Bulleid Pacifics - Past or Present

I'm amazed valve gear ever works at all when you consider that the wheels / axle (from which the motion ultimately derives) are moving relative to the cylinders, and indeed can be moving in opposite directions on either side! Imagine building an internal combustion engine in which the crank shaft was free to move essentially in an arc behind the cylinder; and moreover in which across two cylinders, it could also move in that arc in opposite directions simultaneously ...

Just to nuance that latter point: although the rebuilt locos were excellent, they were also heavier, and that had the effect of preventing their going west of Exeter. (I believe the lightweights finally made it to Plymouth only in the early 1960s with track upgrades, and never went to Ilfracombe / Bude / Padstow etc).

That's an important point when you consider the situation as Bulleid would have seen it ca. 1938 - 40 when he was formulating ideas. At that point, there were still considerable numbers of elderly Adams and Drummond locos working on the Withered Arm, with sustained gradients of around 1 in 70/80 on the line to Padstow, and as steep as 1 in 36 / 1 in 40 to Ilfracombe. The most modern locos seen were Maunsell N class moguls which, excellent as they were, still represented a World War 1 era design, essentially already 25 years old in design terms by the outbreak of World War 2. Most of the other locos west of Exeter were 40 - 50+ years old. So new motive power was desperately needed, and the locos in rebuilt form, had they been built that way from the start, were not the solution.

Tom

 

I'm not so sure it was a total failure. The fact that we have so many of the light Pacifics that have survived with the original design AND are still working with it, indicates it's not a total failure. I do agree that the original design required improvement, of course. GSN will likely have to incorporate some detail changes to go mainline.

 

Really good point Tom, thank you for that. Much must be made of the situation Bulleid found himself in at the Southern and of course, the wartime dilemma.

 

I suspect, in the case of the Southern, most especially on the SE Division, the effects on the pw of fast, heavy locos was perhaps of rather greater concern than the other three lines. Certainly, come proposals culminating in the Merchant Navy class, the notion of a leading truck led to some exchanges between mechanical and civil engineering departments.

Maybe some clues are to be found by looking to the earlier genesis of Maunsell's Nelson and Schools classes?

 

There’s a world of difference between using them intensively on high speed services with crews of varying abilities with less than ideal maintenance facilities and the cosseted life they now lead with only intermittent use.

 

I'll be fascinated to see what you've come up with Simon. Bulleid is definitely a higher priority than Gresley.

The SR had 140 pacifics. What did they actually do with them on an average peak day and an average off-peak day? Given the amount of work the LNs and Arthurs got through, I've often wondered what proportion of the 140 were rostered for duty, what proportion were available but not rostered, under maintenance, in works etc. And how that compares with the other regions.

My prejudice is that the originals did not meet availability standards which was what justified the (not very expensive) rebuilding programme. But maybe my prejudice is wrong!

 

John, there's twenty years worth of records, observational and statistical, where the original Bulleid Pacifics are concerned. They're clearly not in the "failure" category as a class of locomotives. They live a cosseted life today, granted, but they didn't before, and they still worked and did good work.

 

I think there's a level of truth in that. My gut feeling is the evidence coming to light in my research is strongly suggesting there was merit in the rebuilding scheme, and in their original designs. Not an either/or.

 

I agree, but then I would, being a firm believer in the improvements being made to the P2, Clan etc.

I’m no mechanical engineer, but I’d have thought the chains could be replaced by telescopic carden shafts driven by something like the crown wheel of a motor vehicle differential?


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I’m not sure it would be easy, exactly.

This is a view of the valve gear on a light pacific.



Imagine using shafts in place of the two chains. That shaft would need support, at which point - where do you spring that support from? The various moving bits of the valve gear get in the way. By contrast, a chain is supported on the pinion gears at each end, but essentially floats in free space in between.

At the very least, the simple task of linking the two gears with a shaft rather than a chain would seem to require a really substantial change of layout of various stretchers in order to provide support to the shaft.

Tom

 

Like I said, I’m no mechanical engineer


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How much lighter was a V2 compared to an A3? Perhaps Bulleid would have done better to have built a slightly smaller class 6 2-6-2 rather than a class 7 pacific, when class 7 power was hardly needed for locos replacing elderly Adams and Drummond engines!

 

I think that would have been vetoed by the Civil Engineer who had a distinct animus to fast services run by locos with leading pony trucks. He had previously I believe vetoed a wide-firebox Maunsell 2-6-2 design and gave a very political “yes” that meant “no” to a Bulleid proposal for a 2-8-2 that ultimately became the Merchant Navy.

Tom

 

But how many high speed services were there on the SR at that time, and how many of them fell to the older locos that Bulleid was looking to replace with the WC/BB classes? What's more, under wartime conditions speed was not a priority. Bulleid could have drawn on his experiences with the V2s on the LNER and pointed out that they were very successful on secondary services.

 

Isn't that a case of "know thine enemy"? One has to decide which battles to fight.

 

About one axle lighter; when V2s did come to the Southern they were used on MN routes, having if anything a slightly higher axle load at the time. They also had different pony trucks by the time they reached the SR, the swing links fitted when Bulleid knew them having been found out by poor wartime track.

The Southern civil engineer would probably have pointed out that even the Rivers were fine on the ECML.

 

It may be worth bearing in mind that Bulleid had experience of shaft driven valve gear with the first of the P2 class, plus other poppet valve locos on the LNER. There would have to be design changes to be done betwixt the frames; but it can be achieved, as demonstrated by the first series of Caprotti Black Fives that had axle driven shafts between the frames.

Cheerz,

Alan

 

Any idea of using a cardan shaft drive would really require all new cylinders. Such drives are rotary and need a rotary valve gear, not a reciprocating one, which piston valves are. The idea of changing direction of transmission twice and the nfrom rotary to reciprocating to use a piston valve would make it even more complex to little advantage, if any.

 

Wasn't Bulleid's original plan to use some arrangement of spur gears (and cardan shafts?) to operate the same scaled-down Walschaerts as he actually adopted?

 

As a director of GSNCIC, I'm beholden to follow the wishes of my shareholders who want to see an "original" Merchant Navy; whilst I am exploring some minor tweaks, redesigning the valve gear to a completely different arrangement goes too much against what we're trying to do as a project, which is recreate Bulleid's original design. It would also bring in a lot of engineering effort and expense in terms of design and certification that would be hard to justify against that aim.

On a lighter note regarding what could have been, there might be drawings/sketches out there, if you look hard enough. I recently came across an early drawing in the BPLA/NRM collection showing the arrangement of the three chains/drivers which clearly shows the crank axle was originally a self-balanced design which at some point was revised to the unbalanced design, sowing the seeds for Crewkerne