Flyng Dragon PCP Air Tube Failure

[rsterne]

I don't, as yet, know what tolerances will be "officially" recommended by the GTA.... but I don't think it will be much longer before we know that.... Your gun is "tighter" than the one I have for my son.... I'm not scrapping it, at least not yet.... I would be happy to have a receiver on the minimum (tight) side of the tolerance, especially with a tube approaching the minimum (loose) side....

Bob

[Rabbit\Squirrel Killer]

So the tube left in one piece (nothing remains in the breech) Finding the tube will reveal details, however you would expect the threads on the steel counterpart to have even less damage than the receiver (aluminum) being the weaker part.
I have seen separation of two parts where there was barely any damage to the threads on both parts (pressure was in the 1200psi realm). I was shocked to find no sheared threads, and could only conclude that the outside part expanded to the point of letting go of the inserted part.

I am wondering what the expansion rate differences (if any) between the two metals would do?
I imagine the softer metal breach may expand faster when heated by the sun than the harder metal tube causing a loosening fit of the threads for a period of time. I have read that aluminum expands at nearly 5 times the rate of carbon steel, I don't know that to be true. I do know that that would vary with specific alloys.  The looser fit would minimize the damage as they parted. 
Just a thought from someone who has had a gun in the sun heat considerably.  Don't leave your guns in the sun at all, is my advice and general practice.

[rsterne]

The thermal expansion coefficient for aluminum is 12.3 x 10^-6 per degree F, and for steel is 6.7 x 10^-6.... In other words aluminum expands twice as fast as steel.... We are working with a 7/8" diameter, so if we calculate the expansion for a 50*F change in temperature (ie 70*F to 120*F) on that diameter, we get....

Aluminum 0.0000123 x 0.875 x 50 = 0.00054"
Steel 0.0000067 x 0.875 x 50 = 0.00029"

The difference is 0.00054 - 0.00029 = 0.00025" (a quarter of a thou).... I don't think that is the cause.... From freezing to boiling (180*F difference), the difference in expansion is still less than 0.001"....

Bob

[Rabbit\Squirrel Killer]

The thermal expansion coefficient for aluminum is 12.3 x 10^-6 per degree F, and for steel is 6.7 x 10^-6.... In other words aluminum expands twice as fast as steel.... We are working with a 7/8" diameter, so if we calculate the expansion for a 50*F change in temperature (ie 70*F to 120*F) on that diameter, we get....

Aluminum 0.0000123 x 0.875 x 50 = 0.00054"
Steel 0.0000067 x 0.875 x 50 = 0.00029"

The difference is 0.00054 - 0.00029 = 0.00025" (a quarter of a thou).... I don't think that is the cause.... From freezing to boiling (180*F difference), the difference in expansion is still less than 0.001"....

Bob

I did not think it could be "the cause" but that it could be a contributing factor to a already near to failed thread fit.  Kinda "the straw that broke the camels back" so to speak.  I have used heat to remove ceased bolts many times. That is what got me thinking it could contribute to a bad fit by loosening it just a little more.
Granted if it is that close it would fail soon anyway.

[rsterne]

If a quarter thou expansion made the difference (even a thou).... then IMO the tolerances were WAYYYYYYYYYY to close to the limit to start with.... PCPs should be dealing with a 3:1 safety margin....

Bob

[Rabbit\Squirrel Killer]

If a quarter thou expansion made the difference (even a thou).... then IMO the tolerances were WAYYYYYYYYYY to close to the limit to start with.... PCPs should be dealing with a 3:1 safety margin....

Bob

Very true, I was looking at it as more of away to explain the breech threads being more or less in-tacked.

[rsterne]

Again, if they were only overlapped by a quarter thou (or even a thou), they were going to fail anyway.... All the difference in expansion could do is mean that a quarter thou less material would be shaved off the aluminum by the departing tube, if it simply pulled out....

Bob

[Rabbit\Squirrel Killer]

Again, if they were only overlapped by a quarter thou (or even a thou), they were going to fail anyway.... All the difference in expansion could do is mean that a quarter thou less material would be shaved off the aluminum by the departing tube, if it simply pulled out....

Bob

Exactly, a little more thread left behind. I can only look at the pics and not measure them but the threads were not even close to stripped out like reason would say that the softer metal should be, if the fit was tight at all. The harder tube had to be under sized in relation to the breech and ready to let loose when bumped. I read this whole thread tonight, and am glad I did not buy one a while back, I almost did.
I also will reexamine all my converted Crosmans, to remain confident that I have every detail right and that there is no visible signs of fatigue or wear. 

[AirCats]

drained the pressure all the way down this afternoon, by firing cheap daisy pellets through it. This gun is very efficient, took me almost 40 pellets to drain that 600 psi (daisy flat pellets were around 750 fps at 600 psi, by the way). Emptied the last little remaining air by pushing the striker with a screwdriver.

Took it apart and measured the tube OD and receiver ID:
*Tube thread OD, receiver end: at the beginning of thread (the edge that goes into the receiver and press against the flat seal) is 0.849; near the end of thread is 0.862"
*Tube thread OD, Foster-nipple-cap end: 0.861" all the way.
*receiver thread ID: 0.820"

not sure if it means anything........the difference between tube OD and receiver ID is 0.029~0.042" ( I suppose that indicates the amount of thread engagement? ); in the thread spec posted by Bob, class 6h/6g, the OD/ID difference is from 0.026" (min OD - max ID) to 0.042" (max OD - min ID).


 

[tkerrigan]

If I understand this correctly, with internal minor Dia. of .825 and external major Dia. of .847 measured with a good quality dial caliper, it needs parked?  Just my luck if so, it has held air for a year and stacks pellets at 16 yards.  Regards, Tom

[avator]

Again, if they were only overlapped by a quarter thou (or even a thou), they were going to fail anyway.... All the difference in expansion could do is mean that a quarter thou less material would be shaved off the aluminum by the departing tube, if it simply pulled out....

Bob

Exactly, a little more thread left behind. I can only look at the pics and not measure them but the threads were not even close to stripped out like reason would say that the softer metal should be, if the fit was tight at all. The harder tube had to be under sized in relation to the breech and ready to let loose when bumped. I read this whole thread tonight, and am glad I did not buy one a while back, I almost did.
I also will reexamine all my converted Crosmans, to remain confident that I have every detail right and that there is no visible signs of fatigue or wear.

Just to be clear... the latest set of pics that were posted by Dave are NOT of the gun involved in the incident. As bad as these threads are, this tube did not "launch". Even tho the potential is quite evident.

[avator]

I have a question... can the ID of the tubes safely be expanded to create a better/safer fit? I mean, we really aren't talking about a whole lot here are we? Would the threads be further compromised if stretched just enough?

[ezman604]

We appreciate the enthusiasm with draining, disassembly and measuring of the components. Post or save that data for now. But PLEASE wait a bit longer for the inspection procedure to be posted before asking questions about is mine safe or not.
Thanks!!!

[Wood Boats]

How could the issue be compounded by imperfect thread pitch engagement?  This happened at Lufkin Industries after they started sourcing castings out of China.  Tapped holes that would not properly accept made in USA bolts.

If something like this could be the case then everything would feel right during initial assembly of the parts.  The damage to the threads we are seeing could very well have occured during initial insertion of a made and threaded in USA tube into made and threaded in China receiver.  The new tubes would have to be threaded in and removed for visual inspection to know any damage had occured.

I cannot believe Flying Dragon would assemble parts with visibly damaged threads.  Inconsistent OD of the aftermarket tubes would only aggravate the issue.

[rsterne]

I am somewhat surprised that guys are finding receiver threads that are under tolerance.... ie less than 0.8235" ID.... However, that is NOT a real problem as long as the tube threads in by hand....

The tube threads should have no significant taper to them.... other than 1 thread at the end, and maybe a couple of threads where the threads stop.... In between they should be round (within a thou or so) and consistent in diameter over the length of the threads....

DO NOT stretch the tube to increase the diameter.... Doing that means you have exceeded the yield strength of the steel, which could either lead to it cracking where it is thin (ie at the bottom of the threads), or breaking where a crack had already started.... REALLY BAD IDEA....

"Believe" what you like, but there should be NO tubes out there that measure less than 0.858" OD, IMO.... If you think that threading a steel tube into an aluminum receiver, by hand, would cause the damage shown in the pic, please enlighten me as to how....

The problem with trying to figure out the thread "overlap" is that we don't know the exact profile of the threads on either the tube or the receiver.... It is the pitch diameters, and how closely they line up, that governs the actual overlap.... Think of it this way.... In a perfect thread, the area in shear on each part is half the total area of the overlap at the contact point (the pitch diameter).... However, as the female thread gets larger, and/or the male thread gets smaller, the contact point moves towards the points of the threads on both parts.... This puts less area in shear, and this greatly increases the stress on the metal, and reduces the force required to make it shear off....

The other danger (IMO much more important) is that as the thread is cut deeper, there is less wall thickness left on the inside of the tube, below the valley of the thread.... In the FD-PCP (and the XS-60c), this area is subject to "hoop stress" because it is under pressure, in addition to "tension stress" in the tube trying to pull off the end of the receiver.... The weakest point is right where the tube emerges from the receiver.... Not only is the tension stress the greatest, but the tube wall has NO support from the receiver to prevent it from failing from the internal pressure.... As you cut the threads deeper, the wall gets thinner, and the stress goes up.... The easiest way we have to ESTIMATE how thick the wall is, is to measure the OD of the tube threads....

The nominal thickness of the 7/8" OD FD-PCP tube is 0.065".... If the 22mm threads were cut to the maximum specs (the threads being ~0.022" deep), the tubing wall left would be 0.039" (60% of its original strength).... If the threads are cut to the minimum OD, then the remaining tube wall is only 0.033".... That means that if the thread OD is barely within spec, HALF of the original strength of the tube is gone.... If the threads measured 0.850", the tube wall remaining would only be 0.029".... That is less than 1/32" of metal to contain the pressure AND take the stress trying to pull the tube out of the receiver.... Not near my face, thank you....

Bob

[William]

How could the issue be compounded by imperfect thread pitch engagement?  This happened at Lufkin Industries after they started sourcing castings out of China.  Tapped holes that would not properly accept made in USA bolts.

If something like this could be the case then everything would feel right during initial assembly of the parts.  The damage to the threads we are seeing could very well have occured during initial insertion of a made and threaded in USA tube into made and threaded in China receiver.  The new tubes would have to be threaded in and removed for visual inspection to know any damage had occured.

I cannot believe Flying Dragon would assemble parts with visibly damaged threads.  Inconsistent OD of the aftermarket tubes would only aggravate the issue.

You may be somewhat correct,
But from experience those pics of the bad threads on the tube were not damaged during installation, the aluminum threads on the receiver could possibly have been damaged by the Steel threads on the Tube! I have cut a lot of threads over the years on metal and aluminum and those threads on the tube was definitely damaged while being made and not after the fact. A set of worn dies, no lubrication or over sized/undersized material or dies will cause the damage as seen in the pics above on the tube shown here.

You can say what you want (and that goes for anyone) those AG's need to be nationally recalled, this is not about seeing who got lucky and managed to hit the lottery and got a safe gun that has no flaws in workmanship, I have already seen listed here that most of the PCP's mentioned are not within an agreed or set tolerance.

Just wait until one of them things goes off like a rocket again and hits someone. Flying Dragon as far as I am concerned should have pulled the plug and done a recall already.

I make no comment on what happened or how it happened, due to I don't know what caused the failure, but I see that their is something wrong somewhere on all/or most of these AG's!

Yes I would own one and buy one, but I would inspect it before using it to see if the tolerances were within spec's first, the problem is most average people don't have the ability to know if their gun is within a safe margin or not.

I have had all my high-end AG's tubes off for resealing and cleaning and believe me they all have perfect threads and fit as smooth as buttered glass with no wiggle or tight spots while going together. Thats the way all of them should be.

William

[Ribbonstone]

OK…made me look.  Didn’t really buff it clean the parts, just a quick swipe with a clean rag.
Generation 1 rifle (one of the last half dozen .22 Gen 1).

Receiver runs something about .827-.828”. 



Tube thread runs something between .855-.856”.



Not the cleanest threads, but pretty well formed without flats/skips.

Think I'm keeping this one in (1500psi) service.

[rsterne]

Receiver size is nice, and the threads look OK.... The tube threads aren't the greatest, but no big chunks missing.... Being on the small side of the tolerance is a bit of a concern, but at least you know what you've got....

Bob

[stalwart]

But from experience those pics of the bad threads on the tube were not damaged during installation, the aluminum threads on the receiver could possibly have been damaged by the Steel threads on the Tube! I have cut a lot of threads over the years on metal and aluminum and those threads on the tube was definitely damaged while being made and not after the fact. A set of worn dies, no lubrication or over sized/undersized material or dies will cause the damage as seen in the pics above on the tube shown here.

You can say what you want (and that goes for anyone) those AG's need to be nationally recalled, this is not about seeing who got lucky and managed to hit the lottery and got a safe gun that has no flaws in workmanship, I have already seen listed here that most of the PCP's mentioned are not within an agreed or set tolerance.

Just wait until one of them things goes off like a rocket again and hits someone. Flying Dragon as far as I am concerned should have pulled the plug and done a recall already...

William

I have a little, reoccurring, waking nightmare. Goes like this:

You just had a gun fail in your hands... your first realization is that you just got away with scratches and splinters, and relief floods in. Then your second realization hits you: You just launched a 2# missile, with a Wad Cutter tip, into a neighbor's backyard 5th birthday party...

[Kailua]

How could the issue be compounded by imperfect thread pitch engagement?  This happened at Lufkin Industries after they started sourcing castings out of China.  Tapped holes that would not properly accept made in USA bolts.

If something like this could be the case then everything would feel right during initial assembly of the parts.  The damage to the threads we are seeing could very well have occured during initial insertion of a made and threaded in USA tube into made and threaded in China receiver.  The new tubes would have to be threaded in and removed for visual inspection to know any damage had occured.

I cannot believe Flying Dragon would assemble parts with visibly damaged threads.  Inconsistent OD of the aftermarket tubes would only aggravate the issue.

You may be somewhat correct,
But from experience those pics of the bad threads on the tube were not damaged during installation, the aluminum threads on the receiver could possibly have been damaged by the Steel threads on the Tube! I have cut a lot of threads over the years on metal and aluminum and those threads on the tube was definitely damaged while being made and not after the fact. A set of worn dies, no lubrication or over sized/undersized material or dies will cause the damage as seen in the pics above on the tube shown here.

You can say what you want (and that goes for anyone) those AG's need to be nationally recalled, this is not about seeing who got lucky and managed to hit the lottery and got a safe gun that has no flaws in workmanship, I have already seen listed here that most of the PCP's mentioned are not within an agreed or set tolerance.

Just wait until one of them things goes off like a rocket again and hits someone. Flying Dragon as far as I am concerned should have pulled the plug and done a recall already.

I make no comment on what happened or how it happened, due to I don't know what caused the failure, but I see that their is something wrong somewhere on all/or most of these AG's!

Yes I would own one and buy one, but I would inspect it before using it to see if the tolerances were within spec's first, the problem is most average people don't have the ability to know if their gun is within a safe margin or not.

I have had all my high-end AG's tubes off for resealing and cleaning and believe me they all have perfect threads and fit as smooth as buttered glass with no wiggle or tight spots while going together. Thats the way all of them should be.

William

   I did some talking to a fella who had bought one of the first FDPCP rifles when it first came out on the telephone.  I no longer have his number and I don't think he visits this forum.  I hope his rifle is okay to shoot.