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Bulleid Pacifics - Past or Present

Discussion in 'Steam Traction' started by 34007, May 13, 2008.

  1. andrewshimmin

    andrewshimmin Active Member

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    I'm writing this without too much thought and after a whisky or two, but...
    Surely heavy lumps of metal going round and round (coupling roads) also go up and down and they round and round...? So they do contribute to hammer blow? What am I forgetting?
     
  2. Hermod

    Hermod Member

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    The counterweigths opposite crankpins.
     
  3. twr12

    twr12 Active Member

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    The reverser on an original Bulleid Pacific is fine to access for maintenance. It is mounted high up on the inside of the l/h main frame plate above the left hind driving wheel.
    With the loco over a pit, you can easily access the reverser off a ladder or steps to attend to glands. When you need to change the leather cup seals in the oil cylinder, you do need to disconnect the reverser from the weighshaft in the box, but that can be done from the rear access door to the box.
    There is a removable panel in the cladding to acces the steam valve.
     
  4. 21B

    21B Well-Known Member

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    Really the only issue with the designs "as built" was the valve gear. The rest of the design is more or less good to excellent. I think the theory that they had to be rushed is a good one, and certainly Nelson is much trickier to get to steam than a Merchant which with heavier and heavier trains would have been a consideration. I just cannot understand the rationale for the valve gear, unless it was an attempt to save materials / process time.
     
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  5. Big Al

    Big Al Resident of Nat Pres Staff Member Moderator Friend

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    Can't add a great deal on this to what has already been said by folk like @Jamessquared and @Bulleid Pacific.

    Let's just think for a moment about the Bournemouth Belle that, before WWII intervened was loading up to 12 Pullmans of around 500 tons gross in the peak season and struggling to and from Bournemouth on a tight schedule with a Lord Nelson up front. Just a few minutes in the mess room at Nine Elms would have informed OVSB about the motive power needs of the Southern for its biggest trains. And this is before you start to talk about the other prestige trains like the ACE and Golden Arrow.

    Faced with the recommencement of the service whenever the war ended, Bulleid was probably driven by the need for an urgent solution that didn't allow much time for development and there was of course the limitation of the materials available at the time. So the Merchant Navy Pacifics that emerged were almost examples of 'work in progress' rather than the finished article. Personally, I think that it is remarkable that such a competent machine was produced over the period in question and it says quite a lot for the ingenuity of the designer. Yes, there were a few creative ideas that probably could have done with a little more development time and I personally think that the steam reverser is one example. I share the view that it wasn't as simple as 'Ashford good, Bulleid bad'. Both were problematic and maintenance critical.

    The account by Jim Evans (Nine Elms Top Link driver) of a run from Bournemouth to Waterloo on a summer Sunday excursion with one stop at Southampton is worth another read if you have his book. At the time he was a fireman and he describes standing on the platform at the Central with his driver, Joe, to be greeted by ten coaches coming around from the West headed by Q Class 30548. Yes, they took the train to London - remarkable! - and only dropped a little time on the relaxed excursion schedule but it was not without drama and the reverser was part of the problem. His view was that there was so much play in the linkage that the pointer in the cab would never remain still. The harder that the engine was worked - and it needed working hard to keep the blast on the fire and make it steam - the more it leapt back and forth. 35% could mean anything from 25% to 45% and so all you had was the roar from the chimney to help you judge.

    Fortunately we have examples of the original Bulleid design on the system for us and younger generations to marvel at. They are distinctive and unique. It's a pity that one is not on the main line currently as the prospects for Tangmere do not look good and it may well be City of Wells that we will need to turn to in hope.
     
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  6. 8126

    8126 Member

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    The quick and dirty answer is that things going round and round, like crankpins and coupling rods, can be balanced by other things going round and round on the other side of the circle, like balance weights. This does not lead to hammer blow, but if you don't balance these parts you will have hammer blow from the vertical component of the imbalance forces. These parts are normally categorised under rotating balance.

    Things going back and forth horizontally, like pistons and crossheads, cannot be perfectly balanced by rotating things. If you do, to stop the engine shaking the train back and forth, you create a vertical imbalance which was not there previously. This is hammer blow.

    Three and four cylinder engines have pistons and crossheads at the correct opposed phase angles to cancel the reciprocating forces, provided the motion is the same mass for all cylinders. However, because the cylinders are on different centrelines they will still try to make the engine nose from side to side. Pre-Bulleid, designers therefore still included reciprocating balance, so there was hammer blow at individual wheels and axles even if the full engine hammer blow was zero. At Holcroft's instigation, Bulleid had the reciprocating balance removed from a Schools, which with its short wheelbase could be expected to be worst affected. No issues were found, hence the decision to omit reciprocating balance from the Pacifics.
     
  7. DismalChips

    DismalChips New Member

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    As a relative newcomer (or come-backer) to steam and all this, there's something I've wondered about Bulleid pacifics: what are the bars across the bottom of the wheels? They look adjustable. I've never noticed them on any other type of engine (although I wouldn't be surprised if others did) but I can't figure out what they do.

    There's probably an obvious answer, I realise.
     
  8. 8126

    8126 Member

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    Brake rigging? Quite common.
     
  9. Spamcan81

    Spamcan81 Nat Pres stalwart

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    It is indeed part of the brake rigging.
     
  10. DismalChips

    DismalChips New Member

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    Thanks very much.
     
  11. Big Al

    Big Al Resident of Nat Pres Staff Member Moderator Friend

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    As an aside, if you want to see evidence of the forces at work in a steam loco then stand where you can see the front of a LMS Black 5, to name just one example of a two cylinder beast, under strain with a decent load. The buffer beam will move to left and right.
     
  12. S.A.C. Martin

    S.A.C. Martin Part of the furniture

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    Contemporary to Bulleid, the first of the Thompson B1s used V2 wheel sets - which were balanced for the V2s. These were later changed to a spec appropriate and the original batch later gained balanced wheels.

    I think it's fair to say that you treat each design on its own merit - and where the Bulleid Boxpok wheels perhaps didn't require them, other classes definitely did. There was some experimentation on the B1s before settling on an agreed set of figures for balancing, incidentally.

    I would love to see that - would be genuinely astonished. I know there's meant to be some flexibility in the chassis overall but pushing out the buffer beam? Must be quite a sight.
     
  13. Bulleid Pacific

    Bulleid Pacific Part of the furniture

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    It's not a visible indicator that the chassis is flexing to a great extent; instead, it is a manifestation of a two-cylinder locomotive's propensity to 'waddle' along the track when put under strain because there isn't an inside cylinder to balance things up.
     
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  14. ragl

    ragl Well-Known Member

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  15. 8126

    8126 Member

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    Nothing to do with the wheel design, but using V2 wheelsets on a B1 would be unlikely to turn out well. The feature of the Bulleids that allows the omission of reciprocating balance is having three cylinders, although you can do it with a conventional four cylinder arrangement too - it just wasn't done.

    A two-cylinder engine is always a compromise between longitudinal oscillation that shakes the train or vertical hammer blow that damages the track and everything underneath. The Urie 4-6-0s are a rather infamous example; they were superseding Drummond four-cylinder engines, so to ensure they ran as smoothly the design team gave them lots of reciprocating balance. Since their motion work was built along battleship lines, they had truly fearsome hammer blow, which was discovered during the Bridge Stress Committee investigations. I believe the reciprocating balance was rather scaled back on these and the subsequent Maunsell versions.

    The only palliatives are to make the motion as light as possible, which wasn't necessarily that easy with '30s material technology, or to adopt the French Saumon levers; great big valve gear drive rods 180 degrees out of phase with the connecting rods, which produced some rather frightening forces on the return cranks and were only an option with rotary cam gears.
     
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  16. Jamessquared

    Jamessquared Nat Pres stalwart

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    There was a fairly significant change of design of the reverser during the lifetime of the locos that, amongst other things, improved accessibility.

    As built, in the early locos, the reverser was mounted on a frame stretcher between the driving and trailing axles, just to the left of the loco centre line. The steam cylinder was at the front, butting right up to the oil bath; the hydraulic cylinder behind and the crank that transferred the motion to the valve gear was attached to the the piston rod between the two cylinders, where it connected to an intermediate reversing shaft below the frame stretcher (outside the oil bath), which then took the motion to the main reverser shaft, with the reach rods passing into the oil bath. There was a gap of only about 18 " between the top of the stretcher and the underside of the boiler, so access must have been very difficult.

    That arrangement can be seen in the well known diagram that originated in the BR Black Book. Interestingly, in that book it is labelled as "West Country class", though in a paper I have about the reversers written by the late Tony Deller, he suggests it was only the first ten Merchant Navies, 21C1 - 21C10 that were arranged like that - does anyone know?

    (Top diagram on this page: http://www.bulleidsociety.org/21C123/21C123_Valve_Gear.html; original version here where it is clearly - but erroneously?? - labelled "West Country": https://en.wikipedia.org/wiki/Bulle...gear#/media/File:Bulleid_chain_valve_gear.gif)

    In the later engines, the arrangement was completely redesigned in the way you describe (with the reverser mounted on the inside of the left hand frame); the piston rod from the reverser passed directly into the oil bath which presumably became somewhat easier to seal before connecting via cranks to the reverser shaft.

    That arrangement is shown in the bottom diagram on this page: http://www.bulleidsociety.org/21C123/21C123_Valve_Gear.html

    The control arrangements were also modified; in particular, the drawing office implemented a rotary hydraulic valve that was derived from Stirling practice that was much less prone to leak, reducing the tendency for the gear to creep due to leakage from the hydraulic cylinder.

    So my reading is that the original layout was problematic in at least four ways, all of which were improved over time. Firstly the location moved from above a frame stretcher to the side of the frame, improving access and therefore promoting maintenance. Secondly, the connection from the reverser through the oil bath was simplified, reducing the tendency to leak oil. Thirdly, the hydraulic control valve was improved, reducing leaks and therefore reducing the tendency to creep. Finally, the installation of a pilot valve in the steam supply made the operation slower and therefore more controllable. I can't help wondering whether the reputation for poor reversers was actually primarily an issue with the first series Merchant Navies, but the reputation has ended up attached to all the locos.

    Tom
     
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  17. 30854

    30854 Well-Known Member

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    Query: How, if at all, did the original 135° crank arrangement on Maunsell's Nelsons impinge on balancing? Is there any truth in the tale that this layout had an adverse effect on coal consumption and if so (sorry if it's a daft question), why would that be?
     
  18. twr12

    twr12 Active Member

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    That drawing in “The Black Book” must have created many enemies of Bulleid Valve Gear!
    The drawing is rubbish and very confusing.
    It’s easier to explain Bulleid Valve Gear as a modification of Walschearts; where the bottom end of the eccentric/return crank rod is attached to the back end of the Union Link, the two rods being moved by the 3 throw crankshaft at the same speed as the driving axle; mimicking the standard arrangement of return crank rod rocking the expansion link driven by the return crank and the crosshead moving the Union Link at the same speed.
    As the Valvegear is miniaturised to fit in the box, it drives the valves via rocker shafts with small in/big out levers to give commensurate Valve Movement.
    The modified valvegear in the box is set up as for inside admission valves, the rockers are single acting, the Valve Movements are a mirror image of the valvegear and therefore the valves move as for outside admission, which original Bulleids and outside cylinders on detuned Bulleids indeed are.
     
  19. 8126

    8126 Member

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    The 135 degree layout is less perfect in its mechanical balancing than a conventional four-cylinder layout, but a lot would depend on the actual design. In theory, it's better than a two cylinder layout with identical cylinders and motion (and thus half the power), so not too bad.

    I'm not sure, but you seem to be implying the arrangement was changed - not quite so. 850 retains the 135 degree crank angles and still has that rapid shuffling exhaust beat to go with it. Holcroft (again) was the big supporter of this layout and favoured three cylinders for similar reasons. More even torque was cited, but more relevant to your question: he was of the opinion that the rapid beat was smoother than the big blasts of the conventional layout (most four cylinder engines have the same number of beats per rev as a two cylinder engine), leading to less unburnt coal carried over from the fire. There was some evidence from trials with the prototype three cylinder N1 that it was more economical than the standard N-class when worked hard; he opined this was the reason. There were also trials with 135 degree crank angles on a Drummond 4-6-0 (0449) that supposedly gave coal savings, to prove out the arrangements before building 850.

    On the Nelsons themselves, matters were not very conclusive. One of the class (865) was built with conventional crank angles, it being a relatively straightforward change. It retained this feature to the end of its days, but none of the Nelsons were converted to match it. The implication was that it didn't seem to matter either way, whether regarding economy, smooth starting or acceleration.
     
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  20. 30854

    30854 Well-Known Member

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    @8126 may thanks for the comprehensive reply. It was your comment about comparisons between balance issues on 3 and 4 cyl machines which set me off on this train of thought. I confess I was under the impression that the 135° arrangement was an experimental original feature (can't remember the title of the book, but recall a comment, years ago now, that "coal comsumption shot sky high" - and people complain about Wikipedia!). The more even nature of the blast at 8 beats per revolution seems perfectly logical, thouh the lack of any appreciable difference in smooth delivery of torque is a bit mystifying.

    AIUI, Bulleid's revised draughting was the most signiificant alteration to the Nelsons, but were any other modifications made over the years? (btw ..... I spotted the smoke deflectors already!).

    Trying to get my head around the balancing issue, I suspect I'm mixing up different forces in my mind and thinking of having coupling rods 180° out of phase with adjacent cranks on 2 (inside) cylindered machines and the resultant twisting strain on axleboxes.
     

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