5/3 or 4/3 air valves, like the 5/2 or 4/2 air valves, are used to power double acting air actuators.

The actuators could be air cylinders, could be rotary actuators… any air-driven device that requires air to be supplied to alternate ports in order for the device to cycle.

The 5/3 or 4/3 series are directional-control valves. The 5/3 and 4/3 body design allow compressed air to flow to one port of a double acting air actuator while simultaneously allowing air to exhaust from the other port on the same air actuator at the same time.

By shifting the internal flow paths of the valve, the 5/3 and 4/3 air valve sends compressed air alternatively to each of the two actuator ports and exhaust from the other, thus allowing the double acting air cylinder to function.

The valve shown in the following image has 5 air ports. It may be a 5/3, or it may be a 5/2 configuration. You cannot tell the difference from looking at the valve body. The valve schematic, which is typically shown on the side of the valve, is the only way you can determine if the spool is two position (a 5/2) or three position, (a 5/3) unless it is identified as such by the vendor.

A 5/2 double solenoid air valve

A 5/2 – 3 position – double solenoid air valve


Three Position Valves

The “extra” position inside a 5/3 or 4/3 air valve means that the internal spool can be shifted to a center position. The typical spool movement is end to end inside the valve. With a two position valve, the spool shifts from end, across middle, and to the other end. In the three position body style, the spool can be positioned to stop in the middle location to accomplish a specific goal.

Each of the three spool positions is selected to accomplish a desired result in the action of the air cylinder.

Since valves with three position spools 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.

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 spool to the center position automatically. It is normally when the 5/3 or 4/3 valve is “at rest” that the third of the three positions comes into play.

Three Positions – Three Results

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 also cannot flow from either actuator port to either exhaust port as those flow paths too are blocked. Supply, actuator and exhaust ports are all closed.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 dead.

Open Center

When the 5/3 or 4/3 air valve is shifted into its 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 spool 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, this is facilitated.

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 air circuit 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 .

By exerting pressure on both sides of the piston inside a rodless ( band, magnetically couples or cable type) 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 spool configurations.

5/3 Valve Numbers

The first number in a valve designation will identify the number of working ports that the valve has. Therefore, the 5/3 or 4/3 air valve will have either five or four working ports respectively.

The 5/3 valve body ports are; one supply, two air cylinder and two exhaust ports.

The 4/3 valve body ports are: one supply port, two air cylinder ports, and one exhaust port.

The second digit in the valve designation indicates how many positions that valve can have. The 5/3 or 4/3 will have three positions.

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5/3 center close valve default position