First Piston Question

[Dornep]

This will be my first pneumatic spud cannon, not counting a crappy ball valve cannon I built a few years ago.

This gun came about from a eureka moment I had earlier today. I have pic's of my piston below. The gun will be a coaxial barrel sealing piston design and I'm wondering if you have ever seen of a piston like mine and if it worked ok or not.

My gun has a 3" SCH80 chamber and 1.5" barrel. I'm using a 2" reducer bushing and coupling as a piston. The way the reducer bushing is made, the 1.5" barrel slides right into the groove in the bushing. I've filled this groove with RTV Silicone and I'm wondering if it will work to seal the barrel from the chamber. Also how far should my piston be allowed to travel for best performance?

[Hubb]

The piston may be a little heavy. The silicone will work, as long as it is completely smooth to allow for a complete seal. As far as piston travel, a lot of members here say that .25% of the barrel's ID is the way to go.

[Dornep]

A little heavy, as in it will tear the gun apart or too heavy to slide back and forth? I have a rubber bushing (the type they mount car bodies to the frame with) that I'm planning on using as a bumper.

Also, do you have any idea how long it will take half inch thick silicone to completely cure? The package says 24 hours but I think thats for a thin bead and not a deep cast.

[Hubb]

The piston is probably too heavy to move properly. It will probably still move, but it will move slow and will be less efficient than a lighter valve.

Wait the 24 hours on the silicone and check to see how dry it is. If you think you need to, wait some more.

[chartreusesnot]

why do you have to fill the piston at all. the way these work is that gas gets around them or through them right? you just need it to be tight enough that the initial pressure will seat it... or you could use a very big spring.

[clide]

A heavy piston will be a little more abusive on the gun, so make sure you have a good bumper, but it wont have a significant impact on performance. Although if you are concerned you could probably cut a few inches off the back of the piston.

[Dornep]

I'm having trouble understanding how the piston is going to stay in place if the air pressure in the chamber/pilot area is the same. If I set the barrel on the piston and blow in the barrel it lets air out around the sealing face, but if I put some weight on the barrel and blow it's an airtight seal.

When I fill the pilot area it should press the piston against the barrel and create a seal, but once the air pressure in the chamber equalizes to the pressure in pilot area what will keep the piston forced against the barrel?

I'm going to go to the local hardware store and pickup another 2" coupler and bushing and try to make a lighter better sealing piston just incase this one doesnt work.

[jackssmirkingrevenge]

I'm having trouble understanding how the piston is going to stay in place if the air pressure in the chamber/pilot area is the same.

The pressure might be the same but the area is different.

The force on the pilot side is over the full area of the piston, but on thechamber side you have to subtract the area of the barrel's inner diameter.

[Dornep]

Thanks! I never considered that the area was different. That cleared it up for me nicely.


Now a few more questions for the veterans.

If my piston is flush fitting inside the 3" pipe it wouldn't be a good idea to have fittings protruding into the pilot area would it? The piston would destroy the gun if it hit the fitting right?

I know it's not a good idea, but just how bad is it to tap threads into the end cap? If I was able to put my pilot area exhaust valve in the end cap it would solve a lot of problems with the protruding fittings issue.

[jackssmirkingrevenge]

The piston flies back with considerable force, since yours is on the heavy side even at low speeds it would carry substantial momentum. The only thing you want it hitting when it flies back (and remember, for a 1.5 inch barrel, it only has to move back 0.375 inches for maximum efficiency, anything more is a waste and allows the piston to accelerate more and put further strain on your launcher) is a bumper (spring/rubber etc.) that will absorb the impact.

I personally would have no problem with say threading a blowgun directly into the endcap provided you secured it with good epoxy.

[Dornep]

Thanks for the quick reply

That answer all the questions I have for now. Should have something to look at by tomorrow.

Thanks

[jackssmirkingrevenge]

I'm concerned that the blue cap on your piston is really going to hurt performance as it's going to dramatically reduce your flow into the barrel - isn't there some way you can plug that hole with something flush?

[Dornep]

I'm sure I could make it flush but I'm concerned about using other barrels with this piston since it won't seal unless the barrel is 1.5".

I bought a 2nd coupler and a 2" plug that will work the same way but I will have to find another way to seal it. But I will be able to use different sized barrels if I do it that way.

[jackssmirkingrevenge]

Ideally you want a flush piston with a disc of rubber bolted on over its whole sealing surface, that way you can use any size of barrel you want.

[PinHead]

As far as piston travel, a lot of members here say that .25% of the barrel's ID is the way to go.

I believe that you mean 25%, not .25%.

By the way, what's the history on this measurement, and who came up with it?