When there is no incoming pilot air through ports e and f, i.e. when these ports and the end spaces of spool 6 are connected to the atmosphere, spring 7 slides out two pins 8 (Fig. a). The length the pins protrude from the ends of spool 6 is limited and adjusted by special nuts and, consequently, spool 6 is located accurately in the central position. In this position, ports 2 and 3, connected to the ends of the operating cylinder or other device, are closed off. Port 1, to which compressed-air from the air main is deliv- ered, is also closed off. When pilot air is admitted through port e, spool 6 is shuttled to the left by the compressed air. At this, port 2 is connected to port 4, which leads to the atmosphere, and compressed air from port 1 is admitted into port 3. If port e is connected to the atmosphere and pilot air is admitted through port f, spool 6 is shuttled to the right and port 3 is connected to the atmosphere through port 5, while compressed air from port 1 is admitted into port 2. The principle of the valve is shown schematically in Figs, b, c and d.
$3671$SHP,FC$