Sir Nigel Gresley - The L.N.E.R.’s First C.M.E.

I would think the problem is more likely to be with the theory than the empirical experience. After all its not enough to move the train, it also needs to be accelerated. I suspect the detail dynamics might be quite complicated. I don't find it impossible to imagine that the maximum torque value is a factor in accelerating the train at low speed, especially if one considers that at low speeds the individual piston thrusts may be perceptible in the front of the train.

 

Very grateful to Robin for sharing the papers concerned, and I have been reading them at length over the last two days.

The paper that has surprised me the most (but not in a good way) is Harrison's address, "the gathering of the new crop" in which he outlines LNER Pacific development as being 1923-1941 and from 1949...! By way of completely ignoring all of Thompson's work, and Peppercorn's A2, in favour of just the Peppercorn A1.

He makes the claim regarding the Peppercorn A1:

This entire statement strikes me as rather crass.

Firstly, Gresley's final locomotive design, the V4, came at a time of maintenance and availability crisis on the LNER due to the perilous conditions of the Second World War. This locomotive design had conjugated valve gear. There is no indication whatsoever from anything Gresley's design team were doing at that point that any new steam locomotives were going to come out with anything other than three cylinders and conjugated valve gear.

Secondly, three sets of valve gear had been introduced by Edward Thompson, and continued under Arthur Peppercorn. These two engineers changed the status quo, recognising that Gresley's conjugated gear was not handling the additional factors of wartime well. This is crucial - without Thompson's change to the direction of engineering, and without Peppercorn supporting that change, you don't get the Peppercorn A1 design that proved so successful in terms of mileages and availability.

I feel strongly that Harrison is playing fast and loose with his words there. The evidence looks pretty strong to me that Gresley was unlikely to change his views on conjugated valve gear. The last of his locomotive designs that had been designed and built without it was converted into a three cylinder conjugated locomotive (W1) and he had converted some into conjugated machines (B16/2) so the evidence against that statement is rather strong I feel.

 

According to his autobiography, Holcroft was looking for a symmetrical arrangement, and that is what the diagram in Locomotive Adventure shows (and most of the Examples in his patent). It should be noted that the transverse levers each side are 1:2 not 2:1.
As for the N1, a further difference from the Gresley arrangement is that the inside valve slopes and is parallel with the inside cylinder rather than parallel with the outside valves, hence the slope of the driving rod outside the cylinders. The difference in angle is accommodated in the swinging link behind the cylinders.

 

I wonder if it's how the power comes on. By analogy, a 2-cylinder motorbike has much more of an apparent torque curve in the sweetspot of it's rev-range, which feels much more powerful than a comparable 4.

Speculating vigorously now, but a 3cyl steam engine will presumably be getting the next cylinder on steam earlier than a 2cyl, which changes how you feel the power? That could then percolate into a different action with how you add on the power.

Have you any experience on 4- cyl steam to add to the mix?

 

I'm afraid that I've never had the privilege of driving a four cylinder loco or the challenge of oiling one up.

 

And of course a further significant change was that Thompson & Peppercorn opted for divided drive, in contrast to Gresley's preference (shared by Bulleid) for unified drive onto the middle axle. But I suspect that it would not be possible to prove that either unified or divided drive had the advantage in terms of maintenance and availability.

 

Absolutely.

On the contrary, we know divided drive was better in terms of maintenance and availability, and we can evidence that by the Use of Engine Power document, and the engine record cards. The Peppercorn A1s records alone compared to the best Gresley conjugated class (A4) proves that beyond reasonable doubt. That doesn't mean the A4 is poor, mind! All the LNER Pacifics in the grand scheme of things are excellent for mileages and availability compared to the rest of the country.

 

I would love to see the evidence base around that. Mileages--- you can't really compare with the MNs running to Dover and Bournemouth and back. So that only leaves the Duchesses and Princess Royals as close comparators (possibly the Scots, Castles and Kings on a broader base?). Availability--- aren't there problems because different companies used different definitions ( eg available at some point within the 24 hours)?

 

Certainly. We have the results of the exchange trials, 1948, and much primary evidence in the way of records for annual mileages of the classes you mention below.

The Peppercorn A1 with roller bearings was the highest mileage per year and between overhauls of any of the British Railways Pacific locomotives - a fact that has been highlighted by numerous journals.

Only where the LMS are concerned, but even then you can (with a bit of spreadsheeting!) create comparable figures.

I'll reword that to the LNER Pacific classes remain competitive with other railways' similar locomotives.

 

After studying Gresley's patent (thankyou) and other materials I'm basically comfortable with what I wrote. I think its sound to identify five types of conjugated gear relevant to this topic.
1) is Holcroft's original patent, symmetrical with three principal levers and never to my knowledge used on a full size revenue earning locomotive. Gear behind the cylinders. Only really suitable for 4-4-0s.
2 and 3) are Gresley's patent, asymettrical and using
2) two principal levers, a long 2:1 and a shorter symmetrical one, with the valve off the centreline. I don't know enough about LNER locomotives to know if it was ever used.
3) a system of rockers, used only on the prototype O2 2-8-0
4) Gresley/Holcroft, with exactly the same levers/linkage as 2, but with the positions of valves and cranks modified for use when the inside cylinders have a different inclination to the outside. Widely used. When positioned in front of the cylinders taking the drive from extended valve rods.
5) Holcroft on SR, used on three locomotives, later removed. Basically a variation on Gresley's gear, not a development of Holcroft's original. Drive from behind the cylinders to gear in front of them. I wouldn't be surprised, incidentally, if this gear fell under Gresley's patent, but I'm in no position to judge.

 

Does your data actually allow for that degree of disambiguation to say that divided drive was better than unified? I know the state of the axleboxes was a common deciding factor in whether a loco was shopped or not, so I'm happy to accept that the A1s ran greater mileages because their running gear stayed in better condition, but I still don't think there's an adequate control experiment in there, because the A4s had conjugated gear. I believe they had a noted predilection for running hot on the RH driving box (as in if they ran hot it was that one, not that they were serial offenders). That's not a unified drive artefact; I believe it was attributed back to the back-driving of the RH valve gear from the conjugated gear, when running at speed (which the A4s did more than most of the conjugated gear classes). Combined with the over-work of the middle cylinder, the RH bearing took a significantly greater load. Now, the combination of the two is of course a consequence of them being unified drive, but if they'd had three sets of valve gear (like a rebuilt Bulleid) would they still have had a problem? Would a divided drive engine with conjugated gear have coped better? Unfortunately, there was never an Eastern Region equivalent of the Bulleids and the B17s were not sufficiently similar in power or duties, so we don't have the data.

My contention might seem slightly nit-picky, but while you absolutely can say something along the lines of: "From 1948 to 1966 the A1s ran consistently higher mileages with higher availability than the A4s, making them better revenue earning assests in that period," following up with: "because of their divided drive," is not supported by data that isolates that factor (and even with the bit I'm saying is fine, there's an implicit assumption of sufficiently similar duties). That sort of data is actually really hard to get, in my experience statistical studies often run into the mire of trying to make claims with a degree of precision which the collection of the data cannot support. A locomotive is a system; sometimes perfectly conceptually reasonable features are let down or overloaded by some other feature which may be conceptually bad or just badly designed. As @Jimc likes to say: "It's the detail where you fail."

 

What I find interesting about your reply Jim is that you're acknowledging that Gresley 's patent is separate to Holcroft's, which it hasn't always been when reading secondary sources. I think I would still challenge the idea that 4) can be part attributed to Holcroft, I think the description of 2) and 3) in Gresley's patent, and the drawings, make it clear that 4) is an option already in Gresley's valve gear prior to their conversation, and as such should be attributed to him singly.

However, I accept that it is likely going to be down to interpretation on that, and ultimately one could argue it doesn't make much difference: there were only three locomotives with Holcroft's gear, and hundreds with Gresley's by 1948.

 

There's absolute merit in what you say, but I think we can say that the divided drive locomotives had better availability than the conjugated valve gear locomotives: because we have enough data to show that was the case.

The question you are posing is whether we can put that difference in performance down to conjugated gear versus divided drive, and I think we would find real difficulty in not arguing that when you consider:

• The LNER Pacifics have more components and design in common between their classes and sub classes than otherwise though
• We have one large report on the performance of conjugated valve gear locomotives and another giving over 600,000 individual data entries on the whole LNER fleet allowing us a very real and clear analysis of the conjugated classes versus everything else between 1941-1946.
• We have many of the conjugated and divided drive Pacific locomotives' engine record cards that allows us to see mileages/failures/failures rates and that is where we can see pretty reasonably the reasons for failure.

We do know the conjugated classes performed better post-war, and why, but the divided classes also performed exceptionally and can be shown quantifiably by way of their mileages/availability.

The argument used to be for me that we had to argue against conjugated valve gear. Getting close to the data and the primary evidence has turned that on its head for me: I remain to be convinced that divided drive with three sets of valve gear wasn't the way to go, and the excellence of the Peppercorn A1 in service with British Railways by way of mileages and availability show that it had the edge on the Gresley A4 with conjugated valve gear.

And - you'll forgive for saying this I hope - but we do have a very clear comparison: it was 4470 Great Northern as an A1/1 versus an A4 Pacific. The two locomotives are identical up to the valve gear arrangement and front end and Great Northern's engine record card, mileages and post war performance show categorically that the divided drive setup was more robust. No question of that.

And yes, if given proper maintenance conjugated valve gear will work and do good work; but things get superseded - that's engineering. Understanding that an A4 designed in the 1930s was not as good an asset from the basic vantage point of a new locomotive coming into service in the early 1950s is not denigrating the former design for not being good enough but recognising it was a very important part of the overall development to get to that final A1 design.

 

No I wouldn't say that. Patent infringement is a tricky business, and I don't consider myself competent to judge on what might or might not be infringed, and in any case I haven't seen Holcroft's patent. I'll happily say that Gresley's design is significantly different to Holcroft's published drawings. I'll also comment that Gresley took advantage of the offset valve position to produce a much simpler and presumably for that reason an arguably superior design. But validity/applicability of patents? Sorry, not going there: I know (at least some of) my limitations!

To my mind a powerful argument in favour of Holcroft's contribution being important is that Gresley never built another locomotive with the valve gear as per 461. Holcroft's claimed contribution, to adjust the crank angle to match the cylinder inclination and with piston and valve rods not parallel is not AFAICS mentioned in the patent.

 

But (bold quote) that's the point I'm trying to make. 4470 didn't have the conjugated gear in its divided drive form either. Therefore it's very difficult to distinguish "divided drive not conjugated" from "unified drive not conjugated" because the only LNER data points for the latter are B16/3s and not a particularly good point of reference for the Pacifics. The question of whether unified drive or divided drive have any intrinsic reliability advantages is difficult to resolve because you change too much in going from one configuration to another. In an LNER context, the "unified and conjugated" valve gear classes appear to have suffered on reliability, even though as compared nationally they seem to have done pretty well. But as others have said, I don't think you can compare mileage stats between K-X/Edinburgh and Waterloo/Exeter or Waterloo/Weymouth either.

 

But Generally as far as I can make out loco's were changed regularly on the ECML with long runs, and non stop ones in particular being the exception rather than the rule.

 

As @8126 says, there are two separate differences; divided versus unified drive, and conjugated valve gear versus three separate sets. A difference in availability could be due to one of those, or the other, or both.
Intuitively it does seem that a crank axle is subject to greater stresses than a driving axle with cranks on the wheels but no crank in the middle, and the occasional crank axle failures would tend to confirm that; but actually confirming that rigorously and then analysing the consequences for the axle boxes would be challenging.

 

I’ve got Holcrofts patent here too…! Available in sections on the web and in search engine at the NRM.

My personal view is that when comparing the patents together their only unifying factor is the fact their gears drive the centre cylinder. Holcroft's also appears to be designed

No.461's valve gear is only different by way of adjusting the piston angle on the outside cylinders and deleting the rocking shafts to the inside cylinder, and No.461's type of valve gear is not that shown in the patent.

That shown in the patent is not No.461's gear but much closer to the final Gresley design design, albeit without the three bar slide bar arrangement of his conjugated gear locomotives (more traditional walschaerts with the two bar slide bar is shown for the outside cylinders, but that's a by the by).

I may be misreading Jim, so forgive me for not perhaps following you entirely, but if you look at figures 2 (for the first description) and figure 6 (for the second description) in Gresley's patent, Jim, the setup is drawn as that you describe. Those two drawings do show what became the final Gresley conjugated valve gear.

Indeed, Gresley's patent points out that there are two specific variations of his valve gear illustrated, and can be read by specifying the drawings 1, 2 and 3, in accordance with 4, 5 and 6 showing the alternative setup, with the piston and cranks adjusted. Holcroft may have made that suggestion to Gresley, most certainly, later on, but the drawings in his patent and the descriptions for those drawings pre-date that conversation, and show he was in the ball park already, no?

My apologies if I am not quite understanding what you are getting at!

 

There are other arrangements. Something close to (2) above was used by Gresley in the D49s and B17s; the three cylinders are (roughly) at the same level, with valves above and the conjugated gear behind the cylinders. This is possible in the B17 because of the divided drive, but not possible with all three cylinders driving the second axle because there is no space behind the cylinders.
The system used by the Germans in the G12 2-10-0 should be added. I have had the opportunity of travelling behind the preserved 58.311 and investigating the conjugated gear. The geometric principle is identical to Gresley's, but arranged very differently in that the 2:1 and 1:1 levers are replaced by rocking shafts across the locomotive located back by the second coupled axle with long rods connecting to the valves. The beat seemed completely even when the locomotive was running. Of course there is plenty of space in the German locomotive with small wheels and a high pitched boiler and I don't think there would be room in a British Pacific for something similar. I don't know how the gear in other German locomotives was arranged.
It should be added that Holcroft's patented system could equally well be used in (e.g,.) 4-6-0s or Pacifics with divided drive.