Button Head Cap Screws
Button Head Cap Screws
(OP)
I am being asked to verify the structural integrity of an airborne structure. Some of the hardware is held on by button head cap screws.
I understand that button head and flat head cap screws are weaker than regular socket head cap screws, due to the heads shearing off around the socket. Is there a conversion somewhere that lets me predict the strength of a button head cap screw?
The Unbrako guide quotes screw torques calculated from proof stresses of 65,000psi on screws that, otherwise, are 170,000psi. They use 30,000psi on their stainless steel screws, which is the same value they have for their regular screws.
I understand that button head and flat head cap screws are weaker than regular socket head cap screws, due to the heads shearing off around the socket. Is there a conversion somewhere that lets me predict the strength of a button head cap screw?
The Unbrako guide quotes screw torques calculated from proof stresses of 65,000psi on screws that, otherwise, are 170,000psi. They use 30,000psi on their stainless steel screws, which is the same value they have for their regular screws.
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JHG





RE: Button Head Cap Screws
RE: Button Head Cap Screws
These are commercial quality button head cap screws in 18-8 stainless steel. The system is physically located in another office a long way away, so I cannot inspect it. I am hoping that they cannot mess up annealed stainless steel.
I did not design the thing. I am trying to explain it.
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JHG
RE: Button Head Cap Screws
RE: Button Head Cap Screws
RE: Button Head Cap Screws
RE: Button Head Cap Screws
When everybody gets done theorising, then it's time for you to sally forth into the Real World. Go and 'ask the parts'. Tensile tests of a sampling of your fasteners, maybe the chemical testing mentioned.
RE: Button Head Cap Screws
RE: Button Head Cap Screws
Not just a fasteners-in-a-cardboard-box type of delivery from UPS.
RE: Button Head Cap Screws
Fortunately, we are not making very many of these things. I am hoping I or some other person will get to re-design it before we make another one, and use the proper screws, along other things.
The approval people seem to be accepting them. Fortunately, the the screws are stainless. It will be hard to counterfeiters to make something weaker.
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JHG
RE: Button Head Cap Screws
? WTF
Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.
RE: Button Head Cap Screws
Good button head screws have such shallow sockets that they are a pain to use, but they are strong.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Button Head Cap Screws
Please paint this contraption pink, with orange stripes; so that I know not to board and/or stand underneath it.
RE: Button Head Cap Screws
RE: Button Head Cap Screws
I participated in that discussion. 18-8 is a good description of the screw, as per that discussion.
I have commercial stainless steel screws. I did my calculations with 30,000psi as my yield stress, as per the Unbrako manual. I have a reasonable safety factor. I ignored connections to nearby structures, to keep my calculations simple. This should make my results more conservative.
If I had assumed an ultimate stress of 170,000psi, or even Unbrako's reduced stress, I would have to worry about counterfeit screws.
The hardware sucks, but we are well within its limits. The device is an "applpiance" mounted inside the aircraft. It is not primary aircraft structure.
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JHG
RE: Button Head Cap Screws
Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.
RE: Button Head Cap Screws
Then why do the screws have to be stainless if they're indoors? Especially 18-8 stainless??
I just went through this for a customer, who wanted to replace some steel screws with stainless ones, and they were mounted externally. It was rather easy to prove that anyone can easily ruin the wrenching hex in the head, or even torque the head right off a 1/4" SHCS, if it's 18-8. Wouldn't that make you popular if it happens to the guy in the field? We stuck with steel, despite cosmetic corrosion.
If there still is a compelling reason to use stainless screws, then look up the ANSI B## something spec that they should conform to, and inspect for quality/conformance. Inside the cabin, your appliance becomes a potential missile, and needs to be restrained against all flight and emergency landing loads. If the appliance weighs 3 pounds, then maybe there isn't as much to worry about. If it weighs 30 pounds, on the other hand...
STF
RE: Button Head Cap Screws
It is an optical system, and we are in the habit of using stainless. This thing flies at low altitude over the ocean, as well, so rust is an issue.
Given the iffy type of screw we are using, I don't want to make any assumptions that might prove to be wrong. If I assume 18-8 stainless with a yield of 30,000psi, I won't get any surprises when this thing gets sourced from China. The screws are large by our standards, 5-16-24UNF, so there is lots of strength. When I get to design stuff like this, I use lots of 8-32UNC or 10-32UNF screws, usually MS27039 series.
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JHG