23 replies to this topic

[Jilliop]

I recently tried to make a simple SNAP with a standard clothespin trigger using 3" PVC as the plunger tube. For whatever reason, once it was all assembled, the Plunger head never got a seal, and always had friction, so that even with my [k25] spring it moved slowly. Is there something wrong with 3" PVC, or is it simply something that I'm doing wrong? I was using a custom plunger head made of wood with a groove for the O-ring. It was turned on a lathe. So, any ideas?

[BOSS9]

'Jilliop', on 03 Sept 2011 - 2:38 PM, said:

I recently tried to make a simple SNAP with a standard clothespin trigger using 3" PVC as the plunger tube. For whatever reason, once it was all assembled, the Plunger head never got a seal, and always had friction, so that even with my [k25] spring it moved slowly. Is there something wrong with 3" PVC, or is it simply something that I'm doing wrong? I was using a custom plunger head made of wood with a groove for the O-ring. It was turned on a lathe. So, any ideas?

The plunger seal is just a problem of bad implementation. The larger the plunger tube, the more air it pushes, obviously, and pushing more air is harder to do than pushing a little air. A [k25] is not powerful enough to use in a 3" plunger tube. You may need something even stronger than a [k26].

[Bchamp22795]

Yeah I agree with Boss. Its not gonna be anything fantastic. In fact, a not perfect seal may be better than a perfect one. Kind of like a HAMP. Its not perfect, but it rapes. You'll need something crazy like a K14 nested over a [k26] to get the plunger head to move as fast as a normal snap. But that will be a pain to pull back. Unless you wanna be cool and add pulleys to help pull your plunger rod (Not actually suggested)

[roboman]

'Bchamp22795', on 03 Sept 2011 - 4:52 PM, said:

Unless you wanna be cool and add pulleys to help pull your plunger rod (Not actually suggested)

Actually, that's not a bad idea. If you could implement a block-and-tackle system correctly, you could make the blaster really easy to cock, while keeping a 3" diameter PT.

Also, the O-ring on your plunger head may be the problem. In order to get O-rings to create a perfect or near-perfect seal, they have to be large enough to solidly contact the walls of the tube. This creates quite a bit of friction, even with lubricant, in most cases. A better option would be a skirt or U-cup.

[kyle9]

You should use something like a k45 spring nested in orange modwork,longshot,or PAS springs, that would help while not being too difficult to prime

Edited by kyle9, 03 September 2011 - 01:00 PM.

[Carbon]

I don't buy the argument that the spring is too weak. Based on what you described (no seal, yet high friction), I'd say that you're fighting against irregularities in the ID, something that a close fitting plunger head and O-ring can't deform enough to compensate for. You should be able to generate pressure just by holding your hand against the bushing and pressing the plunger rod. If ghat isn't happening, then you have issues that a stronger spring won't solve.

That said, I'd recommend cutting a custom plunger seal from neoprene, or using a skirt seal...something that can confirm to the PT walls better.

Question: how long of a plunger stroke are you using? Considering the increase in volume with a 3" PT, the stroke won't need to be very long.

Also, something to consider: 3" may be running into diminishing returns of PT size. A smaller PT has less volume, but accelerates faster due to lower plunger mass. The greater mass of a 3" plunger may negate volume benefits.

[Daniel Beaver]

Quote

You'll need something crazy like a K14 nested over a [k26] to get the plunger head to move as fast as a normal snap.

Plunger head speed is not the be-all-end-all of power measurements. 3" PVC gives you something like 6 times the area, and so the plunger head only needs to achieve a fraction of the normal speed to air velocity in the barrel. BUT, you have to remember that we are working with a compressible fluid (air), and cannot ignore friction for any practical thought experiment. Doom has more to say about this, and it is worth searching for his posts.

Quote

A smaller PT has less volume, but accelerates faster due to lower plunger mass. The greater mass of a 3" plunger may negate volume benefits.

This is an important consideration. You also have to worry about added friction, because of the longer circumference of plunger seal.

[SgNerf]

So far in my own blaster designs, i've tested out 75mm / 3 inch ID and even up to 100mm / 4 inch ID plunger tubes (the designs end up looking more like handheld bazookas ).... just by moving the plunger manually, i've also noticed that the larger the tube ID, the more effort it takes to push the air through the eventual 1/2 inch ID final exit to the barrel.

I'm not a physics expert (at best only caveman level knowledge)... but what i figure is that when a plunger tube's ID is many times larger than the exit, the oversized ratio creates a sort of natural "air restriction", so the air flow gets slowed down as the higher volume of air has to squeeze through a much narrower opening.

I guess that probably also explains why a long narrow ID pump is easier pump to compress air than a large ID pump.

Anyways, one important thing i have learnt when testing various plunger tube volumes... to get longer ranges out of larger plunger volumes, you'll need to match them with much longer barrels to capitalize on the extra air volume, and that tends to result in rather cumbersome and unwieldy designs (2-3ft length barrels ain't exactly easily maneuverable), so you'll also need to work out and test the ideal plunger to barrel volume combinations.

[Jilliop]

I believe Carbon is correct here, but also the spring might just be too weak. I'm not convinced though, because when I had the plunger head without an O-ring and tried to prime it, it flew forward, took out my bushings, and rebounded back to my foot of a nearby wall. So I'm going to go with the bad seal from friction, and not the bad springs. However, I'm not a pro modder, and don't do homemades well (as Carbon and I have discussed), and there are probably many underlying causes of my failure with SNAPS. And the PT was about 13-14 inches long, without bushings or any other crap on the ends. Thanks for your help everyone but I'm not quite ready for major SNAP projects yet. And I did make 2 HAMPs today too. 2" PTs.

[taerKitty]

My AtomicSplat uses a Reactor as a plunger tube, and has an appx ID of 2.25". At that diameter, with a fairly smooth interior and a cup seal, the stock blue-and-silver Splat spring wasn't enough to get a decent plunger head action from it. I had to swap it out for a beastly ACE #199 spring to get it useable.

In short, I'd say at 3", you're better off HAMPing it into a ramrocket.

[durka durka]

I wouldn't go quite as far as to say that you should eliminate the catch and spring parts of the blaster, but I do think you should consider a hamp style seal. I would focus on maximizing plunger speed (something the blaster will lack) rather than air volume (something the blaster will have plenty of).

In other words, sacrifice a little of the blaster's inherent advantage to balance out some of its disadvantages.

[KaneTheMediocre]

You need to match the volume to the task at hand, and the force for the distance / volume, area. Large bore springers like this will need a much more powerful spring to reach the same pressure as a smaller bore blaster in proportion to their cross sectional area. The flipside is that they need much less draw to get the same volume.

So, unless you need lots of volume (for XL ammo, or lots of regular sized), 3" pvc is more trouble than it's worth. As noted, the seal perimeter length is much longer, which makes it harder to seal, AND there is a larger error on the ID--Most PVC pipe is far from even on the inside, although smooth ID pipes do exist.

[Jilliop]

Again thanks guys I didn't realize there was that much to consider! And I'll probably just make another HAMP. That will be my 3rd. And subsequently, my third homemade. I suck at homemades. So where, in your opinion, does the normality of SNAPs resume? At 2", 1.75", 1.5"?

[taerKitty]

1-1/4" - 1-1/2", I would wager. I haven't seen 1-3/4" PVC. As I said earlier, I think 2" is too large an ID for any hand-primable spring (i.e. no levers, gears, pulleys, motors) to do anything decent with. I could be wrong.

[238232]

'taerKitty', on 04 Sept 2011 - 8:04 PM, said:

1-1/4" - 1-1/2", I would wager. I haven't seen 1-3/4" PVC. As I said earlier, I think 2" is too large an ID for any hand-primable spring (i.e. no levers, gears, pulleys, motors) to do anything decent with. I could be wrong.

Not exactly a SNAP, but Captain Slug's 2-11 calls for a plunger tube with an ID of 2".

[BOSS9]

'238232', on 05 Sept 2011 - 02:11 AM, said:

Not exactly a SNAP, but Captain Slug's 2-11 calls for a plunger tube with an ID of 2".

PVC sizing is nominal, polycarb tube sold by McMaster is labeled in actual dimensions. 2" PVC is larger than 2" in diameter.

EDIT: Thanks Bchamp22795, the difference is indeed small.

Edited by BOSS9, 04 September 2011 - 10:34 PM.

[238232]

Good catch, forgot about that.

Edited by 238232, 04 September 2011 - 10:14 PM.

[Bchamp22795]

'BOSS9', on 05 Sept 2011 - 03:06 AM, said:

PVC sizing is nominal, polycarb tube sold by McMaster is labeled in actual dimensions. 2" PVC is much larger than 2" in diameter.

http://www.harvel.co...ch40-80-dim.asp

According to that, the Average ID is actually not that much different.

[105nerf]

Are you sure you lubed it because i have used 3 inch pvc before and it worked fine so lube it a ton and tell me if it works.








,105nerf

[Jilliop]

Yes I lubed it, but the plunger head is largely wood. Wood doesn't lube terribly well.

[roboman]

'Jilliop', on 17 Sept 2011 - 6:34 PM, said:

Yes I lubed it, but the plunger head is largely wood. Wood doesn't lube terribly well.

I'd be more worried about lubing the rubber part, rather than the wood itself. The wood shouldn't even be contacting the PVC - the O-ring should be the only point of contact.

[Shrapnel]

'Jilliop', on 03 Sept 2011 - 2:38 PM, said:

I recently tried to make a simple SNAP with a standard clothespin trigger using 3" PVC as the plunger tube. For whatever reason, once it was all assembled, the Plunger head never got a seal, and always had friction, so that even with my [k25] spring it moved slowly. Is there something wrong with 3" PVC, or is it simply something that I'm doing wrong? I was using a custom plunger head made of wood with a groove for the O-ring. It was turned on a lathe. So, any ideas?

I have had the same kind of problem with my homemades. Sometimes it depends on what material you make the plunger head out of. If you want you could try the same method of plunger head with polycarbonate , this will make it even stronger, and also you might want a spring stronger than a [k25], i would suggest a k12 with a [k26] nested inside. Good luck.

[Daniel Beaver]

'Shrapnel', on 29 Nov 2011 - 06:21 AM, said:

also you might want a spring stronger than a [k25], i would suggest a k12 with a [k26] nested inside.

Yes, because I want a blaster with 84 pounds of draw force. That's practical.

[Langley]

'Shrapnel', on 29 Nov 2011 - 06:21 AM, said:

I have had the same kind of problem with my homemades. Sometimes it depends on what material you make the plunger head out of. If you want you could try the same method of plunger head with polycarbonate , this will make it even stronger, and also you might want a spring stronger than a k25, i would suggest a k12 with a k26 nested inside. Good luck.

The last post in this thread was from September. That's two Necros in a row.

'Daniel Beaver', on 29 Nov 2011 - 6:14 PM, said:

Yes, because I want a blaster with 84 pounds of draw force. That's practical.

You say that now, but by the next time I see you, you'll have something with compound bow-arms and a hand crank.