Valve Gear Simulators


Today we have access to some extremely useful bits of computer software which can exactly simulate the movements of the valve relative to the position of the piston and enable us to design the ultimate valve gears. Early designers had to use long and tedius calculations to work out valve positions but later ones did have access to full size models in which the various linkages etc. could be adjusted to give the optimum results. I remember seeing one in the Silk Mill museum in Derby some years ago for Walschaert's gear. LBSC had to make do with pins and bits of tin and cardboard!

Note that the modern computer simulators are not really meant to be a design tool. They are more for proving an initial design drawn out on paper or designed by calculation or a spreadsheet and then enable the various dimensions to be 'tweaked' to optimise the design.

The first simulator software I came across was the famous one by Charlie Dockstader who originally came up with the idea for proving the valve gear on one of his friends locos. Charlie's has by far the most numerous of valve gear designs ( there are 67! ) and must cover virtually every valve gear ever designed. Charlie passed away in November 2011 so there will be no more updates but his software is in the public domain and is now hosted by Bitter Creek Western Railroad. It can be downloaded here:

Charlie Dockstader Simulators

It's not a simulator I use a lot now as it asks for a lot of information not really needed to show the valve events but it is the only simulator available for a lot of the valve gears. Note that none of the valve gear examples have been optimised and the events are not perfect! Note you need to download all of the individual zip files or the one large one.


Professor Bill Hall devised two simulators, one for Stephenson's and one for Walschaert's, and these are quite easy to use requiring only basic dimensions. The software can be downloaded from Don Ashton's website:



My favourate software is that designed by Dr. Allan Wallace which covers Walschaert's, Stephensons, and Baker. Allan's software is a little trickier to use, especially the Stephenson's where it asks for certain dimensions you can only get from an accurate drawing or CAD. This software is also available from Don's website on the link given above. I particularly like the distribution diagrams that this software produces.


There is another Walschaert's simulator appeared on the scene recently but I haven't tried it myself:

Valve Gear Constructor

The website is in German and the program requires a user code to be obtained before you can use it.


One important thing you must do with any of these simulators is to use the correct dimensions for the data entries. All the dimensions are in relation to the axis of the cylinder and valve gear which are not necessarily those given on the drawings of the frames for example. Positions for weighshafts, expansion link pivots etc. are normally shown square to the frames but if the motion centre line is angled relative to the frames, these dimensions will be incorrect for the simulator. The correct dimensions will either have to be calculated or, much easier, taken from a CAD drawing as shown below.

The first drawing is for Juliet with Stephenson's valve gear and shows the position of the weighshaft and link hanger pivot as given on the drawings. The simulators ask for the vertical and horizontal distances from the axle.


Dimensions relative to frame


However, the motion centreline is at an angle of approximately 5.28° to the frame so the required dimensions have to be taken relative to this and are significantly different:


Dimensions relative to motion centreline






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