Re: ATM Spider hub twisting (was: Aluminium truss structures)

[Aplanatic@aol.com]

Tom,

The difference in the two spider configurations is simple.  In 
the + case rotation of the secondary causes a *second order* 
change in the length of the vanes with rotation, and in the 
rectangular case ( >< ), rotation causes a *first order* change 
in the length of the vanes.

Since most of the resistance to rotation should come from 
tension in the vanes, a first order effect will be much better 
(stiffer) than a second order one.

This is a very general principle that can be applied to many 
situations that come up in designing telescope mechanics, 
i.e., try to use members in tension or compression instead 
of relying on bending strength.

Dave Rowe.