tisdag 21 mars 2017
The strange case of the Lord Nelsons
The Lord Nelson class should have been an immediate success. Under Maunsell's leadership, the Southern Railway had a first-rate design team with a good record of producing successful new designs and improving existing ones. The prototype was thoroughly tested before series production was put in hand in 1928. The picture shows 854 Howard of Effingham at Waterloo; the locomotive must have been almost brand new, and smoke deflectors had not yet been fitted.
Yet they were disappointing, and despite efforts to improve them, only really came near their potential when Bulleid took over from Maunsell as CME of the Southern Railway in the late 1930s. Then came the war, and afterwards there were so many Bulleid Pacifics that with only 16 locomotives in the class there was little opportunity for them to show their potential. They had very long fireboxes and the rear half of the grate was level, and for that reason they were difficult to fire. Four sets of valve gear for the four cylinders was necessary as the cranks were set at 135 degrees to give a more even beat, but it sounds like an unnecessary complication. The prototype, 850 Lord Nelson, was preserved, and has been restored for main line operation.
One wonders why the design did not closely follow the GWR Castles? The Southern Railway's design team included Holcroft, who had been at Swindon from 1906 until he moved to the South Eastern and Chatham at Ashford in 1914, and would have been familiar with the extended testing and development work which produced the Star class, the predecessors of the Castles.
Background to the design
Extracted from J. Inst. Locomotive Engineers Volume 38 (1948)
In December 1924, Maunsell wrote to the CMEs of the other British railways to ascertain the maximum axle load permitted on their lines. Hughes had submitted three designs on the LMS where the driving axles carried a load of 20 tons which, at the time, was the extreme load permitted on the LMS. Gresley stated that the highest axle load in use on the LNER was 20 tons 16 cwt., and Collett, from Swindon, said that 20 tons was the maximum built to at the time, but new engineering work would permit loads up to 22 tons. Maunsell investigated every channel both at home and abroad to see whether such an engine of the power required could be built within the weight laid down by the Civil Engineer.
The original scheme for an engine which ultimately led to the construction of the Lord Nelson had an axle load of 21 tons 10 cwt., which was 17 cwt. in excess of the 20 tons 13 cwt. of the final Lord Nelson. The cab was rather a departure from the usual Southern Railway cab, being more in keeping with the old North Eastern. To enable the weight to be reduced, the boiler barrel was shortened by approximately 10 in., and this enabled the King Arthur tubes to be used as the distance between the tubeplates was identical.
The improvement made by the alteration to Engine No. 449, particularly with regard to the saving in coal, was so marked that it was decided to incorporate the arrangement in the new "Lord Nelson" class. The arrangement provides a more uniform torque and more regular effect on the firebox draught than is customary and enables the engine to be worked more heavily without fear of 'breaking up" the fire.
The revolving and reciprocating parts were kept light by using high tensile steel, Vibrac, and the balance weight in the wheels was reduced in consequence: this produced a much lighter hammer blow and influenced the Civil Engineer in accepting an axle loading up to 21 tons.
The boiler was large, and a new feature for engines built at Eastleigh was the provision of a Belpaire firebox. The superheater was the Maunsell type with air relief valves. The boiler has probably the widest type Belpaire firebox that could be used within the limitation of the SR loading gauge, consistent with a clear view from the cab; also the longest firebox it is possible for a fireman to conveniently fire. The grate is virtually in two sections, the rear portion being horizontal as a landing and the forward portion sloped. This caused a definite break in the fire and on occasion led to indifferent steaming when inexperienced firemen are used to fire the locomotives.
The Lord Nelson boiler was originally fitted with steel and copper water stays in the firebox. Copper stays were used on the firebox side for the top six rows and the outer end rows only, the remaining stays being of steel. The steel stays were afterwards replaced by Monel stays and this was probably the first application of Monel stays as standard practice to locomotive fireboxes in Britain. The stays are fitted with steel nuts on the inside of the inner firebox.
To enable the engine to be built to the weight allowed by the Civil Engineer, great care was exercised, both in design and actual building. Certain parts normally left as forged or cast were machined to keep within the weight. So much care was exercised that the engine was actually well within the weight when completed, so the remainder of the class did not receive similar treatment. After the balancing of the engine had been calculated at Eastleigh, the figures were submitted to Professor Dalby, who agreed that the engine balance as shown would be very satisfactory in running. Cocks notes that the Southern was unlike the other members of the Big Four: in its intensive passenger services, its electrification and its quest for punctuality. On the rebuilt E and D classes he noted that these shared the large N class piston valves.
...contact with German engineers (in 1930) had an important sequel. They were full of enthusiasm for the solid-headed piston valve with plain rings which had come into use in Germany, and they brought over drawings. This led to their general introduction on the Southern Railway. A snag was struck, however, when these valves were applied to " Schools " class engines working on the Eastern Section.. Certain trains to Cannon Street or Charing Cross have to make a stop at No. 7 platform, London Bridge, which is on a curve and has an up gradient of 1 in 100. To make matters worse, there are catchpoints immediately in the rear of long trains, so that setting back more than a few yards is prohibited. Great difficulty began to be found with the Schools " class in starting their trains and an investigation was made. These engines, in common with the Nelson " class had a lead of ¼ in., but as the solid heads of the valves had a small clearance in the liners, some pre-admission in excess of lead steam started as far back as the first ring, when it passed the port edge. The amount of steam leak was insignificant when on the move, but on starting a heavy load enough leakage occurred to cause a negative turning moment and so seriously affect the tractive effort.
The remedy adopted for this state of affairs was to reduce the lead and transfer the point of cut-off from the edge of the head to the first ring, turning down the diameter of the head in advance of the ring to expose the side of the ring to steam.. This reduction in diameter of the head can only be small in amount, otherwise the reduced bearing surface of the ring in its groove leads to excessive groove wear.
While this alteration ameliorated conditions at starting in the case of the " Schools " class, it was applied also to the " Nelson " class in accordance with the policy of standardisation of parts. In my opinion this was most unfortunate and quite uncalled for from the performance aspect, and I attribute to this the blight which seemed to descend on the " Nelsons " after their earlier brilliance. The alteration did not matter so much to the " Schools," which are customarily worked at a 25 per cent, cut-off and part regulator, but the Western working of the " Nelsons " with full regulator and short cut-off was another matter altogether. Not only was the area of opening to steam restricted by the projecting edge of the head beyond, the first ring but lead steam was reduced as well, so that port opening was much smaller than before, with the same travel. This state of affairs remained until the front end was modified by Mr. Bulleid in recent years.