wlkr wrote:...... as the man said, refering to the booked engine, "that things no good, it's so far out of line you'll be lucky to get it to Finsbury Park"! Or words to that effect but with plenty of colour!
I believe from reading (because I have no mechanical engineering experience) that setting up an alignment of the cylinder with the big end was a critical measurement toward the end of the build or overhaul, and was done with piano wire (sic), or perhaps optically with a theodolite if the plant had one. So the quote above has some resonance in my brain.
That required continuous measurment of the centre of the cylinder bore with the cylinder walls at both front and back. I can only surmise that some adjustment in the cradle or saddle of the cylinders was possible. How the erectors got on where the cylinders were cast en bloc with the frame escapes me. Maybe that technology solved these sorts of problems.
With the integral cast steam loco frame, it's exactly as the casting of a large diesel engine block and fitting the major moving parts. Precision location and machining out of the cylinder bores and axle/crankshaft bearing locations in the casting, likewise location and machining the axle/crankshaft journals for the necessary alignments.
Lot more precision for the diesel of course: in the steam loco, whatever the centre location of the axle and thus crank might be, since it was in suspension in the frame, once the loco was underway the axle moved relative to the cylinder, and the frames flexed, and the rods deflected, so the actual stroke in the cylinder was continually varying. Amazing it ever worked at all isn't it?
I believe that when K J Cooke was appointed as CME of BR Eastern and North Eastern Regions in 1951 he introduced the GWR method of opticle alignment of components when assembling locomotive frames and motion. Presumably the previous methods weren't precise enough for a Swindon man.
When you consider that they were trying to line up coupling and connecting rods and also various parts of the valve gear it must have been a devil of a job as the axle boxes all allowed independant vertical movement of the bearings.
I also remember reading that it wasn't unknown for 3 cylinder locomotives to have slightly different diameters on all 3 cylinders after a heavy overhaul due to requiring different amounts of machining required to clean up the bores. I would assume that this would give slightly different power outputs which would presumably also add twisting forces into the frame. Whether this would be enough to have much impact I have no idea. Perhaps someone with more engineering knowledge than myself could comment.
How to put this; those relaively small dimensional variations hardly mattered at all in a machine as basic as the steam engine. Wear to the valve gear in service would introduce similar scale of change in valve events between overhauls. It will be recalled that in an experiment to restrict the extra power developed in the A4 centre cylinder due to the overrun of the conjugating gear, Thompson had the centre cylinder deliberately lined up to be smaller in diameter, 17" instead of the nominal 18.5", and the four locos so treated ran perfectly well.
