South African Class MH 2-6-6-2 explained

South African Class MH 2-6-6-2
Powertype:Steam
Designer:South African Railways
(D.A. Hendrie)
Builder:North British Locomotive Company
Serialnumber:20958-20962
Buildmodel:SAR Class MH
Builddate:1915
Totalproduction:5
Whytetype:2-6-6-2 (Prairie Mallet)
Uicclass:(1'C)C1'hv4
Driver:3rd & 6th coupled axles
Leadingdiameter:NaN0NaN0
Coupleddiameter:480NaN0
Trailingdiameter:NaN0NaN0
Tenderdiameter:340NaN0
Wheelbase:70feet
Engine Total:43feet
Coupled:8feet per unit
Tender Total:16feet
Tenderbogie:4feet
Over Couplers:79feet
Height:12feet
Frametype:Bar & Plate
Axleload:18lt
Leadingbogie/Pony:7lt
Coupled 1:16lt
Coupled 2:16lt
Coupled 3:17lt
Coupled 4:18lt
Coupled 5:18lt
Coupled 6:18lt
Trail Bogie/Pony:15lt
Tenderbogieload:Bogie 1: 27lt
Bogie 2: 23lt
Tenderaxle:13lt
Weightondrivers:105lt
Locoweight:128lt
Tenderweight:51lt
Locotenderweight:179lt
Tendertype:MP1 (2-axle bogies)
Fueltype:Coal
Fuelcap:101NaN1
Watercap:4250sigfig=3NaNsigfig=3
Fireboxtype:Round-top
Firearea:53square feet
Pitch:7feet
Diameterinside:5feet
Lengthinside:220NaN0
Smalltubediameter:168: NaN0NaN0
Largetubediameter:25: NaN0NaN0
Boilerpressure:1800NaN0
Safetyvalvetype:Ramsbottom
Totalsurface:3211square feet
Tubearea:2961square feet
Fireboxarea:250square feet
Superheaterarea:634square feet
Cylindercount:Four
Hpcylindersize:200NaN0 bore
260NaN0 stroke
Lpcylindersize:NaN0NaN0 bore
260NaN0 stroke
Valvegear:Walschaerts
Valvetype:HP Piston
LP Richardson balanced slide
Coupling:Johnston link-and-pin
AAR knuckle (1930s)
Tractiveeffort:48370lbf @ 50%
Operator:South African Railways
Operatorclass:Class MH
Numinclass:5
Fleetnumbers:1661–1665
Deliverydate:1915
Firstrundate:1915
Withdrawndate: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]

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.[1]

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]

Characteristics

The main bar frames, NaN0NaN0 thick, were machined from a 2feet 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]

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.[1]

As built, the boiler pressure was set to blow off at 2000NaN0, which gave the engine a tractive effort of 53750lbf at 50% of boiler pressure. The setting was later reduced to 1800NaN0, which reduced the tractive effort to 48370lbf at 50% of boiler pressure.[1]

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 3feet gauge.[1]

Its 105lt 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 48370lbf. The Class MH was almost certainly capable of exerting more than 60000lbf tractive effort at starting.[3]

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.[3]

Notes and References

  1. 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.
  2. North British Locomotive Company works list, compiled by Austrian locomotive historian Bernhard Schmeiser
  3. https://sites.google.com/site/soulorailway/home/system-7-1/soul-of-a-railway-part-21-witbank-line-by-les-pivnic-eugene-armer-peter-stow-and-peter-micenko 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.