‘Eagle’ was one of that group of early broad gauge engines that are frequently referred to collectively as ‘Brunel’s freaks’. In some cases this epithet was rather unfair and several of these engines went on to have long lives as branch-line engines.

 

After I wrote about my 3D-printed model of ‘Eagle’, @Lacathedrale commented on my photograph “Will you be able to retrofit the running gear to Eagle?”.  I agree that it does looks very sparse under the frames!

 

My 3D-printed model of ‘Eagle’

 

Recently, I made a 3D model of the running gear under ‘North Star’, after which I commented wryly that “Sadly, after all this effort, almost everything disappears under the superstructure of the locomotive, from normal viewing angles. Next time, I must choose an engine with more-visible ‘works’.”  I now realise that, because of its unusual placing of the front axle, ‘Eagle’ does show off more of its underside than most engines, so it makes a good choice!

 

‘Eagle’ was built by Sharp, Roberts & Co. and the cylinders and motion contained many innovations, devised by the engineer Richard Roberts. Although rarely remembered today, Roberts has been described as “one of the two greatest inventors of spinning machinery” – the other being Samuel Crompton.

 

‘Eagle’ was initially built with piston valves but these were not a success, probably because of steam leakage due to differential expansion of the metals used, A contemporary drawing by E.T. Lane shows that the engine was fitted with Sharp’s own valve gear, with its characteristic forked gabs showing beneath the frames. Another unusual feature of ‘Eagle’ was that, according to Ahrons book ‘The British Steam Locomotive 1825-1925', p.39: “The exhaust passages of Sharp's engines of 1839-1842 were peculiar. Both exhausts were discharged from the cylinders into a cubical box placed midway between the inside cylinders, and the blast pipe was connected to the top of this box, which was intended to act as a form of " air vessel " to equalise the blast. It caused considerable back pressure in the cylinders.” I assume that this is the box shown in the Lane drawing behind the forks of the valve gear. I have also read that one of the defects noted by Gooch was the “choked blastpipe”.

 

‘Eagle’ drawn by E.T.Lane in 1848

 

I found a description and drawing of Sharp’s valve gear in the book ‘Railway Machinery’ by Daniel Kinnear Clark, dated 1855: “… with two transverse shafts, as in Stephenson’s, the one for back gear was placed below the other and worked by short back levers and links – a modification by which all the gabs were raised and lowered at once … by placing the forward gab above the pivot, it was permitted to lie solidly in gear, although the gearing has not the self balancing character of Stephenson’s and, moreover, should the fore gab, by an accident, have fallen into gear, while the engine was placed in back ear, rupture was inevitable”

Sharp’s Valve gear

 

Creating a 3D Model

 

By drawing on the experience I had gained by modelling Stephenson’ gab gear, I felt I could now tackle filling that yawning void beneath the boiler of my model of ‘Eagle’.

 

I started with a somewhat ‘bull at a gate’ approach by using my usual methods of tracing over the various rods that make up the valve linkage but soon found that, without a plan view this was going to be a tricky task. I also realised that Clark’s generic sketch of the gear needs considerable modification in order to fit on a real engine.

 

For example, the description by Clark states there were two transverse shafts arranged vertically. The problem is that if I take his drawing and attempt to apply it directly to my model of ‘Eagle’, I find that the upper rod would have to pass through the boiler! Similarly, the drawn arrangement requires the lifting link for the lower eccentric rod to pass through the weigh bar.

 

I soon realised that a more cautious approach was needed!  I started by creating those components that have their locations determined by the positions of the cylinders and the driving axle. These are the coupling and piston rods, the valve operating rockers, and the weigh bar, as shown below:

 

My 3D model of the Major Components of the motion

 

That has provided a ‘base line’ around which the components of the reversing gear can now be added.

 

Whereas, in the case of ‘North Star’, there were four inside frames on which to attach bearings for the various cross-shafts and slide bars for the cross-head, these do not appear to have existed in the case of ‘Eagle’. According to Arman in his book ‘The Broad Gauge Engines of the GWR, Part One’, describing ‘Eagle’:“Because they were broad gauge engines, the wheels were placed outside the frames and the redundant inner frames were dispensed with”. 

 

So, whereas Stephenson had spread out his gear across the width of the engine, it appears unlikely that this was case with ‘Eagle’, since supports were not readily available.  The layout that I have devised is based entirely on my own speculation, so may not (or may) have any resemblance to the ‘real’ engine.

 

Layout of the eccentric rods.

 

In view of the wide gaps between the valve rods and the frames, I decided that both eccentrics could be placed close together, outside the connecting rods, as shown below (left hand side only):

 

Proposed Locations of eccentrics in my 3D model

 

Now, it was time to find out whether the reversing gear could be fitted in, without parts crashing into one another.

 

My first step was to see if the two vertically-arranged cross-shafts could be fitted into the space between the boiler and the cross-heads. I also checked that the short stroke (14 inch) allowed sufficient space for the crank shaft to clear below the boiler (just).

 

 

After clearing that first hurdle, I then had to find space for the various links between the cross-shafts and the eccentric rods, so that either forward or reverse motion could be selected. Fortunately, I noticed that Lane’s drawing shows a curved rod, running down below the frame, to lift the lower (reverse) eccentric rod. This solved the problem of collision with the weigh bar.

 

Following the schematic of Sharp’s gear, as drawn by Clark, I arrived at the layout of the mechanism shown below:

 

Arrangement of my model of Sharp’s valve gear

 

I now had to ensure that space could be found for all the rods in the gap between connecting rods and the frames. After some shuffling of the various parts, I managed to fit everything in without collisions. Next, having got everything sorted on the left hand side, It was time to copy everything over to the right – ‘mirroring’ parts where necessary. The result is shown below:

 

 

My overall arrangement of ‘Eagle’s motion

 

To see how this all fits in with my existing model engine, I took the view from below the boiler:

 

My 3D-model viewed from below

… and from above, with the boiler removed:

 

My 3D model from above.

 

As I remarked at the outset, this arrangement is purely my own interpretation of how the mechanism might have been laid out on the real engine. Actually, I feel pleased to have go this far, as it has involved thinking about some of the engineering problems for myself, rather than slavishly copying other people’s drawings.

 

It will now fill some of the void under the boiler of my 3D printed model of Eagle:

 

My 3D model of ‘Eagle’, now with Sharp’s Valve gear

 

Mike