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Best British Locomotive

Discussion in 'Steam Traction' started by Hermod, May 12, 2017.

  1. paulhitch

    paulhitch Part of the furniture

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    This is, IMHO, much too much of an assumption to make. The I3 was directly derived from the B4 with some alteration to driving wheel diameter. It had a notorious tendency towards frame cracks which of course has nothing to do with superheating save for the fact that the apparatus permitted more power to be developed with consequent additional stress on the frames. A typical I3 seemingly had extensively patched frames and these palliative measures would have been costly.

    The I1 spent its time on lighter slower duties than the I3. I am not aware if the I1 owed more to G.N.R. practice in frame design than that of the I3, which could be a factor.

    Boiler maintenance charges were said to to be much reduced in a superheated boiler. This was attributed to the great reduction in feedwater consumption. Of course, the capital costs of superheating were not inconsiderable but, obviously, this is a separate issue from maintenance costs. The business of stopping services supposedly getting no benefit from superheating is to my mind, rather an old chestnut.

    PH
     
  2. 30854

    30854 Well-Known Member

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    The subject area of (plate) frames cracking is an interesting one. Perhaps a better understanding would come from looking at long lived classes where it wasn't a noted problem and at design and manufacturing modifications known to have reduced a class's susceptibility to frame cracking.
     
  3. Hermod

    Hermod New Member

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    The Caledonian class 60 had no cracks.The standards quite some.Are there some class 60 frame,hornblock drawings left?
     
  4. paulhitch

    paulhitch Part of the furniture

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    Starting with the "Brighton", Stroudley designs were much less prone to cracked frames than the newer Billinton ones. This I have seen attributed to the hornstay design.

    G.W.R. design seems to have to have produced problems with frame cracks.

    The L.N.E.R. A3/A4 situation is an interesting one. The A3s suffered so badly that a spare set of frames was kept at Doncaster. For the A4s, someone had given some real thought and re-designed the arrangement so as to avoid sharp radii and this was a success. The real surprise is that no-one thought to "retro-fit" the A3s with the improved arrangement and thus gradually eliminate the problem over a period.

    PH
     
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  5. Forestpines

    Forestpines Member

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    At what point in history did engineering and materials science gain a proper mathematical understanding of the principles of stress concentration? My feeling is that it wouldn't have been until very late in the development of steam, but my knowledge of the subject is limited to one book from the late 60s ("The New Science Of Strong Materials, Or Why You Don't Fall Through The Floor" by Gordon, a surprisingly entertaining book).

    It strikes me that avoidance of frame cracking before such principles were fully understood would be more by luck than judgement.
     
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  6. Bill Drewett

    Bill Drewett New Member

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    I think the great leap in understanding came from the research that was done at Farnborough following the in-flight break up of two DH106 Comets in the early fifties. The original Comet had square windows, the redesigned one, oval windows. All commercial jets since have followed suit. Of course the effects are noticed much sooner in a pressure vessel made of 1/2 inch thick aluminium!

    (I absolutely agree about Gordon's book. Reading it at school helped point me towards Mech. Eng. at University).
     
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  7. 30854

    30854 Well-Known Member

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    No argument from me! AFAIK the first proper investigations specifically aimed at problems associated with stress fracturing came with work into why WWII 'Liberty Ships' suffered so many failures. Even then, it took over a decade before catastrophic failures led to several fatal crashes of Comet airliners before the study metal fatigue began to be recognised as worthy of in-depth study in it's own right. (Edit: I see this exemplar has already been noted while I was beavering away on this lot)
    Although Stroudley era locos were less powerful than later designs, it would be interesting to learn which other factors may have contributed to longevity of frames.

    I know the top corners around hornstays were (and remain) an area prone to cracking and recall that radiused corners were identified as an improvement, but can't recall when this was first noted. Many grades of steel are known for issues surrounding machining, with the effects of resultant hardening and brittleness clearly visible under modern electron microscopes and some newbuild websites specifically comment that frame cutting techniques for modern new builds have been modified accordingly.

    It's obviously unreasonable to castigate steam era CME's for not having the benfit of electron microscopes (which probably won't stop some from doing so anyway), but the example of certain designs less prone to fractures seem to have been less well studied than one might expect.

    Clearly, more powerful rebuilds and developments of earlier designs involve laying down more power than original components were ever expected to handle (Super-Clauds come to mind and in light of previous comments, it's probably safe to add the I3 to that list), but despite decades of the same issues surfacing time and again, it's an area which seems to have remained under developed.

    It's interesting to note later American moves from bar frames to cast steel beds (adopted for certain export designs by Beyer Peacock post WWII), though I don't really have a handle on how successfully these coped with the stresses generated by large locos.

    That book by Gordon is going on my shopping list. Thanks for the heads-up.
     
  8. 8126

    8126 Member

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    The big change with the A4 frame design was the horn stay detail. On the A3 and A1 this simply tied together the base of the horn block casting, on the A4 it was also attached to the frame plate itself. This is more of a nuisance to fit, but from the point of view of the frame plate (and thus the corners at the top of the horn gaps) it's much stiffer, because the load is only going through two interfaces rather than four (frame-stay-frame instead of frame-hornblock-stay-hornblock-frame). That reduces the stresses at the top of the horn gap considerably, because more of the load is taken by the stay and less by bending of the plate.

    The LMS re-learned this lesson independently with the Black Fives and Jubilees, and undertook a considerable (although not complete) programme of modifying the frames of both classes to allow for a new stay design. If you see mention of the "Horwich hornclip" that's what is being referred to.

    Regarding some other comments, it's a mistake to think there was no understanding of fatigue before the jet age - in fact a lot of the early research into fatigue problems was driven by the coming of the railways and the failure of axles in particular. Cast iron also taught painful lessons about the dangers of sharp corners. However, railway rolling stock tends not to be designed to the limit of stress, it's designed with stiffness in mind and no railway designer ever had to pay as much attention to weight as an aircraft designer. Aircraft have to be light, so are designed to maximise the utilisation of strength of a material. In addition, mild steel is wonderfully tolerant material, with an endurance limit below which fatigue failures will not occur; if you're designing for stiffness a lot of the structure will naturally fall below this limit. Aluminium alloys are not so tolerant, lower stress just means more cycles to failure. Since aircraft failures have much more potential for catastrophe, suddenly a much more in depth knowledge of fatigue was required, combined with better analysis techniques.
     
  9. Bill Drewett

    Bill Drewett New Member

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    This all makes sense, and my knowledge of railway engineering is more limited than aerospace. It does beg the question though, if locomotive designers did understand the effect of stress concentration on fatigue life, they'd have realised they could eliminate frame cracking with a few more generous radii. Why didn't they?
     
  10. LMS2968

    LMS2968 Well-Known Member

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    One reason was possibly because you have to take the metal from somewhere, and that wasn't always an available option. The main stress area was the top corners of the horn gaps, particularly at the driving horns. As 8126 said, removing the stress there by attention at the lower end was a better option. George Hughes at Horwich had realised this early on, but other railways, including the LMS following his retirement, took time to learn the lesson. Compare the Crabs with the Black Fives, for instance, although these also had thinner frame plates to add to their woes.
     
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  11. andrewshimmin

    andrewshimmin Active Member

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    I seem to remember that the "old Crewe school" of engineers who were apprenticed at Crewe in the Ramsbottom/Webb era were taught to understand metals properly. Webb took that sort of thing very seriously because it was, if you'll pardon the pun, cutting edge stuff in his day and made all the difference to what could be achieved. But I think stronger materials coming in by the early 20ty century made people blase, and fatigue wasn't well understood. Still isn't, often!
     
  12. 8126

    8126 Member

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    Since we've drifted towards the discussion of design techniques, analysis and metallurgy in a railway context, there's a very interesting (to me, at least) story in Phil Girdlestone's book* on the SAR Class 25. I don't have it hand right now, so here's how I remember it.

    The SAR had experienced troubles with broken connecting rods as their successive main-line 4-8-2 classes got bigger, completely consistent with basically designing by rules of thumb which fall over as the design gets bigger. Anyway, when the 4-8-4 class came around, a gentleman by the name of Wynn Douglas was tasked with the connecting rod design. Douglas had been in the US during the war and was, according to another member of the design team quoted in the book, the best theoretical engineer in the design team with a strong interest in late US steam practice. So, the connecting rod was duly designed in accordance with his best engineering understanding and everyone was very confident that there would be no connecting rod problems with the new class.

    You can probably guess where this is going...

    When brand new 25s started breaking connecting rods there was much consternation. So the SAR threw lots of resources at the problem, including fitting strain gauges to the connecting rods of a 25 in controlled tests over real track. They discovered a few things they'd not really expected, like just how much the connecting rods could be twisted when the loco heeled over as it went through crossovers at moderate speed (as Loubser put it in the anecdote, the tendency is to think of stress causing strain, whereas sometimes a component is forced into a shape, thus causing stress), but fundamentally they couldn't find a load condition that should cause the failures experienced. In addition, although the failures were occurring close to the position you'd expect, they weren't quite there.

    An examination of the grain structure eventually gave it away. One of the Henschel machinists had made a mistake on a batch of rods and got a welder to build back up the affected region, which he had done very well such that simple visual inspection of a sectioned item wouldn't give it away. The trouble was, the connecting rods were alloy steel (EN14, I seem to remember) and the region built up was therefore the wrong material and had ruined the heat treatment around it. Hence the failures.

    *Camels and Cadillacs - If you have any interest in a case study of how a well-regarded loco from the days of steam came to be, and all the engineering thinking and teething troubles along the way, I can recommend this book.
     
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  13. Tuska

    Tuska New Member

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    Many people would have loved to see GNR Class A1 1470 Great Northern survive into preservation.

    The fact that Edward Thompson deliberately butchered that triumph of a loco before taking retirement, is proof he was in some way disgruntled over a feud, and allowed his petty hatred and contempt of Gresley, to interfere with his rationality and better judgement as an engineer.

    You have to be a very small man to take out your frustration on something that can't fight back.
     
  14. Matt37401

    Matt37401 Part of the furniture

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    Oh Please! Leave it alone will you, this has been done to death in the Edward Thompson thread! Go and have a look at that, there's a rather more balanced look at those points you make over there!
     
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  15. Eightpot

    Eightpot Part of the furniture

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    Just for the record, the Comet 1s and 2s had a slightly rectangular side window with 3" radius corners. The cracks started from rivet holes in the 'doubling' plates surrounding the aperture. In the light of further wreckage being recovered some time after the Inquiry there was some doubt as to whether the failure initially started at the cabin side window or the ADF one on the top of the fuselage. There was serious consideration that the failure of one caused the failure by near simultaneous shock effect of the other, this happening in fractions of a second so it was impossible to decide as to which one failed first.
     
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  16. Jimc

    Jimc Well-Known Member

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    Well, take a horn guide for instance. Lets say (making some numbers up) the frames are 2 feet deep and the slot for the horn guide needs to have 12 inches of parallel side for movement. Add a half inch radius and the frames are 11.5 inches deep above the hornguide. OK, lets say then we want to make the radius 3inch. Now we only have 9 inches frame depth above the hornguide...
     
  17. 30854

    30854 Well-Known Member

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    .... which just goes to show quite how extensive is the redesign dictated by one seemingly simple modification. Anyone care to comment on other knock-on implications? Suspension comes to mind for one.
     
  18. Tuska

    Tuska New Member

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    A lot of drama llamas on the Internet aren't there?

    Back to locomotives, children.
     
  19. Matt37401

    Matt37401 Part of the furniture

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    Please explain.
     
  20. Bill Drewett

    Bill Drewett New Member

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    I understand the geometry. I would have thought that an understanding of stress concentration would lead to the following design logic:
    1. How much axlebox movement do we need? 12".
    2. What radius should the top corners of the horn gap have (to prevent cracking)? 2".
    3. What minimum depth of frame do we need? 12".
    4. Therefore total frame depth must be 26".
    In your example, why do you start with the overall frame depth? Shouldn't that be the result of the calculation rather than a starting condition? (Of course deeper frames cost more and are heavier, but that's also factored into the design decision: lifetime cost of deeper frames v. cost of frame replacement during design life of product).

    Also, here's another thought. By chamfering the top corners of the axlebox to match the radii at the top of the horn gaps, one could keep the original frame depth and still retain the original 12" of axlebox movement.
     
    Last edited: Nov 11, 2017

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