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In the position of spool 3 shown in Fig. a, port 1, to which compressed air is supplied from the main, is connected to port 2, and port 4 to port 5, which leads to the atmosphere. When pilot air is admitted through port 8, spool 3 is shuttled to the left and remains in this position, held by the friction forces, even after port 8 is connected to the atmosphere. At this, compressed air from port 1 is admitted into port 4, and port 2 is connected to port 6, which leads to the atmosphere. To return spool 3 to its initial position, pilot air is admitted through port 7. Directional valves of this type are most widely applied in various machines with systems of in-travel control. The principle of the valve is shown schematically in Figs. b and c.