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About piston valves

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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About piston valves

Unread postAuthor: Hubb » Fri Apr 09, 2010 1:49 pm

Piston valves are always fun to talk about, right? Well, here's a question for us to ponder on:

Everyone says that the actual piston should be a bit larger than the output of the valve, right? Well, why is that exactly? I know the common answer is so that the chamber's pressure can act against the valve to force it in open, but doesn't the uneven pressure created by dumping the pilot already do that? Has anyone ever actually tested this to see if it does or does not apply?

My thinking is that, with a lightweight piston, the chamber air is not needed to force the valve open, as the uneven pressure presented to the valve upon the exhaust of the pilot will be enough to open it. This means that the output of said valve can be made larger, which will increase the flow rate of the valve. It also means that a valve that is to be built around a specific output diameter can be made to a smaller profile.

Now, some of you may say that by having the chamber air push open the valve, that the valve will open faster. I see no reason to doubt this, but what kind of performance gains can be had with any type of launcher by having the valve go from opening in .4 milliseconds to having the same valve open in .36 milliseconds?
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Unread postAuthor: dewey-1 » Fri Apr 09, 2010 2:39 pm

Perhaps the only piston type valve I can see that works with the same outlet port size as the piston OD is Techs QDV.

All other barrel sealing types require a smaller porting size than the piston to get the pressure differential to force it open with air release piloting.

Now if the outlet porting was equal to piston OD a mechanical means of actuating it would be required. Like the QDV pull shaft.
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Unread postAuthor: iknowmy3tables » Sat Apr 10, 2010 2:05 pm

you're talking about barrel sealing pistons right?
Hubb wrote:Everyone says that the actual piston should be a bit larger than the output of the valve, right? Well, why is that exactly? I know the common answer is so that the chamber's pressure can act against the valve to force it in open, but doesn't the uneven pressure created by dumping the pilot already do that?

no, what do you think the pressure in the pilot is uneven to? the force of the pressure in the pilot is supposed to be uneven to the force of the pressure on the other side of the piston, if you have to 2 o-ring seals of equal size then that's just a QDV, if you hypothetically had pressurized air behind the a QDV piston then but then vented it it would stay in the same place and it wouldn't move unless you created a vacuum below atmospheric pressure in the pilot, you need some force/pressure displacement to make the valve open

however if you are thinking of a sealing face, then I'd understand because a sealing face needs some pressure in the pilot to seal, so then valve would be capable of opening initially, there's a diagram of what I'd imagine this to be like below

I'm really interested in better understanding the true performance difference caused by having initial force from pressure in the chamber vs force from pressure on the piston only after it's opened, it was really bugging me when I posted this thread
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equal diameter piston and output.PNG
is this perhaps what you had in mind?
equal diameter piston and output.PNG (15.51 KiB) Viewed 1392 times

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Unread postAuthor: psycix » Mon Apr 12, 2010 3:48 pm

Could work, but only if you can size the piston properly.

This piston valve will open after the pilot volume has dropped to almost atmospheric pressure, when the pressure will leak into the porting as there is not enough pressure to seal anymore.
Without an O-ring or precise fit, this will dump a lot of air through the pilot before finally opening.
However, if you can prevent that, like with the o-ring. This "late opening valve" is also a very fast opening valve.
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Unread postAuthor: Technician1002 » Mon Apr 12, 2010 10:10 pm

That late opening valve is known for difficulties to get it to fire at all. Tight fit, too much pilot leakage, and such can prevent the valve from opening. The thread on Piston Valves in the WIKI covers the topic going from valves with a large diameter ratio, to a close to 1:1 ratio like the one shown, to the QDV, a 1:1 ratio valve. The closer it is to 1:1, the lower the pilot pressure has to be - and the force to start it opening is reduced, so pilot leakage keeping some pressure in the pilot area and sticky pistons are common in the low ratio designs.

The QDV was built due to the relibility issues with a close ratio valve. I wanted the performance, but not the problems. I've been very happy with the result. It never miss-fires. 8)

Link to the Wiki page;
http://www.spudfiles.com/spud_wiki/inde ... ston_valve
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Unread postAuthor: Hubb » Mon Apr 12, 2010 11:31 pm

I see. The main question in this thread was what kind of performance losses would be present in a near 1:1 piston to output ratio. It does seem that the performance would suffer.

From what I can find, it seems that if one would like to have a piston with a 1:1 ratio, it would need to be constructed using an open piston. This is an example of what I mean by open piston. It's a wonder that I don't see more of these types of pistons, as I can see them performing better than standard piston valves.

The above piston is used here for reference. I would have liked to see this valve put to a launcher, but I suppose the builder had just cause.
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Unread postAuthor: Technician1002 » Tue Apr 13, 2010 2:16 am

I considered an open piston, but due to the speed and force the piston has the weight difference between a steel construction and the plastic was comparable and the plastic was less likely to sustain damage. The center shaft connection to the disks is a very high stress area.

Another issue would be how to seal the bolt and nuts where it passes through a disk against leakage. In addition on a 1:1, the use of o rings is difficult to use in that design. For stress, o rings (floating), the center control rod, high mass parts, and other factors, the spool design was not used in my 1:1 designs. It was considered in the design stage. I'll have to see if I still have a prototype I built to toy with the idea. It consisted of a pair of discs cut from HDPE threaded onto a 3/8 inch pipe nipple. This permitted the control rod and cutting of a pair of o ring grooves.

The main concern was safety related if a threaded disk stripped out the threads, the cannon could fire unexpectedly as the piston came apart. This was the primary reason a single solid piston was chosen. Launching out parts of a piston into a crowd in the t shirt launcher competition was a huge safety concern. In a spudgun, down range safety is not as important as having a crowd of people downrange.

HDPE for the volume is about 1/8th the mass of steel. The HDPE piston's mass is widely distributed with no high stress points.

I removed much of the center of the piston in my design to remove mass and limit the amount of space in front of the piston was required for the control rod.
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Unread postAuthor: iknowmy3tables » Wed Apr 14, 2010 4:43 pm

you really should mention that piston used a sealing face and the fact the it is near 1:1 not exactly 1:1.
the performance difference all depends on a lot of things, in this thread I had a very similar concern and most of the factors that I mentioned that could make a chamber sealing valve more likely to out preform it's barrel sealing equivalent are the same factors that may make a near 1:1 ratio barrel sealer valve have inferior performance,

so on a typical piston there is an area that is constantly acted upon by the pressure from the chamber (also countered by the pressure in the pilot) and an area that is acted upon only after the piston opens, and as you know we depend on this area to provide the jump is force that makes barrel sealers effective, however when the piston is open air is rapidly escaping so in cases where pressure does not build up at the outlet the area exposed after the piston moves is less effective at creating the jump in force and the area constantly exposed to chamber pressure is more effective, this is more often the case with lower pressures, lower chamber volumes, extremely light or absent projectiles, and short barrels

I would feel better with a piston to seal ratio of less than 1:1 for a better "initial force" opening the piston
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 14, 2010 6:36 pm

psycix wrote:This piston valve will open after the pilot volume has dropped to almost atmospheric pressure.


I don't see it opening at all as illustrated by iknowmy3tables, there simple is no pressure differential!

It will dump the pilot and remain static.
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Unread postAuthor: iknowmy3tables » Wed Apr 14, 2010 6:43 pm

but it's a sealing face and not a plug, therefore it needs to have a certain amount of pressure in the pilot to push the sealing face and make it seal
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 14, 2010 7:13 pm

iknowmy3tables wrote:but it's a sealing face and not a plug, therefore it needs to have a certain amount of pressure in the pilot to push the sealing face and make it seal


Regardless, if the piston rides in a tube that is the same diameter or smaller than the barrel, it will never open. Take a look at your design in detail and check the pressure vectors, it balances out so no net force on the piston. The most that can happen is that the rear o-ring flexes and pressure in the chamber leaks through the pilot.
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PISTONBALANCE.GIF
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Unread postAuthor: Technician1002 » Wed Apr 14, 2010 7:51 pm

iknowmy3tables wrote: however when the piston is open air is rapidly escaping so in cases where pressure does not build up at the outlet the area exposed after the piston moves is less effective at creating the jump in force and the area constantly exposed to chamber pressure is more effective, this is more often the case with lower pressures, lower chamber volumes, extremely light or absent projectiles, and short barrels


Just an FYI on this subject. If the pilot area is larger (counter intuitive I know ) then when the piston pops open, very little barrel projectile mass or barrel length is required if the piston has little moving friction. My 2 inch QDV still pops open with no barrel attached, no projectile, etc. This can be seen in my O ring video on youtube.

With a sticky piston and little back pressure and relying on the pressure spike when the valve cracks a little, then yes, the valve can crack, leak, and fail to pop.

I have pulled this gently to bleed down the pressure and found it very difficult as most of the time it pops instead of cracking and bleeding down with no barrel attached. It will pop with a 2 inch long 2 inch diameter barrel (valve outlet area) with no projectile.

Link to the video that most members has already seen.
http://www.youtube.com/watch?v=BbDiZiRVrnk
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Unread postAuthor: iknowmy3tables » Wed Apr 14, 2010 7:56 pm

jackssmirkingrevenge wrote:Regardless, if the piston rides in a tube that is the same diameter or smaller than the barrel, it will never open. Take a look at your design in detail and check the pressure vectors, it balances out so no net force on the piston. The most that can happen is that the rear o-ring flexes and pressure in the chamber leaks through the pilot.

I intended to show that they were equal diameter, but I understand

indeed they cancel out and there is no net force, and for the sealing face to seal there needs to be a positive force pushing the piston against the barrel. otherwise air will get through,

I know what you mean, normally there is some force difference from the pressure that helps the piston start moving, but in this case it's initiated by a "leak" of air through the sealing face that isn't sealing.

it's possible that if you used tough fixed non floating o-ring that were very tight there might be enough static friction to hold the sealing face in place, but I imagine typically something should yeild
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 15, 2010 12:20 am

iknowmy3tables wrote:I know what you mean, normally there is some force difference from the pressure that helps the piston start moving, but in this case it's initiated by a "leak" of air through the sealing face that isn't sealing.


If your piston has an equalisation hole, the chamber would have been emptied through the pilot valve before the the piston even contemplates moving.

If you want to make your piston open faster, minimise your pilot volume and piston weight, and improve your pilot valve. For the ultimate in piston valve opening, fuçk pilot volume :D
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Unread postAuthor: Technician1002 » Thu Apr 15, 2010 1:43 pm

The pilot volume has several important considerations in a cannon, so it can't be ignored.

The larger the pilot volume and the smaller the trigger, the longer that pilot takes to vent to where the piston unseats. Small pilot volume and large trigger reduces this time so the amount that leaks out the EQ port is lessened. A really small pilot volume causes the air in the pilot to compress as the piston moves back and as the chamber vents quickly out the barrel, the pilot pressure may cause the piston to close again before opening completely. This is especially true where the valve seat is small and the piston diameter is large. To compensate, a really large and fast trigger valve is often used such as a sprinkler valve.

In mine (QDV) the pilot area is huge and dual chamber, the small area by the bumper and the large area out to the trigger knob. The hole in between for the rod is used as a dashpot to protect the piston from slamming into the bumper too hard and rebounding shut. This is a part of my design I rarely talk about, but it is designed to be a dashpot.
For those not familiar with a dashpot, here is a link to wikipedia;
http://en.wikipedia.org/wiki/Dashpot

Most toasters in the kitchen use a dashpot to allow the toast to pop up but not be ejected from the toaster. Not all are fluid based. Some are air.

it's possible that if you used tough fixed non floating o-ring that were very tight there might be enough static friction to hold the sealing face in place, but I imagine typically something should yeild


With dual floating o rings in mine, it won't fire until the piston is mechanically pulled back enough to break the seal. Then it pops.
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Last edited by Technician1002 on Thu Apr 15, 2010 3:15 pm, edited 1 time in total.
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