Air is supplied from the main through port 1 and, in the position of spool 3 shown in Fig. a, passes through port 2 . Port 4 is connected to port 5, which leads to the atmosphere. At the same time, compressed air passes through central channel 7 and further through metering orifices 8 and 9 to the left and right end spaces of spool 3. Ports 10 and 11 are connected by pipelines to normally closed two-way pilot valves. Therefore the pressure is equal at the two ends of the spool which, due to friction, remains in the initial position. If port 11 is connected through a two-way pilot valve to the atmosphere and if the incoming air through orifice 9 is less than the discharge through the connecting pipeline and pilot valve, then the pressure in the right end space drops almost to atmospheric pressure. The pressure in the left end space remains constant and, owing to the pressure difference, spool 3 is shuttled to the right. When port 10 is connected to the atmosphere and port 11 is closed off, spool 3 returns to its initial position. The advantageous feature of the directional valve is that it operates with a two-way small-size pilot valve of simplest design which is not connected to the compressed air main. The directional valve operates properly only when the pipelines are of definite lengths. The principle of the valve is shown schematically in Figs. b and c.
$3669$SHP,FC$