Bulleid Pacifics - Past or Present

When I have the head space (or the deadline to do more on the issue becomes too large!) I'll be having a serious look at the valve gear design, and surveying the surviving 10 original Light Pacifics to understand what we should be installing on GSN; it will still be the chain drive in an oil bath, but we'll seek to learn from, and incorporate, developments since 1959.

 

Putting my asset engineer's hat on; I'd be looking at the oil bath in terms of its need to flex/expand/contract against the frames, and access for topping up oil. Different material specs might be helpful for the chains in particular, preventing stretching out the links and creating any slack.

 

Those who are coughing up the cash of course should decide what is built. No quarrel with that. But perhaps we should agree to differ over what counts as Bulleid's original design: I would regard the chain-driven version as his later second-choice design.

 

Re the chains, I have read that they "stretched" in use ( Comparing new ones alongside old, longer ones) but thought that this was more due to wear in all the joints rather than material stretching?
It would be a very interesting IT modelling exercise to see how the valve head dynamics alter as the inertia forces start to "whip" the valve gear rather than the valve gear driving the valve itself, particularly with all the running clearances and scaling factors.

 

What design was built?

That is Bulleid's original design.

Many locomotive designs go through significant design changes as they are drawn. If we started with that logic with the Gresley A4s, we'd be looking at a Cock O' The North styled A4 with no bugatti shaped front end.

The chain driven version is what was accepted into traffic. That's Bulleid's "original design". That was what was allowed and built.

 

A fair comment Phil, but a review of the material spec for the links and chains would still be beneficial.

 

There's suggestion that the surviving original Light Pacifics have used new materials that have helped overcome the chain stretching, I've simply not investigated enough to determine what and why. It's probably next but one (or two) on my list of things to look at; hoping to have the crank axle design approved and ordered in the coming months, then we can get the "new" chain driver designed and sort out the chains themselves

 

Fair comment. Designs do indeed evolve between the first concept and what is finally built, and often evolve further during production. And engineering compromises are involved all the way along. I just feel it's a shame that what was built and therefore what is now being restored is inferior to the earlier design, having being adopted only because of the wartime restrictions.

Edit: Pursuing the thought: the final design of the A4s was an improvement, making them better locomotives (for some value of "better"). The final design of Bulleid's pacifics, with the chains, made them slightly worse locomotives than they could have been.

 

Doesn't that assume that the detail design for Bulleid's original preference would have been up to the job, had it ever got that far? Thinking of a certain later project of his, I wonder if the wartime restrictions actually helped make the pacifics better as he and his team were forced to work within those constraints?

I like the idea of unrebuilding an MN (my shareholding in SLL is as a result of the abortive project to just that with 35022), but taking it the next step to speculative engineering might have been doesn't do it for me.

 

Can we tempt you to invest in 35011, if you haven't already? https://www.35011gsn.co.uk/shares.html

 

That’s speculative at best. We have no idea if the original design was “better” because it was never built.

 

I shall speak to domestic facilities management and the accountant to see what might be possible.

She will undoubtedly ask, as any good charity treasurer must, whether you are set up in a way that might allow Gift Aid to be claimed.

 

We're not currently eligible for gift aid, as whilst we're a CIC, we're not a registered charity.

 

Thank you for such a quick answer.

 

Whilst I am not promoting a deviation from the 'Original' design, (such a stance would be contradictory to My views on the 'Barry 10' projects) but from an ENGINEERING point of view, I cannot see why there would be any difficulty in using a carden shaft, possibly in a torque tube in place of the primary (horizontal) chain, and possibly a similar arrangement or a train of (helical) spur wheels for the vertical final drive, of course not having studied the valve gear there may be practical reasons (clearances?) but in principle I cannot see any reason why shaft's could not have been used.

The term 'stretch' in chains basically means 'getting longer' rather than an actual 'elastication' and is mostly caused by wear of the pins and holes in the links. developments over the past decades have improved matters and it would be counter productive not to use the types of chain now available

 

Would there be any point though? As you say modern chains have advanced beyond all recognition, and its only driving the valves. Using shafts gives you all the losses associated with turning the rotation 90 degrees, plus all the issues of finding somewhere to mount the shaft bearings and dealing with suspension movement.

 

Just an observation on the above diagram. If item 5, the intermediate chain wheel is movable and attached to a suitably stiff spring, it could act as a tensioner for the other two chains. I take it I'm not the first to think of this, was it actually done?

 

I've no idea what was originally proposed in terms of a shaft drive but, if using a spur or double helical (not single) gear drive the output shaft would have had to be accurately constrained to run parallel to the axle. This would have require an axle mounted gearbox; not impossible by any means but would require torque arm connecting the box to the frames. A similar but frame mounted box would be required at the output end of the shaft to drive what would in effect be a three-throw shaft driving the valve gear. If the drive was off the middle axle, there's precious little space for this once you take the middle big end into consideration. A better arrangement would probably to use bevel gearboxes and tur nthe drive through 90°. The same axle hung gearbox arrangement would have been necessary but would be a neater arrangement

 

I haven't made any statements about the 'Point' of any such changes, and neither am I promotingt such things, I'm merely stating that from an ENGINEERING point of view it would not be impossible to use shaft drives for this job. With respect I feel that many on Nat Pres spend to much time 'Over Thinking' instead of actually reading what has been written.

 

I'm beginning to wonder if I should've brought the subject of carden shafts up in the first place. At least I never mentioned "The 'L' Word"!

Although in the context of OSVB's work, the subject of shaft driven steam distribution mechanisms is very much a "what if.... ", I'm finding the points mentioned fascinating .... even the allegedly 'overthought' ones. You can always rely on NatPres debates to produce the most unexpected considerations .... and long may that remain so!