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Labtecpowers (soon-to-be) quick dump valve cannon

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Technician1002 » Wed Feb 02, 2011 2:38 am

As long as the offtopic subject is started on the US and the Metric system, I'll toss in my 2 cents worth.

Many Americans are slowly transitioning to metric. There was a big push in the late 1970's during the gas crunch that set the program back into the stone age. Back to that in a moment..

Compatibility with existing rules regulations and infrastructure are keeping much of our measurement on current standards. Dealing with dual standards for the transition is too difficult. This is why our plumbing is still English. Too many building codes would have to change and dual stock of both standards would be too much hassle for retailers.

Hardware stores carry both metric and English hardware. This of course brings it's own set of problems when close but not matching threads are forced..

Grocery stores carry many items in metric sizes such as bottled water, soda pop, etc.

Back to the setback with Gas.. During the gas crisis in the 1970's, many gas pumps became obsolete as the 3 digit display could not be set above $0.99.9/gallon. About 1/2 the gas stations upgraded and put in metric pumps. Price comparison became difficult as finding the better deal and those who did mange to bother with the math found the 37 cent gas was no bargain. It soon became common knowledge the low price gas was much more expensive and even when prices evened out, the general knowledge was that was the expensive gas because you got so little of it. The metric stations quickly converted back or went out of business.

Calculating gas mileage was a problem too. Miles / Litre was neither all metric or all English.

High priced metric pumps was probably the biggest roadblock to the US adopting metric in the middle of a good push to convert.

Metric is making big advances in fields not bogged down by this problem. American cars have Metric engines, so mechanics now have dual tool sets. Consumer electronics is mostly all metric, such as the bolts needed to mount a flat screen TV to the wall bracket, etc.

Despite the passage of laws in the 1970s, the USA has not completely gone to the metric system. We still buy gas by the gallon, land by the acre and rope by the foot.


http://www.ehow.com/learning-the-metric-system/


Specifically on gas

Gas pumps
Metric gas

Gasoline by the liter in Minnesota, from the USMA Newsletter, Sep-Oct 1979.

At its 2–3 May 1979 public hearings in Washington, DC, the USMB considered the issue of gasoline pumps that were unable to handle prices higher than 99.9¢ per gallon. The 107-page report resulting from the hearings concluded that the U.S. would save at least $94 million by converting pumps to sell gas by the liter instead of converting the pumps to handle prices above $1 per gallon.

At its 21–22 June 1979 meeting in Boston, the USMB voted 13 to 1 to approve a resolution to support conversion of retail motor fuel pumps to dispense fuel by the liter:

RESOLUTION

The petroleum retailing industry generally indicates a willingness to dispense gasoline by the liter. Several states are taking independent action in requiring or recommending liter dispensing. Therefore, the United States Metric Board declares that:

This is an opportune time for the development of a planned and coordinated voluntary program of dispensing gasoline by the liter and the Board urges all affected parties to participate in the planning process. It calls atttention to the need for adequate public information in connection with liter dispensing.

Without taking this action, metric usage is likely to proceed in a haphazard fashion leading to public confusion, disparate end results and a negation of the positive cost advantages that a nationally planned and coordinated program offers.


http://lamar.colostate.edu/~hillger/laws/usmb.html
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Unread postAuthor: Labtecpower » Wed Feb 02, 2011 5:01 am

I think patriottism is also a factor (wich I can totally understand :D )

It sounds very confusing when there are different measuring systems in a country :?

when I watched your photos, I saw I have larger ports than your marshmallow cannon.
By how much would this increase performance?
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Unread postAuthor: Technician1002 » Wed Feb 02, 2011 6:59 am

In port size selection, I cut the ports to have twice the cross sectional area of the pipe ID. This permits the valve when fully open to flow at 1/2 the air velocity as the air in the pipe. Increasing the port size beyond that will have little effect on the flow.

The math is easier on the larger cannon with the 2 inch pipe so I'll use that for an example.

The pipe is about 2 inches ID. Using the formula for area the area of a 2 inch pipe is 3.14 square inches. The three ports are 1 inch tall by 2 inches wide for 2 square inches of area each. In total the ports are 6 square inches feeding a pipe with only 3.14 square inches of area. Making the ports larger does little to improve the flow.

The marshmallow cannon uses the same port to pipe area ratio.

Area = Pi times Radius Squared.
2 inch pipe has 1 inch radius.
1 squared is 1.
1 times Pi is Pi.

Using short ports allowed using a short piston which saves mass and improves speed. It doesn't have to move far and speed is related to the mass in the F=MA equation. With the applied force from the air, Acceleration is inversely proportional to the mass. Halving the mass doubles the acceleration.

My ports are small, but not enough to limit flow and not too large to slow a heavy longer piston. Size was chosen as a balance between flow and speed. It is biased for flow more than speed.

My first QDV, the ABS cannon has square ports. I later did the math and found they were larger than needed. My later designs fixed that.

If you look at any 2 cycle gas engine such as a weed eater or chainsaw, you will find this wide short ports is the norm.

Image
A typical 2 stroke engine exhaust port

Your 42mm pipe has a cross sectional area for flow of;
Area = Pi times Radius squared.
Diameter is 42 Radius is 21 Radius squared is 441
Pi times radius squared is 1384 sq mm. How big are your ports?
I'm guessing from the photos your port area is 4,000 to 6,000 sq mm.
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Unread postAuthor: Labtecpower » Wed Feb 02, 2011 7:55 am

My ports are 3.7 x 3.2 cm,and I have three of them.


3 x (3.7 x 3.2)=3552 sq mm.

I won't have lack of flow :D

I'm going to make a piston out of HDPE, as it is 2/3th of the weight of a PVC one...
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Last edited by Labtecpower on Thu Feb 03, 2011 10:16 am, edited 1 time in total.
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Unread postAuthor: Labtecpower » Thu Feb 03, 2011 10:15 am

UPDATE:

I completely sanded the inside of the pipe.
I also removed 0.05 mm of the piston, so it won't get stuck.

I ordered O-rings 43 mm OD, 5 mm thick
Friggin expensive!

I pay about 0,60 euro's per ring, and the postage costs are 10 euro's!
Well, if I want to make this gun work, i'll need them.
I think I order 6 of them, so when one breaks down I have some spare ones.

Picture of the pipe inside:

Image



I think i'm going to cut out the holes in the butane tank soon. I just have to make sure it's completely empty
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Unread postAuthor: Technician1002 » Thu Feb 03, 2011 12:12 pm

Looking great. I'm amazed you could not find any source of o rings locally. Between places to get plumbing supplies, or a well stocked hardware store, or place that has heavy machinery (hydraulic seals), there should have been a place to get them inexpensively. In my local area the rings were less than 1/2 the price of a can of soda pop. The marshmallow cannon was like 20 cents US each and the large cannon 60 cents each. It must be a bummer not having a local source of supplies.

I bought extra o rings for all my cannons. I've never needed them. With a spare piston and rings for each cannon, I use them to show people how the piston with floating rings works.

When cutting into a fuel tank, use all the fuel until the pressure is gone, then drill a small hole in each end and use an air compressor or other air source to replace the fuel vapor in the tank with air. Let it vent for a long time.

Be aware that when welding, you may encounter some oily deposits or other source of fuel that will vaporize when the metal is heated. To avoid an explosive buildup, either flow air or an oxygen free inert gas into the tank to purge the fumes. I found adding dry ice in the tank for welding works wonders.

Purging a Propane tank with an air compressor. Valve is removed and air is run in the tank for a couple of hours.
Image
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Unread postAuthor: Labtecpower » Thu Feb 03, 2011 12:35 pm

I think i'm going to drill two holes, and ventilate the tank with a pump for inflatable stuff.

I really hate not having a supply for O-rings nearby. but, even with the expensive o-rings, this cannon is cheaper than my copper gun, wich cost me about 30 euro's.

I'm now making the fire and reset bar. I found some 8 mm steel bar around the house.

About the piston weight; it weighs 100 grams. I think it's pretty much for a piston. what do you think?
Definately going to install a good bumper :roll:


I'm pretty amazed by the photo's. I made them with a 5 or 6 year old sony ericsson w810i mobile phone :)
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Unread postAuthor: Technician1002 » Thu Feb 03, 2011 12:58 pm

Not bad photos for a mobile phone. Most of my photographs are shot with an older 2.1 Megapixel camera. It has a good lens, great zoom, great image stabilization, and the photos require little editing to cut them down to web size. I don't need a high pixel count for decent photos for online photos.

The way to consider if your piston is too heavy is compare it to the weight of your projectiles. How much do your intended projectiles weigh? If the piston can move fully open using air pressure, how far has the projectile moved with the same air pressure.

I tried to design mine so the piston is fully open before the projectile has moved twice its length. From there the rest of the launch out the barrel is finished with the valve wide open.

Knowing Force = Mass times Acceleration, you can compare the mass of the piston and projectile to see the inverse relationship between mass and acceleration with a given force. I designed so most intended projectiles are more than 1/2 the mass of the piston. This permits them to accelerate at good speeds and the valve to fully open before the projectile has moved very far in the barrel.

Even the piston in the marshmallow cannon only weighs about the same as a couple of marshmallows. This permits the valve to fully open about the same time as the marshmallow has moved twice it's length in the barrel. :D
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Unread postAuthor: Labtecpower » Thu Feb 03, 2011 1:16 pm

My ammo certainly won't be marshmallows :D
more like potatoes, or rotten apples.
Saboted stuff (ball bearings, darts) is something I like too.


By the way, knee pads FTW!
I found an old one in my shed, and decided to cut it up. I want a good bumper for the heavy piston.
I completely finished the reset rod now.

Image

Fits perfectly in the tube!

The reset ring is completely retracted in the bumper and tube when the piston is pulled back.
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Unread postAuthor: Labtecpower » Fri Feb 04, 2011 12:31 pm

ANOTHER UPDATE! Does this irritate the mods?

I had a lot of spare time today, and I decided to machine some stuff mor my QDV.
I made an aluminium endcap on a lathe, and I cut some threads in it to put it on the pipe.

I also made a nice handle for it out of nylon. It is made in a way that if you push it in completely, the piston is in exactly the right position.

I think I can say "not all 17-year-olds make crappy cannons" :D

Here are some photo's.

Image

Image

Image


Bad news; the O-rings I ordered can't be delivered. They only deliver with a minimum of 20 pieces.
I'll have to find another source.
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Unread postAuthor: Technician1002 » Fri Feb 04, 2011 12:42 pm

Nice job. You are going to like the performance when you get it finished. One note when you weld it, don't weld too close to your ports so you don't warp the metal in that area. I have a little of that with my 2.5 inch as I tried to put the ports right at the end of the tank. It may be best to move the ports about 1/2 inch away from the weld area. Brazing wasn't a problem when I made the 2 inch and 1 inch. Some metal distortion was only a problem when welded.
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Unread postAuthor: Labtecpower » Fri Feb 04, 2011 12:52 pm

I think i'll have it welded by some professional or something.. I dont trust my own welds enough for pressure use.

I wasn't going to put the ports right at the end of the chamber. I want some space :D
The steel where the pipe is made of, is pretty thick. (4 mm) it won't distort very much.

The pipe has to be sanded completely before welding, as it is galvanized. I don't want to get sick of the zinc fumes :pukeleft:
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Unread postAuthor: Brian the brain » Fri Feb 04, 2011 3:36 pm

Does this irritate the mods?


Not at all.

As long as the double post is relevant and has something to add.
With a day between them, people might miss the information you've added if you had just edited the first post.

not all 17-year-olds make crappy cannons


I think i'll have it welded by some professional


:wink: [/quote]
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: Technician1002 » Fri Feb 04, 2011 3:53 pm

Just a quick note on the knob, do you have a way to keep it from pulling off when you fire the cannon? I used a metal pin through the knob and rod to secure mine.

If the knob works back on the rod, it becomes possible to push the piston too far forward which causes the possibility of launching the piston. :shock:
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Unread postAuthor: Labtecpower » Fri Feb 04, 2011 4:00 pm

The knob is compression-fitted. I have put it on with a sledgehammer.
I will keep paying attention.

The rod is 7.02 mm, and the hole I drilled in the knob, was 6.8 mm.
I think it is enough for a secure fit.
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