| South African Class GA 2-6-0+0-6-2 | |
|---|---|
| Class GA 1649, later renumbered 2140, circa 1921 | |
| Power type | Steam |
| Designer | Beyer, Peacock and Company |
| Builder | Beyer, Peacock and Company |
| Serial number | 5941[1][2] |
| Model | Class GA |
| Build date | 1920 |
| Total produced | 1 |
| Configuration | 2-6-0+0-6-2 "Double Mogul" Garratt |
| Gauge | 3 ft 6 in (1,067 mm) Cape gauge |
| Leading wheel diameter |
30 in (762 mm) |
| Driver diameter | 48 in (1,220 mm) |
| Wheelbase | Total: 58 ft 7 in (17.856 m) Engines: 9 ft (2.743 m) coupled 16 ft 2 in (4.928 m) total |
| Length | 65 ft 6 in (19.964 m) |
| Height | 12 ft 10 in (3.912 m) |
| Frame | Plate frame, 33 ft 3 in (10.135 m) between pivot centres |
| Axle load | 17.8 long tons (18.1 t) on 5th driver |
| Weight on drivers | 104.7 long tons (106.4 t) |
| Locomotive weight | 133.85 long tons (136.0 t) |
| Fuel type | Coal |
| Fuel capacity | 9 long tons (9.1 t) |
| Water capacity | 3,350 imp gal (15,200 l) front 1,250 imp gal (5,700 l) rear |
| Boiler | 6 ft 9 in (2.057 m) int dia 11 ft 8.25 in (3.562 m) int length 7 ft 9 in (2.362 m) pitch |
| Boiler pressure | 180 psi (1,240 kPa) |
| Firegrate area | 51.8 sq ft (4.812 m2) |
| Heating surface: Tubes |
279 tubes 2 in (50.8 mm) ext dia 40 tubes 5.25 in (133 mm) ext dia 2,342.2 sq ft (217.598 m2) |
| Heating surface: Firebox |
211.3 sq ft (19.630 m2) |
| Heating surface: Total |
2,554.5 sq ft (237.321 m2) |
| Superheater area | 526.5 sq ft (48.913 m2) |
| Cylinders | Four |
| Cylinder size | 18 in (457 mm) bore 26 in (660 mm) stroke |
| Valve gear | Walschaerts[3] |
| Tractive effort | 47,390 lbf (210.8 kN) at 75% boiler pressure |
| Career | South African Railways |
| Class | Class GA |
| Number in class | 1 |
| Number | 1649, renumbered 2140 |
| Delivered | 1921 |
| First run | 1921 |
| Withdrawn | 1938 |
| Disposition | Retired |
In February 1921 the South African Railways placed a single experimental Class GA Garratt articulated steam locomotive with a 2-6-0+0-6-2 wheel arrangement in service. It was the first Cape gauge Garratt to enter service in South Africa.[1][3]
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The Class GA experimental main line articulated locomotive was one of altogether five Garratt locomotives ordered by the South African Railways (SAR) from Beyer, Peacock and Company (BP) in 1914. The rest consisted of the first of the eventual seven Class GB 2-6-2+2-6-2 branch line locomotives and three narrow gauge Class NG G11 2-6-0+0-6-2 locomotives. Production was disrupted by World War I, however, and BP was only able to deliver the narrow gauge locomotives in 1919 and the Cape gauge locomotives in 1920, after cessation of hostilities.[1][3]
The Class GA was numbered 1649, but the running number was later changed to 2140. The locomotive was erected in the Durban shops and placed in service in February 1921. It was superheated, with a Belpaire firebox, a plate frame and Walschaerts valve gear.[1][3][4]
A powerful steam locomotive is problematic on Cape gauge single line light rail track. Train lengths were limited on such lines because conventional locomotives had been enlarged to the limit of their possible power due to restrictions on axle loading. Alternative solutions would either be double-heading longer trains or re-building and re-laying large parts of the lines to accommodate heavier locomotives. Either method was expensive, especially across the type of mountainous terrain that was enountered inland from all South Africa’s major ports, and in such conditions the Garratt design had distinct advantages.[5]
A Garratt is actually two separate engines combined in a double articulated format, thereby providing multiple powered axles over which the total locomotive weight is spread. This in turn results in a more powerful locomotive since, compared to a tender locomotive of similar total mass, a much larger percentage of the locomotive’s total mass contributes to traction.[5]
Unlike tender locomotives, Garratts are bi-directional, which eliminates the need for turntables or triangles, also known as wyes. The fact that they did not need to be run through to terminals to be turned around also made increased operational flexibility possible.[6]
Probably the greatest advantage of the Garratt is the fact that, with its boiler and grate area suspended between two engine units without the need to leave room for driving wheels and cylinders, wide and deep fireboxes with large grate areas and large diameter boilers were possible. On a Garratt the boiler could literally be dimensioned up to the full cross section of the loading gauge. With each set of cylinders and driving wheels constituting a separate engine unit, the end result was two locomotives in one with one huge boiler that needed only one crew. A Garratt is therefore a single locomotive with double the tractive effort and, with the weight distributed over a long and flexible multi-axle wheelbase, a lower axle load.[5][7]
However, the Garratt design has some inherent drawbacks, the first being a diminishing factor of adhesion over long distances. As water and coal is consumed, the weight over the drivers is reduced, thereby reducing their factor of adhesion, the ratio of weight on drivers to tractive effort. Therefore, as the weight on the drivers decreases, the locomotive has less adhesion and becomes increasingly prone to slipping. With Cape gauge Garratts this was usually overcome by the use of auxiliary water tankers behind the locomotives, which enabled the onboard water tanks to remain filled longer and hence kept factor of adhesion high farther.[3][8][9]
Another drawback is the risk of the locomotive tilting over on tight curves, especially on 24 inches (610 millimetres) narrow gauge.[8]
The Class GA, which had apparently been designed as a direct competitor to the Class MH 2-6-6-2 Mallet, was placed in trial service on the Natal main line. It closely matched the Mallet in terms of tractive effort, boiler capacity, grate area and axle loading, which made it a locomotive of equal power but with 46 long tons (46.7 tonnes) less weight. Tests were carried out with varying loads on various sections of the line, while comparative tests were carried out with the Class MH Mallets.[1][3][8]
The Garratt took greater loads than the Mallet and its running times were better, while its coal and water consumption were lower. With the Garratt’s superiority established, no further Mallet locomotives were ordered by the SAR. During the remaining years of the South African steam traction era, whenever the use of an articulated locomotive was desirable for flexibility, reduced axle loading and high tractive force, the Garratt type was chosen.[1][3]
During the trials it was found that the absence of trailing carrying wheels on the engine units was a disadvantage since it led to excessive flange wear on the driving wheels. As a result number 2140 remained the only representative of its Class. All subsequent Garratt models of the SAR were equipped with trailing Bissel trucks.[1][3]
The locomotive worked across Van Reenen’s Pass between Ladysmith and Harrismith for most of the rest of its service life, until 1938 when it was withdrawn from service because of a cracked frame and scrapped.[3]
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