The SZD Class AA20 was a one-off experimental 4-14-4 steam locomotive constructed in the Soviet Union by Krupp and the Voroshilovgrad Locomotive Factory in 1934 for the Sovetskie Zheleznye Dorogi (SŽD). Two locomotives were set to be built, but due to the construction of the railway's more powerful FD Class, only AA20-1 was built, leaving the second AA20 incomplete.
Wheel arrangement
Under the Whyte notation for the classification of steam locomotives, 4-14-4 represents the wheel arrangement of four leading wheels, fourteen coupled driving wheels (seven axles) in a rigid frame, and four trailing wheels.
Equivalent classifications in other notations would have been:
- AAR classification: 4-G-4
- UIC classification: 2'G2' (also known as German classification and Italian classification)
- French classification: 272
- Turkish classification: 711
- Swiss classification: 7/11
- Russian classification: 2-7-2
History
Prerequisites for the appearance of a steam locomotive
By the 1930s On Soviet railways, the requirements for traction and train speed increased significantly. The existing steam locomotives of type 0-5-0 (series E) and type 1-5-0 (series Ye), the adhesion weight of which did not exceed 85 tons, were no longer able to fully cope with the increased volume of transportation. There was an urgent need to replace them with much more powerful locomotives.
Different groups of specialists proposed different solutions. Some suggested leaving five driving wheel pairs on the locomotive and only increasing the load from the axle on the rails, while strengthening the railway track. Others insisted on keeping the load from the driving wheel sets within 20 tons, while increasing their number. Both groups of specialists did not take into account that at that time the cars were mainly equipped with a screw harness ( automatic couplers on Soviet railways would be installed en masse only in 1934), which could withstand a force not exceeding 20 tf.