The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet (and thus, the name is properly pronounced in the French manner, "Mallay").
In the Mallet locomotive, there are two powered trucks. The rear is rigidly attached to the main body and boiler of the locomotive, while the front powered truck is attached to the rear by a hinge, so that it may swing from side to side. The front end of the boiler rests upon a sliding bearing on the swinging front truck.
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Compound expansion
Mallet's original design was a compound locomotive, in which the steam is used twice, first in a set of high-pressure cylinders, and then in a set of low-pressure cylinders. This confers certain thermodynamic advantages, and also worked well with the Mallet design. Steam was fed from the steam dome down to the aft, high-pressure cylinders - the exhaust steam from those being fed forwards in a pipe with a swiveling joint - to the forward, low-pressure cylinders. The exhaust steam from the larger low-pressure cylinders is exhausted through a slit in the sliding bearing in the top of the swiveling truck and thus to the smokebox above, and the blastpipe (US: exhaust nozzle) and chimney (US: stack). The difference in size between the high and low pressure cylinders can be seen in the picture of the 2-6-6-2 Mallet.
Unlike the case of the rigidly-framed locomotive, the Mallet design is easier to build as a compound, since steam and exhaust pipes are needed for both pairs of cylinders when it is built as a simple. When built as a compound, the only flexible steam pipes that are needed are the ones that deliver low-pressure steam from the rear cylinders to the front.
Simple expansion
Mallet's original patent specifies compound expansion. However many locomotives, particularly in the United States, were built to Mallet's articulation design but without employing compounding (for instance the Union Pacific Big Boy). The term "Mallet" is generally, if somewhat incorrectly, applied to locomotives built with the articulation system specified in the patent. Strictly, however, only compound locomotives are true Mallets[1].
Size
Mallet's original design was intended to allow a medium-size locomotive to better negotiate the tight curves of a narrow gauge railway, but the design was particularly attractive to railroads in the United States because it permitted locomotives to be built to sizes impossible with a single, rigid frame. It was introduced to the USA by the Baltimore & Ohio Railroad in 1904[2]. The 4-8-8-4 "Big Boy" used by Union Pacific Railroad and the 2-6-6-6 Allegheny of the Chesapeake and Ohio Railway are generally regarded as the largest steam locomotives in the world; the 2-8-8-2 Y6b class of the Norfolk and Western Railway can be considered the ultimate development of the true Mallet compound locomotive. These engines were built through World War II, with the last Y6b being constructed in 1952. However, outside North America, the Mallet type had generally been superseded by the Garratt locomotive by the mid 1920s.
References
- ^ "A Big Boy is Not a Mallet". http://www.steamlocomotive.com/bigboy/. Retrieved September 15, 2009.
- ^ Ransome-Wallis, P. (1959). Illustrated Encyclopedia of World Railway Locomotives (2001 republication ed.). Dover Publications, Inc. pp. 500-501. ISBN 0-486-41247-4.
External links
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