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Specifications
Specifications
Configuration:;
• Whyte	2-6-6-2 (Prairie Mallet)
• UIC	(1'C)C1'hv4
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.	48 in (1,219 mm)
Trailing dia.	33+1⁄2 in (851 mm)
Tender wheels	34 in (864 mm)
Wheelbase	70 ft 10+1⁄4 in (21,596 mm)
• Engine	43 ft 7 in (13,284 mm)
• Coupled	8 ft 8 in (2,642 mm) per unit
• Tender	16 ft 9 in (5,105 mm)
• Tender bogie	4 ft 7 in (1,397 mm)
Length:;
• Over couplers	79 ft 5 in (24,206 mm)
Height	12 ft 10 in (3,912 mm)
Frame type	Bar & Plate
Axle load	18 LT 4 cwt (18,490 kg)
• Leading	7 LT 9 cwt (7,570 kg)
• 1st coupled	16 LT 16 cwt (17,070 kg)
• 2nd coupled	16 LT 19 cwt (17,220 kg)
• 3rd coupled	17 LT 6 cwt (17,580 kg)
• 4th coupled	18 LT 4 cwt (18,490 kg)
• 5th coupled	18 LT 4 cwt (18,490 kg)
• 6th coupled	18 LT 4 cwt (18,490 kg)
• Trailing	15 LT 3 cwt (15,390 kg)
• Tender bogie	Bogie 1: 27 LT 10 cwt (27,940 kg); Bogie 2: 23 LT 11 cwt (23,930 kg)
• Tender axle	13 LT 15 cwt (13,970 kg)
Adhesive weight	105 LT 13 cwt (107,300 kg)
Loco weight	128 LT 5 cwt (130,300 kg)
Tender weight	51 LT 1 cwt (51,870 kg)
Total weight	179 LT 6 cwt (182,200 kg)
Tender type	MP1 (2-axle bogies)
Fuel type	Coal
Fuel capacity	10 LT (10.2 t)
Water cap.	4,250 imp gal (19,300 L)
Firebox:
• Type	Round-top
• Grate area	53 sq ft (4.9 m2)
Boiler:;
• Pitch	7 ft 10+1⁄2 in (2,400 mm)
• Diameter	5 ft 11 in (1,803 mm)
• Tube plates	22 ft (6,706 mm)
• Small tubes	168: 2+1⁄4 in (57 mm)
• Large tubes	25: 5+1⁄2 in (140 mm)
Boiler pressure	180 psi (1,241 kPa)
Safety valve	Ramsbottom
Heating surface:
• Firebox	250 sq ft (23 m2)
• Tubes	2,961 sq ft (275.1 m2)
• Total surface	3,211 sq ft (298.3 m2)
Superheater:;
• Heating area	634 sq ft (58.9 m2)
Cylinders	Four
High-pressure cylinder	20 in (508 mm) bore; 26 in (660 mm) stroke
Low-pressure cylinder	31+1⁄2 in (800 mm) bore; 26 in (660 mm) stroke
Valve gear	Walschaerts
Valve type	HP Piston; LP Richardson balanced slide
Couplers	Johnston link-and-pin; AAR knuckle (1930s)

Type and origin
Type and origin
Power type	Steam
Designer	South African Railways; (D.A. Hendrie)
Builder	North British Locomotive Company
Serial number	20958-20962
Model	SAR Class MH
Build date	1915
Total produced	5

South African Class MH 2-6-6-2
South African Class MH 2-6-6-2
	SAR Class MH no. 1661, c. 1915
Power type
Type and origin
Power type	Steam
Designer	South African Railways; (D.A. Hendrie)
Builder	North British Locomotive Company
Serial number	20958-20962
Model	SAR Class MH
Build date	1915
Total produced	5
Configuration:;
Specifications
Configuration:;
• Whyte	2-6-6-2 (Prairie Mallet)
• UIC	(1'C)C1'hv4
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.	48 in (1,219 mm)
Trailing dia.	33+1⁄2 in (851 mm)
Tender wheels	34 in (864 mm)
Wheelbase	70 ft 10+1⁄4 in (21,596 mm)
• Engine	43 ft 7 in (13,284 mm)
• Coupled	8 ft 8 in (2,642 mm) per unit
• Tender	16 ft 9 in (5,105 mm)
• Tender bogie	4 ft 7 in (1,397 mm)
Length:;
• Over couplers	79 ft 5 in (24,206 mm)
Height	12 ft 10 in (3,912 mm)
Frame type	Bar & Plate
Axle load	18 LT 4 cwt (18,490 kg)
• Leading	7 LT 9 cwt (7,570 kg)
• 1st coupled	16 LT 16 cwt (17,070 kg)
• 2nd coupled	16 LT 19 cwt (17,220 kg)
• 3rd coupled	17 LT 6 cwt (17,580 kg)
• 4th coupled	18 LT 4 cwt (18,490 kg)
• 5th coupled	18 LT 4 cwt (18,490 kg)
• 6th coupled	18 LT 4 cwt (18,490 kg)
• Trailing	15 LT 3 cwt (15,390 kg)
• Tender bogie	Bogie 1: 27 LT 10 cwt (27,940 kg); Bogie 2: 23 LT 11 cwt (23,930 kg)
• Tender axle	13 LT 15 cwt (13,970 kg)
Adhesive weight	105 LT 13 cwt (107,300 kg)
Loco weight	128 LT 5 cwt (130,300 kg)
Tender weight	51 LT 1 cwt (51,870 kg)
Total weight	179 LT 6 cwt (182,200 kg)
Tender type	MP1 (2-axle bogies)
Fuel type	Coal
Fuel capacity	10 LT (10.2 t)
Water cap.	4,250 imp gal (19,300 L)
Firebox:
• Type	Round-top
• Grate area	53 sq ft (4.9 m2)
Boiler:;
• Pitch	7 ft 10+1⁄2 in (2,400 mm)
• Diameter	5 ft 11 in (1,803 mm)
• Tube plates	22 ft (6,706 mm)
• Small tubes	168: 2+1⁄4 in (57 mm)
• Large tubes	25: 5+1⁄2 in (140 mm)
Boiler pressure	180 psi (1,241 kPa)
Safety valve	Ramsbottom
Heating surface:
• Firebox	250 sq ft (23 m2)
• Tubes	2,961 sq ft (275.1 m2)
• Total surface	3,211 sq ft (298.3 m2)
Superheater:;
• Heating area	634 sq ft (58.9 m2)
Cylinders	Four
High-pressure cylinder	20 in (508 mm) bore; 26 in (660 mm) stroke
Low-pressure cylinder	31+1⁄2 in (800 mm) bore; 26 in (660 mm) stroke
Valve gear	Walschaerts
Valve type	HP Piston; LP Richardson balanced slide
Couplers	Johnston link-and-pin; AAR knuckle (1930s)
Tractive effort
Performance figures
Tractive effort	48,370 lbf (215.2 kN) @ 50%
Operators
Career
Operators	South African Railways
Class	Class MH
Number in class	5
Numbers	1661–1665
Delivered	1915
First run	1915
Withdrawn	1940

The South African Railways Class MH 2-6-6-2 of 1915 was an articulated Mallet-design steam locomotive.

In 1915, the South African Railways placed five Class MH Mallet articulated compound steam locomotives with a 2-6-6-2 wheel arrangement in coal hauling service.^[1]^[2]^[3]

Manufacturer

During 1914, the requirement for locomotives with a high tractive effort to cope with the increasing volume of coal traffic between Witbank and Germiston led to the introduction of a heavy Mallet compound superheated engine with a 2-6-6-2 wheel arrangement.^[2]

The Class MH Mallet articulated locomotive was designed in detail in the locomotive drawing office in Pretoria under the personal direction of D.A. Hendrie, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1910 to 1922. The draughtsman, specially detached for the work, was J.R. Boyer who was later to become the Chief Locomotive Draughtsman of the SAR. Five of these very large locomotives were ordered from North British Locomotive Company and delivered in 1915, numbered in the range from 1661 to 1665. They were erected in the Salvokop shops in Pretoria and were placed in service in September 1915.^[1]^[2]^[3]^[4]

Characteristics

The main bar frames, 4+1⁄2 inches (114 millimetres) thick, were machined from a 2 ft 2+1⁄2 in (673 mm) wide solid. The hind part of this frame was rigidly secured to the boiler through the high-pressure cylinder saddle castings and terminated just in front of the firebox outer throat plate. From this point rearwards, the frame was of the plate type and arranged to carry the spring gear and other fittings for the trailing Bissel truck.^[1]^[2]^[3]

The locomotives were superheated and had Walschaerts valve gear, controlled by steam reversing gear. The cylinders and steam chests were formed in three separate castings. The high-pressure cylinders were arranged with piston valves, while the low-pressure cylinders were arranged with Richardson balanced type slide valves, arranged above the cylinders. The steam chest covers of the low-pressure cylinders were designed with inclined joint faces to facilitate the handling of the valve and refacing of the ports during servicing.^[2]

As built, the boiler pressure was set to blow off at 200 pounds per square inch (1,379 kilopascals), which gave the engine a tractive effort of 53,750 pounds-force (239.1 kilonewtons) at 50% of boiler pressure. The setting was later reduced to 180 pounds per square inch (1,241 kilopascals), which reduced the tractive effort to 48,370 pounds-force (215.2 kilonewtons) at 50% of boiler pressure.^[1]^[2]

At the time of their introduction, the Class MH was the largest and most powerful locomotive in the world on Cape gauge. It attracted the attention of locomotive engineers throughout the world as an outstanding achievement for locomotive power on 3 feet 6 inches (1,067 millimetres) gauge.^[1]^[2]

Its 105 long tons 13 hundredweight (107,300 kilograms) adhesive weight and the SAR's ultra-conservative practice of reporting a Mallet's tractive effort at only 50% of boiler pressure resulted in a much lower than actual starting tractive effort of 48,370 pounds-force (215.2 kilonewtons). The Class MH was almost certainly capable of exerting more than 60,000 pounds-force (270 kilonewtons) tractive effort at starting.^[5]

Service

They were initially placed in service on the coal line between Witbank and Germiston as intended to supplement the other Mallets already working on that line. In the 1930s they were transferred to Natal to work on the line between Vryheid and Glencoe, also hauling coal. They were outstanding in their performance and remained in Natal for the rest of their service lives until they were all retired and scrapped by 1940.^[1]^[3]^[5]

Illustration

Class MH 2-6-6-2 Mallet no. 1661, c. 1915

References

^ ^a ^b ^c ^d ^e ^f Holland, D. F. (1972). Steam Locomotives of the South African Railways. Vol. 2: 1910-1955 (1st ed.). Newton Abbott, England: David & Charles. pp. 30–32. ISBN 978-0-7153-5427-8.
^ ^a ^b ^c ^d ^e ^f ^g Espitalier, T.J.; Day, W.A.J. (1945). The Locomotive in South Africa – A Brief History of Railway Development. Chapter VII – South African Railways (Continued). South African Railways and Harbours Magazine, May 1945. pp. 350, 356.
^ ^a ^b ^c ^d Paxton, Leith; Bourne, David (1985). Locomotives of the South African Railways (1st ed.). Cape Town: Struik. p. 87. ISBN 0869772112.
^ North British Locomotive Company works list, compiled by Austrian locomotive historian Bernhard Schmeiser
^ ^a ^b Soul of A Railway, System 7, Western Transvaal, based in Johannesburg, Part 21: Witbank Line by Les Pivnic, Eugene Armer, Peter Stow and Peter Micenko. Captions 3–4. Archived 24 October 2020 at the Wayback Machine (Accessed on 4 May 2017)