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MRA

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Hi Folks

I’m a newbie to all this, though I have machining and welding experience, applied mostly to old motorcycle restoration. I’ve been given about half of a Rob Roy 3.5” gauge 0-6-0 tank loco (a common Martin Evans design apparently suitable for a beginner...), and I’m reading a lot and thinking about making a start.

I’m thinking about the (copper - assembled with a mixture of silver solder and oxy-acet brazing) boiler, since the work so far on this has gone a long way but is a little non-standard. I’ve had various suggestions along the lines of ‘I’d start again if I were you’ – and although I may well do this, I don’t like to be frightened of something I’m working on, preferring to have a clear idea of what is what.

I've had a read around on these boards, and looked over the stickies at the top of this section.

So – first question – do Evans’s ‘Model Locomotive and Marine Boilers’ (Argus 1988) and Harris’s ‘Model Boilers and Boilermaking’ (MAP 1971) still give a fair impression of good practice, or have things changed? I have, and have read, both…the former is particularly useful in containing a number of formulae to calculate barrel thickness and stay spacing, which I’ve been comparing to the standard design – but I’m in no position to know if the information is considered current, or obsolete.



Thanks
Mark
 

b.lindsey

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Welcome Mark,

There are others here far more qualified to speak to your boiler questions so I will refrain from that, but am certain you can get some good advice here. Glad you found the site and will look forward to hearing more about your ventures.

Regards,
Bill
 

Herbiev

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G'day Mark. Can't help you with the boilers but welcome to the forum.
 

rkepler

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MRA said:
So – first question – do Evans’s ‘Model Locomotive and Marine Boilers’ (Argus 1988) and Harris’s ‘Model Boilers and Boilermaking’ (MAP 1971) still give a fair impression of good practice, or have things changed? I have, and have read, both…the former is particularly useful in containing a number of formulae to calculate barrel thickness and stay spacing, which I’ve been comparing to the standard design – but I’m in no position to know if the information is considered current, or obsolete.
They're current for the technologies discussed. As long as you stay with what they recommend you won't have any problems either in making the boiler or in getting certification. If you do anticipate certification you might want to find a club and discuss things with them before you go a lot further - it needn't be frightening and they'll be able to assure if you can continue with the boiler as it is (and you might find someone to hold the other torch when you're doing the big stuff).
 

MRA

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Thanks for your replies folks. I have spoken to the local club, who are very friendly and have been generous with time and advice on the brief occasions we’ve been able to meet. But perhaps you’ll understand that as a newcomer it’s hard to say ‘are you sure?’ or ‘why is that?’ in person, to a guy who may up formally inspecting the thing, without coming over like someone with an attitude (or perhaps I’m just bad at that sort of thing). I went away rather discouraged – which is why I’d appreciate help in getting my understanding straight here. After all, my interest is in learning about this thing / new skills / processes – not really in ‘owning’ a working model – though that might be a nice end result one day.

OK, first element of 'non-standardness' of the boiler is that it has been rolled and butt-brazed with an internal strap, rather than made from seamless drawn tube as in the drawing. Thickness is 1/16", as per drawing, and OD is 3.25" as per drawing.
Evans gives a formula for the bursting strength of a boiler barrel...in ASCII maths(!) it goes:

P=D*F*WP / (S*R*C*T*2)

where
P=required plate thickness (")
D=barrel OD (")
F=safety factor - taken as 8
WP = working pressure (psi)
S=ultimate tensile strength of metal (25,000 psi for copper)
R=riveting allowance (1 for seamless, 0.8 for brazed / silver soldered barrel)
C=corrosion allowance (1 for copper)
T=temperature allowance (0.8 quoted in book for WP of 60-80 psi)

So for D=3.25, WP=80 P comes out at 0.05” for solid-drawn or 0.065” for rolled-and-brazed. 1/16” is 0.0625 – so it looks like with a small cut in WP (to 77psi…though I suspect 3 psi difference to standard would be ‘in the noise’ as far as setting safety valves goes…) a safety factor of 8 could be maintained with the brazed barrel, which might then be useable.

Am I missing anything yet?
Cheers
Mark
(Oh – I should have started by saying I’ve read and digested the disclaimers – I’m very happy to take advice on the basis that it’ll be my fault for listening to it! :)).








 

HS93

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I think you will need a copy of the UK/eu boiler code ,No other will do , unless you build to it, your boiler is un insurable and so will not be allowed to be steamed, it is worth following the code as it only has the user builder in mind, to use a boiler in public you should have insurance
ill find a link to it and post it.

read this for starters I am lookin for a copy of the full blue book

http://www.16mm.org.uk/newsite/insurance/Examinaton_Testing_of_Boilers_Nov_2008.pdf

Peter
 

Blogwitch

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Peter has raised a very good point.

Here in the UK, insurance for steam boilers is mandatory if it is going to be steamed in a public place.

What some people don't realise that a public place is anywhere where someone else can be present. So even in your own home, if someone is in the same room as yourself and the steaming boiler, that is classed as a public place, so you must have insurance.
What it is to cover is not yourself, but anyone else, be it your wife or girlfriend, children or even mates. If the boiler was to let go, you could find yourself facing a large lawsuit if anyone other than yourself is injured.
Think how faithful your wife or girlfriend would be if they were offered £500,000 to cover their scars. Some would be off like a shot, sunny beaches and champagne, leaving you to pick up the bill for the rest of you life if insurance wasn't in place.

John
 

brozier

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Making a boiler tube by rolling is fairly normal in the UK, you have no choice when making a tapered barrel.

Is the reinforcing strap also rivetted? My clubs boiler tester has stated he wants rivets and a strap on a black five boiler tube.

If you don't have rivets, why not propose adding an external strap with rivets.

One important question - are you an active member of the club in question? I would suggest you will get more help if you become an active member and pay the usually reasonable subs.

Most clubs don't like people who just turn up for a boiler test and don't contribute although it sounds like they have been very patient with you so you may have got lucky.

It is worth joining a club anyway - I did and always get fired up again after going to a running day or whatever....

On a final note although Roby Roy is billed as a beginners loco the assembly is pretty fiddly and requires a certain order so if you can find someone at the club who has built one you would be well advised to talk to them.

It is worth having a search on the web for drawing errors to e.g. http://www.astronomiainumbria.org/advanced_internet_files/meccanica/easyweb.easynet.co.uk/_chrish/robroy.htm

Cheers
Bryan
 

MRA

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I understand the safety and insurance angles, and the fact that inspection will in the end be through a club etc. I also understand people here wanting to emphasise those points. (If the missus ever wants to take me for large sums, I'm sure she'll think of something :p).

Perhaps I should have added that with other life pressures, this is a thing ('just for me') which gets looked at in odd scraps of time which no-one else has a call on - days out to club meets are very difficult to come by. Hence my interest in on-line discussion, and difficulty with acquiring any level of club commitment. I guess if club people here think 'in that case this is not a pursuit you're likely to find yourself able to get too far with' - then that might be a helpful message to hear, bluntly put, at this point - or indeed the opposite.

Bryan - strap is not also rivetted. I get the point about external strap, though smokebox tubeplate and tubes are all in...

Peter - thanks for that link - I'll look it up as soon as time allows.

cheers
Mark
 

MRA

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Read and digested, Peter. I note that for butt-strapped boiler barrels, the inspector is required to look at the joint during inspection...removing the smokebox tubeplate (silver soldered in) to make that possible on this one, sounds tricky...

Another 'formula' question from Evans. For 1/16" plate he suggests 5BA stays. Stay spacing is then calculated bearing in mind the minimum diamater of the stay and the tensile force it is placed under, bearing in mind the area of plate to which it is attached - or perhaps I should say, for which it is responsible (since force=pressure x area).

Chapman tells me the root (or core) diameter of 5BA is 0.098". Taking this as 'D', in:

Pitch(") = D^2 * T * 3 / (WP*F*4)

where the other terms are as defined in the earlier equation I quoted, then:

Pitch = 0.28"

But the Rob Roy drawings specify specify side stays on a 4 by 4 grid with horizontal spacing 3/4" and vertical spacing 5/8". This seems odd, since these drawings are by the same author as the book quoting the equations (in fact a copy of the RR boiler drawing is reproduced in the boiler book...)
 

Dan Rowe

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Mark,
There is not enough information in post #5 to work this problem with out a copy of the book. The variable 'T' in the staybolt equation is the ultimate tensile strength or 'S' in post #5.

I got the same answer as you got but I have never been very happy with that equation. We are interested about the root cross sectional area of the staybolt and at the very least Evans has reduced Pi to 3 to roughly calculate the area with out any mention of this in the text.

K. N. Harris has an engineering approach to the staybolt calculations. On page 30 he gives an approximate guide for plate thickness and pitch then works out a sample problem for 1/2" square pitch with 80psi pressure. Table 6 gives the root area and the safe working load for several sizes of staybolts. K.N Harris recommends a safety factor of 10 for staybolts and table 6 is approximately worked to that safety factor.

If we use the pitch stated for the design of .75" x .625" each staybolt supports an area of .469 square inch. This times the working pressure of 80psi gives us 37.5 pounds each stay has to resist. Using Table 6 the closest option is 2BA with a safe working load of 41 pounds.

What is the size of the staybolt in the Rob Roy boiler drawing?

Dan
 

Dan Rowe

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Mark,
Reading "Model Engineers Handbook" by Tubal Cain made me think about the number of threads engaged with the plate. 2BA is too course a thread for 1/16" plate. 2BA is listed as an option for 2.5mm plate in the AMBSC.

Looking at the AMBSC 5BA staybolts in 1.6mm plate have a maximum pitch of 14mm at 550kpa.

Dan
 

MRA

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Dan -
Thanks for your interest in this. Yes, Evans mixes his symbols between equations, and I didn't notice - sorry! (I lecture in engineering, and students hate that kind of thing :) ).

On the Rob Roy drawing, the stays are 5BA, at the pitches I quoted. In the book (which, for a 'beginner' design, Evans sees fit to make available along with drawings) he says (p77) '...they can be made of monel metal, drawn gunmetal, or copper...use 3/16" dia material, turned down to 0.126" and thread 5BA in the lathe, but allow the die to cut a little oversize towards the head...'.

No mention is made of changing the stay pitch based on their material (and tensile strength) as might be anticipated...

I've got the table on p30, Harris - 1/2" spacing for 6 or 5 BA stays in 1/16" plate, copper stays. So less fussy than Evans' calc, but more fussy than his drawing!

It appears no-one agrees...but at least he gives a method. I'll work through your calcs following Harris. Any boiler inspectors like to comment on what they'd do with a RR boiler built to the drawing? :)

Unfortunately mine is not built to the drawing - here is the final point of departure:

Inner firebox wrapper 1/16" - check.
Outer firebox wrapper (made by slotting, bending and extending sheet forming barrel - 1/16" - check.

BUT - throatplate, firebox tube plate, inner backplate (sorry, know that's the wrong term) and backplate are all 3/32 on drawing, but 1/16" on mine.

So question is - if a box section made from two 1/16" sheets is OK and 'stayable' (notwithstanding problems deciding the correct stay pitch above) at the 'sides' - why not at the two ends of the firebox? Other designs seem to use same gauge all the way round...


 

Dan Rowe

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HS93 said:
I think you will need a copy of the UK/eu boiler code ,No other will do
Mark,
I think Peter is correct for UK boilers. I failed to locate a copy of the code online but it has been a while since I looked.

Tubal Cain's book has a slightly modified form of Martin Evans staybolt equation. Instead of ultimate tensile strength he gives a maximum allowable staybolt working stress range for copper (2500-3000) GM (3500-4500)and monel (6500-7500). This makes the formula c= 0.75d2f/p. He indicates to use the smaller figure for the smaller staybolts.

c=pitch ins.
d=internal diameter ins.
f=working stress Lbf/sq.in.
p=pressure Lbf/sq.in.

So if we draw a circle through the 4 points of the Rob Roy staybolt pattern and inscribe a square on that circle to get an equivalent square pitch of .69". I worked out Tubal Cains version of the formula solving for p, and got 26psi for copper and 35.5psi for GM and 67.9psi for monel 5BA staybolts.

I also checked what Kozo Hiraoka had to say about this in his article "The Safety of Copper Boilers" He states that the root diameter of the thread should be 0.1" or greater and that is where his graph truncates. We are very close with a 5BA but he does not have a spacing less then 3/4" for the circle diameter. This is based on the ASME code and only 3 points have to be on the circle so an equilateral triangle of 3 points is acceptable. The 80psi line would need a pitch circle just smaller than 3/4"

The numbers in Kozo's graph agree reasonably close in this case to the AMBSC only the AMBSC requires 4 points on the circle all less than 180 degrees apart.

Dan
 

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