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The directional-control nature of 5/3 and 4/3 valves allows them to supply compressed air to one port of a double acting air cylinder while simultaneously allowing air to exhaust from the other port on the same air actuator.
By shifting the 5/3 and 4/3 air valve air can be sent alternatively to each of the two actuator ports and exhausted alternatively from the other, thus allowing the double acting air cylinder to function.
You may recall from reading some of the other pages on this site that the first number in a valve designation will identify the number of working ports that the valve has. Therefore, their valve designation indicates that the 5/3 or 4/3 air valve will have either five or four working ports respectively since the first number is a 5 or a 4.
The 5/3 ports are; one supply, two air cylinder and two exhaust ports.
The 4/3 ports are: one supply port, two air cylinder ports, and one exhaust port.
The second number in the valve designation indicates how many positions that valve can have. The 5/3 or 4/3 will have three positions.
The “extra” position inside a 5/3 or 4/3 air valve means that the spool can be shifted to a center position. Each of the three positions is selected to accomplish a desired result in the action of the air cylinder.
Since three position air valves are more expensive than their two position counterparts, the selection of a three position valve will be deliberate. The circuit designer will have a particular scenario in mind for the action of the air cylinder when the valve that controls it is shifted, and that circuit will require the selection of a specific three position valve to accomplish the goal.
Normally, a 5/3 or 4/3 valve will supply air to one cylinder port and exhaust it from the other when the valve is in one position. When it is completely shifted, the air flow path through the valve changes, and now supply air goes out to the other actuator port, while the other valve port becomes an exhaust path, similar in function to the 2 position air valves.
A 5/3 or 4/3 valve will normally have two internal spring actuators that, when the valve is not being operated by an external valve actuator, shifts that valve to the center position; the third position.
So, it’s when the 5/3 or 4/3 valve is “at rest” that the third of the three positions comes into play.
In that third, or center position, there are three things that can happen to the air flow through that particular valve.
Blocked Center
In this position all the valve ports are blocked. Air cannot flow through the valve to either actuator port as the supply path to those ports is closed. Air cannot flow from either actuator port to either exhaust port as those flow paths too are blocked. The exhaust ports are blocked.
In this position, since air cannot travel through the valve to the air cylinder or from the cylinder back through the valve, then when the valve shifts into “Blocked Center”, the air cylinder will freeze. That is the intent of the circuit designer when selecting a “Blocked Center” 5/3 or 4/3 valve. When this valve is “at rest”, they want the cylinder to be frozen. Air cannot get in or out of the cylinder, and it stops.
Open Center
When the 5/3 or 4/3 air valve is shifted into it’s center position in an “Open Center” three position style valve, the supply line to the valve is blocked, and both cylinder ports are open through the valve to exhaust.
With this selection, the circuit designer has decided that when the valve is “at rest”, it will be necessary to move the cylinder rod (and of course the end of rod tooling) by hand, or perhaps another operation will move the rod and tooling, and since there is no air on either side of the piston inside the cylinder, this can happen relatively easily.
Pressure Center
In the “Pressure Center” position, air will flow from the supply to both air actuator ports, and the exhaust port(s) are blocked.
In this scenario the designer wants to have air to both sides of the air actuator when this valve is at rest.
The air actuator might be a rodded air cylinder, but it also might be a rodless type , perhaps being used in a vertical application.
By exerting pressure on both sides of the piston inside the rodless cylinder, the end of rod tooling can be held in one location. As an added advantage, if there are small leaks in the lines or through the seals of the rodless cylinder, a pressure center valve means that the air pressure will be maintained inside the cylinder regardless of small leaks.
Not all valve manufacturing companies offer three position valves, and those that do, not all of them offer all three of the possible valve configurations.
Drawing a 5/3 or 4/3 is a bit of a challenge. Here’s how!
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