Square captured nuts in sheet metal

[fortjp500]

I saw this at work today. It's a .812 square nut (.413" deep/tall) in a slot that measures 0.875. The material is mild steel 1/4" thick. Calculating the shear from the clamp load is pretty straightforward. I'm wondering more about let's say the square nut and bolt threads are seized, and you apply removal torque. How does one calculate the max torque that the square cut can rotate against the sheet metal before it yields? Is there a conventional method that is straightforward, or would you just use FEA?

 

[SWComposites]

by "it" you mean the when the sheet yields? seems like you could work out by hand the bearing surface and pressure due to the nut rotation and from that estimate the torque required to yield the sheet.
but this looks like a really crappy design.

 

[fortjp500]

Yes the sheet. There is only edge contact when the nut rotates on the edge of the sheet.

 

[3DDave]

It's a common detail of laser cut sheet metal and plastics. The bearing of the nut will be only on the corners of the sheet metal and only on the very end as any deflection of the sheet will unload the contact everywhere else.

I would guess the first thing to happen is that the friction from the nut bearing surface will be slightly less on one side than the other and that will walk the nut out of the slot or make the loading when it tries to turn and wedge very unsymmetrical.

The best approach is to prevent the nut from sticking; use a brass nut or brass bolt or use anaerobic locking compound of the lowest strength to lubricate a little on the way in and resist corrosion or go the whole way and slather it up with anti-seize compound. If it gets stuck then use a grinder to take the head off rather than

 

[rb1957]

well, it is a cheap design ... "crappy" is a little judgmental.

Accurately calculating the "maximum" capacity of pretty much anything is very difficult. In this case there'd be significant yielding and out-of-plane deformation ... presumably the sheet would fail (though possibly the nut could).

This is not intended for significant load. It does have the nice feature that an intelligent user would see the sheet deforming and probably stop torqueing the head. I imagine that it is meant to clamp another sheet under the head ?

 

[fortjp500]

well, it is a cheap design ... "crappy" is a little judgmental.

Accurately calculating the "maximum" capacity of pretty much anything is very difficult. In this case there'd be significant yielding and out-of-plane deformation ... presumably the sheet would fail (though possibly the nut could).

This is not intended for significant load. It does have the nice feature that an intelligent user would see the sheet deforming and probably stop torqueing the head. I imagine that it is meant to clamp another sheet under the head ?

Correct