Walschaerts valve gear was the most popular valve gear used on European steam locomotives in the 20th century. There is an article in Wikipedia about Walschaerts valve gear which includes an animation showing how it works. Now, in most electric models of steam locos which have Walschaerts valve gear the radius bar is fixed in the neutral position (see the simplified drawing below). This is an understandable compromise for the manufacturer because it makes the model simple (the radius bar does not need to move at all) and its position is correct when the locomotive is stationary and almost correct when the loco is running at speed in either direction. But when the prototype loco starts moving forwards, the radius bar is first set to the full forward position and, as the loco gathers speed, it is raised steadily towards the neutral position. And when the prototype loco starts moving backwards, the bar is first lifted to the full reverse position; as the loco gathers speed, it is gradually lowered towards the neutral position. The following simplified drawings show how these positions appear. Incidentally, the reversing mechanism varies widely between different locomotive types.
Now I think it would be fairly easy on an electrically powered model steam loco to make the radius bar move automatically in a good approximation of prototypical fashion. Clearly some intricate modelling would be needed to make the radius rod free to pivot while attached to the expansion link and the reversing mechanism. But the only electrical change needed is to add a solenoid in series with the loco's motor. Sorry, I could not begin to suggest how many turns the coil will need; this will need to be found by experiment. Ideally, of course, it should not present such a load as to interfere with the loco's performance. Thankfully it has little mechanical work to do. See the drawing below.
The solenoid has an armature running through it which is free to slide left and right. Fixed on the armature is a permanent magnet. The two springs return the armature to its neutral position when the solenoid is not energised. As the loco begins to move forwards, initially there is a surge of current because the motor is barely turning and therefore producing almost no EMF. All the power appears across the solenoid which is connected so that it attracts the magnet, moving the armature to the right. As the loco gains speed, the motor EMF rises, opposing the power supply and reducing the current in the solenoid, so that the spring draws the armature back towards its neutral position. If the loco stops and restarts in reverse, the surge current in the solenoid is in the opposite direction, repelling the magnet and moving the armature to the left. As the loco gains speed, the current falls and the spring draws the armature back towards the neutral position. A mechanical link on the end of the armature connects to an axle across the loco chassis attached to the reversing mechanism which moves the radius bar on each side.
I confess that I'm not sure whether a solenoid powerful enough to move the radius rods would draw so much power that it interferes with the loco motor. Also, if the loco is powered by a controller which produces pulses rather than steady DC, would this cause the radius rods to vibrate?
I have not built the above model and so cannot guarantee that it works. If you build it, please let me know the results.
© Roger Amos 2012