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Making a pump....

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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Making a pump....

Unread postAuthor: sco77 » Sun Aug 14, 2011 1:25 am

I was making a pump and was wondering how badly the dead space would impact performance? It is currently in a working state but not complete enough to do pressure tests..

I also need a way to make a check valve, I currently have a washer screwed into the threads of the T and epoxied as well as the coupling epoxied and screw tight into it. I was thinking maybe a bolt on a spring with one of those rubber grommet nuts on the end that meets up with the washer?
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Unread postAuthor: Mr.Tallahassee » Sun Aug 14, 2011 2:18 am

You could probably make a check valve with some copper fittings. A 1/2''-1/4'' copper bushing put in backwards after being polished should work. Just put in a washer stack with a sealing surface on it and a groove cut to let the air flow past the washers better. Just make sure not to cut too far so that the face seals.
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Unread postAuthor: Crna Legija » Sun Aug 14, 2011 3:25 am

I personally would not make a check valve, its is very dangers if it fails under pressure, the pump will turn into a air cylinder and shoot the handle straight up. BTB knows too well what its like, his broke his jaw when the handle smacked him in the head.
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Unread postAuthor: warhead052 » Sun Aug 14, 2011 9:55 am

One thing to do, assuming you live in the states, is goto lowes or home depot and buy a check valve, they are "rated" to 200 psi, but I am assuming you can use them higher than that.
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Unread postAuthor: saefroch » Sun Aug 14, 2011 10:19 am

The female-female check valves from mcmaster aren't all that expensive, and there are some rated to 1,000psi. By inserting a little spacer at the end of the pump before the check valve, you can get the dead volume down pretty far.
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Unread postAuthor: LikimysCrotchus5 » Sun Aug 14, 2011 2:45 pm

Mcmaster check valves are a pretty good price just as saefroch mentioned, and will probably last you a long time. I very much recommend it.

Now that I have been re-enlightened to this idea, I think I will finish my pump :clock: (One emoticon i havent used yet)
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Unread postAuthor: sco77 » Sun Aug 14, 2011 4:57 pm

I give up on it, I will just find a fridge compressor and maybe someday buy a PCP pump and replace the connector with a QD.
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Unread postAuthor: Lockednloaded » Sun Aug 14, 2011 6:52 pm

I'd really recommend saving for the PCP pump, I have one and I love it, not only can you charge your guns to 3600psi, but also you can charge HPA tanks
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Unread postAuthor: sco77 » Sun Aug 14, 2011 7:02 pm

Lockednloaded wrote:I'd really recommend saving for the PCP pump, I have one and I love it, not only can you charge your guns to 3600psi, but also you can charge HPA tanks


Do they have standard threads on the hoses that you can use to swap out for standard connections (like regular disconnects, etc) ?
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Unread postAuthor: Crna Legija » Mon Aug 15, 2011 1:11 am

sco77 wrote:
Lockednloaded wrote:I'd really recommend saving for the PCP pump, I have one and I love it, not only can you charge your guns to 3600psi, but also you can charge HPA tanks


Do they have standard threads on the hoses that you can use to swap out for standard connections (like regular disconnects, etc) ?


IIRC its 1/8 npt on most pumps, so yeah its a stranded size but your better off using high pressure the high pressure one the come with it because they disconnect easier and wont wear out quickly
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Unread postAuthor: Lockednloaded » Mon Aug 15, 2011 1:34 am

I made a simple adaptor with a male PCP QD in a 1/8"-1/4" bushing to a female 1/4" QD
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Unread postAuthor: Technician1002 » Mon Aug 15, 2011 9:24 am

wondering how badly the dead space would impact performance?

Air compresses. The dead space can contain a large portion of your compressed air which won't go out the check valve then re expand in the cylinder so the pump will not take in more air to pump. The pressure you can get with a single stage pump is limited by the amount of dead space.

Here is an example. Let's say you have a pump where the dead space is the same volume as the displacement of the piston. On air intake, the cylinder will contain 1 Atm of pressure. On the compression stroke, air is pumped out the check valve. As the chamber pumps up to pressure some of the air is compressed in the dead space and then re-expands so as the chamber comes up to pressure, less air is pumped on each stroke. When the chamber is up to 2 Atm (about 15 PSI), the pump will no longer pump air. It will compress the air in the pump to 15 PSI and that will re-expand in the cylinder instead of being pumped out to the chamber. The smaller the dead space the higher the pump is able to pump.
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Unread postAuthor: sco77 » Mon Aug 15, 2011 11:31 am

I just went and ordered a PCP pump and a couple male fittings for guns... I will probably also get a fridge compressor when I get a hold of one for free but hopefully the 3 stage pcp pump will be able to reach 300-500PSI fairly quickly on small 20~ CI chambers.
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