Some Major Types of Articulation
Garratt: boiler slung between two independent engine units.
Mallet: rear engine unit fixed, front engine unit pivoting.
The Mallet type was originally introduced to facilitate the layout of a compound locomotive, as in the illustration above. Here high-pressure saturated steam is led directly from the dome to the rear cylinders. The low-pressure exhaust from these cylinders passes through a receiver to the low-pressure cylinders on the front engine unit. These cylinders exhaust diredtly to the stack.
Because of this history, some purists maintain that the only true Mallets are compounds. They then refer to simple locomotives with the same engine unit layout as "simple articulateds" (illustrated below), although this description could apply equally well to the Garratt and Shay pictured here.
Meyer: locomotive with frame riding over two pivoting engine units.
Shay: cylinders alongside firebox, driving cardan shaft geared to axles of trucks. The boiler is offset to the fireman's side of the locomotive, while the cylinders and drive mechanism occupy the engineer's side.
© 2005-2008 J.D.H. Smith