Re: Questions regarding thread diameter and pitch for special design case wtih limited thread length

John2005 wrote:

I have a 3/8" diameter steel rod that is threaded horizontally into a
vertical steel support at one end, and that has a moderate vertical
load applied to the rod at the other end, 5" or 6" from the support.
The load tends to deform, pry or bend the threads out of the support.

The threaded end of the 3/8" rod necks down to a 5/16 diameter by 24
pitch thread, and the necked down portion that the threads are applied
to, can only be .183" long Maximum. The actual threads won't quite be
.183" long because they probably cannot go completely flush to where
the rod necks down.


You should make the threaded part a couple threads longer than
mating hole depth to allow for tolerances and so the necked area
won't jam in the hole.

The threaded end of the rod is screwed into the steel support until it
bottoms out, right where the rod necks down...

(3/8 - 5/16 = .0625 / 2 = .03125) so there is a 1/32" wide portion of
the rod OD that bottoms out on the steel support.

I want the threaded connection to be as strong as possible so the rod
supports the largest load possible. Would I be better off going with an
extra fine 5/16 diameter X 32 pitch thread?


Finer thread would be better, but with decent strength material
you'll have plenty of capability for most applications.   5/16-24
thread with 50 ksi material can carry over 2000 lb in shear.

Are there any disadvantages of using extra fine threads that I should
be concerned with?


It would be easier to cross thread the smaller threads.

Since the rod is basically cantilever mounted horizontally, and the
loads are vertical, it seems that the amount of material that bottoms
out on the steel support may effect things.


The threaded part is in bending and shear either way.

For example, if I use a 3/16" diameter thread, a 10-32 thread, or a
1/4" diameter thread, then this produces more "neck down" on the 3/8"
OD rod than the 5/16" diameter thread, and this provides more surface
area to "bottom out" on the steel support.


That's not a good idea to try and count on that extra surface
area.  Depending on fit and tolerances, it may not be there.
And if the rod does bottom out, you won't be able to remove it
easily if needed.
.

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