South African Class MF 2-6-6-2

CSAR Mallet 2-6-6-2 Superheated
South African Class MF 2-6-6-2

300px SAR no. 1620, ex CSAR no. 1016, c. 1920

Type and origin ♠ Locomotive numbers 1619-1627 ♥ Locomotive numbers 1629-1633 Power type Steam Designer American Locomotive Company Builder American Locomotive Company Serial number 49115-49123, 50039-50043 Model CSAR Mallet Build date 1910-1911 Total produced 14

Specifications Configuration 2-6-6-2 (Prairie Mallet) Driver axle 3rd & 6th coupled axles Gauge 3 ft 6 in (1,067 mm) Cape gauge Leading dia Coupled dia 46 in (1,168 mm) Trailing dia Tender wheels ♠ 34 in (864 mm) ♥ 30 in (762 mm) Wheelbase ♠ 65 ft 6 in (19,964 mm)  • Engine 40 ft 3 in (12,268 mm)  • Coupled 8 ft 4 in (2,540 mm) per unit  • Tender ♠ 17 ft 11 in (5,461 mm) ♥ 17 ft 10 in (5,436 mm)  • Tender bogie ♠ 4 ft 7 in (1,397 mm) ♥ 5 ft 10 in (1,778 mm) Length:  • Over couplers ♠ Height Frame type Bar Axle load  • Leading  • 1st coupled  • 2nd coupled  • 3rd coupled  • 4th coupled  • 5th coupled  • 6th coupled  • Trailing  • Tender bogie ♠ each ♥ av.  • Tender axle ♠ ♥ av. Adhesive weight Loco weight Tender weight ♠ ♥ Loco & tender weight ♠ ♥ Tender type ♠ XS (2-axle bogie) ♥ XM4 (2-axle bogie) Fuel type Coal Fuel capacity ♠♥ 10 LT (10.2 t) Water cap ♠ ♥ Boiler:  • Pitch 7 ft 9 in (2,362 mm)  • Diameter  • Tube plates  • Small tubes 151:  • large tubes 25:  • Firebox type Round-top Boiler pressure 200 psi (1,379 kPa) Safety valve Ramsbottom Firegrate area 49.5 sq ft (4.60 m2) Heating surface 2,615.6 sq ft (243.00 m2)  • Tubes 2,459.6 sq ft (228.50 m2)  • Firebox 156 sq ft (14.5 m2) Superheater Schmidt  • Heating area 559 sq ft (51.9 m2) Cylinders Four High-pressure cylinder size 18 in (457 mm) bore 26 in (660 mm) stroke Low-pressure cylinder size bore 26 in (660 mm) stroke Valve gear Walschaerts Couplers Bell link-and-pin

Performance figures Tractive effort 45,900 lbf (204 kN) @ 50%

Career Operators Central South African Railways South African Railways Class Class MF Number in class 14 Numbers CSAR 1015-1023 SAR 1619-1627, 1629-1633 Delivered 1911 First run 1911 Withdrawn 1939

The South African Railways Class MF 2-6-6-2 of 1911 was a steam locomotive from the pre-Union era in the Transvaal.

In 1911, the Central South African Railways placed nine Mallet articulated compound steam locomotives with a 2-6-6-2 wheel arrangement in service. In 1912, when they were assimilated into the South African Railways, they were renumbered and designated Class MF. Five more of these locomotives were delivered in November 1911 and were numbered directly onto the South African Railways roster.

Manufacturer

Nine Mallet articulated compound steam locomotives, which were delivered to the Central South African Railways (CSAR) in 1911, were built by the American Locomotive Company (ALCO) in 1910 and 1911. They were erected at the Durban workshops in March 1911 and were very similar to the experimental Class MD which had been delivered in 1910, except that the new locomotives were equipped with Schmidt superheaters. The engines had Walschaerts valve gear and Type XS tenders, with a coal capacity of 10 long tons (10.2 tonnes) and a water capacity of . They were numbered in the range from 1015 to 1023.

Compound expansion

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, via the superheater. Their spent steam is then fed to the larger low pressure cylinders, which drive the front set of coupled wheels.

Characteristics

The most noticeable external difference from the experimental Class MD engine was the arrangement of the main steam pipes, necessitated by the superheater. Instead of being led vertically down from the dome directly to the high pressure cylinders, an internal pipe was taken from the regulator valve in the dome to the superheater in the smokebox, from where it was brought back to the high pressure cylinders by external steam pipes, arranged under the running boards.

South African Railways

When the Union of South Africa was established on 31 May 1910, the three Colonial government railways (Cape Government Railways, Natal Government Railways and CSAR) were united under a single administration to control and administer the railways, ports and harbours of the Union. Although the South African Railways and Harbours came into existence in 1910, with Sir William Hoy appointed as its first General Manager, the actual classification and renumbering of all the rolling stock of the three constituent railways required careful planning and was only implemented with effect from 1 January 1912.

In 1912, these locomotives were renumbered in the range from 1619 to 1627 and designated Class MF on the South African Railways (SAR). Five more of the type had been delivered from ALCO in November 1911, but these were numbered in the range from 1629 to 1633, directly onto the SAR roster, marking the introduction of the SAR renumbering scheme under the Union programme. These locomotives were delivered with smaller Type XM4 tenders, with the same coal capacity as the Type XS tenders of the first nine engines, but a smaller water capacity of .

The new locomotives were erected in the workshops in Pretoria, rather than Durban. The erection of engines away from ports was an innovation which was probably the outcome of experience gained with the Class MD, which had been erected at Durban. While the locomotive was brought to the Transvaal, difficulty was experienced at Drummond and Laing's Nek, where the drain angles on the cab roof struck the sides of the tunnels.

Service

The Class MF joined the experimental Class MD on the coal traffic line between Witbank and Germiston. They were acquired to improve the traffic flow on this line, with its ruling gradient of one in one hundred (1%), where the increase of traffic and the resultant congestion was causing considerable delays en route, which led to excessive hours of duty being imposed on crews. When the Mallets replaced the existing Class 11 locomotives on this line, train loads could be increased from 900 long tons (914 tonnes) to 1,600 long tons (1,626 tonnes).

The Mallets proved to be capable of handling the heavy coal loads. In May 1914, Class MF no. 1620 was used in a test run from Witbank to Germiston, hauling fifty-five bogie wagons with an all-up weight of 1,626 long tons (1,652 tonnes). The 80 miles (129 kilometres) journey was accomplished in seven hours, which included several stops.

Modification to simplex

In 1923, engine no. 1620 was converted to a simplex locomotive in the Salvokop shops in Pretoria. Five more, numbers 1621 to 1623, 1631 and 1632, were also converted to simplex locomotives in 1925. Three of these, numbers 1620, 1631 and 1632, were then transferred to East London, where more powerful locomotives were needed, since the mainline had a heavy gradient of one in forty (2½%) which started right at the end of the East London departing platform, making it an extremely difficult section. They appear to have not been up to the task in the Eastern Cape, however, since by 1926 they were observed there, staged out of service.

They were all withdrawn from service and scrapped by 1939.

Illustration

The pictures serve to illustrate the compound and simplex versions of the Class MF.

• Class MF no. 1620 a.jpg

  SAR no. 1620, ex CSAR no. 1016 as compound engine, c. 1920

• Class MF no. 1623.jpg

  SAR no. 1623 as simplex engine, c. 1930

• 

  SAR no. 1627, ex CSAR no. 1023 as compound engine, c. 1912