111 The Great Bear

There are some who say even the standard 7MT would have benefited from three cylinders even with 250 psi and 2 x 28 in cylinders. I wonder just how big the cylinders could have been within the LNER loading gauge.

 

Well, here you are gentlemen. The Bear with a shorter boiler with a combustion chamber, a slightly smaller grate to make room for a bigger cab, and a (rather crudely) enlarged standard tender. Oh and 1400 class trailing axle suspension. Its worth noting, if I hadn't already, that the King boiler is to an appreciable extent a narrow firebox equivalent of the 'Bears.

 

That would mean extending the frames, which in turn would make the back end heavier, and it was already on the limits. There would be more redesign involved than I am capable of having a reasonable shot at..

 

Or something not dissimilar to a Stanier Princess Royal?

Tom

 

This is where we start needing a proper engineer to do our speculation for us. But I'm ignorant of all the design factors, I don't even know which runs heavier per foot, a barrel full of water or a firebox full of coal. Superficially there's an awful lot more Duchess behind the last pair of driving wheels than there is Great Bear, but the Duchess has two tons more weight on each driving wheel than the Bear, so it must have been balanced somehow. If the Bear was redesigned to the King weight limits it would be a lot easier, but if you have the King weight limits you have the King, and don't need a pacific for the GWR traffic anyway...

 

As I understand it, 2 x 26in cylinders. I will go back and get the exact quote for you tonight.

The idea of a 7MT with three cylinders puzzles me - it already exists. It was the Peppercorn A2 (6ft 2in drivers with three cylinders intended for mixed traffic).

 

The A2 was designated 8P/6F so seems to have been passenger biased. The Britannia took the principle of " two cylinders unless you can't get enough TE with just two" to the limit and maybe just beyond.
Perhaps they should have perpetuated the A2, but you could say that of a lot of the later big 4 locos.

 

The barrel vs firebox question is harder, but I'd be fairly certain that a combustion chamber section is lighter than an equivalent barrel section. Steel's only ~8x heavier than water, after all, so a combustion chamber would probably have served a useful secondary purpose of getting weight off the back end of classes to which it was applied. I would also suggest that the Duchess arrangement, with the outside cylinders pushed forward, makes the loco a bit more front heavy, thus potentially relieving the trailing truck of a bit of mass.

Also, look at the position of the trailing truck axle on a Duchess (or indeed an LNER or Southern Pacific) - it's usefully further back than on the Great Bear, closer to the rear of the firebox than the middle. If you assume as a starting point when setting the springs on a Pacific that the coupled axles are to be adjusted to the maximum axle loads the Civil Engineer will accept, the bogie and trailing truck then have to take the rest of the weight and provide the correct moments about the centre of mass to balance the moments from the coupled axles. Pushing the trailing truck back increases its lever arm about the centre of mass, so more of the weight needs to go onto the bogie and less on the trailing truck - German classes seem to take this to an extreme. So, push that 1400 axle back a bit and the Bear could probably have a half decent cab.

I've never really liked 4-6-4 tender engines as a concept - they always strike me as an awful lot of weight not being used for adhesion relative to their potential power. Even the NYC Hudsons, working on a fairly level route, were only really kept viable by boosters.

 

The 1400/4800 trailing axle, like that of the Atlantics, doesn't allow any side control. (Although it doesn't seem that the standard radial truck Churchward used had very much sideplay since Holcroft restricted the cylinders to 15 inch to clear the bogie yet the Castles with a longer coupled wheelbase were able to carry 16 inch cylinders).

 

Yes, Hudsons were known for being "slippery" and the booster was a sort of bail-out for that condition, beautiful brutes that they were. Once up to track speed though, they could really fly!
USA Ten Wheelers were
put out to pasture too soon. More was to be had from them given the basic engine they were. The Pennsylvania RR G5s models were a powerful example in their own right, with 24X28" cyls,118 tons engine weight,40,000+ tractive effort and modest 205 lbs boiler pressure. Wish Gresley or another British engineer could have had a free hand in "tweeking" those G5s engines even if they could not have run on British rails. It would have been interesting to say the least...........A 3 or 4 cyl G5s @250 lb. boiler pressure??

 

Drawing of Great Bear with narrow firebox. (Boiler barrel as actual Great Bear, but Standard 7 firebox)

 

Sorry this is so late. Here is the quote in question. This is taken from Steam World no.62 of 1992.

 

Thanks for the info. Hmmm 21 in cylinders, 6 ft 8 in wheels, 26 in stroke, original bp 200 psi. I make that 24,365 lb tractive effort at 85% boiler pressure against the original 29,918 lb. An A3 boiler would raise that to 26,802 lb. The mods would have been significant - and all for a weaker loco.

 

Tractive effort isn't everything, to be fair. With two cylinders and walschaerts valve gear we introduce the age old argument of maintenance against performance and what trade offs there are.

It's an interesting "what if" - but it so happens I agree in principle with Gresley on this. The Raven Pacifics were a dead end - any development would be to make the most of the existing good parts of the locomotive. Why bother if the future known standard is going to be the Gresley Pacific?

 

The LMS would have taken them into the works, outshopped A1s with the same numbers and called them rebuilds...

Tom

 

This is something I got into studying for the book I've been writing (sorry, end of gratuitous plug). The important word is renewal. Its an interesting word because the shades of meaning attached to it have changed over the decades and more. When I looked at a modern OED the various shades of meaning of renewal pivoted around repair and taking something that was old and fixing it up so it was like new again. But when I looked up dictionaries from the end of the 18th and beginning of 19thC the shades of meaning were rather different, and the word meant something much more like replacing the old one with a new one. So when you put a new handle and a new blade on grandfather's axe you were renewing it.

So this is my best current understanding, principally related to the GWR and bearing in mind that I am *not* a money person.

In railway account terms the renewal fund was a sum of money that was put aside in order to fund replacements of things that were worn out. So when an 1850 0-6-0 was clapped out and needed replacing in 1890 the process was renewing it. If there were some components on the old one - wheel centres were often an example - that could be reused without compromising the end result then they might be reused to save some money, but the general intention was that a worn out 1850 design was replaced with a brand new 1890 design, but one was a direct replacement for the other and in the early days on many lines they would even keep the same number.

A rebuild, on the other hand, didn't come out of the renewal fund money, it came out of the funds allowed out of revenue for maintenance. But a really major rebuild might use just as many new parts as a renewal, it was just that the way they were managed in the books was different. Railway enthusiasts confuse rebuild, repair and renewal horribly and seem to have a definite tendency to use the words interchangeably. I mean, who cares about the budget. But this area of budgets had a huge effect on policy - hell, at the Bluebell you own one of the classic examples...

 

Moving the trailing axle back also improves the ashpan capcity

Moving the trailing axle back also helps with the ash pan arrangement - ash is less likely to get caught on the "hump" over the axle.