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, port 1, to which compressed air from the main is delivered, is connected to ports 2 and 3 which, in turn, are connected to the ends of the operating cylinder or other device. Thus both ends of the operating cylinder are under pressure. 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 continues to pass from port 1 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.
$3672$SHP,FC$