Design for 2-4-2T with CSB & split frames

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Guy Rixon
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Joined: Sun Mar 27, 2011 6:40 pm

Design for 2-4-2T with CSB & split frames

Postby Guy Rixon » Wed Dec 18, 2013 2:05 pm

I am planning a build of a LNWR 2-4-2T "Mansion House" tank, for which I have an old kit (not the current LRM one). Since this would be the first P4 locomotive I complete, I'm after some advice on the chassis design.

I want to spring all the axles for best ride and to pick up from the radial trucks, because relying on just the four coupled wheels for pick-up seems iffy. Therefore, I'm planning to use split frames, so as to avoid wire pick-ups dragging on the unpowered wheels. I already have the wheels (Sharman, I think), and they have plastic centres, so I will use shorting wires, probably from the Bill Bedford range.

I want to use CSB suspension for the coupled wheels, made from High Level parts. I plan to fit a High Level slim-line gearbox. Should I be using High Level's narrow hornblocks on the driven axle, to make room for the insulating gaps in the split axle, or would the standard ones be OK?

If I can control the finished weight (and it's a brass kit, so most of the weight is likely to be ballast) and CoG, I assume that the High Level goodness will get the ride height correct for me. It's a very symmetric loco, so I expect the CoG to be very near central.

For the radial axles, the kit provides working, radial carriers for the bearings. I plan to use these and to suspend each one with two wire springs. The springs would have one fixed end and one end bearing vertically against and adjustable stop, being free to bend horizontally. Thus the springs would both carry some weight and control the side-to-side movement of the axles. The adjustment would let me tune the proportion of weight on these axles.

I don't know how heavy to make the loco. The CSB page and spreadsheets on the CLAG site imply that 100g per coupled axle is good. If I put 100g on each coupled axle and 50g on each radial truck would that be plausible?

Does anybody see any big problems with this approach?

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Mark Tatlow
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Joined: Tue Dec 02, 2008 11:24 pm

Re: Design for 2-4-2T with CSB & split frames

Postby Mark Tatlow » Wed Dec 18, 2013 2:26 pm

If you set up the holes for the CSB's using the High Level jig from the notional centre height for an axle and use their hornblocks then yes the chassis will be set up at the right height.

You will definately want the space saver hornblocks on the driven axle to both keep the breaks in the axle clear of the hornblock bearing and also to give yourself enough room for the gearbox.

A good point to consider is to go for a gearbox that has a grubscrew final drive as it is then posssible to release it from the driven axle (not all of them do). If you stick with High Level, go for thei "+" models as these include the feature and their standard ones don't.

The implications of the use of CSB's is that it transfers the total load of the loco over all of the axles (as per the calculation spreadsheets on the CLAG site - so you don't need to worry about what is loaded by what.
Mark Tatlow

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Will L
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Joined: Sun Jul 20, 2008 3:54 pm

Re: Design for 2-4-2T with CSB & split frames

Postby Will L » Wed Dec 18, 2013 3:41 pm

guyrixon wrote: ... The CSB page and spreadsheets on the CLAG site imply that 100g per coupled axle is good. If I put 100g on each coupled axle and 50g on each radial truck would that be plausible?


The 100 gram settings on the spread sheets is largely arbitrary, best to see what you can get in. The weight split is OK but these days I might try for a bigger proportion of weight on the coupled wheels. You only need enough weight on the radial axles to ensure they stay on the track reliably. You won't need a spreadsheet to work out the fulcrum points, it just needs to be symmetrical, with one dead centre and the outer two between half to two thirds as far the other side of the axle. You can then use the spreadsheet and the loco weight to work out what size of wire to fit.

Will


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