Although the GWR had concentrated on the 4-6-0 wheel arrangement for high-value traffic since the turn of the century and the Star class seemed to be sufficient for express traffic for a long time, George Jackson Churchward designed a Pacific locomotive. One can only speculate about the reason for the development of “The Great Bear”, whether the largest locomotive in Great Britain was to be built simply for advertising purposes or Churchward personally had the construction carried out in order to be able to carry out tests with a particularly large boiler.
For the first time in Great Britain, the choice of wheel arrangement fell on the 4-6-2 (Pacific) in order to be able to accommodate a wide firebox over the trailing axle. At 23 feet, the boiler barrel reached an extreme length that was hardly reached by other locomotives later on. At 15 inches, the diameter of the four cylinders was chosen so large that it was just possible to maintain the loading gauge and allow the bogie to move freely. The outer cylinders were at the level of the rear axle of the bogie and drove the second coupled axle, while the inner cylinders were at the level of the bogie pivot and drove the first coupled axle.
Schematic drawing with dimensions
Locomotive Magazine, February 1908
The number 111 was the largest steam locomotive in Great Britain at the time and, despite the sixth axle, achieved an axle load of 20.8 tons. At the time, the only GWR route capable of handling such heavy loads was the Great Western Main Line from London-Paddington to Bristol. Despite the very limited usability, the one-off had great value for the public perception of the GWR.
From a technical point of view, however, the locomotive was anything but a success because the evaporation capacity was limited. This can be attributed to the fact that the heating surface of the firebox was very small compared to the heating surface of the smoke tubes. Here the tube heating area was 18 times the firebox heating area, while this ratio was in the 11 to 13 range for most Pacifics. Thus, relatively little direct radiant heat could be absorbed by the firebox, while the smoke tubes, which were too long, were inherently problematic and had more surface area than would have been necessary to absorb the heat from the smoke. For this reason, brick arches, combustion chambers or thermosiphons were built into the combustion chambers elsewhere to increase the direct heating area.
The “The Great Bear” remained in service, but suddenly lost importance in 1923 with the introduction of the more powerful Castle class. A year later it was decommissioned. As late as September 1924 some parts of the locomotive were used in the manufacture of a Castle class example. This also got the number plate of 111, but was given the name “Viscount Churchill”.