BUILDING A 3-INCH TRACTION ENGINE
For his latest project Jan-eric hits the road, building a traction engine inspired by a full-size example, but designed to work rather than follow a particular prototype...
As many readers probably know, I’ve been a Live Steamer for two decades by now, having built three 7¼-inch gauge steam locomotives, as well as some smaller steam ‘toys’, and also a couple of battery diesels, all described in this magazine over the past years.
The steam locos provide the most fun, both during building, as well as letting my passengers and myself experience the enjoyment of real steam operation during run days, either on my own track around our summer house, or at the track at our Railway Museum.
However, you do need a track when you want to run any kind of locomotive! This means I only have those two locations available for running unless I disassemble my ‘portable’ track and put it up at another temporary location – I’ve done this a few times but it’s really quite a hassle. Furthermore, the M6 joining bolts on the track pieces are already quite worn, and probably won’t stand many more disassemblies before they have to be renewed – a giant task, considering the more than 100 track pieces...
This fact, as well as inspiration gained during a visit to the Science Museum in London some years ago, where I could inspect a beautiful Aveling & Porter traction engine, (Photo 1), encouraged me to design and build a ‘freelance’ version of a traction engine in 3-inch to the foot scale (in other words one fourth of full size). This could be run in any suitable location with relatively flat ground, obviating the need for any track! In size, it would be comparable to my steam railway engines, and should be able to pull several passengers – provided that I would also build some kind of wagons, of course.
“In size, it would be comparable to my steam railway engines, and should be able to pull several passengers...”
The traction engine
As many readers will be aware traction engines became popular for agricultural and heavy hauling purposes in the mid-19th century, and were manufactured well into the 1920s, when gasoline and dieselpowered tractors took over, albeit gradually. Steam-powered traction engines were still used, at least in some locations, into the 1950s and even later.
In the UK, the use of such engines became uneconomical due to the high axle-weight taxes levied on all commercial haulers. This led to most of these engines being scrapped or simply abandoned. Fortunately, a large number have been preserved and restored, and can be seen in museums all over the world. Many are in operating condition, and are fired up and run during steam festivals.
Their designs varied, in addition to pure ‘haulers’, some were used as ploughing engines, having a plough-pulling, rope-winding drum under the boiler or on the hind axle. Others were elaborately decorated with a canopy and flashy paint work, and fitted with an electric generator. They were used as ‘showman’s engines’, providing mechanical power for carousels or other fairground rides, as well as electricity for long strings of light bulbs at fairs and exhibitions.
The American and English engines were clearly different in design; you can tell this by their wheels alone. The US engines (Case, Avery, Advance, Reeves and others) almost always had spokes made from round steel and welded to the wheel rim, while the British engines (such as those built by Burrell, Fowler, Aveling & Porter, Allchin, Marshall and Ransomes, to mention just a few) all had spokes made from flat steel stock, bolted to a typical ‘double T-profile’ in the rim of the hind wheels.
Another notable difference was the often much larger flywheels as well as the more forward positioning of the cylinders on the British engines. The US engines usually had longer boilers, and a smokebox that protruded quite a bit forward over the front wheel. Case in point: Case!
I chose the British design as a general prototype for my model, and gained inspiration for the details from many different manufacturers, so I’m not building an exact scale model, rather something that generally looks British. This really shouldn’t matter to readers, since I’m definitely not writing a proper construction article – my intention instead is to show the techniques I have used in design and fabrication of my model.
I hope that some of the ideas shown may be of use to other builders of miniature engines, regardless of exactly what they are building – many of the methods can be used for locomotives, too. Therefore, I will not provide anything more than very general measurements, nor any detailed drawings. If you intend to model a particular engine, there are companies providing detailed drawings and sometimes even castings for many of the different British prototypes.
Design methods
Having decided to build the traction engine in 3-inch to the foot scale, I was faced with a big problem: how do
I make the huge wheels? They would be more than 450mm, 18 inches in diameter, twice what my little hobby lathe could handle – it is one of those ubiquitous, low-cost Asian ‘9 x 18’ lathes. The only way possible to produce such large wheels in my little workshop would be to make the wheel rims from steel plate, formed around other pieces of plate cut to the correct circular outline.
You may remember my recent locomotive articles, describing how I used either laser or plasma cutting to speed up both the design and the actual construction of two 71/4-inch gauge steam locomotives as well as a battery diesel. Now, I am once again using the time-saving methods of CAD (Computer-aided Design) and CAM (Computer-aided Manufacturing) – with the help of my model engineering friend Kustaa Nyholm and his home-brew plasma cutter, we cut most of the parts for the traction engine from 3mm steel plate.
I used a simple, free 2D CAD program to draw all the parts to be plasma-cut, first scanning an old outline drawing of a Marshall traction engine that looked particularly pleasing in my eyes. This drawing formed the background for the parts to be traced in the CAD software.
Making changes here and there, both for technical and constructional reasons, as well as incorporating some parts and shapes from other Britishstyle traction engines, I produced the general outline seen in Figure 1. This drawing already shows the shapes of some of the plates to be cut, the four-shaft gearing, as well as a tentative arrangement of the boiler’s tubes and fire-door.
The total length of the engine,
“A big problem: how do I make the huge 450mm, 18 inches in diameter, wheels – twice what my little hobby lathe could handle...”
when finished, will be 1,180mm, almost 4ft, while the width, measured over the hind wheel hubs, is a little under 600mm/2ft. At the time of writing, I estimate the final weight to be at least 120 kilograms, in other words 260lb. The boiler diameter is 160mm, or 6.3 inches – maybe a bit smaller in proportion than most typical British traction engine boilers, but I did happen to have a coppernickel tube of that size ‘in stock’, so my choice was predictable...
Figure 2 shows most, but by no means all of the parts that I have drawn in the CAD software. These are all to be cut with my friend’s plasma machine. Our raw metal retailers normally supply steel plates in sizes starting from 1,000 by 2,000mm (3.3 by 6.6 ft) as delivered from their wholesalers, or sometimes even much larger than that, if the plates are coming directly from a steel mill. Since Kustaa’s plasma machine ‘bed’ can hold material of a maximum size of 300 x 1000mm (1 by 3.3 ft), I paid the supplier to have the plates cut to strips of this size before delivery.
This also meant that I had to break up some of the largest parts of the engine into several smaller pieces of plate, later to be joined by TIG welding; more about that in a forthcoming article. Altogether, we used a dozen or so of these steel plate
strips, weighing 7kg (15lbs) each. Only a small fraction of this material became waste, thanks to placing all the parts as closely as possible in the CAD layout.
Photo 2 shows the plasma machine when it has just started cutting, beginning at the lower left corner of the 3mm (a tad under 1/8-inch) thick steel plate. The cutting speed is around 35mm (11/2-inch) per second, so it doesn’t take very long to cut the whole plateful of parts.
The bottom of the machine contains a water tray, since there are a lot of sparks flying around, requiring protection both for the workshop (note the aluminum spark-arresting plate at the extreme right in Photo 3 – it is an old offset printing plate) as well as for the operator and also the onlooker, in other words yours truly.
I’m smiling in the photo, but you can’t see it because of the face mask – a necessity in the atmosphere of steam and metal dust generated in the cutting process! Ear protection was required too – the compressor and the plasma torch are extremely noisy. After we had cut five plates during one session, my mask was all brown with rust, Photo 4!
Photo 5 shows the parts cut from one single plate, ready to be picked up with a magnet (at this stage, the parts are hot), while Photo 6 shows the waste that was left of five steel plates
after the parts had been removed.
There is one finished part at right in the photo; the long strip with rectangular slits will become the rim of one front wheel. The slits will secure the outer ends of the wheel spokes. Many of the parts for this engine were designed with the same tab-and-slot method I used for my previous loco projects, making the final assembly a breeze, almost like putting together jigsaw-puzzle pieces!
TIG welding
The two most useful tools in my workshop are a TIG welder and – hold your breath now – an angle grinder! Together, they enable me to build relatively complicated assemblies in a very short time, albeit not with the precision that is required by most dyed-in-the-wool, true model engineers, who enjoy making museum-quality miniatures.
My TIG welder can provide 160 Amps pulsed DC, it has a HF ‘spark’ start of the arc – and it can also be used as a ‘stick’ welder by changing the electrode holder. Welding in TIG mode, it of course also needs a tank of Argon gas with regulator and valves. The total cost for such a system is around £1,500 these days, a lot more than when I bought it more than ten years ago. Cheaper units made in Asia are available, but they lack many of the features this Eu-made unit has.
With TIG, some parts can simply be melted together, others need filler metal in wire form. In Photo 7, I’ve started fusing a steel strip to the inner
edge of a rim of a hind wheel, using no filler – note the C-clamps holding the strip securely in place, thus avoiding any gap. Passing the TIG torch over the junction area, the pieces are securely fused together. The edge will later be smoothed with the angle grinder.
The two long and flat, perforated pieces which were needed to form one wheel rim (these parts are coloured dark yellow in the third section at left in Figure 2), were first joined together into one long strip by TIG welding (a complete piece, more than 1.4 metres/4.5 feet long, could not be cut in Kustaa’s plasma machine), and then ‘rolled’ to circular shape in a professional metal workshop. The welded joint was of course cleaned up with the angle grinder before the rolling operation – any difference in plate thickness at a weld would wreak havoc in the rolling process! The final splice was again welded, in order to form a continuous, unbroken circle of steel for the wheel rim.
Note the flat ‘ring’ lying under the rim in Photo 7 – it consists of four parts, also coloured dark yellow in the top left section of Figure 2. In the photo you can also clearly see the welded joins connecting the four pieces into a circle.
I had ground the outer edge of this ring free of any imperfections, to be as circular as possible, and in Photo 8 you can see it welded in place, under the aforementioned edge strip. Here I have used a substantial amount of TIG filler wire in order to get a nice ‘fillet’ in the weld, which imitates the slightly rounded inner corner of the original T-profile rims used in the full-size traction engines.
Uppermost, the photo also shows that I have tidied up the outer, previously fused ‘double-thickness’ edge of the wheel rim with the angle grinder. Note the smoothly ground splice of the wheel rim at lower right. The wheel will later be painted with a rather thick rust-proofing primer paint, so the remaining grinding marks and the slight unevenness of the welds will hardly be visible.
In this way, I was able to produce a reasonably round wheel, just from pieces of flat plate as in Photo 9. Here, only one side is finished, soon there will be another strip and ring welded to the back side of the wheel. Measuring the diameter at several points, I noted that this large wheel was no more than 1.5mm (1/16-inch) out-of-round anywhere – good enough for my purposes, considering the rough method of construction!
This photo also shows the TIG equipment: the yellow welding machine on the shelf, the automatically darkening helmet (almost totally blocking the view of the Argon tank and its regulator behind it), as well as the special TIG gloves, with fingers made from thin, very supple chamois leather. A TIG arc contains very intensive ultra-violet radiation, so properly covering all your bare skin is a necessity. I use a leather welding apron, since even a light-coloured shirt transmits enough UV to cause a good sunburn...
Getting a grip
A very typical feature of British traction engines are the strakes on their hind wheels, absolutely necessary in order to get a grip in loose soil. On US engines, these were often made of angle iron, protruding quite a bit more from the rim, like the ‘spuds’ that can
be attached to British wheels.
There are 32 of these strakes on each hind wheel in my model. They were all plasma-cut from the same 3mm steel plate as the rims, and were cut with two holes each, aligning with holes in the wheel rim. The strakes are coloured light green in the top left section of Figure 2. Short pieces of round steel welded into the holes in the strakes were fitted into the holes in the rim, and again welded, now from the inside. Photo 10 shows this, carried out halfway around on one wheel. The welds on the strakes were later ground smooth, of course.
All in all, I needed four wheels; the two front wheels, at a bit under 280mm (11 inches) in diameter, are significantly smaller than the hind wheels, and have only 10 spokes each, instead of the 16 on the larger wheels, and they have no strakes. In Photo 11, among the mess in my tiny basement workshop, you can see all four wheels with spokes attached.
Photo 12 is a close-up of a tentatively assembled hind wheel; the hub is still missing. The hub will contain ball bearings – I plan to use only self-lubricating, completely sealed bearings in this project. The construction of the wheel hubs will be described in a later article.
In order to continue building the traction engine, I will next have to design and build the boiler, since, unlike its counterpart in a steam locomotive, the boiler is an important element of the mechanical structure holding all the other parts of the engine together.
“Any difference in plate thickness at a weld would wreak havoc in the rolling process...”