Re: [ATM] Wire Spider Vibrations

["Bruce Sayre" <sayre@foothill.net>]

See http://www.foothill.net/~sayre/22-in.%20binocular%202.htm#Top%20end for
a description of wire spiders used on my 22" binocular.

I originally used 0.02" spring temper stainless steel wire. Spring temper
wire (or music wire) has the most resistance to bending, which I thought
would be helpful in keeping the wires straight under lower tension.
Unfortunately, it's also easier to break or kink. During telescope transport
a few years ago, vertical jiggling caused two breaks in the wire.

I changed to 0.040" wire, which solved the problem. I think 0.032" diameter
would be a good compromise between resistance to breaking and shadow
profile.

In my design, there are effectively four strands going from each of the
three outer attachment points on the top end, threaded through six
attachment points on the hub. The hub's holes are rounded to minimize the
need for extreme wire bending. This arrangement completely eliminates all
axial rotational vibration. Compared to Mel's offset hub, this construction
seems like overkill, but I had to live with my legacy hub from earlier
spider vane days. 

Serendipitously, I threaded the wire so that the old hubs provided offset
attachment points. I think you could use the same approach with any
commercial three-vane spider: cut off the vanes but leave a stub for a pair
of holes. Thread wire through the holes, and -- voilà! -- an offset hub wire
spider.

Longitudinal (up and down) diagonal motion in my spider has to be eliminated
by tightening the "web" to increase wire tension. Mel's crisscrossed
attachment seems intuitively likely to reduce this motion, but some amount
of tension is still required to stretch out the strands. Mel also avoided
wire breaks around his hub's bolts by using a softer wire.  

My wire is not attached at the hubs, which makes positioning the diagonal
with respect to the mechanical axis of the tube easy. Once I slide the
diagonal assembly into position and equalize the tension, I tighten the open
end acorn nuts to clamp the wire against the hubs. (Being able to locate the
diagonal axis arbitrarily is a plus for my binocular because it's an
effective way to adjust the eyepiece focusers so they are aligned parallel
to the ground.)

Spring steel and almost any other kind of wire you can imagine can be
purchased from McMaster-Carr  (http://www.mcmaster.com/); pages 3511-3512 of
their online catalog. Looking at their catalog is also a useful way to learn
about wire. The parts I have mentioned are 89085K14 (0.032") and 89085K15
(0.04").  
 
Bruce Sayre
mailto:sayre@foothill.net
http://www.foothill.net/~sayre


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