SpudFiles

Hey, i'm new to these forums but not so new to the world of pneumatic spudguns. I'm currently building by 3rd spudgun, and so far its going to have a 4" by 10' barrel and a chamber maybe twice the size.

However, since i'm looking to maximize power i figured that it would be necessary to build a 4" diameter barrel-sealing piston valve (housed in a 6" T) in order to achieve maximum efficiency of air-release. I plan on (hopefully) pumping this up to at least 120 PSI, maybe even more if i ultimately construct it well enough, but my primary concern was the lubrication for the piston valve, which is the crucial part of this gun.

So does anybody have any suggestions as to what kinds of lubrications should i use for the piston valve? Perhaps which last longest, are able to maintain seals under high pressure, or have the lowest friction coefficient? Thanks

KY jelly i think

... seriously?

Also, i forgot to add this, but i was wondering if some lubrications work better with different kinds of rubber?

I use some stuff from my paintball gear. Best stuff is "Hater Sauce." Works really well but smells a bit.

KY jelly dries out. Use petroleum jelly. If not that (due to possible incompatibility) then use crisco or margarine.

I use plumbing lubricant picked up from my hardware store.

Petroleum Jelly? As in vaseline? That stuff is pretty thick, i've used it on some valves in the past and it doesn't move too quickly, and from my experience the seal was inconsistent.

I've bought some plumbing lubricant actually, it seemed to be what i was looking for, but i haven't tried it out yet. Do you find that you need to reapply it very often? and does it have a low friction coefficient?

KamranGo wrote:However, since i'm looking to maximize power i figured that it would be necessary to build a 4" diameter barrel-sealing piston valve (housed in a 6" T) in order to achieve maximum efficiency of air-release.

That sounds like a hell of a task for a 3rd spudgun. It would be very much better to go with a 1 or 2" piston and a much smaller launcher.
Valves that large are a nightmare to make and maintain, and are very likely to damage themselves if you don't know what's what really well. I can't immediately think of any valve larger than 3", and if someone experienced hasn't already done it, you have to ask why.

I really think you should go down to a smaller size. I'm fairly experienced, having built 10 piston valves now (all in small sizes of copper, not quite the same), but I certainly wouldn't start to build a 4" valve until I had a least built two good 2" valves, and probably a 3" one as well.

white lithium grease.

I agree with Ragnarok I have made about 8 spud guns and I dont have a single gun that uses pipe over 3in. I tend to keep myself small because of the area I live in.As ragnarok said you should make some smaller piston valved cannons to get a little more experienced with piston valves then when you are more experienced consider attempting this. A 4in piston valve is a tough task.

Eh well, i'm ambitious and bored.

I dont think it should damage its self though. The only way i could see it doing that is by the back of the piston slamming into the endcap of the pilot chamber, and thus that repeated shock causing eventual deterioration and ultimate failure. But i'm going to be putting some sort of padding behind the piston to absorb the shock of its recoil. Other than that, the piston its self is going to be very structurally reinforced. I'm using a threaded steel rod about 3/4" diameter to connect the barrel-sealing component to the piston component.

I have been worried that maintaining it will be a slight hassle, so i've been trying to think of ways of connecting this monstrous valve that will allow easy access to the piston for servicing. But overall i don't see why no one else has attempted a 4" diameter piston valve, or why it would exactly be so difficult. Maybe perhaps its their complicated designs that are inhibiting them? My design is really quite simple, i'm not using any O-rings, no need for a lathe or anything, so thus far it has been very easy to construct.

bigbob12345 wrote:I agree with Ragnarok I have made about 8 spud guns and I dont have a single gun that uses pipe over 3in. I tend to keep myself small because of the area I live in.As ragnarok said you should make some smaller piston valved cannons to get a little more experienced with piston valves then when you are more experienced consider attempting this. A 4in piston valve is a tough task.

4" piston is easy because a 3" end cap works perfectly as a piston. 3" pistons are a bit*h

By the way, if you want to know how i'm constructing my piston i've attached two plumbing test plugs such as this: http://www.tooled-up.com/artwork/ProdImage/TB28377.jpg to opposite ends of a threaded steel rod (by removing the middle bolt which you can see in the image, and sliding them on to the rod). They are then secured in place (and their diameter is also adjusted to fit the 4" diameter) by screwing (nylon?) nuts on opposite sides of each plug, under which a rubber washer is also placed to ensure an airtight seal. If this wasn't enough to ensure the seal is perfect, i also plan on using epoxy or silicone sealant on the nuts to hold them in place and ensure that no air escapes between whatever cracks there may be.

cwazy1 wrote:4" piston is easy because a 3" end cap works perfectly as a piston. 3" pistons are a bit*h

That is a 4" housed piston, not a 4" porting piston.

@KamranGo: You have no idea how fast these things can come back, and if you're adding in a 3/4" steel rod, that's going to add a lot of weight to allow it to really damage itself.
I had damping for the piston in one small launcher, and it still tore loose a zealously tightened copper compression fitting. Luckily I spotted the problem before it became dangerous, but in PVC, it will just break with no warning.

The other problem is the sheer volume of air escaping can recreate such a huge recoil that it can snap valves in half that diameter if even a tiny mistake is made. Seriously, trust me, a 4" porting valve is going to be both a nightmare and potentially highly dangerous.

I think you should really re-evaluate and start small, because I don't think you'll get a 4" to work without a lot of hard toil, but if you do, kudos to you.

hmmm, ok. I think the steel rod is actually smaller than 3/4" but yes the piston in its entirety is has a significant weight. I figured that when the pilot air chamber is released, that since the air escapes more slowly than the air in the regular chamber there would be a small "cushion of air" to absorb a decent amount of the recoil of the piston, but if these things really snap back like you say they do i'm definitely going to reassess my design.

However, i'm still a little bit hazy as to where you experience a lot of structural failure? or where exactly are points to be wary of? I want to still pursue building this valve and i was a little bit confused where you had experienced problems before. Though, i think this topic is starting to digress.