New Rockford, North Dakota 58356.
I found the Frank Burris story ‘How to design a slide
valve’ in the Sept.-Oct. ’72 issue very interesting. I am
building my second model, a 40% size of my 25 HP George White, his
diagrams may be of help to me, I still have my valve to make. I
wish he had included some ideas on suitable ‘inside lap’
rather than too much.
Since I have seen more than one instance where model builders
using the single eccentric valve gear have had trouble getting the
proper dimensions of valve, I will try to explain how I would do
this. I hope my efforts will not be a case of ‘the blind trying
to lead the blind.’
First, there is only one width of valve that is right for
single-eccentric gear, woolf etc. I would advise making the valve
last, after the gear is completely assembled and adjusted,
(eccentric properly located, pedestal correct height, reach rod
right length etc.). Now, temporarily fasten a thin piece of
aluminum or even thin cardboard to the valve stem so it will move
across the valve ports when the engine is turned over. Now, with
the reverse lever in one of the corner notches turn the engine to
either center, (turn it in the direction the lever is set for) and
set your dummy valve for the lead you want at the port leading to
the end of the cylinder where the piston is. Now turn the engine to
the other center, (lever in the same notch) if the lead at this end
is too much your valve is not wide enough, if there is no lead it
is too wide.
Now with the lever in the other corner, turn the engine the
other direction and check your leads, if they are nearly the same
that of course is good, if too far off recheck your centers, height
of pedestal, lost motion etc. Possibly you may have to make a
compromise in the width of the valve just a trifle short for one
motion and a little long for the other. The point of cut-off (best
measured in crosshead travel) is very important, if too far off,
possibly a little compromise with the proper lead might help. Now
for ‘inside lap ‘you can cut an exhaust cavity in your
dummy valve and check the exhaust opening, point where compression
starts etc.
There is some precise geometry involved in the design of a
single eccentric valve gear. If I tried to go very far into this it
would really be a case of the blind leading the blind.
I find articles like Frank’s interesting, also ones
discussing valve gears, tandem and cross-compounds, double and
single simples etc. I happen to like compounds. The Marsh valve
gear is my favorite, I am sure there are many engine men that
disagree with me. How about some opinions of which would have the
advantage as far as backpressure is concerned, the tandem or the
cross compound. The big piston being a half-travel when the small
one is at one end could be either an advantage or disadvantage. I
have run both, liked both. Of course I like any smooth running
steam engine.
The fourteenth annual Central North Dakota Steam Thresher show
in September 1972 was well attended and the weather was
satisfactory. Would like to see you there Sept. 21-22-23, 1973.
