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 is connected to port 5, which leads to the atmosphere. When pilot air is admitted through port 8, the piston linked to spool 3 shuttles the spool to the left, overcoming the resistance of spring 7. At this, port 4 is connected to port 1, and port 2 to port 6, which leads to the atmosphere. The spool remains in this position as long as pilot air under pressure is admitted to port 8. When port 8 is connected to the atmosphere, spring 7 returns spool 3 to its initial position. The advantageous feature of the directional valve is that upon an accidental drop in pressure in the pilot (control) system connected to port 8, spool 3 is shuttled to the position shown in Fig. a. This can be employed to put the operating cylinder or device in a safe position upon a failure of the control system. The principle of the valve is shown schematically in Figs. b and c.
$3673$SHP,FC$