Flyng Dragon PCP Air Tube Failure

[moorepower]

IMHO the photos and other evidence point to a spiral failure of the thread root.    The tube is quite possibly hanging in a tree, the threads having turned into a Slinky(Tm).   I have seen this type of failure in low pressure steam and HP water piping, the pipe corrodes and erodes from the inside until only the threads are left.   The pipe is then removed simply by pulling on it( if it hasn't already removed itself)  The damaged thread tops is the main indicator for my diagnoses.

.                                                                                                                                                There is no room for anything but the tube to go through the barrel band. If its not on the threads it won't make it out of the gun case. A crack in the receiver would crack the paint.

[rsterne]

Actually, the idea of a spiral thread failure makes some sense if the thread in the tube was, for example, cut way too deep, leaving too thin a wall to survive the tensile load.... Imagine if you will the thread turning into what looks like a spring, or as said, a slinky.... It wouldn't have to be any larger than the barrel band.... What doesn't make sense is that not even a small piece of the spiral broke off and remained behind, or inside or caught up in the case.... but I suppose it is possible.... Remember, one of the statements is that no metal fragments were found inside the case.... I have removed damaged screws by drilling them out until all that is left is the thread, and sometimes they collapse and pull partially out of the threaded hole.... but usually break off in pieces.... They can, however, leave the female thread perfectly intact if you drilled straight and carefully....

Here is a possible failure mode, typed as I work my way through it.... The stress in the steel remaining below the root of the tube thread would be the highest right where the threads on the tube enter the receiver.... a failure at that point would logically result in a crack, starting there, that would follow the root of the tube thread in a spiral as the tube rocketed away from the receiver.... As the thread became progressively detached from the stub of threaded tubing still in the receiver, it would wedge up along the ramp of the female (receiver) thread outboard of it, collapsing slightly, and leaving a "spring", still attached to the tube, that was smaller in OD than the barrel band, and small enough to only damage the tops of the female threads as it was yanked out of the hole.... Yes, I think this is entirely possible.... That it could happen and leave NO trace behind boggles my mind, however....

Bob

[MJP]

Similar failures have been seen when tapered fittings are used in normal threads not ment for those.
Marko

[sixshootertexan]

IMHO the photos and other evidence point to a spiral failure of the thread root.    The tube is quite possibly hanging in a tree, the threads having turned into a Slinky(Tm).   I have seen this type of failure in low pressure steam and HP water piping, the pipe corrodes and erodes from the inside until only the threads are left.   The pipe is then removed simply by pulling on it( if it hasn't already removed itself)  The damaged thread tops is the main indicator for my diagnoses.

This is exactly what I was saying in my post #49.

[rsterne]

sixshooter, I completely missed/misunderstood that about it uncoiling.... Checking the trees is definitely a good idea....

Bob

[Geoff]

another edited angle ... enlarged photo, lightened and sharpened

click on it to see larger

[Kailua]

Could it be a defective tube?  Where the wall of the tube was too thin (edited) at the threaded end.
http://www.gatewaytoairguns.org/GTA/index.php?topic=83461.0

[bradyman1]

I guess no new photos or additional info pertaining to the guns whereabouts are available. I think it was petroleum based lubes used on one of the fittings. Those lubes then worked their way into the tube. The fill which occurred just prior to detonation caused the oils to be pressurized, vaporized,  and ultimately ignited causing the tube to shoot off like a scud missile.

[Kailua]

I guess no new photos or additional info pertaining to the guns whereabouts are available. I think it was petroleum based lubes used on one of the fittings. Those lubes then worked their way into the tube. The fill which occurred just prior to detonation caused the oils to be pressurized, vaporized,  and ultimately ignited causing the tube to shoot off like a scud missile.

So you are saying this can happen in storage?

[bradyman1]

Highly unlikely but I suppose anything is possible.

[vigilandy]

How would you go about checking tube fitment in the receiver?  Degas first, then?

1. Wiggle it as is? 
2. Unscrew (to break red thread locker) then tighten down again then wiggle it?

I understand that the rifle failure is pending investigation.  As a precautionary measure, I'm degassing and grounding my rifles. NOT blaming anyone at this point, just playing it safe. 

[ezman604]

I guess no new photos or additional info pertaining to the guns whereabouts are available. I think it was petroleum based lubes used on one of the fittings. Those lubes then worked their way into the tube. The fill which occurred just prior to detonation caused the oils to be pressurized, vaporized,  and ultimately ignited causing the tube to shoot off like a scud missile.

THIS is the thoughts of several of us. Supposedly the remains are on the way to Mike how will examine it himself but says he will also take it to the university for forensic metallurgy examination. Will make sure ANY new information is posted.

[Tater]

THIS is the thoughts of several of us. Supposedly the remains are on the way to Mike how will examine it himself but says he will also take it to the university for forensic metallurgy examination. Will make sure ANY new information is posted.

It's nice to hear he's doing his due diligence to investigate. He has a good reputation.

[PakProtector]

What I'd like to do is examine a fresh off the boat set of rifles. Measure thread pitch both external and internal and see what the spread is. As mentioned by vigilandy, re-assemble them with clean threads and see what sort of clearance exists between the biggest internals and smallest externals. Likely pressure test the worst-case examples...perhaps with air from behind an appropriate blast shield.

It is quite unlikely the missing tube will show up. Knowing what we're dealing with on tolerance stack up will allow us to decide just how bad the situation is.

Has anybody asked Mike, or other experienced builders just what they have seen so far?
cheers,
Douglas

[vigilandy]

What I'd like to do is examine a fresh off the boat set of rifles. Measure thread pitch both external and internal and see what the spread is. As mentioned by vigilandy, re-assemble them with clean threads and see what sort of clearance exists between the biggest internals and smallest externals. Likely pressure test the worst-case examples...perhaps with air from behind an appropriate blast shield.

It is quite unlikely the missing tube will show up. Knowing what we're dealing with on tolerance stack up will allow us to decide just how bad the situation is.

Has anybody asked Mike, or other experienced builders just what they have seen so far?
cheers,
Douglas

Douglas,  do you know if the FDPCP in question had an air tube the was painted flat black or glossy black? I have both but am unsure if there are any differences other than paint color.

Since the Gen 1 have been sold out for a while,  Mike probably won't have any around to measure variations in the manufacturing.  However if enough of us got together,  we could probably get together a decent sample size.  Is there a relatively simple way to take measurements of the threads in the receiver?  The airtube itself I assume can be measured with simple calipers. 

[avator]

You guys do realize that the gen I is an aftermarket tube right? So getting one "off the boat" ain't gonna tell you nothing. Please correct me if I'm wrong.

[PakProtector]

You guys do realize that the gen I is an aftermarket tube right? So getting one "off the boat" ain't gonna tell you nothing. Please correct me if I'm wrong.

Alright, could some of the experienced mechanics( Mike I am at a bare minimum talking to you ) who have worked on these things please step up and tell us what their experience with that particular joint fit is?
cheers,
Douglas

[rsterne]

How would you go about checking tube fitment in the receiver?  Degas first, then?

1. Wiggle it as is? 
2. Unscrew (to break red thread locker) then tighten down again then wiggle it?

I understand that the rifle failure is pending investigation.  As a precautionary measure, I'm degassing and grounding my rifles. NOT blaming anyone at this point, just playing it safe.

I have a Gen 1 that I purchased for my son, and it happens to still be sitting here because it is leaking.... When this happened, I checked and found no pressure in it, and with the tube screwed home against the seal there was very little movement sideways at the front end of the tube.... When I unscrewed it 1 turn, the front end would move about 1/10" side to side, which dividing the tube length (15") by the thread engagement in the receiver (3/4"), a ratio of 20:1, means that the tube threads have a clearance of roughly 0.005" in the receiver threads (0.0025" per side).... I then unscrewed the tube and looked at both the receiver and tube threads under magnification, and both appear reasonable in quality, no missing chunks or obvious problems.... I felt confident that they were within safe tolerances and reassembled the gun....

To the best of my knowledge, no Loctite is used on the tube to receiver threads in the FD-PCP, and it makes no sense to do so, as it is sold as a "tinkerer's gun", so numerous assembly/disassembly cycles could be reasonably expected.... I cannot speak for the parent gun, the XS-60c, but I do know that Mike replaces the original (shorter) tube with one made of larger tubing.... The original is 22 mm OD x 1.5 mm wall (0.866" x 0.059" - ID 0.748"), and the FD-PCP tube is 7/8" OD x 0.065" wall (0.875" OD x ID 0.745").... However, the threads are still Metric, 22 mm x 1 mm, so Mike is obviously having those made somewhere.... While checking the fit of XS-60c tubes in XS-60c receivers is a good idea, it won't tell us what the tolerance is on the tubes that Mike is making / having made.... Shortly after I got my son's gun (early 2014?) I asked Mike what material he was making the tube of, and he declined to tell me, stating that was Proprietary information.... Without that information, doing the math to determine safety factors is not possible.... I heard that Mike was considering having the longer tubes for the FD-PCP made offshore (by Xisico?) but have no knowledge if that happened or not, nor have I ever seen a Gen 2 gun.... Only Mike can supply us with that information, should he choose to....

I cannot recommend what each individual owner of an FD-PCP do since this incident.... All I can do is tell you what I did and the results I found.... I agree with Douglas that anyone who has had one of these apart has the right to comment on the fit of the tube to receiver that they have found, and would value that information considering what has happened.... This should be done in a factual manner, with no blame assessed, but we have a right to the information, one way or the other, IMO....

Bob

[clgraham82]

I cannot recommend what each individual owner of an FD-PCP do since this incident.... All I can do is tell you what I did and the results I found.... I agree with Douglas that anyone who has had one of these apart has the right to comment on the fit of the tube to receiver that they have found, and would value that information considering what has happened.... This should be done in a factual manner, with no blame assessed, but we have a right to the information, one way or the other, IMO....

Bob

Well said!

[Ribbonstone]

No new information...no new theories.

Am going to assume the simplest explaination is the best.

Loose tube threads, probably just a confluence of minimum/maximum variation.

Tube likely took off in one chunk.  Nothing left in the receiver, so not likely a radial separation.  Stock shattering does show a pressure release right about at the receiver/air tube junction.

Barrel band doesn't have anything to do with holding the air tube in place. Doesn't have a cross screw, the tube can slide in and out of it a will, so it being in place doesn't indicate much.

No evidence of soot/detonation on the bright face of the valve or breech, so am going to rule out lube detonation.

LEt go while in a nylon case...so not likely that purely an over pressure.