Newbie with a question

[bigsi]

Let me share an alternative with you. This arrangement is known by several names, including a 'cannon valve' as they are often used in Potato Cannons!


It's made from two concentric tubes. The outer forms the reservoir and the inner a barrel.

To charge, you push air through the hole shown. The pressure moves a sliding piston to the right. The piston has a thin diaphragm (thick rubber works well) bolted to the right hand face. This forms a seal with the end of the barrel but can flex under pressure allowing air into the reservoir.


Once it's charged, you release the pressure in the space to the left of the sliding piston (3 port pneumatic valve with exhaust). The pressure on the reservoir side of the piston is higher than on the charge side - so the piston is shot to the left.


Once the piston reaches the end of it's travel, the diaphragm seals - and the only place the air can go is into the barrel pushing the plunger (or potato!) out.


This gives you a valve with the same port size as the bore of the barrel which opens incredibly fast.

I've used this to fire a potato over 500m at 8 Bar pressure - but I've also used it in the configuration above to move a load attached to the piston rod very fast (known as a 'Jerk Rig').

They are very easy to make, even out of drain pipe (so long as the pressure is not too high). You need to calculate the volume of the reservoir to be about 25% greater than the volume of the barrel for the best power / size ratio.

Also, the sliding piston only needs to move 15mm or so - I drew it bigger to make the diagrams clearer.

At school (40 years ago!) I made one out of a Bic Biro as the barrel and a piece of plastic tube as the reservoir. I used a syringe and two non return valves to pump it up. It fired bits of tissue paper with enough speed to really hurt over the width of a classroom! It nearly ended my time at school prematurely!!! - don't try this at home (school) kids!

Si