Re: [ATM] Favorite Spider Designs

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

I wrote:
> Disadvantages I see are
> 1) Not as sturdy as metal vanes (if you whack it it may break)

Daniel Fundo wrote:
> I have made several and never had one break.  I use 0.013 inch stainless
> steel wire with a breaking strength of 220k pounds.  That works out to
> 40 pounds per wire.  I have tested the wire to more than twice that
> without a failure.

In general, we must admit that a wire spider is ultimately weaker than 
one with metal vanes because it is made of much less metal.  So, if 
something nasty happens to the scope, a wire spider is far more likely 
to break.  I'm talking catastrope - running the cage into your truss 
poles, etc.

I haven't broken one yet, despite hitting one with my elbow while 
unloading stuff from my car.  My Cassegrain spider has survived almost 
two years with no problems at all (even after the elbowing).

I have no doubt the spider is fairly strong when it is first made, 
since the wire is new.  However, with time and a rising number of 
thermal expansion/contraction cycles due to taking the scope outside 
and bringing it in, I wonder if the wire will develop fatigue cracks 
and eventually fail.  Guitar strings do.

If that will eventually happen, the simple solution is of course to 
restring the spider after a few years.

Mike Lockwood
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html

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