You might try clamping some weight on the top of the column. That
will lower the column resonate frequency and may help. If it does,
filling the column with sand will help.

Do you stand on the same concrete as the tube is mounted on?

A rod or cable from the corners of the top plate to the corners of the
base will help a lot if it is the tube flexing. Compression or
tension on the aluminum won't make a difference, but adding to the
structure will.

It would be good to understand what is causing the vibration before
changing to steel. The problem may not be in the column flexing.
Maybe you could clamp a magnet to the column top and figure out a way
to support a coil so you can measure the vibration. We used a
geophone from a surplus store on the BPAA telescope.

Dan


domain (qedude) wrote in message news


I'm trying to kill a low frequency vibration (think a slow sway back and
forth). I agree that sand will kill the high frequency stuff but it's not going
to help in this situation. I'm having a hard time determining exactly what is
causing this vibration. It's not the mount as I can kill the power and there is
still a slow sway to the mount and it doesn't seem to die out. Either I have
created a really good pendulum or I may be getting a low frequency harmonic
from a highway about a 1/3 of a mile away.

I was thinking about running a tensioning rod from each corner down to the edge
of the concrete pier to triangulate the top and help add a little resistance to
the mount bending in tension.

If I can't solve the problem simply in the next couple of weeks I'll probably
just replace everything with steel. I have the steel but the aluminum came
available at the right price (scrap price) and I didn't have to spend the time
to sandblast and paint it like I will the steel. The only reason I am spending
time with it is that it is so close to working. If I can cut the size or the
error or move the frequency lower so that I can guide it out I'll be happy.

Scott