[ATM] Aluminium or stainless steel?

["Gary McKenzie" <garymck@netspace.net.au>]

Hi Richard,
Thanks v. much for the info. Didn't realize the deflection would be so
high...

Basically, my idea was to use 2 50mm pillow blocks seperated by 20 inches.
These have an already made 50mm stainless shaft with a 175mm flange on the
end that has been welded and turned to be square. This was going to be the
ra axis of a german eq, however after using a gem for imaging I've decided I
want to be able to track through the meridian. My idea was to attach an
extension to the shaft, and then at the outer end fit the dec axis of an old
eq-5 mount to carry my C8, and then fit a large cable drive to the ra axis
down low. I would then be able to put the scope in any position without
hitting the mount.

This meant a mount that  I could use with a minimum of work (ie just make a
shaft, fit an already made dec axis and I'm up and running)

However, seems that I'd have to go to a much larger shaft to obtain good
performance - what sort of deflection do you thinhk would be acceptable?

Cheers
Gary




Hi Gary,

Saturday, May 7, 2005, 6:57:01 AM, you wrote:

GM> As part of a mount I am making I need to determine whether to use (from
GM> materials already at hand) a 50mm stainless steel shaft, or a piece of
GM> aluminium tube 85mm in diameter with walls either 5mm or 10mm thick.

GM> I need to be able to support a load of around 15kg at a distance of
500mm
GM> from the nearest bearing. One end is supported and the other end is
free.

Stiffest is the thick walled ally, next the thin walled ally,
worst is the stainless. I've assumed E=70GPa for the 6068 ally
and 180GPa for 304 stainless, but modulus of elasticity varies a
lot for stainless, from 110GPa up to 240GPa, depending on type.

Deflections :

50mm stainless : 0.113mm
85/5mm  ally   : 0.088mm
85/10mm ally   : 0.053mm

All three have small internal stress, but I can't comment on
suitability without knowing what deflection etc you are happy
with. Personally I'd want to do the sums on the bearing...

-- 

Best regards,
 Richard

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