Re: [ATM] Betr.: Re: Wire Spider

["Bob May" <rmay@nethere.com>]

Sheeesh!!! You guys are all wrong!  You need to use the top piano
wire and tune the stretched wires to middle A!
Get up to B and you're too tight and so on.
I'll note for real that you do need to have the wires tight but
you don't need to be able to vibrate them to any high frequency
as once the bends are out of the wire, the secondary won't be
going anywhere unless the outer ring is flexible.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Harro Treur <Harro.Treur@waternet.nl>
To: <atm@atmlist.net>; Mark Cowan <toolontop@yahoo.com>
Sent: Tuesday, May 27, 2008 12:09 AM
Subject: [ATM] Betr.: Re: Wire Spider


> HA ha ha,
> do i notice some people have  hard feelings dismantling a
guitar?
>
> Off course 6 tensioners can be regarded overkill.
> But for me: they were available at the time. Furthermore I am
happy with this freedom of adjustability.
> The weight of the 6 little  banjotensioners is 70 grams
together.
>
> One note: the angle of the wires can be surprisingly shallow on
a 12 inch.
> But i can imagine that a 20inch with heavy secondary is another
story and requires a more steep angle, like Mel Bartel's.
>
> Harro
>
>
>
>
>
> >>> Mark Cowan <toolontop@yahoo.com> 08:52 Dinsdag 27 Mei 2008
>>>
> > Everytime this subject comes up, I cringe, because I know I
will see another
> > arrangement with guitar tuners on it.
>
> You read my mind!  Guitar tuners / banjo tuners / any kind of
machine head is
> overkill several-fold.  It's easy to make a more than adequate
tensioning
> device from about $.50 worth of hardware, and it's a whole lot
lighter.
>
> I've been at this a while, and posted a method for building a
simple low-cost
> tensioned wire spider on this list back in October of '95:
> http://astro.umsystem.edu/atm/ARCHIVES/OCT95/msg00164.html
>
> This has worked beautifully on the 8" f/6 I described,
apparently influencing a
> number of people who built their own versions. ;)
>
> For single ring upper cages, as on a 14" f/4.7 TriDob variant
I'm working on
> (again), you can only get span at the edge by using spreaders
like Mel did on
> his 20".  You don't _need_ span at the edges in smaller sizes,
but the included
> angle to the secondary hub from a single attachment point on
the edge doesn't
> scale fast enough to be stable at larger sizes.  I suspect
that's why Mel
> starts from a span and crosses the wires, effectively doubling
the angle, which
> doubles (at least) the stability.  That crossing point could be
brazed to
> improve it even more...
>
> To all - study Mel's implementation a while and read how it's
constructed.
> Note that he built a jig to hold the secondary assembly in the
correct position
> while installing the wire - but after that no adjustment should
be needed,
> right?
>
> Presently I'm trying to determine conclusively whether 3 spokes
(in-and-out)
> can function as well as the 4 spoke "bowtie" version that's the
leader in my
> full size mockup.  I still have minor longitudinal vibration in
the latter but
> that's it.   I just _want_ to use 3 spokes for symmetry, but
I'll adopt
> whatever proves out the best.
>
> So one thing I'm working with presently for this design is
using the synthetic
> Vectran instead of wire, a fiber stronger than Kevlar (and
steel as well) with
> zero creep. If there are thermal issues with wire (which I
doubt) this won't
> have 'em.  I'm about to string up a continuous version of this
to see if it
> does a better job with the vibration issues.  I'll report back
on that shortly,
> as to how it works, if it works, or if it doesn't.  No easy way
to pass current
> along it though...
>
> And Dan, could you explain this a bit more, maybe with a
picture?  It sounds
> like the "bowtie" I mention but...how many spokes? How many
spikes? Thanks!
>
> > Another point, the most stable arrangement is the bicycle
spoke one, i.e. 2
> > wires connected to 4 points at the center (on the secondary)
and 4 at the
> > outer edge.  The 4 at the center are aligned on a plane
parallel to and on
> > the optical axis.
> >
>
> Best,
> Mark
>
>
>
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