Two Questions- Purpose of Crown Stay & Sequence of Assembly

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Runner

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Hi all,

I thought I knew the purpose of the crown stay, i.e to prevent deflection of the top of the boiler barrel and/or deflection of the top of the firebox. My Martin Evans design has the crown stay rivetted and silver soldered between the firebox top and the boiler barrel, however I have seen a Martin Evans design which shows that the crown stay doesn't touch the boiler barrel. It therefore wouldn't prevent deflection of the boiler barrel. Maybe I need clarification on the purpose.

Secondly, what is the preferred method of assembly of the boiler, particularly in relation to the firebox stays? Martin Evans for my locomotive says that the firebox stays are the last operation, i.e after the foundation ring and backhead are silver soldered. However I have followed the very interesting post "Making a boiler for a 3.5" gauge locomotive" where the firebox stays are completed before the foundation ring and backhead are installed. This method allows access between the inside of the boiler barrel and the outside of the firebox to apply silver solder to both sides of stay as it passes through the boiler barrel and the firebox. Effectively four silver solder operations per stay. For my sequence of boiler construction, you would be unable to see the penetration (if any) of the solder into the gap between the barrel and firebox. Martin Evans recommends the use of caulking solder for the stays, which is not soft solder but a high MP solder. I have silver soldered 3/4 of the foundation ring, but not the backhead. I prefer to use silver solder for the firebox stays but because of the foundation ring prevents access to most of the forward firebox stays between the barrel and firebox, should I silver solder the stays before the backhead and what steps should I adopt to ensure penetration through when having access to only the indside of the firebox and outside of the barrel.


Regards

Brian
 

GWRdriver

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Runner said:
I thought I knew the purpose of the crown stay, i.e to prevent deflection of the top of the boiler barrel and/or deflection of the top of the firebox.
That's true, but to be pedantic the barrel won't deflect, it will want to expand. I hesitate to say it was common, but in full size traction engine practice it was not unusual to see a boiler with true girder stays, that is, where the stays spanned the length of the crown sheet, bearing on the fore and aft heads, and were not attached to the barrel. The reason this was possible is because, assuming the internal pressure acts equally at all points, a tubular form is self-staying. This is not true of external [crushing] pressure. A design in which at least one of the girder stays is attached to the barrel is preferred for model engines. A typical combination is to have two outboard stays which are connected at the barrel and a central girder stay which is not which conveniently leaves the middle open for the throttle assembly.

Secondly, what is the preferred method of assembly of the boiler, particularly in relation to the firebox stays?
I think everyone will have their own method which is dependant upon their skill, experience, equipment, and the design of the boiler. In one instance where I had a particularly deep firebox I silver-soldered the outer wrapper and firebox assembly together before installing the barrel and throat assembly or the back head. This allowed me to see the stays as I soldered them and get back in more easily if I miss one, which does occasionally happen. But there have a number of boilers where I didn't need to do that. In a perfect world I would like to be able to solder in all the firebox stays before installing the outer wrapper, to insure quick and complete solder penetration to the inside stay ends, but so far other considerations have prevented that being practical. Also, I have found it to be an advantage for the silver-soldered firebox assembly to have the flues (as usual), the girder stays (if any) and the mud ring attached at one go.
 

Runner

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Thanks Harry,

it appears that the practice when silver soldering to get penetration through the other side when only one side is accessible, is to make a small nick in the hole which provides a path for the solder to flow. However this is impractical since the hole size is 1/8th inch dia and in the firebox not a hope. Will say a .002" gap between the stay and hole produce the same affect?

Brian
 

steamin

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My boiler fabrication has been involved with steel materials and TIG welding. Before I even considered welding anything together, I sat down and made a step by step list of how the boiler was to be welded together. I was amazed at the mistakes that I made on paper. The list included which edges were to be beveled, what sub assemblies had to be made first, what sides of the various plates had to have the holes beveled for the stay bolts. In other words, the boiler was completely fabricated on paper before I ever cut the first piece of material.

Early in my apprenticeship, my father would sit me down and have me make a list of the machining processes before I ever turned on the machine. He wanted me to be aware of corners and edges that are necessary to maintain to measure off of to make an accurate piece. After a while, the process became second nature for me. Thus I applied to same process to my boiler fabrication projects. Preplanning is the name of the game.

Maybe one day I will get brave enough and maybe have enough heat to silver solder an all copper boiler together. Do be careful and do have fun.
 

GWRdriver

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steamin said:
Before I even considered welding anything together, I sat down and made a step by step list of how the boiler was to be welded together. I was amazed at the mistakes that I made on paper.
Brian,
I do exactly as Steamin does on every boiler I build. I spend time all along the way thinking through the sequence of assembly and soldering. If I can't walk myself mentally through the entire process then I don't start. There are usually just one or two problem areas to work out and once those are figured out the rest falls into place.
it appears that the practice when silver soldering to get penetration through the other side when only one side is accessible, is to make a small nick in the hole which provides a path for the solder to flow.
I hope I understand your question correctly but yes I lightly nick and channel and bevel my boilers all over so that solder has no trouble finding its way into the hole, even the easily accessible ones. The problem with supposing a certain clearance (such as .002") between parts is that it's practically impossible to predictably maintain such a close fit through assembly and soldering. The best thing is to nick or notch the holes or file flats, but if there's too much relief solder will run through the gaps and joints will appear to need more solder when in fact there's probably a solder puddle accumulating somewhere. This doesn't really hurt anything except your pocketbook.
 

Runner

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Thanks Larry and Harry,

sound advice. In my case the "words and music" of Martin Evans (ME) was used in the fabrication of my boiler except that I was unaware that my propane torch was only OK up to but not including the stage of silver soldering the foundation ring. I deviated slightly by silver soldering that part of the foundation ring to the backhead something that was easily done. The backhead with that part of the foundation ring is an outstanding task to complete the boiler. I'm at the stage of requiring another propane torch and another pair of hands to provide enough heat. In having this hiatus in the completion of the boiler I am looking at other other people's efforts and decided to silver solder the stays as opposed to ME's requirement for caulking them. I suppose that caulking solder has greater coverage i.e. gap filler than silver solder. I am therefore wanting to ensure that I am aware of all the ramifications in deviating from ME's requirement.

Your sound advice is helping me get there.

Regards,

Brian
 

GWRdriver

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Brian,
I'm glad I could be of some help. My personal opinion on "caulk" is that the technique of caulking, especially when using soft solder, or with any solder that's incompatible with the primary silver solder used, is to use it mostly as a last resort repair technique. Once a boiler has a non-compatible solder applied it can't then be silver soldered again. Not only will silver solder not stick to those areas but you will burn out other soft solder present contaminating the surrounding joints and at that point a boiler is essentially toast. I've seen a dozen boilers (in for repair) where owners have attempted to use gobs of soft solder not just to plug a weep in an otherwise sound connection, but to try under the guise of "caulk" to re-attach a structural component like a bushing and that doesn't work in the long run. But it does have its place. What I've done some cases, rather than pronouce a boiler dead, is to remake the fitting (bushing , etc) so that the attachment is made by mechanical means (threads, screws, etc) and then caulk that connection and that usually works very well and can extend the life of a boiler quite a bit.
 

Runner

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Thanks Harry,

that is a very useful bit of information about the inability to silver solder over caulking solder. I have purchased Comsol high MP soft solder from the CUP Alloys in the UK for the firebox stays and 842 for the silver solder. The Comsol won't now be used for the primary construction, however in the event of a small leak (this is my first attempt at making a boiler) could it be used there as a last resort, bearing in mind that once used it's the point of no return?

Regards,

Brian
 

GWRdriver

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It could be, but this would be for what we call a "weep", literally just a tear-trickle, from a pinhole . . . but there are other dodges to take care of weeps. Carefully done, peening with a punch will often close a weep, as will a tablespoon of ground ginger thrown into the boiler before it's pressurized. The ginger fiber will find any small leak and try to get out, hopefully unsuccessfully. I'm familiar with the name Comsol, it's been bandied about British ME writing for years, but I don't know anything about its alloy so I can't comment about that.
 

Runner

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Thanks again Harry,

it is obvious that you are proficient at making boilers and in the repair activity aware of a number of workarounds that have proved to be successful in stopping leaks. I don't want to set myself up for failure by anticipating a leak in the finished boiler because so far I am happy with what I have achieved. However if I do have a problem that I can't correct it is comforting to know that there assistance at HMEN in general and your good self specifically.

Brian
 

GWRdriver

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Runner said:
I don't want to set myself up for failure
Many thousands have been built by first-timers so all else being equal as long as you've got enough heat and don't get in a hurry you'll be fine. BTW what boiler are you building?
 

Steam4ian

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G'day Harry

Thanks for the advice about peening and ginger. I have repaired my boiler after tubes started leaking at the firebox tube plate. We resilver soldered the tubes and they look good but I still have two pin hole leaks.

I can just about get to the water side of the tube plate and could apply some 842 silver solder paste but I am reluctant to reheat the boiler incase something comes adrift. I am waiting on Cupalloys to get back to me about delivering OS.

I am also reluctant to apply high melting point solder for all the reasons you have mentioned. It is a last resort. BTW HMP solder and Comsol are 95% Pb 1.5% Ag and 3.5% Sn; melting point 301 degC, 575 degF

I guess the ginger is a miniature version of horse sh!t which was used on full size boilers.

Thanks again

Regards
Ian
 

GWRdriver

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Steam4ian said:
I guess the ginger is a miniature version of horse sh!t which was used on full size boilers.
Ian,
I have always understood that's exactly what it is, on a model scale. My first mentor built steel boilers for 7.5"ga locos a number of years (I only build copper) and on any given boiler it was not unusual for weeps or small blowholes to appear in the best of welds. Rather than hit it with the welder again, or plug it with something, he would do a low pressure steaming and typically within moments whatever dusty debris remained in the boiler would seal the weeps and those that didn't seal in that way usually rusted closed overnight and never appeared again. So many tricks to learn.
 

robmort

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Whilst an old thread, I'd like to add that for a round-top firebox, although crown stays are not needed for strength there, it is often forgotten that if such stays connect the top of the firebox to the outer wrapper they do take the stress off the foundation ring which otherwise takes all the resulting pressure between the top of the firebox and the wrapper, which is a concern.
 

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