Before I started this model, I knew it was going to be peculiar but, as it has unfolded, I realise that I should have thought: “you just ain't seen n-n-n-nothing yet Here's somethin' that you're never gonna forget ”. How such an extraordinary machine, which ran in regular service for about a year, seemed to vanish from the memory so quickly seems surprising.

 

As I tried to make progress with my model, I had a lot of difficulty in working out a plausible configuration for the components of the drive train. For example, in Wood’s Plate XIII, the connecting rod appears to run in front of a frame that it must surely be behind. Some of the supporting structures are unclear; for example, those for the cross-heads and their slides. I realised more research into other engines of the period was needed before I could make much more progress.

 

The huge toothed drum that I described in my previous post was also a surprise – the model displayed in the Museum & Archive at Didcot Railway Centre does not have anything like it but seems to be based solely on a loose interpretation Sekon’s illustration.

 

The model displayed at Didcot Railway Centre (taken through glass)

 

It’s clear that a model based on Wood’s description of 1838 will look very different from this!

 

Modelling the Running Gear

 

Although Wood’s ‘A Practical Treatise on Rail-Roads’, 3rd ed, 1838, states in the text that the driving wheels had a diameter of 5 feet, I was fortunate to realise, before it was too late, that the scale on his Plate XIII actually indicates a diameter of 6 feet. This did not matter when I was extruding from the drawing but became important when the width of the engine needed to be taken into consideration.

 

I now believe that the drawing in Colburn (1871) of ‘Harrison’s engine’, which I showed in Part One, was probably taken from the Patent specification, as it shows many differences, including smaller wheels, from ‘Thunderer’, as illustrated by Wood. Wood stated that “we have given a drawing of one, built by Messrs. Hawthorn of' Newcastle-upon-Tyne, for the Great Western railway”, so this may explain a divergence between his Plate XIII and his text, which was probably derived from the Patent specification.

 

My first step was to lay out the outside frames and fit the spacing of the 6 foot diameter driving wheels to the 7 foot gauge. From Wood’s Plate XIII (1838), I knew that the length of the platform was about 14’ 6”. Now I could visualise how much space was left for the eccentrics and cranks on the driving axle outside the large central drum gear wheel. This in turn determined the placement of the inside frame and the pillars that linked the axle boxes for the shafts of the gear mechanism. I had a lengthy period of trial and error before I settled on the overall layout.

 

Overall Layout of my 3D Model Engine Carriage

 

Next, I constructed a cranked axle, using the same method I previously used for my model of North Star.  I had to find space for the valve-gear eccentrics (one each side) and the axle boxes with their linked support pillars. I referred to Wood’s Plate XIII for the design of these parts.

 

 

I extruded the paired axleboxes, their supporting pillars, and the springs from the drawing, as shown on the left. I used the rotate tool to create cylindrical support pillars from the cross-section drawing.

 

 

 

 

At this stage, I also created the smaller gear wheel on the driving wheel axle, by using exactly the same methods that I described in Part One for the large gear

 

 

 

 

 

 

 

 

 

It was clear that the space was tight but, after more trial and error, I reached the arrangement shown below. Of course, the locations of the cranks also determined the spacing of the cylinders at the opposite end of the platform

 

Modelling the Valve Motion

 

As I described in Part One, the valve gear was extremely conservative for what was otherwise intended to be an advanced concept. There was only one slip eccentric for each cylinder and a framework of rods that harks back to Stephenson’s Killingworth engines. Reversing the gear was a manual operation that could only be carried out when the engine was stationary. A release lever was provided, to disconnect the valves themselves from the eccentrics.

 

I started by tracing the various elements of the left hand side gear, as illustrated in Wood’s Plate XIII. The mechanism for the release lever was partially hidden behind other components but I worked out the mode of operation in the following drawing.

 

Annotated sketch of Valve release mechanism

 

The pivoted lifting rod allowed the eccentric rod to oscillate back and forth and operate the valves by means of the gab (notch) hooked over a pin on the valve operating lever. The lifting rod could be operated manually by means of the cam and a release handle, so taking the gab out of contact with the valves themselves. I extruded the various rods from the drawing and then assembled the gear onto the engine carriage, as shown below:

 

My 3D model of Thunderer’s Valvegear (LHS only)

 

There should be various cross-shafts and supporting structures that I have not yet included, at this stage. As I have commented before, one of the advantages of 3D modelling on a computer is that items stay where they are placed, without supports.

 

I decided not to simply copy the gear to the opposite side but to take account of the different angles of the eccentrics for the two cylinders. I had modelled all the components of the linkage separately, so this as simply a case of repositioning the eccentric rods and the lifting rod.

 

 

With both sets of valve gear in place, I could now add the connecting rods to produce the arrangement shown below:

3D printed engine carriage with connecting rods added.

 

 

Modelling the Cylinders and Steam Pipes

 

Wood’s description of the cylinders states that “one of the cylinders,… is placed horizontally upon a bedplate, … the other cylinder being placed in the same position, parallel thereto, and on the same plane, and upon a similar bed plate on the opposite side.”. In respect of the steam pipes, he wrote “The two carriages are fastened together by the bar, O, but, to compensate for the motion between the two carriages, a peculiar kind of joint is used for the steam pipe, ... and the disçharging pipe,”.

 

I have assumed from this that the two pipes were placed on the centre line of the engine, with additional manifolds to convey steam to and from the two cylinders. My assessment is that the cylinders, valves, and steam pipes were all enclosed by a single casing at the back of the engine carriage so, in the absence of any further information, I created a simple rectangular box. I added some detailing in the form of cylinder end covers and the stubs of the two steam pipes leading to the sliding joint with the boiler carriage. Wood’s Plate XIII also shows various steam pipes emerging from the top of the cylinder casing. I would like to make it a little more elegant-looking but that would be pure speculation on my part.

 

As ‘Work in Progress’ shots, I attach a couple of views of this remarkable machine. I think anyone who saw this ‘in the flesh’ with it’s great revolving drum at the front, must have been somewhat awe-struck.

 

 

 

Supporting Pillars

 

Having sorted out the basics of the machinery, I now had to consider how it should all be supported and held-together. Wood’s description contains virtually no information on this matter, so I have been trying to interpret the glimpses I can get from Plate XIII.

 

My starting point was to model a supporting column in the form shown on the plate, as illustrated below.

 

Supporting column extruded from Plate XIII

 

Having made a 3D model of one column by my usual methods of extrusion and use of the ‘Revolve’ tool, I had to work out a plausible way of deploying copies across the width of my model, to support components that were currently relying on ‘sky hooks’

 

As I surveyed the possibilities, I realised that, by placing columns on each side of the motion on both sides of the platform (4 columns in all), I could use these to support the two weigh bars carrying the valve rockers.. Furthermore, I realised that they could also support slide bars for the cross-heads of the connecting rods. With this arrangement, the handles for manual operation of the valves could be placed close to the outside edges of the platform. The manual release handles could also be placed at the outer edges, to facilitate disengaging the valve gear as required by the reversing procedure. The layout I eventually arrived at is shown below:

 

My arrangement of the Supporting Pillars on the RHS of the engine carriage

 

I had very little evidence to go on when developing this structure but it looks to be a reasonable engineering solution to the overall design of the mechanism.

 

 

That leaves me with various odds and ends – buffer beam, and the like. After adding those, I could set this amazing creation loose on the world:

 

My model of the ‘Thunderer’ engine carriage, rendered in ‘Fusion’

 

How could anyone who saw this in action in 1838 ever forget it? But first we need to add steam to get it going – the boiler carriage will be my next modelling task.

 

Mike