I ended Part Three with the prospect of modelling the many rods and brackets on the underside looming over me. I had intended to write more at that time but found myself struggling to understand how various parts of the engine fitted together. I think all the ‘easy’ bits have now been done, so I could no longer avoid the complex underpinnings.

 

To gain an overview, I ‘mirrored’ one half of the split plan-view from ‘The Engineer’ and then colour-coded various elements – blue for frames, orange for crankshafts, green for valve gear, and red for wheel bearings. I made a couple of ‘corrections’ to the ‘mirror’ process by moving the cranks on one side to represent ‘quartering’. I have repeated this plan as a ‘header’ to this entry. following its use in Part Three .

 

My 3D model overlaid on ‘The Engineer’ plan view

 

I was pleased to find more information, which helped me interpret the various drawings, in an article from ‘Engineering’, 11th Feb.1870 (reproduced in the Broad Gauge Society (BGS) journal ‘Broadsheet’ No.27, Spring 1992). Although the article refers to the ‘rebuilds’, some of the information appears to apply to the original engines as well. I quote:

 

“…. There is also a centre stay for the crank axle fitted with adjustable wedges; this stay is bolted to transverse plate in front of the firebox which ties the frames and assists in supporting the stay;

 

The eccentric sheaves are of cast iron, as are also their respective straps, these latter having cast on the half that receives the rod two ears which with a pin inserted vertically and eye in the eccentric rod make a lateral joint. The valve gear is of that class known as Gooch‘s stationary link. ... The valve spindles are. guided by a cast-iron bracket bolted to the plates which carry the bogie pin and unite the boiler barrel with the smoke-box tube plate; these brackets have each a flat bar of iron or steel fitted for the spindle crossheads to slide on; these crossheads being similar to the piston crossheads. The reversing shaft is carried by two brackets bolted to the bottom slide bars.”

 

Gooch ‘Stationary Link’ Valve gear

 

I then found a lot more useful information in articles by Douglas S Johnson, published in two issues of ‘Broadsheet’, Nos. 83 and 84 (2020), in which he described constructing a model the ‘hard way’, using nickel silver and brass. While very helpful, these articles also provoked great sighs of relief that I was using 3D computer modelling, rather than facing the problems raised by real model engineering.

 

Modelling the ‘Motion’

 

As before, I have tried to follow a ‘line of least resistance’, so decided that the moving parts of the motion were the easiest components to understand and place in their appropriate locations. My hope was that the locations of the various supporting brackets would become more obvious once I had the moving parts in place. One of the great things about 3D modelling in a computer is that individual parts will stay where they are placed, as though on ‘sky hooks’!

 

Sketch of Motion over ‘The Engineer’ Drawing

 

I started with the main drive-shafts between the cylinders and the driving wheel cranks. The rods are simply cylinders, produced by extruding their cross-section drawings. I have simplified the cross head by extruding from a plan view and then set in place two slide bars, above and below the cross head. I show these parts above the ‘canvas’ which provided me with the overall dimensions.

 

My representation of the main drive components

 

These parts will form a static representation of the motion – fully working motion would need metal bars and bearings, which are not on my agenda at present. Because of their prominent locations, they are needed for completing the external appearance of my model.

 

Side view of the Motion in place on my model

 

I followed up by using similar methods to create the various components of the valve gear. I made the profile of the Gooch stationary link by tracing over the above sketch of the valve gear and then created the various rods by simple extrusions from sketches. After creating the various components individually, I moved them into their appropriate locations on one side of the engine and then ‘mirrored’ the whole lot to the other side.

 

My layout of Valve gear components

 

Next, I put the components into the context of the rest of the model (minus boiler and smokebox), to help me to determine where the various supporting structures need to be placed.

 

Setting the Motion in the context of my Model

 

Before I could get much further, I needed to develop a better understanding of how this engine ‘worked’.

 

Overall Engine Structure

 

In most engines, the driving wheels transmit the force needed to pull the train, through a pair of strong plate frames running the full length on each side of the engine. These are linked at the back to a strong drag bar running across the width of the engine and carrying the couplings to following vehicles.

 

In this Pearson engine, the strong plate frames are notably absent. The design has been likened to a road-going Traction Engine but, although there are similarities, they are not the same. In a Traction Engine, the driving wheels are near the back and transmit their forces through a strong frame at the rear end, which carries the necessary draw gear. The boiler in such an engine is a forward extension from the ‘pulling part’ of the engine, carried at its forward end by the steerable front wheels.

 

A different analogy can be found in Brunel’s design for his Chepstow Bridge, in which he took advantage of the considerable strength of an iron tube to transmit both compression and tension forces. In Pearson’s engine, it is the boiler that provides this key structural component, being connected to the central driving axle through the yoke spanning the top of the boiler. As a tank engine, the design was intended to work in both directions. When running forwards the boiler transmitted the driving force in turn to the firebox, through a transverse frame member, and then to the rectangular tank underneath the coal bunker. The rear coupling hook was bolted directly to the back of this tank, which acted as a box girder. For running backwards the forces were carried by two plates riveted to the lower sides of the boiler, which transmitted the forces to the cylinder casting and then by a short shaft to the front coupling.

 

I should point out that the above is my own interpretation after spending several days looking at drawings. If those with more engineering expertise see it differently then I shall be pleased to be corrected.

 

This method of conveying the main driving forces through the boiler would not be permitted now. The fact that even substantial plate frames were subject to cracking under stress, suggests what could happen to a pressurised boiler in similar circumstances.

 

Modelling the Structure

 

It took a lot of head-scratching and poring over drawings before, largely by trial and error, I worked out how everything fitted together. The drawings show a plethora of riveted plates, which took me some time before I could understand their functions and how they fitted within the overall context of the engine as a working vehicle. I’m not sure that I can now recall all the steps that I made (and an account would be very tedious anyway) but the outcome of all my deliberations is shown below.

 

I started with the basic rectangular frame, described Ahrons as “only 8in. deep for the greater part of its length except at the driving hornblocks. An arrangement of angle plates, 2ft. deep, was fastened to the side of the fire-box and to the front of the well tank. From this point to the back buffer beam there was no frame at all.”

 

Next, I had to understand the curved plate that can be seen in ‘The Engineer’ side elevation, extending from the back of the smokebox and riveted along the lower sides of the boiler. I determined that there were actually two of these plates attached on either side of the casting that carries the front bogie mount. Their purpose was, apparently, to transfer tractive forces from the boiler to the front coupling on the engine. I placed them on my model as shown below:

 

Modelling the Front-end Boiler Brackets

 

I could now place the ‘motion’ I described earlier into the context of these brackets and the rectangular frame, as shown below:

 

Setting the motion within the inside frame

 

I could now work out the arrangements for the centre bearing of the crank axle and its fore and aft attachments to the firebox and front well tank.

 

 

Centre-bearing for Crank Axle (outer bearings not shown)

 

It all looks so simple now – it’s hard to take in how long it took me to figure all this out from the drawings I have 🙂

 

Actually, when I put it all together, perhaps it doesn’t look quite so simple! Quite a step up from my previous modelling methods:

 

My model of the ‘Works’

 

 

It’s rather a pity that almost all of this becomes invisible once the boiler and outer frames are in place 😒

 

I also find myself wondering how the real engine was erected, with so many ‘inter-dependent’ parts.

 

My 3D model in ‘photographic grey’

 

There’s not even much to see from underneath because it’s hidden by the well tank.

 

My 3D model viewed from below

 

After rendering in ‘Fusion 360’ my model looks like this:

 

My 3D model rendered in ‘Fusion 360’

 

You’d have to look at this rather carefully to spot any visible differences from my earlier renderings!

 

Now that I’ve teased out most of the internal features, which has been an ‘interesting’ mental exercise, I shall have to return to considering the ‘cosmetic’ appearance. There’s still a lot to be done on the details, such as rivets, boiler bands, and so on … and on.

 

Oh, and brake gear on the rear bogie.

 

Enough for now

Mike