[PRCo] carbon and steel inserts

[Jim Holland]

Below is a forward to the San Francisco Muni History email group which 
apparently came from the trolleycoach group Ken mentioned (forward by 
Richard DeArmond who runs that group)  FYI::

>Here is a part of a discussion on steel and carbon inserts from the 
>etb chat group, which I run:
>
>"I'm not in the line dept. here at IRM but I can offer some observations.
>There is specialwork that is made for wheels, some that is made for shoes,
>and some for both.  Using the wrong kind for the application will result
>in arcing, broken parts, and generally bad performance (e.g. dewirements).
>A city that has a big investment in wheel-type specialwork up in the sky
>is not going to throw it out before they have to, even considering the
>better performance and economy of shoe operation.
>
>"Regarding shoes, there are two types, which don't cooperate well.  They
>are steel and carbon-insert.  The steel shoe, which was the first
>developed, wears the wire a little because it keeps a clean copper surface
>on the bottom.  The use of graphite lubricators does help with this, but
>the fact of the matter is that steel shoes keep the wire pretty much bare.
>Carbon-insert shoes use a bronze holder.  There is a well-known chemical
>reaction, called "filming", that occurs when copper and carbon slide past
>each other in the presence of water vapor and electrical current.  This is
>the chocolate-brown coloration that motor commutators take on after a
>while of use.  This film is low in friction, hard, and electrically
>conductive.  Just the perfect thing for keeping wear on the copper to a
>minimum, whether it's wire or commutator.  After a while of use, the wire
>*and* the carbons wear very little, but until the film is established, the
>copper surface will wear away a carbon quickly.  Now, remember what I said
>about steel shoes, how they keep the surface clean and bare?  That's why
>steel and carbon-insert shoes don't cooperate well.
>
>"In Milwaukee, some trolley coaches that were assigned to routes that
>shared wire with streetcars used one steel and one carbon-insert shoe-
>steel on the (shared) positive, carbon-insert on the (TT only) return.  I
>don't know how many, how long, or other such details, but that's how they
>interoperated."   (Richard Schauer)
>
Jim Holland