Talk:Bulleid chain-driven valve gear
From Wikipedia, the free encyclopedia
 |
This article is within the scope of WikiProject Trains, an attempt to build a comprehensive and detailed guide to rail transport on Wikipedia. If you would like to participate, you can visit the project page, where you can join the project and/or contribute to the discussion. See also: WikiProject Trains to do list |
||
| Start | This article has been rated as start-Class on the quality scale. (assessment comments) | ||
| Mid | This article has been rated as mid-importance within the Trains WikiProject. | ||
 |
This article is within the scope of WikiProject UK Railways.
|
||
 |
This article is within the scope of the Locomotives task force. |
[edit] Image
A nice diagram of the mechanism appears in Handbook for railway steam locomotive enginemen, page 109. As a UK government publication of 1957 unfortunately it's not out of copyright until the end of the year, but if nobody's found another I'll upload it then.--Old Moonraker 15:33, 21 July 2007 (UTC)
-
- Yes, that would be good, as I'm not very knowledgeable on copyright, apart from the Leader photographs, of which I was lucky enough to have a copy of a book that stated that the copyright had expired... Cheers.--Bulleid Pacific 18:01, 21 July 2007 (UTC)
[edit] Chain stretch?
I have always had my reserves about imputing the strange valve events to chain stretch. As Bulleid himself pointed out, [1], this would be fairly predictable and could be allowed for. On the other hand I have always been dismayed by the fact that many of the components of Bulleid’s gear are “anti levers” that multiply movement and effort rather than reducing it as they should ideally do. Now anyone with a little experience of descriptive geometry will know how when plotting a series of arcs, the most minute error in centring the compass accumulates at a truly astonishing rate. For this reason in planning and laying out any valve gear, you would have to compensate error in the pins centres as much as possible by proportioning your levers so as to reduce effort and travel (the long end drives the short end). Precisely the opposite happens in many components of Bulleid’s gear; the worst offender would likely be the 3 : 8 multiplication of effort of the final rocker to the valves, plus the union link between the rocker end and the twin girder arrangement spacing the valve heads. This combined IMO would would impart very strange accelerations to the valve and always tend to make valve events totally unpredictable. It would also increase resistance to movement and so increase wear on the pin joints, chain stretch etc. The only person to broach this problem seems to have been of all people, Bulleid’s son H.A.V.[2]. — Has anyone else done so?
To quote Brian Reed (not exactly a Bulleid fan) [3]: "Valve proportions and and events were not impeccable even in new condition (my italics, as it proves that the problem has to come from valve gear proportions). In service, the events were almost unpredictable, and could not be maintained through the 36,000 miles between complete valve and and piston examinations (...)."
Pin wear is a secondary issue though, and only aggravates the main one: the magnitude of its effect is a symptom and not the cause, which as I say, is a geometrical issue.
A point that should not be lost sight of is that design of mechanisms was something of a lost art in Bulleid’s day. You have to go back to the Eighteenth and Nineteenth Centuries to find its heyday when truly wonderful and often purely artistic feats were performed by draughtsmen (Bulleid still had a good one from the old school on his staff — Harold Holcroft, but could not get on with him).
One more point on Bulleid's gear. They were said to have had the reverser based on an LSWR model, greatly inferior to the SECR one, invented by James Stirling, well proven and in general use since his time[2]. But once again, look at the design and proportions of the bell crank governing the position of the radius rod, itself working in a very short expansion link so the tiniest adjustment of the die block position would have made considerable difference to the valve travel (not to mention die slip). What an idea then to make the reverser bell-crank give slight multiplication of effort thus making the travel of the steam server piston even shorter than necessary.
All told the Bulleid Pacifics had many really excellent points but were marred by some catastrophically bad ones which does not include the chain drive — or at any rate, it was not a serious one; the worst were valve gear layout and the reverser. Resolve these problems, plus a LEMPOR exhaust (easily fitted) — and perhaps less easily, compensated springing and you would be on the way to the sort of engine Bulleid really intended. If they could improve Duke of Gloucester performance out of recognition without materially changing the appearance, it might be be worthwhile trying a similar “monkey gland treatment” with one of the remaining Spam Cams.--John of Paris 11:05, 7 August 2007 (UTC)
