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Pilots for Pistons

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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Pilots for Pistons

Unread postAuthor: Secret Squirrel » Mon Nov 29, 2010 1:29 pm

By now I've built a handful of piston valves in various sizes, and have encountered one problem in every valve I have made, they always end up needing a much larger pilot valve than I at first expected.

My first valve was a coaxial using 1.25" end cap in 2" pipe sealing against a 1.5" seat. (all of those sizes are nominal sizes of PVC) Due to my inexperience at the time, I at first tried to pilot it with a 1/4" ball valve, which needless to say, was a fail. After stepping my pilot up to a 3/4" ball valve and then a 3/4" sprinkler I got the valve to fire reliably, but only at pressures higher than 40psi, any less and it all hissed out the pilot.

More surprising was my recent copper gun using a barrel sealing piston in a tee. The piston was epoxy (wonder where I got that idea :wink: ) cast inside the 3/4" pipe it would later slide in. Much to my surprise, I had trouble piloting this valve with a blow gun. After ludicrous application of grease I got the valve to work, though not reliably.

One thing all my valves have had in common is a very small piston:seat ratio. I purposely built them this way so that the pilot would have to be almost entirely exhausted before the valve would open, leading to very fast opening times. I found this very effective on the PVC coax mentioned above once I switched to a large pilot valve, but I'm afraid I am losing performance due to air leaking around the piston during the increased time the pilot is open and the valve hasn't triggered.

So in planning my next project (which may turn out good enough to post on here) I'm wondering how big is big enough for the pilot? The valve is going to be a somewhat different design than most pneumatics on this site, I'll post a drawing later. The piston will slide inside either 1.5" or 2" PVC and seal against either 1", 1.25", or 1.5" PVC, I'm still deciding on the ideal ratio.

I don't blame you if you didn't read all that, so to sum up: for 1.25" to 2" size piston valves, what size pilot is necessary for good performance?
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Unread postAuthor: saefroch » Mon Nov 29, 2010 1:36 pm

Secret Squirrel wrote:how big is big enough for the pilot?
You're not going to like this, but it all depends. To pilot a piston valve, flow out the pilot valve must be greater than flow through the equalizer hole, from the chamber back to the pilot. So it really depends on how tightly the piston fits its housing. Make sure that when the piston is forward and sealed that a good portion if its rear end is still firmly seated in the back of the piston housing. I once build a piston valve and was horribly confused why it sometimes wouldn't pilot, and that turned out to be my problem.

Bigger is better, unless you have a check valve inside the piston to prevent flow from the chamber to the pilot. Then it doesn't matter.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 1:39 pm

More surprising was my recent copper gun using a barrel sealing piston in a tee. The piston was epoxy (wonder where I got that idea ;) ) cast inside the 3/4" pipe it would later slide in. Much to my surprise, I had trouble piloting this valve with a blow gun. After ludicrous application of grease I got the valve to work, though not reliably.


When I cast an epoxy piston directly in copper pipe it was a flop and didn't pilot well. However a slightly smaller piston in a syringe tube worked perfectly in my 12 bore coaxial.

Image

I would surmise that the copper pipe is not exactly a perfect cylinder and therefore the casting method is not suitable. If you look at most of my projects, cut down syringes make up the majority of my piston cylinders because they are precision made and smooth - because after all they are meant to have an airtight plunger rin through them - and perfect for such applications.

Regarding suitable pilots,this recent thread should be of interest.
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Unread postAuthor: Secret Squirrel » Mon Nov 29, 2010 1:49 pm

saefroch wrote:
Secret Squirrel wrote:Bigger is better, unless you have a check valve inside the piston to prevent flow from the chamber to the pilot. Then it doesn't matter.


Well as of now the plan is for the piston to be made up of washers on a threaded rod with spacers to create O ring grooves. So with an O ringed piston I shouldn't have too many problems with low pilot flow.

As far as casting pistons in copper pipe goes I'v had too much trouble with that. The first time I used a pipe cutter to cut the pipe (the kind that likes to draw spiral grooves) and it happened to reduce the diameter of the end of the pipe, making the piston impossible to remove. The next attempt I used a hacksaw. The piston was still difficult to get out, and took a bit of sanding before it would slide at all in the pipe. I'll have to agree, copper pipe is not the best material to cast a piston in. Though it hasn't caused me as many headaches as hot glue pistons, they never turn out right for me.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 1:59 pm

On the subject of suitable pilots vis a vis piston construction, here's one from the archives :)

My first "large bore" co-axial from 2004 which I made using the body of a foot pump as a chamber. Because of the fact that I used the pump head (with a floating o-ring) as the piston, I was able to pilot the (2" diameter!) piston with a schrader ;)
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Unread postAuthor: Secret Squirrel » Mon Nov 29, 2010 2:16 pm

That gun you posted looks almost exactly like what I plan on building, just a bit stubbier. I was planning on using a foot long (or thereabouts) chamber and a two foot barrel all 2" PVC with a pair of bushings between to form the sealing face.

The fact that you could pilot such a similar design with a schrader is giving me the confidence to attempt a blow gun as a pilot. If it doesn't work I'll just slap my modded sprinkler on there and I'm sure I'll have no problems.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 2:24 pm

Secret Squirrel wrote:The fact that you could pilot such a similar design with a schrader is giving me the confidence to attempt a blow gun as a pilot.


Note that schrader piloting was only possible because of the o-ring seal on the piston which effectively prevented air from the chamber leaking back into the pilot.
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Unread postAuthor: Technician1002 » Mon Nov 29, 2010 2:40 pm

My Dragon cannon worked fine with a 1/4 inch ball valve. It is a close ratio piston like yours. It has a 2.5 inch PVC barrel with an o ring seat 2.8 inches in diameter. The piston rides in a 3 inch cylinder with an o ring to control leakage. The piston was a 2.5 inch PVC pipe cap, which finally shattered.

Controlling leakage into the pilot, and low friction, is key to getting them to work.
Image
6 inch chamber;
2.5 inch barrel;
3 inch piston;
2.8 inch valve seat o ring;
1/4 inch pilot ball valve.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 3:42 pm

On a slightly unrelated note, while looking through my archives I found pictures of a "QDV" type launcher I had made, this was in 2003 :D
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Unread postAuthor: POLAND_SPUD » Mon Nov 29, 2010 3:48 pm

LOL LOL LOL
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Unread postAuthor: Secret Squirrel » Mon Nov 29, 2010 4:07 pm

jackssmirkingrevenge wrote:Note that schrader piloting was only possible because of the o-ring seal on the piston which effectively prevented air from the chamber leaking back into the pilot.


I figured that was the case. An O ring on the piston has been part of the plan all along. The piston will be threaded rod with washers to support the sealing face and form an O ring groove. Basically exactly like JSR's piston, only longer.
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Unread postAuthor: Lockednloaded » Mon Nov 29, 2010 7:27 pm

for piston pilots it always seems you're either stuck with a blowgun's low flow, a BV's awkward opening, or a modded sprinkler valve's pure overkill. For less than a modded SV, throw on a slide valve of your choice to compact you pilot area and give you a comfortable trigger

BTW, JSR, link to your old thread?
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Unread postAuthor: saefroch » Mon Nov 29, 2010 8:08 pm

Lockednloaded wrote:it always seems you're either stuck with a blowgun's low flow, a BV's awkward opening, or a modded sprinkler valve's pure overkill.
Ah, but there's alway the option of piloting a piston valve with another piston valve and piloting the pilot with a blowgun. :D Due to current aiming issues, I may be going that route.
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Unread postAuthor: Technician1002 » Mon Nov 29, 2010 8:16 pm

Wow another vintage QDV. Did it have a sliding piston on a rod?
That would have been made shortly after the Cell Core test cannon, AKA my apple cannon.

I need to dig through 35mm negatives and see if I can find the date I made my first QDV. It was before I got my first digital camera. I have some prints of it shooting adding machine tape at summer camp -- somewhere.
:roll: I have no idea where the negatives are now. The receipt date for the processing would be a good find.
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Unread postAuthor: mark.f » Mon Nov 29, 2010 10:29 pm

There's always the venerable quick-exhaust valve for piloting pistons. Not complicated, and, indeed, rather intuitive to setup as a pilot (although I managed to hook one on backwards my first time... did I say I was a math major?), and the flow is great for their size.

However, I would definitely take a look at your piston before considering pilot options. The better of a seal you have between the piston and the track pipe, and the more control you have over equalization (the epitome of this being check-valve type equalization), the smaller pilot valve you can conceivably use. I built a 'test' piston last year with O-rings (which incidentally required nothing more than a miter box and sandpaper ((belt sander to speedy things up)) for a 2" piston from 1-1/4" fittings) and a small 1/16" equalization hole which could be piloted with a stock blowgun, although I used a 1/4" ball valve after I was through playing around with it.

Well, if you didn't want to read all that: focus on your piston before your pilot valve. :wink:
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