Thoughts on my piston valve design

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stup86
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Thu Aug 26, 2021 10:29 am

Just to update anyone that is interested...

This project is not dead, but life took over and 2 years passed. I had some issue with the 3D and lost my latest revisions, so will have to revisit the design. After re-reading this entire thread again I have some changes I'd like to make. I will post any changes soon and look forward to anyone's feedback.

It's nice to be back though :)
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Thu Aug 26, 2021 10:36 am

stup86 wrote:
Thu Aug 26, 2021 10:29 am
This project is not dead, but life took over and 2 years passed.
It happens to the best of us, looking forward to seeing progress :)
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Moonbogg
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Thu Aug 26, 2021 2:53 pm

Good to see you back. Designing is a big part of the fun, so I look forward to you working on it some more.
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Thu Aug 26, 2021 4:36 pm

Thanks for the welcome back.

So here is where I am at the moment.

As recommended many moons ago I have added a large washer to secure the rubber face seal to the front of the piston. As before this is attached with an M10 screw.

Pressed into the rubber face seal is a sleeve to allow the screw to clamp the seal tight and not over compress the rubber

I have also moved the O-Ring grooves further apart.
Piston_V6.png
Here is an exploded view for clarity :D
exploded_view.jpg
I plan on performing some more stress analysis on the piston before cutting metal, although I'm not entirely sure how to setup the analysis within SolidWorks (in particular what fixture face I should be using) - if anyone has any guidance that would be greatly appreciated.
Piston_render_1.jpg
Regards

Stuart
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Sat Aug 28, 2021 3:46 am

Something else I'd like someone's opinion on is the faces the chamber pressure and pilot pressure will be acting on

Apologies for the metric units but currently I have the following:

Chamber side of piston (235.62mm2):
Chamber_face_area.png
Pilot side of piston (2019.46mm2):
Pilot_face_area.png
I had a value of 1200 psi from HGDT for a 10x mix so I designed the piston to have a relatively small area to act upon (chamber side). WIth the current area of the pilot side (I can't really increase this) it requires a pilot pressure of around 133psi - from my calcs the pressure either side of the piston would equalise at chamber pressure approximately 1140psi.

Can anyone confirm this?

Regards

Stuart
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Moonbogg
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Sat Aug 28, 2021 11:02 pm

Based on your shown areas, I got 1143 psi chamber pressure for equalization, so the same that you got. It looks like this is an estimate though because it looks like you clicked the surfaces and there are chamfers on them, so the actual diameters will be a little larger, but it should be pretty close as it is. It also depends on the sealing face and if the whole rubber will be against the port. If the rubber diameter is larger than the edge of the port, then the calculations won't be accurate as the equalization would happen at less than the 1140psi because there would be more surface for the chamber pressure to act on at the front of the piston.
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Sun Aug 29, 2021 1:42 am

Thanks Moonbogg

That does give me confidence that I'm on the right track.

I will double-check all the areas that the pressure will act on.

I also had an idea the other day about using a spring to keep the piston sealed. Has anyone done this?
spring concept.png
Early concept, if this is a viable option I would need to resign the piston and find a suitable spring!

Obviously the downside is the spring isn't variable so a different spring would be needed for a given mix.

I'm also not sure how the spring would operate during operation. Would I get any valve bounce?

Regards

Stuart
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Sun Aug 29, 2021 5:05 am

> Obviously the downside is the spring isn't variable so a different spring would be needed for a given mix.

A threaded boss at the rear can increase or reduce preload on the spring within a certain range, although this will also reduce spring travel.
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Sun Aug 29, 2021 10:20 am

mrfoo wrote:
Sun Aug 29, 2021 5:05 am
A threaded boss at the rear can increase or reduce preload on the spring within a certain range, although this will also reduce spring travel.
I did think about some adjustment to preload the spring, but I'm not sure I can find a suitable spring that would cover 1x to 10x mixes whilst also not reducing spring travel too much.

I'll continue to work on the concept and see where it goes. Thanks for your input :)

Regards

Stuart
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Mon Aug 30, 2021 12:34 pm

Does anyone know if MrCrowley is still active on this forum?

I have some questions regarding the constuction of his cannon and it appears the majority of the links to his Mjöllnir cannon are dead.

Currently I'm having concerns with the fittings that I can get in the UK. Its turns out that the fittings I have are only rating to 150psi :shock:

Any help on this would be greatly appreciated

Regards

Stuart
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Moonbogg
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Mon Aug 30, 2021 5:48 pm

Are the fittings aluminum? I'm not familiar with the fittings you are going to use. I also don't know why commercial fittings are given the pressure ratings that they have. With PVC it's obvious, but if galvanized iron says 150psi then that seems strange to me but I'm sure there's some reason. Maybe it has to do with potential length of pipe assemblies, bends and turns, welded connections, water hammer effects and increased stress from pipeline geometry. Also, maybe those pipes are intended to be used according to an industrial standard where the pumps and other equipment is rated for 150psi, so they just match it. Just guessing here though. Oh, also if those are rated for gas then a low rating might make more sense.
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Mon Aug 30, 2021 6:04 pm

Moonbogg wrote:
Mon Aug 30, 2021 5:48 pm
Are the fittings aluminum? I'm not familiar with the fittings you are going to use. I also don't know why commercial fittings are given the pressure ratings that they have. With PVC it's obvious, but if galvanized iron says 150psi then that seems strange to me but I'm sure there's some reason. Maybe it has to do with potential length of pipe assemblies, bends and turns, welded connections, water hammer effects and increased stress from pipeline geometry. Also, maybe those pipes are intended to be used according to an industrial standard where the pumps and other equipment is rated for 150psi, so they just match it. Just guessing here though. Oh, also if those are rated for gas then a low rating might make more sense.
Hi Moonbogg,

The fittings I'm currently using are malleable cast iron. They are also available in stainless steel. Both rated at 10bar/150psi.

My first thought was similar to yours, if the fitting has no need to be rated higher then it would be rated to match.

But, they I saw some fittings rated to 3000psi. These fittings, as expected, are much beefier come with a much beefier price tag! (£200 for 2" tee, compared to £6 for cast iron fitting)

Ideally I can get some info on if the fittings I'm intending to use have been used before...
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