Air is supplied from the main to port 1 and further through a groove in spool 8 and channels in the body into space 3. Acting on shoulder 4 of spool 5 it shuttles this spool to the position shown in Fig. a. Ports 6 are for admitting air into the directional valve with the direction of flow controlled by the pilot valve. With spool 5 in the position shown, air is admitted through ports 6 and passes through the internal channels of spool 5 and port 7 to the right end of directional valve spool 8. This end space is connected to the atmosphere through small-diameter orifice 9. But since incoming air through port 7 exceeds the discharge through orifice 9, the pressure in the right end space rises. The left end space of spool 8 is connected to the atmosphere through a small-diameter orifice. Owing to the difference in pressure at its ends, spool 8 is shuttled to the left. This connects port 2 to port 10, which leads to the atmosphere, and, consequently, the pressure in space 3 drops. Port 11 is connected to port 1 and the compressed air acts on shoulder 13, tending to shuttle spool 5 to the right. But, at this time, spool 5 is still held by compressed air entering through ports 6 and, since the area of shoulder 14 is larger than that of shoulder 13, spool 5 remains in the position shown. When the pilot valve is released, ports 6 are connected to the atmosphere and pressure shuttles spool 5 to the right. The next time the pilot valve is operated, air from ports 6 passes through channel 15 and port 16 to the left end space of spool 8, shuttling this spool to the right to the position shown in Fig. a. When the pilot valve is released again, the pressure on shoulder 4 shuttles pilot valve spool 5 back to the position shown in Fig. a. Thus, spool 8 is shuttled upon each pulse of pilot air. The principle of the valve is shown schematically in Figs. b and c.
$3668$SHP,FC$