Looking at the frames of my ‘Rob Roy’ model, I realised that there is a very visible ‘yawning gap’ between the frames, which really needs to be filled with something. It never bothered me with ‘narrow’ gauge ‘double-0’ models but a 28 mm gap is a different matter altogether.
The instructions for the Broad Gauge Society (BGS) kit of the Gooch Goods, on which I have based my model, contain very little information beyond: “note: the kit provides for two slidebars per cylinder but it was probably a four-bar arrangement”
It soon proved much more difficult than I had anticipated, to find out, with any certainty, what should be there. Drawings of these early engines are few and far between and, where they do exist, tend to be in rather poor condition and difficult to ‘read’.
Fortunately, several fellow members of the BGS came to my aid and I was directed towards a ‘general assembly’ drawing from the National Railway Museum (NRM), which certainly met those criteria of difficulty mentioned above. It did, however, start me off on a line of enquiry, which answered many questions.
My initial examination of this drawing revealed that the leading pair of driving wheels were flange-less, which I had already anticipated in my model, since it was needed to provide sufficient clearance between the pairs of wheels. On the prototype, it was doubtless desirable, to enable these locomotives to negotiate even moderate curves.
More surprising is that the centre line of the cylinders appears to be above the centre line of the driving wheels but the cylinders do not appear to be inclined. This may be down to the rather poor quality of the drawing and the inclination would only need to be about 3°. On balance, this small inclination seems to be the more likely arrangement.
Being a general arrangement drawing, the detail of the motion is difficult to discern from the NRM drawing, because there are many overlapping details. The Waverleys, however, were built around the same time as many of the Gooch Goods locomotives, so it seemed reasonable to me that the arrangement of the motion should share many features with those engines.
Locomotive design at that period was still heavily influenced by Stephenson’s ‘Patentee’ locomotive, which set the pattern for design throughout much of the Victorian period. Here was the prototype of the classic British ‘Victorian’ locomotive, with two inside cylinders mounted between the frames. There is a very detailed description of the components of Stephenson’s engine in G Drysdale Dempsey’s book “A Rudimentary Treatise on the Locomotive Engine” (download from the Internet Archive)
I found this book very useful in helping me to understand many of the construction details of the early engines. For example, there are detailed drawings of the cross-heads and many other parts of the motion, accompanied by information about the four slide-bars that were used.
Gradually, I began to build up a detailed ‘picture’ of how the motion of the Waverley class was probably put together. This was helped further by detailed drawings of a Gooch Goods engine ‘Pyracmon’, in the Atlas of Plates, by Dempsey.
‘Pyracmon’ had a layout of the valve gear that was superficially similar to the more familiar Stephenson-Howe motion but with the important difference that the expansion link was mounted on a fixed pivot, while a radius rod attached to the piston rod could be raised or lowered to reverse the gear. This arrangement is attributed to the Gooch brothers and was applied to many of the Gooch locomotive designs for the GWR.
Taken together, these various sources of information allowed me to attempt a rough sketch of how the motion of the Waverley class engines was probably laid out. By overlaying details from the ‘Pyracmon’ drawing onto the NRM drawing, I established that the overall layout of the components was very similar in the two designs.
One notable difference was that, whereas the feed pumps for the boiler were driven by eccentrics on the driving axle in ‘Pyracmon’, they were driven from the cross-head on the Waverleys.
My very basic sketch, below, shows the overall layout of the motion between the frames of the Waverley-class locomotives. A centre stay ran the length of the engine, between the back of the smokebox and the front of the firebox. My sketch shows the components on one side of this stay, which were mirrored on the opposite side.
The four slide bars are shown in red. These were attached to the back of the cylinders and to angle brackets that were fixed to the motion plate, running across the engine.
The Gooch valve gear components are shown in blue. The reversing action is conveyed to the valve rods via a rotating shaft running across the engine and operated by the reversing control lever, which ran along the side of the locomotive to the footplate.
Moving parts are shown in grey and the fixed mounts for the reversing link and the reversing shaft are black. The reversing shaft was also attached by trunnions to the slide bars.
Simple Sketch of Gooch Motion (LHS only)
NB I now think that the reversing shaft should be above the motion in the Waverley class, not below, as shown in this sketch, which is based on the Gooch Goods.
More information to follow in my next post
I must emphasise that this is my own interpretation of the references quoted above, so there may well be points of detail that I have missed. I intend to use it as a basis for producing a few simplified components, to fill in some of that yawning gap between the frames of my model.
EDIT (5th Jan) - I have re-assessed the possibility of a small inclination of the cylinders since first posting.