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

[PeterL]

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

[lloyd-ss]

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.

[rsterne]

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

[oldpro]

 This is good news to any beeman chief owners.

[lloyd-ss]

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

[Nvreloader]

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

[lloyd-ss]

.................................

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

[int3man]

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 :- )

[Gippeto]

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

[lloyd-ss]

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

[rsterne]

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

[Gippeto]

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

[rsterne]

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

[Eddie_E]

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.