[TheForge] Which Drill Rod?

[Bruce .]

Dave,

I don't doubt your expertise, but my intuition (like Pete's and Jerry's) is
screaming otherwise!

I suspect I know why:  Take a dead soft wire -- it bends real easy.  Take a
hardened music wire of the same diameter -- it doesn't bend nearly so
easily.  What's the catch? -- We typically exceed the elastic limit of the
dead soft wire -- very easy to do.  I learned relatively recently that it
is not so hard to exceed the elastic limit of hardened music wire:  Springs
are wound from hardened wire, not from the annealed wire!

Anyway, I may still need hardened wire or rod, but now I can calculate that
a 3/16" rod will be five times the stiffness of the 1/8" rod.  This may not
help for the pulley shaft, unless I choose to drill out all the pulleys,
but it should do fine for the other application.

I plan to start with what's on hand for this latter case and see whether a
problem develops.  McMaster won't be open till Monday anyway.  (Eat your
hearts out, those of you who can't "will call" from McMaster!)

Bruce

Bruce
NJ


On Sat, Nov 30, 2013 at 3:42 PM, David E. Smucker
<davesmucker at hotmail.com>wrote:

>
>
> Jerry,  The flexing of a piece of steel, in this example heat-treated O1
> vs.
> as purchase O1 is the same -- it depends only on the modulus of the
> material
> -- not the strength.  The heat-treated will be much stronger but have no
> difference in stiffness.  Now if this "shaft" was heat-treated 2024
> aluminum
> it would be roughly as strong as the annealed O1 but only 1/3 the stiffness
> because aluminum has a modulus 1/3 that of steel.  It is very important not
> to confuse modulus of elasticity with strength.  OK what has a higher
> modulus than steel?  Tungsten is about twice that of steel, so the
> suggestion to make the shaft from a 1/8 dia tungsten TIG welding electrode
> is a great idea.
>
> Of course the cheapest way to make a stiffer shaft is to increase its
> diameter.  The polar moment of inertia goes up by the 4th power.  So a 1/4
> inch shaft of the same width and same loading is 16 times stiffer than a
> 1/8
> shaft.  It is also stronger but that goes up only by the cross sectional
> area.
>
> Detailed design of such shafts is basic to many machine designs and is key
> for example to the design of a gearbox where shaft deflection under load is
> a huge factor in the mesh and wear of the gear teeth.
>
> Dave Smucker
> Brasstown, NC
>
>