The South African Railways Class MA 2-6-6-0 of 1909 was a steam locomotive from the pre-Union era in the Natal Colony.
In 1909, the Natal Government Railways placed a single Mallet articulated compound steam locomotive in service. In 1912, when it was assimilated into the South African Railways, it was renumbered and designated Class MA.
Manufacturer
As a result of a visit to the United States of America by Natal Government Railways (NGR) Locomotive Superintendent D.A. Hendrie in 1909 to study Mallet type locomotives and also American locomotive design in general, the American Locomotive Company (ALCO) supplied the NGR with two experimental locomotives which represented a radical departure from previous NGR locomotive designs. One was the NGR Class B America D, later to be designated the Class 3A on the South African Railways (SAR), while the other was the first Mallet type articulated compound steam locomotive in South Africa.
Mallet advantages
The main advantages of the Mallet locomotive type were articulation and compound expansion.
Articulation
The object with an articulated locomotive is to distribute traction power over a larger number of driving wheels with a flexible wheelbase. On the Mallet type, the boiler was fixed to the frame of the rear engine unit, as on a regular steam locomotive. The front engine unit carried the boiler barrel by means of a large supporting bracket, attached to the underside of the barrel and carried on a slide, fixed to the front engine unit's frame. On curves, the front engine unit could move several inches laterally under the boiler barrel. Side control springs acted as a centering device for the engine unit.
Compound expansion
In a compound expansion (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, from which spent steam is then fed to the larger low-pressure cylinders which drive the front set of coupled wheels.
Characteristics
The NGR 2-6-6-0 Mallet locomotive had Walschaerts valve gear, a bar frame and used saturated steam. It was the first articulated compound steam locomotive to enter service in South Africa and its design and large proportions represented a big advance in motive power on Cape gauge. At the time, it was the most powerful locomotive in the country.
Copper steam pipes, in internal diameter, were arranged from each side of the steam dome and passed vertically down the outside of the boiler to the steam chests of the high-pressure cylinders. The exhaust from the high-pressure cylinders passed to a receiver pipe, in internal diameter, arranged along the centre line of the locomotive and with a ball-joint at the connection with the high-pressure cylinder casting. The front end of the pipe was fitted with an expansion joint near the connection which led to the steam chests of the low-pressure cylinders. The exhaust from the low-pressure cylinders was carried by a pipe, also in internal diameter, with flexible connections leading into the smokebox.
The high-pressure cylinders were fitted with diameter piston valves and had a bore of , while the low-pressure cylinders were fitted with Richardson balanced slide valves and had a bore of . Both sets of cylinders had a stroke of .
An intercepting valve was arranged in the high-pressure cylinder casting, through which the engine could be worked in simplex mode when required. Upon starting, the valve would feed boiler steam directly to the low-pressure cylinders for a few revolutions of the coupled wheels, just sufficient to give the engine additional power at starting, after which it automatically cut off and the working would become compound. By means of a steam cock in the cab, the driver could also admit boiler steam to the low-pressure cylinders for additional power while ascending steep inclines.
The tender rode on diamond-frame four-wheeled bogies and was equipped with a glass water gauge on the side of the tender tank, the first time such a device had been used in South Africa.