[Amps] conductivity problem

[R.Measures]

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