South African Class MB 2-6-6-0

NGR Mallet 2-6-6-0 1910
South African Class MB 2-6-6-0

NGR Mallet no. 337, SAR Class MB no. 1602, c. 1910

Type and origin Power type Steam Designer American Locomotive Company Builder American Locomotive Company Serial number 48337-48341 Model NGR Mallet Build date 1910 Total produced 5

Specifications Configuration 2-6-6-0 (Denver) 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 9 1⁄4 in (18,523 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 69 ft 1 in (21,057 mm) Height 12 ft 5 3⁄16 in (3,789 mm) Frame type Bar Axle load 16 LT 14 cwt (16,970 kg)  • Leading 7 LT 15 cwt (7,874 kg)  • 1st coupled 15 LT 5 cwt (15,490 kg)  • 2nd coupled 14 LT 10 cwt (14,730 kg)  • 3rd coupled 12 LT 9 cwt (12,650 kg)  • 4th coupled 11 LT 10 cwt (11,680 kg)  • 5th coupled 11 LT 16 cwt (11,990 kg)  • 6th coupled 16 LT 14 cwt (16,970 kg)  • Tender axle 11 LT 5 cwt (11,430 kg) average Adhesive weight 82 LT 4 cwt (83,520 kg) Loco weight 89 LT 19 cwt (91,390 kg) Tender weight 45 LT (45,720 kg) Total weight 134 LT 19 cwt (137,100 kg) Tender type 2-axle bogies Fuel type Coal Fuel capacity 10 LT (10.2 t) Water cap 4,000 imp gal (18,200 l) Firebox type Round-top  • Firegrate area 42.5 sq ft (3.95 m2) Boiler: ​  • Pitch 7 ft 6 in (2,286 mm)  • Diameter 5 ft 8 in (1,727 mm)  • Tube plates 16 ft 7 1⁄2 in (5,067 mm)  • Small tubes 260: 2 1⁄4 in (57 mm) Boiler pressure 200 psi (1,379 kPa) Safety valve Ramsbottom Heating surface 2,700.8 sq ft (250.91 m2)  • Tubes 2,546.2 sq ft (236.55 m2)  • Firebox 154.6 sq ft (14.36 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 Couplers Johnston link-and-pin

Performance figures Tractive effort 44,810 lbf (199 kN) @ 50%

Career Operators Natal Government Railways South African Railways Class NGR Mallet SAR Class MB Number in class 5 Numbers NGR 337-341 SAR 1602-1606 Delivered 1910 First run 1910 Withdrawn 1924

The South African Railways Class MB 2-6-6-0 of 1910 was a steam locomotive from the pre-Union era in the Colony of Natal.

In November 1910, the Natal Government Railways placed five 2-6-6-0 Mallet articulated compound steam locomotives in service. In 1912, when they were assimilated into the South African Railways, they were renumbered and designated Class MB.

Following on the satisfactory performance of the experimental Mallet compound steam locomotive which had been acquired by the Natal Government Railways (NGR) in 1909, a further five similar locomotives were ordered from the American Locomotive Company (ALCO) in 1910. They were delivered in November of that year and numbered in the range from 337 to 341.

Like the previous Mallet locomotive, these five had Walschaerts valve gear, bar frames and used saturated steam. They differed little from the previous Mallet, basically only in respect of their larger boilers which made them slightly heavier and their tenders with a 1 long ton 10 hundredweight (1.5 tonnes) larger coal capacity.

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.

In the compound Mallet locomotive, 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 which drive the front set of coupled wheels. 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.

These five locomotives joined the first Mallet in banking service, working heavy coal trains between Estcourt and Highlands on the Natal mainline. To relieve coupler strain, they were used in three-locomotive trains and, in this manner, were able to haul 750 long tons (762 tonnes) up the one in thirty (3⅓%) gradients on that line.