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Piston-in-a-tee questions

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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Piston-in-a-tee questions

Unread postAuthor: PVC Arsenal 17 » Thu Aug 16, 2007 8:55 am

I'm looking to build a 1.5" piston valve housed in a 2" tee. Before I start buying parts I need to know a few things.

Is it true that the port diameter should be as close to the piston diameter as possible? Or should it be smaller?

How much travel should a 1.5" piston have?

Would I need equilization holes if the piston was contained in 2" sch80? (very close fit)

Should the sealing face be halfway in the tee? In other words, if you looked into the tee through the middle socket, you would see the piston touching the barrel halfway through.


That should be all. Thanks!
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Unread postAuthor: judgment_arms » Thu Aug 16, 2007 9:26 am

1.Don’t know.
2.I believe it to be about ¾ of an inch.
3.I’ve never used equalization holes and I use cast pistons, meaning very close fit.
4.Maybe not half way but it should stick in a little maybe 1/3 the way.
Hope that helped.
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Unread postAuthor: homedepotpro » Thu Aug 16, 2007 11:58 am

1. no there needs to be more surface area on the back of the piston (in the pilot are) the in the front port.
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Unread postAuthor: frankrede » Thu Aug 16, 2007 12:23 pm

3/4" sounds a bit off, considering my 2" piston valve had 1/2" piston travel.
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Unread postAuthor: Panzerfaust » Thu Aug 16, 2007 1:00 pm

I think someone said that a piston only needs to move back like .175 of it vent diameter or something similar. My mauler only moves back like .25 inches so i dont think it has to move very much. As long as your piston is not airtight you wont need an equilization hole, but it will fill faster if you have one. A hole will also harm performance some.
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Unread postAuthor: judgment_arms » Thu Aug 16, 2007 1:01 pm

Yeah, ¾ of an inch is off… just did the math, it’s about 3/8 of an inch.
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Unread postAuthor: spud yeti » Thu Aug 16, 2007 1:06 pm

I have heard that the general travel should be approx. 1/4 of the pistons diameter.
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Unread postAuthor: MrCrowley » Thu Aug 16, 2007 3:05 pm

frankrede wrote:3/4" sounds a bit off, considering my 2" piston valve had 1/2" piston travel.


I think it was clide who said he uses the '1/4 of the barrels diameter for piston travel' rule in co-axials, not tee valves. I think he said for tee-valves he usually gives it a bit more space.
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Unread postAuthor: PVC Arsenal 17 » Thu Aug 16, 2007 3:09 pm

homedepotpro wrote:1. no there needs to be more surface area on the back of the piston (in the pilot are) the in the front port.


really?
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Unread postAuthor: noname » Thu Aug 16, 2007 3:12 pm

Regarding the post right above this one, yep. I figured this out when I used a 3" long 1/4" nipple connected to a 1/4" ball valve for a pilot. The chamber was 1 1/2" and the barrel was 3/4." I downsized the back of the piston so I would have a tiny pilot volume, and the back of the piston would fit inside the 1/4" nipple. It didn't work.
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Unread postAuthor: PVC Arsenal 17 » Thu Aug 16, 2007 3:32 pm

So in other words...

Edit: F*ck I switched the labeling. Top diagram is BAD, bottom is GOOD.

Edit 2: Rmich said:
rmich732 wrote:EDIT: One question... why not step up the valve porting to 1.5" instead of 1"? Not only would this have greater flow (increasing performance), but because barrel sealers actuate faster when the barrel port diameter is closer to the piston diameter, that would further improve performance.
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Unread postAuthor: randompkguy » Thu Aug 16, 2007 3:51 pm

I just built a piston valve in a 3" tee, and my piston fits so close that I have to oil it so that it slides, and I have no equalization holes...it seems to work fine (no o-rings just a rubber front disk)

As close to the outside diameter as possible is good, id say that even having the outer diameter of the barrel seal the same size as the piston is fine, because that way you still have about 3/16ths to an 1/4 inch of sealing surface, and all of the force works to push the piston against that surface...

In my valve I couldn't put the barrel seal in the middle because the inside of the tee isn't the same diameter as the inside of the 3 inch pipe(the piston fit in the 3 inch pipe), so if the inside of the tee is the same diameter as your pipe, then you can but if its not then I wouldn't bother because the piston will have trouble sliding back into a smaller hole if its a such a tight fit...instead just recess the barrel seal about 1/8th inch into the pipe
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Unread postAuthor: PVC Arsenal 17 » Fri Aug 17, 2007 10:30 am

So a 1.5" piston sealing against a 1.5" pipe is fine? Or should I step down the pipe to 1.25"?
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Unread postAuthor: Modderxtrordanare » Fri Aug 17, 2007 1:17 pm

PVC Arsenal 17 wrote:So a 1.5" piston sealing against a 1.5" pipe is fine? Or should I step down the pipe to 1.25"?


A 1.5" piston will work sealing against 1.5" pipe. (My piston was a 1.5" cap, so it's a little bigger and thats why it works.)
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Unread postAuthor: PVC Arsenal 17 » Fri Aug 17, 2007 2:00 pm

Modderxtrordanare wrote:
PVC Arsenal 17 wrote:So a 1.5" piston sealing against a 1.5" pipe is fine? Or should I step down the pipe to 1.25"?


A 1.5" piston will work sealing against 1.5" pipe. (My piston was a 1.5" cap, so it's a little bigger and thats why it works.)


my piston was going to be a plug, so the diameters are exactly the same.
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