Re: Kilian's mount: Parts are getting purchased.

Bob Madden (madden@netcom.com)
Fri, 28 Apr 1995 18:27:45 -0700 (PDT)

Somehow I keep forgetting my structures lectures. It is true that stiffness is a function of ^4, but isnt the moment of inertia a function of the OD minus the ID which will make a tubular section be less stiff than a solid. It is just for stiffness to weight ratio that hollow sections become much better. I somehow have a feeling that this thread is implying hollow is the answer. There is a trade where hollow becomes best, but I have a difficulty with small diameters.

Bob

):-{])) <---- madden@netcom.com madden@svpal.org Remember amateur astronomers: "keep looking for the next Universe"

On Fri, 28 Apr 1995, Bob Luffel wrote:

> >
> > Bratislav, I liked your post on tube stiffness and the 4^ power. Now that
> > surely affects deflections purpendicular to the axis, but what about
> > torsional deflection and vibration when using stepper motors? I guess
> > there are 3 degrees of deflection, axial, longitudinal and torsional.
> > Just speaking about the shaft or tube, not the mount. Longitudinal is a
> > throw-a-way. Ah, the art of amateur telescope making can get into the art
> > of structural analysis, can it not?
> >
>
>
>
> The polar moment of inertia of a shaft (or tube) is also to the 4th power,
> so torsional stiffness is increased to the same degree as
> transverse bending.
>
> I have always wanted to drill out the center of my German Equatorial's
> stainless dec. shaft to lighten it, but drilling long straight holes can be
> a challenge (though gun-makers solved this one long ago...).
>
>
>
> Bob
>