16 replies to this topic

[nerfturtle]

So, I was sitting in science today, I had just finished the lab, and I was playing around with one of the syringes we were using. I plugged the tip with my thumb, pulled back on the plunger, and let go. I was suprised to see that it flew back with amazing speed, and I started thinking about how that could be used on a nerf gun. This is what I came up with.

I know it's not so clear so I will clarify. There are two chambers. Both have plungers in them. The plungers are attatched at the back. The only difference between these are the fact that the front of one of them is sealed up. So, you pull back on both of them and they catch on a clothespin catch, or something like that. There is now a vaccum in the lower chamber, so when you release the catch both the plungers will fly forward. One of these plungers goes to a barrel, and launches a dart.
The one main problem I see with this is that it is hard to maintain a vaccum, you need a perfect seal. But otherwise I am eager to start this project, I just wanted to see what you guys think first.

[Prometheus]

Doesn't seem very feasible, unless you intend on using cheap aftermarket components. definitely not feasible as a pure homemade; using syringes or some other pre-fabricated device would be a necessity.

[SHADOW HUNTER ALPHA]

I've been testing this concept with syringes from the pet store (the plastic-tipped ones for feeding small pets), but I eventually decided that the concept wasn't really worth it. The plunger system will be double the height, which is not ideal for many homemades. The seal for the vacuum tube, like you mentioned, will also have to be perfectly formed and maintained, as well as the plunger seal itself. This is especially true of guns to be used in wars, because there is generally a long period between cocking and firing during stand-offs and maneuvering along which air might leak into the vacuum tube and negate any potential for a snap-back motion.

The way I saw it was that I could have double the size of a normal plunger tube for a risk in reliablility. It's a great novelty, and if you can make and use one, more power to you. However, it doesn't present a very practical approach to hurling foam as of now.

[Prometheus]

I'm gonna further explore this, I'll be using a PETG barrel, some big syringes (I know some H & K students in the human sciences program), and the body will be made from 3-D printed ABS. The only problem is fire control. Any ideas anyone?

[Quilan Fett]

Clothespin trigger.

[nerfturtle]

Wow, I've always wanted to use a 3D printer, they look so cool. Does your school have one? What do you mean by fire control? If you mean a catch then I see no reason that a clothespin trigger would not work. I was also thinking about the issue of having the vaccum decrease over time, and came up with a solution. You could have whatever was sealing the vaccum chamber be a chack valve, so if air leaked in, you could just push it out via the check valve.

Edited by nerfturtle, 22 October 2007 - 09:49 PM.

[SHADOW HUNTER ALPHA]

Ratcheting Catch System I:

I) Blue: Plunger tube and barrel assembly

II-III) Brown: Plunger and Vacuum Rod respectively

IV) Green: Vacuum Tube

V) Light Blue: Catch notches

VI) Red and Gray: Catch and trigger bar respectively

Step I: The upper of the two colored diagrams. The plunger and vacuum tube are pulled back, and the ratcheting catch has moved up into the notches to hold the assembly in the cocked position.

Step II: The lower of the two colored diagrams. The catch has been pulled down as the trigger is pulled back. As the catch pivots with the backwards motion of the trigger bar, the sear (upper part of the catch that engages the notches) is pulled free and the plunger and vacuum rods rush forward.

[six-five-two]

I still like the clothespin trigger idea better.

[Prometheus]

Wow, I've always wanted to use a 3D printer, they look so cool. Does your school have one? What do you mean by fire control? If you mean a catch then I see no reason that a clothespin trigger would not work. I was also thinking about the issue of having the vaccum decrease over time, and came up with a solution. You could have whatever was sealing the vaccum chamber be a chack valve, so if air leaked in, you could just push it out via the check valve.

We have a 3-D ABS printer, it uses the stereolithograph file from a CAD design the part.

I'm using 3 syringes, one as the plunger and 2 to propel the plunger syringe. I just need to do some calculations on volume when I get them, but the two syringes combined will have a larger volume than the third one.

[CaptainSlug]

A vacuum is hard to maintain and requires a very tight seal. Tight seals create a lot of friction unless you use the right materials. And even then if you keep a vacuum you would only have 15psi of actuation to work with. Frankly it's NOT going to outperform a spring.

[umdlancer]

Turtle, you know that using a syringe as a classical plunger tube assembly would work out much more effectively than using two syringes to form a vaccum-driven mechanism, right?

[Prometheus]

A vacuum is hard to maintain and requires a very tight seal. Tight seals create a lot of friction unless you use the right materials. And even then if you keep a vacuum you would only have 15psi of actuation to work with. Frankly it's NOT going to outperform a spring.

i got thinking, and this is the death knell of the project. If I permanently seal the end of the syringes, what happens when I break the seal, and that end is still closed? I now have lost my vacuum, and it wont work anymore.

[Maeric]

Yes.... But you could make a spacer at the bottom and de-cocked the pressure in the vacuume could be at a -20 or so already... that might work.... But other than that it is completely unfeasible. Just use a spring.

P.S.: Sorry if I made some grammar mistakes... I'm not feeling quite well today...

[Prometheus]

Yes.... But you could make a spacer at the bottom and de-cocked the pressure in the vacuume could be at a -20 or so already... that might work.... But other than that it is completely unfeasible. Just use a spring.

P.S.: Sorry if I made some grammar mistakes... I'm not feeling quite well today...

I meant if the syringe plunger ever got pulled out of the tube, then no vacuum at all, unless you fabricate some type of bleed system when you reinsert them.

[nerfturtle]

A check valve. That's what I said before, but it didn't seem too clear.

[Prometheus]

A check valve. That's what I said before, but it didn't seem too clear.

Still a pain in the ass.

[privateer]

Stay away from crack. Have you considered a gas spring (the opposite of a vacuum)? $10 from Mcmaster.

Some of the newer Webley and Beeman airguns use gas springs, but they call it a "gas ram" because it sounds cooler. I'm gonna try to make a snap out of one; it should get here tomorrow.

A gas spring would be the opposite of using a vacuum as a spring, I suppose...

Edited by privateer, 25 October 2007 - 03:07 PM.