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QEV question

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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QEV question

Unread postAuthor: sharpshooter11000 » Thu Mar 08, 2012 2:09 pm

So I ordered a QEV for piloting the piston on my co-axial build but I've come up against some problems.
I won't be able to fill through the pilot port as the piston seals both barrel port and chamber port when closed. Is that normal?
Also the site said the threads were 1/2" BSP but measured with a ruler the ports are 3/4" wide.
Any help is appreciated :wink:
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Re: QEV question

Unread postAuthor: warhead052 » Thu Mar 08, 2012 3:31 pm

sharpshooter11000 wrote:Also the site said the threads were 1/2" BSP but measured with a ruler the ports are 3/4" wide.
Any help is appreciated :wink:


That is normal, try hooking it up to a 1/2" fitting. If that doesn't work, then try a 3/4" fitting.
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Unread postAuthor: sharpshooter11000 » Thu Mar 08, 2012 4:31 pm

Thanks, I read up on BSP fittings and found that if you measure the port and take away 1/4" then thats the size in BSP. A bit strange I think :roll:
But is the piston inside the QEV supposed to seal the barrel port and chamber from the pilot side? It would make it impossible to fill from the pilot side. Should I drill an EQ hole in the piston?
Sorry for so many questions, it's my first QEV!
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Unread postAuthor: warhead052 » Thu Mar 08, 2012 6:55 pm

That's kinda the way it is for NPT as well. I would just poke a small hole in the qev piston for a equalization hole instead of drilling, that way you don't mess up the rubber.
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Unread postAuthor: Lockednloaded » Thu Mar 08, 2012 7:31 pm

I think I probably have the same one as you, it will fit NPT fittings just fine, and the piston has a cup seal so that air can enter the chamber unmodifyed
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Re: QEV question

Unread postAuthor: jackssmirkingrevenge » Thu Mar 08, 2012 10:51 pm

sharpshooter11000 wrote:Also the site said the threads were 1/2" BSP but measured with a ruler the ports are 3/4" wide.


1/2" BSP refers to the inner diameter of the pipe the fittings are made for (British Standard Pipe ;P) not the thread itself.

it will fit NPT fittings just fine


...

some engineering website wrote:While NPT threads are common in the United States, BSP threads are widely used in many other countries.

WARNING: Never, never try to mate a BSP fitting with an NPT or NPS fitting if the pressure holding capability is at all critical.

NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).
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Re: QEV question

Unread postAuthor: Lockednloaded » Thu Mar 08, 2012 11:26 pm

jackssmirkingrevenge wrote:
sharpshooter11000 wrote:Also the site said the threads were 1/2" BSP but measured with a ruler the ports are 3/4" wide.


1/2" BSP refers to the inner diameter of the pipe the fittings are made for (British Standard Pipe ;P) not the thread itself.

it will fit NPT fittings just fine


...

some engineering website wrote:While NPT threads are common in the United States, BSP threads are widely used in many other countries.

WARNING: Never, never try to mate a BSP fitting with an NPT or NPS fitting if the pressure holding capability is at all critical.

NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).


I suppose it's not the safest thing ever, but many members have done the same thing and assured me it'd be alright. Also, before taking anyone's word when my own well being is on the line, I pressure tested the connection to 2000psi (without the piston in) and it didn't even leak. I will only be charging mine to 700psi on a regular basis, so I think it's pretty safe at the moment
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Re: QEV question

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Unread postAuthor: sharpshooter11000 » Fri Mar 09, 2012 2:38 am

warhead052 wrote: I would just poke a small hole in the qev piston for a equalization hole instead of drilling, that way you don't mess up the rubber.


Mine doesn't have a sealing face or anything, it just seems to be hollow plastic (a bit like the hollow plastic balls used in children's ball pits) It also seems to look less like a normal QEV but more like a home made piston valve inside.
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Re: QEV question

Unread postAuthor: jackssmirkingrevenge » Fri Mar 09, 2012 2:46 am

Lockednloaded wrote:I suppose it's not the safest thing ever, but many members have done the same thing and assured me it'd be alright. Also, before taking anyone's word when my own well being is on the line, I pressure tested the connection to 2000psi (without the piston in) and it didn't even leak. I will only be charging mine to 700psi on a regular basis, so I think it's pretty safe at the moment


I'm not saying I haven't smooshed dissimilar threads together either, but it's definitely not recommended ;)

It also seems to look less like a normal QEV but more like a home made piston valve inside.


Let's see a pic before you go poking holes in it.
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Unread postAuthor: sharpshooter11000 » Fri Mar 09, 2012 2:56 am

I'll get one after school today.
I suppose if it goes wrong I could machine a delrin/nylon one which I would feel safer about using at higher pressures.
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 09, 2012 2:59 am

Go ahead and machine a simple synthetic sabot then, make it a close fit and you won't have to worry about equalisation holes.

Worked a treat for mine.

Image
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