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re: bolt bending

re: bolt bending

(OP)
I usually use the basic minor diameter for computing the section modulus of a bolt in bending. It's the smallest diameter, and thus conservative. But is there perhaps a way to be less conservative?

Has anybody else used the effective diameter that can be back-calculated from the stress area of a bolt in tension, and used that for computing bolt section modulus?

RE: re: bolt bending

You can use the shank diameter, if the assembly is designed so that only the shank is subject to bending.

If you're bending the threaded part, I wouldn't use the tensile area, because you don't know where the effective section centroid is relative to the centerline, in general, at any point of interest. I.e., the section modulus sort of wobbles along the length of the thread, as the centroid follows a helix, going both above and below the wrapped centerline.

Can you explore uncommon thread forms, e.g. those with large root radii, or just use a larger bolt?


Mike Halloran
Pembroke Pines, FL, USA

RE: re: bolt bending

(OP)
I'm thinking I probably need to bring up an issue with a legacy design at my company, b/c somewhere along the line, someone decided that loading 100 lbs on a threaded rod in bending was a perfectly good idea. Mind you, it's also a 300 series stainless steel threaded rod as well.

Thanks for the inputs!

RE: re: bolt bending

I have to ask: Have you gotten away with it so far?

Mike Halloran
Pembroke Pines, FL, USA

RE: re: bolt bending

Hi glulambeam

When I do bolted joints in bending, I assume the bolts are infinitely stiff and calculate the bending stresses as increased tensile forces on the bolt see link below and scroll till you see the heading bolts withstanding bending forces:-

http://www.roymech.co.uk/Useful_Tables/Screws/Bolt...

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: re: bolt bending

glulambeam,

Bending the threaded section of a bolt is not good design practice. If I were analyzing this, I would use the minor diameter for my bending and stress calculation because it is conservative, as you note. Also, I would consider that the bottom of the thread acts as a stress concentration, making a conservative analysis value that much more important.

--
JHG

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