Flyng Dragon PCP Air Tube Failure

[rsterne]

As you are probably aware, there are tolerances for the Minimum and Maximum dimensions for every thread.... Here are the tolerances for 22 mm x 1 mm threads, as used in the XS-60c and the FD-PCP.... I don't know the fit class, so I will give both classes for External threads (there is only one for Internal).... I will give the dimensions in both Metric and Inch, for your convenience.... The source is a website called "Engineers Edge"....

Internal Threads (class 6H):
Minor Diam:
Min. 20.917 mm 0.8235"
Max. 21.153 mm 0.8328"
Pitch Diam:
Min. 21.350 mm 0.8406"
Max. 21.510 mm 0.8469"
Major Diam:
Min. 22.000 mm 0.8661"
Max. 22.304 mm 0.8781"

External Threads (Class 6g):
Major Diam:
Max. 21.974 mm 0.8651"
Min. 21.794 mm 0.8580"
Pitch Diam:
Max. 21.324 mm 0.8395
Min. 21.206 mm 0.8349"
Minor Diam:
Max. 20.891 mm 0.8225"
Min. 20.590 mm 0.8106"

External Threads (Class 4g6g): Slightly tighter tolerances than class 6g for the dimensions in red - ie not cut as deep
Major Diam:
Max. 21.974 mm 0.8651"
Min. 21.794 mm 0.8580"
Pitch Diam:
Max. 21.324 mm 0.8395"
Min. 21.249 mm 0.8366"
Minor Diam:
Max. 20.891 mm 0.8225"
Min. 20.633 mm 0.8123"

The original tube for the XS-60c uses a 22 mm OD x 1.5 mm wall tube, and is threaded 22 mm x 1mm pitch.... I just pulled my FD-PCP tube and measured the OD of the tube threads, and the ID of the receiver threads.... The dimensions in blue are the ones that are easy to measure with Digital Calipers.... The receiver I have, was within the stated tolerance on Minor Diameter, the ID measures 0.829-0.830" .... The tube I have, measured 0.8575 to 0.8585" for the back end that threads into the receiver (right on the minimum for the Major Diameter).... and I got 0.8565-0.8575" (slightly under the minimum allowable) for the front end.... Therefore, the tube on my rifle was right at the minimum allowable dimensions.... I did not measure the ID of the front nut/sleeve that holds on the fill fitting.... Remember, this is a sample of ONE....

Bob

[Bill G]

Hey Bob, 
Is the calculator on Engineers edge for thread strength what you use?  basically, I am wondering which formula/ method to use to decide what thread and pitch to use for a reservoir.  Obviously the remaining wall thickness it paramount to containment and is likely the weakest aspect of a reservoir.  I'm undecided on threading vs the shear on the bolt head method.  The threads sure make for a cleaner look on a rifle.  It would seem to me, that the method of using bolt heads under shear, would be better at producing a soft failure than a threaded end cap.  Although, proper design and fit should leave both methods completely safe. 

Bill 

[rsterne]

Not really the place for this discussion.... but there are three failure methods possible with a threaded plug.... If the threads are under pressure (as in the FD-PCP), the thinner wall below the threads weakens the tube, and you must use that thinner wall in your tube strength (hoop strength) calculations, instead of the full tubing thickness.... If the O-ring is inboard of the threads (eg. Disco fill fitting), then the thinner tube wall where the threads are is not under pressure, it is only in tension, resisting the end force on the cap or tube trying to pull the threaded portion away from the reservoir axially.... To calculate the strength of that you use the cross sectional area of the remaining tube wall and the end force.... The third type of failure would be the threads shearing.... The strength of that is calculated using the circumference of the pitch diameter, times the engagement length, divided by two (half that area is in the tube and half in the plug).... using the shear strength of the weaker material, which is 60% of the tensile strength.... This calculation is only valid if the threads are full depth.... the less engagement you have, the less material you have in shear (axially), because the two pitch diameters are different, so where they touch the other half of the thread is not full thickness.... If the threads are on the low side of the tolerance, you need additional length of thread engagement to make up for the lack in shear area of each individual thread.... Properly fitted threads (ie at the maximum tolerances) are incredibly strong and need little length of engagement (only a few threads, generally)....

Bob

[Bill G]

Yea my question was off topic some what.  I realized this when I found the thread on end caps that you had replied to some time back.   

Thanks for the reply

Bill

[bradyman1]

Any updates? Does Mike have the gun in hand? No luck finding the tube I assume?

[zandrew]

When I saw this it made me glad my Hatsan at44 is built like a tank. I am not saying these guns are Not as I have no experience with them. I am not here to try and get my kick in as I hate seeing this. Mike has always been courteous and answered any of my questions regarding his products. Hope its a just fluke situation though.

With the band still intact I wonder if it might have been a valve failure?

[KzooRichie]

Curious if anything has been resolved/discovered.

[rsterne]

Not to my knowledge, and I see that nobody else has even measured the OD of their tube threads or the ID of the receiver threads to see if they meet the standard tolerances I posted.... pretty complacent considering your safety is at stake if there are guns out there that don't meet those tolerances.... JMO....

Bob

[nervoustrigger]

Bob, the one I just took apart to inspect and tune measures as follows:

tube thread major diameter:  0.859" - 0.860"
receiver thread minor diameter:  0.825"

Which puts them both within tolerance though on the low side.  There is just the tiniest bit of play perceptible when the threads are engaged just 2-3 turns.

[vigilandy]

Not to my knowledge, and I see that nobody else has even measured the OD of their tube threads or the ID of the receiver threads to see if they meet the standard tolerances I posted.... pretty complacent considering your safety is at stake if there are guns out there that don't meet those tolerances.... JMO....

Bob

I've taken one apart and with my cheapie calipers (only accurate to two decimal points) I can see that the one I took apart is close to the minimum requirements.  Bought a better caliper but haven't remeasured.  Subjectively,  there seems to be quite a bit of play when the tube is screwed in but not tightened.  In the meantime,  my three have been grounded pending resolution of this issue.  Not being complacent and trying to be safe. 

[rsterne]

Jason, yours is a good one, for sure, I wish mine was like that....

Bob

[vigilandy]

Bob, the one I just took apart to inspect and tune measures as follows:

tube thread major diameter:  0.859" - 0.860"
receiver thread minor diameter:  0.825"

Which puts them both within tolerance though on the low side.  There is just the tiniest bit of play perceptible when the threads are engaged just 2-3 turns.

Jason,  can you clarify what you mean by "tiniest" bit of play?  Is this on the one I sent you?  I appear to have two versions.  One with the matte black tube like the one I sent you and two with gloss black tubes.  The one I had apart was one of the glossy models.  At 2-3 threads engagement the tube has enough play that I would never use the word "tiny."

[Gerard]

If this is helpful, perhaps measuring the maximum and minimum positions at the end of the tube using a standard number if turns of the tube would make for a standard point of comparison?  Say 5 turns in?

[rsterne]

If you are going to do that, I would use fully installed and backed off so that it JUST doesn't touch the seal.... That is the condition the rifle is in when filled.... The end of my tube moves about 0.1" sideways when so installed.... I have heard of much worse, but apparently the owners don't want to post in this thread for fear of criticism....

Bob

[vigilandy]

If you are going to do that, I would use fully installed and backed off so that it JUST doesn't touch the seal.... That is the condition the rifle is in when filled.... The end of my tube moves about 0.1" sideways when so installed.... I have heard of much worse, but apparently the owners don't want to post in this thread for fear of criticism....

Bob

How would this work Bob?  Screw the tube till you feel it tightening up,  then back off a half turn? And measure movement at the fill end?   I'm having trouble feeling when exactly it starts touching the seal. 

It would be nice to have a sample from various owners,  but honestly that wouldn't help me in figuring out the safety margin of my own guns.  Seems like there is a bit of variation in manufacturing. 

I'm not one to criticize Mike either.  He's been nothing be nice and helpful to me.  At this point I'm just being paranoid. I've got no evidence that these guns are out of spec.  Since Mike has sold all his supply he probably can't take measurements himself.

[rsterne]

Your description is what I had in mind.... Since the engagement is about 3/4", whether it was backed off 1/4 turn or 1 turn would make no difference to the sideways play at the fill end, which is where mine moves about 0.1".... Perhaps it would be almost impossible to relate this to whether or not the gun was safe, however.... if in fact any of them besides the one that let go have a problem, which we don't know for sure....

Measuring the OD of the tube and the ID of the receiver isn't perfect either, because it doesn't address the depth of the threads in the two parts.... All it tells you is the amount of interference to prevent the tube from pulling out of the threads.... On my gun that is about 0.0275-0.0295", or about 0.014" per side.... If you use the minimum tolerance on the tube OD (0.8580") and the maximum tolerance on the receiver ID (0.8328"), that gives a minimum overlap of 0.0252", or 0.0126" per side.... The maximum overlap, if the threads were as tight as allowed by the tolerances, would be 0.0416" (0.0208" per side).... Obviously, the more overlap the stronger the joint....

If the failure of the tube in this event was, indeed, a spiral failure with the tube cracking at the bottom of the threads and unravelling out of the receiver like a spring.... the critical dimension is not the OD of the tube threads, but the diameter at the bottom of the threads, minus the ID of the tube.... or more to the point the thickness of the tube at the bottom of the threads at the thinnest point....

Bob

[nervoustrigger]

Andy, the measurements are from one of the glossy-tubed rifles.  I didn't get a measurement of the play before I reassembled, sorry.  It was just barely enough that I could feel it.  All I can say is it felt like the parts were well engaged.

[vigilandy]

Ok,  here's my measurements.  I gotta say that this type of work makes me really wish I had something fancier than a $20 digital caliper from Harbor Freight.  That's something to add to the Christmas list. 

The calipers are supposed to be accurate to 0.001" but I have no way of verifying. 

The threads were measured in terms of overall min/max readings. 
Air tube threads at receiver end measure 0.853" to 0.855"
Threads inside receiver measure 0.820" to 0.822"

Amount of wobble (at the tube cap) was significant but difficult to measure. I basically marked a piece of paper at the start and stop points using the shadow cast from a light almost directly overhead.  The receive and tube were lying directly on the table.  I would give these a plus/minus 0.05" error. 
 
After three turns in from thread engagement: 0.198"
After backing off 1/2 turn from contacting seal (started tightening up): 0.132"

I can see that my measurements are less than what Bob posted as the minimum.  How does a that translate into the tube's ability to hold XXXX psi?

As you all can probably tell, I'm not fully confident in my tools and measurements. I would be happy to send the parts out to someone for verification.  Preferably someone who is properly equipped and knowledgeable.

[vigilandy]

Andy, the measurements are from one of the glossy-tubed rifles.  I didn't get a measurement of the play before I reassembled, sorry.  It was just barely enough that I could feel it.  All I can say is it felt like the parts were well engaged.

My measurements are also from a glossy tubed model.  Clearly there's production variation here because mine has a definite wobble until tightened down. 

[rsterne]

I can see that my measurements are less than what Bob posted as the minimum.  How does a that translate into the tube's ability to hold XXXX psi?

The only one who can determine that is Mike.... it's his responsibility, IMO.... If my tube was 0.003-0.005" under tolerance, I would send it back.... The good thing is that your receiver is also on the small side, so the interference on the sizes (worst case) is (0.853 - 0.822) = .031", or 0.0155" per side.... I would be more concerned about the thickness of the tube below the bottom of the threads....

Bob