South African Class 21 2-10-4

The South African Railways Class 21 2-10-4 of 1937 was a steam locomotive.

In 1937, the South African Railways placed a single Class 21 steam locomotive with a 2-10-4 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 2-10-4 was never built.^[1][2][3]

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 the 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 15 long tons (15.2 tonnes) axle load on 60 pounds per yard (30 kilograms per metre) rail.^[1][2][3][4]

Characteristics

Watson disliked articulated locomotives and his aim was to develop a powerful non-articulated mixed traffic branchline locomotive with an axle load suitable for light rail. To enable it to negotiate tight curves of 300 feet (91 metres) radius, the third and fourth coupled wheelsets were flangeless. In addition, a total side play of 1 ³⁄₄ inches (44 millimetres) was provided on the leading coupled wheels and spherical bearings were fitted to the leading coupling rods. Its 5 inches (127 millimetres) thick 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.^[1][2][3]

A.G. Watson

The end result could be considered as Watson's answer to the Class GF 4-6-2+2-6-4 Garratt locomotive, with the two types having very similar axle loads. The Class 21 carried more water than the Garratt and was about 23 long tons (23.4 tonnes) heavier with 43,700 pounds-force (194.4 kilonewtons) tractive effort, compared to the 34,200 pounds-force (152.1 kilonewtons) of the Class GF. Even so, only one Class 21 was produced and the design was not repeated.^[1][2][3][5]

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.^[1][2][3][5]

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 26 inches (660 millimetres) compared to 28 inches (711 millimetres) 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.^[2]

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 branchlines, 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.^[2]

The spring compensation of the engine was arranged in two groups. The leading Bissel bogie and the first two pairs of coupled wheels were compensated in one group, while the remaining three pairs of coupled wheels were compensated with the trailing bogie. Cross compensation 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 compensating beams. The proportion of reciprocating parts balanced was only 6%, which reduced the hammer blow on the rails to 5 long hundredweight (254 kilograms) per wheel at 50 miles per hour (80 kilometres per hour). The locomotive was noted for its smooth running.^[2]

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.^{[1][2][3][6][7]}

The tender was equipped with a mechanical stoker. The water tank was of welded construction and the coal bunker was of the self-trimming design. Each Bissel truck was compensated with two pairs of rigidly mounted wheels. The eight rigidly mounted wheels were fitted with overhead laminated bearing springs, while the spring gear for each pony truck and two adjacent axles were compensated throughout the springing system, constituting two equal groups. Vacuum cylinders, mounted on top of the tank, actuated brake blocks on the rigidly mounted wheels.^[2]

The tender's wheel arrangement did not prove to be very successful and, with the exception of the Type JV tender, a similar experimental tender which had been built in the Salt River shops in 1936, during the design phase of the Class 21 as a prototype test-model of the Type FT tender, it was not used again on the SAR. Comparison, in working order, with the Type MR tender showed that the Type FT tender weighed 30,128 pounds (13,666 kilograms) more than the Type MR, while it carried the same amount of coal and only 987 imperial gallons (4,490 litres) more water, the difference representing a weight of 9,870 pounds (4,477 kilograms).^[1][2][3]

The Bissel or pony axle design is commonly used on steam locomotives with a single leading or trailing axle, and was later also used on the Class 4E electric and Classes 32-000 and 32-200 diesel-electric locomotives.^[3]

The aborted Class 22

A design for a Class 22 steam locomotive, Watson's final design, was submitted at about the same time. The proposed Class 22 was also to have a 2-10-4 Texas wheel arrangement, but was to have been a heavy mainline version of the Class 21, with an axle load of 22 long tons (22.4 tonnes), the heaviest which current SAR track could bear on its 96 pounds per yard (48 kilograms per metre) mainline rail. It was to have been a massive machine with larger 60 inches (1,524 millimetres) coupled wheels, a larger 80 square feet (7.4 square metres) grate and the larger Type EW tender which was later to be used with the Class 23 locomotive.^[1][2][3][5]

If this engine had been built, it may have been one of the world's most outstanding locomotives. The proposed boiler pressure was 250 pounds per square inch (1,724 kilopascals), a figure never attained on the SAR, and its anticipated tractive effort of 66,406 pounds-force (295.39 kilonewtons) at 75% of boiler pressure would have made it capable of handling loads of 2,200 long tons (2,235 tonnes) on the coal run from Witbank to Johannesburg with comparative ease.^[1][2][5]

The design was a compromise between a 2-8-4 passenger class with 66 inches (1,676 millimetres) coupled wheels and a 2-10-2 freight locomotive with 60 inches (1,524 millimetres) coupled wheels. At the time, however, the demand for general utility locomotive types was so pronounced that no good argument could be put forward for the introduction of a heavy locomotive dedicated to goods working only. Another factor which acted against the project was the insufficient length of the receiving sidings in the yards, which made it doubtful that such a locomotive would have been able to be used to its full capacity.^[2][5]

Although the Class 22 was never built, the class number was not used for another steam locomotive type.^[5]

Service

After trials on the Randfontein-Mafeking line, no. 2551 was mainly used on the line from Pretoria to the Eastern Transvaal, where it took loads exceeding those given to the Class 15CA.^[2][3]

During the Second World War, this locomotive was often used to haul long and heavy military trains and it was made the official mascot of the oldest military unit in Pretoria, the Pretoria Regiment (Princess Alice's Own). The Class 21 was the only SAR locomotive to be honoured in this way by the military.^[8]

At the time, it seemed that ten-coupled engines could not be used as general utility types, suitable for the rather severe curvature which existed on SAR lines, particularly those of 45 pounds per yard (22 kilograms per metre) and 60 pounds per yard (30 kilograms per metre) track, where the curvature was generally more severe than elsewhere. A ten-coupled locomotive could only have 25% more tractive effort than an eight-coupled engine of similar axle load, and with really heavy loads, the Garratt type had definite advantages and was much less severe on the track.^[2]

The Class 21 was scrapped in 1952, after only 15 years in service. During its time in service, it was equipped with smoke deflectors. Neither the Class 21 nor the aborted Class 22 design was subsequently repeated in either its original or modified form, leaving the impression that they represented advanced thinking which appears to have been considered as too far and too fast by Watson's successors.^[3]

Illustration

The main picture shows no. 2551 with smoke deflectors installed, c. 1945, while the following show both sides of the locomotive in the factory gray livery in which it was delivered.

References

Wikimedia Commons has media related to South African Class 21 (2-10-4).

Locomotives of South Africa
Steam	7' ¼" gauge East London 0-4-0VB Table Bay 0-4-0ST Table Bay 0-4-0T Table Bay 0-4-0WT 4' 8½" gauge Cape 0-4-0T Blackie Cape 0-4-2 Cape 2-4-0T CGR 0-4-0ST Aid Natal 0-4-0ST Durban Natal 0-4-0WT Natal Natal 4-4-0T Perseverance 3' 6" gauge Tank engines CGR 0-4-0ST 1873 CGR 0-4-0ST 1874 CGR 0-4-0ST 1881 CGR 0-4-0ST 1881 Coffee Pot CGR 0-6-0ST CGR 0-6-0T CGR 2-6-0ST 1900 CGR 2-6-0ST 1902 CGR 1st Class 0-4-0ST 1875 CGR 1st Class 0-4-0ST 1876 CGR 1st Class 2-6-0ST CGR 1st Class 4-4-0T CGR 2nd Class 4-4-0T CGR 3rd Class 2-6-0T CGR Railmotor CSAR Class C CSAR Class E CSAR Rack 4-6-4RT CSAR Railmotor Durban's Congella Durban's Edward Innes Durban's John Milne Durban's Sir Albert Port Elizabeth 0-4-0ST Kowie Railway 0-6-0T Kowie Railway 4-4-0T Metropolitan & Suburban 4-6-2T NGR 2-6-0T Durban & Pietermaritzburg NGR Class I 2-6-2T NGR Class K 0-4-0ST NGR Class K 0-6-0ST NGR Class K 2-6-0T NZASM 10 Tonner NZASM 13 Tonner NZASM 14 Tonner NZASM 18 Tonner NZASM 19 Tonner NZASM 32 Tonner NZASM 40 Tonner PPR 26 Tonner PPR 35 Tonner Portuguese SAR Class A SAR Class B SAR Class C SAR Class C1 SAR Class C2 SAR Class D SAR Class E SAR Class F SAR Class G SAR Class H SAR Class H1 SAR Class H2 SAR Class J SAR Class K SAR Railmotor SAR Dock 0-4-0ST SAR Dock 0-4-0T SWA 2-8-0T 3' 6" gauge Tender engines CGR 1st Class 2-6-0 1876 BP CGR 1st Class 2-6-0 1876 Kitson CGR 1st Class 2-6-0 1879 CGR 1st Class 2-6-0 1891 CGR 1st Class 4-4-0 CGR 1st Class 4-4-0TT CGR 2nd Class 2-6-2TT CGR 3rd Class 4-4-0 1883 CGR 3rd Class 4-4-0 1884 CGR 3rd Class 4-4-0 1889 CGR 3rd Class 4-4-0 1898 CGR 3rd Class 4-4-0 1901 CGR 3rd Class 4-4-0 1903 CGR 4th Class 4-4-2 CGR 4th Class 4-6-0TT 1880 CGR 4th Class 4-6-0TT 1882 CGR 4th Class 4-6-0TT 1882 Joy CGR 4th Class 4-6-0TT 1884 CGR 5th Class 4-6-0 1890 CGR 5th Class 4-6-0 1891 NGR Havelock NGR Class I SAR Class Exp 1 SAR Class Exp 2 SAR Class Exp 3 SAR Class Exp 4 SAR Class Exp 5 SAR Class Exp 6 SAR Class S SAR Class S1 SAR Class S2 SAR Class 1 SAR Class 1A SAR Class 1B SAR Class 2 SAR Class 2C SAR Class 3 SAR Class 3A SAR Class 3B SAR Class 4 SAR Class 4A SAR Class 5 SAR Class 5A SAR Class 5B SAR Class 6 SAR Class 6A SAR Class 6B SAR Class 6C SAR Class 6D SAR Class 6E SAR Class 6F SAR Class 6G SAR Class 6H SAR Class 6J SAR Class 6K SAR Class 6L SAR Class 6Y SAR Class 6Z SAR Class 7 SAR Class 7A SAR Class 7B SAR Class 7C SAR Class 7D SAR Class 7E SAR Class 7F SAR Class 8 SAR Class 8A SAR Class 8B SAR Class 8C SAR Class 8D SAR Class 8E SAR Class 8F SAR Class 8R SAR Class 8X SAR Class 8Y SAR Class 8Z SAR Class 9 SAR Class 10 SAR Class 10A SAR Class 10B SAR Class 10C SAR Class 10D SAR Class 11 SAR Class 12 SAR Class 12A SAR Class 12B SAR Class 13 SAR Class 14 SAR Class 14A SAR Class 14B SAR Class 14C 1st SAR Class 14C 2nd SAR Class 14C 3rd SAR Class 14C 4th SAR Class 15 SAR Class 15A SAR Class 15B SAR Class 15C SAR Class 15CA SAR Class 15E SAR Class 15F SAR Class 16 SAR Class 16A SAR Class 16B SAR Class 16C SAR Class 16D SAR Class 16DA 1928 SAR Class 16DA 1930 SAR Class 16E SAR Class 17 SAR Class 18 SAR Class 19 SAR Class 19A SAR Class 19B SAR Class 19C SAR Class 19D SAR Class 20 SAR Class 21 SAR Class 23 SAR Class 24 SAR Class 25 SAR Class 25NC SAR Class 26 SAR Katanga SWA 2-8-0 3' 6" gauge Articulated engines CGR Fairlie CGR Kitson-Meyer SAR Class FC SAR Class FD SAR Class GA SAR Class GB SAR Class GC SAR Class GCA SAR Class GD SAR Class GDA SAR Class GE SAR Class GEA SAR Class GF SAR Class GG SAR Class GH SAR Class GK SAR Class GL SAR Class GM SAR Class GMA SAR Class GO SAR Class HF SAR Class KM SAR Class MA SAR Class MB SAR Class MC SAR Class MC1 SAR Class MD SAR Class ME SAR Class MF SAR Class MG SAR Class MH SAR Class MJ SAR Class MJ1 SAR Class U 2' 6" gauge Cape Copper John King & Miner Cape Copper Britannia Cape Copper Caledonia Cape Copper Clara Cape Copper Condenser Cape Copper Scotia Namaqua Copper Pioneer Walvis Bay 2-4-2T Hope 2' & 600mm gauges ACR Class NG-G16A CGR NG 0-4-0T CGR NG 0-6-0T CGR NG 4-6-2T CGR Type A 2-6-4T CGR Type C 0-4-0T NGR Class N 4-6-2T SAR Dutton SAR NG 0-4-0T SAR Class NG1 SAR Class NG2 SAR Class NG3 SAR Class NG4 SAR Class NG5 SAR Class NG6 SAR Class NG7 SAR Class NG8 SAR Class NG9 SAR Class NG10 SAR Class NG-G11 SAR Class NG-G12 SAR Class NG-G13 SAR Class NG-G14 SAR Class NG15 SAR Class NG-G16 SWA Class Ha SWA Class Hb SWA Class Hc SWA Class Hd SWA Jung SWA Zwillinge
Electric	Amcoal Class E38 SAR Class ES SAR Class ES1 SAR Class Exp-AC SAR Class 1E SAR Class 2E SAR Class 3E SAR Class 4E SAR Class 5E s1 SAR Class 5E s2 SAR Class 5E s3 SAR Class 5E1 s1 SAR Class 5E1 s2 SAR Class 5E1 s3 SAR Class 5E1 s4 SAR Class 5E1 s5 SAR Class 6E SAR Class 6E1 s1 SAR Class 6E1 s2 SAR Class 6E1 s3 SAR Class 6E1 s4 SAR Class 6E1 s5 SAR Class 6E1 s6 SAR Class 6E1 s7 SAR Class 6E1 s8 SAR Class 6E1 s9 SAR Class 6E1 s10 SAR Class 6E1 s11 SAR Class 7E SAR Class 7E1 SAR Class 7E2 s1 SAR Class 7E2 s2 SAR Class 7E3 s1 SAR Class 7E3 s2 Spoornet Class 7E4 SAR Class 8E SAR Class 9E s1 SAR Class 9E s2 SAR Class 10E SAR Class 10E1 s1 Spoornet Class 10E1 s2 SAR Class 10E2 SAR Class 11E SAR Class 12E Spoornet Class 14E Spoornet Class 14E1 TFR Class 15E Spoornet Class 16E Spoornet Class 17E Spoornet Class 18E s1 TFR Class 18E s2 TFR Class 19E TFR Class 20E TFR Class 21E TFR Class 22E
Gas-electric	SAR Gas-electric
Diesel	3' 6" gauge Diesel-electric PRASA Class Afro 4000 SAR Class DS SAR Class DS1 SAR Class 31-000 SAR Class 32-000 SAR Class 32-200 SAR Class 33-000 SAR Class 33-200 SAR Class 33-400 SAR Class 34-000 SAR Class 34-200 SAR Class 34-400 SAR Class 34-500 SAR Class 34-600 SAR Class 34-800 SAR Class 34-900 SAR Class 35-000 SAR Class 35-200 SAR Class 35-400 SAR Class 35-600 TFR Class 35-800 SAR Class 36-000 SAR Class 36-200 SAR Class 37-000 Spoornet Class 39-000 TFR Class 39-200 TFR Class 43-000 TFR Class 44-000 2' gauge Diesel-electric SAR Class 91-000 3' 6" gauge Diesel-hydraulic SAR Class 61-000 3' 6" gauge Electro-diesel Spoornet Class 38-000
List of South African locomotive classes South African locomotive numbering and classification

South African steam locomotive tenders
Cape gauge	Type CL Type CZ Type ET Type ET1 Type EW Type EW1 Type EW2 Type FT Type GT Type HT Type JT Type JT1 Type JV Type KT Type LP Type MP Type MP1 Type MR Type MS Type MT Type MT1 Type MT2 Type MX Type MY Type MY1 Type SH Type SK Type TJ Type TL Type TM Type WE Type WG Type X-17 Type X-20 Type XC Type XC1 Type XD Type XE Type XE1 Type XF Type XF1 Type XF2 Type XJ Type XM Type XM1 Type XM2 Type XM3 Type XM4 Type XP1 Type XS Type YB Type YC Type YE Type YE1 Type ZA Type ZB Type ZC Type ZE
List of South African locomotive classes South African locomotive history