. This blocks port 1, and port 2 is connected to the atmosphere through port 3. Channel 7 is connected at all times to the atmosphere. When air supply is cut off to both ports, 4 and 5, spool 6 will remain in either of its extreme positions owing to friction. The principle of the valve is shown schematically in Figs. b and c.
$3691$SHP,FC$