South African Class MG 2-6-6-2 - AbsoluteAstronomy.com

In 1911 the Central South African Railways placed a single experimental Mallet articulated compound steam locomotive

Compound locomotive

A compound engine unit is a type of steam engine where steam is expanded in two or more stages.A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger...

with a 2-6-6-2

2-6-6-2

In Whyte notation, 2-6-6-2 refers to a railroad steam locomotive that has two leading wheels followed by six coupled driving wheels, a second set of six coupled driving wheels, and two trailing wheels...

wheel arrangement in service. In 1912, when it was assimilated into the South African Railways, it was renumbered and classified as Class MG.

Manufacturer

A single experimental locomotive was included with the order for nine Mallet articulated compound steam locomotives that were delivered to the Central South African Railways (CSAR) by the American Locomotive Company

American Locomotive Company

The American Locomotive Company, often shortened to ALCO or Alco , was a builder of railroad locomotives in the United States.-Early history:...

(ALCO) in 1911. It was similar to the other nine locomotives, but used saturated steam, had a mechanical stoker and the coupled wheels on the leading engine unit were of a 5 inches (127 mm) larger diameter than those of the trailing engine unit. It had Walschaerts valve gear and was numbered 1024.

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, steam is expanded just once in any one cylinder before being exhausted through the smokebox.

In the compound Mallet locomotive

Mallet locomotive

The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet ....

, 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 that drive the front set of coupled wheels.

Characteristics

Apart from having a different boiler, there were three major differences between this locomotive and the other nine that were built and delivered at the same time.

Saturated steam

First, it was not equipped with a superheater in spite of the fact that superheated locomotives had already been proven to be more economic and superior in performance.

Driving wheels

The second was the fact that the front set of driving wheels were of a larger diameter than the rear set. In theory this configuration was to result in improved acceleration, with the rear engine unit providing the traction. It was also believed that the difference in frequency between the front and rear cylinder exhaust beats would result in a more even pressure in the receiver pipe and therefore improved steam flow, although exactly how this was to come about was not clear. It was the only South African articulated locomotive to have driving wheels of different diameters.

The unorthodox driving wheel configuration did not have the expected results and, in comparison with the other nine, the locomotive proved to be inferior in performance.

Mechanical stoker

Third, this was the first South African locomotive to be equipped with a mechanical stoker. This device, of the "Street" type, consisted of a coal crusher, hand-fed by the fireman, that was fitted to the front left hand side of the tender footplate and driven by a small steam engine mounted behind the hand brake column.

The crushed coal then fell by gravity into a chute which led to a receiving bin that was fitted below the back buffer beam of the locomotive, from where it was picked up by a bucket elevator travelling in large tubes. The full buckets were carried up in the left hand side tube mounted on the back of the firebox, discharged into a central receiver and then travelled down the right hand side tube.

The bucket belt was driven by another small steam engine, mounted on the left side of the firebox near the top, and the main shaft of this engine had adjustable cams which operated three steam cocks that supplied steam jets to the coal delivery orifices. From the central receiver, a cone shaped tray directed the coal to the right, centre or left sides as required, while the cam-operated steam jets blew the coal into the firebox.

It was a very complicated, cumbersome and extremely noisy arrangement that required the fireman to attend to two auxiliary engines and feed the crusher, in addition to his normal duties. It was removed and the locomotive was converted to hand firing soon after being placed in service.

Service

In 1912, when this locomotive was taken onto the South African Railways (SAR) roster, it was renumbered to 1628 and classified as Class MG. It joined the experimental Class MD

South African Class MD 2-6-6-2

In March 1910 the Central South African Railways placed a single experimental Mallet articulated compound steam locomotive with a 2-6-6-2 wheel arrangement in service...

and the Class MF

South African Class MF 2-6-6-2

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 classified as Class MF...

on the coal traffic line between Witbank and Germiston and remained there until it was withdrawn from service in 1927.

Gallery

The main picture shows Class MG number 1628 in its original SAR livery.