| South African Class 21 2-10-4 | |
|---|---|
| Archive picture of sole Class 21 2-10-4 2551, circa 1937 | |
| Power type | Steam |
| Designer | South African Railways |
| Builder | North British Locomotive Company |
| Serial number | 24379[1][2] |
| Model | Class 21 |
| Build date | 1937 |
| Total produced | 1 |
| Configuration | 2-10-4 "Texas" |
| Gauge | 3 ft 6 in (1,067 mm) Cape gauge |
| Leading wheel diameter |
30 in (762 mm) |
| Driver diameter | 54 in (1,370 mm) |
| Trailing wheel diameter |
30 in (762 mm) |
| Wheelbase | Total: 68 ft 4.0625 in (20.830 m) Engine: 18 ft 9 in (5.715 m) coupled 4 ft 6 in (1.372 m) trailing 36 ft 9 in (11.201 m) total Tender: 12 ft 3 in (3.734 m) rigid wheelset 22 ft 3 in (6.782 m) total |
| Length | 76 ft 10.0625 in (23.420 m) |
| Height | 13 ft (3.962 m) |
| Axle load | 14.95 long tons (15.2 t) on 3rd driver |
| Weight on drivers | 72.85 long tons (74.0 t) |
| Locomotive weight | 106.6 long tons (108.3 t) |
| Tender weight | 69,122 lb (31.4 t) empty 65.8 long tons (66.9 t) w/o |
| Locomotive & tender combined weight |
206,916 lb (93.9 t) empty 172.4 long tons (175.2 t) w/o |
| Tender type | FT * 2-8-2 wheel arrangement * Wheels 34 in (864 mm) dia * Length 32 ft 7.4375 in (9.943 m) |
| Fuel type | Coal |
| Fuel capacity | 10 long tons (10.2 t) |
| Water capacity | 5,587 imp gal (25,400 l) |
| Boiler | 6 ft 2.25 in (1.886 m) int dia 22 ft 6 in (6.858 m) int length 9 ft (2.743 m) pitch |
| Boiler pressure | 210 psi (1,450 kPa) |
| Firegrate area | 63 sq ft (5.853 m2) |
| Heating surface: Tubes |
136 tubes 2.5 in (63.5 mm) ext dia 36 tubes 5.5 in (140 mm) ext dia 3,168 sq ft (294.317 m2) |
| Heating surface: Flues |
26 sq ft (2.415 m2) |
| Heating surface: Firebox |
206 sq ft (19.138 m2) |
| Heating surface: Total |
3,400 sq ft (315.870 m2) |
| Superheater area | 676 sq ft (62.802 m2) |
| Cylinders | Two |
| Cylinder size | 24 in (610 mm) bore 26 in (660 mm) stroke |
| Valve gear | Walschaerts |
| Tractive effort | 45,700 lbf (203.3 kN) at 75% boiler pressure[3] |
| Career | South African Railways |
| Class | Class 21 |
| Number in class | 1 |
| Number | 2551 |
| Delivered | 1937 |
| First run | 1937 |
| Retired | 1952 |
| Disposition | Retired |
In 1937 the South African Railways placed one Class 21 steam locomotive with a 2-10-4 Texas wheel arrangement in service, designed as a mixed traffic locomotive suitable for light rail. A simultaneously proposed heavier main line version Class 22 2-10-4 never materialised.[1][3][4]
Contents |
The Class 21 2-10-4 steam 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 (NBL) 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 that was limited to a 15 long tons (15.2 tonnes) axle load on 60 pounds per yard (30 kilograms per metre) rail.[1][4]
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, the third and fourth drivers were flangeless. It used a Watson Standard no. 3B boiler, the same as that used in the Class 15E, Class 15F and Class 23.[1][4]
The end result could be considered as Watson’s answer to the Class GF 4-6-2+2-6-4 Garratt locomotive, having very similar weight and tractive capacities. 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][4][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 locomotive was delivered with Walschaerts valve gear.[1][4][5]
The tender was an unusual experimental type using 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. The tender’s wheel arrangement did not prove to be very successful and, with the exception of a similar tender built in the Salt River shops for test purposes, was not used again.[1][4]
The pony axle design is commonly used on steam locomotives with a single leading or trailing axle, and was also used on the Class 4E electric and Class 32-000 and 32-200 diesel-electric locomotives.[4]
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 be a heavy main line version of the Class 21, with an axle load of 22 long tons (22.4 tonnes), the heaviest that current SAR track could bear on its 96 pounds per yard (48 kilograms per metre) rail. It was to have been a massive machine with larger 60 inches (1,520 millimetres) driving wheels, a larger 80 square feet (7.432 square metres) grate and the larger Type EW tender that was later to be used with the Class 23 locomotive.[1][4][5]
If this locomotive 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 (1,720 kilopascals), a figure never attained on the SAR, and its anticipated tractive effort of 66,406 pounds-force (295.4 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][5]
The design was a compromise between a 2-8-4 passenger class with 66 inches (1,680 millimetres) driving wheels and a 2-10-2 freight locomotive with 60 inches (1,520 millimetres) driving 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 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.[5]
Although the Class 22 was never built, the class number was not used for another steam locomotive type.[5]
During its time in service, Class 21 number 2551 was equipped with smoke deflectors. It was mainly used on the line from Pretoria to the Eastern Transvaal and was scrapped in 1952 after only 15 years in service. 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 that appears to have been considered as too far and too fast by Watson’s successors.[4]
The main picture shows number 2551 as delivered, without smoke deflectors.
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