GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: lloyd-ss on February 11, 2018, 11:22:22 AM
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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
https://youtu.be/xvSx74F0M4U
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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.
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LLoyd-SS
Outstanding info, excellent video and clear photo's etc. 8)
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
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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...
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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 (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 (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
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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.
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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
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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?
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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. :(
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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..... ::)
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Very interesting test and read. Thanks to Lloyd and Bob for this.
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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
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Thanks for the comments, everyone. The GTA is a great place to share information.
Lloyd
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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
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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
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Excellent Info on the safety margins and failure modes with testing as usual from Lloyd and Bob. Such valuable info for sure.
Mike
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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 (https://www.mcmaster.com/#91290a138/=1avzg91)
Bob
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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!!!
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I've got the screws...if he wants to test them PM me.
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I've got the screws...if he wants to test them PM me.
That sounds like a plan. PM coming.
Lloyd
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I am working on getting the test piece apart so that at least the tube can be re-used for testing the screws that Ribbonstone is sending me.
Here is a pic of the aluminum plug pushed partially back out of the tube. The way the screws sheared off, it is not coming out without a fight. The head that popped off is laying on the tube for reference.
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yea, that cant be good.
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Left-hand drill bit?....
Bob
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Left-hand drill bit?....
Bob
Yup, I should to get a few of those. The need has come up more than once. Seems like one of those things you struggle with for years, and then after you deal with it, you're wondering, "what took me so long?" LH drill bits added to the shopping wish list.
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Thanks for sharing the test results. Very informational.
Have this test been done for the Mrod valve screws?
Peter
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Thanks for sharing the test results. Very informational.
Have this test been done for the Mrod valve screws?
Peter
I did a test of the Marauder type screws in the new lightweight weight M-Rod air tube made by JSA. Even though this wasn't a test of the OEM M-Rod screws, indications are that the screws are more than up to the task. Here is that thread.
https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0)
I also did some testing of the OEM Disco screws and the upgraded alloy low profile SHCS that a lot of folks install in the Disco. rsterne referenced that particular thread earlier in this thread.
Lloyd
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Thanks for sharing the test results. Very informational.
Have this test been done for the Mrod valve screws?
Peter
I did a test of the Marauder type screws in the new lightweight weight M-Rod air tube made by JSA. Even though this wasn't a test of the OEM M-Rod screws, indications are that the screws are more than up to the task. Here is that thread.
https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0)
I also did some testing of the OEM Disco screws and the upgraded alloy low profile SHCS that a lot of folks install in the Disco. rsterne referenced that particular thread earlier in this thread.
Lloyd
Thanks Lloyd. I did see that test that you performed. It showed the tube failing but the screws were still intact. I'm curious about there strength with this current test you did for the screws specifically. The reason for my asking is seeing what the max fill pressure these screws can withstand repeatedly.
Peter
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Thanks for sharing the test results. Very informational.
Have this test been done for the Mrod valve screws?
Peter
I did a test of the Marauder type screws in the new lightweight weight M-Rod air tube made by JSA. Even though this wasn't a test of the OEM M-Rod screws, indications are that the screws are more than up to the task. Here is that thread.
https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0)
I also did some testing of the OEM Disco screws and the upgraded alloy low profile SHCS that a lot of folks install in the Disco. rsterne referenced that particular thread earlier in this thread.
Lloyd
Thanks Lloyd. I did see that test that you performed. It showed the tube failing but the screws were still intact. I'm curious about there strength with this current test you did for the screws specifically. The reason for my asking is seeing what the max fill pressure these screws can withstand repeatedly.
Peter
The Marauder rifle is rated for a 3,000 psi max fill pressure by Crosman, and that should be observed. The screws will handle a lot more force than that before FAILING, but you should never operate any equipment close to its failure point. Allowing at least a 3.5 to one safety factor is a recommendation made by some engineering organizations. So, if the screws fail at a fill pressure of 10,500psi, they a safe maximum operating pressure is 3,000 psi.
I have never tested the Marauder screws to failure, and honestly, I do not know if they are the weak link in the pressure system of the Marauder. Something else might very well fail before the screws do.
Lloyd
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Hummm...
Another situation where to get 3 screws you have to buy 100.
Wish we could get a group effort going to share with each other
for example, I have 90 (kept 10) 1/8 BSPP bonded seal washers used in our HPA fittings sitting on the shelf looking for good homes. Paid $14 for them at a hydraulics supply company.
H, I would even pay the $1 shipping to put 5ea into an envelope so other guys don't have to pay $5 ea ripoff price charged by the AG outfits.
I have a Chief and have no plans to hotrod it but would change the screws for 'peace of mind' but it irks me to buy n waste most of a box of screws
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Hummm...
Another situation where to get 3 screws you have to buy 100.
Wish we could get a group effort going to share with each other
for example, I have 90 (kept 10) 1/8 BSPP bonded seal washers used in our HPA fittings sitting on the shelf looking for good homes. Paid $14 for them.
H, I would even pay the $1 shipping to put 5ea into an envelope so other guys don't have to pay $5 ea ripoff price charged by the AG outfits.
I have a Chief and have no plans to hotrod it but would change the screws for 'peace of mind' but it irks me to buy n waste most of a box of screws
Carl, Yes, I agree. It is so easy to exchange ideas back and forth on the forum and the internet, wouldn't it be great if we could do the same with little odds and ends of special hardware. Somebody might volunteer on the Chief screws. We could have a hardware swap at the Funshoot. ::)
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There is a reason for the large safety margins used on pressure vessels, Peter, and you hit the nail on the head with your question....
The reason for my asking is seeing what the max fill pressure these screws can withstand repeatedly.
I highlighted the word REPEATEDLY because there is a big difference between the test Lloyd performed on the screws (a single load to failure) and what happens with cycling a pressure vessel hundreds or thousands of times, where the repeated stress may fatigue the material.... With steel, if you keep the stress applied to less than half the tensile strength, the cycle life is almost infinite.... Titanium is similar.... Aluminum has no such fatigue stress threshold, it continues to fatigue even if the applied stress is low, but at a much reduced rate.... In practical terms, if you keep the stress in most aluminum alloys below half the tensile strength, the cycle life is so great you can ignore it (perhaps millions of cycles)....
When a company designs a pressure vessel, and state a Maximum Safe Working Pressure (MSWP), they usually take into account a generous cycle life, and maybe even the possibility of some corrosion.... If you fill to a higher pressure, you are reducing the number of cycles to possible failiure, and that is the primary reason it is a bad idea.... You might get away with it once.... a hundred times.... or a thousand times.... but simply by using more pressure than the specified MSWP you are pushing it.... or at least shortening the safe lifespan....
Bob
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x2 on what Bob just said in Reply # 30.
Lloyd
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There is a reason for the large safety margins used on pressure vessels, Peter, and you hit the nail on the head with your question....
The reason for my asking is seeing what the max fill pressure these screws can withstand repeatedly.
I highlighted the word REPEATEDLY because there is a big difference between the test Lloyd performed on the screws (a single load to failure) and what happens with cycling a pressure vessel hundreds or thousands of times, where the repeated stress may fatigue the material.... With steel, if you keep the stress applied to less than half the tensile strength, the cycle life is almost infinite.... Titanium is similar.... Aluminum has no such fatigue stress threshold, it continues to fatigue even if the applied stress is low, but at a much reduced rate.... In practical terms, if you keep the stress in most aluminum alloys below half the tensile strength, the cycle life is so great you can ignore it (perhaps millions of cycles)....
When a company designs a pressure vessel, and state a Maximum Safe Working Pressure (MSWP), they usually take into account a generous cycle life, and maybe even the possibility of some corrosion.... If you fill to a higher pressure, you are reducing the number of cycles to possible failiure, and that is the primary reason it is a bad idea.... You might get away with it once.... a hundred times.... or a thousand times.... but simply by using more pressure than the specified MSWP you are pushing it.... or at least shortening the safe lifespan....
Bob
And that's the big question Bob. I'm looking at having a titanium tube made at 25 5/8" inches for my rifle ( I know, I know. Why not put a bottle on it?). I want this rifle to look like a traditional marauder but under it's looks, have a bark/bite like no other. There's just so much custom parts on this rifle, it's not even funny. Anyway, that's the reason for wanting stronger screws. My max fill would put me between 3400-3600 psi but safety is my number one priority.
How would you go about tackling this Bob if you were to do it exactly the way I've mentioned. Am I missing anything. Depending on thickness of tube I might have to worry about sear engagement, fill assembly strength, bolt/hammer lug breaking if not fully closed etc..... Just trying to do this the right way.
Peter
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Thanks for sharing the test results. Very informational.
Have this test been done for the Mrod valve screws?
Peter
I did a test of the Marauder type screws in the new lightweight weight M-Rod air tube made by JSA. Even though this wasn't a test of the OEM M-Rod screws, indications are that the screws are more than up to the task. Here is that thread.
https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=138623.0)
I also did some testing of the OEM Disco screws and the upgraded alloy low profile SHCS that a lot of folks install in the Disco. rsterne referenced that particular thread earlier in this thread.
Lloyd
Thanks Lloyd. I did see that test that you performed. It showed the tube failing but the screws were still intact. I'm curious about there strength with this current test you did for the screws specifically. The reason for my asking is seeing what the max fill pressure these screws can withstand repeatedly.
Peter
The Marauder rifle is rated for a 3,000 psi max fill pressure by Crosman, and that should be observed. The screws will handle a lot more force than that before FAILING, but you should never operate any equipment close to its failure point. Allowing at least a 3.5 to one safety factor is a recommendation made by some engineering organizations. So, if the screws fail at a fill pressure of 10,500psi, they a safe maximum operating pressure is 3,000 psi.
I have never tested the Marauder screws to failure, and honestly, I do not know if they are the weak link in the pressure system of the Marauder. Something else might very well fail before the screws do.
Lloyd
Thank you very much for your responses Lloyd. I truly hope I'm not derailing your thread even though it's pertaining to valve screws of a different rifle? Sorry, if I am and will start a new thread if you would like me to do so.
Peter
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Peter, Yes, you've described a whole 'nother thread about air tube design. Send me a PM if you like.
Lloyd
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Peter, Yes, you've described a whole 'nother thread about air tube design. Send me a PM if you like.
Lloyd
Thanks Lloyd. My apologies. PM sent.
Peter
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There are a LOT of things to look at, from the tube strength to the threads at the ends, to how the valve is anchored, not only for the shear strength of the screws, but the forces they put on the tube wall.... Assuming the tube is up to the job, and that the ID and wall of the tube is the same as an MRod (and we should never assume).... I would simply add a 4th screw like I did in my latest (6mm) build.... Best to have Lloyd take a look at the whole reservoir as a SYSTEM.... As you say, there are many OTHER things to consider, and it is surprising how many people forget that, while focusing on just one item....
Bob
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There are a LOT of things to look at, from the tube strength to the threads at the ends, to how the valve is anchored, not only for the shear strength of the screws, but the forces they put on the tube wall.... Assuming the tube is up to the job, and that the ID and wall of the tube is the same as an MRod (and we should never assume).... I would simply add a 4th screw like I did in my latest (6mm) build.... Best to have Lloyd take a look at the whole reservoir as a SYSTEM.... As you say, there are many OTHER things to consider, and it is surprising how many people forget that, while focusing on just one item....
Bob
Bob , we often discuss the tube wall and its ability to withstand longitudinal load.. But , the stress lengthwise is calculated as a factor of 1/2 that of the hoop stress. My question is ( LLOyd might chime also ) , in what fashion would the failure occur if the tube wall gave in to the rearward pressure from the screws ?? For some reason , if the screws don't shear, I cannot see a catstrophic failure in this fashion,.. I may be way off, hence I'm asking
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The video where Lloyd tests the new aluminum tube from JSA will give you your answer.... Admittedly it happened early because of the proximity of the bottom screw hole to the sear slot.... but the screws plowed through the tube, ripping slots until the O-ring vented.... We call this type of failure "exceeding the bearing load" of the tube wall, and it shows up first as a buckling or stretching of the tube wall where the screw heads are pushing on it.... or as a "tearout (shear) failure" if the hole is too close to the end of the tube.... The failure in the aluminum tube Lloyd tested started as the latter (on the bottom hole), but ended as the former (on the other two holes)....
While you are correct that the londitudinal stress on the tube FROM THE AIR PRESSURE INSIDE is only half the hoop stress from that pressure.... the localized stress on the compression side of the holes in the tube is MUCH greater because the entire load on the valve is concentrated on just the area of the (three) holes....
Bob
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makes perfect sense.thx . I think the equations are better applied to scuba tanks and large propane tanks where the load is assumed symmetrical.. whereas , that 1000 plus lbs resting on .100 sq inches is certainly not as ideal. going to search Lloyds youtube now
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There are a LOT of things to look at, from the tube strength to the threads at the ends, to how the valve is anchored, not only for the shear strength of the screws, but the forces they put on the tube wall.... Assuming the tube is up to the job, and that the ID and wall of the tube is the same as an MRod (and we should never assume).... I would simply add a 4th screw like I did in my latest (6mm) build.... Best to have Lloyd take a look at the whole reservoir as a SYSTEM.... As you say, there are many OTHER things to consider, and it is surprising how many people forget that, while focusing on just one item....
Bob
Thanks a bunch Bob for you reply. As not to derail Lloyds original post, I'll start a new one.
Peter
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Ribbonstone sent me some high strength alloy steel socket head cap screws to test as a replacement for the OEM Beeman Chief valve retention screws. The new screws (M4-0.7 thd x 5mm long) appear to be about 2/3 stronger than the OEM screws. That is a significant improvement.
https://youtu.be/RZ9BJbaPlFY
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Excellent test and video, Lloyd.... So at the 2,000 psi MSWP recommended for the Chief, that puts the safety margin up from 2.5:1 with the stock valve screws, all the way to 4.2:1 with the high-tensile ones from McMaster Carr.... Personally, I would be much happier with that, it allows a much larger margin for fatigue.... We have no idea what the tube material on the Chief is, so compared to a Disco tube we still would not be OK using a higher fill pressure with the screw upgrade.... but those screws would provide a 3.5:1 safety margin, as used by the ASME, at 2,400 psi.... The nice thing is they are a "thread in" replacement for the originals.... and a better fit in the holes in the tube as well....
Anyone got a Beeman Chief tube they would like to donate to see it destroyed?....
Bob
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This is good news to any beeman chief owners.
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Here is a follow-up picture to the video, with the sheared OEM screws, and the high strength alloy steel socket heads. I found the 3rd broken socket head after I uploaded the picture.
There are a couple of additional things that are worth mentioning.
With the alloy steel screws, only 2 of them actually sheared off. The 3rd one easily screwed out of the tube and appears to still be in good condition, even when viewed under 2x magnification. After the first 2 screws sheared off and the tube shifted, the load released from the 3rd screw before it had a chance to shear off. You can see that happening if you watch the pressure gauge in the video. It is very important to note, however, that if the tube had been filled with air, the force would not have been released, and the 3rd screw would also have sheared. At least 99% sure about that.
The other thing worth mentioning is the shear failure mode of the pan head (Phillips head) screws. All of them sheared off at the deepest indentation of the Phillips recess, and the head of every screw has a hole through the center of it. To me, that shows that the actual SHEAR load bearing cross section was more of a tube shape, rather than a cylinder. When I tested the Crosman Discovery screws a few years ago, the same sort of failure was evident. I retested the Disco mock-up using Low Profile high strength alloy steel socket head screws (similar to what is used in the P-Rod), but because the test fixture had a smaller hydraulic jack on it, I was unable to run the test with all 3 Low Profile screws installed at the same time.
I am suspicious that the Low Profile heads might fail in the same manner that the Phillips heads did, albeit at a much higher force.
I think I will try some additional tests with the Low profile socket heads. I have them in both 8-32 and 10-32. Might possibly test some other screw types, too. Don Cothran asked me a while back about what sort of difference a stainless steel valve body might make. I honestly don't know if it would do better or worse, but I do have a hunch.
More tests to come.
Lloyd
(http://i226.photobucket.com/albums/dd79/loyd500/Beeman-after-test_zpslzghqhov.jpg)
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Lloyd
Very interesting info and results of screw types and failures.
Question,
Would the flat head socket screws with the tapered heads, counter sunk with part of the taper into the base material,
and the top of the head flush with the tube, greatly change the shear strength, relating to psi levels?
Tia,
Don
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.................................
Question,
Would the flat head socket screws with the tapered heads, counter sunk with part of the taper into the base material,
and the top of the head flush with the tube, greatly change the shear strength, relating to psi levels?
Tia,
Don
Don, that is a good question and I don't really have an answer. The taper would certainly change the dynamics of the loading and might be more likely to "shoot" a screw head at me when it fails.
I have some 10-32 x 3/8 flat head with 82 deg c'sk, with #2 Phillips, plain stainless steel.
Also have some 6-40 x 1/4 flat head with 82 deg c'sk, with a shallow .078" hex key socket, high strength alloy steel.
The tapered head on the 10-32 is about .125" tall, and on the 6-40 it's about .105" tall. The taper would give two disadvantages that I can think of, one is that it also puts the screw in tension, which might try to pull the screw out of the aluminum thread in the valve body, and the other is that the steel tube would act more like a shear knife on the screw. But I don't know. Both screw tapers are long enough to get all the way through a .065" wall tube, so there would be some taper embedded into the valve body. Maybe I can try some of those during the test, too. All the tests would be done on Discovery size tubes, 7/8" O.D. x .065" wall, with .740 dia valve body material, because I have lots of that.
Could possibly get a database going on the various screws and their shear characteristics.
Lloyd
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Lloyd
Very interesting info and results of screw types and failures.
Question,
Would the flat head socket screws with the tapered heads, counter sunk with part of the taper into the base material,
and the top of the head flush with the tube, greatly change the shear strength, relating to psi levels?
Tia,
Don
I think that I can shed some light on the Flat head Question. I built Wheelchairs lifts for Buses for 30 years, We used 3:1 for Static, and 6:1 for Dynamic saftey factors based on the Ultimate material strength. The problem with the Flat heads is the consistancy of the hole centerline to centerline. So instead of full contact around the head you end up with a bunch of fasteners with hight point or side load on each and every fastener. You are much better to screw the fastner into the valve and then try to shear the head off. Even with CNC Machining you are off just a little and then there is the issue of tool chatter on the angle portion of the interface.
Just my Humble opinion.
Michael :- )
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If you're looking for something to test (lol)...how about the effect of clamping force (torque on the fastener) on the shear failure point?
Or seat area...ie valve recess for the screw to seat upon vs no seat (no recess in the valve body)?
Al
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If you're looking for something to test (lol)...how about the effect of clamping force (torque on the fastener) on the shear failure point?
Or seat area...ie valve recess for the screw to seat upon vs no seat (no recess in the valve body)?
Al
OK, all you guys who are asking for more tests have got my attention.
I still have the test setup with the bigger bottle jack in it, but I will have to make up a bunch of test specimens and figure out just what screw configurations to test.
Here is a starting list, so please let me know if you have some other "must test" configurations. The tubes will all be .875 O.D. x .065 wall A513Ty5 cold drawn DOM tubing. This is the same tubing that I did the hydraulic burst test on and it made it to over 15,000 PSI.
For the plugs, I will use 6061T6 aluminum, because that seems to be fairly common for valves and plugs. I might try one or two with a steel plug, but I am not sure yet.
The test pieces will all have 2 fasteners 180 degrees apart so that the load is balanced, and it will also be much easier for me to make 2-hole specimens instead of 3 hole. The tube I.D. is a nominal .745 (.875-(2x .065)) and the aluminum plugs will have a nominal .735 dia to give .005 clearance on the radius, like is common practice for plugs with o-rings. For plugs that are made to test "clamping" force and how it affects the strength, the plugs will be more of a slip fit with about .0005" to .0020 total diametral clearance. That will be so that the tube does not get distorted out-of-round when the 2 screws are torqued tight.
Only test 5 will have the screws torqued to add clamping force.
1) 10-32 low profile socket head screw, high strength alloy steel. Slight spotface on the plug.
2) 8-32 low profile socket head screw, high strength alloy steel. Slight spotface on the plug.
3) 10-32 regular socket head screw, standard strength stainless steel. Full counterbore in the plug to leave .030" head height proud of the tube O.D.
4) 10-32 flat head screw, standard strength stainless steel. Full countersink to leave the head slightly proud of the tube O.D.
5) 8-32 socket head screw, standard strength stainless steel. Tube holes will be drilled to .170 dia give approx .006 clearance around the screw shank. Socket head will be torqued to proper spec.
Depending on how the specimens behave, further testing might be indicated, i.e., different screw configurations or steel center plug.
We shall see. Let me know if you think this will be worthwhile. Thanks.
Lloyd
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Sounds good, Lloyd.... looking forward to the results.... Too many guys think that just using "Stainless Steel" screws is the answer for high strength....
I wonder if there is any significant difference if you did "Test #2" with the screws barely snugged against the spot face, torqued to the proper value, and tightened as tight as you dare....
Bob
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Think you may have misunderstood what I meant. My thought was to test the additive effect of clamping force/friction of the screw head on the valve body. The thinking is that this clamping force has to be overcome before the body of the screw sees any shear force.
Al
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That is exactly what I suggested.... doing Test #2 with three different torque values of the screw against the valve body to see if there is any difference in the load required to shear them....
Bob
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I wonder if the Beeman Commander has all of the screws updated to a larger size and if the parts retrofit the old Chief? It's rated at 3000 psi.