Entry posted by Ian H C
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So, the super stiff springs. I could modify the chassis and fit compensation; don't want to do that. I could try and incorporate an different kind of suspension: don't want to do that either. I want to retain the prototype fidelity of this model. So I'm left with the task of making the suspension work. I should add that on a different blog this subject has come up and Mr Parkins suggests that the springs are mostly cosmetic and the axleboxes allow the wheels to drop into depressions in the track providing a kind of compensation. Well, maybe. But I've set myself the challenge of making the springs work so that's what I'll do. Whereas parts scale down the physics doesn't.
Could the springs be made from another material? I don't aim to get too far into the science, but what we'd want is a material with a lower Young's modulus, a spring that bends more for a given load. Of all the traditional spring materials phosphor bronze has by far the lowest modulus, so we already have the best easily available material. There are obviously materials that are more flexible but they don't make good springs, often because they have a correspondingly low yield strength and would be more likely to bend than spring back. Again avoiding the maths, the deflection of a spring made from a simple strip is proportional to the cube of the thickness. So reducing the thickness of the strip to a half (in this case to 0.2mm) would reduce the spring stiffness by a factor of 8. For a 125g wagon we'd then expect a spring deflection of about 0.65mm. That's close to being practical in 7mm I think. If we look at the actual spring we see that, just like the prototype, there are two leaves that span the distance between the spring seats on the chassis, and a number of shorter leaves that don't, but do increase the effective stiffness of the spring as it deflects more. The spring leaves only work together when they are in contact so if we can separate them they'll work in isolation until they touch. The plan then is to reduce then thickness of the top two leaves to one half of the original thickness and bend them carefully so that when loaded with their share of 125g they are slightly separate from the leaves below. That way we can have the wagon 'floating' on the top leaf and bending under load for short distance until it contacts the leaf below, also thinned to half, and from that point the spring progressively stiffens until it contacts the unmodified leaves and becomes effectively solid. The top two leaves will be less than scale thickness but material science doesn't care about this and I'll have to accept the compromise.
Having got the fuzzy, low wattage science done it's back to making stuff.
Or maybe not. I foresee another problem. It isn't possible to remove the wheels with the bearing holders in the W irons, and it isn't possible to remove the bearing holders from the W irons once the springs are fitted to the chassis. That would mean that from this point in the build the wheels are not removable. You'd want them in for positioning the brake shoes, but I'd want them removable for further work, cleaning up and probably painting. I should be clear that this a personal preference and not a 'mistake' in the instructions. Maybe keeping the wheels in from now on is viable in 7mm, I've no experience to guide me. It's not what I'd want to do in 4mm.
I think the best solution is to have the spring retainer pins on the chassis removable, or at least fitted much later. These are the short lengths of wire that were soldered into parts 25 earlier in the build. I made a tidy job of fixing them and now they have to come out. Pants! And there are 8 of them. More pants!
A relatively easy and accurate way of reducing the thickness of the top two leaves is to solder a couple of 0.2mm brass strips to a bigger lump of brass leaving enough space between them to solder in a spring leaf. There's a really sub-standard photo to illustrate. Then with a big flat file simply file away the phosphor bronze until the file skims the two flanking strips. Unsolder the spring leaf and clean it up. Quicker than it sounds. Be careful though, the thinner springs are more vulnerable to bending, particularly in the centre where the hole is. There is a spare of each of the leaves on the fret so you have insurance.
Because we've thinned two leaves to half thickness we've effectively lost a leaf in the spring buckle. We need a tight fit. The cheat is to make a tiny spacer from 0.4mm etch scrap and solder it to the lower face of the smallest, lower leaf. It restores the depth of the assembly and it sort of vanishes when the spring is assembled. It is arrowed on one of the photos below.
Ok, this time we really are back to making stuff. Thread the spring leaves through the spring holder and line up the holes. I found it easier to line the holes up with a pin first and then a piece of 1mm wire with a point filed on the end. Make a couple of temporary spring retainers for the chassis holes from 0.8mm wire, and long enough to push in and out easily. Drop the axle box into the W iron and rest the spring on the spring seats each side of the W iron. Insert the temporary retainer wires. Bend up the ends of the top 2 spring leaves to approx 90 degrees. They don't wrap around the retainer pins as much as the instructions suggest, check prototype photos for this. Leaf 2 will be slightly too long once bent, so mark where it needs to be cut. Either take it out and cut it now, or leave it until the next time the spring is out.
To set the camber of the springs we need to know where the axle centre should be relative to the solebar with one quarter of the wagon weight on each spring. That should give us correct buffer and coupling heights and a prototypical gap between spring holder and bump stops 26. This works out as 15.5mm from top of solebar to centre of axle. Measuring from the top of the solebar is convenient since we're working with the chassis upside down on a flat surface. To weight the axles I'm using a steel parallel that weighs about 125g. Lucky coincidence.
Lots of faffing now. Springs in and out and tweaked gently until the ride height comes right. Make sure the pin through the spring assembly isn't touching the bump stop on the chassis and preventing the proper spring deflection under load. Get the camber of the top two thinned down leaves correct first. The other unmodified, leaves will be flatter and don't have much influence. When you get the camber of the top leaves correct then take the spring apart and carefully tweak the camber of the unmodified leaves to nearly match the top ones. I say 'nearly match' because in normal running we don't want them in contact with the top two, otherwise we'd be back to very stiff suspension again. Leave them slightly less curved so there's a small gap between the thick and thin leaves. Hopefully a photo will make all this clear.
Once happy with the spring setting I committed heresy and soldered the lower leaves to the spring holder, the 1mm pin and each other. Tiny amount of paste flux in the right place and a minimum of solder. It prevents the leaves rotating around the pin and shuffling around keeps the whole spring assembly stable and looking tidy. Be careful not to get any solder on the top two leaves. I covered them with graphite from a pencil before reassembly and soldering. Pencil lead graphite makes a very effective anti solder coating. Cut off and file the excess length of the 1mm pin. The top leaves need to be free to move on the spring seat as they change length under load, otherwise you end up with a very stiff spring again. Phew - finished one. Three more to do.
And finally, there it is sitting on its wheels with working leaf springs. A quick roll along the workbench shows that it glides along smoothly and the wheels do move up and down to accommodate irregularities. Happy. Surely that's the most difficult bit over?
I'm hoping that when complete, painted and weathered the thinner top leaves won't stand out. We'll see.
7 hours today. Total 39.5 hours.
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