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Barrel Lube

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Barrel Lube

Unread postAuthor: Sonicboom101 » Tue Jul 27, 2010 4:52 pm

When I first started building spud cannons I was told by my friend that I need to lube up the barrel with WD-40. I've been thinking after all this time, is it really necessary to lube up the barrel before you shoot the cannon? I was also thinking could this make the gun projectiles shoot farther?
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Unread postAuthor: ramses » Tue Jul 27, 2010 4:57 pm

It is not strictly necessary, but it may be beneficial. When I shoot things wrapped in duct tape for proper fit on hot days, I use white lithium grease.

So, for rubber, or sticky projectiles, yes, otherwise, no. Keep in mind that WD 40 and other petroleum distillates can erode seals, o-rings, and anything rubber or foam.
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Unread postAuthor: jimmy101 » Tue Jul 27, 2010 5:28 pm

For a typical combustion you really don't won't to be lubing the barrel. The gun will almost certainly perform a bit better with a dry barrel.

In a typical combustion, ammo to barrel friction actually helps performance a bit. You can fiddle around with HGDT to get an idea of how much a reasonable amount of friction will help.
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Unread postAuthor: Sonicboom101 » Tue Jul 27, 2010 7:16 pm

jimmy101 wrote:For a typical combustion you really don't won't to be lubing the barrel. The gun will almost certainly perform a bit better with a dry barrel.


Yeah that's what I figured, that's good to know. Most of my guns are combustion, and all of the barrels smell like WD-40 witch I hate. Next time I fire one of them up without WD-40 and I'll see how well it preforms.. :twisted: Thanks!
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Unread postAuthor: psycix » Wed Jul 28, 2010 3:22 pm

The propulsion force acting on the projectile is so large that the friction force can be ignored.
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Unread postAuthor: jimmy101 » Wed Jul 28, 2010 4:55 pm

psycix wrote:The propulsion force acting on the projectile is so large that the friction force can be ignored.

Nope.

A typical combustion guns only get up to 60 or so PSI. A tight fitting spud might have 10 or 15 PSI equivalent in static friction (especially with a double-beveled spud cutter). Figure dynamic friction is a fair bit less but probably still significant.

The advantage of a fairly high static friction is that it retards movement of the ammo until a larger amount of fuel has burned. With very low friction the ammo starts to move when the chamber pressure is quite low, that wastes barrel length. (Combustion if a pretty slow process.)

Even with a fairly high static friction (like 30 lbs or 10 PSIG) the ammo is moving after only 10% (IIRC) of the fuel has been burned.
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Unread postAuthor: FighterAce » Wed Jul 28, 2010 5:31 pm

A while back I read somebody teflon coated the inside of his barrel.. any word on that?
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Unread postAuthor: Sonicboom101 » Wed Jul 28, 2010 6:16 pm

I've never heard on Teflon coating the inside of the barrel. I don't see a reason for doing that though but keep in mind I've only built a few guns and I've still got a lot to learn.
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Unread postAuthor: jimmy101 » Wed Jul 28, 2010 6:48 pm

FighterAce wrote:A while back I read somebody teflon coated the inside of his barrel.. any word on that?

Only that it was probably a waste of time. For a combustion it is definitely a waste unless you use a burst disk. For a pneumatic it would only help with a very fast valve.
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Unread postAuthor: Technician1002 » Wed Jul 28, 2010 9:01 pm

In testing t shirt launchers on air, the slick 3 inch ABS is a little faster than PVC. You don't want oil on t shirts. Choice of barrel material can sub for lube.
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Unread postAuthor: urgle the danish cow » Wed Jul 28, 2010 9:46 pm

i think it reduces friction of the projectile on the barrel thus increasing velocity. but i suggest trying out gear lube instead. thank me later. cheers
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Unread postAuthor: Sonicboom101 » Thu Jul 29, 2010 10:07 am

Thanks for the suggestion about the gear lube. I'll first try it without lube then with the gear lube when I get my hands on some.
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Unread postAuthor: SpudBlaster15 » Thu Jul 29, 2010 11:44 am

ramses wrote:Keep in mind that WD 40 and other petroleum distillates can erode seals, o-rings, and anything rubber or foam.


I've never had a problem with WD-40 eroding seals or other rubber parts. It's kerosene based, and won't have much of an effect on said components. However, you are correct, there are many harsher petroleum distillates that can swell and even potentially damage rubber.
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Unread postAuthor: psycix » Thu Jul 29, 2010 1:48 pm

jimmy101 wrote:A typical combustion guns only get up to 60 or so PSI. A tight fitting spud might have 10 or 15 PSI equivalent in static friction (especially with a double-beveled spud cutter). Figure dynamic friction is a fair bit less but probably still significant.

Its not just that dynamic friction is less, once the spud starts moving it may "sand" itself down to the size of the barrel.

I think the dynamic friction equals at most a few psi, depending on how you load your spud of course.
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Unread postAuthor: jimmy101 » Fri Jul 30, 2010 7:08 pm

psycix wrote:
jimmy101 wrote:A typical combustion guns only get up to 60 or so PSI. A tight fitting spud might have 10 or 15 PSI equivalent in static friction (especially with a double-beveled spud cutter). Figure dynamic friction is a fair bit less but probably still significant.

Its not just that dynamic friction is less, once the spud starts moving it may "sand" itself down to the size of the barrel.

I think the dynamic friction equals at most a few psi, depending on how you load your spud of course.

Maybe. I know that HGDT only lets you enter the static friction, then it divides that by two and uses the result for the dynamic friction. So, for a double bevel cutter with a static fiction of 30 pounds (in a 2" barrel), that would put the dynamic friction in the vicinity of 15 pounds (~3 PSIG) or so. Not much like you say. But if you remove that friction the performance of the gun may well drop because of the reduction in the static friction.
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