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Can you analyze this .wav

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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Unread postAuthor: boyntonstu » Sun Oct 04, 2009 4:52 pm

psycix wrote:Is that me, or does that piston shove up the T piece so far that it has a direct gap into the pilot volume? Or does the piston have a second piece behind it?
Any chance of a pic of the piston? (Always take pictures from parts before making them unreachable)

Also the piston diameter seems very very close to the porting. This is only recommended when your piston is very tight and your pilot valve is near overkill, because this way the piston opens very late, and more air may leak out of the pilot volume before it snaps open. However, if your piston is very tight and your pilot valve is good, a "snapping" piston gives a faster opening and possibly less bounce due to the pilot volume being at a lower pressure when the valve starts opening.


Image

The 'eye' bolt has been removed.

The piston weighs 1/2 oz.
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Unread postAuthor: Technician1002 » Sun Oct 04, 2009 8:12 pm

First thing I see is the valve seat is centered in the T. The thing you want centered is the OPENING. Move the valve seat to the right so the valve when open has a centered opening. The ledge is choking the flow.

Edit; saw the photo of the piston, now I have other observations. The piston appears to be of a non solid construction so the grooves for the o rings may provide less than an ideal seal. The shape of the grooves would allow the rings to press harder than needed against the cylinder wall when trying to fire the launcher. It is highly likely that is preventing the piston from popping open under high pressure but allowing it to rock back just enough to allow some flow into the barrel. After the chamber pressure has dropped some and the pressure off the o rings is released some, then the pressure between the projectile and piston may be enough to pop it open.

The third item is the large size of the piston. It looks overly long. The air that goes between the piston and projectile pushes them both. The projectile a loose marble will move easily. The piston with 2 rings and maybe the bumper and face rubbing on the cylinder wall along with it having a mass most likely quite a bit higher than a marble will move relatively slow.

My recommendations is to create a shorter piston with only 1 ring for lower friction. Make the bumper part of the pilot housing, not part of the piston to lighten it. Use a solid piston material for ring grooves with a better seal, and make the ring groove walls vertical or close to vertical so pressure between the chamber and pilot doesn't try to bind the rings. Lastly move the valve seat to one side in the T so it doesn't block half the flow.

Edit; Looked up the weight of 100 glass marbles. It is 1 lb 3 oz or 19 oz. This gives each marble 0.19 oz weight. The piston is 2.5 times the projectile weight.
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Unread postAuthor: psycix » Tue Oct 06, 2009 4:04 am

Try it without o-rings, andif it works with just one o-ring (try both grooves, maybe only one of the grooves is bad)
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 06, 2009 4:16 am

As mentioned before I think the crucial problem is that the piston diameter and barrel diameter are too small in relation to eachother, either use a bigger tee or reduce the diameter of the barrel at the breech.
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Unread postAuthor: boyntonstu » Tue Oct 06, 2009 11:48 am

Technician1002 wrote:First thing I see is the valve seat is centered in the T. The thing you want centered is the OPENING. Move the valve seat to the right so the valve when open has a centered opening. The ledge is choking the flow.

Edit; saw the photo of the piston, now I have other observations. The piston appears to be of a non solid construction so the grooves for the o rings may provide less than an ideal seal. The shape of the grooves would allow the rings to press harder than needed against the cylinder wall when trying to fire the launcher. It is highly likely that is preventing the piston from popping open under high pressure but allowing it to rock back just enough to allow some flow into the barrel. After the chamber pressure has dropped some and the pressure off the o rings is released some, then the pressure between the projectile and piston may be enough to pop it open.

The third item is the large size of the piston. It looks overly long. The air that goes between the piston and projectile pushes them both. The projectile a loose marble will move easily. The piston with 2 rings and maybe the bumper and face rubbing on the cylinder wall along with it having a mass most likely quite a bit higher than a marble will move relatively slow.

My recommendations is to create a shorter piston with only 1 ring for lower friction. Make the bumper part of the pilot housing, not part of the piston to lighten it. Use a solid piston material for ring grooves with a better seal, and make the ring groove walls vertical or close to vertical so pressure between the chamber and pilot doesn't try to bind the rings. Lastly move the valve seat to one side in the T so it doesn't block half the flow.

Edit; Looked up the weight of 100 glass marbles. It is 1 lb 3 oz or 19 oz. This gives each marble 0.19 oz weight. The piston is 2.5 times the projectile weight.


Thanks,

I took some caliper measurements and discovered that the piston was not retracting far enough to open the barrel.

After removing the extra washer and at 375 psi I measure .031 seconds and 645 fps.

Much faster and the 72 grain marble splintered the back of the 5/8" plywood.
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Unread postAuthor: boyntonstu » Tue Oct 06, 2009 1:10 pm

Technician1002 wrote:
Edit; Looked up the weight of 100 glass marbles. It is 1 lb 3 oz or 19 oz. This gives each marble 0.19 oz weight. The piston is 2.5 times the projectile weight.


72.9 grain = 0.166 ounce is what I measure.

16/.166= 96.3 marbles per pound. (close)

Your comments sure pointed me in the right direction.

645 fps is what GGDT predicts if you choose the right parameters.

I added some weight for the wadding and estimated 80 gr.

Do my parameters seem OK?
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 06, 2009 1:20 pm

boyntonstu wrote:I added some weight for the wadding and estimated 80 gr.

Do my parameters seem OK?


The weight should be in grams, not grains - divide it by 16 ;)

Also, are you sure your pilot volume is 12 cubic inches, sounds too huge.

Also, are you sure you have any dead volume at all? Doesn't the marble contact the piston face?

And who's Sophie :roll: :wink:
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Unread postAuthor: POLAND_SPUD » Tue Oct 06, 2009 1:57 pm

divide it by 16

or simpler... a 16mm marble is 5g each...

just a suggestion get rid of the gauge (put it opn the compressor - great video BTW) that would lower pilot volume
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Unread postAuthor: boyntonstu » Tue Oct 06, 2009 4:37 pm

boyntonstu wrote:
Technician1002 wrote:
Edit; Looked up the weight of 100 glass marbles. It is 1 lb 3 oz or 19 oz. This gives each marble 0.19 oz weight. The piston is 2.5 times the projectile weight.


72.9 grain = 0.166 ounce is what I measure.

16/.166= 96.3 marbles per pound. (close)

Your comments sure pointed me in the right direction.

645 fps is what GGDT predicts if you choose the right parameters.

I added some weight for the wadding and estimated 80 gr.

Do my parameters seem OK?


GDDT calculates the same results if gm = equivalent gr

Both the starting position and the pilot volume parameter seem very insensitive.
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SophieMayah  Balloon comp 3-17-09.jpg
SophieMayah Balloon comp 3-17-09.jpg (25.18 KiB) Viewed 555 times
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Unread postAuthor: Technician1002 » Wed Oct 07, 2009 2:14 am

Well done. Almost doubled the velocity and increased the power 4X.
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Unread postAuthor: boyntonstu » Wed Oct 07, 2009 11:26 am

Technician1002 wrote:Well done. Almost doubled the velocity and increased the power 4X.


That is what happens when I follow your advice.

The one thing I did not change is the position of the barrel seal.

I believe that the piston is opening wide enough to allow good flow.

Also, there are no elbows between the chamber and the barrel.

I removed the piston bumper and I am designing a fixed pilot bumper.

Please double check my GGDT and the Audacity measurement.

Since I fudged GGDT to achieve 645 fps, am I in the ballpark with my parameters?

What additional speed/power can I expect to gain from 400 psi and an 80 grain projectile?

What would you suggest for my next steps?

Thanks,

BoyntonStu
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Unread postAuthor: Technician1002 » Wed Oct 07, 2009 12:10 pm

boyntonstu wrote:
Technician1002 wrote:Well done. Almost doubled the velocity and increased the power 4X.

That is what happens when I follow your advice.


:D

The one thing I did not change is the position of the barrel seal.


Carefully examine the cross section of the barrel, then find the cross section of the valve opening. If the barrel cross section is larger than the valve opening, then I still highly recommend moving the valve seat. Area of a circle is pi times the radius squared. For the barrel use the radius of the inside of the barrel. For the valve opening find the area of the full circle if it wasn't restricted and 1/2 it due to the pipe protrusion 1/2 way into it. Make your own call based on your measurements. My opinion is based on visual and not measurements. Measurements will give the answer.

I believe that the piston is opening wide enough to allow good flow.

Also, there are no elbows between the chamber and the barrel.


No elbows is definitely very good.

I removed the piston bumper and I am designing a fixed pilot bumper.


It is a great start to making the piston lighter. Below is a recommended trim on the length of the piston. Use the part cut off to be part of the bumper. Drill the center of the bumper so it does not block the pilot flow.

Please double check my GGDT and the Audacity measurement.

Since I fudged GGDT to achieve 645 fps, am I in the ballpark with my parameters?


Audacity looks good. If possible reduce the volume when recording. It is easier to analyze without clipping. Under is better than over. If the whole recording is only 25% amplitude, audacity can turn up the volume for an easy to view waveform, but can't correct a clipped waveform.

GGDT shows the pilot volume is about twice the size of the chamber. If you adjust it to reality, the valve opening time will drop by about 2 ms. If you make the valve seat adjustment as mentioned above, you may increase the valve coef to as high as 40-60%. See what that will do to your graph. :D

What additional speed/power can I expect to gain from 400 psi and an 80 grain projectile?

What would you suggest for my next steps?

Thanks,

BoyntonStu


The next steps is to reduce the leakage past the piston rings. Sanding smooth the face of the wood the air will press the rings against and varnishing it there will reduce the amount of air that leaks out the pilot before the piston moves. High leakage can cause rapid loss of pressure in the chamber before the piston opens. This is especially true on small high pressure chambers. Look at your audacity waveform. How long did it hiss before the boom? GGDT shows the main valve opening at 2 ms. Does Audacity show a longer hiss before the boom. Excess piston blow by delays the boom and during the delay, the chamber pressure is bleeding away. Quick is good. To get quick, use a light piston using the cut shown and reduce the piston EQ leakage to a low value.

You cut off the beginning of the hiss in the Audacity screen capture. What shows is 10 ms, not 2. This shows problems with opening time.
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