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stirrup pump question

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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stirrup pump question

Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 12:36 pm

I'm in the middle of a stirrup pump build, and I believe I did my initial calculations wrong. I'm using 3/4" type L copper wich has an id of .785".
I'm not sure what I did wrong in my calculations to begin with but I was figuring around 155 pounds of force to output 400PSI, but now I have re-done my math and I'm coming up with 193.6 pounds of force to get to 400PSI, ignoring friction of the floating o-ring and other losses an accual engineer might figure in.

I was wondering if anyone else has made a pump out of 3/4 inch copper, and if so what kind of pressure were you able to achieve?
Here is a pic of the piston incase that helps at all.

Thanx for any comments, Lock


<a href="http://s117.photobucket.com/albums/o75/lockmanslammin/?action=view&current=pumpparts2.jpg" target="_blank"><img src="http://i117.photobucket.com/albums/o75/lockmanslammin/pumpparts2.jpg" border="0" alt="pump parts bigger"></a>
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Unread postAuthor: Fnord » Wed Mar 05, 2008 12:41 pm

I did the same thing, but I got 193 psi as a final result.

BTB was able to achieve ~1100 psi with a 3/4 inch pump before it decided to punch him in the face.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 12:44 pm

_Fnord wrote:I did the same thing, but I got 193 psi as a final result.


Same here, if high pressure-low volume is your goal then maybe you should go for some narrower tubing.
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Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 12:51 pm

PUNCHED in the face?! I want to hear about this... what happened? Check valve fail or some thing? what kind of check valve were you using?

And the reason I went with the 3/4" is that my chamber is rather large at around 45 inches or so of 3/4" copper.
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Unread postAuthor: Fnord » Wed Mar 05, 2008 12:51 pm

edit: The check valve blew out, yeah. If he reads this thread you'll probably get a graphic description of what happened :)


Also, Lock, think if you're really going to need 1000 psi for anything. I actually (mostly) finished my high pressure pump but I don't have a gauge that goes up high enough or even a gun to use it on (1/2" copper btw).
(Oh, and Schrader valves apparently do work as check valves, but they limit the max pressure you can get because it takes a lot of force to get them to open)
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Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 12:58 pm

All I'm really aiming for is maybe one charge at around 500PSI just to make sure everything on the gun and pump can take it, and from then on only going to around 400 PSI just so I have somewhat of a safety range. I read somewhere that soldered copper fittings are rated for 400PSI, so as far as I can see that is my weakest link. That is assuming that my homemade brass, spring, bearing and o-ring check valve is up to the task.
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Unread postAuthor: Hawkeye » Wed Mar 05, 2008 1:20 pm

I easily achieved 350 psi with mine and stopped only because it starts to heat up excessively and I only want to push a cheap blowgun so high.
You have to account for the fact that you can slightly bounce your weight on the bottom of the pump stroke (as I have mentioned many times).
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Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 1:24 pm

Hawkeye, did you use a floating oring? Also what type of check valve did you go with?
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 1:35 pm

Hawkeye wrote:you can slightly bounce your weight on the bottom of the pump stroke (as I have mentioned many times).


Not great for the wrists though if you do it a lot...
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Unread postAuthor: Hawkeye » Wed Mar 05, 2008 1:40 pm

Yes a floating o-ring in a pump head comprised of three tap washers bolted to a half inch end cap. The pump rod is half inch copper with the end cap attached to the end. It makes the pump much lighter.

Also my check valve is a tiny thing housed in a 1/8"-1/4" reducer. It is a very small metal spindle with an o-ring and a pen spring holding it lightly shut until pressurized.
The pump end that the air exits has the same fitting you are using on the top of yours with a brass 1/2" bushing with a female 1/8" thread soldered into the 1/2" side.
Then the pump is just threaded onto the reducer with the check valve in it.
The pump can be left on or removed before shooting.
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Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 1:49 pm

I have kind of a similar check valve have a female to female 1/8"hex coupler, with a ball bearing that fits inside nicely, then on one side there is a 1/8" male to male hex coupler. the o-ring sits inside the female hex on the end of the male hex. attached to the other side of the bearing housing is a 1/8" x 1 1/2" pipe nipple with a spring inside and the end notched so the ball won't seal the wrong way. and on the end of that there is an elbow attached that puts light pressure on the spring.
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Unread postAuthor: Gippeto » Wed Mar 05, 2008 2:23 pm

There are many ways to build a pump. I'm confident that a search will turn up several good ideas. This is what I did.

http://www.spudfiles.com/forums/easy-st ... 13277.html
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Unread postAuthor: Hawkeye » Wed Mar 05, 2008 2:51 pm

JSR, I can assure you it is much easier on the wrists than holding a shock pump and maxing it out. In fact I'm not aware of any wrist strain. Push-ups etc. put more stress on the wrists.
I actually only bear down on mine with the heel of one hand.
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Unread postAuthor: dongfang » Wed Mar 05, 2008 2:57 pm

Hi,

.785", that´s an area of 0.48 in^2.

So you have about half a square inch. With 400 pounds per whole square inch, you will have about half that as the force on the piston. I think your second calculation is better.

Anyway, even if that´s too much, keep the pump. It will be much faster than a smaller one for the first 200 or so PSI.

BTW, floating o-ring, what does that mean? What´s the difference bwtween floating and non floating?

Regards
Soren
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Unread postAuthor: Hawkeye » Wed Mar 05, 2008 6:57 pm

A floating o-ring fits the bore of the pump perfectly but rattles around in an oversized groove in the piston head. This is so that it can also act as a check valve. It seals against the upper edge on the down stroke and then moves against the bottom edge (which has slots cut in it so that air can leak past) on the upstroke. It needs a bit of play to allow this to happen optimally.
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