Tony Geary
Pictures by the author

Locomotives with inside working motion might be regarded as the province of the modelling elite but in actual fact creating models with this feature is within reach of any practical modeller. The heart of inside motion is the crank axle and the components for this can be sourced from Laurie Griffin, MOK and Finney7 and there may well be others. I have worked mostly with the Laurie Griffin products.

Essentially the components supplied consist of two cranks with webs and a set of four eccentric sheaves. The cranks and eccentric sheaves are aligned with sections of wire, which pass through a series of alignment holes in the various components. I pin my cranks to the axle for ultimate reliability. The first thing to do is to assemble the items on to the axle and to check that they will fit inside the frames and hornblocks, if they are used.

I thinned and reshaped the crank webs and you can see that one hole was redrilled. Of course, it was the last of the four and the drill broke, so I drilled another hole just to make sure. The steel sheaves were custom made for this model. In the motion kits, they can be turned or cast brass ones. They are numbered so that they go in positioned in the correct order. The alignment wire can just be seen protruding at the bottom right and will be cut off later. The position of the cranks aligns with the corner of the square on the end of the axle.

In most cases I have found it necessary to thin down the webs of the cranks, to enable it all to fit. Once they are the correct size it is a good idea to use a large drill bit held in the hand in a countersinking action, to create a slight recess on the outsides of the crank webs to allow the solder to gather. Some eccentric straps are split and can be assembled after the crank axle is finished. Others are one piece so they must be fitted before the axle is soldered up and it is necessary to check that they rotate freely on the sheaves. Then they can be chemically blackened before assembly to prevent solder taking on the bearing surfaces. It’s probably a good idea to degrease the components, but I must admit, I have never needed to do that. I always check that the assembly is central on the axle. Once this is done, I use a small pillar drill to create a 0.8mm hole right through the crank web and axle, which in this case is Slater’s. I use lubricating oil and withdraw the drill bit often, to clear any swarf. It is important to ensure the drill is vertical and doesn’t miss the web on the other side. The steel Slaters axle is very mild and easily drilled. If the drill breaks off, it can be left in there. With any luck it will be stuck in the steel axle, so will perform the purpose of a pin.

You can see where I overdid the filing on the nearest web but there was still just enough material for the pin. The other pins need to go in on the other crank before it will be ready for soldering.

It is worth checking the alignment of the inside cranks with those on the wheels. If needed, I rotate the axle within the cranks to get it into the right position before it is pinned. Once the axle is pinned, I use a small blowtorch to solder. If I can extract the pins, I try to feed some solder paste and flux into the holes when putting them back. I use a No Nonsense plumbers’ flux, applied with a cocktail stick around all the points where the axle passes in to the webs.

I find there is no need to flux or solder the sheaves. They will be held in place with the alignment pin. More solder paste is then placed around the axle/web interfaces. It will be drawn into the joint by the flux. Once prepared, I place the axle on a brick or tile in a location where the surroundings won’t catch fire. I then heat the axle with the blow torch and, when it is up to temperature, the solder will be seen to run. It is worth having some solder to hand to feed on to the assembly, if it looks like there is not enough in place. Once the axle is up to the solder temperature, I try to avoid overheating as the solder will just run off. After coffee and a chocolate biscuit or two, the blackened artefact will have cooled down sufficiently and I can examine it. It will look pretty messy. Then comes the jolly task of cleaning it up with files, scrapers and emery boards, but being gentle with any bearing surfaces so that they do not get scored. Once it is beautifully polished up, it will be clear if the solder has taken. I’ve not yet had to revisit one.

If the crank has one piece eccentric straps, I use some WD40 or similar to clear any gunge remaining between the bearing surfaces. The aim is to be able to rotate the crank by hand without the straps sticking.

The next task is for me to cut away the sections of axle that are not required within the webs. I generally use a piercing saw as it is gentle on the whole assembly. I use a separate blade for steel and would not use it for brass.

With a good blade and a tidy cut, only a little filing is required to clean up the stub of the axle. The main thing I ensure is that the big ends pass freely through the gap. I like to make the connecting rods detachable. This assembly shows the lower part of the motion, which forms one sub-assembly. The steel end of the axle can be seen within the web and the pin locations are just visible. They can also be thinned down if required.