When there is no incoming pilot air through ports f and e, 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, port 1, to which compressed air from the main is delivered, is closed off and ports 2 and 3 are connected, respectively, to ports 4 and 5, which lead to the atmosphere. Since ports 2 and 3 are connected to the ends of an operating cylinder or other device, in the given position the device is connected to the atmosphere. When pilot air is admitted through port e, spool 6 is shuttled to the left by the compressed air, and compressed air from port 1 is admitted into port 3 while port 2 is still connected to the atmosphere through port 4. If port e is connected to the atmosphere and pilot air is admitted through port f, spool 6 is shut tled to the right and compressed air from port 1 is admitted into port 2 while port 3 is connected to the atmosphere through port 5. The principle of the valve is shown schematically in Figs. b, c and d.
$3670$SHP,FC$