Fig. 60 shows the valve schematic for a 5 ported, three position, Blocked Center, compressed air valve.
I have shown the two internal spring actuators on this schematic which, when any external valve actuators are not being employed, will center the spool in the valve.
Since, at rest, this valve will be in its center position, the air supply - depicted by the black circle and line leading up to the supply port of the valve body - is shown in the center position.
If this 5/3 were left in an energized state, that is to say - that the external valve actuators were solenoids and this valve was at rest with one of the solenoids actuated, then the valve would be shifted to one side or the other, and the schematic for that valve would so depict this.
In the Blocked Center 5/3 valve, the circuit designer has decided that when this valve is at rest the all ports will be shut. No air can flow into the valve, and no air can flow from the two cylinder ports at the top of the valve into or out of the air actuator.
Force = pressure x area. If the surface area on either side of the piston were exactly the same, the piston would be frozen in position by a 5/3 blocked center valve.
However, the cylinder rod reduces the available surface area on one side of the piston. Since air pressure exerts force equally in all directions, and since the rod side of the piston has a smaller surface area upon which for the compressed air to push, the relatively larger area on the non-rod side of the piston will slowly overcome the force of the air on the rod side of the piston, allowing the piston to shift. This means that the rod and the end-of-rod tooling will move too.
In Fig. 61, we show the same type of schematic, but now the center position shows the Open Center flow path.
Compressed air circuit designers will typically use an open center 5/3 valve where there is a need to move the rod and/or rod tooling when the valve is at rest. With both cylinder ports on the valve open to atmosphere, and with the compressed air supply blocked, then the rod and the tooling can be moved easily by hand.
Or, in the event that the valve is solenoid operated, and there is a power failure, the air would dump from the cylinder from both air ports, and the tooling could be moved manually back to a safe-start position.
The last 5/3 air valve configuration is Pressure Center.
Using this valve configuration, the circuit designer has decided that, when this valve is at rest, compressed air will be supplied to both ports on the actuator at the same time.
This will stop cylinder rod travel. Although, if it is a typical rodded type air cylinder, the piston surface area imbalance will allow the cylinder piston to creep.
If the air actuator is a rodless cylinder, or a double rod air cylinder, or perhaps a rotary actuator - all of which have comparable surface areas on both sides of their pistons - then the pressure center position would freeze them solid.
In the event there were air leaks between the valve and the actuator, that the air supply to the actuator through the valve was constantly regenerated, leaks which could cause the piston to shift one way or the other, would be overcome.
I recall one application for a 5/3 pressure center, lever operated hand valve. This client was opening a swinging door on a dump bin, and the three position valve was used to manually extend or retract the cylinder that controlled the door opening. By shifting the valve one way the cylinder would open the door. By using the valve actuator lever to shift the valve the other way, the cylinder would close the door to reduce, and ultimately stop, the flow. By letting go of the valve actuator lever, the valve - in its center position - would feed air to both sides of the door cylinder, holding the door in place. That, over time, the imbalance in piston surface area would allow the piston to move, did not really affect this operation as they simply operated the valve lever to adjust the door position again as required.
By now I expect that you'll have figured out that exactly the same process is used to draw the 4/3 valve. Fig. 63 shows a 4/3 valve with blocked center position. You would simply substitute the schematics for the open center or pressure center as needed, to show all configurations of the 4/3 air valve.
The 4/3 air valve body has only one exhaust port as opposed to two, as shown in the next image.