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BR Standard class 6 No. 72010 'Hengist' and Clan Discussion Thread

Discussion in 'Steam Traction' started by Bulleid Pacific, Nov 23, 2009.

  1. MellishR

    MellishR Part of the furniture Friend

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    I hope "difficult" does not mean "not worth trying". The new Clan will have some significant improvements over the originals, so retaining flawed valve geometry would be a pity if there is any possibility of improving that along with the rest.
     
  2. ianh1

    ianh1 Member

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    Definitely not worth trying and its on the list for when we get to that stage. There's an awful lot going on at the moment and we don't have much spare time. We are close to finishing an initial study of the boiler options as we finalise the last parts of frame assembly. The bogie is well advanced and we expect to cast the cannon boxes before the end of the year.

    We have found that we need to prepare modern CAD drawings for our suppliers to work with. Our copies of the BR drawings have varying quality so its best to provide our suppliers with metric versions that they can read easily. If you do have CAD skills and can help convert our drawings, we'd like to hear from you!
     
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  3. huochemi

    huochemi Well-Known Member Friend

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    Do you know what the issue is Ian? Is it the suspension of the radius rod / lifting link, which was a noticeable design change from the LMS/LNER practice of having a die block working in a slotted extension to the radius rod?
     
  4. std tank

    std tank Member

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    70000 and 70013 have been running OK.
     
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  5. class8mikado

    class8mikado Well-Known Member

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    Of course the one variable available in the Walshaerts connundrum might be to reduce the required travel, Lots of very decent engines out there with nothing like 7 inches...
     
  6. jnc

    jnc Well-Known Member

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    What I wouldn't give to know what the old-timers at Swindon would have made of that piece of kit. Probably pretty much the same as their reaction to optical alignment gear, now that I think of it.

    Noel
     
  7. class8mikado

    class8mikado Well-Known Member

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    Dont have a clue what the actual shopfloor culture/ attitude was in reality, but its always put across as we do things the Swindon way, because its the best way.
    Any tool available to ensure that this was indeed the case would be taken in and used.

    This from Don Ashtons site

    BR CLASS 7

    The BR Class 7 motion owes much to American layout practice with equiangular expansion link swings and a radius rod hanger forward of the trunnion. In retrospect this proves a doubtful way to deal with long valve travels and the events are generally inferior to those produced 20 years earlier. It is of interest to investigate the points of relative sensitivity.

    First the angular swings themselves. In right hand elevation the backset produces an anticlockwise angle of 24.43o and a clockwise one of 24.33o – essentially equal. In full forward gear this accounts for 78.37% cut off (front) and 74% (rear) and a port opening disparity of almost 7/32″. Port opening as a distance, rather than time, is relatively unimportant, though it is interesting to see what it takes to remove this disparity and to reveal the relative insensitivity of backset angle. The backset has to be reduced by half in order to equate port openings, yet the cut offs remain similar at 78.14% and 72.54%. On the other hand, cut offs of 78.92% and 75.53% from a backset angular increase of some 5.5o are little different in equality from standard, giving the same port opening inequality from angular swings of 24.6o and 26.23o. Quite clearly the backset is shown to be relatively insensitive to adjustment, as expected. [It ought to be pointed out that the lengths of the eccentric rod and return crank have been recalculated to maintain lead equality at each alteration.]

    More surprising in this particular case is the resistance of the anchor link to affect results. Even 1″ either side of the 13.8″ given length has but little influence and leaves the hanger arrangement fully in charge of the difficulties. Moving the point of suspension (1″) on the radius rod quickly disturbs the port openings, the cut offs much less so, whereas shifting the weighshaft 1″ horizontally either side equalises port openings at the expense of cut off disparity.

    The real culprit is the enlargement of a design which could adequately cope with shorter travel valves, say up to 6″, but becomes a victim of larger angularities above that. Pitch circle radiuses of 7″ or more compare ill with a Stephenson’s eccentric equivalent of only half of this. The design becomes an exercise in deciding at what running cut off it is best to equalise events without too much sacrifice elsewhere. And the less said about back gear the better!

    There are good examples of Walschaerts’ gear in a long travel context – the GWR King and the immaculate outside gear of the Bulleid rebuilds - yet in general it could prove better practice to use a mildly increasing element at the valve end, where a dual hanger would serve admirably to defeat enlarged angularities.


    Can tell you that a lot of this goes straight past me, and as Std Says, they are doing alright thankyouverymuch with ( to my mind) un refined draughting and foggy Valve events...
     
    Last edited: Nov 20, 2020
  8. Richard Roper

    Richard Roper Member

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    Interesting reading, even if a lot of it goes over my head... Would the Immaculate arrangement of Walschaerts gear on the rebuilt Bulleids have anything to do with it being outside admission for the outside cylinders (it was inside admission for the inside cylinder after rebuilding)?
    Here's a GIF file from The Bulleid Society's website, on their page for 34059... With the layout of outside and inside gear on a Bulleid rebuild. I hope the Bulleid Society don't mind it being reproduced. I will delete if necessary.

    Richard.
    [​IMG]
     
  9. Sheff

    Sheff Well-Known Member

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    As you say, much of that is beyond me. But I’d be much more interested in what was going on between say 15% and 40% where the gear spends most of its time in mainline running.


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  10. Jamessquared

    Jamessquared Nat Pres stalwart

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    Quite so.

    The other point that struck me from the description above was:

    The design becomes an exercise in deciding at what running cut off it is best to equalise events without too much sacrifice elsewhere. And the less said about back gear the better!

    To which my thought was - it's a Britannia. How much time was it envisaged that it would need to run backwards?

    I think sometimes people get too hung up on theoretical deficiencies of valve gear at the expense of practical usage. If the design process involves a series of trade offs - optimise one parameter at the expense of another - then provided the design team get it right where it will spend most of its time, what is the problem? For a loco designed to always be running forwards and spend much of its day running at 60mph+, if the valve gear is good, as you say, between 15% - 40% in fore gear, it is probably of minor consequence if it isn't perfect elsewhere.

    There is a poster on this forum who seems obsessed with some theoretical deficiency of the Maunsell Q class valve gear, despite the fact that I have never seen it ever raised as a noticeable issue with the class in service!

    Tom
     
  11. std tank

    std tank Member

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    The diagrams are interesting, but a bit confusing. The valve gear on the Standards is a similar, but different style of Walschaerts valve gear.
     
    Last edited: Nov 20, 2020
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  12. Steve

    Steve Part of the furniture Friend

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    Another locomotive that similarly suffers from dire back gear events is the SR S15 becaue of the method of radius rod suspension. However, as anyone who has ever been on one of these will readily agree, Maunsell/Urie obviously never intended for them to go backwards.
     
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  13. std tank

    std tank Member

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    Deleted.
     
  14. huochemi

    huochemi Well-Known Member Friend

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    Outside admission is kinder, as the connection of the radius rod to the combination lever below the valve rod gears up the movement i.e. less throw/angularity is needed, whereas with the more usual inside admission, with the radius rod connected above the valve rod, the movement is geared down. It is noticeable also how the die block in slotted radius rod was re-adopted on the rebuilt locos, rather than the lifting links used on the standards. One potential problem with lifting links is that the arc of the swing may mean that the link is pulling the radius rod in the wrong direction e.g. up when the arc of the expansion link is downwards.
     
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  15. RLinkinS

    RLinkinS Member

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    The suggestion of the mild increasing element at the valve end is interesting. It will result in an increase in valve travel without the increased swing of the link which seems to increase the errors. I have only seen this arrangement on the Chinese loco in the NRM. Are there any other examples?

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  16. huochemi

    huochemi Well-Known Member Friend

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    I find some of Don Ashton's writing rather arcane, including this comment. But in answer to your question, Maunsell's rebuild of the E class 4-4-0s, and possibly other classes, incorporated an unequal rocking lever to give increased valve travel -see Holcroft's Locomotive Adventure p98 and the drawing at the back of the book. This was with Stephenson's valve gear.
     
  17. RLinkinS

    RLinkinS Member

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    I had forgotten this, thanks for reminding me. The arrangement certainly worked well on the D1s and E1s. The arrangement on the Chinese loco is quite simple and is a clever solution with little downside


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  18. MellishR

    MellishR Part of the furniture Friend

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    Can any of the experts here explain why the outside gear has the eccentric rod attached to an offset extention on the expansion link (as in many other designs) but the inside gear does not have that extension?
     
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  19. MellishR

    MellishR Part of the furniture Friend

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    While that is true, the relative lengths of the sections of the combination lever are such that the gearing up or down is very small.

    That is one striking difference between the forms of Walschaerts gear on different locos. Presumably the pros and cons must be fairly finely balanced for one form not to have been adopted universally. I would think that the slotted link would give better control of the valve events at the expense of the long sliding movement which is more difficult to lubricate than a pivot.
     
  20. fergusmacg

    fergusmacg Part of the furniture

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    But that piston valve crosshead arrangement as used on the standard arrangement whilst it requires slightly more difficulty in lubrication to a slide (rather than a pivot) it does restrict the lateral forces acting on the piston valve spindle that a pivot arrangement does and can't prevent causing uneven wear and leakage at the piston valve gland (already under higher pressures as outside admission). Now I know this has been a problem on some Merchant Navy locos (following a discussion around Canadian Pacific many years ago with a fitter on the GCR) so perhaps the MN arrangement has better geometry for valve events but it's perhaps not the 'perfect' solution some make out.


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