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Thoughts on my piston valve design

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Thoughts on my piston valve design

Unread postAuthor: stup86 » Thu Jun 13, 2019 5:02 pm

Hi all,

Attached are some images of the designs of my first hybrid cannon.

I've been working on piston valve and wanted people thoughts on the design that I'm at currently.

I plan to use o-rings to seal the chamber from the barrel - anyone had any luck with a design like this?

All comments welcome

NOTE: the images do not show the barrel!

Regards

Stuart
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Attachments
Hybrid section.jpg
Hybrid section
Piston valve section.jpg
Valve section

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Re: Thoughts on my piston valve design

Unread postAuthor: wyz2285 » Fri Jun 14, 2019 5:15 am

The piston won't open.
The area of the barrel side need to be smaller than the area of the pilot side.
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Re: Thoughts on my piston valve design

Unread postAuthor: jackssmirkingrevenge » Fri Jun 14, 2019 9:06 am

wyz2285 wrote:The area of the barrel side need to be smaller than the area of the pilot side.


It is tho.

ratio.png
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Re: Thoughts on my piston valve design

Unread postAuthor: wyz2285 » Fri Jun 14, 2019 10:32 am

Ohhh the o-ring is in the T not the piston. Well yeah that will work, need low pilot pressure tho, loos like a small difference to me.
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Re: Thoughts on my piston valve design

Unread postAuthor: stup86 » Fri Jun 14, 2019 10:39 am

wyz2285 wrote:The piston won't open.
The area of the barrel side need to be smaller than the area of the pilot side.


Thank you for replying.

It was obvious really!

I'm made some changes to the design, incorporating a weld-in bush to reduce the diameter of the piston at the barrel side.

Again, any thoughts and comments are appreciated

Piston valve section V2.jpg
Piston valve section V2

iso piston valve cutaway V2.jpg
iso piston cutaway


Regards

Stuart
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Re: Thoughts on my piston valve design

Unread postAuthor: Moonbogg » Fri Jun 14, 2019 2:53 pm

Stuart,

I like the design. I'd like to see the barrel side diameter be closer to the pilot side diameter, similar to how you had it in your first drawing, but not quite that close. The closer they are, the less pressure you will need in the pilot area. With less pilot pressure, this means less stress and abuse on the piston during the fueling cycle and operation will be easier also. You have to use the surface areas of the piston to calculate how much pilot pressure you'll need for a given opening pressure.
I see a small edge of engagement where the piston seats against the T-housing. I'd like to see a larger step there. If the pilot pressure has to be high, and you have a tiny step of engagement, I'd expect that edge to experience some high stress and abuse maybe. Just a thought.
Also, if you make the front of the piston very close in diameter compared to the back, again, you can have a low pilot pressure. If you increase the volume of the pilot area so the air doesn't compress too much, and if the pressure is low in the pilot, then you don't even have to worry about venting the pilot for each shot, but you will need to have a bumper on the front of the piston as well as the back. I know this works because I just made a piston hybrid based on this principle, but it would change your design by quite a bit so I'm not sure you want to go that way. Just thought I'd mention it.
What mix are you looking at? Based on the piping component design, I'd assume the standard 10X?
Keep us updated.
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Re: Thoughts on my piston valve design

Unread postAuthor: stup86 » Fri Jun 14, 2019 4:28 pm

Moonbogg wrote:I like the design.


Thank you

Moonbogg wrote:I'd like to see the barrel side diameter be closer to the pilot side diameter, similar to how you had it in your first drawing, but not quite that close.


Moonbogg wrote:I see a small edge of engagement where the piston seats against the T-housing. I'd like to see a larger step there.


To address both of these comments, I've updated the design again as shown below:

Piston section V3.jpg
Piston valve detail


There is now a larger face of engagement where the piston seals the barrel and the delta between the piston end diameters has reduced.

What are you thoughts on this?

Moonbogg wrote:if you make the front of the piston very close in diameter compared to the back, again, you can have a low pilot pressure.


The diameter of the piston on the left (barrel side) is 41mm and the right (pilot) diameter is 52mm - is this close enough?

Also, I now have all the o-ring grooves on the piston.

You have to use the surface areas of the piston to calculate how much pilot pressure you'll need for a given opening pressure.


Do you have an more details on calculating this? Any equations etc?

What mix are you looking at? Based on the piping component design, I'd assume the standard 10X?


Let's start with 10x and see where we go. I like to tinker and make things better...

Thanks for all your help so far, please keep the helpful comments coming.

Regards

Stuart
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Re: Thoughts on my piston valve design

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Re: Thoughts on my piston valve design

Unread postAuthor: jackssmirkingrevenge » Sat Jun 15, 2019 8:22 am

stup86 wrote:Do you have an more details on calculating this? Any equations etc?


The force keeping the piston shut is area of the largest diameter x pressure.

Force trying to open the piston is (largest diameter - smaller diameter) x pressure.

Let's say the major diameter of the piston is 2 inches and the smaller diameter is 1.5 inches, and your pilot pressure is 100 psi.

The force keeping the piston closed is therefore 314 lbs.

The net area on the chamber side is 1.374 square inches, so for the piston to open, the pressure inside the chamber must exceed 314 lbs / 1.374 square inches = 228 psi.

What you want is for the opening pressure in the chamber to be as close as possible to the maximum pressure generated by the burning fuel/air mix.

HGDT is a great program to determine this, as it is no longer available online I have attached a zip file.

Details about the program archived.

Nice solidworks drawings by the way, I assume you have access to machining facilities if you intend to build this?

Also, I assume you're aware that this would comfortably exceed 12 ft lbs of energy and be considered as a firearm in the UK, so one is best advised to keep a low profile... which would be a challenge with a large bore hybrid...
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Re: Thoughts on my piston valve design

Unread postAuthor: stup86 » Sat Jun 15, 2019 12:28 pm

jackssmirkingrevenge wrote:HGDT is a great program to determine this, as it is no longer available online I have attached a zip file.


Thank you!

Nice solidworks drawings by the way, I assume you have access to machining facilities if you intend to build this?


Never really got in to Solidworks, this was modelled using CATIA. There is a workshop at work with a metal working lathe, mill and a new CNC mill, so yes I have access, although they are busy, so getting it machined may take some time. However it's not complicated what I'm asking. so fingers crossed.

I've been playing around with KeyShot recently to render images. Here's my first attempt if anyone is interested. Need to look at the lighting though.

untitled.6.jpg
Piston rendering


Thanks for the help.

Regards

Stuart
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Re: Thoughts on my piston valve design

Unread postAuthor: Moonbogg » Sat Jun 15, 2019 3:30 pm

Stuart,

The piston looks better in terms of the diameter ratio. You are using a vented pilot design, so I think the ratio you have is probably optimal enough for that type of valve. Keep in mind that the effective sealing diameter may end up being the step of engagement since the pressure builds so fast that significant leakage to the o-ring may not have enough time to take place during combustion. I'm not sure, but it shouldn't matter that much either way. You may just have to experiment with the actual opening pressure and performance versus your calculated opening pressure and performance.

The next issue I see is the bumper. I assume that little rubber washer in the back of the pilot is a bumper? Or is that used as a guide for the rod? It looks like a bumper. It's going to get smashed to hell and it will be as if it wasn't even there. A 10X mix has a theoretical peak pressure of around 1100psi. Multiply that by the front surface area of your piston to get an idea of how much force will be pushing back on the piston. Hint: It's going to be a lot.

Keep in mind that using a cannon at 10X is probably a real pain in the ass in terms of practicality. If you need 200psi in the pilot for every shot, then that's going to suck, right? You either need a high pressure hand pump or a high pressure air compressor. That's one of the main reasons I like close piston ratios, low pilot pressures and lower mixes: practicality. The increased safety is just a bonus around here of course :lol:

Another key thing to consider here is the piston travel and minimizing it as much as possible. The piston is basically a projectile and every time you shoot the cannon, you are shooting your cannon with itself. The farther the piston travels, the more time it has to accelerate and that means it will slam harder. I'm not sure how big of a deal that really is because I've only really built one decent piston cannon, so my experience is limited there, but I would try to minimize piston travel as a top priority.

A rule of thumb for piston opening distance is 1/4 of the port diameter. So, if the port is 100mm, then the piston can open 25mm and that will be sufficient, although this is assuming the piston is just a flat face up against the port. My design wasn't exactly like that and neither is yours, so your piston will have to open more than the 1/4 rule would suggest. Basically aim for 1/4 the diameter in terms of clearance from the port opening. Your piston sticks inside the port a little, so it needs to open that much plus the 1/4 rule plus just a bit more for good measure (also the bumper will compress, allowing a bit more opening).

If the total piston travel distance is 25mm, then I would aim to have a bumper that's 25mm thick (or a little more). That's not a rule or anything. It just feels right to me having a bumper the same thickness as the piston travel distance. My piston travels 1/2" and my bumper is 1/2" thick. Keep in mind the rubber will expand outward when it gets smashed. Try to have the rubber almost fill the volume in which it occupies so that the rubber will absorb the impact, compress and then fill the void at which point the rubber should then behave like an incompressible fluid, preventing any further piston travel.
If the rubber isn't thick enough or hard enough and if it's allowed to get smashed too thin, then it will be an inadequate bumper and you might end up cracking your pipe fittings or damaging the piston, or both.
Keep us updated. Good luck.

EDIT: For your reference, you may wonder about rubber hardness (durometer). I use a 50 shore A hardness buna rubber for my bumper and sealing face in my 3X hybrid, and it works perfect. That's only 3X though. You'll have to decide on a hardness based on either your gut, experimentation or both.

Shore Durometers of Common Materials
Material Durometer Scale
Bicycle gel seat 15–30 OO
Chewing gum 20 OO
Sorbothane 30–70 OO
Rubber band 25 A
Door seal 55 A
Automotive tire tread 70 A
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Re: Thoughts on my piston valve design

Unread postAuthor: stup86 » Sat Jun 15, 2019 5:54 pm

Moonbogg wrote:The next issue I see is the bumper. I assume that little rubber washer in the back of the pilot is a bumper? Or is that used as a guide for the rod? It looks like a bumper.


Correct, it is a bumper. I'm not exactly sure of the hardness but you're right, it does look woefully inadequate when considering the beating it's going to get. I'll increase the diameter to limit the radial expansion and adjust the length to more closely match the piston travel.

Currently the bumper is 10mm thick and at present the piston has a maximum travel of 40mm. I'll adjust to be as close to 12.7 (25% bore diameter) plus the extra the piston extends into the barrel (~11mm from memory). I was planning on making the bumper by cutting it on the lathe from a larger diameter performance car polybush, so adjusting the diameter and length will not be an issue.

I've tried to keep the weight of piston assembly (pictured rendered above) as low as possible. CATIA currently estimates the total weight at ~400 grams. Unfortunately I don't have a breakdown of the weights currently, but thia can be provided if anyone is interested. My initial idea was to make it as lightweight as possible to reduce the stress on the surrounding components.

Currently, I've only considered aluminium as my piston material, but has anyone experimented with any polymers? Not sure what the shortlist would be that can cope will the pressures involved though. I'm interested in anything that will reduce overall weight and, obviously, is affordable!

As I'm new to HGDT, i'll have a play with that later and see if I can incorporate further design changes into V4.

Thanks again all, especially Moonbogg, for the knowledge sharing.

Regards

Stuart
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Re: Thoughts on my piston valve design

Unread postAuthor: Moonbogg » Sat Jun 15, 2019 10:52 pm

I have included a link to my build log. It has prints for the piston and everything else. I used Delrin plastic for the piston. Works surprisingly well. Seriously, I was expecting some nasty issues with my piston, but to my SHOCK it actually works just as I hoped it would.
Can you use Delrin or similar for a 10X cannon? Perhaps you can. I think a guy around here used 3D printed plastic parts for his 10X hybrid, although the core was still a steel rod. If the pilot pressure is high, the plastic might get deformed and damaged at the seating edge. Not sure. You can do some force/kpa calculations and compare to the yield strength of your plastic of choice. Or, just go with aluminum and lighten it up. Honestly, at 10x I don't think a bit of a heavy piston will be too much of an issue. Yours looks at least as good as the others I've seen around here and those performed great.

hybrid-cannons/topic26921-30.html
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Re: Thoughts on my piston valve design

Unread postAuthor: mrfoo » Sun Jun 16, 2019 2:21 am

stup86 wrote:I'm interested in anything that will reduce overall weight and, obviously, is affordable!

Depending on dimensions, I might be able to sort you a block of teflon or two, for the cost of postage from cheese-eating surrender-monkey land. Drop me a PM if you're interested.
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Re: Thoughts on my piston valve design

Unread postAuthor: stup86 » Sun Jun 16, 2019 5:15 am

mrfoo wrote:Depending on dimensions, I might be able to sort you a block of teflon or two, for the cost of postage from cheese-eating surrender-monkey land. Drop me a PM if you're interested.


Thank you. Once I've finalised the piston design, I'll send you some dimensions and we'll sort something out. :D
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Re: Thoughts on my piston valve design

Unread postAuthor: stup86 » Sun Jun 16, 2019 5:31 am

There's something about the orientation of the piston that isn't sitting right with me.

I feel that having a design where the piston moves vertically, essentially sealing the chamber rather than the barrel would be better. As a picture is worth a 1000 words, here we go:

Cannon_alternative.jpg
Alternative piston orientation


What's the general consensus of the 2 designs featured?

Regards

Stuart
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