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PVC Tee QDV build

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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PVC Tee QDV build

Unread postAuthor: aEx155 » Thu Mar 03, 2011 2:00 am

I'm working on a QDV valve that is housed in a 1 1/4" tee, with 1" ports for the barrel and chamber. Eventually this will be in my friends birthday present : a 2" NERF vortex football launcher

The hardest part so far has been modifying the tee to allow the 1 1/4" pipe to go through. I also had to modify the 1" female NPT adapters because the sockets on the tee were a bit shallow (even though it is marked NSF-PW)

Image Image Image

Here are some cellphone pictures so you can see some of the work I've done. I cut the port to be a little larger than 1" square, since I'm using 1" pipe (inner area is ~0.85 sq. inches). The piston is made from 3/4" fittings and 1 3/8" OD o-rings.

The main problem I'm having is the fact that the o-rings for the piston are getting stuck on the port I cut into the side of the pipe that will be inserted into the tee. No matter how much I beveled the inside edge, the o-rings get caught between the side wall and the piston.

I need to make a new pipe, so I'm wondering: should I try and go with beveled edges, or should I leave a strip of PVC in the center to support the o-ring? Maybe a different port size?
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Unread postAuthor: mark.f » Thu Mar 03, 2011 8:08 am

How much squeeze is there on your O-rings? I would recommend reducing the compression on them and seeing if that helps (I don't know if you made the piston with a lathe or not).

Otherwise, looks good. However, what's the reason for modifying the tee? Couldn't you just have one piston on the pipe in one side, and one on the other side, with the large gap working as the port itself?
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Unread postAuthor: Gaderelguitarist » Thu Mar 03, 2011 8:20 am

It's possible that by cutting out that large chunk, you've relieved structural stress on the pipe, allowing it to warp. Mark's suggestion is also quite possible.

Since this piece is going to fit inside the tee, and therefore be within your pressure vessel, why not just make the valve pipe with a metal pipe? I realize it's much more labor intensive to get a nice port cut, but it shouldn't warp and will be much stronger than you need it to be.
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Unread postAuthor: Technician1002 » Thu Mar 03, 2011 9:05 am

mark.f wrote:Couldn't you just have one piston on the pipe in one side, and one on the other side, with the large gap working as the port itself?


I abandoned that design long ago as a floating ring is expanded under pressure to make a tight seal. When it comes out of the end of the pipe, the ring expands enough to be removed from the groove. My early coaxial was built that way and o rings were lost when used above 45 PSI. I considered it a failure. My later designs fixed that problem. My ABS cannon is the first properly working QDV I built.

For the ring getting stuck, it indicates the ring is too big. The wide part of the ring should remain down in the groove at all times with only the edge bulge sticking out to make a seal against the pipe.

Photo showing the fat part of the ring always remains in the groove. This is the piston in my Marshmallow Cannon.
Image
The front ring as a floating ring has sagged to the bottom so the top side shows no protrusion. Even off center the fat part hanging out the bottom has not extended outside the groove.

This photo has rings that are too large. I use this to make it clear the rings are loose and floating in the groove. The rings used on the working cannons are only slightly larger than the piston and just large enough to contact the cylinder walls.
Image

To add the "bars" between the ports to properly retain the ring, you don't want the ports too wide. Making them less than 1/3 the pipe circumference works for me in the smaller sizes.

Another recommendation is to have very little gap between the piston and cylinder wall. I machine my pistons for very little gap. They are just small enough to slide freely without sticking or dragging.
Image
This way you can use the smallest possible o ring without the need for it to extend too far above the surface to work.

I would recommend building it in a larger T and use a pair of reducers to hold the barrel/valve cylinder and use 3 ports instead of one. A 2 inch T would work nicely for this.

I have future plans to build a 2 inch iron QDV in a 3 inch T so the chamber can be changed for testing. Large for low pressure for fruit and small high pressure for insane pop can launches.
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Unread postAuthor: aEx155 » Thu Mar 03, 2011 9:52 am

To be able to hold pressure and not leak the pipe itself will eventually be glued into the tee. (that is the only way it will work when you think about it) Therefore it will have to be plastic. I am not worried about the pipe warping. The problem is the fact that the large o-ring is able to expand into the port and it won't go back in without some convincing, which won't be possible within a tee.

@Technician1002

For 1 1/4" pipe, I'm in a tight spot as far as o-ring sizes. The ID of the pipe is 1.360". That is right between 1 5/16", which is too small, and 3/8", which is what I'm currently using, and is the closest I'm going to get without being too small (even in 1/32" increments).

I also don't want to use a larger tee because this project has already gotten expensive and its physical size is already bigger than I was expecting.

I need approximately 1 square inch (actually only ~0.83) of port area: would I be fine if I cut two 1/2" wide by 1" long ports in my pipe with 1/4" between them?
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Unread postAuthor: Crna Legija » Thu Mar 03, 2011 10:17 am

At work we have a kit for making orings, its just a pair of pliers that can cut a V on one side and a < on the other then you super glue them to gether. shouldn't be to hard with a stanley knife.

looks like this at then and
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Unread postAuthor: Technician1002 » Thu Mar 03, 2011 12:01 pm

The in between sizes is one of the big reasons I have not made a floating ring for my ABS cannon. The next size larger is too big. It currently has the smaller ring which I expanded by layering some electrical tape in the bottom of the o ring groove. It does make the piston harder to pull.
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Re: PVC Tee QDV build

Unread postAuthor: Gun Freak » Thu Mar 03, 2011 5:30 pm

aEx155 wrote:Eventually this will be in my friends birthday present : a 2" NERF vortex football launcher

WOW you are an awesome friend :D :D
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Re: PVC Tee QDV build

Unread postAuthor: aEx155 » Thu Mar 03, 2011 7:27 pm

Gun Freak wrote:
aEx155 wrote:Eventually this will be in my friends birthday present : a 2" NERF vortex football launcher

WOW you are an awesome friend :D :D


We actually found out about our hobbies when I brought a small ball valve cannon to school in 8th grade for demonstrating pressure differential. He told me about an ABS potato cannon he and his dad built.

Later that year I gave him a sprinkler valve for his birthday. Some time later we combined his sprinkler valve and my piston valve (with the sprinkler valve as the pilot) and we shot nail darts (which I also showed him how to make) For his birthday this year I gave him a NERF Vortex football. He knew what was in store.

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@Technician1002
I have some 1 5/16" o-rings that are barely too small. I will try and put some tape underneath and see how that goes.

Should I go with narrow slots for ports (and floating o-rings) or tight o-rings (with a clear port)?
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Unread postAuthor: Gun Freak » Thu Mar 03, 2011 8:20 pm

Why can't you get correctly sized rings? I found ones that fit my pipes fine at the hardware store (Ace).
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Unread postAuthor: clemsonguy1125 » Thu Mar 03, 2011 8:27 pm

Looks good, Im planning something similar. Your running into the same problems I am.
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Unread postAuthor: aEx155 » Thu Mar 03, 2011 8:59 pm

Gun Freak wrote:Why can't you get correctly sized rings? I found ones that fit my pipes fine at the hardware store (Ace).


The o-rings I have are from ACE hardware. That isn't the problem. The problem is that, out of the available sizes (ranging from 1 5/16" to 1 7/16" in 1/16" increments) the only one that is close is 1 3/8", which is a tad too big.

1.360" = 1 1/4" PVC pipe ID
1.3125 = 1 5/16 OD
1.375 = 1 3/8 OD

I do have some o-rings that I can try cutting and gluing, I'll try that before I cut a new pipe. Will the super glue prevent a seal at all?
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Unread postAuthor: Gun Freak » Thu Mar 03, 2011 9:03 pm

If you are careful gluing, you should be ok. I'd heard of people doing that to make the right size o-ring. However, I think an oversized ring would work... I mean it's worth a try :P
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Unread postAuthor: aEx155 » Thu Mar 03, 2011 9:25 pm

Gun Freak wrote:If you are careful gluing, you should be ok. I'd heard of people doing that to make the right size o-ring. However, I think an oversized ring would work... I mean it's worth a try :P


By Technician's standards, the o-ring I am using is oversized. If I have a wide port the o-ring expands into it and gets caught.
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Unread postAuthor: clemsonguy1125 » Fri Mar 04, 2011 8:22 pm

Did your piston require any type of lathe work, or is it just pieces cut and glued on a pipe. Thanks
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