LMS 2P 4-4-0

The RCTS book on the Hughes and Stanier 2-6-0s says that other nicknames for the Stanier locos were lobsters and camels. I guess there were plenty of diesels with mollusc nameplates.

BTW, the RCTS book does not have the story of the initial design having the cylinder axis higher than the driving axle centre which you mentioned, nor does Langridge. Where is this information from please? Presumably they looked at the drawing of the 43xx as a starting point.

 

After being the SMF Archivist for over twenty years, a lot of information has come to me, the origin of which I cannot now trace. However, in this case I do know: it's ina hand-written latter from Eric Langridge himself, see image. I've included only the first of three pages. Eric was involved only in the boiler design , which explains his uncertainty about the cylinder height.

I had a lot of input into the RCTS book, and while there is a lot in there, there is a lot that isn't: you can fit only so much between the covers. The nicknames given above were spotters' names. Having never been a spotter, I don't use them and stay with the railway terminology.

I don't think any of the 43XX made its way into the design, which was Horwich and, despite comments higher up the thread, came down from the Horwich Crabs with the exception of the boiler. Apart from the wheel arrangement, there is almost no similarity between the two classes.

 

Might as well include the third page for completeness!

 

A decapod is a type of crustacean, which could be confusing.

 

Thank you, that is interesting, it's a pity that issue did not make it into the book. The RCTS series is a bit trainspotter-ish in places but I guess that is where the market is. Langridge's comment is a bit sweeping as the Swindon "Desaxe" arrangement only applied to some classes, as I noted above.

 

You need to remember that Eric originally started work on the LSWR, and they and the GWR were arch enemies. His writing has a lot of anti-GWR bias in it.

 

Corrected it for you

Tom

 

Returning to seriousness: I've never really thought about this before. But if I understood it correctly, if the cylinder axis is above the driving wheel centre line, doesn't that mean that the crankpin will traverse slightly more than 180° when the piston moves from the back to the front of the cylinder; and slightly less than 180° in moving from the front to back. Which, if the wheel speed is constant, means one piston stroke takes longer than the other. Did that have any practical measurable impact (for example, in the relative distribution of power between the two strokes; or in settings of the valve timing) or was it one of those "in theory yes, in practice negligible" sorts of issues?

Tom

 

My thought was that where they do look completely different is around the cylinders and motion, which presumably was where the "crabs" name came from.

 

There are two theories on the Crab nickname: the large, high-mounted cylinders and associated gear reminded people of the claws of that animal; and the big cylinders under load with offset thrusts tended to push the back end from side to side, and this sideways movement reminded people of a crab's sideways walk.

You pays your money and takes your choice!

 

Interesting point, it certainly does something to the geometry, which one could perhaps explore by drawing it out. Initial thought is that at front dead centre, the crank pin would be slightly above the horizontal centre line, and at rear dead centre, slightly below it i.e. it tilts the axis of the motion slightly, which was possibly compensated in the angle of the return crank/eccentrics. I noted on the brief discussion on piston speed in another thread that, in examining the connecting rod/piston motion, you quickly get to the rather uncomfortable conclusion that (absent a connecting rod of infinite length) the piston completes the front half of its motion in less than 180 degrees/half the rotation of the crankpin, and vice versa for the rear half, and this principle would presumably be unchanged.

Of course, because the driving axle is sprung, this is not a fixed relationship anyway. It is probably one of several reasons why you don't want to jack up a steam loco and let it operate with the axleboxes at the bottom of the horns, which would create a centre line offset, and you may find the piston over-running, apart from damaging the bearings.

On valve gear drawings, certainly of LMS locos, you often get a table of values (which someone has presumably laboriously worked out by hand) of the port openings, cut off, lead etc for the front and rear ports at various levels of nominal cut-off, and there are inevitably differences between the two. I think as with much steam loco engineering, it was a compromise and (in the case of valve gear) inequalities were probably minimised for the most commonly used cut off. The differences are minimised at 30% cut-off on the Fowler 2-6-4T (being the drawing I happen to have in front of me).

 

I once spent a day assisting with valve setting the H class. (I did the grunt work, not the clever stuff...) In essence, if you take each single valve / cylinder in isolation, you had four numbers you were trying to optimise (the port openings at each of front and back dead centre, in each of full forward and back gear) but only two parameters you could vary (the length of the fore gear and back gear eccentric rods). And then you had to take into account that although you did the job cold, the valve spindle would expand when hot, which had more effect at the front of the cylinder than the back.

I tend to bear that in mind when you get comment on fora about certain locos sounding well or badly adjusted. It’s as much art as science!

Tom

 

The cylinder centreline on Gresley's A1/A3 and A4 Classes is 1/2" above the centreline of the driving wheels. I don't know whether his other designs were similar.

 

Remember also height of the cylinders will vary to the centre of the driving wheels, due to how much water is in the boiler/firebox and coal/ash is in the grate. The GWR 43XX is listed 4 1/4 tones heaver (1 1/4 per driving wheel) when the engine is full against empty.

 

The whole difference between full and empty seems of little relevance. How far do the wheel centres move up and down relative to the frame and cylinders between maximum and minimum levels in the gauge glass? And how much with tyre wear? And how much movement is likely due to dropped joints or other irregularities in the track out on the line, when the loco is working hard? (Shed yards may have much worse track, but a loco wouldn't be doing much work there, so piston forces and valve events wouldn't be critical.)

 

Question: into how much detail does this (fascinating) subject discussion need to go before it's more properly moved to M.I.C ?

The breadth of knowledge among our merry throng never ceases to amaze me. It's one of the greatest pleasures of being a member .... even for a certified (or should that be 'certifiable'?) ignoramus like me.

 

I've a vague recollection in the back of my mind that doing this gives better valve events in forward gear at the expense of back gear. Could be totally wrong, though.

 

I spent a bit of time calculating that ...

These are the rather scrappy graphs. For orientation, I assumed a crank of arbitrary half-throw (i.e. half piston stroke) of 1; and a con-rod length of 5; and I assumed the cylinder axis at the same level as the axle. At front dead centre, the cross head is therefore 6 units in front of the axle (the length of con rod + half the crank throw). 180° later, it is 4 units in front at back dead centre (the con rod length minus half the crank throw). By a bit of trigonometry and Pythagoras, you can then calculate how far in front of the axle the the cross head is over a complete revolution; you get this graph. (o° is front dead centre; 90° is the crank vertically up, 180° crank to the rear; 270° crank verticlly down):



Where is the piston at half stroke? Since the furthest forward of the crosshead is 6 units in front of the axle and furthest back 4 units, half stroke is when the crosshead is five units in front of the axle. Expanding the graph in that region gives:



From which it can be seen that the piston is at half stroke when the crank has rotated about 84°. Since the graph is symmetrical above and below, it means that it takes about 168° of rotation for the piston to cover the front part of its stroke, and therefore about 192° to cover the back part, as @huochemi states.

The numbers will be slightly different depending on the ratio of con rod length to crank half throw; for example, if they are in the ratio of 8 to 1, the same numbers are about 172° and 188° - but the principle is the same.

To do: the same calculation with the piston axis offset above the axle centre line. But first I've got a copy of Bluebell Times to edit this weekend (anyone spot the procrastination?) ...

Tom

 

Angularity is well known and increases as the con rod length reduces in comparison to the stroke. We spent a long time on this when I was a student as some just could not understand why the crank wasn't perpendicular with the piston at half stroke. That was on internal combustion engines, where the stroke / con rod length tends to be much lower so the angularity more pronounced.