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Structural vs Non-structural machine screws: Does it really matter?

Tandem46

Well Known Member
Looking at machine screws on Aircraft Spruce, they list:

MS24693C Non structural stainless machine screws - 55,000 PSI
MS24694C Structual stainless machine screws - 85,000 PSI

Ok, big difference of 30,000 PSI. But 55,000 PSI seems more than sufficient. When I think of all the screws on the RV, I cannot remotely imagine any of them being subjected to anywhere near 55,000 PSI. What am I missing? Mainly asking out of curiosity.

As an aside, what would one use the MS51959? Other than being 82? vs 100? for the 24693/26694, they look the same.
 
Structural

You have to consider the shear load on the bearing surface area.

For a very simplistic 2D example; look at the cross sectional area of an AN3 bolt that is bearing on a .032 sheet. The bolt is .187 in diameter and it contacts the sheet which is .032. The contact area is .187 x .032, or .005984 in^2.

If you apply a 300 pound shear force to that bolt and it can act only on the contact surface, you arrive at 300 lb / .005984 in^2 or 50,133 PSI.
 
Looking at machine screws on Aircraft Spruce, they list:

MS24693C Non structural stainless machine screws - 55,000 PSI
MS24694C Structual stainless machine screws - 85,000 PSI

Ok, big difference of 30,000 PSI. But 55,000 PSI seems more than sufficient. When I think of all the screws on the RV, I cannot remotely imagine any of them being subjected to anywhere near 55,000 PSI.

SNIP

What do you base that on? The stress threaded fasteners are exposed to during proper torquing is much higher than you might be imagining. In many applications, the highest stress a bolt sees is during torquing.
 
Ok, big difference of 30,000 PSI. But 55,000 PSI seems more than sufficient. When I think of all the screws on the RV, I cannot remotely imagine any of them being subjected to anywhere near 55,000 PSI. What am I missing?

You are missing the fatigue threshold behavior of steel. A correctly torqued AN3 bolt, for instance, sees ~40 ksi of stretch. The number of cycles the two grades of steel can take under that load differs by orders of magnitude.

My latest kit does include a number of non-structural fasteners, but they are mostly for attaching plexiglass and fairings where they are torqued "by feel" with a screwdriver.
 
If you don't know what hardware to use, check your plans or contact Van's support. In some cases it can be critical. Please don't attempt to do your own stress analysis - speaking professionally for a moment, it's considerably more involved than it seems.

I don't use either of these. The heads get buggered up too easily. I use at least 125 ksi fasteners and prefer 160 ksi ones just for the ability to avoid those hassles. In every case that I've seen so far, these are equal to or exceed the strength of the fasteners specified in the plans. Do try to avoid 180 ksi or above, though.

There are several sources for the strength data. The gold standard is the spec itself. Do a search for that. Aircraft Spruce often lists the strength and so does General Aviation Hardware.

Dave
An old retired aerospace structural engineer
 
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