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QDV semi auto idea

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QDV semi auto idea

Unread postAuthor: thebestnoobcake » Tue Feb 02, 2010 10:27 am

This is just a random idea that popped up into my head. I decided to stick it up here for criticism/approval before I go to sleep.

It is a really crappy drawing and thus I apologise in advance.

It is just a normal qdv but with a spring!
When the valve is closed, the trigger (orange) holds the valve in place against the pull of the extended spring (aqua). I have never used this kind of valve, but according to Mr. Technician's video on youtube, the compressed air gives the piston a bit of a push. The piston would compress the spring, which, having more potential energy (from the air) can return the spool back to its original position. The valve should close soon after the extra air is vented from the barrel.

I stuck an extra spring on the drawing for extra dampening purposes and the end could have a vent. The vent would act as a dashpot depending on the size of the hole.

If there are any glaring technical problems (or this has been thought of and laughed to death elsewhere), please tell me.
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Unread postAuthor: kjjohn » Tue Feb 02, 2010 12:25 pm

This is exactly like the piston design for the hybrid I'm building. It could work, but for pneumatics, I would say it is inferior to regular piston valves, which can be made semi-auto with a simple 3-way valve and a blow-forward bolt. This design would not completely release the air in the chamber as a regular valve would.
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Unread postAuthor: MrCrowley » Tue Feb 02, 2010 12:35 pm

It's actually more a Chamber Sealing Piston Valve than a QDV...
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Unread postAuthor: thebestnoobcake » Tue Feb 02, 2010 12:42 pm

Isn't that what a QDV is? It seals the chamber.
I thought that piston valves generally have a sealing face and have a pressure differential across them which is utilised to open them.
QDV is a spool valve which, when closed has no forces (kinda) on it.

is that about right?

@kjjohn: Does all of the air need to be exhausted? If it does, then the spring could be swapped out for one with a lower k value. I was thinking that the last bit of air doesn't really do much and it would make subsequent repressurisation a tad quicker (I only have a bike pump lol)
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Unread postAuthor: POLAND_SPUD » Tue Feb 02, 2010 12:56 pm

well the QDV is in fact a chamebr sealing piston (and the barrel sealing piston at the same time) :D
but yeah I agree that it's more like a spool valve

I've been thinking about something similar myself...
if you replace the 'trigger valve' there with a 3 way valve you could build a semi (If you don't know how just ask)

but if you find the way to cut off air supply to the chamber when the piston if open you could achieve full auto with it
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Unread postAuthor: thebestnoobcake » Tue Feb 02, 2010 1:00 pm

@POLAND_SPUD: You could stick another spool on the rod which would block off the air supply. cos aren't spool valve usually used for this kind of thing?
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Unread postAuthor: kjjohn » Tue Feb 02, 2010 1:01 pm

A QDV is a piston valve, it is just a more efficiently designed one. Normal QDVs have a sealing face and a pressure differential, and so do piston valves. Before a piston valve or QDV is triggered (pilot volume dumped), there are no differential forces on it (meaning that the pressure on either side of it is the same).

In terms of air being exhausted, no, not all the air has to be exhausted, but you don't want to have more than a few psi left in the chamber after each shot. Working with springs and pistons can be tricky, if you want the piston to open at a specific pressure, you have to look at surface area of the piston, strength of the spring, rate of compression of the spring, etc. Personally, I would just go with a normal piston valve or QDV. It would open quicker and be far more efficient.

edit: poland_spud beat me to it! Like he described, I would scrap the spring and use a 3-way valve.
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Unread postAuthor: MrCrowley » Tue Feb 02, 2010 1:04 pm

Just to clarify, I meant it was more a typical chamber sealing valve than a QDV, because if we are talkign about Tech's QDV's, they're as POLAND_SPUD described them. I guess this is more like a hybrid of the two.
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Re: QDV semi auto idea

Unread postAuthor: velocity3x » Tue Feb 02, 2010 1:29 pm

thebestnoobcake wrote:When the valve is closed, the trigger (orange) holds the valve in place against the pull of the extended spring (aqua). I have never used this kind of valve, but according to Mr. Technician's video on youtube, the compressed air gives the piston a bit of a push. The piston would compress the spring, which, having more potential energy (from the air) can return the spool back to its original position. The valve should close soon after the extra air is vented from the barrel.


I'm confused by your description. In one sentence you say "pull of the extended spring" . In another sentence you say "The piston would compress the spring". Exactly what type of spring do you plan to use.....compression or extension?
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Unread postAuthor: POLAND_SPUD » Tue Feb 02, 2010 1:44 pm

You could stick another spool on the rod which would block off the air supply. cos aren't spool valve usually used for this kind of thing
yeah you could... the biggest advantage of that is that you can combine both the main valve and the 3 way valve (which allows you to achieve semi automatic fire) into one valve and can control it with a simple blowgun

That could be useful for large bore semi automatic launchers. However, large bore semi auto launchers are not very practical because of their size, air consumption etc. and you might find that it is actually easier to build a large bore semi auto gun with a normal piston valve and a 3 way valve


It's not that QDVs or modifications of QDVs can't be useful...
I've been trying to come up with a design that could be easily and cheaply machined from a block of aluminium, more less using the same technique as here

That machined block is used for loading ammo and joining all the main parts in this but as you can see it uses a QEV valve

obviously it seems like a good idea to combine both the main valve and the loading mechanism into one block of aluminium with just a couple of ports for the barrel, chamebr and the triggering valve...

you could then mount the aluminium block stock on a gun frame that looks like an M4

@velocity
I think there are two springs there... one of them pulls the piston to open and the other one is used as a bumper and also serves to reset the piston
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Unread postAuthor: thebestnoobcake » Tue Feb 02, 2010 2:21 pm

@veolcity: Can a spring not work in compression and extension?

In the ready to fire position, the spring will be in tension, enough to pull the piston/valve/spool enough to allow the chamber to vent into the barrel. The air will accelerate both the projectile and the piston. The piston will compress the spring further, storing enough (hopefully) energy to return the piston to its original position.

I like POLAND_SPUD's idea because I was thinking that having the sear projecting into the back of the barrel was not such a good idea.

@kjjohn: the QDV's sealing face is not really a "face".. or is it ??
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Unread postAuthor: kjjohn » Tue Feb 02, 2010 2:33 pm

What velocity was asking is what type of spring you are using: is it an extension spring (the kind you pull apart), or a compression spring (the kind you push together)?

If you are intent on using a spring, I would go with POLAND_SPUD's idea, but it would be cheaper and simpler to just do a normal piston or QDV with a 3-way valve.

A QDV has a sealing face, it is just a little different from a normal piston. On a regular piston, the face of the piston itself is the sealing face. On a QDV, the sealing face is a smaller piece of rubber or other material that is extended outwards from the main piston, and which seals the barrel (hence the name sealing face). If you have not done so already, look up Quick Dump Valve in the Spudfiles wiki, it will give you a more detailed explanation and a diagram of how a QDV works.

edit: Here is the link to the wiki page: http://www.spudfiles.com/spud_wiki/index.php/Quick_Dump_Valve
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Unread postAuthor: POLAND_SPUD » Tue Feb 02, 2010 4:54 pm

QDV's sealing face is not really a "face".. or is it ??

that is extended outwards from the main piston, and which seals the barrel

if I get it right you described this
Image

but it think he meant tech's QDV,
Image which uses two orings...

maybe it wasn't the best idea to describe both of those valves in one article as it makes it kind of confusing

@thebestnoobcake
I don't think there is any reason for you to try my design...
I guess it could work and I know it sound kind of stupid when I suggest that it might not be the best idea to try it out but let me explain...

I posted it ages ago in one of technician's threads on his QDV launchers... it wouldn't be difficult for him to mod one of his gun in this way

of course you could to copy his design but I don't think it's as simple to build as he claims it to be as it requires you to weld a couple of parts
[youtube]http://www.youtube.com/watch?v=No3nI07mlQo[/youtube]
you could modify his design slightly so that it will look more like a traditional coaxial piston gun


but the point is that it would be simpler and cheaper to buy a QEV, 3 way valve and an air cylinder... that's what I used on all versions of my semi auto

sure you'd' have to buy a couple of parts but at least it works well and is really simple to build... [youtube]http://www.youtube.com/watch?v=Azyx72whFiE&feature=player_embedded[/youtube]

you could convert that from semiauto to fullauto with the use of a solenoid valve & timer or with a pneumatic timing circuit

anyway building a semi or full auto gun is real PITA unless you use spherical and consistent in size ammo such as steel ball bearings... I used marbles on previous versions of that gun and I had a lot of problems with them just because some of them were oversized and caused jams...

also keep in mind that semi/fullauto guns consume large amounts of ammo and air - so don't assume that 200 BBs would last you one week

to sum it up, before you start building a semi/full auto you just have to realize that it will cost a couple of times more than a single shot gun and that it might be a frustrating experience
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Unread postAuthor: thebestnoobcake » Thu Feb 04, 2010 4:48 am

My original idea was to have the performance of the QDV with the added bonus of having semi automatic operation.
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Unread postAuthor: tghhs » Thu Feb 04, 2010 5:35 am

Really the QDV in the wiki, (not techs) will have the same amount of force applied as a regular piston valve. I don't Think it should be there.

The person who did it was very foolish and neglected the force on the 'sealing' face that would cancel out the force (in the incorrect theory) of the extra surface area on the main 'piston'.

This basically means that newbies will build weaker less reliable valves.

Techs valve on the other hand, will act completely differently and is the true QDV.
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