| South African Class MD 2-6-6-2 | |
|---|---|
| CSAR 1001, later SAR Class MD 1617, circa 1910 | |
| Power type | Steam |
| Designer | American Locomotive Company |
| Builder | American Locomotive Company |
| Serial number | 46581[1] |
| Model | CSAR Mallet |
| Build date | 1910 |
| Total produced | 1 |
| Configuration | 2-6-6-2 Mallet |
| Gauge | 3 ft 6 in (1,067 mm) Cape gauge |
| Leading wheel diameter |
28.5 in (724 mm) |
| Driver diameter | 46 in (1,170 mm) |
| Trailing wheel diameter |
28.5 in (724 mm) |
| Wheelbase | Total: 65 ft 6 in (19.964 m) Engine: 8 ft 4 in (2.540 m) per coupled set 40 ft 3 in (12.268 m) total Tender: 4 ft 7 in (1.397 m) bogie 17 ft 11 in (5.461 m) total |
| Length | 73 ft 7.25 in (22.435 m) |
| Height | 12 ft 10.375 in (3.921 m) |
| Frame | Bar frame |
| Axle load | 15.025 long tons (15.3 t) on 4th driver |
| Weight on drivers | 86.6 long tons (88.0 t) |
| Locomotive weight | 100.725 long tons (102.3 t) |
| Tender weight | 56.7 long tons (57.6 t) |
| Locomotive & tender combined weight |
157.425 long tons (160.0 t) |
| Tender type | 2 axle bogie Wheels 33.5 in (851 mm) dia |
| Fuel type | Coal |
| Fuel capacity | 10 long tons (10.2 t) |
| Water capacity | 5,000 imp gal (23,000 l) |
| Boiler | 6 ft 0.125 in (1.832 m) int dia 19 ft 10.125 in (6.048 m) int length 7 ft 9 in (2.362 m) pitch |
| Boiler pressure | 200 psi (1,380 kPa) |
| Firegrate area | 49.5 sq ft (4.599 m2) |
| Heating surface: Tubes |
271 tubes 2.25 in (57.2 mm) ext dia 3,169 sq ft (294.410 m2) |
| Heating surface: Firebox |
156 sq ft (14.493 m2) |
| Heating surface: Total |
3,325 sq ft (308.903 m2) |
| Cylinders | Four |
| High-pressure cylinder size |
18 in (457 mm) bore 26 in (660 mm) stroke |
| Low-pressure cylinder size |
28.5 in (724 mm) bore 26 in (660 mm) stroke |
| Valve gear | Walschaerts[2] |
| Tractive effort | 45,900 lbf (204.2 kN) at 50% boiler pressure[1] |
| Career | Central South African Railways South African Railways |
| Class | Class MD |
| Number in class | 1 |
| Number | CSAR 1001 SAR 1617[1][3][4] |
| Delivered | 1910 |
| First run | 1910 |
| Withdrawn | 1926 |
| Disposition | Retired |
In March 1910 the Central South African Railways placed a single experimental Mallet articulated compound steam locomotive with a 2-6-6-2 wheel arrangement in service. In 1912, when it was assimilated into the South African Railways, it was renumbered and classified as Class MD.[1][2][3]
Contents |
The first Mallet articulated compound steam locomotive on the Central South African Railways (CSAR) was ordered for test purposes from the American Locomotive Company in 1910. It was numbered 1001 and placed in service in March 1910, and with its full working order weight of 157 long tons (160 tonnes) it was the heaviest locomotive in the world working on Cape gauge at the time. It had Walschaerts valve gear and used saturated steam.[1][2][3]
In a compound locomotive steam is expanded in phases. After being expanded in a high pressure cylinder and having then lost pressure and given up part of its heat, it is exhausted into a larger volume low pressure cylinder for secondary expansion, after which it is exhausted through the smokebox. By comparison, in the more usual arrangement of simple expansion, steam is expanded just once in any one cylinder before being exhausted through the smokebox.[1][5]
In the compound Mallet locomotive, the rear set of coupled wheels are driven by the smaller high pressure cylinders which are fed steam from the steam dome. Their spent steam is then fed to the larger low pressure cylinders that drive the front set of coupled wheels.[1][2]
At low speed the locomotive proved to be capable of handling heavier trains than any of the existing CSAR fleet of goods locomotives, but at higher speeds of 25 to 30 miles per hour (40 to 48 kilometres per hour) it could not compete successfully because of excessive wear on the moving parts that resulted in failures and, as a consequence, high maintenance cost. It has been surmised that, had it been superheated and equipped with larger diameter driving wheels, the results may have been much better.[1]
The Mallet was acquired as an experiment, the ultimate object being to improve traffic flow on the 80 miles (129 kilometres) coal line between Witbank and Germiston. The increase of traffic and the resultant congestion on this line with its ruling gradient of one in one hundred was causing considerable delays en route that led to excessive hours of duty being imposed on crews. The intent was to determine the feasibility of replacing the existing Class 11 locomotives on this line with more powerful Mallets, in order to be able to increase train loads from 900 long tons (914 tonnes) to 1,600 long tons (1,626 tonnes).[1]
In 1912, when the locomotive was taken onto the South African Railways (SAR) roster, it was renumbered 1617 and classified as the sole Class MD. It remained in service on the line between Witbank and Germiston until it was withdrawn from service and scrapped in 1926.[1][2][3]
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