Re: [ATM] Public Terrestrial Telescope: Re: Replies Regarding FocuserBearing Material

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

Your experience with shrinkage is a noted one.  What happens is
that the gear has been made too thin and the stress from the
shrinkage causes a failure of the plastic when the stress gets
too large.  One cure for that is to not put the gear on a metal
shaft but rather a plastic shaft of the same material which also
shrinks along with the gear.  The other is to make the gear thick
enough to withstand the stress until the center section cold
flows to the shaft.  Thus, a square shaft where the gear is
mounted loosely to the shaft is a lot better than a press fit to
the round shaft.  Even a D shaped shaft works a lot better than
the round shaft.

If you think that the shrinkage of Nylon and Delrin is bad over
time, you should see the shrinkage right out of the casting
machine!  What's worse  is that the casting temp, dwell time and
tpressure of the shot changes the actual shrinkage of the part
from the original mold size!  Even the amount of recycled plastic
makes a difference.
Bob May
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