Edward Thompson: Wartime C.M.E. Discussion

IMO, this well observed statement goes to the core of the issue .... and isn't relevant to just Gresley's locos. More than a few issues with famously divisive designs down the years seem to come back to appropriate driving and firing techniques, be it Webb compounds, LNER or Southern pacifics, or 6MT 'Clans'. It also too frequently seems to be an aspect of in depth testing which receives disproportionately little consideration in otherwise comprehensive official reports.

Sometimes, on the other hand, if it looks like a Drummond 4-6-0 and sounds like a Drummond 4-6-0, chances are it'll perform like a Drummond 4-6-0!

 

Gresley Pacifics were built with 65% cut-off, but were modified to 75% post WW2.

 

Peter - I agree - that’s precisely my point.

 

I'm another one who thinks that Fred is way off the mark with his statement. As you say, it is the norm for most drivers to put the rack into full gear for starting, just to make sure that you get steam to at least one cylinder. It isn't always necessary, though, and a loco will often start with it notched up. I do know some who will try it, and if it doesn't move when the regulator is opened, will wind it down until it does. Even being in full gear doesn't guarantee that you will start and it is quite common to have to reverse slightly to get away in forward. Perhaps Fred is confusing conjugated gear with Caprotti gear? I believe that on the earlier Black 5 Caprotti engines you had to wind it into full gear when reversing to re-set the valves. That was unnecessary on the British Caprotti fitted locos i.e. 71000 and Standard 5's.

You asked about Gresley pacifics and full gear. Eightpot has already confirmed that they were limited to 65% when built. I'm sure I have read somewhere that Gresley believed that, with a three cylinder loco, you did not need 75% and limiting it to 65% reduced the stresses on the loco. Of course, this theory does not hold water and three cylinder locos are even worse at starting than two or four cylinder ones for reasons discussed previously on Nat Pres. One thing I'm not certain about is whether the Gresley pacifics only had the cut off increased for forward gear, with back gear remaining at 65%. I say this because the indicator on A4's is marked at 75% in fore gear but only 65% in back gear and experience with 60007 indicates that it can be quite reluctant to start in back gear at times.

 

65%.

 

I'm guessing pacific tanks would be less likely to slip than their tender counterparts, due to very different weight distribution, making me wonder whether (say) the LBSCR J class 'pacifics' were more or less susceptible than the L class 'baltics'? I ask, as 'excessive non-adhesive weight' is a charge I've heard raised against both 0-6-4Ts and 4-6-4Ts (though oddly, never against 2-6-4Ts).

Not, AFAIK, noted for being particular offenders, but were the A4s any more (or less) prone to losing their footing than the rebuilt W1 4-6-4 (or 4-6-2-2 .... if you prefer)?

 

But you had said, just a few posts earlier, "the worst offender is the Peppercorn A2". I presume Peppercorn's A2s had the same boiler pressure and cylinders as his A1s, so somewhat higher TE for much the same adhesive weight. Other things being equal that would make them somewhat more likely to slip. Also, as discussed here before, a 3-cylinder loco may not have much TE until steam reaches a second cylinder, at which point the TE will increase suddenly. That could be enough to start a slip with any of the LNER or Bulleid Pacifics.

 

Six were converted during WW2 with the remaining 28 following between 1946 and 1957 according to https://www.lner.info/locos/A/a4.php

 

I can appreciate regulators positioned thus will have very different characteristics to those in the more traditional position, for the reasons stated, but surely the question has to be one of drivers becoming familiar with and accounting for this. I don't recall too many complaints about uncontrolled slipping by 9Fs.

 

I'm willing to be corrected - given my lack of mechanical experience - but my understanding of the conjugated valve gear is that on starting it needs to be in full forward (or reverse) so that the first revolutions of the driving wheel to set the gear in train. I also understand that many drivers found Gresley Pacifics liked a cut-off in the range 40 - 50% when climbing gradients - especially those like Holloway Bank when departing from Kings Cross. At Kings Cross in particular the start-off in full gear and the subsequent quick change to a lower cut-off figure could cause any Gresley Pacific to slip - and often did. This may be a "fantasy from the lineside" but I have to admit that I rarely saw a Gresley Pacific slip in service but that may be simply due to the good enginemanship of the drivers who regularly drove them and knew how to handle the LNER Pacifics in BR days.

 

Could you explain a bit more? I don't understand what there could be to set in train.

 

2 quotes that are mutually incompatible and contradict each other."Worst offender"?! "The Peppercorn A2".

 

I've listened to this earlier, and have to say it is extremely interesting and insightful indeed. There's more information contained in that one interview than I've read in several books - Which is why I can't wait to get my hands on a copy of Simon's book!

Richard.

 

The W1 was noted for being particularly sure footed on its trials against the P2s in the early 1940s: which ultimately may have been a factor in the P2s being rebuilt as Pacifics later on.

 

The 2 to 1 linkage that connected all cylinders ?

 

Selective quoting again Julian. Quelle surprise!

 

As I say well worth a listen, some of Simons detractors should listen to it, they might actually learn something!

 

That gear was no different from any other Walschaert type gear in its operation and required no particular skill or technique from the driver.

 

I remember reading somewhere that if the joints of the conjugated gear were worn, the middle valve could undertravel at low speeds, and overtraval at high speeds. So if you got unlucky and stopped in a position where the middle piston was in the optimum position for starting the train, it might make a difference.

I've also wondered whether the low back pressure of the double Kylchap might make a difference when slipping. Other exhausts, with a higher back pressure, would have a greater throttling effect which could help control slipping.