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On the previous page you know that we have drawn all of the flow path configurations for the center position of the 5/3 valve. Now it’s time to put all the schematics together into the actual single valve schematic.
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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 it’s center position, the air supply – depicted by the black circle and line leading up to the supply port – 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 the circuit needed this valve to be at rest with one of the solenoids actuated, then the valve would be shifted to one side or the other, and the schematic 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.
A cautionary note here; if the blocked center 5/3 valve is selected to try to freeze the piston inside a normal, rodded air cylinder, over time, the piston will still tend to creep a little.
One side of the piston in a typical air cylinder has a piston rod in the center of it and that rod extends out the end of the cylinder.
Force = pressure x area. That means that if the surface area on either side of the piston were 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.
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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 shut off, then the rod and the tooling can be moved easily.
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 configuration is Pressure Center.
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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, with the piston surface area imbalance referred to earlier, will act by stopping the cylinder travel.
If the air actuator is a rodless cylinder, or a double rod air cylinder, or perhaps a rotary actuator – all of which have comparably 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, air 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 it’s 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, didn’t really affect this operation as they simply operated the valve lever to adjust the door 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.
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