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Bulleid Pacifics - Past or Present

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

  1. Allegheny

    Allegheny Member

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    I'm just wondering at what speed would centrifugal force cause the oil to be retained in the BFB pockets?
     
  2. Jamessquared

    Jamessquared Nat Pres stalwart

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    I'm slightly struggling with that - why worn chains would cause a timing issue.

    You have a pair of chains, driving at a gear ratio of 1:1. So one rotation of the crank shaft ultimately results in one rotation of the jockey shaft, which drives the valve gear.

    If the chain "stretches", the effect is that it will ride higher up the gear teeth on each wheel. That has lots of deleterious effects, but doesn't fundamentally change the 1:1 relationship between crank shaft and jockey shaft rotation - you can't suddenly get one turning at a different rate to another. So the timing shouldn't get affected in that way. The only possible cause I can see would be that wear would increase slop, but that happens in all valve gear as it wears.

    I still tend to the view that the real issues were with the reverser, and in particular a combination of lack of precision control (relative to a screw reverser) and the normal issues with steam reversers if they don't get regular maintenance, an issue that was complicated in the case of the first ten Merchant Navies by poor positioning. For current occasional use, a better-than-BR maintenance regime ought to eliminate both issues of chain slop and reverser maintenance, meaning the lack of precision control remains the one significant weakness.

    Tom
     
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  3. Allegheny

    Allegheny Member

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    Sure, the wheels will both rotate at the same speed, however if the length of chain between the wheels on the side which is tension increases slightly , the jockey wheel will be a degree or two behind where it ought to be, compared with where it would be if the valvegear had been set with a new chain.
    I hope this makes sense.
     
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  4. srapley

    srapley New Member

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    Excellent explanation
     
  5. srapley

    srapley New Member

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    Thinking it through, I'd suppose it happens in the following way. Loco is stationary, oil drips into pockets. As the loco starts to move, the pockets rotate, oil spills out of them onto the webs of the wheel & other parts of it's surfaces due to gravity, coating or lubricating them for want of any other word. As the wheel rotates faster, centrifugal forces start to dominate over gravitational forces, and the oil starts to spray out, with some of it coating the underside of the boiler. There would be a speed above which the oil would be retained in the BFB pockets, but I'd imagine a lot if not all the oil would have been removed by the described process before the locomotive gets to that speed. The splashers fitted to some of the first series MNs may have reduced this behaviour, but they were removed from the design to reduce weight
     
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  6. srapley

    srapley New Member

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    We're restoring a pacific, not building one... :)

    Somewhere in the design evolution was a 4-8-2, which I have a model of (look at my profile pic), though I need to fix the lining on it as one line is missing (and the nameplate is too high up & the number is bizarre)
     
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  7. Jamessquared

    Jamessquared Nat Pres stalwart

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    Yes, I see that. I guess the question then is (1) is that material relative to other sources of lost motion or imprecision (for example, an air pocket in a reverser cataract cylinder) and (2) how did it compare with, say, worn motion parts on a loco with more conventional valve gear (which could also cause an analogous effect, for example if a die block was worn in the expansion link).

    Tom
     
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  8. Allegheny

    Allegheny Member

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    Can oil leak from the axleboxes when the locomotive is running?
    I'm just thinking that if pockets fill up when running, then the brakes are applied, the sparks occur at the same time that oil starts to fall out of the pockets. However, having said that, I would imagine the oil would have to soak something which acts as a wick before a fire can occur.
     
  9. 30854

    30854 Resident of Nat Pres

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    A most praiseworthy effort to crowbar the thread back on topic there, sir! :Pompus:

    Good luck with that one! ;)
     
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  10. Jimc

    Jimc Part of the furniture

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    I'm not sure why oil leaking from the oil seals round the axle wouldn't end up on the wheels too. I'm also having trouble visualising there being enough axle box oil spilt to cause the problem, whereas the oil bath clearly contains a more than adequate supply.

    To put some numbers on the other question, according to a quick web search typically the maximum stretch permitted on an industrial roller chain before replacement is 3%, and I plugged that onto a drawing of the Bulleid gear and got about 15 degrees of rotation for 3% chain stretch. Obviously every time the gear was adjusted in service this would reset.
    I don't recall ever having cam chain stretch as something we considered when I was involved with street motorcycles (racers are another matter), but now I think about it they had tensioners that would tend to minimise any effect, so the engine designers had dealt with the problem for us.
     
    Last edited: Apr 19, 2022
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  11. Jamessquared

    Jamessquared Nat Pres stalwart

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    So why didn't Q1s have a reputation for fires? They had BFB wheels and similar cladding round the boiler - and would also have put in some pretty hard braking at times on unfitted trains; the one thing they lacked was the oil bath.

    Tom
     
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  12. Allegheny

    Allegheny Member

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    With motorcycles the angle error of the jockeywheel is not relevant. The other issue with Bulleids is that the chain has to drive the valvegear when running in either direction, which will make tensioning more tricky, although I don't think impossible.
     
  13. Big Al

    Big Al Nat Pres stalwart Staff Member Moderator

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    I wouldn't necessarily cite Cecil J Allen as an authority on engineering matters but he gives chapter and verse on the problems generated by the oil bath and its ability to send oil in various directions, not to mention the tendency for condensation and water to contaminate the oil. As I said, one could hope that under a heritage maintenance regime, many of these problems could be managed.
     
  14. srapley

    srapley New Member

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    My understanding is a lot of the problems with the oilbath were solved after the MN rebuilding program was complete, either by BR or by the heritage movement. The other related issue (fires) has also been reduced by changes in brake block materials by BR, and by keeping the underside of the locomotive cleaner in preservation. We (GSNLRS) will be aiming to replicate these changes as much as possible on GSN as we unrebuild our beast.
     
  15. Cosmo Bonsor

    Cosmo Bonsor Member

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    Regarding chain tension, the main problem you are trying to solve is maintain the angle of the jockey shaft in relation to the crank shaft. This is exactly the same problem in an IC engine. There must be a tensioning device on a chain which wears significantly and on which timing depends. It can be automatic or manually adjusted. In an IC engine the tensioner is on the un-driven or slack side. This means the wear in the chain changes the relative angles of the crank and camshaft. Other chains may be left un-tensioned. For example in one of my bikes the cam chain and balancer chains have manually adjusted tensioners but the oil pump chain has no tensioner. The angles of the oil pump drive is unimportant, but the other two are. Tuners take care to ensure the cam shaft(s) are in the correct relationship to the crankshaft by using slotted cam gears or other devices.

    The Bulleid method is subtly clever IMHO. The tensioner adjusts for both chains and more importantly it works on both the driving side of the chain, which is under tension and the slack side. This means that as you tighten the chain the gears stay in phase so there is little or no timing error as @Allegheny says. It is best practice to have the slack, which there must be, on the top of a chain drive, as it is when the engine is going forwards on the first horizontal chain. The valve gear setting will take the slack in the chain and the lost motion into account and be optimised for forward running, obviously, and the backwards running would have been allowed to take care of itself. Unfortunately, one of the experts who taught me valve setting is no longer here to ask.

    As for lost motion, there are a lot of pin joints in the valve gear. Were there any other designs of gear for three cylinder engines with lots of pin joints? Were there any issues that Bulleid would have known about? He asked mischievously.
     
    Last edited: Apr 19, 2022
  16. MellishR

    MellishR Resident of Nat Pres Friend

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    Then again, that project does have a thread of its own.
     
  17. 30854

    30854 Resident of Nat Pres

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    My dear chap, oodles of commom digressions have (often current) threads of theirs own. Since when's that ever stopped us?
    :Resistanceisfutile:
     
  18. 8126

    8126 Member

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    A few years ago 34007 was sounding like the beat to some particularly experimental music. It still seemed to go, because it ran in that condition for quite a while, but I don't think I've ever heard anything sounding quite so odd while not being failed.

    As I recall, when the MHR dug the concrete out of the smokebox floor, they found that the 3:8 levers, which magnify and transfer the motion from the gear to the valves, had heavily worn pivots, with the usual attendant lost motion. The pivots to these levers are pretty heavily loaded, they must each as a bare minimum see 11/3 times the force required to move the valve, which is a pretty significant multiplier. For reference, in a slow speed case (ignoring the lever dynamics) the single main pivot of the Gresley 2:1 lever will see 3x the force required to operate the middle valve, and there's one big one of them, accessible without taking up the smokebox floor. The short end pivot at the centre of the 1:1 lever is then at 2x the middle valve actuating force.

    Even if the 3:8 levers are in perfect nick, any lost motion error further up the chain (excuse the pun) must inherently be magnified, so keeping clearances tight in the pins assumes greater importance than with a conventional gear.
     
  19. MellishR

    MellishR Resident of Nat Pres Friend

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    All of which implies that what Cox/Stanier said about the Gresley gear applies a fortiori to Bulleid.
     
  20. Jimc

    Jimc Part of the furniture

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    That's maybe a little overstated. I'm no kind of expert on this sort of detail, but according to my limited understanding there's nothing intrinsically terrible about having rockers that amplify the valve travel. The GWR did it with some of their 2 cylinder classes and their valve events are rarely questioned.
    The issue with the Gresley gear, as I understand it, is that lost motion from both sides is conjugated together, giving double the lost motion that the equivalent amount of wear in a more conventional layout would give. The Bulleid pacifics, OTOH, have no connection between the different sets of gear so nothing of that kind can occur. The Q1's stephenson gear also had rockers with a similar magnifying effect to the pacifics and I haven't heard that criticised. If the location of the levers on the Pacifics is such that they are difficult to maintain adequately then that's certainly a design flaw, but one related to detail engineering rather than intrinsic to the concept. But I welcome correction from those with more expertise than I have.
     

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