[Amps] conductivity problem

[Will Matney <craxd1@ezwv.com>]

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. What we 
done was make some copper brushes out of hard copper bar which were 
spring loaded in a holder. They were either turned in a lathe or 
machined on a mill so the faces were completely square with the mating 
surface. We used to mount this assembly on the trunnion of a welding 
positioner. They came into contact with the back side of the bull gear 
which was then attached to the machines table thus providing a ground. 
We also connected the ground or hot leads, which ever the application 
needed, directly to the copper brush(es). We also used coil springs to 
force the contact. Flat springs will break easily over time. You want 
kind of heavy springs for this too so the brush won't try to "hop" on 
rotation. You would be better to make these or have them made at a local 
machine shop as we (Pandjiris) were expensive. If your running the brush 
for any hot wires, you have to insulate the brush holder. This can be 
done using some micarda tubing or something similar like a glass/epoxy 
tube. 10 amperes at 230 Volts will take a good size brush (2300 watts). 
That's 65 amps at 35 volts which is a similar welding application. In a 
carbon brush, probably 1" wide X 1/2" thick for a long life application 
which is really not that long using carbon. If you could make one using 
a hard copper alloy, an aluminum or a phosphor bronze, it would last for 
ages. Plus, bronze and copper have the ability of acting like a bearing 
and are used in sleeve bearings all the time. The last thing is the 
other rotating body which the brush needs to make contact with should be 
extra smooth. A machined finish of around 50 micro inches would be a 
minimum and can be done on a lathe or mill without grinding the surface.

If your application already has a brush holder, try having some phosphor 
bronze brushes made to replace the old carbon brushes. That would be a 
big cost savings over making an assembly from scratch. I can say this, 
the brushes would almost outlast the contact surface and or actually 
wear a ring in it before replacement.

I could design the brush set here but you would have to retain someone 
else  to make it. I still have all the engineering info that Pandjiris 
came up with on this and could dig it out. If this will help, let me know.

Will Matney


Good day Gentlemen,
I am constructing a device that will permit the continuous rotation of a shaft which will have a circuit on one end that will require power at 230 volts ac.
I do not want to use conventional carbon brushes and slip rings and I have elected to go down the mercury annulus ring connections which will obviate the brush wear and need for replacement in a location it will be difficult to access.
My question is how do I calculate the current conductivity of the mercury to pass possibly ten amperes.  This calculation will have to take into account the ring "bath" of the liquid and the size of the electrodes that will pick up the current.
Additionally, does anyone have any information on the possible corrosion effects between mercury and other metals that I should steer clear of bearing in mind the electrodes to be used for picking up the power.
I am aware of the amalgam action of mercury with certain other metals but is there any sort of corrosive attack that might take place.
Thanks in advance for any answers and if it preserves the bandwidth of the reflector then I will look forward to your advice direct.
Regards,
Clive GW3WEQ 


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