The South African Railways Class 21 2-10-4 of 1937 was a class of steam locomotives used in South Africa.
In 1937, the South African Railways placed a single Class 21 steam locomotive with a Texas type wheel arrangement in service, designed as a mixed traffic locomotive suitable for use on light rail. A simultaneously proposed heavier mainline version Class 22 was never built.
Manufacturer
The Class 21 2-10-4 Texas type locomotive was designed by A.G. Watson, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1929 to 1936. It was built by the North British Locomotive Company in Glasgow and delivered in 1937. Only one locomotive was built, numbered 2551. At the time, the design represented the maximum power obtainable from a ten-coupled non-articulated locomotive which was limited to a axle load on rail.
Characteristics
Watson disliked articulated locomotives and his aim was to develop a powerful non-articulated mixed traffic branch line locomotive with an axle load suitable for light rail. To enable it to negotiate tight curves of radius, the third and fourth coupled wheelsets were flangeless. In addition, a total side play of was provided on the leading coupled wheels and spherical bearings were fitted to the leading coupling rods. Its bar frames extended from the front to the hind buffer beams and were stayed by cross stretchers of a light fabricated construction. It used a Watson Standard no. 3B boiler, the same as that used in the Classes 15E, 15F and 23.
The result could be considered as Watson's answer to the Class GF Garratt locomotive, the two types having very similar axle loads. The Class 21 carried more water than the Garratt and was about heavier with tractive effort, compared to the of the Class GF. Even so, only one Class 21 was produced and the design was not repeated.
Watson's design called for cylinders with rotary cam poppet valve gear, but since the locomotive was still under construction when Watson retired, his successor as CME, W.A.J. Day, made use of the opportunity to alter the specifications. Thus, in the year following Watson's departure, the Class 21 locomotive was delivered with Walschaerts valve gear.
The cylinders were interchangeable with one another and with those of the Classes 15F and 23, even though the piston stroke of the Class 21 was compared to in respect of the other two classes. On the Class 21, the difference in stroke was made up by providing deep spigots on the cylinder covers.
The 2-10-4 Texas type wheel arrangement had not been tried before on the SAR. To distribute the axle loads to within the maximum allowed for branch lines, the ten-coupled arrangement with a four-wheeled trailing bogie was required. To further assist with keeping within the permissible weight limits, the cab was of welded construction and all footplates and running boards were made of duralumin while the axles and crank pins of the coupled wheels were hollow and the connecting rods were of special high tensile steel.
The equalised suspension of the engine was arranged in the customary three groups. The leading Bissel bogie and the first two pairs of coupled wheels were equalising in one group, while the remaining three pairs of coupled wheels were equalised with the trailing bogie, on each side (not cross-connected); thus three groups. As is usual, cross equalisation (LHS to RHS) was provided between the leading Bissel bogie and leading coupled wheels. Instead of pins, case-hardened cotters were provided for fulcrums of the bearing spring equalisinging beams. The proportion of balanced reciprocating parts was only 6% which reduced the hammer blow on the rails to per wheel at . The locomotive was noted for its smooth running.
Tender
The Type FT tender was an unusual type which used six pairs of wheels in a 2-8-2 wheel arrangement, with the leading and trailing wheels in Bissel-type pony trucks and the rest of the axles mounted with a rigid wheelbase. Like the leading and trailing wheels of the engine, all tender wheels were fitted with roller bearings. The rationale behind the design was that if the tender frame could be carried on more points represented by the rigid and Bissel wheels instead of on only two bogie pivot centres, the frame could be made lighter.