Interpreting the Valve Motion

 

At the end of my previous post, I commented on the surprising layout of the valve gear, as shown on the Lane sketches. The Works drawing of the engine, as originally designed, shows a more usual arrangement, with the weigh-bar placed under the boiler behind the smokebox.

 

Following further research, however, I have re-interpreted the drawings of Aeolus by E.T. Lane and have, therefore, revised this post on 16th April.

 

According to the recently published book by Brian Arman: ‘BG Engines – Part 3’, several engines built during the 1840s were fitted with a regulator that was placed immediately above the cylinders. Drawings of, for example, the ‘Prince’ class show that the regulator was operated by levers from a shaft across the front of the smokebox, by means of a control on the right-hand side of the footplate.

 

I had not been happy with my previous thought that these components might have represented a ‘throw back’ to much earlier type of valve gear as used on Stephenson’s ‘Planet’ series of engines. It appears that Gooch started fitting his fixed link expansion gear from 1843, so I now suggest that the rebuilt Aeolus may have been one of the first engines to have been fitted with this gear. Far from being a ‘throw back’, this engine may have a been a test-bed for the new arrangement, which was fitted to Great Western and other engines shortly afterwards.

 

This change of interpretation does not actually affect the design of my model very much, it is simply that the parts that I have modelled serve a different purpose from what I initially had thought. The new interpretation is much more convincing than my earlier thoughts.

 

Initially, I produced every piece of the regulator mechanism as a separate ‘body’, so that I could move them all around independently to what seemed to be the most appropriate positions. The plausible outcome looked as below:

 

Potential Layout of regulator mechanism in ‘Fusion 360’

 

The next requirement is to provide supports for all these gubbins and, on this matter, I find the available sketches and drawings very confusing.

 

Lane’s sketches, which were clearly used as the basis of the much later G.F.Bird drawings, show what appears to be a rather massive support structure carrying a shaped boss to hold the transverse shaft, presumably in the centre-line of the locomotive. There is no clear indication of how the outer ends of the shaft were supported. As an interim measure, I decided to add support brackets extending forward from the smokebox, as shown in drawings of  ‘Prince’, but there is no indication of such supports in the Lane sketches. There also seems to be a pillar supporting horizontal bars but with no indication of where these were placed laterally. Was there perhaps a curved rail around the front of the engine?

 

20th Century drawing by G.F. Bird, derived from Lane’s sketches

 

 

Another puzzle is provided by those rather splendid curved side-brackets shown on the original Works drawing of ‘Aeolus’. It is not clear whether these were retained when the smaller wheels and consequently lower outside frames, were fitted during re-building. It is also not clear where they appeared along the length of the engine, although the photograph of ‘Vulcan’ suggests they were at the front of the smokebox and rear of the firebox, with intermediate straight supports to the sides of the boiler. Bird seems to have assumed this in his drawing but it not clear on Lane’s sketches.

 

Adding Surface Details

 

One of the pleasures of using 3D modelling tools is the ability to add rows of rivets, either in regular arrays or following a defined path, by using a few keystrokes and the ‘pattern’ commands.

 

For the frames, I drew one rivet and then used the rectangular pattern tool to create a 2 x 36 array for all the rivets on one side and then the Copy command to replicate them on the other side. For the smokebox front, I drew one rivet and then told them to follow a path around the edge of the smokebox – job done! With these additions, my 3D model in ‘Fusion 360’ looks like this:

 

Assembly of parts within ‘Fusion 360’

 

Preparation for Printing

 

As I have pointed out before, there is a difference between a 3D model and a printable 3D model.

 

My approach is to break the complete model down into several parts that can be printed individually and then assembled as a ‘kit’. In deciding how to separate the parts, I try to ensure that each part has a flat surface which can be laid on my FDM printer bed. An advantage of this approach is that the print times for individual parts can be quite short so, if some re-design is necessary, it can be done without having to re-print the entire model.

 

Smokebox Front

 

I felt that the most demanding task for my printer would be to reproduce the details on the smokebox front, including the supports for the valve gear. For this reason, I did a test print of this part first, since the overall success of my model depends on how well this region can be represented. After printing, my first trial looked as below (Note that I shall print the weigh bars and levers separately.):

 

My first test 3D print of the Smokebox Front

 

There were a few stray strands of filament that had to be carefully removed with fine tweezers but overall, I was very pleasantly surprised to see how well the details had been rendered, including the supports for the valve gear and the protruding piston rods, etc.

 

Further Components

 

Encouraged by this first trial (which took only 10 mins to print), I continued to select and print the various other components.

 

I have learned to keep the various parts in the form of individual ‘bodies’ within ‘Fusion 360’, so that I keep as many options as possible open for printing. In this case, the front of the smokebox, shown above, is a ‘body’ in its own right, separate from the rest of the smokebox. The back of this part is completely flat, to lie on the printer bed, while the front has some very fine details, including the piston rods and eccentric rods, protruding from the front surface.

 

Most of the other parts are simpler and very similar to the equivalent parts on models that I have constructed before. For example, the boiler, smokebox body, and firebox appeared as below, immediately after printing:

 

My 3D Printed Boiler Components on Printer Bed

 

I am especially pleased with the way the combined dome/safety valve cover has turned out. I printed this in two parts: the fluted barrel and the curvaceous top cover. I provided each part with dowels and mating sockets, so that they plugged together on top of the firebox. After fitting together, the printed parts look like this:

 

My 3D-printed Dome/Safety Valve Cover on Firebox

 

I am reminded of Brunel’s statement in a letter to the engineer T. E. Harrison on 5th March 1838. Forgive the sexist remarks but I am quoting verbatim: “Lastly let me call your attention to the appearance - we have a splendid engine of Stephenson's, it would be a beautiful ornament in the most elegant drawing room and we have another of Quaker-like simplicity carried even to shabbyness but very possibly as good as engine, but the difference in the care bestowed by the engine man, the favour in which it is held by others and even oneself, not to mention the public, is striking. A plain young lady however amiable is apt to be neglected. Now your engine is capable of being made very handsome, and it ought to be so.” [MacDermot, History of the GWR]

 

I do think this dome is very handsome and has printed very well!

 

Now, I have all the parts needed to complete the boiler assembly. The major parts all plug together, while the smokebox front is glued to the flat front-face of the smokebox itself. I have not yet attached the weigh-bar and rocking levers, to actuate the valve rods, which can be seen protruding from the front face:

 

My Model ‘Aeolus’ Boiler

 

This is a good place to pause before embarking on the running gear, which will support these completed parts. That will be my next post.

 

Mike