>He also ran into a similar issue trying to compound his RH-based
>mallet. As I see it the problem with building a compound out of two
>identical engines is simple. Once you get the loco moving the rear set
>will be pushing normally while the fronts will be acting as dead weight
>due to not being able to produce enough power from the cooled steam. The
>steam leaves the rear set at a lower presure and temperature than it
>enters (else the loco doesn't move) and then it needs to do the same at
>the front set. If you're running identical bore/stroke on both sets of
>running gear, the front set looses power when compaired to the rear set
>and doesn't do much in the way of pulling. When you account for friction
>(and that the Ruby isn't all that strong to begin with, well mine
>isn't...) this may explain why it's not working on steam but is fine on
>air. Giving the rears a higher presure and letting the steam expand into
>the fronts (which need to have more piston to push on... and more room to
>fill) may well allow this setup to work.
>
>I get the feeling it's just that we gauge one'ers don't normally run high
>enough boiler pressure to make this system work, but I don't
>know. :) I'm thinking 50-60 PSI should work.
>
Not only do you need different sized cylinders for compounding to work, you
need a good superheater as well as higher pressure. I suspect the Ruby has
no superheater and though I don't know about the RH, I would suspect it does
not when something as expensive (and as well thought of) as the Aster Mikado
is also totally lacking in that department.
If you look at the previous generation of designers and writers (I am
thinking of Martin Lewis, Martin Evans, LBSC, and Henry Greenly in
particular) you will find that they all used 80-100 psi (and even 120 psi in
a few cases) in everything from 1/4" to 1 1/2 " scale. Martin Evans, for
example, calls for 100 psi in his O gauge Royal Scot.
All these designers (with the exception of Lewis) also used superheaters
even in the smallest gauges. The only exception that I actually have in my
possession is in the blueprints for LBSC's 2 1/2" gauge Olympiade. Though no
superheater is shown the size of one of the flues, the position of the steam
pipe, and the position of the steam spider (it's a 3 cylinder loco) show
that he mounted one in the boiler. This is a variant on typical LBSC
behavior. He was notorious for publishing a set of plans or a whole book and
then building his personal version of the engine quite differently.
For some reason, (liabilty concerns?) it seems as if these low steam
pressures and no
superheaters have become typical of modern RTR and fully-machined kit locos.
J. Alexander all over again.
All of this is just a long-winded way of saying--Do it! Get whatever books
you don't have, measure what's commercially available, do the calculations,
and hope that Ruby and RH and whatever else will work. I wouldn't be
surprised, though, if you have to get the tools eventually and scratch
build--a lot.
Victor Lacy
