>Does anyone have any drawings or details of the single cup resin balance (I
>believe John Tye built and used one).
>I have the article which appeared in "The Europa Flyer" (issue no. 12)
The photos I took of Jon's balance at one of the seminars were originally
just for my own interest, kind of visual notepad. I hoped the one I later
picked for publication with the theory article might show enough detail to
allow construction of a replica, but perhaps not.
I've still not constructed my own version of this balance, so this posting
is THEORY. If you get an answer from someone who has done the PRACTICE,
that could be a lot more reliable!
I'm embarrassed to say I can't immediately put my hands on the original
prints, so what follows is based on the published picture. The materials &
methods of construction are not really important as long as a few
principles are observed. The main beam should be stiff and light. Jon's was
a piece of wood (ply?) with lightening holes. An aluminium extrusion would
be OK, provided you had the resources to attach the other parts to it. The
pivots are the heart of any balance, and should be as free of friction as
possible, and level in the transverse axis (front to back). Looks like
Jon's pivot was just a spindle through a block of wood attached to the
beam, but it might have had ball races on it. The whole thing was supported
on a chipboard base with 2 uprights at the rear. The beam pivot was on the
left-hand one, and the right-hand one had a couple of stops to limit beam
movement. A pointer on the beam lined up with a fixed mark on this upright
when the beam was in balance. The trunnion supporting the resin cup
platform was liberated from something like a hamster wheel or some sort of
child's toy. A chunk of lead (folded sheet) rested on the beam at the left
end for initial balancing, and the pegs for the balance weights (nuts) look
a bit like what you'd get if you knocked in some 6-inch nails and took the
heads off them. They should be at right angles to the beam axis, and could
be vertical as in the theory illustrations rather than horizontal. Probably
best to keep everything close to the central axis of the beam, so as not to
give the pivots a hard time.
All sounds a bit agricultural, but you can make it as posh as you like
without improving the performance significantly. And you can't argue with
the quality of Jon's results!
A favourite old method of getting low-friction pivots is to mount a couple
of razor-blades with their edges up AND PARALLEL. Then make sure they're
level and lay a steel rod across them as the axle of your beam. Pretty good
for low-mass applications (eg static balancing of model aeroplane
propellers), but not sure if it would stand up to continued use with this
design. Some sort of knife-edge is probably the easiest way to get low
friction at low cost, and the knife-edges can be made replaceable if wear
is a problem. Getting & keeping the whole thing at right angles to your
local gravity field could be problematic with 2 separate knife-edges.
Hope this helps
Rowland
... that's Rowland with a 'w' ...
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