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Concept
Garratt locomotive
Summary
A Garratt locomotive is a type of steam locomotive invented by British engineer Herbert William Garratt that is articulated into three parts. Its boiler, firebox, and cab are mounted on a centre frame or "bridge". The two other parts, one at each end, have a pivot to support the central frame; they consist of a steam engine unit – with driving wheels, trailing wheels, valve gear, and cylinders, and above it, fuel and/or water storage.
Articulation permits locomotives to negotiate curves that might restrict large rigid-framed locomotives. The design also provides more driving wheels per unit of locomotive weight, permitting operation on lightly engineered track. Garratt locomotives produced as much as twice the power output of the largest conventional locomotives of railways that introduced them, reducing the need for multiple locomotives and crews.
The principal benefit of the Garratt design is that the boiler and firebox unit are slung between the two engine units.
As Oswald Nock wrote, the Garratt type holds several advantages over the Mallet type:This [the Mallet] was so designed to provide a very large engine unit, to be managed by a single crew, but to spread the dead weight over many axles and thus avoid excessive loads on the track and under-line bridges, and at the same time retain flexibility of wheelbase to facilitate operation on severely curved sections of line. The Mallet, having driving-wheel units beneath the boiler, retains the limitation in dimensions inherent in the orthodox type of steam locomotive, while the immense length of some of these machines is itself a handicap. Except with oil firing, ... one cannot put the cab in front. H.W. Garratt ... patented ... the idea of having a single large boiler slung on a cradle carried on two entirely separate engine units. ... The boiler could be developed to ideal proportions, unfettered by any wheels beneath it. It could be kept short, and of large diameter, which is the best possible form for securing a high rate of evaporation.
Official source
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