South African Class MA 2-6-6-0

NGR Mallet 2-6-6-0 1909 South African Class MA 2-6-6-0
NGR Mallet no. 336, SAR Class MA no. 1601, c. 1909
Type and origin Power type Steam Designer American Locomotive Company Builder American Locomotive Company Serial number 46044 Model NGR Mallet Build date 1909 Total produced 1
Specifications Configuration: ​  • Whyte 2-6-6-0 (Denver)  • UIC (1'C)Cnv4 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 60 ft 2 3⁄4 in (18,358 mm)  • Engine 33 ft 2 in (10,109 mm)  • Coupled 8 ft 4 in (2,540 mm) per unit  • Tender 17 ft 10 in (5,436 mm)  • Tender bogie 5 ft 10 in (1,778 mm) Length: ​  • Over couplers 68 ft 2 3⁄8 in (20,787 mm) Height 12 ft 5 1⁄8 in (3,788 mm) Frame type Bar Axle load 13 LT 18 cwt (14,120 kg)  • Leading 6 LT 17 cwt (6,960 kg)  • 1st coupled 12 LT 14 cwt (12,900 kg)  • 2nd coupled 13 LT 18 cwt (14,120 kg)  • 3rd coupled 13 LT 6 cwt 2 qtr (13,540 kg)  • 4th coupled 13 LT 2 cwt (13,310 kg)  • 5th coupled 12 LT 18 cwt (13,110 kg)  • 6th coupled 13 LT 16 cwt (14,020 kg)  • Tender bogie Bogie 1: 20 LT 7 cwt (20,680 kg) Bogie 2: 22 LT 10 cwt (22,860 kg) Adhesive weight 79 LT 14 cwt 2 qtr (81,000 kg) Loco weight 86 LT 11 cwt 2 qtr (87,960 kg) Tender weight 42 LT 17 cwt (43,540 kg) Total weight 129 LT 8 cwt 2 qtr (131,500 kg) Tender type 2-axle bogies Fuel type Coal Fuel capacity 8 LT 5 cwt (8.4 t) Water cap 4,000 imp gal (18,000 l) Firebox type Round-top & combustion chamber  • Firegrate area 40 sq ft (3.7 m2) Boiler: ​  • Pitch 7 ft 4 in (2,235 mm)  • Diameter 5 ft 4 in (1,626 mm)  • Tube plates 17 ft 10 1⁄2 in (5,448 mm)  • Small tubes 230: 2 1⁄4 in (57 mm) Boiler pressure 200 psi (1,379 kPa) Safety valve Ramsbottom Heating surface 2,574 sq ft (239.1 m2)  • Tubes 2,422 sq ft (225.0 m2)  • Firebox 125 sq ft (11.6 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: Richardson balanced slide Valve travel HP: 5 in (127 mm) LP: 5 1⁄2 in (140 mm) Valve lap HP: 1 in (25 mm) LP: 7⁄8 in (22 mm) Valve lead HP & LP: 3⁄16 in (5 mm) Couplers Johnston link-and-pin
Performance figures Tractive effort 44,810 lbf (199.3 kN) @ 50%
Career Operators Natal Government Railways South African Railways Class NGR Mallet SAR Class MA Number in class 1 Numbers NGR 336, SAR 1601 Delivered 1909 First run 1909 Withdrawn 1927

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 2-6-6-0 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.

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.

The main advantages of the Mallet locomotive type were articulation and compound expansion.

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.

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.