South African Class MC 2-6-6-0

The South African Railways Class MC 2-6-6-0 of 1912 was a steam locomotive.

South African Class MC 2-6-6-0
SAR Class MC no. 1608, c. 1912
Type and origin Power type Steam Designer North British Locomotive Company Builder North British Locomotive Company Serial number 19577-19586 Model NGR Mallet Build date 1912 Total produced 10
Specifications Configuration: ​  • Whyte 2-6-6-0 (Denver)  • UIC (1'C)Cnv4 as built (1'C)Chv4 (no. 1612 & 1615) 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. 45+1⁄2 in (1,156 mm) Tender wheels 30 in (762 mm) Wheelbase 59 ft 3 in (18,059 mm) ​  • Engine 33 ft 5 in (10,185 mm)  • Coupled 8 ft 4 in (2,540 mm) per unit  • Tender 16 ft 6 in (5,029 mm)  • Tender bogie 4 ft 6 in (1,372 mm) Length: ​  • Over couplers 66 ft 9+3⁄4 in (20,364 mm) Height 12 ft 5+3⁄16 in (3,789 mm) Axle load 14 LT 13 cwt (14,890 kg) ​  • Leading 8 LT 15 cwt (8,890 kg)  • 1st coupled 14 LT 5 cwt (14,480 kg)  • 2nd coupled 14 LT 5 cwt (14,480 kg)  • 3rd coupled 14 LT 7 cwt (14,580 kg)  • 4th coupled 14 LT 10 cwt (14,730 kg)  • 5th coupled 14 LT 11 cwt (14,780 kg)  • 6th coupled 14 LT 13 cwt (14,890 kg)  • Tender bogie Bogie 1: 21 LT 14 cwt (22,050 kg) Bogie 2: 21 LT 16 cwt (22,150 kg)  • Tender axle 10 LT 18 cwt (11,070 kg) Adhesive weight 86 LT 11 cwt (87,940 kg) Loco weight 95 LT 6 cwt (96,830 kg) Tender weight 43 LT 10 cwt (44,200 kg) Total weight 138 LT 16 cwt (141,000 kg) Tender type TM (2-axle bogies) Fuel type Coal Fuel capacity 8 LT 5 cwt (8.4 t) Water cap. 4,000 imp gal (18,200 L) Firebox: ​  • Type Round-top  • Grate area 42.5 sq ft (3.95 m2) Boiler: ​  • Pitch 7 ft 6 in (2,286 mm)  • Diameter 5 ft 8 in (1,727 mm)  • Tube plates 16 ft 2+3⁄4 in (4,947 mm)  • Small tubes 258: 2+1⁄4 in (57 mm) Boiler pressure 200 psi (1,379 kPa) Safety valve Ramsbottom Heating surface: ​  • Firebox 154 sq ft (14.3 m2)  • Tubes 2,462 sq ft (228.7 m2)  • Total surface 2,616 sq ft (243.0 m2) Cylinders Four High-pressure cylinder 17+1⁄2 in (444 mm) bore 26 in (660 mm) stroke Low-pressure cylinder 28 in (711 mm) bore 26 in (660 mm) stroke Valve gear Walschaerts Valve type HP Piston, LP Slide Couplers Johnston link-and-pin
Performance figures Tractive effort 44,810 lbf (199.3 kN) @ 50%
Career Operators South African Railways Class Class MC Number in class 10 Numbers 1607-1616 Delivered 1912 First run 1912 Withdrawn 1933

In 1912, the South African Railways placed ten Class MC Mallet articulated compound steam locomotives with a 2-6-6-0 wheel arrangement in service in Natal.

Manufacturer

To augment the Mallet locomotive fleet operating across the more difficult sections of the Natal mainline, the South African Railways (SAR) placed an order with the North British Locomotive Company for ten compound steam locomotives which were very similar to the Class MB. They were delivered and placed in service in May 1912, designated Class MC and numbered in the range from 1607 to 1616.

Compounding

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.

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. By comparison, in the more usual arrangement of simple expansion steam is expanded just once in any one cylinder before being exhausted through the smokebox.

Characteristics

Like the previous Mallets, these ten compound locomotives had Walschaerts valve gear and used saturated steam. The high-pressure cylinders of the hind engine unit were equipped with piston valves while the low-pressure cylinders of the front engine unit were equipped with slide valves.

Their Type TM tenders were the same as those of the Classes 3 and 3B, with a coal capacity of 8 long tons 5 hundredweight (8.4 tonnes) and a water capacity of 4,000 imperial gallons (18,200 litres). The locomotives differed little from the previous Mallets in size, power and performance and may for all intents and purposes also have been classified as Class MB. It would seem that, compared to the Cape Government Railways which tended to group locomotives in the same Class which were dissimilar even to the extent of having different wheel arrangements, the early SAR at times took locomotive classification to the other extreme.

Unlike the Class MB on which the sandboxes were placed on top of the boiler in accordance with American practice, the sandboxes of the Class MC were arranged on a different system, the advantages of which were questionable. The sandboxes for the hind engine unit were secured to the running board just to the rear of the high-pressure cylinders, while those for the front engine unit were placed on the upper sides of the boiler just to the rear of the smokebox.

Superheating

The Class MC were satisfactory locomotives but like the earlier Mallet compounds, they would have performed better if they had been superheated. Two of them, numbers 1612 and 1615, were later equipped with new boilers with superheaters and their coupled wheels were retyred to a diameter of 46 inches (1,168 millimetres). No others were modified in this manner.

Service

The Class MC joined the Class MA and MB fleet on the Natal mainline, working heavy coal trains between Estcourt and Highlands. In later years, some were transferred to the Cape Western System where they served into the 1930s as banking locomotives up the Hex River Railpass between De Doorns and Touws River. Others were transferred to the Witwatersrand for general service and to haul coal on the Witbank-Germiston line. They were withdrawn from service during 1933.