Barrel to throatplate joints

[baggo]

Hi all,

My brother and I are about to build a new boiler for Rob Roy (as the one that came with the chassis was a pile of scrap!) and also I've just acquired a Simplex chassis complete with a Reeves boiler kit. Both these boiler designs use a simple butt joint where the throatplate meets the boiler barrel (rather than a flanged joint) and I was wondering if such joints are still considered acceptable?

LBSC used to use plain butt joints on his boilers (many of which are still going strong) so there can't be that much wrong with them.

It wouldn't be much trouble to put a second flange on the throatplates and so considerably increase the joint strength or even put a thickening strap on the barrel where the throatplate butts up to it.

Any suggestions/comments would be appreciated,

John

[ukjimoo]

I have recently constructed a traction engine boiler with a simple butt joint between the throatplate and the barrel from an approved design and the boiler has been certified by my club, so i guess it must still be acceptable.

I think that this is the only part of a boiler where a simple butt joint is acceptable and its because the outward forces or stretch on this joint are very low compared to the tube plate for example.
I think that the strength of a butt joint arranged in this way is actually very strong because the outward force on the throat plate is pushing it hard against the curved back edge of the barrel.

Jim

[greasemonkey]

HI John
I built a simplex boiler earlier this year but modified the design slightly. If you make the outer firebox wrapper from a seperate piece of copper plate to the barrell and use a piece of tube inside the boiler as a joining ring, then the bottom part of this ring can be flanged down to strenthen the joint between throat plate and barrell. The other thing I did was to fit rod stays and not girder stays tio the firebox crown.

Andy

[davidimurray]

Hi John

I don't see that there would be that much benefit. Throatplates were were originally flanged for the purpose of riveting. Also it can be argued that they require further strength due to the physical size and mass and also due to the much larger expansion that is likely to occur.

My big concern with this sort of arrangment would be making the flanged joint. Consider the situation where you have a 'butt' joint (I actually insert the tube into the throatplate with about 1/32" inside the plate) where a 3/16" barrel joins a 3/16" throatplate and the joint is silver soldered. If you then did a flanged joint, you could hit a problem where the 'hole' is too small for the barrel - as a result you could end up machining the hole to make it larger and thinning the plate where the joint is made - a 3/16" barrel to a 1/8" flange !

When I used to design boilers (riveted and welded steel locomotive boilers). I used to specify everything as flanged and then end on butt welded. This was to prevent cracking that could occur in the corners. I doubt this ould be a problem with 'model' boilers and if you look at most steel model traction engine boilers they tend to be square corner boxes.

Hope that makes some sense.

Cheers

Dave

[baggo]

Hi Guys,

Thanks for your replies. I think that I'll stick with the published design using the butted joint as it is certainly simpler to do. My major concern was that it may not get past the boiler inspector under the new regs but as Jim's just had one certified then it is obviously still ok. As suggested, the joint is virtually only in compression rather than tension.

I dug my Allchin boiler out the other day and noticed that that also uses a butt joint although I thought it was flanged (well it was 30 odd years ago when I made it!) and there's a lot more stress on a traction engine boiler than a loco one.

Thanks,

John

[Shawki Shlemon]

Hi baggo
May be I am too late but better late than never . I know that this may not apply in UK but in Australian boiler code for copper billers, a simple butt joint for throat on round top boilers is allowed provided is brazed and not silver soldered .

[baggo]

Hi Shawki,

It's never too late!

We have already done the Rob Roy boiler, in fact it's getting near the testing stage, but I won't be tackling the Simplex boiler until next year.

I am surprised that the Australian code insists on a brazed joint rather than silver solder - we are always told that a properly silver soldered joint is at least as strong as the copper.

An article in a recent Model Engineer actually casts doubts on the suitability of brazed joints in copper boilers due to de-zincification of the brazing alloy by electrolytic action. Apparently this can also happen to a silver soldered joint but to a much lesser degree.

The only real objection I have come across to the plain butt joints is that they can suffer from stress fractures causing actual cracks in the joint eventually leading to the boiler having to be scrapped. It's suggested that these fractures actually form whilst the boiler is under construction due to unequal expansion and contraction of the different parts of the boiler during soldering and cooling.

John

[Shawki Shlemon]

Hi baggo
In the Australian code the only brazing allowed is for the butt joint on the throat plate for round top boilers . I built few round top boilers and I formed a throat plate with flange and silver soldered it . It is not as hard as it seems .When I started the hobby I had no experience in silver soldering or sheet metal work, but when there is will there is a way . I have so far built 25 boilers for 2.5" , 3.5" , 5" , 7.25", lorry & traction engine . If I can do it everybody can . One may spoil a few bits ,I am guilty of that but so what that is life .
I recently built a boiler for a NSW Z 17 4-4-0 engine and is exactly like simplex boiler only .5" shorter .I formed the throat plate satisfactorily . By the way when you open the barrel for fire box make sides uneven ie one side complete and does not need extension ,extension only on one side .It is easier .
The trick of silver soldering boilers are :
1- Clean
2- Make room for solder to run (no tight joints )
3- plenty of flux
4- ample heat
5- Small step at a time then re-clean (people get tempted to do too much get carbon in in joints and stay holes that are difficult to clean)
6- ask for help if you need it . It is OK .

[waggy]

One of the most helpful things I ever made was / is an upright stand to which I can clamp a propane torch at any height for background heat when soldering a boiler.
It consists of a 30" length of 3/8" square steel welded end on into the centre of a 1/2" thick, 8" round piece of steel as a base. Both pieces obtained from a scrap yard, nothing special. You need a bit of weight in the base for obvious reasons.
To enable clamping the torch I silver soldered a 1" x 2" brass "flag" to the stem of the torch. A suitably adjusted mole grip does the rest!
A simple yet effective way of giving you both hands to apply the prime heat and solder without help from somebody else.

Waggy.