I think the usual way is to have a butt strap inside the tube, this makes it easier to get a good seal with the smokebox.

HTH,

Rod

Ive never checked this before but my 'industry standard' excel spreadsheet concurs with Jason's observation, there is a 20% reduction in operating max psi for a silver soldered joint (god only knows who comes up with these figures as if properly made it is at the very least as strong as the parent metal).

I suppose the 20% allows for the fact that the joint may not be properly made, hard to be 100% sure that solder has flowed completly through the joint particularly if rivits are a bit too tight or there is the odd gap within the joint (out of sight) that is too large for the solder to bridge.

But it's no longer a (seamless) tube so at 80% it is effectively 18g tube.

As I said speak with your inspector as you are deviating from the published design, he may be happy for you to carry on, he may not. Best to find out now before investing more time and money into the boiler.

I put the numbers through what is probably the same spreadsheet that Fizzy refered to and you should still be OK with what you have and still be over a decent factor of safety BUT don't take my word for it check with the boiler inspector.

As Duncan says the stay spacing on the flat areas is the one to watch, what stay spacing do the drawings show and also what thickness is the firebox

Hi Jason and all – dont know if you have already seen it but the Ozzy boiler fed (cant remember their correct title) have published test results of different boiler end plates under pressure, from fully flanged to just a plate plonked over the end of a tube and poorly silver soldered on – result…no sign of any failure in any joint even when it was evident that there had been no visible signs of solder penetration. Reassuring if nothing else!

Just my own personal observation but all the 3.5"g miniature loco boilers I have come across have been 3/32" or 2.5mm outer wrapper and outer firebox. A joggled joint on the barrel was quite common years ago but is now frowned upon and as Jason says a seperate outside butt strap on same, but there are many miniature locos running around without this feature.

As others have said, check first with your club boiler inspector.

Cheers,

Julian

Edited By julian atkins on 09/05/2017 00:01:39

Mick, you still need to watch what type of stay you use, as I said headed stays just come over the 5 but unheaded eg plain rod or poking a rivit through the hole will only give about 4.

Once again check with your inspector before proceeding with caution.

As promised, the extract from the book by Martin Evans;

'Model Locomotive and Marine Boilers, M Evans, 1988 (reprinted 2011)'

P27,

P = D x F x WP / S x R x C x T x 2 where P is plate thickness (inches), D is outside diameter (inches), F is safety factor, WP is working pressure (lbs per sq inch), S is UTS (lbs per sq inch), R is rivet allowance, C is corrosion allowance and T is temperature allowance.

According to Mr Evans…. Safety factor is between 6 and 10 with 6 too low and 8 is acceptable., UTS for copper is 25000 lb per sq inch, no corrosion allowance for copper, temperature allowance of 0.8 for copper working at 80 lb per sq inch (Rob Roy working pressure).

Rivet allowance for silver soldered joint is 0.8.

Plugging in the numbers gives;

P = 3.25 x 8 x 80 / 25000 x 0.8 x 1 x 0.8 x 2

= 0.065 inches. so the 1/16" is fine considering the range of the safety factor.

Jason – does the lower safety factor for stays apply to the other parts like this barrel thickness?

I understand that Mr Evans has his critics but I would be really interested to know what data any inspector actually use these days. It is all very well saying 'go see an inspector' – but what do inspectors use? This should not be wrapped in a black cloak or worse – a computer spreadsheet/ software.

Phil H