Spring 7 holds spool 3 in the position shown in Fig. a. At this, port 1, to which compressed air is supplied from the main, is connected to port 2, and port 4 to port 5, which leads to the atmosphere. To switch over the valve, it is necessary to apply a mechanical force to head d of pusher 8 which exceeds that exerted by spring 7. When spool 3 is shifted to its left position, air from port 1 is admitted into port 4, and port 2 is connected to port 6, which leads to the atmosphere. If head d is released, pusher 8 and spool 3 are returned by spring 7 to the position shown in Fig. a. The principle of the valve is shown schematically in Figs. b and c.
$3677$SHP,FC$