Testing Beeman Chief Valve Screws to Failure- SECOND VIDEO added reply #42

[lloyd-ss]

Bob Sterne asked me to do a test of the valve screws that come in the Beeman Chief PCP. Bob, being the super conscientious guy that he is, had upgraded the screws in his gun and was curious about the originals. I think the Chief is rated at a 2,000 psi max fill pressure.

Please note that this is an informational video only. It is not official, nor is it intended to provide any design information. It is just for fun.
Also, please note that in the video, the gauge is reading pounds of force on the three screws. IT IS NOT THE SAME AS PSI IN THE AIR TUBE.

I enjoy doing the testing; Bob will explain the details.
Lloyd

 

[Ribbonstone]

Also interesting to me (As a Chief owner) at 4:25.


I did take Bob's advice and changed the screws for the best ones he could find (lets just say "Mc-KNOWN Company"...am a little to PO at them for their non shipping to Canada so they won't get an "honorable mention"), but as I didn't counter sink the heads into the valve body, am not going to count that as being any stronger than the original screws.

[Nvreloader]

LLoyd-SS

Outstanding info, excellent video and clear photo's etc. 

You and several others provide a vast wealth of info, for this site.

I for one greatly appreciate all that you provide for us.

I am locked in on your info.

Thank you.

Tia,
Don

[grand-galop]

Thank you LLoyd!!  This test is not formal but will get the fog out about the Beeman Chief and the security factor surrounding the block and valve...

[rsterne]

First of all, a big THANK YOU to Lloyd at the AirGunLab for performing these tests for me.... I got a Beeman Chief for Christmas and on examination the small valve screws, and their rather poor fit in the holes in the tube, concerned me.... When I removed the screws, I found that they were Metric 4 mm x 0.7 mm with threads all the way to the head, and installed in the valve on a tiny flat that wasn't even deep enough to allow the head to seat completely against the valve body for it's full width.... The tube is 26 mm OD x 22 mm ID, so that is a 2 mm wall, which looks to be plenty for the recommended 2000 psi fill pressure, but of course we don't know the material either the tube or the screws is made from.... I ran the calculations on the screws, and those results were in Reply #41 on my thread on the Chief, you can see them here.... https://www.gatewaytoairguns.org/GTA/index.php?topic=136973.40

I didn't have the Grade 12.9 screws from McMaster Carr that would fit, and they were still a bit below what I like to see when I am doing my safety calculations, so I drilled out the valve to accept 10-32 alloy steel SHCSs which I was completely confident in.... I then contacted Lloyd to see if he would test the originals, he agreed, and I mailed them out to him.... The results of that test are in the video above.... Please note, I am NOT an Engineer, and this information is not presented as a criticism of Beeman.... However, I will make some comments about what I look for in terms of safety factor when building my own PCPs, and you can draw your own conclusions....

Pressure vessel testing began back in the early days of steam engines, when boilers exploding was a big concern.... Steel at that time was brittle compared to what we have today, and the plates were riveted together.... It was decided that boilers should be built to withstand five (5) times their working pressure to avoid catastrophic failure.... As steels and manufacturing techniques improved, that was reduced to a safety factor of 4:1, and then eventually to the 3.5:1 used by the ASME (today?) for pressure vessels.... Information on the various standards used around the World is hard to come by for the layman (read me), but it appears the ASME have two safety margins, the 3.5:1 to failure, and a factor of 1.5:1 to "yield", which is where the metal "stretches or bends" in a "plastic" manner, without returning to its original dimensions.... Lloyd found an interesting ASME/API paper not long ago which stated that the allowable stress (load) cannot exceed 2/3rds of the "Yield" strength of the material (equivalent to a safety factor of 1.5:1 using Yield) and could also not exceed 1/2 of the "Tensile" (ultimate) strength of the material (equivalent to a safety factor of 2.0:1 using UTS).... These factors are extremely low, and not something I would ever design to, but they do show the differences between different standards.... Most of you will have heard of periodic "Hydrostatic testing" of SCUBA and SCBA tanks (usually every 5 years) to insure their continued safety.... This is usually done to 5/3rds the working pressure, and there are limits for how much the tank can expand during the test, and it must return to its original size after the pressure is released.... This is testing to insure the material is not "Yielding" (permanently deforming) at the test pressure, and means that the tank is operating at a safety margin of at least 1.667:1 to yield.... Materials subjected to repeated stress cycles "fatigue" or lose strength over time, so this periodic testing checks for that, and also for things like cracks or corrosion that would make the tank unsafe....

Some Engineers, and many people working in industries like the Oilfields, use a minimum safety margin of 3:1 using the Yield strength as a design point.... There is a constant debate about whether such a high margin is necessary, but their (valid) point of view is "better safe than sorry".... If such high safety margins were applied to aircraft, they would be too heavy to ever get off the ground (1.5:1 is common in that industry).... and yet we ride on them all the time, and only rarely does one suffer a materials related failure.... So, what do I use, personally, as a DESIGN POINT?.... I use a minimum of 3:1 using UTS, and prefer 3.5:1 (only because I was able to find that in an ASME document).... In order to avoid concerns about metal fatigue, I use a minimum of 2:1 to Yield for things like the pockets in a tube where the valve screws bear, and 2.5:1 to Yield for everything else.... I determine these numbers using the published UTS (Tensile strength) and Yield strength of the materials, and the standard calculation methods for the application.... Lloyd and I both have spreadsheets to aid in those calculations.... We both use only 60% of the UTS when calculating the Shear strength, which is the standard procedure, even though most things will not fail at that number (more like 67-75%)....

So when did the valve screws in the Chief fail?.... They started to Yield at about 3,000 lbs. of force, and failed at 3,400 lbs. in this test.... The ID of the tube is 22 mm (0.866"), which means the area is 0.589 sq.in.... The pressure inside the Chief that would cause the screws to yield (bend) would be (3,000 / 0.589) = 5093 psi.... and they failed at (3,400 / 0.589) = 5,772 psi.... Using the recommended 2,000 psi fill pressure, that gives a safety margin of 2.55:1 to yield and 2.89:1 to failure.... The yield point met my personal standards.... but I am glad I replaced them with the 10-32 screws as I am certain they will exceed my 3.5:1 goal.... Lloyd actually tested the exact screws I am using a year or two ago in a similar test, and they survived a force of 7,500 lbs. before yielding (6.4:1 in the Chief at 2,000 psi)....

https://www.gatewaytoairguns.org/GTA/index.php?topic=72672.0

Note that in those tests (a summary of the results is in Reply #23), the standard Disco screws also yielded at a force of 3,000 lbs., but because the tube ID is smaller that load would occur in a Disco at 6,800 psi, that works out to a safety margin to yield of 3.4:1.... Lloyd also tested some alloy steel low-profile SHCSs from McMaster Carr in the Disco (same size as stock, just better material) and they yielded at 4,200 lbs. force.... That shows what good quality screws can do, compared to the stock ones in a Disco.... We didn't test the Grade 12.9 replacement screws from McMaster Carr that would fit the Chief.... but if I were a betting man, I would bet they would do better than the stock screws.... In my estimation, based on when the stock screws yielded, I think they are about the equivalent of a Grade 5 screw (92 Ksi Yield, 102Ksi Tensile).... Grade 12.9 Metric screws are rated at 170 Ksi Tensile....

Based on the test of these valve retaining screws, I personally would recommend that you do NOT exceed the 2000 psi fill pressure recommended by Beeman on the Chief.... Since we do not know the material the tube is made from, and have not tested it, that would be prudent in any case.... It wouldn't be a bad idea to carefully inspect the valve screws every time you remove them to make sure they show no signs of distortion, bending, or partial shearing.... and replace them with new should that be observed....

Bob

[Rob M]

Thanks Lloyd and BOB.. this lends some credibility to the Chinese PCP manufacturers for those who perhaps  weren't so sure..  Some will misconstrue this comment , and that's okay.safety first.

[lloyd-ss]

Thanks for the nice and thorough write-up Bob, and especially for pointing out that many different standards can be applied depending on what field you are in or what country, or what you are personally comfortable with.  But always, never exceeding the manufacturer's recommended fill pressure is the right thing to do.

Of all the testing that I have done of OEM style equipment from airgun manufacturers, I am glad to say that nothing I have tested scared me as being unsafe. And I don't think I personally have heard first hand of any accident where the fault lied totally with the manufacturer. But compressed air is very dangerous and must be treated with knowledge and respect.
Lloyd
 

[Ribbonstone]

It's a 2K PCP.  Likely the original screws will serve just fine at 2K, even though Phillips heads don't inspire an abundance of confidence.   

Changing just the screws to the "Mc-master-we-don't-ship-to-Canada" screws MIGHT offer a little more strength, but not the strength of counter sinking the screw heads (as Bob as done) below the shear line. (Mentioned for the Disco/Maxi shooters that believe just swapping screws makes for a 3K PCP).

Inexpensive PCP's have changed....if you want a 3K PCP, buy a 3K PCP.

Personally, one of the attractions of the Chief is the 2K fill. Stuff done to increase that fill pressure just puts in in an "also ran" track-team line up.

SERIOUSLY: If making a 2K PCP run at 3K is such a great idea, why not the posts about making a 3K PCP run at 4K?

[lloyd-ss]

SERIOUSLY: If making a 2K PCP run at 3K is such a great idea, why not the posts about making a 3K PCP run at 4K?

Right you are! Now that really DOES scare me.  

[gendoc]

It's a 2K PCP.  Likely the original screws will serve just fine at 2K, if you want a 3K PCP, buy a 3K PCP.

absolutely.....

[canadian_shooter]

Very interesting test and read. Thanks to Lloyd and Bob for this.

[TF89]

That is some awesome testing and explanations LLoyd and thank you Bob for the informational PSA.

I'm just amazed at the level of expertise on this site.

Dave

[lloyd-ss]

Thanks for the comments, everyone. The GTA is a great place to share information.
Lloyd

[rsterne]

If someone has three of the McMaster Carr alloy-steel screws and wants to send them to Lloyd.... maybe we can twist his arm to see if they are better?.... I would like to see the safety margin to hard failure increased if possible, and they should do that....

Bob

[lloyd-ss]

Bob, Are you thinking M4 SHCS, to be used with a shallow counterbore (maybe 1mm deep) into the valve body?
Or just high strength alloy steel pan head screws with no additional c'bore?
Lloyd

[Buldawg76]

Excellent Info on the safety margins and failure modes with testing as usual from Lloyd and Bob. Such valuable info for sure.

Mike

[rsterne]

Lloyd, I was thinking of just screwing them into the Beeman valve, so that guys could just upgrade the screws only.... just to kick the safety margin up a bit.... They might have to Dremel a small recess in the stock, but most guys can handle that.... Milling a pocket is a whole other can of worms (but even better, of course).... These are the screws I found....

https://www.mcmaster.com/#91290a138/=1avzg91

Bob

[oldpro]

 I seriously thought they would fail around 2500PSI!!! Ive seen them(the hardware) and I was almost certain they wouldnt make it to a safety margin I would be comfortable with. Glad your back and active again Lloyd!!!

[Ribbonstone]

I've got the screws...if he wants to test them PM me.

[lloyd-ss]

I've got the screws...if he wants to test them PM me.

That sounds like a plan. PM coming.
Lloyd