South African Class MD 2-6-6-2
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CSAR no. 1001, SAR no. 1617, c. 1910
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| Type and origin | |
|---|---|
| Power type | Steam |
| Designer | American Locomotive Company |
| Builder | American Locomotive Company |
| Serial number | 46581 |
| Model | CSAR Mallet |
| Build date | 1910 |
| Total produced | 1 |
| Specifications | |
|---|---|
| Configuration | 2-6-6-2 (Prairie Mallet) |
| Driver | 3rd & 6th coupled axles |
| Gauge | 3 ft 6 in (1,067 mm) Cape gauge |
| Leading dia. | 28 1⁄2 in (724 mm) |
| Coupled dia. | 46 in (1,168 mm) |
| Trailing dia. | 28 1⁄2 in (724 mm) |
| Tender wheels | 33 1⁄2 in (851 mm) |
| Minimum curve | 350 ft (107 m) |
| Wheelbase | 65 ft 6 in (19,964 mm) |
| • Engine | 40 ft 3 in (12,268 mm) |
| • Coupled | 8 ft 4 in (2,540 mm) each |
| • Tender | 17 ft 11 in (5,461 mm) |
| • Tender bogie | 4 ft 7 in (1,397 mm) |
| Length: |
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| • Over couplers | 73 ft 7 1⁄4 in (22,435 mm) |
| Height | 12 ft 10 3⁄8 in (3,921 mm) |
| Frame type | Bar |
| Axle load | 15 LT 0 cwt 2 qtr (15,270 kg) |
| • Leading | 7 LT 14 cwt 3 qtr (7,862 kg) |
| • 1st coupled | 14 LT 9 cwt 3 qtr (14,720 kg) |
| • 2nd coupled | 14 LT 2 cwt 2 qtr (14,350 kg) |
| • 3rd coupled | 13 LT 17 cwt 1 qtr (14,080 kg) |
| • 4th coupled | 15 LT 0 cwt 2 qtr (15,270 kg) |
| • 5th coupled | 14 LT 11 cwt 1 qtr (14,800 kg) |
| • 6th coupled | 14 LT 10 cwt 3 qtr (14,770 kg) |
| • Trailing | 6 LT 7 cwt 3 qtr (6,490 kg) |
| • Tender axle | 14 LT 3 cwt 1 qtr (14,390 kg) av. |
| Adhesive weight | 86 LT 12 cwt (87,990 kg) |
| Loco weight | 100 LT 14 cwt 2 qtr (102,300 kg) |
| Tender weight | 56 LT 14 cwt (57,610 kg) |
| Total weight | 157 LT 8 cwt (159,900 kg) |
| Tender type | 2-axle bogies |
| Fuel type | Coal |
| Fuel capacity | 10 LT (10.2 t) |
| Water cap | 5,000 imp gal (23,000 l) |
| Firebox type | Round-top |
| • Firegrate area | 49.5 sq ft (4.60 m2) |
| Boiler: |
|
| • Pitch | 7 ft 9 in (2,362 mm) |
| • Diameter | 6 ft 1⁄8 in (1,832 mm) |
| • Tube plates | 19 ft 10 1⁄8 in (6,048 mm) |
| • Small tubes | 271: 2 1⁄4 in (57 mm) |
| Boiler pressure | 200 psi (1,379 kPa) |
| Safety valve | Ramsbottom |
| Heating surface | 3,325 sq ft (308.9 m2) |
| • Tubes | 3,169 sq ft (294.4 m2) |
| • Firebox | 156 sq ft (14.5 m2) |
| Cylinders | Four |
| High-pressure cylinder | 18 in (457 mm) bore 26 in (660 mm) stroke |
| Low-pressure cylinder |
28 1⁄2 in (724 mm) bore 26 in (660 mm) stroke |
| Valve gear | Walschaerts |
| Valve type | HP Piston, LP Slide |
| Valve travel | HP 5 in (127 mm) LP 5 1⁄2 in (140 mm) |
| Valve lap | HP 1 in (25 mm) LP 7⁄8 in (22 mm) |
| Couplers | Johnston link-and-pin |
| Performance figures | |
|---|---|
| Tractive effort | 45,900 lbf (204 kN) @ 50% |
| Career | |
|---|---|
| Operators |
Central South African Railways South African Railways |
| Class | Class MD |
| Number in class | 1 |
| Numbers | CSAR 1001, SAR 1617 |
| Delivered | 1910 |
| First run | 1910 |
| Withdrawn | 1926 |
The South African Railways Class MD 2-6-6-2 of 1910 was a steam locomotive from the pre-Union era in Transvaal.
In March 1910, the Central South African Railways commissioned a single experimental Mallet articulated compound steam locomotive with a 2-6-6-2 wheel arrangement. In 1912, when the locomotive was assimilated into the South African Railways, it was renumbered and designated Class MD.
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. 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.
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 (simplex), steam is expanded just once in any one cylinder before being exhausted through the smokebox.
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 which drive the front set of coupled wheels.
The bar frames of the rear engine unit and its high-pressure cylinders were rigidly attached to the boiler, with the engine’s wide firebox extending over the coupled wheels. The front engine unit and its low-pressure cylinders were not rigidly attached to the boiler barrel, but carried the boiler on a single sliding bearing which was placed between the leading and intermediate coupled wheels. The underside of the smokebox was flattened to provide clearance between it and the cylinder assemblies of the front engine unit. The two engine units were coupled together by a single vertical pin connection, arranged at a point on the centre-line of the high-pressure cylinder saddle casting. This arrangement gave the locomotive a short rigid wheelbase of 8 feet 4 inches (2,540 millimetres) which made it capable of traversing curves of short radius.
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