Kerr Stewart 4420 - Live steam and 3D printing

[Trevor Thompson]

So some details of the modifications to the steam system:

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You can see that I have added a spigot onto the bottom off the bend where the steam exits the boiler, and the pressure gauge attached to it. Also in the top right you can see the water filler which sticks through a hole in the body. The gauge cock is also visible, but you can see it better in the next photo:

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In this photo you can see the pipe leading from it to the left. The pipe is held in place with lock tight - because I can't get the cock off thew boiler without removing the pipe. Opening the cock requires a spanner(for now) - but at least I won't burn my finger opening it when it is hot.

The pressure gauge is helpful when you try to find steam problems. Before the modifications the boiler was raising about 10psi - just enough to get it to run. When I blanked off the steam pipe at the motor end the boiler got up to 35psi easily - at which point the pressure relief valve operated and kept the pressure down at 35psi. So all of that works. With the motor reconnected it was reaching 20psi easily and when I moved the reverser the pressure dropped to 15psi, and of course the motor rotated (once the condensate had worked its way out of the system).

I could see that no steam was leaking from the reverser (which surprised me) but it was from the cylinder faces. The springs holding the cylinders together are just what I had in the spares box, so some stiffer ones are on order.

Trevor

[Trevor Thompson]

With the desire to make a more permanent job of this loco, I have redesigned the body, and reprinted it.

I could have just reprinted the Sketchup generated .stl files from before - however Fusion 360 produces much better .stl files - so I redrew it. My new printer produces better quality prints, and Simon alerted me to the ability to "iron" the top surfaces. So the new body:

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I have been over generous with the primer, and I have a bit of cleaning up to do, plus some stay filament threads to clean off. It is always easier to see what needs sanding when it has been primed. But I can already see that it is a much cleaner and smoother set of prints.

You can see that the smokebox is missing as I was contemplating making that from brass. Having come across the concept of a ceramic burner, the plastic smokebox looks feasible again. I have a redrawn smokebox printing at the moment.

Trevor

[Trevor Thompson]

It has taken a while to gather all of the components required for the chassis, partly because the information provided by the gear retailers isn't complete. If I had known the pitch circle diameter for the gears it would have been easier. So lots of trial and error and I got to the point where the gears on the intermediate shafts provided the correct wheelbase for the 6 wheel chassis, and the bearing recesses in the inner frames made the gears mesh correctly:

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So the simplest view, to show you the 7 gears and the shafts they run on, and how they all make the wheels go in the same direction, when the steam motor is attached. It was a bit of a fiddle because things were not all for the same size shafts, the gears for the steam motor are for a 4mm shaft, the new gears are for a 1/8 inch shaft, and the bearings for a 3mm shaft. Anyway it all rotates freely!

You can see how the inner chassis, the cosmetic outer chassis, and the footplate fit together here:

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The sides of the inner chassis are held to the spacers using 2mm self tapping screws. The inner frame as a unit is held to the footplate with 2 8BA bolts. The outer frame is glued together and fitted to the footplate with 4 self tapping screws, going down through the footplate into the corners of the outer frame. On this redesign everything critical is assembled with mechanical fastenings.

At the moment it is all in bits for painting - and I am just beginning to put it back together:

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One thing I haven't mentioned is that the centre axle is slightly longer than the others and can slide sideways, 1mm each way, in the bearings - rather than taking the flanges off the centre axle wheels.

I am building up the paint thickness on the steel footplate before it is finally assembled.

There is just the bracket to hold the steam motor onto the inner frame which is being printed, measured and modified to get to the final fit onto the inner chassis. The idea being that the motion is a subassembly, which is easily removed as a unit.

More to follow

Trevor

[Keith S]

I haven’t logged in for a while, and I am very pleased to see work on this fascinating model is continuing.

[Trevor Thompson]

I mentioned a couple of posts back about experimenting with burners. I was successful but not in the way that I expected.

This is the experimental burner I made, a ceramic burner to fit into the 15mm diameter boiler tube:

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The original burner is in the background.

It, worked in that it acted as a flame thrower, burning very quietly, and easily filling a 15mm pipe with flame. However it wasn't going to solve my problem - which was flames going all the way to the smokebox and heating that up until it glowed - and melting the plastic. My hearing isn't what it was (I blame series land rovers) and I prefer to be able to judge how far to open the gas valve by listening to the sound level - since I can't actually see the flame except when it is hitting the smokebox.

I went back to my original burner and experimented with jets. I found that I could fully open the gas valve with a NO 3 JET and the flames remained in the boiler tube. This is the burner with the No 3 jet at a normal opening:

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and fully open:

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Having put it back together and steamed it I can confirm that the boiler is still steaming well, and the smokebox remains comparatively cool. So that problem is solved.

Going back to the new chassis. One improvement being built in is increasing the gearing on the steam motor. My first iteration has a 2:1 reduction, but the loco was running too fast, so this time it is a 4:1 reduction which I hope will be an improvement.

So back to mounting the steam motor.

Trevor

[Old Man Aaron]

That is bloody good.

Where did you get the gears? I need to stop messing around and figure out some homebrew slomos..

[GTB]

Where did you get the gears? I need to stop messing around and figure out some homebrew slomos..

Here in Oz you should be able to find a range of metal gears off the shelf in your nearest rc model car shop. If you don't have one of those nearby, Ebay is another and likely cheaper source.

As Trevor pointed out, the rc car manufacturers don't supply design data. Usually all you get is a part no. and the car model(s) the gear fits. Which is why I pay the higher price and buy gears from RS Components, who do supply the tech data.

Graeme

[Trevor Thompson]

Graeme has a valid point about buying from a proper supplier. However when the gears are less than £1 each, I can buy a range of gears for £10. I just had to wait for them to travel half way around the planet! and a bit of trial and error.

Really depends on how many gears you want!

And I have spares for the next project. A DIY SloMo is a good idea.

Trevor

[Trevor Thompson]

The reassembly is progressing well.

After some trial and error to position the bracket accurately, I have printed the rear frame stretcher and the bracket for the steam motor as a single component. The boiler, gas tank, and the chassis are all assembled to the footplate. Some slight adjustment of the pipework was required because the boiler and steam motor are slightly closer together than in the original model. Slight modification of the jet carrier was also required for the same reason:

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The body fits over the top of it all accurately:

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I have turned up a more realistic chimney and fitted it, and am adjusting the smokebox to ensure an accurate fit. Some heatproof insulation to add at that end in due course.

Trevor

[Old Man Aaron]

Chipping away.

[Trevor Thompson]

I have printed the final version of the smokebox cover;

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to protect things in case the real smokebox does ever get too hot I have added high temperature insulation under the footplate, and inside the printed smokebox cover:

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You can also see the magnets in their holders at the bottom of the smokebox cover, which hold the cover in place on the steel footplate. The only disadvantage of that insulation under the footplate is that it is visible in the foot holes in the outer frames. A bit of black card might eventually hide that.

So I think that 4420 is now finished, apart from a cover to stop the gears getting grit in them. It just remains to get her out on the railway and run her.

Trevor

PS If anyone wants me to put the updated .stl files and the footplate plan on the forum - just let me know.