The South African Railways Class 6L of 1904 was a steam locomotive from the pre-Union era in the Cape of Good Hope.
In 1904, the Cape Government Railways placed its last two 6th Class bar-framed steam locomotives in service. In 1912, when they were assimilated into the South African Railways, they were renumbered and designated .
Manufacturer
The Cape Government Railways (CGR) placed its last order for two 6th Class locomotives with Neilson, Reid and Company in 1903. In that same year, Scottish locomotive builders Sharp, Stewart and Company and Dübs and Company merged with Neilson, Reid to form the North British Locomotive Company (NBL). The two locomotives, Neilson, Reid works numbers 6324 and 6323 and built at the Hyde Park works of the former Neilson, Reid, were therefore delivered with NBL works numbers 15889 and 15888.
Characteristics
The two locomotives in this order were experimental and were the first South African locomotives to be built with superheaters and piston valves with internal admission. Like other second generation 6th Class locomotives with bar frames, they had high running boards with no need for driving wheel fairings. The two locomotives were delivered in 1904, numbered 909 and 910 for the Western System of the CGR.
Cylinders
The pistons, with a bore of , were the largest yet used on the 6th Class. The piston valves, arranged above the cylinders, were cast iron, in diameter and actuated by Stephenson Link motion through rocking shafts. The balance weight was replaced by a compression spring encased in a cast-steel cylinder.
A pressure relief valve was fitted to the cylinder covers and snifting valves were fitted to the steam chests. Lubrication of the cylinders and piston valves was by means of a Ritter six-piston oil press, which was arranged in the cab at the right hand side of the engine and actuated from the trailing crank pin.
Firebox
The boiler barrel was telescopic in form, with the larger diameter adjacent to the firebox. The front portion of the firebox was supported from the shell by means of partial girder stays, fitted with expansion links. The remaining portion was supported by through stays, each partially encased in an iron tube filled with cement. The stays in the sides, the tubeplates and backplate were copper, while the 158 tubes were iron.
Schmidt superheater
These locomotives were the first engines to be built in Britain with a Schmidt superheater, which was of the smokebox type and consisted of 61 solid drawn-steel tubes. The superheater tubes were encased in a box which was built up of thin steel plates, adapted to the shape of the outer series of tubes, and ended at the steam collector box in two narrow vertical dampers. The whole superheater occupied two-thirds of the total length of the smokebox. To ensure that the superheater tubes received ample heat, a large flue of diameter was installed in the lower part of the boiler between the firebox and the front tube plate, where it connected to the casing of the superheater.
The 61 tubes were bent into concentric rings and arranged in vertical planes about apart, each plane containing alternately two rings and one ring, with the one ring of a diameter intermediate between that of the other two rings. The first nine rings nearest the tubeplate, however, were arched in decreasing radii from the front of the flue to form a chamber between the inner and two outer rings into which the gases from the large flue first passed.
The inner series of tubes were bent up at the ends and expanded into two cast-steel collector boxes, attached to the smokebox on each side of the chimney. The right hand side box was divided into two chambers by a central partition, with half the superheater tubes expanding into each of the chambers. The partition on the tube plate side of this box was connected to the regulator pipe, while the front partition was connected through the steam pipes to the cylinders. The left hand side steam collector box had no partition.
The action of the superheater was simple. Upon opening of the regulator, saturated steam was admitted to the rear chamber of the right hand side steam collector box, from where it passed through the tubes to the left hand side box in a slightly superheated condition. From there it passed back to the front chamber of the right hand side box through the other half of the tubes, and then, in a superheated condition, to the cylinders.