400psi piston vavle

[ALIHISGREAT]

well i have ditched the 'ultra valve' idea for obvious reasons in favour of a normal piston valve which i plan to make 'ultra' by building it well ( i hope)

so my new design will be in a 28mm-22mm copper reducing tee. the piston (red) will then be inside some 28mm copper which extends out the back of the tee which will then have a 28mm compression fitting with a 1" thread on the other end which will then lead to a qev or another piston (haven't decided yet ) then on the other side of the tee (the 22mm side) some a 20mm marble barrel (copper) will be stuck inside some 22mm pipe which will then go into the tee and the 20mm barrel will protrude into the tee so the piston can seal on it.

now i need soem help choosing piston material, i think either PTFE rod or UHMWPE rod from this page will be the best http://www.directplasticsonline.co.uk/ i think the ptfe is lighter but i'm not sure. also will a 25mm piston fit tight enough in 26.4mm id pipe?

edit: just checked prices and UHMWPE is far cheaper hmm...

oh and i already realise this is probably very similar to HEAL but i figured as there are no pics of the insides of HEAL i haven't copied although even if i had copying is complimentry

[jackssmirkingrevenge]

It won't work as it is, your effective piston diameter is smaller than your barrel diameter.

[MrCrowley]

In other words, your sealing face needs to be larger then the seat (barrel) for a piston to work, and it needs to be in contact with pressure, you need an area on the piston that has pressure pushing it back.

[ALIHISGREAT]

In other words, your sealing face needs to be larger then the seat (barrel) for a piston to work, and it needs to be in contact with pressure, you need an area on the piston that has pressure pushing it back.

yeah thats what i suspected but i wasn't sure, so i will bore out a comp fitting to allow the 22mm to slide through so it protrudes slightly fromt he end of the reducer but other than that it should be alright? i don't quite understand what you mean by saying i need an area on the piston with pressure on the back because the air will be on the back of the piston?

[MrCrowley]

You need area on the sealing face of the piston that isn't covered by the seat(barrel) so that there is air pressure pushing the piston back, that is what makes a piston push back when there is no pressure behind the piston pushing it forward.

It's all about equal and unequal forces.

[ALIHISGREAT]

You need area on the sealing face of the piston that isn't covered by the seat(barrel) so that there is air pressure pushing the piston back, that is what makes a piston push back when there is no pressure behind the piston pushing it forward.

It's all about equal and unequal forces.

ahh yeah got it, so how much piston travel do you think i would need for a 20mm barrel? would having the seat halfway back in the tee be a good place or would i need it further foreward as the piston cannon travel back into the tee it can only stay in the middle 'section' if you know what i mean

[jackssmirkingrevenge]

Let's do this mathematically:

For a 3/4" barrel at 400 psi, the pressure is acting on a "free" area of 0.44 square inches, with a force of 177 lbs. Looking at your diagram, it is this force pushing the piston to the right and keeping it sealed.

If you empty the pilot chamber, you removed the force of the air acting on the 10mm guide rod, and area of 0.12 square inches, meaning a force of 49 lbs. This means that the force keeping the piston closed is less, but still positive at 128lbs, meaning there's no force to open the piston.

[ALIHISGREAT]

Let's do this mathematically:

For a 3/4" barrel at 400 psi, the pressure is acting on a "free" area of 0.44 square inches, with a force of 177 lbs. Looking at your diagram, it is this force pushing the piston to the right and keeping it sealed.

If you empty the pilot chamber, you removed the force of the air acting on the 10mm guide rod, and area of 0.12 square inches, meaning a force of 49 lbs. This means that the force keeping the piston closed is less, but still positive at 128lbs, meaning there's no force to open the piston.

ahh yes i can see that being a slight problem

edit: so i can solve this problem by either making the 'guide rod' a big diameter like 22mm on a 22-25mm piston sealing on a 2omm seat?

[jackssmirkingrevenge]

In order to work it would have to look something like this:

edit:

so i can solve this problem by either making the 'guide rod' a big diameter like 22mm on a 22-25mm piston sealing on a 2omm seat?

As long as the guide rod's bigger than the seat (3/4" is about 19mm) you should be fine. Piston travel should be around 5-10mm max.

[ALIHISGREAT]

ok thats good so a 22mm piston would seal on a 20mm seat or is that cutting it a bit fine?

[jackssmirkingrevenge]

is that cutting it a bit fine?

Quite - you need a small pilot volume, tight fitting piston and good flow pilot valve for that setup to work reliably.

[Hotwired]

What comes around is likely to come around again some other time

[ALIHISGREAT]

i supose i will have a rethink whilst in barcelona because my design on't work the only other way i can see of doing it is to make a 'regular' barrel sealer i supose i could make it super light and low friction or something to get more impressive opening times or i could try and replicate what a qev does and have a rubber-ish piston-diaphgram instead of a conventional piston but i think this would be hard to do and it would be hard to get a piston that can take 400psi repeatedly

[Hotwired]

it would be hard to get a piston that can take 400psi repeatedly

Get a piston QEV. Unscrew it. Remove piston thing. Insert custom piston. Screw it together again. Use at 400psi.

[jackssmirkingrevenge]

i think this would be hard to do and it would be hard to get a piston that can take 400psi repeatedly

My epoxy pistons do that all the time, it's not such a big deal.