RE: modal frequency, 12' 1/2-13 screw and buckling

I forgot to mention buckling. The screw will stay straight when the nut is
pulling it straight, but when putting the screw under compression it can
buckle. A long skinny column is subject to Euler buckling, this is pretty
straight forward stuff. for example see
http://physics.uwstout.edu/StatStr/Columns/colse61a.htm

with steel, 12'long .4" diameter, the buckling stress is 17 lb. If the nut
was loaded with over 17 lb while it was compressing the screw at full
extension, the screw would buckle. Bad, very bad. Control
acceleration/decceleration!

Further, if unsupported over 12', the .5" midspan deflection of the screw
would reduce the buckling stress much lower. Now it doesn't act like a
column, but rather like a very skinny piece of bent wire.

I'm starting to think that it may be necessary to tension the screw to give
it any axial strength at all.

Am not yet able to calculate load stiffening (tension) effects with FEA, but
hopefully in a couple of days problems will be cleared up.

If this is off topic, please let me know.

Elliot