Nick Dunhill's workbench CR 0-4-4T Wrexham Tanks

Steph Dale

Western Thunderer
Roxey have heard marked left and right in their gearboxes. I presume the gears are Markits.

Yep - they're the same gears we use in some of the Finney7 gearboxes (with ball-races!), I'm not certain the largest of them is actually large enough for Nick's application.

Steph
 

Overseer

Western Thunderer
Spur gears shouldn't be too hard to buy off the shelf, a search for 0.5 mod spur gear, for example should find suppliers selling to robotics enthusiasts etc. Brass gears seem less readily available at the moment but steel and various plastics are common. The stock gears at the size you are looking at will probably be 4 or 5 mm wide, you could cut them in half to make 2 gears from each. Different numbers of teeth in each gear is good practice. I would definitely add a rigid arm connected to the gearbox with an axle bearing on at least one side of the axle gear, to reduce noise and increase the service life. Spur gears tend to want to force themselves apart and with a long axle the leverage results in more rapid wear in the bearings or fatigue of the axle, likely to be a problem with a fabricated crank axle. It may need to be a split bearing on the centre of the axle to get it in.
 

Isambarduk

Western Thunderer
Spur gears shouldn't be too hard to buy off the shelf
Not been finding it too easy so far!
To date, the best we have in brass spur gears is
Motionco but we would need to use two different sizes to achieve the required separation, which should not be insurmountable.
The stock gears at the size you are looking at will probably be 4 or 5 mm wide, you could cut them in half to make 2 gears from each.
Yes, the Motionco examples are 6mm wide so, with a thin parting-off tool, two should be possible from each gear.
It may need to be a split bearing on the centre of the axle to get it in.
The gear on the cranked axle is going to double up as one of the outer crank webs. The whole arrangement is so short and stiff in this application that it's hard to envisage any significant problem with maintaining alignment nor with wear or fatigue.
David
 

Overseer

Western Thunderer
Ah, I was visualising the gear replacing one of the inner crank webs instead of the outer. The outer position means the spur gears will be adjacent to the frame bearing so adequately supported.
 

Isambarduk

Western Thunderer
... we would need to use two different sizes to achieve the required separation, which should not be insurmountable.
Sadly, I find that we cannot use two different sizes (the spec is so restricted because of the tight space available) so this option is insurmountable. Our best bet now (well, our only bet, really) is a clock-building model engineering friend who is going to have a go at cutting a couple of pairs. Watch this space.

David
 

Nick Dunhill

Western Thunderer
....anyway before we get mired in the solution to my power dilema I think I better build a model. I have the bare bones of a chassis with the stays in the correct places and a cylinder block with slidebars. I bought some Gresley cast hornguides in the hope that I would be able to modify them, but the amount of modification is a bit too excessive so I'll either find an alternative or scratchbuild some. In the meantime the sensible thing to do is to build up.


I had my suspicions that the Redcraft etches were a 4mm blow up. I have had experience of blow-ups (see my NGG16 build) and it's wasn't good. The Redcraft etches didn't clear the first hurdle.


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The inner edges of the footplate etch didn't reach the frames (by a long way.) The splasher fronts, that fold up, are designed for a 4 mm wheel so are too big in diameter and too close to the edge of the footplate (by a long way.) Also the centre of the splashers does not match the centre of the front wheel (by a long way.) I could have modified them but it was easier to make new ones by a long way. Maybe I'm being a bit unfair and the Redcraft etches are designed for a 4mm RTR chassis.


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That's a lot better and I get rivet detail too.


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I have checked some other parts on the Redcraft etches. The body is about 1 mm too wide but I can modify that. I now live in hope rather than expectation. (By the way if anyone wants a set of etches for the chassis and bogie for this build PM me, they go together very well.)


The buffer beams did at least fit and, although a bit long, the valances (or hanging bars for those with a GWR-19 affliction) do fit. As you can see I added some footplate brackets and most of the angle iron that holds everything together.


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Next, the postie arrives with more L section and I finish the above......
 

Nick Dunhill

Western Thunderer
...interesting where the journey takes us sometimes. I had intended this week to build all the brake equipment between the frames (cylinders, plumbing, tanks etc..) I made a vacuum reservoir tank from telescoping tube as I don't have a lathe (no neeed for comment here Mr DLOS!) and it took a while to cut nice angular cylinders by hand. So I attached the assemblies to the frames and my immediate thought was that there would be a conflict between the tank and the bogie.

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So I downed tools on the brake system and thought it'd be a better idea to build the bogie first. I have built a few pre grouping bogies recently and they all seem to be built along the same principals. I guess this loco had a fixed bogie and was effectively a 0-4-(2+2) arrangement. The bogie carries roughly half of the loco's weight so doesn't have any vertical springing between the chassis and bogie bolster. All the springing is in the bogie axleboxes and equalising beams, and there is just rotational and lateral movement at the chassis/bogie pivot. As I am going to have to make the second driving axle rigid to accommodate the patent Dunhill/Beeson/Smith drive mechanism, so the bogie will have to have some vertical springing or the driving wheels will lift off the rails on undulating track (I'm sure none of you have any of that, but just in case...)

So I have learned that buying a cast spring that is similar in size to the original brings inevitable re-designs to the eqalising beams (and almost every component on the bogie.) I decided to get Mickoo to etch the parts for the spring, together with the other components of the bogie, to test the hypothesis that it would be easier to fabricate individual springs from etched leaves than bugger about making a cast spring fit the rest. Well possibly maybe is the answer. I filed etching cusps off dozens of etched leaves, bevelled the edges, filed a taper on the ends and carefully bent them into a gentle curve.

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The bogie chassis had phosphour bronze wire side control fitted.

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The springs and equalising beams did take a while to make but do look better than a 'nearly' cast spring.

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I had to assemble the bogie chassis on parallels blocks as the wheel axles were standard 3/8 tharg and the axles blocks accept 5/32 thargs. My mistake I forgot to get the axles machined. I think the finished bogies justify the effort.

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Back to the GA and I noticed a plinth between the bottom of the chassis and the bogie bolster. I decided to make a pair, but a millimetre shorter so I can accommodate some vertical movement. Some experimentation with my dwindling stock of springs will be necessary! (Anyone know where to get very small numbers of smallish compression springs?)

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Next week I go back to the brake system, via the couplers (if they ever turn up!)......
 

simond

Western Thunderer
Nick,

I understand your drivetrain dilemma (the gears look very nice), but I fear you’re going to have another one if the middle axle is fixed and the others are sprung. It’ll involve the thing nodding vigorously as it accelerates and brakes, or rocking very noticeably as it trundles along.

I had this experience with my 1366 tank, the first I’d built with springing, and I resolved it by using weaker springs in the middle axle. I had a similar problem with a Springside 14/48xx which has a ludicrous design, the drivers are fixed and the trailing axle (remember Springside models are cast w/m and heavy) under the fat end of the model is sprung with a telescope affair. It “wheelied” when accelerating! I resolved that one by a single point rocking pivot for the trailing axle and twin beam compensation for the drivers.

Perhaps a fixed pivot for the bogie, centre of gravity behind the driving axle, and very lightly sprung leading axle?

atb
Simon
 

Nick Dunhill

Western Thunderer
Hi Simon
I'm interested to hear your thoughts as I've not had the 0-4-4T conundrum yet. I simply have to fix the driven axle and the front axle will be sprung downwards but sit on a bump stop. I really dont want to go down the route of making rocking beams as I'm determined to stick to the GA as close as money will allow. I'm going to put all the lead in the front of the boiler between the drivers. If I experiment with a suitable spring on the bogie pivot I'm hoping it doesn't wheelie. As you said the Springside 14XX was all cast WM and heavy at the rear, this is all brass sheet.
 

Richard Spoors

Western Thunderer
(Anyone know where to get very small numbers of smallish compression springs?)

Hi Nick, Lee Springs Ltd have a detailed supply of compression springs if you need a specific size and load. They can be a tad expensive though.
Compression Springs Search Results
I liked your loco springs. My German wagon kits have DIY springs that you make up from a phosphor bronze sheet where each spring leaf is cut with a CNC miller.
Cheers

Richard
 

Scale7JB

Western Thunderer
I think it depends on whether you plan the loco to ride on the springs or on the stops.

I simply use the Slaters wagon buffer springs, or for something a bit stiffer their loco hornblocks springs.

JB.
 

7mmMick

Western Thunderer
Hobby holidays will supply horn block springs. Phil does two strengths, they’re dead handy.

Mick
 
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