On Mon, 20 Apr 2015 10:42:34 -0400, "Jim Wilkins"
wrote:

"Jim Wilkins" wrote in message
...
"Aristatos"
wrote in message
roups.com...
replying to Jman, Aristatos wrote:
Jman wrote:

I need a steel rod/s. precision ground, polished, turned, heat
treated
whatever
is most effective. 3/8" OD and a length of 4'. I need to know
what kind of
steel (1144, Stressproff, 4140 TGPHT would have the least amount
of
sagging,
twisting, and flexing (wasn't sure on the scientific terms for
these). Can
you
help me out with that one? Or maybe point me to a graph or an
equation to
work
it out?
Thanks in advance for your time,
Jeremy
p.s in the future I will need an eight or 10 foot rod



The sagging could be from a weight of 50 pounds to 100 pounds,
twisting
would be
from high torque stepper motors

thanks again people, just slightly confusing with the different
numbers
and what
not. And I should have specified the sagging from load, not
length.

thanks again, I was worried no one would answer, new here.

thanks


Stronger or harder alloys simply follow the same line further up
before permanently deforming, at the point where the lines turn
toward horizontal. This is why greater strength or hardness doesn't
provide greater stiffness.


The Wiki describes elastic behavior in more detail, with examples:
http://en.wikipedia.org/wiki/Young%27s_modulus

It doesn't mention that the values are the force per unit area
required to stretch the sample by 100%, or to twice its length, which
can't be done in practice since it would break or become thinner,
decreasing the area.
http://en.wikipedia.org/wiki/Talk%3AYoung's_modulus
"it may be better to leave it out, to avoid confusing people, or at
least putting it down the bottom in some sort of "vaguely interesting
but complicated stuff" section, well away from the main definitons."

Some common materials like Tungsten are stiffer than steel, but also
heavier so they'd sag about the same if held horizontally. Beryllium
would be a great structural material if it wasn't poisonous.

You could try gently and unobstrusively bending a 3/8" steel rod in a
hardware store to get a hands-on feel for how stiff steel is, and
isn't.

-jsw


Coat hangers are good for that, too. They're what I use to demonstrate
the principle to people. When you're talking about how even the
crappiest steel has the same elastic properties as, say, 200,000
psi-yield music wire of the same diameter, it makes a good demo. Steel
doesn't come much crappier than coat hangers.

--
Ed Huntress