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Tornado

Discussion in 'Steam Traction' started by Leander's Shovel, Oct 20, 2007.

  1. W.Williams

    W.Williams Well-Known Member

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    Why are you asking that? You already know the answer. Are we really going to have to go back around this? CAn we not?
     
  2. Steve

    Steve Part of the furniture Friend

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    I am asking because you said "....subsequent fatigue failure in the motion." That is news to me and is a fundamentally different thing to something failing due to overload for whatever reason. Any fatigue failure has to be a cause for concern and I feel that it is important to have clarification from you.
     
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  3. W.Williams

    W.Williams Well-Known Member

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    You do understand that fatigue comes in two forms yes?
     
  4. flying scotsman123

    flying scotsman123 Resident of Nat Pres

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    Presumably a fatigue due to poor material or fatigue due to excess force beyond the design requirement being applied, the fatigue you mean being the latter?
     
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  5. Steve

    Steve Part of the furniture Friend

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    No, but I'm sure that you can enlighten me. In my book and being simplistic, fatigue is the initial formation of micro-cracks due to a combination of stress changes and frequency and amplitude of these changes. Fatigue cracks are frequently associated with stress concentrations and tend to grow over time until a point is reached where the remaining material has insufficient strength to carry the load, at which point failure occurs. Fatigue failures are not simplistic overload failures due to tensile, bending or shear stresses and are generally associated with poor design or manufacture. I'm not aware of either being the case with Tornado or of there being any evidence of fatigue, which is why I asked my initial question.
     
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  6. W.Williams

    W.Williams Well-Known Member

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    Ah young loon, much to learn. https://files.asme.org/IGTI/Knowledge/Articles/13048.pdf ;)

    Micro cracks exist in nearly everything. It's crack propagation that is the root cause of Fatigue failures. Which can either be HCF or LCF in nature. High or Low cycle.

    This is where you are missing something as you are only thinking of HCF. LCF failures are categorised by precisely what you outline.

    It has nothing to do with the design or manufacture of these parts. Tornado is not in the dock for any design or manufacturing errors as I have been at pains to point out further up.

    A stuck valve will lead to bending in to the combi lever, because the peer wee combi lever isn't up to the job of stopping the motion and wheels @ 90mph...

    This is evidenced in the bluing of the area surrounding the failure of the lever, of which the photos up thread show.
     
  7. MellishR

    MellishR Part of the furniture Friend

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    If the valve seized suddenly, the combination lever would have broken immediately. If the valve spent a little while getting stiffer and stiffer, but still moving, that would have put increasing stresses on the combination lever, which might have suffered fatigue failure even without the valve seizing totally. I don't know which it was, but it hardly matters. The root causes and the end result are the same in both cases. The root causes have been addressed and the broken parts have been replaced. The only uncertainty is whether the Trust have given up any intention of future 90 mph running. I would hope not, simply for the reason why it was planned in the first place, to facilitate pathing on lines that are increasingly busy with trains travelling at 90 mph or considerably faster.
     
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  8. acorb

    acorb Member

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    Tornado is still a 90 mph plated locomotive. Speaking to someone who would know last year, she has run well into the 80's on more than one occasion since the incident. As I don't know the specific circumstances I won't elaborate further. However, there is as far as I know nothing stopping her from hitting 90mph again, it just requires correct route and more importantly, stock.

    What limits her generally is the strange rule that mk 1 stock is limited to 75 mph when hauled by a steam engine, but can run at 90 or 100mph with diesel / electric. If anyone has a logical explanation for this I would be pleased to hear it.

    Where I would agree with you is I think it will be a while before we see a 90mph advertised trip, but if she is put onto a rake of mk2 or mk3 coaches on a suitable route there is to my knowledge nothing stopping her.
     
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  9. flying scotsman123

    flying scotsman123 Resident of Nat Pres

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    I seem to recall from a debate at the time that it was purely an administrative oversight, the way things were worded just happened to mean that it wasn't allowed, and the issue had never come up before because neither had the circumstances. Hopefully as and when Tornado has another crack at it that inconsistency will be addressed now that it is known about.
     
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  10. 8126

    8126 Member

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    Like @Steve, I'm not sure I'd characterise the Tornado gear failure as low cycle fatigue. Low cycle fatigue generally implies repeated stresses above the elastic limit (but below UTS) in some part of the structure due to either exceeding the design operating envelope or inadequate design, causing rapid crack propagation and hence failure. What happens with a seized valve is repeated forcible displacement of the (stuck) combination lever beyond the elastic limit, but not far enough (due to the geometry of the system and characteristics of the metal) to cause failure on the first cycle. It's much more like work hardening, with consequent reduction in breaking strain and thus failure when the component is (again forcibly) pushed past the reduced breaking strain. If I have a piston seize on my car and it spits a connecting rod out I'm not going to call that a fatigue failure of the connecting rod, the only difference is that the combination lever on Tornado was ductile enough to delay the inevitable by a few cycles. You couldn't specify any metal alloy to take that load case with that component form and not be subject to gross plastic deformation, unless it was strong enough to break something else first.

    If failure had been due to excess loads on the gear purely from over-speed, with no seizing of the valve, but occurring over a relatively small number of cycles with plastic deformation due to inadequate strength, then I would agree it was low cycle fatigue.
     
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  11. W.Williams

    W.Williams Well-Known Member

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    Let me introduce you the concept of ductility. Ductility says your statement is incorrect.

    There is no such concept as "failure from overspeed" as you put it. If its not LCF then what is it? Because all the evidence says it is.

    Plastic deformation is not due to inadequate strength, it's a result of excessive load. That load happened because something that is supposed to move, stopped moving.
     
  12. 8126

    8126 Member

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    If I spin a mechanism (like for instance one of Pratt and Whitney's finest turbines) at a speed inducing dynamic loads which the system cannot bear, I'm struggling to see how the excessive speed is not the cause of failure. One might even call it failure due to over-speed. I can do this several times inducing dynamic loads below UTS but above yield, and it will fail in low cycle fatigue, due to excessive speed, without any failure to function until it messily disassembles itself. If I have a stronger material for the same weight, the permitted speed will increase.

    The (failed) mechanism driving Tornado's combination lever was capable of providing sufficient load to cause failure in a single cycle, if it had been able to provide enough displacement, which it was not until the material work hardened enough to reduce its ductility and hence its breaking strain. If it had been made from fully hard tool steel it would have failed on the first cycle, or something else would have got there first. The two modes of failure are different.
     
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  13. Steve

    Steve Part of the furniture Friend

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    Well, I admit that I have learnt something in terms of terminology, if nothing else. In my world of engineering such a failure as happened to the combination lever would never be referred to as a fatigue failure simply because it was due to gross overload leading to plastic deformation. Fatigue failures to me are purely low stress high frequency propagating cracks and, I'd suggest that is the thought of anyone who sees the word fatigue.

    I am enlightened. Thank you, but I hope that you refrain from using the word fatigue in this way as fatigue failure to all but the most learned implies something far more concerning.
     
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  14. W.Williams

    W.Williams Well-Known Member

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    Still not related to speed. This same failure could have happened at 30 mph, as a stuck valve will always cause bending in the combi lever leading to a LCF failure if enough cycles are induced to work harden the lever.
     
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  15. 8126

    8126 Member

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    I fear we are arguing over exact technical terminology while not actually disagreeing over what happened.

    I should clarify; I wasn't trying to imply the failure of the combination lever was due to speed as a direct cause in this case (other than quite probably contributing to the valve seizing). I was saying that if in a hypothetical scenario the gear had failed from the dynamic loads of otherwise normal operation at excess speed (from acceleration of the valves and components), with parts of the gear stressed beyond yield on every cycle, that to me would be a classical low cycle fatigue failure. The load will be about the same each cycle, especially if the loads are predominantly tensile and therefore no stress redistribution occurs. In that scenario, if I change the properties of the metal such that no plastic deformation will occur with that applied dynamic loading, the low cycle fatigue failure goes away (although high cycle may still be a factor).

    In the actual scenario, as I commented previously, no metal could eliminate the failure. There was effectively a fixed deformation with massive strain forced on the combination lever by the rest of the mechanism, with a vast load available to overcome any component resistance. If I do a tensile test but occasionally release the load and reapply it on the way up to the UTS, it's not necessarily a fatigue failure when it goes. The route to failure can still be in a ductile mode, with necking until a fast fracture occurs, whereas LCF to me implies failure by rapid crack growth until fast fracture. LCF can be 10^4 or even 10^5 cycles. That would get Tornado a solid few tens of miles....
     
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  16. gricerdon

    gricerdon Member

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    As far as I am aware it has not run well into the eighties since the Ebor Flyer. If it had then I am sure one of us in the timing fraternity would have known. Something in the high seventies is quite common though
     
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  17. osprey

    osprey Member

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    Ah, days on the Denison textile tester, memories
    ...
     
  18. 242A1

    242A1 Member

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    Thank you.
     
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  19. Steve

    Steve Part of the furniture Friend

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    Me too. We had a 50T machine and a 5t machine to play with. Avery's (who owned Denisons) offered me a job there when I left Uni. I often wonder what would have happened if I'd accepted it. Denison's foundry (which used to be connected to the Middleton Railway) is now Facultateive Technologies, who make incinerators and cremators.
     
  20. osprey

    osprey Member

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    Still doing destruction business then!
     

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