Introduction

 

In Part 1 of this series, I described my model of the piston-carriage for the South Devon (SDR) atmospheric railway, based on drawings by Paul Garnsworthy in the Broad Gauge Society (BGS) Journal ‘Broadsheet’Nos 44 and 46.

 

It’s been great to receive so many positive comments – clearly some of my viewers like reading about ‘forgotten’ corners of railway history. They spurred me into getting on with the next phase. Thank you!

 

The carriage body was relatively straightforward, being similar to other broad gauge carriages that I have already modelled. I felt, however, that I should attempt to understand the workings of the system and I find that the best way of doing this is to build a model.

 

There are several drawings in the Patent Application by Clegg and Samuda but these were only intended to illustrate their ideas and contain features that were never used in practical applications of their idea. It was pointed out to me recently by a fellow member on this site @drduncan that the principle is the same as that used in the steam catapults on aircraft carriers in the Navy. It was in that context that I found some very clear drawings that illustrate the principle very well:

 

Slotted Pipe for Steam Catapult

 

In the SDR application, the sealing flap was made from leather strips that were hinged along one side of the slot. This meant that instead of being lifted bodily, as in the illustration above, it had to be raised at an angle, from the side opposite the hinges.

 

As a result of research by members of the BGS, negatives of drawings of the original SDR piston carriage were found in Bristol Museum and re-drawn by Paul Garnsworthy for the BGS Journal ‘Broadsheet’. This was no mean task, as the originals were not only very faded but consisted of a series of partial or split views, typical of the period. There are various plan and elevation views of the mechanism, which I found needed careful study to determine how the various components were arranged (N.B. I’m no expert in reading engineering drawings!)

 

The description in the two ‘Broadsheet’ articles helped to shed light on the various complexities. The following is based on those articles

 

How it Worked

 

The chassis framing beneath the carriage was quite conventional for the period, except for the central bay, where a wide space was left for the atmospheric apparatus. This apparatus was carried by bearings on the wheel axles, with two bearings on the central axle and one each on the outer axles,

 

The atmospheric apparatus. had its own substantial frame composed of iron plates, In the centre these formed an octagon, carrying the two bearings on the middle axle. From this octagon, longitudinal plates extended to single bearings located centrally on the outer axles. Two hangers, pivoted from the octagon, suspended a longitudinal plate, which hung below the carriage and passed through the slot in the top of the main propulsion pipe, set between the running rails. These hangers were angled to raise the leather sealing flaps on the opposite side from their hinges, running along one side of the slot.

 

Once the hangers were inside the propulsion pipe, they were attached to a pair of 11 foot long piston frames. Between these frames there were five rollers, which functioned to raise the hinged sealing flap, fitted along the upper slot of the propulsion pipe, and let it down again, once the plate connecting the pistons to the carriage had passed. There were two additional rollers mounted on the main assembly, above the pipe, to ensure that the flap valve was pressed down firmly after the carriage had passed.

 

Piston rods extended from each end of the piston frames, inside the propulsion pipe, to the pistons themselves, which were located towards each end of the carriage. The two pistons allowed the carriage to be operated in either direction, so that it did not need to be turned at the end of a journey.

 

In operation, the propulsion pipe, set between the running rails ahead of the carriage, was evacuated by stationary steam engines. Atmospheric pressure acting on the piston from the back of the carriage then pushed the carriage and its train forwards. Behind the leading piston, the rollers on the piston frame raised the sealing flap so that the blade connecting the carriage to the pistons could pass through and then, towards the back of the piston carriage, a top roller re-made the seal, so that the pipe could be evacuated again for the next train.

 

Creating my Model

 

Once I had worked out how all the gubbins were supposed to work, I could start to create a model. The drawings show lots of additional rods and levers, not all of which are understood, so I concentrated on the major elements of the system.

 

As usual, I started by importing drawings as ‘canvases’ in ‘Fusion 360’ and extruded the various components by tracing over these. Although the following description may seem straightforward, the actual design and fitting together of the various components involved a great deal of trial and error!

 

I started with the main carriage frame, which provides an overall orientation reference and defined the mounting points for the wheels and axles. I added the springs and axleboxes by the same methods as for my earlier designs.

 

3D sketch of carriage underframe and wheels

 

With these key reference points in place, I could then start to create the atmospheric equipment, described above, around them.  As usual, I extruded these parts from a drawing. I placed axle bearings on the central octagonal plates and on the longitudinal plates, in their proper locations over the three axles.

 

The frame (red) carrying the atmospheric equipment underneath the carriage

 

The next major component to create was the piston frame and pistons, which ran inside the propulsion pipe when the train was operating. This component is made up of several parts: two frames, the piston rods, the pistons themselves, rollers, and the hangers which couple this component to the octagon and thence to the train..

 

 

The piston assembly that runs inside the propulsion pipe

 

In the prototype, the hangers were connected to the octagon on pivots. These allowed the complete piston assembly to be raised clear of the track, in case the piston-carriage had to be attached to a normal train or simply to negotiate points and crossings (which were another problem for the atmospheric system)

 

Having created the various parts separately by tracing over the drawings, I then used the ‘move’ tools in ‘Fusion 360’ to bring them together into the correct alignment. At this stage, it became apparent that several minor adjustments to dimensions were necessary, to ensure that all the parts fitted together neatly in the correct orientations. I made two screen shots of the assembly: first with the pistons in the operating position and then hinged to one side by 45°, in order to clear normal trackwork.

 

 

The Atmospheric Equipment in raised and operating positions

 

I should re-emphasise that these 3-D sketches are simplified, to illustrate the basic operating principles of the atmospheric equipment. The raising of the piston assembly was achieved by pinion gears and shafts, which could be operated by a removable handle, rather like the starting handle on an old-fashioned car. Other control rods could adjust the pressure applied by the flap-closing rollers, as appropriate for the direction of travel. There may also have been linkages to open relief valves in the pistons since, apart from applying the brake, there was no provision for regulating the speed of the train.

 

This last factor once caused a major panic for the crew of a train on the Kingstown and Dalkey Railway, when they forgot to attach the piston-carriage to its train before starting. It is said that the carriage, operating as a ‘light engine’, reached a speed of 80 mph but, fortunately, was stopped before there was a disaster at the top of the incline!

 

In addition to the atmospheric gear, a braking system, controlled by handles in both the driving positions at the ends of the carriage, was fitted to operate on one side only.

 

Printing the Model

 

It was immediately apparent, from the small sizes of some of the components, that a working model in 4mm scale could not be created solely by 3D printing. In particular the various rollers and the piston rods would need metal components.

 

Unless, however, I was also prepared to build some miniature slotted pipe and flexible valve strips, the model would not be operational anyway so, simply as a demonstration model, I felt that a realistic ‘impression’ could be obtained by taking a few ‘short cuts’.

 

These applied mainly to the piston assembly that runs inside the propulsion pipe. I decided to embed the rollers within a solid piston carriage and run a brass wire longitudinally to connect this part to the two pistons. This proved more awkward than it sounds, as I also wanted a flat face, to lay on the printer bed but, after much trial and error, I found a solution that involved splitting the piston carriage into three printable pieces.

 

I similarly embedded the two flap-closing rollers into the upper frame and fitted another brass wire across the length of the octagon, to carry the pivoting hangers. Now, at last, it was time to think about making some trial prints.

 

Once I had a collection of what I considered to be ‘printable’ parts, I passed the drawings to the ‘Cura’ slicing program. At this stage, a few more problems emerged. I still don’t know how it happened but some parts were inclined at very small angles, such as 0.3 degrees. I do know that I have never experienced this problem before so, perhaps, there has been some change to the ‘Fusion 360’ software that allowed this to happen. Fortunately, it could be corrected by simple rotations in ‘Cura’, until the components lay flat on the printer bed.

 

Another problem emerged because I had failed to note that the body of the piston carriage was unusually narrow for a broad gauge vehicle, at only 8’ 4” wide. I should have been warned by the comment in the ‘Broadsheet’ article that “Nevertheless, [the frames] came perilously close to the wheels (7’ 7 ½ ” over wheels / 7’ 8½ ” between solebar flanges”. I had left insufficient clearance between the body sides in my original design to fit around BGS wheelsets, so I have had to widen and re-print the carriage body.

 

I knew that I was taking risks with some of the ‘printable’ parts I created but, in fact, my old Geeetech printer coped admirably with some rather small dimensions and awkward shapes. The first step was to print the main chassis members and fit the wheelsets. This went well, as shown below:

 

My 3D-printed underframe and BGS Wheelsets

 

The next step was printing the octagon and longitudinal members that carry the atmospheric apparatus. I designed these to be linked together by a brass rod passing across the width of the octagon, which I intend to use for suspending the lower parts of the equipment.

 

Since these are all small parts, I added the piston frames to make a ‘set’ of parts for printing together. The complete set printed in just 19 minutes. As mentioned above, I simplified the design of these parts so that they all had flat faces that could lie on the printer bed. The ‘rollers’ are not movable but printed as integral parts of the frames. After printing, the complete set appeared as below:

 

My 3D-printed Atmospheric frame parts

 

I printed the remaining small parts as a separate group, since they all involved more complex shapes, which I anticipated might prove difficult. To minimise the possibility of these parts toppling over while printing, I selected the option in ‘Cura’ to surround each part with a ‘brim’. I prefer to avoid support structures whenever possible, as they can be difficult to remove cleanly – even very thin layers of printed PLA can be remarkably tough!

 

In fact, even the very small ‘hangers’ came out better than I had expected, though the brim was not easy to remove from these. I find the only successful tool for this job is a finger-nail. I show these parts still in place on the printer bed - note that the cylinders are only 11 mm long. Note also the holes in the hangers, to allow them to be suspended from the octagon. The thin strips attached to the hangers are to allow these very small parts to be glued to the piston frame (the one with 5 rollers) shown in the previous illustration.

 

My 3D-printed cylinders and hangers on Printer Bed

 

As I mentioned above, I had to re-print the carriage body a little wider, in order for it to fit over the wheels, after which the whole kit and caboodle looked as below:

 

 

All parts for my model Atmospheric Piston Carriage

 

As with many jobs, it all looks quite simple now it’s done. Perhaps it’s down to my increasing age but this one cost me more “tears and sweat” than anything else I’ve made since I started trying my hand at 3D printing.

 

I shall pause here for a stiff drink and then tackle the job of assembling all those small pieces, which will disappear out of sight underneath the carriage!

 

I’m still thinking about the colour of the carriage. I think it looks nice and rather ‘rustic’ in brown.

 

Mike

 

Feature Photo - William Dawson, "From the Station in St Thomas to the Alphington Meadows", detail