Button Head Cap Screws 1

[drawoh]

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.

--
JHG

 

[Screwman1]

What standard are these parts being made to? The standard (if it's aerospace) will have the tensile strength for the parts, not just the mechanical properties. If they are not to an aerospace standard I would not fly them, there are some really poor quality imported button head commercial parts out there now.

 

[drawoh]

Screwman1,

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.

--
JHG

 

[Metaljon]

We do some aircraft work and when we get small fasteners we section and measure hardness. We also do chemical analysis to determine material grade. That is usually sufficient for any testing qualification.

 

[TVP]

The problem is that the depth of the recess is not specified in commercial standards, which means that you can end up with a very small section between the under head transition and the corner of the recess. This can be compounded by poor concentricity of the recess with respect to the outer diameter of the head. These type of fasteners should not be used in aircraft structures applications.

 

[hella]

Do tensile tests on a sample lot of fasteners.

 

[Duwe6]

"Do tensile tests on a sample lot of fasteners."

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.

 

[Screwman1]

The problem with doing tensile tests on a lot is that the spec for commercial button heads in SS won't keep them consistent so the next lot may be higher or lower. If they go on an aircraft they need to be to one of the aircraft specs., otherwise you are open to failures when the vendor resources the mfg. of the parts to China or India.

 

[racookpe1978]

Well you have to spec the fasteners to an aircraft spec (commercial aviation or MILSPEC/MS/AN spec) but then you have to demand that the receiving facility get that certification paper with each delivery.

Not just a fasteners-in-a-cardboard-box type of delivery from UPS.

 

[drawoh]

racookpe1978,

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.

--
JHG

 

[mfgenggear]

drawoh

? WTF

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[EdStainless]

Make up your own drawing, assume that the socket is deep, use the min tensile (30ski) and min area that you find and see if it looks like they will work.
Good button head screws have such shallow sockets that they are a pain to use, but they are strong.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[imcjoek]

The approval people seem to be accepting them. Fortunately, the the screws are stainless. It will be hard to counterfeiters to make something weaker.

Please paint this contraption pink, with orange stripes; so that I know not to board and/or stand underneath it.

 

[MintJulep]

See thread1103-314965 re calling out "18-8" for hardware.

 

[drawoh]

MintJulep,

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.

--
JHG

 

[mfgenggear]

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.

:>)


Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[Sparweb]

"The device is an "appliance" mounted inside the aircraft. It is not primary aircraft structure. "

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

 

[drawoh]

SparWeb,

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.

--
JHG