GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: TF89 on March 18, 2019, 01:45:34 PM
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I'm out of town and thinking about various ideas since I can't actually do anything air gun related other than think of new ways to tweak what I have.
This question is regarding barrel tensioning.
I have a Discovery and thinking of adding a 11mm ID x 13mm OD carbon tube.
Since I have the BNM breech and shroud I can drill the breech out to allow about an 1" to house the carbon tube inside the breech. I would also add a couple more set screws to hold the carbon/barrel assembly.
I have been off and on playing with the idea of sleeving the barrel and I sanded the ID of a 11mm tube so now it is a very snug slide fit over the barrel. I just haven't taken the next step so the carbon tube has been sitting for a couple of years now.
My question is more about ways to tension the barrel. If I bond the sleeve to the barrel then adding a nut/threads so that I can tighten the muzzle end of the barrel against the carbon sleeve doesn't seem like it would do anything to actually tension the barrel against the breech. Is that correct?
I'm thinking that to try and tension the barrel the carbon tube would need to be able to move so I would be tightening the barrel against the breech and the carbon tube is really acting like a bushing between the two.
Or would it be better to just bond the sleeve and leave it at that?
Dave
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Dave
I know nothing about your equipment,
Question for you,
If you tension the CF sleeve against the action/breech and the end of the bbl,
How much pressure is being imparted on the bbl holdings screws,
will they withstand the force from the constant vibrations from shooting etc?
Food for thought....
Tia,
Don
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That is a good question. Just in the current config I have two set screws with corsponding drilled holes or dimples in the barrel to secure it in place in the breech. I will certainly add more, just not how much would be needed.
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It may affect accuracy if the barrel is permanently attached to the CF sleeve because it may impart some uneven stress to the whole barrel "system". Maybe the stress is at the same angle and equal on each shot and just produces different harmonics?? I am not certain about the effect. On the other hand if the barrel free-floats in the CF tube the barrel will hit the sleeve somewhere. It may not hit the same way every time which screws up accuracy. Kind of akin to putting too many barrel bands on your barrel.
This gif shows PB barrel harmonics but airgun harmonics are similar. This is what your barrel is trying to do.
(https://www.gatewaytoairguns.org/GTA/MGalleryItem.php?id=6292)
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If you are placing the sleeve against the front of the receiver, in order to tension it, then you would not want to glue it to the barrel.... I think that would be slightly better than gluing it and just butting it up against the receiver.... Better yet would be to bore out the receiver to accept the larger diameter sleeve to a depth of at least 1", and then glue the sleeve to the barrel, and insert the sleeve into the receiver.... That way the barrel OD is increased everywhere.... Do NOT just tighten the setscrews against the CF tube, drill through it so that they bite into the steel of the barrel....
If you are going to glue the sleeve to the barrel, you don't want too tight a fit, you need enough room for the glue.... I would use Loctite 638 (green)…. You want the barrel loose enough to fall through the sleeve under it's own weight, no tighter than that....
Bob
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You should check out Jsar. They are selling tensioned barrels. If your determined to diy though, I tensioned a barrel liner into a chromoly tube. I Tig welded one side and threaded another. I only put a small tig tack on one side as I was nervous about distorting the thin liner. Later on when my project again changed directions I tried to break my tack loose. I’m a mechanic and I couldn’t break my little tack loose with a ratchet. Needless to say it is tensioned. If I had a carbon tube like that I would thread both sides, finest pitch you can find tap, die and nut for, and use green locktite rstern suggested, apply tension while it’s wet, and then machine down nuts.. one side smooth for breech, one side thread for your favorite ldc.. 1/2x20?
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Thank you for the tips. I'm planning on drilling the breech about an inch regardless of which direction I go. The questions in my mind are what both of you mentioned. It seems the best way to manage harmonics would be via tension. I have a slight concern about crushing the carbon tube as it is on the thin side. I don't know how many inch pounds to use, 110 or so it what I seem to remember being discussed. Would still need to come up with a way to tension a barrel.
This discovery has a 20" barrel and I'm getting close to 30 fpe. Ran out of pellets before any real testing was completed. Those are waiting for me when I get back in town.
Good to know about the fit for bonding Bob as I was thinking a tighter fit was needed.
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With that thin of a wall on the CF tubing, I would personally not want to tension it.... Gluing it under tension and then removing the tension would be a bad idea, I think the continued stress on the glue line may cause it to creep or fail.... If you will be inserting the CF sleeve into the receiver, I would just glue the sleeve in place, do a post cure if you can, and forget the tensioning....
When I use Loctite 638 (green) I allow to set 24 hours at room temp. (70*F) and then post cure for 3 hours in an oven at 175*F.... Loctite is made for metal to metal, and the bond to the CF tubing (the resin, after all, is a plastic polymer) is not great without the post-cure.... I think eventually (weeks or months) it may reach the same strength as the post-cured product, but I'm not sure about that.... On the other hand, how much bond strength do we really need?.... Fully cured 638 is 4500 psi shear strength, and you will have about 29 sq.in. of bonded surface, so that is about 65 tons to make the sleeve move once cured.... You would surely destroy it first....
Bob
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Thanks for all the information. Now to put in practice.
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Another idea, if you have the clearance, is to do what WAR and/or JSAR does and have snap ring on the breech end of the barrel, securing it in place, a sleeve tighten against (that's not glued), and the tensioning nut/LDC mount... WAR makes just a tensioning nut.
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I guess I have one more question. How much relief do you do for the breech hole? So say the the carbon sleeve is 13mm or 0.511811" O.D, what the the receiving hole ID be?
Thank you,
Dave
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A slide fit.... 0.002-0.004" over usually does the trick.... Chances are a CF tube will not be the same OD in every orientation anyways, unlikely to be within 0.001", let alone 0.000001"....
Bob