Nick Baines

 

Even by nineteenth century stands, Cramptons were unusual locomotives. Only a handful were ever built and run in Great Britain, but in 1848 the LNWR constructed what was to be the largest to appear here. Named Liverpool, it had a service life of only ten years, including an appearance at the Great Exhibition of 1851. For its time, the weight, wheelbase, and the huge driving wheels were exceptional in every way. Accounts suggest that it was too heavy for the lightly-constructed track of the time, and spread the rails, which is certainly consistent with the short life.

There is a lot in the prototype to interest me as a model builder. The Stephenson’s valve gear is outside the driving wheels and driven by a pair of huge eccentrics, and this feature is so obvious that it just has to be modelled correctly. The driving wheels themselves, eight feet in diameter, are striking, and there is an odd collection of leading and carrying wheels. The two front axles have wheels of different diameters, and the centre wheels are flangeless. The outside frames with all their rivets and bolts are another obvious feature.

It is not apparent in the side elevation, but the boiler is not circular. It is egg-shaped, being formed of two semicircles (or almost semicircles), the upper being of larger diameter than the lower, joined together. Clearly the intention was to make the largest possible boiler, but one that also had to fit between the wheels. The firebox too is an odd shape, waisted to fit between the driving wheels, then expanding at the front end where it met the boiler. Internally it is most interesting to the engineer, but thankfully irrelevant to the modeller, with a huge number of narrow boiler tubes, a split firebox, and a species of thermic siphon, the last of these dating almost a century before Mr. Bullied fitted them to his Pacifics.

Altogether it represented something quite out of the ordinary, and the model required a lot of thought and planning. Conventional ideas about a separate chassis and body, the way model locos are usually made, went out straight away. There is not really a footplate at which to split the model at all. The final solution involves a number of subassemblies, bolted together. The “backbone” of the locomotive is the inner frames, spacers, buffer beam, and drag beam. The outer frames and cylinders are attached to this. Other subassemblies comprise the piston rods and slidebars, and the valve gear. The smokebox, boiler, and firebox form another subassembly that bolts directly to the inner frame assembly. The final major component is the driving wheel splashers and the footplate between them.

The construction of the model actually resembles that of the prototype to a much greater extent than is usual. But it was also consistent with an approach to loco building that I had been developing gradually. At one time I followed the common approach of soldering everything up, but more and more that just did not seem the right approach for scratch building. Increasingly I had been using subassemblies for my models. It does require a certain forethought, and of course there are holes to be drilled and tapped to take the bolts that hold everything together. In spite of that, I do find it facilitates the construction, being able to remove and to a limited extent realign parts at will. It also makes the painting so much easier. I took this process further with Liverpool than previously, and it has proved to be the logical development of this strategy.

 

 

 

The inner frames, beams, and spacers, are all cut from 0.7 mm nickel silver on the pantograph milling machine. The frames are an odd shape because of the great difference in size between the driving and carrying wheels, which means that the driving wheel bearings are located in frame extensions above the top edge of the frames, and the carrying wheels below the bottom edge. The front beam and the drag beam are permanently fixed to the frames because there is no easy way or even need to make them detachable.

As the photo shows, the driving wheels run in fixed bearings. The leading axle is carried in a sleeve bearing that floats vertically in slots. When assembled, the bottom of the boiler is in contact with this bearing, and forms a pivot about which the wheel assembly can rock and take up any unevenness in the track. The remaining axles float vertically but are unrestrained. Thus the weight is entirely carried on the leading and driving axles (hopefully more on the driving axle, but that will come later in the story). Given the compensation on the front axle, this should give a stable arrangement. The other axles just go round and carry no weight. The side play of all the wheels is limited by the splashers. On the prototype the central pair of wheels were flangeless, and that was copied on the model. The intention is that the loco will accept a minimum of six foot radius curves. An early track test showed that this requirement was met, much to my relief.

Working backwards from the front beam, there are two spacers with a curved shape. This is because they also support the boiler which is bolted through the holes in the centres of the spacers. The third spacer has small lugs that prevent the motor and gearbox assembly from rotating about the driving axle. The brackets on the outside of the frames are to carry the cylinders.

 

 

 

The outer frames are 0.4 mm nickel silver, also done on the pantograph mill. The step that runs along the outside is soldered along its length, and that also has the effect of keeping them straight. The rivets were done with my rivet press, and the various holes you can see are for the bolts used on the prototype. These will be represented using plastic mouldings from Grandt Line – I don’t do absolutely everything myself.

These frames are bolted to the ends of the buffer and drag beams, using small and hopefully inconspicuous lugs, and to the brackets that also support the cylinders, using 12 BA bolts. The frames had to be removable for access to the wheels. If they were fixed, I would have had to allow the wheels to drop from the frames, which is easy enough for the carrying wheels, but a potential nightmare for the driving wheels encumbered with motor, gearbox, and all the rods and valve gear.

 

 

I will deal with the easy things first. For the motor and gearbox I gave Brian Clapperton, otherwise known as ABC Gears, a copy of the Liverpool drawing on which I had marked up the space in which they had to fit. The motor is in the firebox, then the drive has to go through the ash pan, underneath the footplate, and upwards to meet the driving axle. Brian quickly confirmed that nothing in his standard range would fit such an odd locomotive, but offered to make me a special gearbox, which he did at a very reasonable price. Job done – I did not even have to calculate the gear ratio, Brian did that for me too.

The carrying wheels are Slater’s products. I doubt if the spoke profile is quite right, but they are hardly visible behind the outer frames and life is too short for some things. One set of wheels was mounted in the lathe and the flanges were carefully removed, but preserving the coning. The wheels on one side of the loco are shorted out using copper wire, carefully soldered between the tread and the hub, because I am using the split polarity (“American”) system of pickup, where the loco and tender are of opposite polarity.

The driving wheels are something else entirely. Of course nobody makes an eight foot diameter, eighteen spoke wheel with extended boss, so there was nothing for it but to make them myself. If I had needed a lot of them, it would have been worth making a master for the wheel centre and having it cast in brass (I doubt that whitemetal would have the necessary strength), but I needed so few that I chose to machine them. Because the manufacturing process is not specific to Liverpool, you will find the notes elsewhere on this site.

 

 

The valve gear is driven by eccentrics on the outside of the driving wheels. The eccentrics were made with a single step, as shown in the section below. With two eccentrics mounted back-to-back, this is sufficient to hold the eccentric sleeves in position.

 

The eccentrics were turned from brass bar. This was offset in the 4-jaw chuck and the centre bored for the driving wheel boss to a depth sufficient for all four eccentrics. It was then centred and turned to size. Each eccentric in turn had the step formed, and was parted off. The two eccentrics on each side have to be set 180° opposed to each other. I made up a simple jig to hold them in place and soldered them, before drilling them for the crankpin. Of course some solder got on to the steps where the sleeves run, and had to be scraped away.

Each sleeve and rod was cut from a sheet of 1.6 mm nickel silver to the outside dimensions, clamped on to a faceplate with a backing sheet to avoid damaging the faceplate, and the centre was bored out to be a running finish on the eccentric. This has to be a good fit, otherwise the whole valve gear becomes sloppy. Like the prototype, the sleeve had to be split at the bolted flanges in order to assemble the valve gear. I used a 16 BA nut and bolt for this – this size is just small enough for the purpose. First each flange was drilled 0.85 mm. This sort of thing used to frighten me to death, but gradually I learned how to do it without breaking too many drills (and more significantly, leaving the broken end irretrievably stuck in the part I was trying to drill). A high-speed drilling machine that holds the drill properly centred is important. It should also be equipped with a sensitive feed. I actually use a drill chuck held in the pantograph mill for the purpose. The drill bit should be sharp (of course), and kept lubricated. Ignore the books that tell you brass should be worked dry. For drilling tiny holes, lubricant helps. Almost any fluid is better than no fluid, and in situations like this, I use spit, of which I have a supply conveniently available.

Once drilled, the sleeve was then split at the flanges using the finest piercing saw blade I could find, the cut faces were dressed, and a thin shim was soldered in place to make up the thickness of material lost in sawing. The rod itself was thinned down and correctly shaped. The forked end was made separately and soldered in place. I prefer to make components like this all in one piece, but in this case the forked end would require it to be made from much thicker material, most of which would have to be tediously machined away. Sometimes a compromise is necessary.

Finally the sleeves can be fitted to the eccentrics. Doing up the tiny bolts was a very frustrating process, so much so that I will now do almost anything not to have to disassemble and reassemble them. Actually I cheated. The drawing shows two bolts on each flange. I used only one and that was bad enough. The assemblies were quite tight to begin with because of the last vestiges of solder on the eccentrics, but with the application of oil and a couple of hours running in, they loosened up.

 

 

 

The cylinders are a complicated by the fact that the valve chest and the top of the cylinder assembly is inclined at quite an appreciable angle. The front end is therefore larger than the rear. The basis of the assembly is a thin-walled tube whose outside diameter was turned to be slightly less than the diameter of the finished cylinder. The front and rear plates were soldered to the ends of this tube (everything held together with a bolt through the centre), and then the outer wrapper was soldered in place. I am sometimes asked how I get my corners so sharp. The answer in this case is to make the wrapper slightly oversize, solder it in place, then file it down and finish by polishing it on a piece of wet and dry paper on a flat surface.

As can be seen in the photographs, the slidebars, motion bracket, piston rod, crosshead, and connecting rod form a separate assembly. The piston rod runs in a tube that goes right through the cylinder, and is secured by a nut disguised as the gland on the front end of the cylinder that screws on to a threaded portion of the tube. The alignment of the piston rod was critical because it projects a long way forward of the cylinder to drive the boiler feed pump -- that funny shaped thing at the top of the photograph.

I put the piston rod, conn rod, and crosshead together before assembling them in the slidebars, and it was actually quite fiddly holding the four bars in the correct alignment while soldering them up. Another time, I would make it so that the crosshead can be dismantled and added to the slidebars after rather than before assembly. Sometimes I learn things the hard way.

 

 

 

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