6 replies to this topic

[KaneTheMediocre]

So, I recently decided that as much as I hate lube, skirt seals are awesome, and I'm not very close to figuring out how to make one that doesn't use lube. So i bit the bullet and tried one out. Of course, it sealed awesome. However, it sealed awesome in BOTH directions, which was a problem for me. Sometimes, especially if darts run a bit oversize, hoppered blasters will be difficult to draw because they create a vacuum in front of the plunger, as air has no path to enter into the plunger tube. This isn't usually a big problem, as enough air can get in around the dart to fill the plunger tube. But sometimes, vacuums get made, and darts can get sucked back farther than we'd like them to. Furthermore, if there is a partial vacuum inside the plunger tube prior to firing, the amount of air fired by the power system can be drastically reduced. This problem was dramatically worse for hoppered HAMPs, which is why I made an integrated check valve for the plunger heads
http://nerfhaven.com...showtopic=20030

Skirt seals, until now, have not had this feature, and I really wished that I had it when I was playing with my PAC shooting marshmallows, and on occasion sucking gooey blobs into the plunger tube. Fortunately, they were surprisingly easy to get out (probably thanks to the lube), but I didn't feel like I could consider a blaster to be complete if it had problems associated with plunger vacuum, even if they were only an issue on occasion. I found the solution on the head of the pump for the Buzzbee Big Blast. The check-skirt is based on that design, but with a skirt instead of an O-ring.

Diagram:



Prototype pics:





How to build it: Seriously, look at the diagram. And look at what I did-- I used a saw, drill, and hammer to make this, although later versions will probably be milled for a nicer finish (The PVC is actually cracked in one spot from where I tried to use an oversize drill as a countersink, and I had to remove a piece of the CPVC because it bent inwards during assembly).

Why it's good to have:
No misfires due to vacuum in the plunger tube
No darts or debris gets sucked into the plunger tube
Slightly easier draw

Why it's not necessary to have:
Air can usually get around the dart prior to firing the hopper
A nail or screw will prevent darts from sucking back into the plunger tube (usually)
Misfires due to vacuum in the plunger tube are rare under normal circumstances.

Edited by KaneTheMediocre, 19 November 2010 - 07:05 PM.

[Ice Nine]

Ugh. Just, ugh. Could you be any worse, Kane?

I don't think you could.

21:13 kanethemediocre TELL THE WORLD HOW I HAVE NO FRIENDS AND MASTURBATE WITH SILICONE GREASE

Edited by Ice Nine, 14 November 2010 - 10:14 PM.

[LotusNerf]

This is a very clever way to eliminate vacuum loading. Nice job!

Does the skirt seal slide along the pvc smoothly?

[KaneTheMediocre]

This is a very clever way to eliminate vacuum loading. Nice job!

Does the skirt seal slide along the pvc smoothly?

Not as smoothly as I'd like, there's a bit of stiction when it's all lubed up. When drawn against vacuum, there is a tiny initial pop before the skirt seal moves.

[Y-Brik]

Interesting concept, though in need of minimization (I'm sure you're already working on it). Honestly, I haven't had a ton of problems with this on my +bow (choppered)- I have a CPVC dart guide to avoid vac-jamming, and if I do feel a vacuum in the PT I just open the ball valve a hair to equalize pressure.

[KaneTheMediocre]

So, I made another check-skirt, and this one fixes most (perhaps all?) of the problems with the first one.

The changes are:

1). The plunger rod is pushed all the way through to the end of the plunger head. This strengthens the whole assemblage, and fills volume that would other wise be dead space. I pushed the rod through first, then milled out the slots, but if you need to use a scroll saw you can just saw the slots first, then push the rod through. That said, pushing 1/2" nylon rod through 1/2" CPVC is absolutely ruinous, so I recommend reaming the CPVC out before you start this process--Especially if you're doing the scroll saw method. Remember, the cut up end of the PVC/CPVC is the part that you need to push on / hammer in order to get it to face the right way on the plunger rod, so you want it to be as easy as possible.

2) The skirt travel has been reduced to a hair more than the height of the skirt. So, instead of almost an inch of the draw being used to actuate the skirt, it's more like 1/4". And the overall length is reduced.

3) I didn't use excessive amounts of silicone grease, and I was careful to put is only on the outer rim of the skirt and the plunger tube, NOT between the PVC pipe and the skirt. I suspect that lube getting between the PVC pipe and the skirt would be OK if I don't use way too much like I did the first time, but it's not needed, so don't put any on there.

4) I used a set screw to retain the skirt, instead of a cotter pin. More people keep set screws around than cotter pins, and I like that it doesn't wobble about. I do wish I didn't have to tap it. Normally, I just put screws through plastic without tapping, but this thickness combined with the fury of nylon prevented that from working.

So, now it's more efficient, and the check valve actuates every time the plunger is drawn and fired, rather than only when it encountered a substantial vacuum. See the diagrams!

[Ice Nine]

This is bad and you should feel bad.