[ATM] The efficacy of RTV

["Scott Berfield" <sberfield@xbox.com>]

Just wanted to share a near horror story that has a happy ending due to
the bonding power of RTV.
 
My new (finished in December) 12" "X-Scope"
(http://www.berfield.com/X-Scope.html) was in my observing tent a few
weeks ago when we had some unseasonably hot weather. I had not used it
in a few days due to work schedules and laziness so I am unsure of when
it happened, but when I went out to the tent one weekend I nearly had a
heart attack. The lower struts had failed causing the tube segment
containing the mirror to fall approximately 3 feet to the ground where
it clearly bounced and rolled several feet before coming to rest. The
acrylic dust cover was shattered, and I was scared to look inside, but
the mirror (representing 10 months of work) was unmarked and still
firmly attached to the cell by the 6 blobs of RTV. The only other damage
was a scuff on the wooden end ring. Things could have been MUCH worse!
 
The struts had failed in two different ways. They are carbon fiber
pultruded tubes with threaded inserts bonded into the ends with epoxy.
The ball ends are simple hardwood beads drilled and tapped with 1/4"
bolts threaded and epoxied in. What I think happened was that the heat
caused the metal inserts to expand enough to crack one or two of the the
tubes slightly -- the inserts were able to work loose. One of the other
tubes failed when the threaded rod was able to tear out of the ball. I
think this happened as a secondary failure where once the mirror was
supported by fewer than the normal 8 struts, the stress was enough to
break the rod free. The solution was to re-do the lower struts. They now
have an extra wrap of carbon around the ends to prevent cracking, and
the threaded rods have been replaced by bolts that go all the way
through the ball ends. Tested these by hanging 20 lb weights from them
in a hot-house arrangement (my tool shed) for several days with
alternating cool-down periods. They now are very secure. I have also
replaced the acrylic cover with a polycarbonate shatter-resistant one.
 
The moral of all this is twofold -- One, it never hurts to over-design
for strength and it pays to pre-test. and Two, RTV when properly applied
is a miraculous substance!
 
Scott
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