Thankyou Keith,
I agree with Clark (I usually do for some reason!) Your explanation has
helped me too, in a few of your easy to understand sentences, I am
considerably enlightened about this dam magical valve gear.
Now I realise that my Aster Stirling needs an adjustment of the eccentrics-
as a friend has been trying to tell me! Except I like the "bark"!!
I don't expect to burden you in DH, accordingly, I think we should all
come to Jamaica or -- 62 degree Sunny Nevada for a seminar! Of course in
Nevada, in the summer, all locos are solar powered--Clark forgot to mention
that. You know, 110+ degrees on a cool day! I know!! it's a dry heat!
Geoff.
>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
>
>--
>=======================================================================
>Keith Manison Phone (876)702-0337
>7 Mulberry Close Fax (876)702-0661
>Jacks Hill P.A. Email [EMAIL PROTECTED]
>Kingston 6, Jamaica W.I.
