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.

Table of Contents

5 way 3 position pneumatic valve

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.

5-3 Valves 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
  • Open Center
  • Pressure Center

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.


5/3 Closed Centre Valve Default Position

by Peter Foster
(New Castle, Delaware)

We use a Festo 170248 5/3 closed centre valve to actuate a double acting cylinder.

I understand that if the air or electrical supply is interrupted the cylinder will remain in position (except for some creep due to the different surface area at each end.

Valve circuit schematic
Valve circuit schematic
Festo Valve Schematic
Festo Valve Schematic

My question is if the valve function itself fails (spring/leak etc.) do these valves default to the centre closed position?

We are trying to answer a safety question as to what happens if something goes wrong with the valve. Thanks. I’ve attached the schematic, the valve in question is the one going to the DNG-200 cylinder.


I spent some time working as a territory rep for Festo, so know that if you contact them, they will have a distributor rep by to see you in no time, at least that’s the way it used to be.

Having said that, the best thing to do to determine the answer to your question is to look at the schematic of the valve.

I have uploaded a copy of the Festo 170248 5/3 closed centre valve schematic, and ask you to review it. In particular, look at either end of the schematic, and note the graphic that denotes a spring.

Since there is a spring on either end, if this valve fails due to loss of electricity or air pressure, it is designed to “spring center”, and in so doing, block all ports.

If there were no springs, “sticktion” would come into play, and the valve spool would likely stay in the position it was last in if the power and air disappeared, unless vibration moved the spool. That would not be a safe type of valve configuration to use.

In other words, the valve you have selected is designed to “fail safe” by blocking all ports and stopping any actuators in the event of a power or air disruption.

Hope that helps.

Cheers,

Bill


Festo comms
by: Anonymous

Hi Bill,
Thanks for the response. Festo seem to concur with what you say.

I spoke to them and got a couple of varying answers. They obviously don’t want to commit but the 1st guy said: with no electrical power it centre closes, with no air it centre closes, with a broken spring it will still centre since the springs are designed not to go past centre.
The 2nd guy agreed with the 1st but was less committed in his response and said they couldn’t say how far the spool might move with 1 spring failing due to variable friction etc. which I can also understand. I think it depends a bit on who you talk to.

So overall it looks like we are OK. The rub will come in the safety review when we talk about failure modes and whether any redundancy needs to be built into the system. I’ve seen double pilot valves for presses, lots of bucks and probably an overkill. However, the user does have his hand under this ram a lot of the time so we’ll see.

We are having a Festo rep come in to take a look also.

Thanks again, great website by the way. v. useful.


Thanks for the comment about my website. I’m sure glad you find it useful.

With person safety, the question is, how far do you go?

The valve is supposed to work. If a spring fails, it may or may not.

In order to take that out of the equation, then plumb in double redundancy so that if one valve fails, the other valve will do the job.

The odds of both valves or valve components failing at the time of a power or compressed air outage are probably more remote than you winning the power ball lottery.

Is that safe enough? That’s up to the safety committee.

Festo is a first class company selling extremely well built air components. But even at that, they cannot determine the conditions of use or how a product will ultimately fail, and the ramifications of that. That’s up to the user.

Again, thanks for your kind comments.

B.


Valve failure
by: Doug in s.d.ca

I don’t know your exact application, but perhaps some sort of pusher could be made/used to keep the operator’s hand completely out of the potential danger area?


Good thought, Doug. The device in question is actually called an “anti-tie down” and it’s purpose is to ensure that an operator’s hands are not ever in an area where they could be impacted upon.

B.


Double redundancy
by: Anonymous

Just a follow up on this, the only thing I can see that will give that redundancy is something like one of the Ross safety valves which are usually used for presses.

https://rosscontrols.com

These are designed to return the cylinder to a safe or retracted position if one spool doesn’t move as expected. This seems an overkill for us, I’d like to find a compromise. The problem is detecting the fault once it occurs.


A pressure sensor can tell a system if a particular valve has shifted or not, that can be tied into a blocking check valve that locks a cylinder if necessary.

There are lots of folks in the industrial pneumatic distributor business that can help you design the appropriate circuit and specify the products you need.

Circuit design is out of the purview of this site. If you want some help with that send me a message through the contact page and I’ll provide rates.


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