Locomotive Valve Gear Design & Simulation Software

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Although the valve gear simulation software produced by Charles Dockstader is one of the better known, it is by no means the only such software available. Most published valve gear designs have been produced by somewhat empirical methods, very often by (model) engineers with a less than perfect understanding of the basic principles. Slavishly scaling and copying full size designs is little better, as not only were many of these valve gears imperfect in the first place, model and full size locomotives are required to perform different functions. For example the full size GWR 1500 class (LBSC's Speedy) was designed to potter about Paddington pulling a few empty coaches, most clubs use it as an express passenger locomotive. A close inspection of the 1500 Swindon drawings (with the hindsight of modern computer simulation) will show design problems. There is little point in spending years building a steam locomotive, model or full size, only to find that it is an indifferent performer. Making changes may prove impossible once the locomotive is completed.

It is only with the advent of powerful high speed computers that the actual performance of valve gears can be seen "live", and results of the various valve events simulated in a graphical form. Only 10 years ago the computation for a set of valve gear events took about 15 mins and the graphs a lot longer. Nowadays it is more or less instantaneous, such is the progress of the modern computer. It is however very important that model engineers do not use these simulation programs as design tools per se. The component dimensions and locations etc. should be established by mathematical calculation using proven design rules and formulae. Excel spreadsheets are available to do these calculations in a split second. See Don Ashton's work on Dr. Allan Wallace's website below. The calculated results should be fed into the simulation program so that the results can be seen in real time and graphs produced. Not the other way round.

It should however be noted that there is no such thing as a 'perfect' valve gear. That is probably why there are so many. You can however get very close, provided you approach the problem correctly. The "cut and try" approach is a recipe for failure.

Engineering in Miniature featured an excellent article by Don Ashton (February 2006) on Valve Gear Simulation. Mentioned in the article are simulation programs by Charles Dockstader, Prof. Bill Hall and Dr. Allan Wallace.

For those of you who like reading try Locomotive Valves and Valve Gears by Yoder and Wharren 1993 ISBN 0-9519367-1-9 Camden Miniature Steam Services and the excellent mathematical treatise by Don Ashton - Design Procedures for Walschaerts' and Stephenson 's Valve Gears. Now in its 2nd edition Tee Publishing and Camden.

(1) Link to Dr. Allan Wallace's Valve Gear Site. Highly recommended

(2) Link to the Official Dockstader Valve Gear Site

(3) Link to Professor Bill Hall's Valve Gear Site.

(4) Link to Rainer Radow's Steamboat Valve Gear Site.

Colin Usher November 2007