John2005 wrote:
Hello everyone,

I would like to ask if anyone could please provide some feedback on the
following situation.

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.

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?

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

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.

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.

My guess is that a 5/16-32 thread would stil provide the best holding
power.

I would appreciate any opinions on the best thread diameter and pitch
to use. Unfortunately, I cannot just go with a longer thread.

Thanks
John

Going to a smaller thread will only make it weaker. The threads will
pull out easier under the resulting tension component and the smaller
diameter where it meets the 3/8 diameter is where it will likely fail.
Extra fine threads will not help the pull resistance in the tension
component but the the larger minor diameter will help somewhat at the
junction with the 3/8 diameter. I would not go crazy with and extra fine
thread unless you happen to have the tap and die. Since you are limited
by the rod material the best thing you could do is to drill and tap the
rod as deep as possible and use a extra high strength set screw. The
wall will be only 1/32 but its material is the right place being
farthest away from the neutral axis. You might want drop down to a
1/4-28 screw to give you a little extra wall.