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High Pressure Piston Valve Design-1000psi

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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High Pressure Piston Valve Design-1000psi

Unread postAuthor: vakimo » Wed Mar 14, 2012 7:21 pm

This is the design for the core component of a high pressure launcher I’m working on. It's a basic barrel sealing piston valve, except it's designed for much higher pressures than normal. The goal is 1000psi, and the sound barrier. :D

If any of you see something that could be improved, or especially if something could fail, please let me know!
Any help would be greatly appreciated.
I'm still in high school, so my experience with materials and processes is limited. I plan on building this as soon as I can purchase the stock metal.

I'm really trying to overbuild it to compensate any loss of strength from the machining processes or the assembly. The individual, unmodified fittings and pipe are all rated or calculated for at least 2500psi, some 6000psi. My main concerns are whether or not the joints themselves will hold, and any weaknesses in the components caused by machining or stresses from bolt arrangement.

Height: 8.5”
Bore: 0.75” (might make it 1”)
Barrel Length: 5'
Chamber length: 1.5”x18” (shortened for pics)
Piston: 2”x1.75”

Design:
-Everything except for the fittings, bolts, and screws is 6061 aluminum. The chamber is just copper colored on the pics for contrast.

-The main bolts (labeled as B1) are currently 1/4”, which together have three times the tensile strength than the load from 1000psi. However, I'm not sure how the impact of the piston over time would affect them, or how the tightening down of their nuts would.

-This is only the core component, once I get it finalized I'll add the support structure for it (particularly needed for the recoil) as well as a the loading and firing systems

Seals:
-The piston has a layer of 1/8” thick rubber on either side. The bottom one is pushed out by the two o-rings to form a concave disk. This is so that on impact, the o-rings will compress better than just a layer of rubber, absorbing impact and sealing off the sides of the chamber.
*the o-rings are not meant to seal the piston in the chamber while filling, there will be a small gap*

-All permanents seals (not 3 and 4) are to be to be machined to fit, sealed by J-B Weld, and mechanically tightened. All threaded fitting will use the usual Teflon tape.

*note: there is supposed to be a small gap between the piston and the chamber to allow the air to seep around while filling, I just forgot to add it in.
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Unread postAuthor: al-xg » Wed Mar 14, 2012 8:12 pm

Looks good. Personally, I'd make the barrel seat tube shorter to simplify the flow path from the chamber to the T.

What projectile will you be trying to launch ?

Is that plugged T there to load projectiles into the barrel from the rear ?

Are you machining all the aluminium bits from scratch ?
What machines do you have at your disposal ?
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Unread postAuthor: warhead052 » Wed Mar 14, 2012 9:12 pm

Wow, that looks really good. I don't see anything wrong with the design, apart from what al-xg said. Good work on the 3D part, now lets see the real one!
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Unread postAuthor: vakimo » Wed Mar 14, 2012 9:27 pm

Thanks, is there a minim length needed for the barrel seat tube? some of the tutorials I've watched say to keep it centered of the air chamber for some reason.

I'd start with an aluminum cylinder lathed to a point a few inches long.

Yes it is :wink: for testing I'd probably just load through the barrel, but I'd like to convert it to semi once i get the piston valve working.

All the aluminum parts will be machined from stock metal in the form of flat bar and pipe.

I'll be using community college equipment, so I'm trying to stick with a lathe, metal band-saw, and drill press for this one. The last one i worked on involved TIG welding and water jet cutting.. trying to keep it simple this time :roll:
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Unread postAuthor: Moonbogg » Wed Mar 14, 2012 10:09 pm

Looks good. One thing I would take a look at is the disc used as the port for pressure gauge. I would not secure it with the 4 bolts in shear like that. Instead, I would either thread that part and have it screw into the chamber, or use tie rods to hold it. The bolts in shear are roughly 60% of their strength in tension or so and theres only 4 of them.
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Unread postAuthor: warhead052 » Wed Mar 14, 2012 11:15 pm

Just noticed this. You may want a stronger bumper at 1,000 psi, I don't think a couple of big o-rings will work for this. You may need a spring for this.
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Unread postAuthor: Crna Legija » Thu Mar 15, 2012 12:48 am

The piston could be smaller in diameter only need to be a few mill bigger then the seat that would also make the piston lighter in turn making it move faster and less of a impact on the fill/pilot plate.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 15, 2012 12:51 am

It can shed some weight while still remaining over engineered ;)

The piston is at least twice as long as it has to be, reducing piston weight not only improves performance but also reduces stress on the system.

Consider making it a cup shape:

Image

Just an idea of the potential weight savings from another thread:

Image

This may mean you need to extend the seat tee though.

Also, piston travel is far too great. Aim for half the diameter of the tee seat. Again this improves performance (smaller pilot volume) and reduces stress on the system (piston has less space to accelerate).

To make breaking the sound barrier easier, you might want to get your hands on some helium ;)

edit: Actually, why not just extend the length of the copper coloured chamber slighty and avoid the need to add that tangential chamber completely? Simpler to build, less potential failure points and you will get less turbulent flow too.
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Unread postAuthor: Crna Legija » Thu Mar 15, 2012 1:12 am

jackssmirkingrevenge wrote:The piston is at least twice as long as it has to be, reducing piston weight not only improves performance but also reduces stress on the system.



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Unread postAuthor: jackssmirkingrevenge » Thu Mar 15, 2012 1:20 am

Crna Legija wrote:Copy Cat


- you posted as I was writing my post (looking up pictures takes time :D)

- You suggested smaller diameter, I suggested shorter length ;)
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Unread postAuthor: Technician1002 » Thu Mar 15, 2012 1:31 am

My two cents worth.

In regards to a smaller diameter piston, the second benifit is the pilot pressure must be lower before the balance of force on the piston is upset. This permits the piston to remain closed until much of the pilot pressure is vented so when it pops open, is is not moving into a high pressure pilot. This permits faster operation.

For leakage into the chamber for charging, I don't recommend a gap. It would need to be very small to be functional so QC of the total area is difficult. A better solution is a precise drilled passage. to bypass a piston with a 100% seal. In the how to I have a thread on making a small o ring piston that shows drilling the small port between the pilot area and chamber area.

Due to the high forces at those pressures, plan on having a piston with very limited travel to limit the maximum speed and KE of the piston. This can be significant in a close ratio high pressure design. You will need to contain both the static pressure as well as the peak impact forces as the piston bottoms in the pilot. Proper deceleration zone into a bumper can limit the peak forces.

When testing, start with low pressure and work up looking for signs of material stress from excessive forces.
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Unread postAuthor: vakimo » Thu Mar 15, 2012 8:04 am

Sounds good.

Moonbogg, I don't have a pipe threader that large, to i might have to keep the shear bolts for now unless i can redesign it to include tie rods.

Thanks for the pic jack :wink: I think i might actually be able to get some helium. The tangential chamber is actually 18" long, it just shortened for the pics.

Thanks for the advice everyone, a lot of good information here. ill be making quite a few changes to the design.
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This is badly proportioned, but would something like this be the ideal? or if the weight was reduced enough would a spring even be necessary? the holes through the piston would be smaller to.
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Unread postAuthor: Technician1002 » Thu Mar 15, 2012 8:49 am

At that pressure the ports will need to be tiny, or it won't fire. Below is a photo of the size of the port in the piston for my Mouse Musket. It is drilled from the chamber area to the pilot area.
Image
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Unread postAuthor: vakimo » Thu Mar 15, 2012 9:05 am

Alright. By the way would a nylon piston work for this? its a pain in ass to lathe, but it would be lighter and more shock absorbant
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Unread postAuthor: square_pants » Thu Mar 15, 2012 9:15 am

Thats a pretty admirable idea however it is very ambitious, firstly I have a feeling that your rubber sealing face is going to get ripped straight off during piloting.
Have you considered a piston which is able to physically clamp the edges of the rubber, something a bit like this:

Image

By the way what are these bolts for?

Image

If you have a lathe then wouldn't it be easier to just make those top and bottom holding plate things? that way you wouldn't have to use those bolts and you wouldn't have to worry about air leaking under those rings either.

Also I think you are going to a lot of effort to make the valve high pressure but having a 180 degree flow path for the air is only going to decrease performance (so is the ball valve as a pilot valve in your pics)

Overall though I am not trying to put down your ideas, I think you have put a lot of thought into the configuration of your valve and it would be awesome to see you pull it off.
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