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Brass to mild steel, expansion issues?

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Tony Johnson 215/09/2020 11:35:22
2 forum posts

Hello guys

Can anyone advise on this issue. I have been working on a CAD drawing of an early model (live steam) engine, published in the 1860s. The text isn't easy to follow, but indicates the side frames (originally shown as 1/8th inch) need to be locally thicker to allow the axle boxes to move freely. The frames have now been cut in 3 mm mild steel, I'm thinking of adding (3 to each frame) 2 or 3 mm brass pales to emulate horn blocks / carrying the axles boxes. At the narrowest the steel frames are 15 mm, overall length 400 mm. The proposed brass plates each side would be 2 at 60 mm and one at 100 mm (driving wheel axle). The bottom line is - would there be an issue with thermal expansion re the steel/brass?

CuP Alloys 115/09/2020 20:28:47
258 forum posts

Hi Tony

And the temperature change is?


Paul Kemp15/09/2020 20:43:12
560 forum posts
18 photos

Making the assumption (dangerous maybe lol) that this is a railway loco of conventional construction and thus the frames never reach anything like boiler temperature and the fact small gauge loco's have been using gunmetal horn castings riveted to the frames since Noah was a boy, then absolutely no issue at all.


Nigel Graham 215/09/2020 22:34:57
748 forum posts
16 photos

Expansion / contraction is proportional to both length and temperature change, as well as the metals' co-efficients or exapansion. In these small sizes and the modest temperatures changes a model loco chassis is likely to feel, the length changes will be tiny and the difference between them even tinier.

However, re-reading your question I am not clear what you mean by the dimensions and dispositions of the brass plates. Are those 60mm and 100mm the axle spacings? The change with temperature that matters would be that of the wheelbase, not overall length, so very small; and but the horn-plates you are representing normally extend only a short distance either side of each axle.

As a rough guide the external width of a horn-plate is usually somewhere around 3 X the axle diameter, but I don't know how this particular loco was designed.

It might be a problem only if you make the brass plates extend the full wheel-base plus the overlaps: without calculating, 400mm might be long enough for a bi-metal plate to warp slightly over a few 10s of degrees temperature range either side of the temperature at which it was assembled. Conventional horn-plates separate for each axle, as Paul says, would be no problem though.

Tony Johnson 215/09/2020 23:50:16
2 forum posts

Hi, Keith, Nigel and Paul,

Many thanks for prompt replies to my query.

It is of conventional construction (albeit 1869) if you open Google Books and enter 'The Model Dockyard Handy Book' and scroll down to p.25 the full description is there. The temperatures involved would be as those associated with similar frames of 'modern' design.

Looking at the rough sketch they show at the opening of the article you can see horn plates on a similar model. These are not described or shown on the Figs. So my intention was to introduce them. The axle spacing are 122 mm and the axles 7 mm so where the horn plates are abeam of the axles the width is compatible with Nigel's comments. However, I intend to follow the curve of the frame to the top making the maximum width at that point 60 mm and 100 mm for the driving wheels.

Difficult to describe but hopefully better by reference to the original drawing in the Victorian 'Handy Book'. In other words my brass plates would locally mimic the frame cut outs for the springs, and provide thicker horn plates. You will see from Figs. 39 and 40 the form of the frame, in fact my plates would more or less extend over the whole of that shown on Fig. 40 (that's the 100 mm max section) the other two plates being a max of 60 mm each.

Apologies if it's not explained too well, but your help and advice is much appreciated, It's taken me quite some time to get the rough original drawings into usable CAD form, and the guys at LaserMaster have done a great job working from my DXF files, so I would hate to mess it up at this stage.

From what all say it looks as though my large - and somewhat elaborate horn plates, will be OK. But I'm still open for comments.

Thanks again



Edited By Tony Johnson 2 on 15/09/2020 23:51:03

Edited By Tony Johnson 2 on 15/09/2020 23:57:04

Perko716/09/2020 13:40:20
352 forum posts
25 photos

If the horn plates are rigidly attached (bolted or riveted) to the frames then they will all move together as one. The two different metals will try to expand at their individual rates but will be held captive by the rivets or bolts so both items will move together as a composite assembly. The different expansion rates will result in an increase in internal stress in each item and an increase in shear stress in the connecting rivets or bolts, but we are talking minute quantities at this size.

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