[EMAIL PROTECTED] wrote:
>
> As my understanding of steam engine valve gears is minimal I ought perhaps to keep
>my head below the parapet.
Not at all Art. Valve gears is one of "my things", so perhaps I can add
a little. Walscherts gear nicely separates the two functions any valve
gear has to perform. The first, and most fundamental, is to make sure
the engine goes in the desired direction. The second is to use steam
more effectively buy cutting of its addmission to the cylinder some time
before the end of the stroke so it can perform some of its work by
expansion.
It was soon discoverd that a greater range of cutoff could be built into
a gear if the valves we made with long lap, that is the part of the
valve face that covers the inlet ports was made considerably wider than
the port itself. So if the valve is centered over the ports there would
be, say, an inch (in full size practice) extending beyond the inlet
ports at each end. This raised a problem though. In order to open a long
lap valve at the begining of the stroke the valve has to be moved that
same inch so the port opens as soon as the piston moves through dead
center. This is achived by giving the valve lead, that is its motion
leads that of the piston by the distance of the lap, in this case 1".
Now lead can be built into the setting of the eccentric, and this is
done in gears that have two eccentrics, such as Stephensons. But
Walscherts uses one eccentric, or a return crank that does the same
thing. If this is set for correct lead for forward running it will
actually give lag for reverse, the port will open late. So the return
crank on Walscherts gear is set for no lead, and lead is provided by a
combination lever which takes its motion from the crosshead. This is the
bit that is left out on the "simplified Walscherts" gear.
Roundhouse have nonetheless done a very good job on their valve gear
design and you can drive on the gear to some extent. I drove a
Roundhouse loco with RC gear on the reverser and was able to control
speed using the cutoff with the throttle left at near full setting. That
is the "proper" way to drive, at least for a GWR man like me!
The book "Locomotive Valves and Valve Gears" by Yoder and Wharen is an
excellent work for those of you that are interested. Chapter 11 develops
the Walscherts gear explanation very nicely.
Cheers
Keith
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Keith Manison Phone (876)702-0337
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