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New wadding design and piston update - Also .wav file

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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Valve seat seal

Unread postAuthor: Technician1002 » Tue Oct 20, 2009 8:12 pm

I'm up again. A few comments on the piston. If you have a piston that is difficult to seal, like this one, lapping in the valve is a solution. Make a long piston out of wood dowel that will fit in there and either use fine sandpaper or valve grinding compound and gently rotate the dowel in the valve area (snug fit) to wear the valve sealing face. With a little valve grinding compound on the valve seat then the piston face is mated to the valve seat in the same fashion.

Smaller valve seats are easier to do. Larger seats due to the size of the sealing area are much more critical to complete this step. In the attached photo, this process on the Dragon is shown. Note the sets of black marks on the radius of the valve seat. These are sharpie pen marks. The valve seat was lapped and sanded true to the valve cylinder by use of a piston with a sandpaper face. The seat was sanded until all the sharpie marks wore off evenly. When I started, several wore only a little. I marked often and checked progress until they all wore evenly. It is how I got the dragon to seal. Not the mark on the lower right is completely sanded off. It was my high spot. That is a 2.5 inch valve seat using a 3 inch piston. In the photo you can see one of the marks is worn down more than the rest. This was a high spot that not only provided problems sealing, but put a twist on the piston so it wanted to stick. This may be an issue you are having.

A proper opening valve has the air pressure surge right after the end of the hiss without any hesitation. The second waveform is my Mouse Musket discharge. Note the pressure wave is right after the end of the hiss showing the valve opened fully when it opened.

Am I the only one that can look at a waveform? Help the man out. The wave has been downloaded only 6 times, twice by me.
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PICT235.JPG
Dragon valve seat polishing marks.
Musket Expanded.png
End of hiss is followed by a strong pressure wave as the valve pops all the way open.
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 9:29 pm

I agree with your technique of valve grinding the seat.

OTOH Most rubber sealing washers can conform to a slightly uneven surface.

I use wet and dry paper to polish the seat and although not perfect, I felt that it was good enough to seal.

I have seen the washer bent out of shape after a well sealed shot.

Therefore the support and/or backing seems to be the problem.

So far, the best was Oak piston/nut/rubber/washer/nut.

Instead of Oak piston/rubber/washer/nut.

With nuts on both sides of the rubber, I was able to use 2 thin (11/32) wrenches to tighten the washers against the rubber.

With 500 psi parts are subject to high forces.

Perhaps I should try one of these MRR pistons?

Image

I don't understand what the spring does.

Photo captured from:

[youtube]http://www.youtube.com/watch?v=JPfLwk7W0Gc[/youtube]

BoyntonStu
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Unread postAuthor: jor2daje » Tue Oct 20, 2009 9:54 pm

Have you tried adding mutiple layers of rubber for the sealing face? On my first piston valve I didnt sand the sealing face smooth enough, but I put two layers of rubber so it deformed enough to seal nicely.
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 10:02 pm

jor2daje wrote:Have you tried adding mutiple layers of rubber for the sealing face? On my first piston valve I didnt sand the sealing face smooth enough, but I put two layers of rubber so it deformed enough to seal nicely.


Thanks,

Good idea.

What kind of rubber solved your sealing problem?
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Unread postAuthor: jor2daje » Wed Oct 21, 2009 12:03 am

I used these rubber tiles that I'm pretty crazy about, but I think most rubbers like neoprene should work well.
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Unread postAuthor: Technician1002 » Wed Oct 21, 2009 12:22 am

boyntonstu wrote:
I don't understand what the spring does.

Photo captured from:

BoyntonStu


There is no spring that I could see. The piston is made up of stacked washers and nuts on a bolt. Air filling from the pilot closed the valve.
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Unread postAuthor: boyntonstu » Wed Oct 21, 2009 6:46 am

jor2daje wrote:I used these rubber tiles that I'm pretty crazy about, but I think most rubbers like neoprene should work well.


Rubber tiles?

What are those?

How thick?
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Unread postAuthor: boyntonstu » Wed Oct 21, 2009 6:52 am

Technician1002 wrote:
boyntonstu wrote:
I don't understand what the spring does.

Photo captured from:

BoyntonStu


There is no spring that I could see. The piston is made up of stacked washers and nuts on a bolt. Air filling from the pilot closed the valve.


Ooops! Wrong spectacles.

You are correct, (as usual).

Why use stacked steel washers? What purpose do they serve?

Do they fit the chamber and therefore scrape the copper?

Wouldn't rubber washers be better?

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Unread postAuthor: Hubb » Wed Oct 21, 2009 7:29 am

I used this technique to solve my sealing problems. Now, I use this technique on every piston valve I make, whether I need it or not.
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Unread postAuthor: boyntonstu » Wed Oct 21, 2009 12:55 pm

I tried a piece of thin (~3/16") open cell foam behind the rubber, bubbles.

I hot glued the screw and nut, perfect seal! NO BUBBLES.

4 shots. No bubbles.

This is a shot of 4 ply cotton brief material with marble exposed at 550 psi.

I can't make heads or tails from it.


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10-21-09 cotton wadding 550 psi.rar
(62.24 KiB) Downloaded 40 times
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Unread postAuthor: Hubb » Wed Oct 21, 2009 1:13 pm

Yeah. That foam you use is probably better than the felt. I will have to "borrow" that idea the next time I build a piston.
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Unread postAuthor: boyntonstu » Wed Oct 21, 2009 1:36 pm

Hubb wrote:Yeah. That foam you use is probably better than the felt. I will have to "borrow" that idea the next time I build a piston.


Working together we can get 'er done!

I am working on a home built photo-transistor chronograph.

At the moment the signal is too weak.

Audacity and Technician tells us more about our cannons than any other tool.

So in a sense, I am glad that it didn't work.

Perhaps we should have a thread showing our Audacity sounds?
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Unread postAuthor: Technician1002 » Wed Oct 21, 2009 1:48 pm

boyntonstu wrote:I tried a piece of thin (~3/16") open cell foam behind the rubber, bubbles.

I hot glued the screw and nut, perfect seal! NO BUBBLES.

4 shots. No bubbles.

This is a shot of 4 ply cotton brief material with marble exposed at 550 psi.

I can't make heads or tails from it.


BoyntonStu


The hum is gone. The start of the hiss to the pow is about 0.06 seconds. Nice and fast. The waveform is clipped. The shot is very fast. I can hear the impact, but I too am having trouble seeing it in the waveform. It's fast. The target is hit quickly. I think that the small barrel and high pressure is creating a large high frequency component so the large long step I see with my cannons is not readily visible. Is there a way to turn down the mic a little during recording so the waveform doesn't peak so bad? It is LOUD. Nice sound. I like it.
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Unread postAuthor: boyntonstu » Wed Oct 21, 2009 2:52 pm

Technician1002 wrote:
boyntonstu wrote:I tried a piece of thin (~3/16") open cell foam behind the rubber, bubbles.

I hot glued the screw and nut, perfect seal! NO BUBBLES.

4 shots. No bubbles.

This is a shot of 4 ply cotton brief material with marble exposed at 550 psi.

I can't make heads or tails from it.


BoyntonStu


The hum is gone. The start of the hiss to the pow is about 0.06 seconds. Nice and fast. The waveform is clipped. The shot is very fast. I can hear the impact, but I too am having trouble seeing it in the waveform. It's fast. The target is hit quickly. I think that the small barrel and high pressure is creating a large high frequency component so the large long step I see with my cannons is not readily visible. Is there a way to turn down the mic a little during recording so the waveform doesn't peak so bad? It is LOUD. Nice sound. I like it.


It is a blast to shoot!

I am using type M and I do not plan to go beyond 600 psi.

The ball valve trigger goes to about 75% open and it's gone!

Can you guess where the sound of the hit shows on Audacity and estimate the speed based on traveling 20 ft with the mic in the center?

I am recording with the Audacity volume set at zero.

Perhaps if I place the mic inside an oven mitten, the sound will be more delineated?

Thanks guys, you all deserve credit for this cannon.

BoyntonStu
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Unread postAuthor: boyntonstu » Wed Oct 21, 2009 3:47 pm

Another shot with mic wrapped in a towel and inside a glove.

There should be at least 2 bangs after the muzzle blast.

The marble went right through the 5/8" plywood target and it dented a 3/8" sheet positioned about 7" behind it.

What a blast!

These were the parameters and I tend to believe the 800+ fps 120 fpe!
Image

I water test for piston leaks before every shot. No leaks.

BoyntonStu
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10-21-09 cotton wadding 550 psi. through plywood.rar
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