Air is supplied from the main through port 1 and further through internal channel 7 of spool 3 to the left end space of the spool (Fig. a). When pilot air is admitted through port 8, then, since the cross-sectional area of piston 9 is considerably greater than that of spool 3, the spool is shuttled to the position shown in Fig. a. At this, 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. When port 8 is connected to the atmosphere, the pressure acting on the left end of spool 3 shuttles it to the right. This connects port 1 to port 2 and port 4 to port 5, which leads to the atmosphere. The advantageous feature of the directional valve is the absence of a mechanical spring, and this reduces the probability of valve failure. In addition, upon an accidental drop in pressure in the pilot (control) system connected to port 8, the valve is put into a definite position. 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.
$3674$SHP,FC$