South African class 12A
The 4-8-2 locomotive of the class 12A was the last design by D.A. Hendrie for the South African Railways. It was based on the class 12 that had been introduced before World War I. The biggest change was that the area of the firebox was enlarged by 30 percent. All in all, they had the maximum size that was possible for a Mountain locomotive on cape gauge.
Between 1919 and 1929, North British built 48 and Henschel built 19 more. At the beginning, they were used for coal trains on the line between Witbank and Germiston. Later they were also used for passenger trains and spread over all South Africa. Starting in 1943, a total of 44 were rebuilt to class 12AR. Since there was no Watson standard boiler in a suitable size, they got a completely new boiler.
All in all, both the unrebuilt class 12A and the rebuilt class 12AR were reported to be very free steaming and powerful for a cape gauge Mountain. Between 1947 and 1953, North British built eight similar locomotives without superheater for industrial operators. In SAR service, the remaining class 12A and 12AR locomotives had all been moved to shunting service by 1977. The last ones were withdrawn in the eighties.
Technical data
General
| Property | 12A | rebuilt 12AR |
|---|---|---|
| Built | 1919-1929, 1947-1953 | |
| Manufacturers | North British, Henschel | |
| Wheel arrangement | 4-8-2 (Mountain) | |
| Gauge | 1,067 mm (Cape gauge) | |
Dimensions and weights
| Property | 12A | rebuilt 12AR |
|---|---|---|
| Length | 20.41 m | 21.21 m |
| Wheelbase | 9.78 m | |
| Total wheelbase | 17.86 m | 18.78 m |
| Service mass | 100.6 t | 100.8 t |
| Adhesion mass | 70.3 t | 70.1 t |
| Total mass | 152.5 t | 167.9 t |
| Rigid wheelbase | 4.12 m | |
| Axle load | 17.6 t | 17.7 t |
| Water capacity | 19,321 l | 27,278 l |
| Fuel capacity | 10.2 t (coal) | 12.2 t (coal) |
Boiler
| Property | 12A | rebuilt 12AR |
|---|---|---|
| Grate area | 3.763 m² | 3.809 m² |
| Firebox surface | 19.417 m² | 15.236 m² |
| Tube heating surface | 213.77 m² | 217.208 m² |
| Evaporative heating surface | 233.187 m² | 232.444 m² |
| Superheater surface | 43.293 m² | 44.594 m² |
| Total heating surface | 276.5 m² | 277.0 m² |
Power plant
| Property | 12A | rebuilt 12AR |
|---|---|---|
| Driver diameter | 1,295 mm | |
| Boiler pressure | 13.1 bar | |
| Expansion type | simple expansion | |
| Cylinders | two, 610 x 660 mm | |
Power
| Property | 12A | rebuilt 12AR |
|---|---|---|
| Power source | steam | |
| Estimated power | 969 kW (1,317 PS) | |
| Starting effort | 211.0 kN | |
Calculated values
| Property | 12A | rebuilt 12AR |
|---|---|---|
| Factor of adhesion | 1 : 3.27 | 1 : 3.26 |
| Optimal speed | 27 km/h | |
| Power-to-weight ratio | 9.6 kW/t | |
| Heating Surface Equivalent | 416.9 m² | 397.8 m² |
| Cylinder volume/HSE | 1 : 924.76 | 1 : 969.03 |
| Adhesive weight/Total | 1 : 1.43 | 1 : 1.44 |
| Tractive force/Heating surface | 1 : 15.94 | 1 : 15.90 |
| Grate area/Heating surface | 1 : 73.47 | 1 : 72.73 |
| Firebox/Tube surface | 1 : 11.01 | 1 : 14.26 |
| Total/Superheater | 1 : 5.39 | 1 : 5.21 |
