In a comment on my previous post @Mikkel wrote “I never know what's next on your blog Mike”. Actually, I feel much the same – I never know where a whim will take me next!

A week ago, the thought of a Bristol and Exeter Railway (B&ER) engine was nowhere in my mind and then @Annie posted some splendid photos of Pearson’s magnificent 4-2-4 Broad Gauge tank engines.

B&ER No.42 4-2-4T designed by James Pearson

It wouldn’t be true to say these engines have never crossed my mind but they were always rapidly consigned to the ‘too difficult’ box. Now, having built a few of Brunel’s so-called ‘freaks’, I have learned a lot more about 3D design and the capabilities of 3D printing. So – time to have a go ? …

Back issues of the Broad Gauge Society (BGS) journal ‘Broadsheet’ provide a wealth of information, including both drawings and photographs of several versions of these engines. There is also a lengthy biography of James Pearson in ‘Broadsheet’ No.78 (Autumn 2017) which, in addition to providing useful career information, also busts a few myths.

Before his appointment as Locomotive Superintendent, at age 34, on the B&ER, Pearson had a spell as the engineer responsible for operations of Brunel’s ill-fated atmospheric system on the South Devon Railway. That system has been the subject of another of the many deviations followed by my Blog.

Almost all references to Pearson suggest that he was a Quaker which was supposedly the reason behind his adoption of black as the colour for his locomotives. The BGS biography found no evidence to support that assumption and reports that Pearson was Christened at his local Parish Church in Blackburn. It seems more like that the livery arose from the well-known concern for economy, which characterised much of the B&ER operations.

There are also many stories of his 4-2-4 engines that do not stand up to close examination. According to an article in ‘Broadsheet’ No. 48 (Autumn 2002), they were originally ordered specifically for duty on the Yeovil branch and not, as often reported, for the Exeter expresses. Clement Stretton in his book ‘The Development of the Locomotive 1803 to 1896’ (published 1896) stated that he remembered a 9 foot single tank running smoothly at 60 mph and that “Mr Pearson many years ago informed the writer that his engines had been officially tested at 81 mph,” Sadly, many of Stretton’s claims have subsequently been shown to be inaccurate, so these statements should be treated with caution.

Towards a Model

I have commented before that making a model is a very good way of learning how the original engine was constructed. In this particular case, even a cursory examination shows that this engine had several very unusual features. For example the boiler carried a ‘yoke’ to which suspension rods from the driving wheels were attached. There were other brackets between the boiler and the very light frames, which only extended from the back of the smokebox to the front of the firebox. Thus, the boiler itself had to bear many of the forces associated with a locomotive engine – a practice that would not be approved now!

I had to start somewhere and, with so many peculiarities, it was hard to choose. As a ‘gentle introduction’, I decided to start with the two bogies, which were attached to the rest of the engine by central ball and socket joints – no side-play allowed! I found some old frame drawings reproduced in ‘Broadsheet’ No.49 (Spring 2003) so, following my usual practice, I loaded these as a ‘canvas’ into ‘Fusion 360’.

The collection of drawings included a plan view of the bogie and elevations of some of the main members. I started by tracing over one of the diagonal members and creating a solid ‘body’ in ‘Fusion 360’. I then made a sketch of the profile of the central pivot mounting and used the ‘Revolve’ tool to create a socket to receive the ball suspended below the engine itself. The next step was to align the diagonal members and the central bearing over the plan view, included in the drawings, as shown below:

1. Tracing the Bogie components                                                          2 Arranged over Plan Drawing

The rest of the frame comprised a very lightly built rectangular arrangement of plates, with the horn blocks attached at the four corners. I traced and extruded these components into the structure shown below:

Bogie Assembly extruded over the ‘Canvas’ in Fusion 360

I have made plenty of 4-foot diameter wheels before, so it was a straight-forward task to create these and fit them into the horn-blocks for a rendered portrait of the complete bogie in ‘Fusion 360’. According to Ahrons, in a lengthy description of these engines in his book ‘The British Steam Locomotive 1825-1925’, each bogie wheel had an independent india rubber spring.

My 3D model of one of the two bogies

That’s one small step* – or, perhaps, two, since there will be similar bogies at each end of the engine! It seems that I am now committed to working out how to fit together all those brackets and levers that hold together the complete engine.

Mike

* or one bite of the elephant as @Mikkel once described it