In my previous post about the tender I modelled for ‘Viper’, I ended with the comment that “The outside frames and, especially, those solid-plate wheels look far more like the wheels of the engine ‘Ajax’, built by Mather, Dixon & co. - but that leads to another story …”

 

Now I can take up this story by looking at the design of ‘Ajax’, sometimes referred to as a ‘super freak’ amongst the motley collection of locomotives that Brunel ordered for the nascent GWR. As originally delivered, Ajax had driving wheels of 10 ft (just over 3 m) diameter, built up not with spokes but out of a number of solid iron plates. It was their similarity to the solid-disk wheels on the tender for ‘Viper’ that set me thinking about how to model these huge driving wheels, using Autodesk ‘Fusion’ software.

 

 

There is a YouTube Video by Anthony Dawson about Ajax, which discusses the various disputes concerning the actual appearance of Ajax, which took place towards the end of the 19th century. Experience has shown me, however, that the recollections of ‘old enginemen’ can be very suspect – including even those of Daniel Gooch - so I prefer to use contemporary sources whenever possible.

 

Correspondence of the time provides plenty of evidence that the driving wheels really did have a diameter of 10 feet, including illustrations of their use to create a trolley for moving the statue of the Duke of Wellington to Hyde Park Corner, shown in the Illustrated London News, dated 3rd October 1846.

 

 

 

 

 

 

Once again I am indebted to the young draughtsman, E.T.Lane, who died at the age of 20 but, fortunately for us, left a unique legacy of drawings of early GWR rolling stock. These included a drawing of ‘Ajax’, showing the details of its remarkable solid-disk wheels.

 

 

E.T.Lane - Drawing of ‘Ajax’

 

 

Modelling the Driving Wheels

 

My model driving wheel is a simple extrusion from a series of concentric circles, representing the hub, rim, and flange (referred to as a ‘flanch’ in early 19th-century writings). I added the lines separating the six individual plates that made up the prototype wheel, using the ‘circular pattern’ tool to ensure that these lines were regularly spaced.

 

Then I drew and extruded a single rivet and used the ‘circular pattern’ tool to create a ring of rivets just inside the outer rim of the wheel. A complication is that the pattern is interrupted at each junction of the plates, so I had to manually delete those six rivets. Next, I used the ‘rectangular pattern’ tool to create the lines of rivets along the edges of each plate. The following illustration shows screen shots at each stage of this process:

 

 

Stages in creating my 3D model of the ‘Ajax’ Driving Wheel

 

Thank goodness for computer modelling – there are 372 rivets on each wheel and, of course, I need two wheels. With the ‘Fusion’ software, it was the work of a few moments but I would not relish the thought of pressing these rivets out individually!

 

My 3D model of ‘Ajax’ Driving Wheel

 

The carrying wheels were of the same design as the wheels I made for the tender of ‘Viper’, although larger in diameter.

 

Creating the Valve Gear

 

Lane’s drawing clearly shows the characteristic fork of Carmichael’s valve gear, which I have previously created for my model of Viper.  I copied the main components of the gear from there and applied them to the current model. Some adjustment of the layout of the parts was needed to match the locations visible in the Lane drawing. Fortunately, I had modelled all the components of the valve gear as separate ‘bodies’, so it was possible to adjust the lengths and positions of the various rods to match the dimensions of ‘Ajax’.

 

When assembled in this new location, the valve gear, with its single eccentric for each cylinder, appeared as shown below:

 

 

Carmichael Valve gear applied to my 3D model of Ajax

 

 

Salter Spring Balance

 

In previous models, I have usually omitted the Salter spring balance safety valves but decided to add these here, as they are such a prominent feature on Lane’s drawing. They turned out to be an interesting mini-project in their own right, involving a mixture of ‘pipes’ and extrusions in ‘Fusion’.

 

It took some time to create the whole assembly but the important point is that these safety valves are now available to me as ‘components’ for fitting to many other engines. Although I try as much as possible to make sure that dimensions are within the capability of my 3D printer, I suspect these items will need to be fabricated from brass wire in a ‘real-world’ model.

 

3D model of Salter Spring Balance

 

 

Chimney Cap

 

Lanes drawing shows a wire mesh ‘spark arrester’ on top of the tapered chimney. I decided to create this by means of the ‘Pipe’ tool. I drew the path of one loop of wire then turned it into a ‘pipe’. After placing this first loop across the diameter of the chimney, I used the ‘Circular Pattern’ tool to create the complete ‘bonnet’. This is another component that is probably not amenable to 3D printing!

 

 

My 3D model Spark Arrester

 

 

Completing the 3D Model

 

It was only when I started to assemble the remaining major components of the model – boiler, smokebox, and firebox – that I came to realise what a peculiarly proportioned engine the prototype ‘Ajax’ actually was.

 

Despite those huge driving wheels, the boiler was tiny, even by the standards of the time – only 3 feet in diameter and 8’ 6” length!  Perched high above the driving axle, leaving room to clear the cranks, it does look rather ridiculous. The knock-on is that the firebox and smokebox also have to be placed very high, in comparison with other engines of the period. Apart from their elevation, these parts were conventional and I created them by adapting the proportions of components I had already made for other models.

 

The elegant brass-domed boiler fittings were also produced by my usual method of drawing the profile and then using the ‘Revolve’ tool to create the cylindrical bodies. I added wooden cladding to the sides of the domes by first creating a single plank and then using the ‘Circular pattern’ tool to create evenly-spaced rings of cladding. (referred to as ‘cleading’ in early 19th century writings)

 

Completing the model involved adding a collection of small items, most of which I could copy from previous models – springs, boiler supports, buffers, etc. I noticed that Lane’s sketch shows a vertical line above the centre of the driving wheel splasher. I have interpreted this as a brace to help support the very tall splashers. In the absence of any further information, I created a simple hoop between the two sides of the engine. There are several other bracing rods between the horns supporting the axles.

 

 

 

Seen in isolation, this engine looks reasonably sensible, if rather ungainly. It has been reported that when first built “these two 10ft. wheel engines had a projecting front, and the splashers

covering the wheels above the frames were made to represent paddle boxes of a steamboat. For these reasons, Dr. Lardner says, they were generally known as the ‘boat engines’”. If they were used initially, these features must have been removed by the time Lane made his sketch.

 

Comparisons

 

When placed alongside a contemporary engine of more conventional design, the absurdity of the huge driving wheel, together with the very small boiler, becomes much more obvious. It is hard to comprehend that the boiler of ‘Ajax’ is actually smaller than that of ‘Viper’!

 

My 3D models of Viper and Ajax

 

Although the tender for Viper does have some design features in common with those of Ajax, as mentioned in my previous post, it is clear from my models that they could not have been used together, with such a difference in footplate heights.

 

Very little is known about the tender for Ajax but it seems likely that its body was placed on a raised platform, possibly in a manner similar to that of the Firefly replica tender at Didcot, which has a wooden plinth.

 

 

Firefly Replica Tender with Wooden Plinth

 

To finish, I placed Ajax and Vulcan together in front of my model of the old Paddington Engine house.

 

 

Ajax and Vulcan at Paddington Shed 1840

 

 

Mike