All-Ports Blocked / Closed Center Valve

GA

yes, as already stated, the leackage accross your valve will allow the cylinder to move, and as slready stated you could get a more precice valve for more $$ with less leakage (drips/min)

Also as stated, try a double acting cylinder (which may or may not work depending on application.)

in short bhrescobar answered it. GA from me

Thank you all for the input.

The reason I need to use an all port blocked valve is because in my application, I need to be able to hold the cylinder in its fully retracted position.

It is a single acting cylinder and the air is supply to the front port to retract the cylinder. The spring on the back will push the cylinder out once the air is exhausted from the front port.

In my application, speed is critical. I tried with a double acting cylinder and it is too slow. I also don't have any flow control and also it is a spool valve because I was told that it works faster the pilot-operated valve...

Any more suggestions?

Many thanks

We have used quick dump valves with much success to increase the actuation speed of double acting cylinders.

Yep. That's what I mean... pressure on one side, vent the other. Then, invert it to move the opposite way.

I notice not only the cylinder slowly creeps out when it is stopped at its fully retracted position, but also slowly creeps in when it is already fully extended.

This is interesting considering while the cylinder is extended, it is so by the power of the spring. The only way the cylinder could creep against the spring is if you are getting pressure to the cylinder even when all ports blocked (valve leakage). this must be a very leaky valve. I'm guessing you are using a spool valve currently which you can't really expect any load holding due to the leakage rate of the spool.

I suggest you get PO check valves (pilot operated check valves) to do the load holding, and leave the spool valve to perform only what it is capable of, (directing flow)

Use a Single Solenoid 2 Position Spring Return valve and connect it so the cylinder is normally extended by its Spring. That would Fail to Brake On.

To run the machine energize the solenoid to hold the Brake Released.

I suggest still putting PO check valve/s to eliminate the creep due to leakage in the valve.

RVZ717 wrote:

"I suggest still putting PO check valve/s to eliminate the creep due to leakage in the valve."

This will only work if the valve is changed to "P" Blocked, "A" and "B" to "T".

We always put in PO checks for any workholding cylinder to eliminate any creep, it works beautifully on a 3 pos 4 way valve with center pos all ports blocked. maybe if i have a bit of free time, ill sketch a few schematic examples.

Bang Bang valves tend to leak less than spool, (drops per minute(at least when talking fluid))

There are many ways to skin a cat (sorry Del, no offense) the question to be asked, is how precise does the machine need to be? What ammount of creep is acceptable? How long is the cylinder stopped in each position?

$$-vs-quality...

Del needn't feel slighted. The phrase refers to skinning catfish. I know of three ways, and I'm sure there are many more.

Interesting, I was just commenting to a friend about how i diden't really understand the saying that well. catfish eh? three ways? thats two more than i know.

1. Separate the meat from the backbone, then put your knife under the skin at the tail and run it toward the head.

2. Cut through the skin at the tail and separate a small distance. Grab with pliers and pull.

3. use a fish skinner; a roller that does the same as 2, but is neater unless you know what you're doing.

Hi RVZ717,

Thanks for your suggestion.

Can I modify my spool vavle into a PO checks? Or I have to buy a new one?

Thanks

Chin

you have to use the PO checks in addition to your spool valve. I will attempt to generate a small schematic for you. The check valves are fairly inexpensive, and you will be able to utilize your current valve/cylinder

Hello All,

There are two effective soltions that I can suggestion for this prolem and I've applied both to many such applications.

Sol#1

Use a double acting cylinder with two independant 3/2 normally closed valves(poppet sytle). You can sequence them so that the one valve will work as a air spring with a fixed pressure(ie spring force). Similar to a mechanical spring but with the ability to control the force via the pressure. With two independant valves you eliminate intenal leakage and hence creep.

If you have to use a single valve with mid position blocked, them you must incorporate the PO checks as suggested. What you have to do is activate the pilot ports of the PO checks from the opposite valve port (a cris-cross circuit).

Sol #2

Use a 3/2 normally closed valve on the retract port of the single acting cylinder. If you can, mount the valve directly on the cylinder port. This will minimize lag and increase reaction ans speed. If not, get the valbve as close to the cylinder as possible.

Hi Chin

You are using a spool valve. Most of them leak. You need to make a change.

1. Change your cylinder to a double acting cylinder instead of spring extended and hold it in either position with air pressure using only two positions of the valve.
2. Or you can use the cylinder you have, apply and hold pressure to keep the spring compressed (retracted) and exhaust pressure to allow the spring to extend the cylinder naturally.

A blocked center valve may not be ideal for your application. Try a thee way (3 ports) valve.

Tom

I recommended the PO checks as a easy solution to your problem, without having to change out your cylinder/valving. True there are many solutions to the problem, you simply have to make a choice as to which avenue you want to travel down. most of the solutions provided to you here are viable, and should be considered. the ultimate decision will be made depending upon your current circumstances. how much $$ you wish to spend, how you want the end result to operate, and how many parts you wish to replace.

not knowing the application makes it hard to figure the best solution, but in general if it were my problem, i would install 2 PO check valves criss crossing the pilot ports, this is only the simplest solution to my knowledge.

Why don't you just use a double acting cylinder and a 5 port valve instead?

Also you can use a piloted check valve on the port.

What brand are you using, since I've never really had any creep issues with Festo solenoids.

I first tried with double acting 5 port valve but the it is slow. And hence, I use spring return instead. I use MAC valve.

How does a PO check work in this case? Can any body help me with a quick schematic diagram? My schematic is attached. Many thanks

Hi Chin

A picture is worth a thousand words. Keep one solenoid energized to allow the spring to extend the cylinder. Switch to keep the other solenoid energized to compress the spring and retract the cylinder. Keep one or the other solenoid energized at all times and ignore the blocked center position.

Yup - that has the same action as a 3/2 valve.

Many Thanks, Tom!!

Thanks a lot, RVZ717!

Chin

In reality, for a spring return cylinder, you only need one POcheck. also, if you are only running the cylinder in a fully retracted, fully extended position, you can do it by changing your valve to a 2 POS 3Way (i believe this has been suggested) If you need to stop and hold the cylinder in multiple locations, there are again more possibilities. The correct answer really depends on your application. Pneumatics are a bit different than hydraulics, due to the compressability of the air. In hydraulics adding the PO checks would be the first, cheapest, possibly best solution, while in pneumatics you may have multiple solutions that can yield relatively the same result. The bottom line best answer is fully dependent on your specific application.