something like equivalent to a 10,000 psi solid rivet.
The problem with machine screws is the tremendous weakening at the
thread root due to stress concentration. For a little #6 screw I'd guess
that it magnifies the stress a couple of times higher than you might
guess from just taking the minimum cross section of the screw. There is
not as much magnification with a larger #8 so someone decided #8 is the
smallest size to be called "structural" which is a good rule of thumb.
Then there is the effect of threads cutting into aluminum when subjected
to shear which can result in a lot of movement if the loads are
reversing. If it is only the instrument panel in plain sight, I
wouldn't worry about it but one can always rivet and bolt a 3 part test
section up and pull it apart to see what happens. It might be like the
countersunk rivets where material thickness has a large effect on how it
fails.
I agree that it is probably just as well to avoid powder coating
structural aluminum . I mentioned once that I hold aluminum in bare
hands (no gloves) when power sanding it for the same reason. Too hot to
hold means let it cool as far as I'm concerned.
Ken
Dave Ricker wrote:
Hi Guys
I'm putting a lot of work experience to good use tonight, let's talk about
stainless steel screws and rivets now ;-)
OK, seriously, we use a lot of stainless fasteners to on that (shipboard) crane
I talked about in my other mail this evening and the material strength (yield
strength, not breaking) we use for the fasteners is the annealed (there's that
word again!) strength which for stainless steel is about 30,000 psi. We use
that value unless we buy "rated" strength fasteners because you can't depend on
it being anything greater.
Without doing any numbers, I expect that #6 screws would be stronger than the
rivits in the condition which I understand that MAM uses for their calculations
(ie without the mandrel for strength). The aluminum is pretty soft in those
rivets, the breaking stress is probably the 25 - 30, 000 psi range with yield
probably down in the 10 - 15,000 psi range. If you replace the rivets with
screws at one for one than I can't imagine it being less robust.
The only thing I might do differently would be to use machine screws vs sheet
metal screws (if I understood that right Eric), they'd have a better pull-out.
FWIW, you probably have enough that it doesn't matter but it's worth keepin an
eye on them to make sure none loosen.
Cheers,
Dave
Eric Fogelin wrote:
I used #6 stainless sheet metal machine screws in nutplates using the same
hole spacing specified in the manual for the rivets.
That is a lot of screws. Dick Wampach wrote in a separate response that
these should be structural.
I don't have the engineering background, so take these comments with a grain
of salt and check further before you go with screws.
As I recall the shear strength of the 1/8" avex rivets we use are about
25-30,000 psi. I think that a #6 stainless machine screw is higher.
If true, you could use fewer screws, save weight and time installing nut
plates. You should use nutplates as everyone has mentioned, the panel is
structural.
At the end of the day, I think fabricating, painting your panel and testing
all of your avionics and instruments prior to final panel install is 90% of
the benefit. If you screw in place, you may get a little benefit in the
future if you make massive changes. If you don't think you will make big
changes, don't do the nutplate work and just final rivet when everything
checks out.
Eric
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