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The Ported Piston plunger head

Premier O-ring plunger head!
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#1 T da B

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Posted 01 April 2014 - 01:56 AM

Hello lady and gentlemen,

After finally getting around to thoroughly testing this prototype, I'm very excited to present you all with some dope new technology that you may not have seen before! The idea is not mine, but I'm "porting" it over from our sister sport--Airsoft! First of all, let's quickly go over the advantages and disadvantages of regular old O-ring plunger heads:

Advantages:

  • Incredibly cheap
  • Take up very little real estate
  • Easy to maintain
  • Long-term reliability
  • Low friction

Disadvantages:

  • Sealed breeches don't allow air into the plunger (vacuum loading)
  • More machining required
  • Tight tolerances to get a working plunger head

The Ported Piston, or PP is a modified O-ring plunger head that eliminates the first disadvantage off the list. Here is a picture to illustrate:
pp_zpsffebf996.jpg
Of course, you have to imagine a dart plugging up the right side of each plunger to understand the real benefits!

Writeup:
For this write-up, I'm building a plunger head for 1 1/2'' OD polycarbonate tubing out of 1/2'' HDPE (cutting board). Start by cutting out a disc of HDPE with a diameter about equal to the ID of your tubing. Mark the center and draw 3 lines through the middle (or more if you want more than 6 ports). Try to keep the lines equidistant from each other.
IMG_2396_zpsf212013b.jpg

Drill a 1/8'' hole through the center and tighten on a 1 1/2'' 6-32 bolt. Chuck in it in the drill press (if you don't have the luxury of a lathe) and file/Dremel it down until it slides into your tubing nicely:
IMG_2370_zpsf073b38b.jpg

Now use a rough file and a small square one to get a nice hamburger shape. Make sure the track is wide enough to allow the O-ring to slide up and down a little. Test the fit in your tubing constantly!
IMG_2397_zps615fd322.jpg

Now mark the holes on top--mine were 1/4'' from the edge. Mark the holes around the perimeter that will meet up with the ports on top--they should be lined up with the O-ring when it is slid downwards all the way:
IMG_2400_zps3a69552e.jpg

Now drill the holes in the top to a depth of around 11/32'' with a 1/16'' drill bit:
IMG_2402_zpsc4c95966.jpg

Now drill the holes along the perimeter to meet the previous ones. I centered the disc as best as I could in my small vise and used my drill press. I literally looked in through the upper hole until I saw the drill bit appear at the end of the small tunnel.
IMG_2407_zps4faee451.jpg

To see if your ports are functioning properly, blow through the hole on top of the disc. Clear out any swarf with a paper clip--it is imperative that the passages be clear of debris!
IMG_2415_zpsf2bdc43f.jpg

IMG_2414_zps01f6f2f6.jpg

Here you can see what the plunger head looks like when you prime the blaster--the O-ring slides upwards, exposing the ports, then fresh air can flow from behind the plunger up through the ports and into the plunger tube for the next shot. The O-ring is a 1 5/16'' OD O-ring from Ace.
IMG_2436_zps66c1f9e2.jpg

And when you fire, the O-ring slides backwards, covering the ports. As an added bonus, air rushing in through the top ports pushes the O-ring outwards, improving the seal!
IMG_2439_zps9a1758f6.jpg

Slapped on a plunger rod:
IMG_2422_zps00f8129d.jpg

And into my Ported Piston Plusbow (PPP):
IMG_2441_zps647a5993.jpg

Final Thoughts:
After hundreds of shots worth of testing with various barrels, I'm getting the same performance out of my PPP as my skirt seal RainbowPump! Even with sealed breeches and rapid firing, there is no drop in velocity. Though this plunger head design required a lot of effort, the result is well worth it.
IMG_2458_zpsbbe545fd.jpg

This was the max fps--the average was 260-270. It was achieved with a 12'' slide breech and 1.25'' #6 slugs. I hope you guys found this as awesome and intriguing as I did. No go forth and make yourselves a PP!
~T


Edited by Aeromech, 23 November 2015 - 03:10 AM.

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#2 Buffdaddy

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Posted 01 April 2014 - 06:05 AM

Um...Nerf blasters already use this. Seriously, look at a Nitefinder plunger head, or any plunger head in general. They're already designed in this fashion to allow air to enter the plunger tube, then seal off upon firing.

The issue isn't really about this being a new technology. It's about people not using it - using the wrong size o-rings, wrapping the plunger head with teflon tape, and just going for 100% seal in general without considering the effects.
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#3 shmmee

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Posted 01 April 2014 - 08:47 AM

I like it! Yes nerf already uses this on some of their blasters, but anyone who forgets to put a dart stop in and sucks a dart into their plunger tube should be a fan too. I know I've been there before. Having other plunger head options is also a great thing. All too often we end up with forcing the only O-ring we have on hand. Having the option to machine our own gives us options.
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#4 Daniel Beaver

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Posted 01 April 2014 - 09:41 AM

Good writeup. As Buffdaddy mentioned, regular nerf guns use a system like this, but it's not too common to see it in homemade blasters.
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#5 T da B

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Posted 01 April 2014 - 01:56 PM

I've never seen a Nerf gun ported in this manner. I've seen plunger heads with a hole from top to bottom (Stampede) and plunger tubes with holes or slits (Longshot), but those designs all sacrifice power on the forward stroke to eliminate vacuum loading. I'd be very surprised if Nerf went through the trouble of making a top-to-side ported plunger head.
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#6 Azrael0987

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Posted 01 April 2014 - 05:26 PM

A hack saw blade works well for lathing plastics. I've used one before to ghetto lathe an o-ring plunger head out of 1/4 inch polycarbonate.
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#7 Thailyer14

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Posted 01 April 2014 - 09:00 PM

Very clean good job !
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#8 Guest_Just Some Bob_*

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Posted 02 April 2014 - 11:57 AM

I've never seen a Nerf gun ported in this manner. I've seen plunger heads with a hole from top to bottom (Stampede) and plunger tubes with holes or slits (Longshot), but those designs all sacrifice power on the forward stroke to eliminate vacuum loading. I'd be very surprised if Nerf went through the trouble of making a top-to-side ported plunger head.


Be surprised, then: open a Firestrike.

You just didn't know what you were looking at.
They are almost all of this way. The Firestrike even has the holes through the face like yours. The ones with the slots often also have an air channel that goes part way under the o-ring. Just to break the seal, in one direction only. None of them "sacrifice power" to do it, either. Almost all single acting air pumps do the same. Slots are generally simpler to manufacture, and probably fine for most home builders.

You invented this independently. That's NOT a bad thing. It just means you could improve your powers of observation.

Edited by Just Some Bob, 02 April 2014 - 12:01 PM.

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#9 T da B

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Posted 02 April 2014 - 03:02 PM

You invented this independently. That's NOT a bad thing. It just means you could improve your powers of observation.


Thanks for the feedback, Just Smug Bob. So Nerf ports their plunger heads. That's wonderful--if you actually use stock plunger heads. What separates this design from anything Nerf has ever put out is the fact that multiple ports along the perimeter expand the seal in all directions while firing.
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#10 KaneTheMediocre

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Posted 02 April 2014 - 05:57 PM

This looks WAY easier than the check-skirt, which I abandoned later for a flap on the top of the plunger head. For reasons I'm not going to explain in detail, the latter strategy has given me lots of trouble with 3d printed plunger heads, but I expect this to be substantially easier. If this works out, I'll probably switch to o-rings (from skirts), saving $5 for any blaster that I do.

I'm skeptical that the holes contribute anything to "spreading" the seal, but if you get good seal, it doesn't really matter.

And I didn't know that existing Nerf blasters use these on their plunger heads--I basically stole the check-skirt design from the big blast pump head. In hindsight, the need is fairly obvious for any blaster with a single barrel or sealed turret.
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#11 Draconis

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Posted 02 April 2014 - 07:49 PM

Thanks for the feedback, Just Smug Bob. So Nerf ports their plunger heads. That's wonderful--if you actually use stock plunger heads.


Hey, no need to lash out because you are embarrassed. This is partially our fault, as a community. Most of us in the position to come up with a design innovation such as this just take for granted the nearly universal ability to observe existing designs. On behalf of the rest of us, I apologize for not making it abundantly clear how engineers solved this problem thirty years ago.

What separates this design from anything Nerf has ever put out is the fact that multiple ports along the perimeter expand the seal in all directions while firing.


Except that they don't. If there is any space between the o-ring and the head, then it all becomes the same fluid flow and makes no difference. If there is no space between the o-ring and head, then yes, you could build pressure inside that inner chamber. However, the pressure would have a direct relation to the area of the front ports, which is at least an order of magnitude smaller than the area on the outer edge of the o-ring, where the air would be working against better sealing by trying to lift the o-ring away from the cylinder edge. But this also would not allow the desired one-way-valve action.

What I suggest is actually tapering the interior edge of the channel that the O-ring rides in. In this case, as the cylinder is compressed, the friction between the outer edge of the o-ring and the cylinder wall will draw the o-ring back. The increased diameter of the head at the back will mechanically force the o-ring out, causing it to seal better. Just like is done in multiple blasters and hand-operated air pumps.
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#12 T da B

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Posted 02 April 2014 - 11:19 PM

My imperfect filing left a little taper in there and the results are indeed beautiful. Chrony results have been posted!
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#13 KaneTheMediocre

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Posted 04 April 2014 - 10:20 PM

A 3d printed adaptation has done well so far:

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#14 T da B

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Posted 05 April 2014 - 11:54 PM

Nice! Glad to see it's working well for you, too.
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