Boiler expansion brackets

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Old England
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Boiler expansion brackets

Post by Old England »

Hello Folks,
I was wondering what the typical rear boiler mounting looks like on the larger 3/4" scale locomotives. I know in the UK some builders are happy to rely on the smokebox joint, various pipework and the cab to hold the boiler in place but I won't be doing that. I'm currently scheming out something which will be part of the rear foundation (mud) ring but I'd be interested to see what other people do.
This leads to another related question: How do people attach their cab to the loco other than at the back? I'll be adding a support just behind the boiler (also the boiler support/expansion bracket) but it seems full size practise was to attach to the boiler.
Thanks,
Carl.
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Bill Shields
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Re: Boiler expansion brackets

Post by Bill Shields »

Brackets on the boiler to mount the cab and let the cab float.

Do you specifically want pictures?

in the world of dead simple, there is this way.

holes in the back head below the mud ring
removable screws in the drag beam.
bracket 1.jpg
bracket 1.jpg (72.82 KiB) Viewed 1376 times
bracket 2.jpg
bracket 2.jpg (57.87 KiB) Viewed 1376 times
as you can see, the cab is attached directly and only to the boiler -> both of which lift off the chassis for easy transport.
Too many things going on to bother listing them.
Odyknuck
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Re: Boiler expansion brackets

Post by Odyknuck »

Yes lol
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Dick_Morris
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Re: Boiler expansion brackets

Post by Dick_Morris »

This is the prototype for my model and what I will use. Instead of sliding, they flex. On the prototype they are 1/2" plate.
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James Powell
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Re: Boiler expansion brackets

Post by James Powell »

I just took a look at my Caribou (it's sitting here at work with me...), and it appears as though the front of the firebox is tied to a stay plate similar to the one in Dick Morris's photo. The rear of the firebox is "wedged" between a set of brackets that holds the cab on, with a U shaped plate forming the cab front. So, the whole thing could well not be screwed into the boiler, but the firebox end is able to expand on the frames and yet still not fall out. The front, I would assume, is mounted to the smokebox via a collar, as that's how dad has done several other engines. I have a photo of the boiler off the engine, but I don't know where it is.
Odyknuck
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Re: Boiler expansion brackets

Post by Odyknuck »

I know it depends on how long the boiler is and other variables however how much movement do you typically get with our Hobby Locomotives?
James Powell
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Re: Boiler expansion brackets

Post by James Powell »

Well, scale wise, it'll be relatively proportional, and I know that on full sized steam tractors (traction engines) that use the boiler as the mainframe, you have to allow for up to 1/16" of difference between cold and warm on the valve gear...so probably 5-10 thou on a 3.5" loco. That being said, it's basically an unstoppable force in terms of effort available, something IS going to bannana if the boiler is rigid at both ends. I'd bet on the frames...or the brackets if they are "lightweight".
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Bill Shields
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Re: Boiler expansion brackets

Post by Bill Shields »

It is an easy calculation.

Multiply the coefficient of thermal expansion of the boiler shell material times the length of the boiler times the temperature of the output steam.

For copper it is 16 x 10-6 m/ m°C

Then do the same for the smokebox (entire length thereof), using 450F. This gives a margin of safety unless you know the exact exhaust temp (which many do not know)

For 100 psi steam, I use 300F (convert to °C and get 150)

Then add them together.

It is a very rough number, but you can be sure you will not exceed it.

I just use .125" for all my 3 5 and 4 5 gauge locos. Used .1875 on my 1.5" gauge 2-8-0.

Example: a 24" copper boiler, 150 degrees C above ambient (generous) will grow .0014 meters or about .006", which is close enough to the value shown above for just the boiler. Then add in smokebox growth.

Double boiler length, double the growth for you hernia gauge folks.

Build from steel ...very different numbers...IIRC steel is less than half of that for copper
Too many things going on to bother listing them.
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Old England
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Re: Boiler expansion brackets

Post by Old England »

Bill Shields wrote: Tue Mar 28, 2023 4:55 pm Brackets on the boiler to mount the cab and let the cab float.

Do you specifically want pictures?

in the world of dead simple, there is this way.

holes in the back head below the mud ring
removable screws in the drag beam.

bracket 1.jpg

bracket 2.jpg

as you can see, the cab is attached directly and only to the boiler -> both of which lift off the chassis for easy transport.
Thanks for the rapid response Bill, I'll be attaching my cab to the frame though, just my personal preference.
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Old England
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Re: Boiler expansion brackets

Post by Old England »

Dick_Morris wrote: Tue Mar 28, 2023 7:49 pm This is the prototype for my model and what I will use. Instead of sliding, they flex. On the prototype they are 1/2" plate.
Great photo Dick. I think the USRA 0-8-0 locos used this method but only at the rear of the firebox, it looks like a lot of these brackets were laminated from 1/4" sheets for flexibility.
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Old England
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Re: Boiler expansion brackets

Post by Old England »

James Powell wrote: Tue Mar 28, 2023 10:49 pm Well, scale wise, it'll be relatively proportional, and I know that on full sized steam tractors (traction engines) that use the boiler as the mainframe, you have to allow for up to 1/16" of difference between cold and warm on the valve gear...so probably 5-10 thou on a 3.5" loco. That being said, it's basically an unstoppable force in terms of effort available, something IS going to bannana if the boiler is rigid at both ends. I'd bet on the frames...or the brackets if they are "lightweight".
We don't want a banana! Based on the coefficient of expansion of copper my boiler will expand 12 thou for an increase above ambient of 100°F
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Old England
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Re: Boiler expansion brackets

Post by Old England »

James Powell wrote: Tue Mar 28, 2023 8:10 pm I just took a look at my Caribou (it's sitting here at work with me...), and it appears as though the front of the firebox is tied to a stay plate similar to the one in Dick Morris's photo. The rear of the firebox is "wedged" between a set of brackets that holds the cab on, with a U shaped plate forming the cab front. So, the whole thing could well not be screwed into the boiler, but the firebox end is able to expand on the frames and yet still not fall out. The front, I would assume, is mounted to the smokebox via a collar, as that's how dad has done several other engines. I have a photo of the boiler off the engine, but I don't know where it is.
Hi James,
The bracket at the front of the firebox is the method drawn by Martin Evans for Caribou. I don't have the drawings for this loco but Evans describes attaching a piece of 1" X ⅞ brass angle to the back section of the mud ring. The cab floor is attached to this and the cab built up from it, so very different to normal UK practice.
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