[34C]

In 4mm there are plastic kits available for the Coronation articulated twins, although they are something of a job to make up. The bodysides are one piece transparent mouldings which require very careful masking to enable painting: but which are thereby capable of replicating very well the flush exterior of the prototypes. http://www.cooper-craft.co.uk/00carriages.html#kirk

There may well be etched brass kits available, but I have to confess boundless ignorance on that score.

But is there a kit of the Dynamometer car? Don't recall ever seeing a model.

 

[34C]

The real claim for the A4 class is how small they were (due to the UK loading gauge constraints) yet still able to get into the 110+mph club in company with larger wheeled and grate designs such as the 05 and Hiawatha. If, at this late date, there is to be a historical revision, it should go to the Hiawatha 4-6-4. They worked the fastest regular schedule assigned to steam, and did it reliably: daily achieving speeds in the 100 - 110mph range as a necessity to keep time.

 

[34C]

Although specifically in the case of the Gresley pacific design, the centre big end was simply understrength for half of its' duty cycle at higher power outputs. The flat back strap of the big end bearing stretched during the inside piston's forward power stroke, causing the big end bearing to pinch on the crank journal, leading to overheating. The failures occurred on power, or more precisely when high power was being produced. Mistakenly this was ascribed for a long time to lubrication failure, and that was where the effort was directed. It was eventually rectified by putting a reinforcing rib around the back strap to increase its' stiffness (now a 'T' shape in cross section); and the problem was fixed.

 

[34C]

Mr Nock's opinions should be treated with some caution. The optical frame alignment was certainly very helpful in reducing clearances on bearings, and the locos were noticeably quieter in action when erected with this technique (something that distinguished Swindon's products from its' introduction from the other railway's workshops output); and the objective evidence in the form of reduced rates of bearing wear in service from the more accurate assembly is well attested. But the inside big end design was revised in 1945, well ahead of optical alignment; and this reduced incidences of overheating in that bearing to what was typical of all plain bearings operating at similar loading on UK locomotives.

General UK practice in loco bearings was pretty primitive up to WWII. The plain bearing was universal, fit and finish largely a matter of tradition. The wartime experiences of US designed locos with grease lubricated bearings, and the automotive bearing design of tanks and other heavy vehicles built in railway workshops had an influence on moving practise forward; rather late in the day it has to be said. Really any loco in the 2,000+ IHP class should have had roller bearings on driving axles and big ends at minimum. But the railways were all short of cash so had to concentrate on what was most urgent, and had large workforces of fitters distributed around their systems to deal with the heavy ongoing maintenance demand of steam locos...

 

[34C]

David, Quite so. And you will recall that Gresley was the prime mover in the quest for such a rolling road plant to be built in the UK. Even then without stop motion high speed photography, analysing the failure mode would have been problematic. The problem was solved the old fashioned way: careful inspection of the failed components, and engineering insight and analysis of the forces at work.

And then even a rolling road may not have been all the help that might be supposed. There are well documented instances of severe bearing heating, draughting difficulties, failures in combustion rate achievement and consequent poor steaming, and truly frightening wheelslips on such test plants; all atypical of the tested locomotive when on the rails. Some features of the basic Stephenson locomotive were a serendiptous synergistic outcome of the vehicle on rail interaction while in motion, and the different environment of the rolling road lost much of this synergy.

 

[34C]

QUOTE (72C @ 18 Dec 2008, 22:18) <{POST_SNAPBACK}>.. Gresley (I think) consulted André Chapelon to improve the draughting of the A4's but I am not totally sure.
Gresley and Bulleid were very much indebted to French practise in steam circuit design. Once Gresley had his A1 pacific running, most of the improvement that culminated in the Kylchap ejector fitted A4, was based on the work of Messrs Chapelon and Lancrenon, and concentrated on eliminating internal resistance in the steam circuit. The French railway engineers had obtained their pre-eminence in this field by experience with compound locomotives, which in their early days largely lost the efficiency gained by more efficient expansion in the cylinders, by the extra work the steam had to do in the more complex steam circuit. It is estimated that 70% of the drawbar horepower gain of the Kylchap A4 over the single chimney A3 derives from the steam circuit improvement, the remainder coming mainly from the increased boiler pressure and superheater area.

M. Andre Chapelon in particular maintained a strong interest in UK locomotive design, and influenced the original W1 compound; and also tried to persuade Collett of the virtues of a large trapezoidal grate 'Super King' 4-8-0 to supply the GWR with a heavy express engine of comparable power output to a Pacific, but better suited to the curvature and gradients of the lines West of Exeter.