South African Class 12A 4-8-2

South African Class 12A & 12AR 4-8-2
Class 12AR no. 1535 at Maraisburg, 27 April 2009
Type and origin ♠ Class 12A as built with a Belpaire firebox ♥ Class 12AR rebuilt with a round-topped firebox ♣ Type MP1 tender - ♦ Type MT tender Power type Steam Designer South African Railways (D.A. Hendrie) Builder North British Locomotive Company Henschel and Son Serial number NBL 21738-21757, 22751-22765, 23891-23903 Henschel 21046-21051, 21428-21440 Model Class 12A Build date 1919-1929 Total produced 75
Specifications Configuration: ​  • Whyte 4-8-2 (Mountain)  • UIC 2'D1'h2 Driver 2nd coupled axle Gauge 3 ft 6 in (1,067 mm) Cape gauge Leading dia. 28 1⁄2 in (724 mm) Coupled dia. 51 in (1,295 mm) Trailing dia. 33 in (838 mm) Tender wheels 34 in (864 mm) Wheelbase ♣ 58 ft 7 1⁄4 in (17,863 mm) ♦ 61 ft 7 3⁄8 in (18,780 mm)  • Engine 32 ft 1 in (9,779 mm)  • Leading 6 ft 2 in (1,880 mm)  • Coupled 13 ft 6 in (4,115 mm)  • Tender ♣ 16 ft 9 in (5,105 mm) ♦ 20 ft 5 in (6,223 mm)  • Tender bogie ♣ 4 ft 7 in (1,397 mm) ♦ 6 ft 2 in (1,880 mm) Length: ​  • Over couplers ♣ 66 ft 11 5⁄8 in (20,412 mm) ♦ 69 ft 7 in (21,209 mm) Height ♠ 12 ft 10 in (3,912 mm) ♥ 13 ft (3,962 mm) Frame type Plate Axle load ♠ 17 LT 6 cwt (17,580 kg) ♥ 17 LT 8 cwt (17,680 kg)  • Leading ♠ 17 LT 6 cwt (17,580 kg) ♥ 17 LT 14 cwt (17,980 kg)  • 1st coupled ♠ 17 LT 6 cwt (17,580 kg) ♥ 17 LT (17,270 kg)  • 2nd coupled ♠ 17 LT 5 cwt (17,530 kg) ♥ 17 LT 4 cwt (17,480 kg)  • 3rd coupled ♠ 17 LT 6 cwt (17,580 kg) ♥ 17 LT 8 cwt (17,680 kg)  • 4th coupled ♠ 17 LT 6 cwt (17,580 kg) ♥ 17 LT 7 cwt (17,630 kg)  • Trailing ♠♥ 12 LT 12 cwt (12,800 kg)  • Tender bogie Bogie 1: ♣ 27 LT 10 cwt (27,940 kg) ♦ 32 LT 18 cwt (33,430 kg) Bogie 2: ♣ 23 LT 11 cwt (23,930 kg) ♦ 33 LT 2 cwt (33,630 kg)  • Tender axle ♣ 13 LT 15 cwt (13,970 kg) ♦ 16 LT 11 cwt (16,820 kg) Adhesive weight ♠ 69 LT 3 cwt (70,260 kg) ♥ 68 LT 19 cwt (70,060 kg) Loco weight ♠ 99 LT 1 cwt (100,600 kg) ♥ 99 LT 5 cwt (100,800 kg) Tender weight ♣ 51 LT 1 cwt (51,870 kg) ♦ 66 LT (67,060 kg) Total weight ♠♣ 150 LT 2 cwt (152,500 kg) ♥♣ 150 LT 6 cwt (152,700 kg) ♠♦ 165 LT 1 cwt (167,700 kg) ♥♦ 165 LT 5 cwt (167,900 kg) Tender type ♣ MP1 (2-axle bogies) ♦ MT (2-axle bogies) MP, MP1, MR, MS, MT, MT1, MT2, MX, MY, MY1 permitted Fuel type Coal Fuel capacity ♣ 10 LT (10.2 t) ♦ 12 LT (12.2 t) Water cap ♣ 4,250 imp gal (19,300 l) ♦ 6,000 imp gal (27,300 l) Firebox type ♠ Belpaire - ♥ Round-top  • Firegrate area ♠ 40.5 sq ft (3.76 m2) ♥ 41 sq ft (3.8 m2) Boiler: ​  • Pitch ♠ 7 ft 9 in (2,362 mm) ♥ 8 ft 6 in (2,591 mm)  • Diameter ♠ 5 ft 10 1⁄2 in (1,791 mm) ♥ 6 ft (1,829 mm)  • Tube plates ♠ 18 ft (5,486 mm) ♥ 19 ft 4 in (5,893 mm)  • Small tubes ♠ 159: 2 1⁄4 in (57 mm) ♥ 119: 2 1⁄2 in (64 mm)  • Large tubes ♠ 24 5 1⁄2 in (140 mm) ♥ 30: 5 1⁄2 in (140 mm) Boiler pressure ♠♥ 190 psi (1,310 kPa) Safety valve ♠ Ramsbottom - ♥ Ross Pop Heating surface ♠ 2,510 sq ft (233 m2) ♥ 2,502 sq ft (232.4 m2)  • Tubes ♠ 2,301 sq ft (213.8 m2) ♥ 2,338 sq ft (217.2 m2)  • Firebox ♠ 209 sq ft (19.4 m2) ♥ 164 sq ft (15.2 m2) Superheater: ​  • Heating area ♠ 466 sq ft (43.3 m2) ♥ 480 sq ft (45 m2) Cylinders Two Cylinder size 24 in (610 mm) bore 26 in (660 mm) stroke Valve gear Walschaerts Valve type Piston Couplers Johnston link-and-pin AAR knuckle (1930s)
Performance figures Tractive effort ♠♥ 41,840 lbf (186.1 kN) @ 75%
Career Operators South African Railways Class Class 12A & 12AR Number in class SAR 67, Industrial 8 Numbers 1520-1550, 2103-2138 Delivered 1919-1929 First run 1919 Withdrawn 1980s
The leading coupled axle had flangeless wheels

The South African Railways Class 12A 4-8-2 of 1919 was a steam locomotive.

Between 1919 and 1929, the South African Railways placed 67 Class 12A steam locomotives with a 4-8-2 Mountain type wheel arrangement in service. Between 1947 and 1953, eight were also built for industrial use.

The Class 12A was the final locomotive design by D.A. Hendrie, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1910 to 1922, and one of his finest. It was an improved and larger version of his Class 12 locomotive, with larger diameter cylinders to increase tractive effort and a redesigned boiler.

Between 1919 and 1929, altogether 67 of them were built on five orders, 48 by the North British Locomotive Company (NBL) in Scotland and 19 by Henschel and Son in Kassel, Germany.

At the time, these locomotives were of the maximum size permissible for the Mountain type on Cape gauge. They were designed primarily to supplement the Class 12 locomotives which were already working on the coal line between Witbank and Germiston. Since these engines had a slightly heavier axle load than the predecessor Class 12, it was foreseen that they would be able to take a slightly increased load on the Witbank-Germiston section.

The locomotives were built on 1 ¹⁄₂-inch thick (38-millimetre) plate frames. They were superheated and had piston valves, Walschaerts valve gear and Belpaire fireboxes which included combustion chambers. The boilers of the first two batches of 1919 and 1921 were equipped with Ramsbottom safety valves, while works pictures of the remainder show that they were equipped with Ross pop safety valves. The feedwater supply of the third and subsequent batches was by a top feed arrangement through copper pipes which were run from the running boards on each side to the top of the boiler ahead of the steam dome.

The original Class 12A superheater header was arranged with the Gresley type air valve which was introduced to prevent the risk of superheater elements being burned or damaged by overheating while engines were drifting. The valve automatically opened when the regulator was closed and allowed outside air to be drawn into the superheater elements. Even though experience showed that the heated air which then entered the steam chests and cylinders was harmful to cylinder lubrication with no apparent effect on the life of the superheater elements, it was some years before the use of these air valves was finally discontinued.