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 4, and port 2 to port 6, which leads to the atmosphere. Spool 3 is switched over to either of its two positions manually by means of lever 7. The spool can be linked to any mechanism (cam, lever, etc.) for shifting. When spool 3 is shifted to its right-hand position, air from port 1 is admitted into port 2, and port 4 is connected to port 5, which leads to the atmosphere. The principle of the valve is shown schematically in Figs. b and c.
$3678$SHP,FC$