[Buffdaddy]

I had this idea, among many others, trying to make an entry to the recently announced NIC Homemades Creation Contest. Instead of the usual path of "work on something until it's awesome, then post results later", I thought, "Why not make the whole thing public?" While I'm unsure of the moderators' opinion on a thread of this type (they could simply say "MAKE YOUR OWN BLOG" and close it, or like it, I have no clue), I think that doing something in this fashion would be beneficial for all, as all can share insights as to how to build, what' doesn't work, what does work, and in the end have much more knowledge imparted to the NIC.

So without further ado, I'll outline my thinking and progress thus far. (First post coming in a few minutes)

[Buffdaddy]

First things first: For my homemade escapades, I decided to try my hand at piston valves. We already have plenty of springers, and while the "normal" airguns (a la an At2K tank) are rather lacking as well, we have things like the P-VAT. So this is relatively uncharted territory.

Piston valve tanks (also known as backpressure tanks) are the things powering the Hornet, Big Salvo, Sonic Bazooka, and SSPB, among others. The mechanism relies on pressure differentials. For easier explanations, I'll simply add the links below:

Spudfiles
Secret Strike Internals

You fill the tank, equalizingthe pressure throughout, with air essentially leaking past /around the piston to the rest of the tank. The initial pumping seals off the barrel. Once you've pressurized, you open a valve to release the air behind the piston. With the front of the piston suddenly under more force, the piston flies back, sealing off the leak, and letting the rest of the air go down the barrel.

There are several different methods of piston construction. Spudding (potato launching, for the uninitiated) often involves large piston valve setups. The massive amount of air that needs to be moved, however, means that you need a nice ball valve or some other method to quickly leak out the air - not ideal for Nerfing. The cmmon nerf designs look more like the following from the Autopsy thread:

Autopsy Thread

In these, there is often a rubber tip on the front of a plastic tube, which sits inside another tube ("I'm a dude, playing a dude, disguised as another dude!"). It operates on the same principle, but is meant to maximize the tank space used for launching the projectile.Once the tiny tubes get sealed off by the rubber piece, the rest of the air comes out the barrel. This is a reason why Big Salvo tanks are so awesome. In addition, you leak less air to start the process, which means you can use things like blast buttons. (Use the search function, there have already been some efforts in this area).

So that sums up what I'm trying to accomplish: a working homemade piston valve, with relatively cheap and easy construction. Whether it's possible or not, I don't know. Well, actually, I do, it should be possible. The real question is whether I can make a working design(s), or if someone else has the insight to do it. It's a totally open area for Nerf innovation, and we may end up with several different designs.

In any case, let's make this a learning experience, shall we?

[Buffdaddy]

A bit more mechanism talk, before getting into actual attempts (and my first two failed miserably, I assure you).

Piston valves require a free-moving piston, generally speaking. A few variations require o-rings for sealing things off, but usually you have a slightly-not airtight piston, as this allows leakage. You then rely on quick air dumping to drive the piston back before too much air leaks back around the piston, and nothing happens. Alternatively, you could make the piston air-tight, but place a check valve in the piston to allow one-way air movement. This of course means you'd better have excellent seal and lubrication. Bu we're talking about the NIC, don't we already do this to maximize the power of our springers?

Furthermore, there is a relationship between the size of the piston and the size of the barrel. Let's assume an overly large piston, and a small barrel you've sealed off. You start to fire the piston, and it does so easily (you have massive surface area driving the piston back). But there's the possibility of the pressure relieving out the barrel faster than the pilot area, and so the piston seals up again without all the air coming out.

Now shrink the ration of piston diameter to barrel diameter, approaching close to a 1:1 ratio. You start the process, and air leaks out the blast button or what have you. This process is slow and will leak a bit more air in the process to get the piston moving, but once it does, the piston should fly back the rest of the way and allow the remaining air to evacuate via the barrel.

There are a few ways to mitigate the weaknesses of different designs, but you should just remember the basics. You need a fast way to bleed air (in relation to tank size; remember the spud cannons need ball valves and all that), a good piston with slight but not serious leakage, and good seals on the barrel.

(Post will be edited if needed for info correction/addition)

[Just Some Bob]

I've been thinking along the same lines for a while. The biggest challenge to me has always been locating two readily available plastic tubes that slide freely and yet almost seal. Certainly one could use brass, but I choose not to. Another factor that catches many people up is that the outlet opening for the barrel must have a smaller area than the piston cross section. People who have opened up the outlet on backpressure tanks often learn that lesson the hard way - Hornet tanks in particular. If the areas are close, the piston will move sluggishly or not at all. To get it to really snap, there needs to be a significant difference, and a larger piston volume means a larger release of triggering air "wasted" out the back. But that's what it takes to get performance. The pistons in the Jobar/Cobra/Pango are about 3/4" while the blast outlets are more like 3/8" (double the diameter = four times the area).

Edit: I see you posted again while I was composing. No, you will never be able to get even close to a 1:1 ratio.Such a piston will never move. The relative vacuum impinging on the portion of it exposed inside the barrel will keep it closed. Also, many excellent spudguns have been built with pistons having an air-tight seal. The trick there is to put a checkvalve inside the piston as well.

This post has been edited by Just Some Bob: 21 May 2011 - 03:59 PM

[Buffdaddy]

I know, this has been a problem. If you look carefully, there are several fittings that fit inside other diameter tubes (1 1/4" PVC fittings in 2" pipe, or 3/4" fittings in 1 1/4" Pipe), but ever then, the difference is great enough that E-tape or some other method would be needed to get it to the right fit. As for the nerfy version, there are a few possibilities. Notably, 19/32" Brass goes into 1/2" PVC (usually), but it's close enough for air to not leak past. Filing a few channels on the inside of the PVC wall should solve that problem....but I have another sealing issue on that design.

There's multiple possibilities, but those are the ones I'm testing right now.

On a more humorous note, attempt one was, inf fact, just having the piston sit against the front wall. So essentially, barrel area = piston face and it didn't work. But the way I set it up led to a lot of other ideas in the process, so it was a good thing.

OBOB: I realize those things, but thanks for clarifying! There is a good balance you have to find, between how much air you bled, how fast the pilot flies closed, etc. It's just seeing what works And I did list the check valve possibility, I am going to try that on one design that failed earlier.

This post has been edited by Buffdaddy: 21 May 2011 - 04:16 PM

[andtheherois]

Very interesting idea. I can't wait to see a completed version. Since you're making this for the homemades thread I'm going to guess you're making this large enough to be a standalone blaster. What I'm wondering however, is that if your idea works, will you be able to scale them down to a small enough size so that we can integrate them into other blasters or at least the more popular blasters/shells with enough space?

[Just Some Bob]

Overall size? The backpressure tank designs I'm working on are based on 1.25" PVC as the exterior, and less than 7" long. In general, it should be easy to lengthen or shorten most homemade designs, just as it is with springers, but without any cutting of springs. I'm hoping for reasonable power from a 4" chamber, but results haven't been good yet. A lot depends on finding the same reducer bushings that I already have a few of, and that's been difficult. Newer ones all seem to have the "webbed" parts on the outside, cutting too much into chamber volume for the same overall length.

So yes, fitting one inside a NF shell is rather unlikely. But it's not impossible that, if I can get it working, it might be scaled down to 1" PVC. Unfortunately the reducer bushings tend to be solid, again removing chamber volume, so it might have to be 7" long to get any power. Won't know until I try.

This post has been edited by Just Some Bob: 21 May 2011 - 04:39 PM

[Langley]

Just Some Bob, on 21 May 2011 - 04:35 PM, said:

Won't know until I try.

I think that pretty much sums up the thread. I don't see the problem with a build log, but maybe you could hold off on posting until you have something built that you can log? What you're doing is cool and all, but right now it looks suspiciously like an idea thread.

[Buffdaddy]

Be assured, I'm posting the actual stuff...but in the morning. It's nearly midnight, here.

But for now, does this satisfy your curiosity?

This post has been edited by Buffdaddy: 21 May 2011 - 07:42 PM

[shardbearer]

Yeah, barrel face=sealing face does not work cause there is no air in front of it. Extend the barrel backwards a bit and it should work like a charm. And a 1:1 ratio does work, but needs mechanical actuation instead of pneumatic. This is probably better for nerf as no air is wasted as a pilot volume. For an example, search for the quick dump valve (usually called a QDV) on spudfiles, it uses a piston floating on a rod to improve opening time.

[Buffdaddy]

Apologies for not starting the build log part as soon as I could, but we'll rectify that.

I've been going along the spud cannon line of thinking for the past few days. I have two designs for it, with roughly the same chamber volume for firing the dart. I'll show the process on the second (and working) version.

Here's the pieces of the chamber, laid out for viewing pleasure:

1 1/4" PVC bushing, coupler, 3 inches of 1 1/4" PVC, another coupler, another bushing. The e-tape wrapped thing you see?

This is a 3/4" PVC endcap, wrapped in a bit of e-tape. A craft foam bumper gets placed on the front, for sealing off the barrel.

This is the rear bushing of the chamber, with a craft foam ring to seal off the piston when it moves back. I could only find 1 1/4" to 3/4" bushings, so you need another bushing to get down to the 1/2" PVC we all know and love.


For the front, we have 1/2" CPVC hammered into 1/2" PVC. The CPVC sticks about 2 3/4" into the chamber, and gets capped with a 1/2" to 3/4" bushing. Why? To have a nice flat surface for sealing with the piston.

For the rear of my blaster, to reduce space for testing, I have a 3/4" PVC cross with a dual-action bike pump shoved in the bottom, a 3/4" ball valve on the back, and a McMaster OPV on top. Hardly war worthy, but good for testing.

This shows the larger version of my piston, made with 2" PVC parts (it failed....). I'm using 2-part epoxy to seal it off; something I can use sparingly to get a decent seal, but get apart with a pair of channel lock pliers.

This rig is hardly war-worthy, but it at least proves the concept. Pumping to 50 psi, I'm getting about 70 feet with bb stefans. There's obviously more work to do.

[Just Some Bob]

here is my other design:


It relies on the builder being able to find a PVC check valve constructed such that you can attach a trigger rod:

my local OSH hardware stores have had about 50% of their stock be this style. I've also seen them at Ace.
I've been able to make it fire without modding any of the PVC parts shown, other than the plug on the far left (Unless glue counts, then the left two were "modded" but the rest were just threaded together, because I wanted to be able to return most of it if the concept failed). The plug just gets a hole through it, for the trigger rod, which attaches to the threaded end of the pin through the valve. An o-ring over the rod, and a PVC disc that compresses it a little, completed the back seal. The beige piece on the far right is a CPVC coupler, where I plugged in a pump with a CPVC end for pressurizing, then removed that (to then insert a barrel). Popped great in testing, even a little too powerful for my own preferences.

But this is not back-pressure, and I haven't worked on it at all for a year.

[Buffdaddy]

I'm currently revising the design, and going the normal, nerfy piston valve direction to boot.

On the old: It would make sense to invert the sliding cap. That way, you have a smaller area of air you have to suck out to actuate the piston, and you can get more power from the same size cylinder, or the same from a smaller one.

New discovery: 3/4" CPVC endcaps slide nicely on 1.2" PVC. You'll see it tomorrow.

[SgNerf]

Just Some Bob, on 22 May 2011 - 11:10 AM, said:

here is my other design:


It relies on the builder being able to find a PVC check valve constructed such that you can attach a trigger rod:

my local OSH hardware stores have had about 50% of their stock be this style. I've also seen them at Ace.
I've been able to make it fire without modding any of the PVC parts shown, other than the plug on the far left (Unless glue counts, then the left two were "modded" but the rest were just threaded together, because I wanted to be able to return most of it if the concept failed). The plug just gets a hole through it, for the trigger rod, which attaches to the threaded end of the pin through the valve. An o-ring over the rod, and a PVC disc that compresses it a little, completed the back seal. The beige piece on the far right is a CPVC coupler, where I plugged in a pump with a CPVC end for pressurizing, then removed that (to then insert a barrel). Popped great in testing, even a little too powerful for my own preferences.

But this is not back-pressure, and I haven't worked on it at all for a year.

Just Some Bob,

You'll be glad to know that using a check valve as an air-tank blaster trigger does work!

3DBBQ from Taiwan successfully created his JSPB V6 design based on such a concept, and it even includes a semi-auto firing feature (ie. pump up the air tank, then can fire 6-7 shots individually in sequence), and that was done by simply positioning a ball valve behind (only slightly opened) as a rudimentary adjustable air regulator to control the air flow from the main tank space to the secondary tank space.

This was 3DBBQ's V6 design (imho, the dude has amazing blaster style and colour schemes):



You can see the video if it operating here:



http://sites.google....oduce/jspb/2010

And his check valve trigger build guides (the photos should explain everything quite clearly, no need to translate):

http://sites.google....print/triggerT1

http://sites.google....print/triggerl1

He posted up that design back in end-2009 (i think he might have also posted something about it on NerfHaven too, not sure though)... when i saw it back then i immediately drove out to get the parts (we seem to have similiar types of pipes, joints and valves in SG too) and built a version of it to try out... and it works! It fires in semi-auto style too, just like in his video. Anyways, here was my replicated build:



Main difference is mine uses a tire valve instead, 'cos i preferred to have the option to connect it to a manual pump or portable air compressor.

BuffDaddy, sorry for "hijacking" your thread with a different air-tank blaster trigger setup, just sharing some info to add on to Just Some Bob's post.

This post has been edited by SgNerf: 23 May 2011 - 01:10 AM

[Buffdaddy]

SgNerf, on 23 May 2011 - 12:28 AM, said:

BuffDaddy, sorry for "hijacking" your thread with a different air-tank blaster trigger setup, just sharing some info to add on to Just Some Bob's post.

HOW DARE YOU!!!!!!

No matter, we're kinda just sharing our experiments here.