Victorian Railways AA class
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| Type and origin | |
|---|---|
| Power type | steam |
| Designer | Eugene Siepen |
| Builder | Phoenix Foundry, Ballarat |
| Build date | 1900–1903 |
| Specifications | |
|---|---|
| Configuration: |
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| • Whyte | 4-4-0 |
| • UIC | 2'Bh2 |
| Gauge | 5 ft 3 in (1,600 mm) |
| Driver dia. | 6 ft 1 in (1.854 m) |
| Adhesive weight | 32 long tons (33 t; 36 short tons) (roadworthy) |
| Tender weight | 41.2 long tons (41.9 t; 46.1 short tons) |
| Total weight | 91.1 long tons (92.6 t; 102.0 short tons) |
| Firebox: • Firegrate area |
23 sq ft (2.1 m2) |
| Boiler pressure | 200 psi (1,379 kPa) |
| Heating surface | 1,439 sq ft (133.7 m2) |
| Cylinders | Two, inside |
| Cylinder size | 19 in × 26 in (483 mm × 660 mm) |
| Valve gear | Stephenson |
| Valve type | piston valves |
| Performance figures | |
|---|---|
| Tractive effort | 20,850 lbf (92.75 kN) at 80% WP MEP |
| Career | |
|---|---|
| First run | 1900 |
| Last run | 1932 |
| Disposition | All scrapped |
The AA class was an express passenger locomotive that ran on the Victorian Railways between 1900 and 1932. The largest, heaviest and most powerful 4-4-0 steam locomotive to run in Australia, it was the final development of this locomotive type in Australia.
In 1900, Victorian Railways still ran express passenger trains with the successful but ageing B class 2-4-0 which dated back to 1862. The 'New A' class 4-4-0 of 1889 had proven to be a successful design, as had its close cousin and exact contemporary the X class 0-6-0. With traffic needs continuing to grow, the VR drafting office decided to take the best features of the New A and X class, but enlarge the locomotive with a larger, higher pressure boiler and larger cylinders.
Innovative design features of the AA class included air sanding equipment, improved Gresham No. 9 injectors, horizontal grid spark arrestors, steam for auxiliaries supplied directly from the dome, and a sloped ashpan to provide easier cleaning.
With a comparatively high 185 pounds per square inch (1.28 MPa) boiler, later increased to 200 pounds per square inch (1.4 MPa), it was considered impractical to use traditional slide valves. A trial installation of overhead piston valves on New A class locomotive No. 422 demonstrated considerably greater efficiency and reduced maintenance, and the AA became the first of many subsequent VR locomotive classes to be built with piston valves.
With an axle load of 16.85 long tons (17.12 t; 18.87 short tons), the AA had reached the design limits of size and power possible with a two coupled axle locomotive type with Victoria's relatively light track infrastructure. Its high tractive effort and limited adhesive weight (due to axle load constraints and only two powered axles) combined to produce a less than ideal factor of adhesion of 3.5. Future VR locomotive designs were based on three or four coupled axles.
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