A typical steam locomotive transmits power from the boiler to its cylinders.
These cylinders are on either side of the boiler and make up the steam engine
that powers the driving wheels using a system of interconnected rods and
cranks. This system, as applied to a railway locomotive, was initially
developed in 1804 in Great Britain by Richard Trevithick. It was substantially
improved in the system used by George Stephenson on his small 1829
locomotive, Rocket, the prototype for most future steam locomotives.
The system of
horizontal driving
rods and the long
rigid wheelbase
typical of more
traditional mainline
steam locomotives
is clearly visible on
Canadian Pacific 2858 (670006*). (CSTM)
In the typical steam locomotive, there are two groups of rods, each connected
to one or more cylinders located on either side of the locomotive. These rods
convert the pistons’ reciprocating (back and forth) motion into the
driving wheels’ rotary motion. Because the tractive effort (pulling
capacity) of the locomotive is partly dependent on the number and diameter of
driving wheels, engines with high tractive effort tend to have many connected
driving wheels, making the wheelbase comparatively long and rigid. For
example, some of the largest steam locomotives in Canada had rigid driving
wheelbases of 22 feet (6.7 metres). Moreover, the majority of steam engines
haul their fuel and water in separate tenders behind the engine. The tenders
contribute to the overall length of a steam locomotive and to the dead weight
the locomotives have to pull.
This type of locomotive design is suited for large public railways where hauling
capacity, range and speed are important. Stable tracks are required to allow
conventional steam locomotives to haul heavy loads quickly. To meet stability
requirements, grades seldom exceed three percent (a rise of 3 metres over
100 metres) and the tracks are constructed with long gentle curves.
Moreover, since the tracks are permanent, railway companies invest
considerable sums of money in their construction and maintenance.
* The numbers in brackets are the accession numbers of artifacts held by
the Museum.