T i m wrote:
On Tue, 28 Nov 2006 21:14:14 GMT, Guy King
wrote:

The message
from T i m contains these words:
But it all works (so far) but I wondered if I could replace the SS bar
with *tube* (of the same diameter) and would that better / (stiffer)?
No. Tube is stiffer per weight, but not stiffer per diameter. The only
way to make it stiffer is to use a stiffer metal or a larger diameter or
a different geometry.

Hmm, thanks 'guys' g and that makes sense (now I think about it).

Ok, on the geometry then (and this is probably a stupid thought) what
about a 'X' section inside a tube (I'm guessing it would still be a
function of the overall x sectional area)? This probably sounds like a
perpetual motion question ;-)

So, failing a 'yes' to the above, is there a simple formula (rule of
thumb) for how much 'stiffer' any particular steel would be for a
percentage increase in diameter please?

Say I go from 3/8 to 1/2" for example, would it be 25% or 33% or
something else stiffer?

Think its broadly proportional to diameter squared..
I.e. the load ends up at a greater diameter spread over a greater area..

Hard steel drill shaft is the least resistant to bending..usually
stainlesl too.

And a total pig to cut. No saw will dio it - needs a grinder.

All the best ..

T i m