John Cockcroft - Gazette - November 2018 (Vol 20 No.9)

One of the most common problems with models of locomotives that include Walschaerts valve gear is movement in the return crank on the driving wheel. To be frank, I don’t know all the technical terms in the valve gear, and because all it does on a model is: a) look right, and b) not stop the loco from running smoothly. This crank sits on the end of a crank pin with the coupling and connecting rods behind it and it is supposed to remain at a fixed angle and rotate so as to impart movement to the valve gear so that it moves like the real thing. It is important that it does not slip or move, or wobble, or cause obstruction.

On the real loco the crank is very securely fastened and can be removed for maintenance. I tried to be clever and make it screw on the pin and stop tightly just at the right point – but I found it a pointless and frustrating exercise and have concluded that the best way to secure these annoying items is to solder them to a crank pin nut and the pin itself. This is because when you think of it, even if you destroy the pin when you have to take down the motion, hopefully a rare action, the crank pin is very cheap to replace and can be cut off in the worst case scenario - so solder it I do! Using Slater’s wheels the danger is that you can melt the plastic wheel by heat transmission down the pin. I alleviate this by using a thicker 10BA crank pin which replaces the supplied 12BA pin, and tapping the hole in the wheel 10BA. You then have to tap the brass top hat bushes 10BA which prevents any looseness in them and in some cases they can even be reversed, with the flange outside to act as a retainer for the coupling and connecting rods. By the way, the same technique can be used to make flush coupling rod retainers on leading crank pins where clearance behind the slide bars is an issue. So in practice I have found that by not dwelling too long with the soldering iron the heat sink provided by the 10BA pin, bearings, connecting rod and coupling rods protects the wheel. But don’t be daft. Pause and let things cool if you have to mess about. I use 145 degree solder and pre-tin the back of the crank and sometimes the pin and nut. You may have to adjust it a bit, but let it cool before each adjustment – and anyway you have to test the chassis to establish when it is right.

By using this method there is little chance of the crank coming loose and it should remain where it is supposed to be for years. Which it will - if you have cast iron wheels with the pins tightly screwed into them. But, if you use Slater’s wheels, it is possible for even a 10BA screw to move in the plastic wheel. This is because unlike real locos, models are routinely grabbed by the Great Hand of God, and the valve gear – being furthest out - frequently ends up with the full weight of the loco wrenching it round on its return crank. So it needs to be much more secure than it would have to be if the loco always remained on the track. Here is how I try to alleviate this brutal state of affairs. As I noted above I replace the valve gear crank pin with a 10BA screw tapped into the Slater’s plastic centred wheel. Some Slater’s wheels have a rebate at the rear of the wheel so that the head of the crank pin screw is recessed into the wheel. When this not present I always use a suitable sized drill in a pin chuck to make a countersunk hole as it is not a good idea to have something sticking out near the axle bearing. I screw the 10BA bolt into the crank pin hole as tight as I dare without stripping the thread. Then I take a short section of 0.7mm handrail wire and bend it into an L-shape with the long side maybe 6mm to 8mm and the short side up to 5mm (these dimensions are not critical but the short side should not protrude out of the front face of the wheel and the long side should fit neatly into the slot in the head of the crank pin screw).

Next I drill a 0.7mm or 0.8mm hole into the rear of the wheel in line with the slot in the crank pin screw head and about 2 to 3mm from it. The short arm of the L-shaped wire can then be pushed into the hole with the longer arm in the screw slot and a quick blob of 145 degree solder secures it. The crank pin will not move unless you hit the other end with a hammer and strip all the threads in the wheel! Unless you are very clumsy it will be very difficult to pull the return crank out of position when you lift the loco. In fact it is more likely that you will bend the valve gear than move it out of place.

I have dealt with enough wobbly valve gear to realise that not everyone knows this technique but I apologise if I have patronised those who do.