South African Class FD 2-6-2+2-6-2

South African Class FD 2-6-2+2-6-2
Class FD no. 2323, later renumbered no. 674
Type and origin Power type Steam Designer North British Locomotive Company Builder North British Locomotive Company Serial number 23294-23297 Model Class FD Build date 1925 Total produced 4
Specifications Configuration: ​  • Whyte 2-6-2+2-6-2 (Double Prairie)  • UIC 1'C1'+1'C1'h4t Driver 3rd & 4th 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) Wheelbase 58 ft 7 in (17,856 mm) ​  • Engine 18 ft 7+1⁄2 in (5,677 mm) each  • Coupled 8 ft 6 in (2,591 mm) each Pivot centres 36 ft 6 in (11,125 mm) Length: ​  • Over couplers 65 ft 7+3⁄4 in (20,009 mm) Height 12 ft 9+1⁄4 in (3,893 mm) Frame type Plate Axle load 12 LT 7 cwt (12,550 kg) ​  • Leading 10 LT 11 cwt (10,720 kg) front 10 LT 4 cwt (10,360 kg) rear  • 1st coupled 12 LT 7 cwt (12,550 kg)  • 2nd coupled 12 LT 3 cwt (12,340 kg)  • 3rd coupled 12 LT 5 cwt (12,450 kg)  • 4th coupled 12 LT 1 cwt (12,240 kg)  • 5th coupled 12 LT (12,190 kg)  • 6th coupled 12 LT 1 cwt (12,240 kg)  • Trailing 10 LT 16 cwt (10,970 kg) front 9 LT 14 cwt (9,856 kg) rear Adhesive weight 72 LT 17 cwt (74,020 kg) Loco weight 114 LT 2 cwt (115,900 kg) Fuel type Coal Fuel capacity 5 LT (5.1 t) Water cap. 2,500 imp gal (11,400 L) front 1,300 imp gal (5,910 L) rear Firebox: ​  • Type Belpaire  • Grate area 41 sq ft (3.8 m2) Boiler: ​  • Pitch 7 ft 6+1⁄4 in (2,292 mm)  • Diameter 6 ft (1,829 mm)  • Tube plates 10 ft 8 in (3,251 mm)  • Small tubes 190: 2 in (51 mm)  • Large tubes 32: 5+1⁄2 in (140 mm) Boiler pressure 180 psi (1,241 kPa) Safety valve Ramsbottom Heating surface: ​  • Firebox 178 sq ft (16.5 m2)  • Tubes 1,552 sq ft (144.2 m2)  • Total surface 1,730 sq ft (161 m2) Superheater: ​  • Heating area 371 sq ft (34.5 m2) Cylinders Four Cylinder size 15 in (381 mm) bore 24 in (610 mm) stroke Valve gear Walschaerts Valve type Piston Couplers Johnston link-and-pin AAR knuckle (1930s)
Performance figures Tractive effort 31,690 lbf (141.0 kN) @ 75%
Career Operators South African Railways Class Class FD Number in class 4 Numbers 2320-2323, renumbered 671-674 Delivered 1926 First run 1926 Withdrawn 1949

The South African Railways Class FD 2-6-2+2-6-2 of 1926 was an articulated steam locomotive.

In 1926, the South African Railways placed four Class FD Modified Fairlie articulated steam locomotives with a 2-6-2+2-6-2 Double Prairie type wheel arrangement in service.^[1][2][3]

The Class FD Modified Fairlie locomotive was designed and built for the South African Railways (SAR) by the North British Locomotive Company in 1925, to the specifications of Colonel F.R. Collins DSO, Chief Mechanical Engineer of the SAR. Four locomotives were delivered in 1926, numbered in the range from 2320 to 2323. They were later renumbered in the range from 671 to 674.^[1][2][3][4]

The Class FD was a heavier and more powerful version of the experimental Class FC which had been placed in service in 1925. It was the Modified Fairlie equivalent of the Class GD 2-6-2+2-6-2 Garratt which was similar in both size and mechanical respects, hence the Class FD designation of these Modified Fairlies. The FA and FB classifications were never used by the SAR. The Class FD locomotives had Walschaerts valve gear, plate frames and were superheated. They proved to be powerful locomotives and good steamers, but they were less successful than their Garratt equivalent.^[1][2][3]

They suffered from the same shortcomings as their Class FC predecessor. The 65 feet (19,812 millimetres) long rigid frame resulted in severe overhang on sharp curves and was also prone to metal fatigue and cracking, brought about by the long frame overhangs at the front and back beyond the engine unit pivot centres. The overhangs, laden with the water and coal bunkers of which about two-thirds of each extended beyond the respective pivot centres, tended to oscillate in an up-and-down motion while the locomotive was in motion.^[1][5]

In addition to this rather serious defect, the pivot bearings were also subject to quite rapid wear since they carried a considerable additional load compared to those on the Garratt as a result of the water and coal bunkers which were mounted on the main frame instead of on the engine units. This resulted in increased frequency of maintenance and as a consequence, increased operating cost.^[1][3][5]

The locomotives joined the sole Class FC in service on the Cape Midland System, working on the Grahamstown branch. They were scrapped by 1949.^[1][3]

The long rigid underframe of one of the Class FD locomotives was used to construct a footbridge across the single track at Poet’s Corner Halt on the old Natal mainline between Pinetown and Rossburgh junction in Durban. The underframe forms the main girder of the footbridge.^[6]

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  Class FD at Port Alfred, 3 September 1942

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