On Sep 1, 2004, at 6:35 AM, Will Matney wrote:
Clive,
I used to be a sales engineer for Pandjiris, Inc. out of St. Louis. We
manufactured automated welding equipment and welding positioners. This
is something we researched in depth as we had to use brushes holding
welding current on many pieces of moving machinery. Mercury is hard to
contain, especially when it gets one bit hot (it will and can boil).
We tried using this in spindle bores of rotating positioners like our
competition did using shaft seals to hold the mercury. Of course if
something isn't done, the welding current will go through the
bearings, and thus ruin them quickly. Mercury has the problem of
expanding too much when it gets warm or hot thus blowing seals. This
in turn leads to leakage of the mercury and is a big no-no. We got
orders all the time from our competitions un-satisfied customers. Now
there's some companies who also make these "rotating grounds" but I
don't know how much they will or can guarantee no leakage. Carbon
brushes have disadvantages also. They are brittle, wear out quickly,
and heat up quickly.
-- Graphite brushes are self-lubricating, so they do not wear out as
quickly as carbon brushes. If the current burden is such that a brush
heats up, the brush is not large enough. Multiple graphite brushes are
used in 600hp series-wound DC motors used in diesel-electric railroad
locomotives. Brushes are just another maintenance item, like
spark-plugs and oil filters.
...
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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