From: brownp@cs.unc.edu (Peter Brown)
Newsgroups: rec.railroad
Subject: Re: compound locos (was Re: High Speed Rail Query)
Date: 7 Nov 92 22:29:18 GMT
	<1992Nov5.200421.14679@mintaka.lcs.mit.edu>
	<1992Nov6.064845.19703@sq.sq.com>
Organization: UNC Department of Computer Science
In-reply-to: msb@sq.sq.com's message of 6 Nov 92 06:48:45 GMT

In article <1992Nov6.064845.19703@sq.sq.com> msb@sq.sq.com (Mark Brader) writes:

[truly excellent discussion of various types of compound locomotives
mostly deleted]

I have only a couple of nitpicks:

   As has been noted, another arrangement was a 2-cylinder compound, with
   the high- and low-pressure cylinders on opposite sides.  The correct name
   for these is cross compounds.  They were the simplest of all types of
   compound, but probably had the least smooth action since steam was
   only drawn from the boiler twice per revolution of the wheels.

They also had the problem that it was difficult to get the forces from
the two cylinders properly balanced (boiler pressure varies, and
anyway the two pistons can't be the same size), resulting in
maintenance headaches (at least in American practice).  This may have
been a reason why the compound Garratts Mark mentions didn't use
cross-compounding.

   There were two types of compounds where the high- and low-pressure
   cylinders acted synchronously so that some parts could be shared
   between them; again, smoothness was traded for reduced complexity.
   One kind, already mentioned, had the two cylinders' pistons on a
   common piston rod, so the cylinders were one behind the other;
   this was called a tandem compound.  The other kind had the cylinders
   one above the other, with two piston rods driving onto a common
   crosshead.  *This* was the Vauclain compound, named for Sam Vauclain.
   In the Vauclain compound, and I expect the tandem also, a single
   casting would used for the pair of cylinders.

Actually, the tandem compound (high-pressure cylinders in front of
low-pressure cylinders) had the two sets of cylinders as separate
castings, at least in some cases.  The Santa Fe had some locomotives
of this type, which were not successful because of the difficulty of
renewing the packing between the two sets of cylinders; for this
process, the front set of cylinders had to be swung out of the way,
which clearly wouldn't have been possible had they been a single
casting.  (There may have been other maintenance tasks which required
this, too, but my source only mentions renewing the packing.)

Something Mark doesn't mention about the Vauclain is that the high-
and low-pressure cylinders also shared a common valve chest
(presumably rather more complex than a standard valve chest);
generally this was a piston valve (as opposed to the slide valve
standard in American practice in the 1890's), mounted inboard of the
high-pressure cylinder and actuated by inside valve gear.  You can see
the front of it pretty clearly in photos of the early Vauclain
compounds.

Source for all of this: Alfred W. Bruce, _The Steam Locomotive in
	America_, but I'm citing from memory, so mistakes could well
	be mine, not Bruce's.

The Vauclain-Mallet Mark describes sounds truly funky.

Peace,
--Peter
--
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Newsgroups: rec.railroad