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Edward Thompson: Wartime C.M.E. Discussion

Discussion in 'Steam Traction' started by S.A.C. Martin, May 2, 2012.

  1. huochemi

    huochemi Part of the furniture

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    When you say the axleboxes were "fabricated, not cast", what exactly do you mean (and where is that info from)? Typically LNER axleboxes were solid bronze, cf the standard GWR / LMS box which was steel with a pressed in brass (but "fabricated" would be an unusual term to apply to describe that). ISTR that in one of its earlier overhauls, perhaps the first, the Talyllyn Railway had the contract to overhaul the boxes of 61264 and I remember seeing them on the big lathe (Churchill?) that the TR had then just acquired (which sat in an annex of the North Carriage Shed), and they certainly looked like big chunks of bronze. The attached link has some images of the boxes of 61264: https://www.thompsonb1.org/2016/06/wheelsets-delivered-back-to-grosmont/
     
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  2. S.A.C. Martin

    S.A.C. Martin Part of the furniture

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    I may have actually got my parts mixed up! I am now checking my notes and I see "fabricated stretcher plates" is underlined, not axleboxes. My understanding was that axleboxes were also fabricated. Leave this with me to fact check myself! :oops:
     
  3. huochemi

    huochemi Part of the furniture

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    I also looked at Part 2B but could only see the reference to fabricated stretchers.
     
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  4. S.A.C. Martin

    S.A.C. Martin Part of the furniture

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    So diving into my references and notes: the stretchers to the main frames were fabricated, the guides for the axleboxes were fabricated also, but not the axle boxes themselves, which were, drum roll please...standard from the Gresley V4 design.
     
  5. huochemi

    huochemi Part of the furniture

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    Do you have the source for that? (the guides being fabricated?) Do you mean just the horn guides for the rear axle? Part 2B notes that there were hornblocks on the leading and driving wheels and horn guides on the trailing wheels (a fairly common arrangement as it gives more room for the firebox). A fabricated hornblock sounds unusual. The reference to the V4 is because of the identical dimensions (8 3/4" dia. x 9" long journals), not the design per se which in both cases were likely to have been LNER standard.
     
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  6. S.A.C. Martin

    S.A.C. Martin Part of the furniture

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    Rodger P. Bradley in his research "The LNER Bongos" states the following:

    upload_2021-6-11_14-57-23.png


    Fair comment!
     
  7. huochemi

    huochemi Part of the furniture

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    Thanks. I could not see where in your extract Bradley refers to the horn guides?
     
  8. pete2hogs

    pete2hogs Member

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    Thompson's designs were - and I mean no denigration by this - Gresley designs adapted for war time expediency. The B1 is a natural descendent of the K2.

    I personally only have three criticisms of Thompson's designs -
    1: The front end layout of his Pacifics.
    2. Excessively small wheels on the L1 chasing an inappropriate combination of outer suburban fast engine and low speed short haul mineral engine.
    3. Too many pointless - but very small volume - rebuilds. A1/1, Q1, D, etc. Nothing wrong with the O1 or the O4/8 -or of course of the brilliant B12/3 and D16/3 before he was CME.

    I could come up with more criticisms of Gresley, frankly.
     
  9. Richard Roper

    Richard Roper Well-Known Member

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    Interesting that the Darlington batch had frames 2" shorter than the NBL batch. I wonder where the 2" was lost? I assume it was at the front end.

    Richard.
     
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  10. Hermod

    Hermod Member

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    Gresley and Bulleid were once taken by a Nord 141 tank with Cossart valves built 1933 to Paris in failed Pacific time and was wildly impressed.
    The Nord tank had some special external balancing that made 1600mm drivers 6.5 rev per second in daily use.
    When Thomson designed L1 for same kind of fast suburban traffic he chosed 1575mm drivers and did not put the balancing rods on .
    Not a bridge to far but two rods to short maybe.

    http://www.antiqbrocdelatour.com/les-anciens-trains-de-legende/images/La-141-TC-Nord-20.jpg
     
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  11. S.A.C. Martin

    S.A.C. Martin Part of the furniture

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    I left it overnight as I thought I was being stupid, and just not seeing it. What I see now is that I read across two paragraphs.

    I think I am maybe going insane. Holiday needed. Send help.
     
  12. 35B

    35B Nat Pres stalwart

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    We all need a holiday! Commiserations sent, but the tank too low to send more!


    Sent from my iPad using Tapatalk
     
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  13. jnc

    jnc Well-Known Member

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    A while back there as a discussion (starting about here), about whether the cause of the P2 axle failures had been correctly identified. While looking for the answer to a query (on the P2 thread) about the choice of the Lentz vale gear, I ran across two posts from the P2 organization about the axle failures. One (the design study), had already been mentioned here; the other, a news story, hadn't. Just thought I'd preserve it here.

    Re-reading all that, I thought about the analysis of the historic failures a bit more, and I have to say, @Eightpot's theory is not facially nonsense; and without more details than are in the 'design study' page, it is impossible to say if Mott MacDonald's FE model included the factor picked up by Eightpot - the increased side/bending axle loads incurred by the longer 4-axle Mikado design than by the shorter 3-axle Pacific design, when going around a given tightish curve. (Mott MacDonald's theory of why the P2's had so many axle failures was apparently a combination of the P2's higher maximum axle torque, and better footing - Pacific's would slip if the torque on the axle was too high. All very correct - but was there yet another factor too?)

    I enquired of someone with considerable experience of using FE models to predict crack initiation and propagation, and she agreed that while modern (within the last 20 years) FE models can do cracks, the correctness of their predictions depends in part on the accuracy of the load model fed into them. (Another factor is the details of how a particular FE model predicts crack initiation; 'simple' FE models that predict cracks simply take the peak forces at any point, and compare them to the bulk material properties; e.g. if the material can withstand X Knewtons/mm^2, and the FE force model predicts 1.2X KN at some place, a crack could start there.) If the load model used is incomplete, though, that will effect the accuracy of the prediction, though.

    Mind, I don't think there's much cause for concern, though. If the curviness of the line was a factor in the historic failures (along with the Mikado's longer wheelbase), I don't think 2007 will be living on a curvy line. The axle has also been beefed up, too.

    Noel
     
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  14. Sheff

    Sheff Resident of Nat Pres

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    Unless my memory is failing me, I thought that the principal cause of failure identified was cracking initiated by sharp corners in wheel seat machining - maybe keyways and/or shoulders?
     
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  15. jnc

    jnc Well-Known Member

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    Yes, that was part of it - at least one old picture showed that that one's crack started in a keyway corner. But that makes sense; there are stress concentrations in corners, so that's where you're likely to see the material's strength exceeded, and a crack thus started. (And the Trust's redesign includes "a better keyway design", which will presumably reduce those stress concentrations.)

    But the other part is the question of 'where did that stress com from'? And that's what @Eightpot (and now I) are wondering about. (And, for engineering design purposes, someone will need to know 'how large is it'?) The previous recent analysis apparently attributed it to the torque transmitted through the axle from the cylinder (via the connecting rod) to the driving wheel. The suggestion now is that there was an additional "rotating bending force [applied to] the axle" by the longer Mikado as it went around a corner, which also contributed to the total stress (which eventually caused the failure). I find this quite plausible, if all the failures happened just outside the axle-box (especially as the axle is 9-5/8" in diameter there, and only 8-1/4" in diameter in the section adjacent to the inside cylinder crank web).

    FWIW, I just showed that picture of the failed axle to the structural engineer whom I had consulted, and once I explained how all the associated pieces fitted together (the axle-box, etc) her immediate reaction was 'oh, yeah, if there's a sideways force being applied to the axle outside the supporting bearing, you'd expect to see a failure right there'.

    Now, it's more complex, because there's already (and always was) a large force applied there to the axle at a right angle to the rotational axis of the axle, which is the weight of the locomotive (through the wheel). (Which is probably why that part of the axle, outside the axle-box, is significantly larger in diameter than the inner part, now that I think about it.) And probably the greater driving torque was also a factor in the overall stress on the axle.

    It's not clear how all these things fit together. According to the LNER Web-site, the P2's put 20 tons on the driving axles, and the A2/2's were 22 tons.... And I don't know the diameter of the crank axle outside the bearing on the A2/2's, to know if they're the same or not (and thus directly comparable). But one has to wonder if the longer driving-axle wheelbase on the P2's, and the resultant greater load going around a corner, was part of why they had several crank-axle failures - and the A2/2's didn't.

    Now, this is just a theory, and it might easily be wrong. And in case it's not clear, I'm not trying to cast any aspersions on any engineers, past or present. I just want to know what the answer is to a very long-standing puzzle.

    Noel
     
  16. 30854

    30854 Resident of Nat Pres

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    I wonder ..... Would it be a theory which, with a potentially significant point having been identified by @Eightpot, could now be subjected to computer modelling, or is construction now so advanced there's little point?

    Given there were quite a few, did any other 8-coupled UK designs (none of which, I realise, had such large coupled wheels) suffer failures which could reflect the issue highlighted? For some reason, my mind is drifting towards Mr. Bulleid's proposed SR 2-8-2, abandoned in favour of the eventual MN design.
     
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  17. Jimc

    Jimc Part of the furniture

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    One thing I think I noticed - and it needs a competent railway engineer to state whether this is nonsense or real phenomenon, is that odd numbers of wheel might be worse for alignment on curves than even numbers. If you consider three pairs of wheels then the centre pair are on the apex of the curve, but with four pairs the centre wheels are off the apex, so it seems to me the actual lateral displacement of the wheels is less **for the same wheelbase**. On the other hand a longer wheelbase is of course worse than a short one. I presume actual length of wheelbase is by far the dominant factor though. At least it explained to me why the centre pair of wheels on a 9F are the flangeless ones. But maybe that affects "track spreading" more than anything else?
     
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  18. Eightpot

    Eightpot Resident of Nat Pres Friend

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    I'd like to thank jnc (Noel) for confirming that my theory isn't as crackpot as it appears to be to some. As he confirms in the P2 Group's analysis they only referred to the torque applied to the axles, with no mention of the sideways forces. I freely admit that the mathematics and computer studies are somewhat beyond my understanding, me being more of a practical 'hands-on' type.

    To me the real villain here is the fact that as the pioneer of railways as we know them, that in hindsight (a wonderful thing!) our load gauge profile simply wasn't large enough to cater for future requirements. Essentially we are restricted now to a width of less than 9 feet over cylinders and 13 feet in height. Compare this with Germany starting railways in 1835 they could have 10' - 4-1/2" (3150 mm) over cylinders and a height of 14' - 11" (4550 mm) over the chimney. With the P2s to get the power from 21" cylinders other things like axle side-play and bearing widths were very restricted. If, say the cylinders were made smaller and the piston rod centres widened, along with a boiler pressure increase to 250 psi - which didn't come until the A4s in 1935 - it may have been possible to put some flexibility into the chassis. The P2s were really pushing the boundaries and with a lot of things in engineering, it is all a matter of compromise.

    To give an idea of an increased load gauge possibilities the German Class 52 Kriegslok 2-10-0 has a width over cylinders (600 mm/23-5/8" bore) of 3000 mm/9' - 10" which permits a 1"/25 mm each side of centre side play (50 mm/2" total) on the 1st and 5th coupled axles plus thinned flanges on the centre driving wheels, and this combined with a Krauss-Helmholtz truck - essentially turning the chassis into a 4-8-0 - made for a very flexible loco on curves.

    For the P2s what also did not help was that they were confined to a route of only 130 miles long split into two sections of 60 and 70 miles at Dundee. This would, with engine changes there, result in them standing around at depots quite a lot and yet still burning coal.
     
    Last edited: Jun 16, 2021
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  19. Matt37401

    Matt37401 Nat Pres stalwart

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    Wasn’t Mr Bulleids 2-8-2 vetoed by the Civil Engineer?
     
  20. Jamessquared

    Jamessquared Nat Pres stalwart

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    Yes and no :)

    There was a lot of back-and-forth, but Ellson, the Civil Engineer, (remembering the Sevenoaks accident) was averse to pony trucks on large engines. In 1938, the drawing office got out a design with a Helmholtz leading truck. With that modification, the Civil Engineer said "yes" to having two locos built for trial; and on that basis, Bulleid said "no", not wanting to just build a trial loco. "[Bulleid foresaw] a future filled with more arguments about engines", in the words of HAV Bulleid, "and regretfully reverted to a Pacific".

    So in essence Ellson said "yes" to a 2-8-2 provided it was just a trial, and Bulleid said "no" to a trial. Of course, Ellson may just have been smart in finding a way to block something he didn't want without actually being the one doing the blocking! (That is supposition on my part).

    Tom
     
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