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Homebrew Backpressure Semi Auto Valve

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#1 atomatron

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Posted 01 January 2010 - 03:14 PM

This should answer any questions that anyone would have about duplicating this (If not then ask):
Posted Image
Posted Image
^a few changes: there is now an end cap with a hole on the end of the CPVC that is closest in the picture that hold a spring to return the valve back to it's starting position, the valve is also situated in the shell such that when the trigger is pulled it pushes the valve back.


Here's an overview of the valve system before and after firing:
Posted Image

AT2K Pump
Super soaker Tank
Tee
Steel
Neoprene
And black = CPVC and tubing
the squiggily thing is a spring
Grey is the pressurized space

More detail on the valve, names of the parts are given:
Posted Image
(^It says rubber but it's a neoprene washer)

Just a few notes:
-Tightening the nuts gets the neoprene washers to seal
-Make sure that the CPVC you're using is smooth on the inside (better seal)
-The holes going to the tubing should be pretty small, cross line leakage is bad
-This should be lubed with silicone grease (not spray stuff, if you can)
-The spring should be pretty strong and thusly expect the trigger pull to be tough
-One could just as easily use a bicycle pump, a solid PVC tank or a magstrike bladder, this version doesn't get to high pressures
-This same sort of setup could be altered for a system like cheesy's:
http://nerfhaven.com...showtopic=18411
-Venom used the same parts on his attempt at a back pressure trigger:
http://nerfhaven.com...topic=15372&hl=
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#2 Samzilla

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Posted 01 January 2010 - 03:24 PM

How finnicky is the trigger though? Is it pretty reliable, and also would a normal hornet one work?
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#3 atomatron

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Posted 01 January 2010 - 03:37 PM

How finnicky is the trigger though? Is it pretty reliable, and also would a normal hornet one work?

I haven't had any issues with it yet, you could probably modify a hornet trigger like cheesy did.
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#4 venom213

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Posted 01 January 2010 - 06:03 PM

Heh, I found this very funny as I was looking through it, and then when I saw the link to my topic I realized why it was all so similar. Glad to see my design in use. I had actually thought about using that design to make something that accomplishes the same thing as this but I never made it since my last project based around the rubber washer on a screw design failed.
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#5 TantumBull

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Posted 03 January 2010 - 03:50 PM

Amazing, amazing work atomatron. I'm in the process of replicating this valve with parts from SMDTG pumps. It's kind of a funny coincidence actually; I'm working on something extremely similar to your DTB. Hopefully your valve will work out for me to save me 13 bucks on a valve from McMaster. Really great work all around, including the DTB.

I do have one question, though. I know you mentioned that the trigger pull would be hard, but even without the spring in mine, there's quite a bit of friction. Then again I haven't lubed it yet... Anyway, on that DTB, with the improved seal on from the salvo tank plus the friction from the valve, is the trigger pull uncomfortably hard?

Edited by TantumBull, 03 January 2010 - 06:03 PM.

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#6 Eh Watt

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Posted 03 January 2010 - 07:26 PM

Dude, great job on that. I look forward to seeing this at the next war. Hopefully.
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#7 Fome

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Posted 05 January 2010 - 09:20 PM

I'm in the process of replicating this at the moment but I'm having some troubles.

Either my rubber washers are slightly too big or my CPVC is slightly too small. The friction (even with lube) from the moving bits is way too much for the trigger pull. Sometimes the rubber washers will even pull off of the washer sandwich. The spring (a fairly hefty one) doesn't even budge it at full compression.

I've tried taking off rubber washers, loosening the nuts, tightening the nuts, sanding/filing out the insides of the pvc, adding a beefier spring, etc.

Any ideas?

#8 Zorns Lemma

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Posted 05 January 2010 - 09:39 PM

Is this dry assembly or when you have pressure running through the system?

If it is dry assembly, then take a 1/2" bit to your CPVC to get it to be actually 1/2" and not .49x" ID.
If it is during wet operation, then most likely your spacing is incorrect so there is a pressure difference in your trigger system adding $bignum lbs of force to it.
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#9 Fome

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Posted 05 January 2010 - 10:25 PM

Dry.

Unfortunately I don't have access to a lot of my tools right now so drilling the CPVC will have to wait.

I did, however, play around with some other sizes of tubing and the piston fits perfectly inside 9/16" brass. Very little friction yet airtight seal. So yeah, my rubber washers are just way too big. Odd because they said 1/2" OD at the hardware store.

#10 atomatron

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Posted 05 January 2010 - 10:54 PM

I do have one question, though. I know you mentioned that the trigger pull would be hard, but even without the spring in mine, there's quite a bit of friction. Then again I haven't lubed it yet... Anyway, on that DTB, with the improved seal on from the salvo tank plus the friction from the valve, is the trigger pull uncomfortably hard?

Yes, though the large trigger on the dtb helps it's still uncomfortable.


Fome; I'm sure that using brass instead of cpvc would work. Though I would worry about the holes in the brass going to the air tank chewing up the rubber if they aren't sanded well on the inside.
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#11 Fome

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Posted 06 January 2010 - 01:53 AM

Fome; I'm sure that using brass instead of cpvc would work. Though I would worry about the holes in the brass going to the air tank chewing up the rubber if they aren't sanded well on the inside.


My concern too. I'm also unsure about how I'm going to attach the airline tubing. Right now I have some 1/2" PVC e-taped/gooped over the brass piece that I'm going to use. My idea is that by drilling through both to make the holes there should be enough "depth" for the airline tubing to take hold without actually going into the brass.

We'll see.

Thanks for putting this up, atomatron.

#12 3DBBQ

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Posted 06 January 2010 - 09:54 AM

Posted Image


What is the yellow part?
Why does the loss of the air can also be launched?
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#13 ggk

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Posted 06 January 2010 - 11:52 AM

That yellow part is a backpressure air tank. Most likely it is representing an air tank from a big salvo.
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#14 atomatron

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Posted 09 January 2010 - 04:50 PM

Here is a picture of the final setup:

Posted Image

I had to add some material to the trigger so that the valve could function smoothly, It's basically a brass plate epoxy puttied to the trigger:
Posted Image
Posted Image


The 'piston' out of the valve:
Posted Image


And the finishing touch:
Posted Image
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#15 smeagolsaur

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Posted 15 January 2010 - 12:49 PM

Looks good. Can you post a video of it in action?
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#16 Zack the Mack

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Posted 15 January 2010 - 02:18 PM

Why do you use two O-rings? It seems like the one furthest from the tank could be eliminated, and the friction reduced, if you capped the PVC pipe.
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#17 cheesypiza001

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Posted 15 January 2010 - 03:07 PM

Why do you use two O-rings? It seems like the one furthest from the tank could be eliminated, and the friction reduced, if you capped the PVC pipe.


Doing so would make all of the pressure from the bladder push on the trigger which would make the trigger extremely difficult to pull. By using two O-rings and keeping the back open, the pressure is cancelled out and therefore there is no air pushing directly on the trigger.
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#18 atomatron

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Posted 15 January 2010 - 03:27 PM

Looks good. Can you post a video of it in action?

I'll see what I can do.

Why do you use two O-rings? It seems like the one furthest from the tank could be eliminated, and the friction reduced, if you capped the PVC pipe.

Doing so would make all of the pressure from the bladder push on the trigger which would make the trigger extremely difficult to pull. By using two O-rings and keeping the back open, the pressure is cancelled out and therefore there is no air pushing directly on the trigger.

What he said.
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