16mm Projects

Roy Plum and Alan Poxon : Péchot-Bourdon Locomotive of WW1

Péchot-Bourdon Locomotive of World War 1

The Locomotive……..   a brief description by Alan Poxon

In 1882, Colonel d'Artillerie Prosper Péchot patented a system of portable 600 mm gauge railways that could provide modern logistical support to the French Army. This système Péchot used rolling stock with two, three or four axle bogies to move artillery pieces and supplies into prepared positions. Motive power could be provided by soldiers or horses but, with the increasing size of the guns, it was clear that steam locomotives were needed.In 1884, Prosper Péchot was introduced to Charles Bourdon, an engineering consultant, who later went on to become a Professor at the École Centrale, the Engineering University of Paris. Together, they set about designing a powerful locomotive that could work on tight curves, hastily laid track and keep within the système Péchot axle loading. They were influenced by Paul Decauville who had been impressed by the Double Fairlie locomotives on an 1879 visit to the Ffestiniog Railway.

Despite Robert Fairlie's 1864 patent, the locomotive that Péchot and Bourdon designed was to all intents and purposes a Double Fairlie. Minor differences included changes to the steam dome and a simplification of the steam and exhaust piping. The Péchot-Bourdon locomotive was much shorter than the Ffestiniog Double Fairlies and sported two large spark-arrester chimneys. Fairlie died in 1885,  Péchot and Bourdon took out their own patent on the design in 1887.

When adopted by the military it was designated the locomotive articulées modéle 1888. To everyone else it was simply the Péchot-Bourdon. Despite reservations in some quarters about the locomotive being overly complex for military use, the French Army had 52 examples built between 1888 and 1906. These locomotives were largely employed in northern France, servicing networks of forts and defensive positions.

From 1914, the trench warfare on the Western Front led to a massive expansion of the 600 cm military railways and motive power was in dramatically short supply. France turned to the Baldwin Locomotive Works in Philadelphia with an order for 280 Péchot-Bourdon locomotives, the first one hundred of which were delivered just two months later. A further 15 were built by the North British Locomotive Company in Glasgow. With a length over buffers of only 18 foot 11 inches, the Péchot-Bourdon is less than 60% the size of the Ffestiniog Double Fairlies which allows the loco to negotiate the tightest of curves. Short boilers and a central steam dome were also necessary to safely climb and descend the steep inclines associated with the French fortifications in place prior to World War One. The locomotive's unique appearance was also enhanced by stacks of the briquette fuel, a large water lifter on the fireman's side and canvas curtains over the cab doors.

Only two Péchot-Bourdons have survived into preservation. One is housed in the Dresden Transport Museum in Germany and a second is to be found at the Pozega Railway Museum in Serbia. This latter locomotive was loaned for display in Paris in 2021.

The Model……  by Roy Plum

Having been interested in “Trench Railways” for some time, I’d drawn, 3D printed and assembled quite a few locos and items of rolling stock used by the British and Canadian armies.  The French Army prototypes always seemed a bit “quirky” and “odd-ball” to me, and hadn’t been able to worm their way into my imagination as had the British Narrow Gauge items. Blame Alan Poxon for championing the French trench railway rolling stock.  He’d e-mail me a photo every now and then, usually of some strange item of rolling stock, with the comment “How about one of these, then?”

I resisted until I saw a photo of a French field gun being transported  on 2 bogies and a couple of planks (I think that’s what it was, we’re going back a year or two now!) and I though…why not?  The French system of Trench Railways was a very “modular” system, using the same basic wagon frame and bogie design for 2, 3 and even 4 axle bogies… so if I drew it once, I could print all sorts of rolling stock based on the same basic design.

I produced the “simple gun transport” and this was followed by a quite complex rail-mounted gun, then a bogie well wagon, and then a huge gun barrel carried on 4x four axle bogies.

Up to now I had no French loco to pull this stock, and original photos depicted a wonderfully “odd-ball” loco that looked like a Fairlie but with a marked Gallic make-over:

It would be quite something to model….  but it looked ever so complex.  I’d had a go at 3D printing internal combustion locos, but a very small double-ender steam loco with outside valve gear….  Could it be done?  Could it be made to operate?? 

A live steam version would be something wonderful, but maybe an electric 3D printed one would be a good start? In a moment of weakness at the National Garden Railway Show (some years back) I’d purchased an excellent book by Malcolm Wright’s wife, Sarah, entitled “Colonel Pechot: Tracks to the Trenches” and this described and provided drawings of all the Pechot rolling stock and the Pechot-Bourdon loco.  Unfortunately, not all the drawings were scaled, and some stock seemed to have oval wheels, suggesting that the original works drawings hadn’t been scanned too carefully. Nevertheless, armed with these drawings, some data about key dimensions (e.g. diameter of wheels, loco length and height) and as many photos as I could find, I commenced drawing the loco.  As I drew it, I was thinking about how it would break down into component parts (just like an “Airfix” kit) that would be “printable”.  By “printable” I mean small enough to fit on my printer bed without warping, small enough to print in an acceptable time (say 4 hours max per part) and with careful orientation of the major printed faces (to try to minimise the cursed layers that are often so obvious in filament printed items). Here’s how the loco started to look:

Being a “double ender”, I only had to draw one end as the other end would just be a copy with a few “tweeks”. The next stage was to explode the drawing to produce a set of “piece parts” that could be printed.

You might notice that I often draw the same part with a number of variations, as I try initial prints and often find that I can make improvements to produce better versions. Here is the body printed, assembled and a smear of car body filler added to “fill the gaps” and “smooth the curves”.

So far I had a body, but what about the bogies?

The Bogie Challenge…..I checked the dimensions of the body and found it to be incredibly small….  So much so that I thought I might have made an error in scaling the drawings for 16mm.  Would an electric motor fit in the bogie?  How would it drive the axle…  by worm reduction or via gearbox plus bevel gears?  How fast would it run? Would there be enough room for the bogie mechanism in the boiler & side tanks?  What would be the minimum radius the loco could traverse?

I discovered that Roundhouse loco driving wheels were a suitable size for the loco, but all their axles are suited to dual gauge (45mm & 32mm) so are too long for my model.  I decided to be brave and chop out a suitable length from the centre of each Roundhouse axle.  The two halves were then drilled down the centre axis and a pin inserted to (try and) keep it all lined up.  A 3D printed jig ensured that the square ends of the axles retained their “true” quartering.  At the risk of offending the serious engineers here….  it was all held together with Superglue!! Here are the shortened axles: 

The largest size of MFA motor that would fit between the wheel back-to-backs was identified (An RE360…  width 27.5mm) and represented in my drawing, as were Roundhouse wheels and axles. 

Next question…. what gearing system to use?I re-read several 16mm Project write-ups on the Yorkshire Group website to see what others had used, and saw mention of HPC gears.  A perusal of their website revealed tables of sizes, combinations, materials and ratios that made the average book of logarithmic tables look like child’s play!  Eventually I decided to try a 40:1 reduction ratio with a steel worm and a brass gear.  The brass gear would have to be drilled out slightly to fit the Roundhouse axle but, hey-ho, it’s only money if it all goes wrong, ain’t it?

Drawing the bogie arrangement, it looked like this:Or exploded, like this:

For anyone wanting to try something similar, after a few trials the final chosen arrangement was a delrin plastic worm (Part No ZW0.5-1) and gear (Part No ZM0.5-40/1).  The worm had to be mounted on a brass shaft to adapt its 5mm dia hole to fit the 2.3mm dia motor shaft.  Delrin was eventually selected over steel and brass to try and reduce the running noise of the arrangement…  and it’s much cheaper!

In designing anything that involves mechanical parts I try to keep everything as sub-assemblies that can be screwed together (usually with 6 or 8BA nuts and bolts).  By avoiding glue, if things go wrong I can update the design and replace sections without having to throw a whole assembly away and start again.  This comes in useful if I find an idea just isn’t working, or if a part needs making stronger or slightly different in design to complement other changes that might be required. 

In this case the sub-assemblies of the whole bogie are held together by just 6 bolts.  Removing these allows replacement of the motor, the gears, the axles, the wheels, the cylinders and the motion if required. Here is the first attempt at the bogie frame with the Roundhouse parts added.  Notice that I hadn’t quite left enough space for the flange on the rear wheels, so a bit of gouging with a hot soldering iron was required.

This error was corrected on subsequent builds!

I don’t seem to have photos of the complete built-up bogie, but there’s a video of the initial design being tested here:


This test was performed prior to taking a hacksaw to the (expensive) Roundhouse axles.

In the video clip you can also see 4 of the screws that hold the bogie assembly together.  The other 2 screws fix the motor to the frame.

The final stages involved coming up with a design that fixed the bogies to the upper body and allowed them to pivot.  There was very little clearance anywhere, but it all came together somehow! Each bogie is held in place with 2 screws, one hidden under each water tank filler cap.

The RC gear and battery was crammed into the central firebox / boiler section, once again with very little clearance.  The loco runs on an 11.1v LiPo battery and I get around 50 minutes running time. Here are a couple of “finished” locos (although they haven’t had the complete dummy valve gear added, even now):

A Seuthe smoke generator has since been fitted into each chimney, providing the necessary smoke effects. A video of the model loco pulling an appropriate train is here: https://vimeo.com/690605025…and for those interested in the prototype Pechot system, here are some video clips that I’ve found on the web:  https://vimeo.com/666285421

After a few runs it was decided that a suitable sound system was required.... but that’s another story.

…and maybe, one day, even a live steam version???   Don’t hold your breath!

August 2022


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