In this first article, I’ll explore how I designed, built and timed simplified outside Walschaerts valve gear for my model of Russell. I will look at which components of the full-size gear we will replicate and then design these for the locomotive.
For the second article, I’ll look at how I designed inside Walschaerts for a model of Jubilee 1897.
In the final part of the series, I’ll move away from designing for a model and look at the full-sized gear in more detail as well as having a quick look at Stephenson’s and other similar valve gear.
1. Outside Walschaerts A look at Walschaerts and a quick explanation of the various pieces
So, looking at the photo of my model of Russell you can see that the working elements of the design comprise: –
- The return crank. This is rigidly fixed to the crank pin on the drive axle crank (i.e. the one carrying the connecting rod). If we were instead looking at an inside framed loco, the crank pin would be attached directly to the wheel. The return crank’s function is to provide forward and backward movement for the valve gear as two cranks together rotate around the axle.
- The eccentric rod. This is attached to the return crank and transmits the forward and backward motion to the expansion link.
iii. The expansion link. This links to a radius rod which can move up and down the slot in the expansion link. The expansion link rotates around the centre of the slot so if the radius rod is at the top the forward and backward movement is reversed enabling the loco to go backwards.
- The radius rod. The radius rod links the expansion link to the valve rod and is raised or lowered via a lifting link.
- The valve rod. This moves the valve inside the valve chest over the live steam and exhaust steam ports. On my model of Russell, I’ve used Roundhouse cylinders so there’s little more to say at this stage. In part 2, where I make my own cylinders, I’ll look at valve design in more detail.
The full-size valve gear has more working components, in that the valve rod is also driven from the crosshead via the union link and combination lever. On my model and on commercially produced versions of the motion these parts are replicated purely for cosmetic purposes (or on some models not replicated at all). Movement of the crosshead does not move the valves as the radius rod and valve rod are directly connected.
Why simplified?
I have examples of models built by Accucraft, Merlin and Roundhouse that use Walschaerts driven only from the return crank. So why?
This drive provides movement to the valves, enabling live steam to be admitted to and exhaust steam to be vented from the cylinders at the right time. The expansion link provides a means of reversing the loco and in the full-size gear the ability to reduce the cut off and use the steam expansively. We need this part of the drive as we obviously want our loco to both move and to be reversable. With our relatively crude valve chest designs however our ability to ‘notch up’ is virtually non-existent.
The drive from the crosshead provides constant lead to the valves. Lead is the amount by which the live steam port is open prior to the piston reaching the end of its travel. So, steam is entering the cylinder before the piston’s stroke is reversed and means that steam is quickly available to propel the piston in the other direction. On fast moving engines, this steam also provides a cushioning effect for the mass behind the piston. For this reason, larger lead is provided on express engines compared to shunting locomotives and it is essential for smooth running at speed.
I’ve read several model engineers forums about lead and there is a division between those who copy the prototype and provide lead and those who say that it is unnecessary and don’t. Those choosing not to provide lead have the view that admitting steam prior to completion of the stroke provides resistance and that as mass doesn’t scale in direct proportion, we don’t need any cushioning. Both camps say their models run perfectly well (although I did read one comment about a loco with lead setting off in the wrong direction).
In short, lead is not needed in our scale and its inclusion may be a hindrance.
2. Design for the model Calculation of the various elements
We will start with the expansion link and calculate how long the slot needs to be. The valves on Roundhouse cylinders require a nominal throw of 4.0mm with an upper limit of 4.8mm. I chose a figure of 4.4mm reasoning that any small loss through the various joints would bring it closer to the nominal figure.
According to Martin Evans (see bibliography) the expansion link should not rotate more than 25° either side of centre so 50°in total. If the gear rotates more than this, it will become difficult to reverse due to increased friction. I chose to use a slighter lower figure of 45°, partly to give a margin and partly because I thought, at the time, I’d need to calculate the circumference and 45° being ⅛ of a circle would have made the maths easier. I later realised that the movement would be the straight line between the end points (i.e. the chord) and not the segment of the circumference (the arc).
I decided to make the slot in my expansion link 2.2mm wide which meant that the corresponding pin in the radius rod would be the same diameter.
With these figures we can calculate the length of slot required. About 50 years ago I would have done this with sin tables but these days there are any number of online chord calculators. Hopefully you will be able to follow the calculation in the diagrams.
The next step is to decide where the eccentric rod will connect to the expansion link so that we can then calculate how much this (and therefore also the return crank) needs to move. This will depend on the model and if copying a prototype how close to scale you can make it. On my model quite a bit of compromise between scale and practicality was required and I required a gap of 3mm from the bottom of the expansion link slot (see diagram).
We can now calculate the required length of the return crank. The diagram shows a right-angled triangle with the wheel/axle crank one of the two adjacent sides. The length of this crank is driven by the stroke of the cylinders. In my case the Roundhouse cylinders have a stroke of 15.875 mm. The remaining adjacent side is taken from the figure we have just calculated. Both cranks rotate around the axle so the figures we will use will be 50% as we want the throw from the centre (i.e. the radius not the diameter). We can then calculate the hypotenuse which will be the length our return crank needs between its pin centres.
This may well be a different length to the original scaled down as the dimensions and spacing of the ports in our valve chests will not correspond to the full size loco.
3. Some construction notes
I won’t give a blow-by-blow account of the construction but just relate a few things that maybe of interest.
Radius rod. Its length is scaled from the original and the curve of the expansion link slot should correspond to this radius.
Expansion link. The radius of the slot was marked in a piece of steel, chain drilled and then filed. This piece was one of five riveted together, partly inspired by Brian Wilson (see bibliography), but constructed differently. The pivot was drilled 2.2mm this being the width of the slot in the expansion link, enabling the shank of a drill of this size to be put through the assembly to aid accurate construction. The expansion link is connected to the motion bracket and pivoted on either side. Hopefully the photo will explain this better than my words.
Return crank. Drilled on a milling machine, using one axis on the x & y table to ensure accuracy.
4. Timing
The return crank is set at 90° to the axle crank; remember the right-angled triangle we used to calculate its length. By convention, when the locomotive is going forward the radius rod should be lowered in the expansion link and, for locomotives with slide valves, this will mean that the return crank will be in 90° in advance of the wheel crank. However, Russell in common with early Hunslet use of Walschaerts is one of many exceptions, it is arranged with the return crank 90° behind the axle crank and therefore the radius rod is raised for the loco to go forward.
In the final article, I’ll compare the way full Walschaerts is set up for inside and outside admission valves. This affects not only the return crank but also the combination lever.
I’ve found getting the return crank in exactly the right place difficult as I find it hard to visualise the 90° between the two cranks. I devised a method of doing this by measurement against a fixed part of the loco, the principle being that if the measurement is right, then the return crank must be in the right place. On Russell I used the motion bracket and the end of the eccentric rod.
The next step is to look at the valve chest and by disconnecting the valve rod from the radius rod and turning the valve rod before reconnecting, the position of the valve over the ports can be adjusted. I do this by rotating the wheels both forward and backward with the radius rod in the appropriate position and equalising how far the live steam ports are open when the wheels are at top and bottom dead centre. In these positions the valve will be either fully forward or fully reversed.
If the motion has been built correctly it should be possible to do this with no more than a small amount of compromise.
Bibliograpghy
- Model Locomotive Valve Gears, Martin Evans - Tee Publishing
- Steam Trains in Your Garden, Brain Wilson - Revised edition by Wilson Locomotives
- Building Bagnall 0-6-0T – ‘Dennis’, Keith Bucklitch - Serialised in 16mm Today, February 2001
- Roundhouse Engineering – SS02 Walschaerts
- Lap & Lead in Slide Valves, Charles A Purinton
- The Walschaert Locomotive Valve Gear, W W Wood - Internet Archive