Flyng Dragon PCP Air Tube Failure

[Gerard]

Yup, flat topped threads like those are unlikely to hold even half what full-depth threads can do. Not at all surprising it popped free under pressure. The only surprise I see is that it held beyond the first attempt at filling.

[rsterne]

Here is a drawing, to scale, showing the tolerances for the threads, and where the failed tube sits by comparison....



The red line is the threads in the receiver, at maximum diameter permitted by the tolerances.... The black line is the threads on the tube, at nominal diameter, and the blue line is the threads on the tube at minimum diameter permitted by the tolerances.... The green horizontal line is the OD of the failed tube, measured across the threads.... Note that there is only a 0.005" overlap between the top of the tube threads and the bottom of the receiver threads.... At the minimum allowable tolerances on the parts, that overlap would be a minimum of 0.012".... These only applies when the tubes are centered.... If the threads are additionally a loose fit, then the tube can move sideways in the receiver, reducing that overlap even further....

While I cannot state that tube threads at the minimum OD tolerance, and receiver threads at the maximum ID tolerance, will not fail.... stalwart did run a test to failure on such a setup, and it did not fail before the valve screws sheared at 6000 psi.... We now have one set of data where a failure did occur.... If you measure the OD of the threads on your tube, and the ID of the threads on the receiver, and they are not within tolerance, I personally wouldn't continue to use the gun.... The thread tolerances can be found here.... http://www.gatewaytoairguns.org/GTA/index.php?topic=99476.0

Bob

[birdmove]

    Can I ask how far that part flew through the air before hitting down?

[Tater]

Good follow up guys.
Thanks for the pics Douglas, and for the explanation and measurements Bob.

[Bobbieone]

    Can I ask how far that part flew through the air before hitting down?

When I read this thread last year they could not find the tube , don't know if it has turned up since  ?  .

[uncle paulie]

See Post #336...

pv

[PakProtector]

    Can I ask how far that part flew through the air before hitting down?

About 50 yards. The angle at which it was discharged is unknown, and would of course have a great effect on potential distance.
Douglas

[Kowboi]

I'm glad I finally saw this thread. I have two of the 1st gen rifles that have been out of service for a while. I was getting ready to start using them again but after reading this thread I immediately degased both of them...geuss I'll  go from there.

[Ribbonstone]

Finding the tube helped put to rest at the mechanics of how it happened.  We've all had miss-matched screw threads either pull loose or strip under stress; seems like the same kind of event.

Would be better to know than to guess/hope.  Take it down, measure the threads, then decide if you air it up again.

[Kowboi]

That was the plan...degassed awaiting disassembly and inspection. I only have cheapo harbor freight calipers though.
Edit:
And I need a battery.

[larspawn]

Hey Douglas,  I haven't been shooting much for a couple years but started into the AGs again. Do you still do fun shoots out at your place?  If so maybe I'll make it out some time and sweet talk you into measuring the threads on my FDPCP. For now I'll degas it. It's been sitting for 2 years gassed!  Yikes.

Andy aka LARSPAWN.

[moorepower]

From the looks of the photo, it would not take a caliper to see the threads were too shallow. The tube probably wiggled when it was tight on the Oring.

[Kailua]

I have a question.  When turning threads on tubing is the O.D. of the thread measurement supposed to be smaller than the tubing's O.D.?  It looks like it from the picture shown.

[Ribbonstone]

The OD of the tube pretty much has to be less than the ID of the receiver.  If they were the same, would need a rather long pipe wrench to screw them together.  All things that screw together have to have a little slack (or it's a "crush" fit that isn't coming back apart without some heat or some damage).  In this rife's case, you HAVE to unscrew the air tube in order to get the valve out, so there has to be something besides that "crush" fit in order to do basic repairs (can consider a "crush" fit as a one-time fit....it won't be that tight the 2nd time).

Which does bring up a wear factor.  The more often you unscrew the tube and screw it back on, the sloppier the fit becomes.  Can't really avoid that, it's just in the nature of things screwed together tightly.

Took 14 or 15 pages of posts before I got off my lazy-butt and took mine apart to do the measurements.  I understand just how disagreeable it is to take apart a non-leaking, shooting just fine, air rifle can be and resisted it for a long time.


In this case, give in to the urge.

http://www.gatewaytoairguns.org/GTA/index.php?topic=96148.msg933666#msg933666

Didn't come out to "max", but did come out with enough "bite" to the threads that I kept it in service (strict 1500psi limit...the Chinese pump sold with the rifle is good about that, as 1500psi is just where the green section starts).

Had considered other reasons for failure, but the photos rule those out.  At least 1 full thread hangs out, unsupported by the breech, so at that point the tube strength is only as strong as the depth of the metal from the bottom of the groove...which is an UNIMPRESSIVE thickness (something like .04-.045")...which pretty much explains the 1500psi rating.  HAd thought that it might have radially cracked at that thread (esp. as it "went off" when removed from a pickup bed...and might have been dropped in its case).;

Photos prove that is NOT THE CASE...it wasn't a bending/impact stress...seems to have been a shallow thread tube.

[larspawn]

So are we saying these are not safe at all and should be destroyed?  Sorry I'm late to the thread but getting very concerned that I should get rid of this. There's no solution?

[Kailua]

I have a question.  When turning threads on tubing is the O.D. of the thread measurement supposed to be smaller than the tubing's O.D.?  It looks like it from the picture shown.

Oops missed Bob reply 341.  So the next question is.  So what caused the O.D. of the threads to be reduced in diameter so much?   Was it turned in a wrong or faulty die?  Or some kind of wear on the thread?

[OleTomCat]

I have a question.  When turning threads on tubing is the O.D. of the thread measurement supposed to be smaller than the tubing's O.D.?  It looks like it from the picture shown.

Oops missed Bob reply 341.  So the next question is.  So what caused the O.D. of the threads to be reduced in diameter so much?   Was it turned in a wrong or faulty die?  Or some kind of wear on the thread?

The tube was probably turned undersized then threaded.

[uncle paulie]

What we are saying is that each of us must make our own decisions regarding these guns. On the last two pages of this thread there is all the info. you would need to make an informed decision. The recommended tolerances are posted. You must make careful measurements of your gun. If it is within the limits recommended, you would likely be fine ONLY filling to 1500psi. If you are in doubt, you can contact the dealer and see what recourse you may have with him.

pv

[Taso1000]

Gentlemen,

Is it then the consensus that it was a badly machined, out of specification tube that was missed in quality control and that the system, as designed, is safe?

Thanks,

Taso

[uncle paulie]

Read post above yours. You are the only one who can make the final decision as to whether or not your rifle is suitable for use.

pv