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Step 7Understanding Pistons
I will now try and teach you the principles behind pistons, how they work and why.
A piston is a cylindrical object that moves inside a piece of pipe. It has a sealing face, which seals against the barrel and a bumper which softens the impact on the cannon fittings when it is fired backwards.
A piston needs to have a tight fit with the pipe it slides in, otherwise it wont work to it's full potential.
In a pneumatic cannon, air is filled in from behind the piston, because the piston is a tight fit the air pushes the piston forward and it seals against the 'seat' which leads to the barrel. Because the seat is sealed, no air can go through it and into the barrel. The air now leaks around the piston and fills up the chamber to a desirec pressure.
The air has more force pushing on the back of the piston, keeping it sealed, then on the front because of the area differential. The air can only act on what area is exposed and since the seat takes up some of this area, there is less force acting on it pushing it back.
Once you remove the air behind the piston by exhausting the pilot volume (area behind piston) with the sprinkler valve (which lets the pilot volume out into the atmosphere) the area acting on the back of the piston pushing it forward, is gone. Now there is only a force acting on the front, pushing the piston back. The piston is sent back at incredible speeds in a split second into the 'bumper' which reduces the impact. Now the seat is open and the air pressure from the chamber flows into it and out into the barrel launching your projectile. This happens in a split second and is usually quite loud.
This is an animation of what happens, I made the same one with 2 different speeds so you can see what happens every frame.
Okay it seems that instructables is speeding up the animations, so to see slower ones just go here instead:
http://i75.photobucket.com/albums/i290/MrCrowley45/Piston5fps.gif
http://i75.photobucket.com/albums/i290/MrCrowley45/Piston2fps.gif
A piston is a cylindrical object that moves inside a piece of pipe. It has a sealing face, which seals against the barrel and a bumper which softens the impact on the cannon fittings when it is fired backwards.
A piston needs to have a tight fit with the pipe it slides in, otherwise it wont work to it's full potential.
In a pneumatic cannon, air is filled in from behind the piston, because the piston is a tight fit the air pushes the piston forward and it seals against the 'seat' which leads to the barrel. Because the seat is sealed, no air can go through it and into the barrel. The air now leaks around the piston and fills up the chamber to a desirec pressure.
The air has more force pushing on the back of the piston, keeping it sealed, then on the front because of the area differential. The air can only act on what area is exposed and since the seat takes up some of this area, there is less force acting on it pushing it back.
Once you remove the air behind the piston by exhausting the pilot volume (area behind piston) with the sprinkler valve (which lets the pilot volume out into the atmosphere) the area acting on the back of the piston pushing it forward, is gone. Now there is only a force acting on the front, pushing the piston back. The piston is sent back at incredible speeds in a split second into the 'bumper' which reduces the impact. Now the seat is open and the air pressure from the chamber flows into it and out into the barrel launching your projectile. This happens in a split second and is usually quite loud.
This is an animation of what happens, I made the same one with 2 different speeds so you can see what happens every frame.
Okay it seems that instructables is speeding up the animations, so to see slower ones just go here instead:
http://i75.photobucket.com/albums/i290/MrCrowley45/Piston5fps.gif
http://i75.photobucket.com/albums/i290/MrCrowley45/Piston2fps.gif
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Feb 11, 2010. 6:36 PMknex luver123
says:
i had never thought of this method, my friends have made the easy ball valve cannons, and i liked them alot, and wanted to build one, but never took the time, boy will i show them when i make this.
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Feb 11, 2010. 7:07 PM
MrCrowley (author)
says:
MrCrowley (author)
says:
you will really have to take your time with this, not only are they expensive, they are difficult to build but easy to screw up. Many things can make the piston not work properly, so you have to do your research and make sure you know what you're doing. Check out www.spudfiles.com.
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