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Bluebell Motive Power

Discussion in 'Steam Traction' started by Orion, Nov 14, 2011.

  1. misspentyouth62

    misspentyouth62 Member

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    Thanks for explaining 8126
    So would monal stays be used predominantly within the preservation movement would you say?
    I have some recollection of seeing the heap of removed stays at GCR Loughborough when 34039 Boscastle was having firebox repairs or more probably, firebox removal for replacement and I had the impression that they were copper? Maybe I got this wrong though.
     
  2. Jamessquared

    Jamessquared Nat Pres stalwart

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    Monel stays have been used at least back to the 1930s in locomotive fireboxes.

    You are right about the impact of differential expansion. Very crudely, as I understand it, in a conventional firebox where the inner and outer are rigidly locked together at the foundation ring, the stresses of differential expansion gradually increase as you go higher in the box. Therefore, the top corners (of a Belpaire firebox) are particularly stressed. Each heating / cooling cycle puts a stress on the stays and eventually they may fail through fatigue, possibly exacerbated by weakening through corrosion. There is considerable design subtlety in how in particular the top of a firebox is supported.

    The LMS (and quite possibly the other companies) defined recognised “breaking zones” around the top corners that were a particular concern, and at various times did experiments with copper, steel and monel stays to look at how they lasted (and therefore ultimately the impact on boiler maintenance costs). The issue is complicated by the fact that when you replace a stay, you then have to enlarge the stay holes and re-tap them and fit a larger stay; there were standard stay head diameters but enlargement can only be done a certain number of times before you need to start again.

    Tom
     
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  3. misspentyouth62

    misspentyouth62 Member

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    Thank you Tom.
    A probable silly question but given that the weight of the inner box is supported at f/ring and from so many stays, what would happen if there were less or no stays at the breaking zones such as corners and crown? Do the stays limit inner from outer firebox expansion that would otherwise exacerbate cracks & breakage?
     
  4. MellishR

    MellishR Well-Known Member Friend

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    Differential expansion is merely one of the complications that need to be dealt with. The fundamental purpose of stays is to withstand the boiler pressure. Any region of a firebox with too few stays would burst.
     
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  5. misspentyouth62

    misspentyouth62 Member

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    Thank you MellishR.
    I find it incredible that an old stay can be removed without damaging the firebox and new holes drilled and tapped exactly in alignment to insert new stays - and then repeat another 2000 times! Similarly I would love to watch how holes are drilled and perfectly aligned for a new inner firebox. What happens if the holes are slightly out of alignment? Would the holes be weld-filled and re-drilled inner & outer?
     
  6. Jamessquared

    Jamessquared Nat Pres stalwart

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    The weight of the inner firebox is supported via the foundation ring. The stays aren’t there to support the weight, but to resist the pressure trying to push the inner and outer firebox apart. So removing stays to avoid the problem of stay breakage might have some rather interesting results! (Old loco crew joke: What steps to you take if you see no water in the gauge glass? Answer: bl@@dy large ones!)

    The design of boilers took into account the tensile strength of the materials and from that could work out the number of stays required, taking into account acceptable safety margins etc. There was also a lot of (probably somewhat empirical) consideration of the precise shapes of the plates, corners etc. the aim being to provide adequate safety while minimising initial construction and subsequent maintenance costs. To which you could add a whole suite of other requirements to optimise the steam raising potential of the boiler, concerning such things as tube number, length and diameters, and for superheated boilers, the balance between tube and superheater surface. Then layer on how changes to design change both the total weight and weight distribution - some boiler designs had special high tensile strength steel plates because doing so meant they could be thinner, giving a worthwhile weight saving.

    Apart from Monel stays and high-tensile boiler plates, improved water treatment (leading to reduced corrosion) and copper welding (meaning the old process of replacing entire inner fire boxes after a certain time could be replaced with repeated replacement of parts, typically lower half sides) were some of the other technological changes of the inter-war years that changed boiler maintenance practice.

    All in all, quite a complex process; and also one where design might improve incrementally. It’s frequently noted that there were 800+ Black 5s made, but by no means does that mean there were 800+ identical locos: the boiler design being one of the areas where there was quite significant variation through time.

    Tom
     
  7. mdewell

    mdewell Member

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    I expect very long drills and taps (to reach through both inner and outer plates, as don't forget the threads also have to align so that a new stay can be screwed through from one side (i.e . you cannot tap inner and outer ends seperately.))

    Edit: Found the following which shows work in progress on LMS 45407 (video posted in April 2011)
     
    Last edited: Nov 8, 2019 at 9:46 AM
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  8. misspentyouth62

    misspentyouth62 Member

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    Thanks mdewell. I too found this video on YT and found it interesting as was another which showed application of 'flexible stays' in use on boilers being built and maintained in the US. These looked to me made of steel and as their name suggests, allows a degree of movement between inner and outer fireboxes.
     
  9. Kylchap

    Kylchap Member

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    Makes you realise just how much work goes into a firebox overhaul.
     
  10. Steve

    Steve Part of the furniture Friend

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    Flexible stays are used in the UK, as well.
    And why they cost so much.
     
  11. Cosmo Bonsor

    Cosmo Bonsor New Member

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    Hi.

    The subject of boiler stays comes up now and again.

    The main reasons for replacing stays are breakage of stay, poor boiler plate condition needing new plate work and burnt away heads in the firebox.
    There might be other reasons like the outside end of the stay corroding during a sojourn in South Wales.

    For those of you who’ve never had the fun of copper stay replacement here’s what you do.

    Chisel each end off the stay flush with the plate.
    VERY carefully clean up the chiselled end so you can see the full circumference of the stay.
    Centre punch and with a windy drill, drill into the stay the thickness of the plate plus a bit.
    Use a 3/4” drill or the biggest you can without hitting the plate threads.

    Now for the fun bit.

    Using a straight punch, break out through the side of the stay around its circumference.
    You may punch through where it has broken if you are lucky.
    With a round nosed chisel, (pre grouping ones are made of very good steel, SECR ones are best!) collapse the remaining threads into the hole you drilled. That’s why you go deep, to give you room.
    You have to kind of ‘peel’ the thread away from the plate. Sometimes this is easy, other times it can be a real pain.
    Repeat for the other end. When you punch through the sides, the bit in the middle will fall down. Where it ends up depends on which way up the boiler is and chance.

    OK, you now have old threads in the plates and/or a newly drilled hole in new plate.
    You pass a reamer that has a very gentle taper over its roughly 2’ length. That’s for side stays, crown stay tools are even longer.
    The reamer is sized to the boiler tap you are using. It will remove nearly but not quite all of the threads in the hole.
    You now insert the next size tap. It will be 1/16” bigger than the hole you have.
    If you look at one, it has a very long taper on it with the full form thread only on the last bit and the thread is continuous.
    The tap will pick up on the last little bits of thread in the hole and as it is the same pitch, 11tpi is usual (12 tpi is the other less common one) it cuts the new thread in exactly the right position.
    This is the magic that keeps the threads in pitch and removes least material.

    Repeat until you have a good thread in both plates.

    There’s more to consider, and a lot of detail and variation I have missed out. I may elaborate if anyone is interested.

    The video is good but doesn’t really show the removal part. The forming of heads bit is good. We did it slightly differently, I suspect there may regional differences like much else on steam engines.

    I’ve replaced a lot of stays in my time working on engines, I will be happy never to pick boiler tools again.

    Hope you find this interesting.
     
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