SECR N1 Class - Design and Construction

Design and Construction

For detailed information on the Holcroft valve gear and numbering variations, see: Holcroft valve gear and Livery and numbering

Maunsell and Holcroft completed plans for a new 3-cylinder locomotive in 1919. Designated N1, the new design was an attempt to increase the operational flexibility and power of the SECR 2-6-0 without substantially increasing axle loading. Holcroft's previous employment as an engineer involved with the GWR's 4300 class meant that the N1 used the same Churchward features as the N class to aid standardisation and the manufacture of parts. These included the use of long-travel valves for free running at speed, right-hand drive in the cab and 5 ft 6 in (1,680 mm) driving wheels. The retention of the 2-6-0 wheel arrangement meant ample accommodation for the N class' tapered boiler with Belpaire firebox. As with the N class, detail design on the N1 was left to another of Maunsell's assistants, James Clayton, who brought functional Midland Railway influences to the design: the loading gauge-friendly shape of the cab, the tender and the large-diameter smokebox. The smokebox housed a large superheater, regulator valve and snifting (anti-vacuum) valves. The N1 also retained Walschaerts valve gear on both outside cylinders.

The main design differences with the N class included the cab front, exhaust arrangements, and the addition of an extra (inside) cylinder between the frames to drive the centre driving axle. Clayton had revised the layout of the cab front spectacle plates (small windows on the front of the cab) to improve forward visibility. This incorporated two large single panes of glass either side of the boiler instead of the four smaller windows used on the N class. The inside cylinder was to be actuated by Holcroft's design of conjugated valve gear, which consisted of mechanical links fitted to both sets of outside Walschaerts gear, eliminating the need for a separate set of valve gear between the frames to operate the inside cylinder. The result would be a reduction in weight and the amount of mechanical equipment located in this inaccessible part of the locomotive.

Holcroft’s valve gear design was also an attempt to address the problems associated with Gresley’s conjugated valve gear, which was prone to variations in valve events caused by heat expansion of the valve spindles within the pistons. The design utilised the motion of the outside valve rods (the rods transmitting the motion of the driving axles to the valves, such as the combination lever) instead, although the restricted space between the back of the outside cylinders and the front driving wheels made it impossible to locate the rocking arms controlling the conjugating mechanism in the vicinity. Holcroft’s solution was to move the mechanism to a position above the pony truck and ahead of the cylinders and link it to the outside valve rods with extension levers, which had to be accommodated within the strict SECR loading gauge tolerances. This necessitated a new design of cylinder block whereby the outside steam chests were set inwards and the size of the cylinders reduced to 16 in × 28 in (406 mm × 711 mm) stroke and diameter. The sides of the outside cylinders consequently sloped inwards towards the top in a configuration similar to GWR 2-cylinder locomotives, thus providing ample space in which to accommodate the extension levers. The inside cylinder and conjugation assembly were set at an incline of 1 in 8 to clear the front pony truck, leaving space for a third set of independent Walschaerts valve gear between the frames should Holcroft’s conjugation mechanism prove unreliable in service.

Another issue concerned the strength of the locomotive frames around the cylinders. Taking inspiration from the 3-cylinder locomotives designed by Vincent Raven for the North Eastern Railway, Holcroft suggested that all three cylinders and associated steam passages should be cast in a single block, with rectangular holes cut into the frames to enable the outside cylinders to project through them. However, the SECR’s established practice of using outside steam chests on locomotives with outside cylinders prevented this, and the design team resorted to cutting a gap in the frames to accommodate the outside cylinders, which were then secured in place with a bolted splice plate. Production limitations at the Ashford works foundry also prevented the manufacture of a single casting that incorporated all three cylinders. This meant that the cylinder castings were split into two sections, with the left and inside cylinders forming a single unit that would be bolted onto a separate right-hand cylinder casting. Holcroft’s new cylinder block design also incorporated a separate saddle casting, a configuration that allowed the N1 cylinder patterns to be used with any diameter of smokebox, creating a standard component with potential applications on future locomotive designs.