Scottish Railways - Steam Locomotives

A steam locomotive is powered by a reciprocating steam engine, i.e., a power unit operated by the expansion of steam which is admitted into a cylinder and moves a piston to and fro.

The piston motion is transmitted to the driving wheels of the locomotive.

The drive mechanism and control gear of a steam locomotive: The steam, generated in a smoke tube boiler or flue boiler, is delivered through a pipe to the slide-valve chest.

From here the slide valve admits the steam to the cylinder alternately through two openings so that it enters first on one side and then on the other side of the piston and thus pushes the latter to and fro.

The steam enters the cylinder on one side of the piston, and pushes the piston in the opposite direction.

At the same time, the exhaust steam on the right-hand side is being pushed out of the cylinder by the piston and is discharged through a duct from which it escapes through the chimney into the open air.

The piston rod connects the piston to the crosshead, which moves backwards and forwards in a guide.

Attached by means of an articulated connection to the cross-head is the driving rod whose other end is connected to the driving pin on the driving wheel.

In this way the reciprocating (to-and-fro) motion of the piston is transformed into the rotary motion of the wheel.

There are usually two or more sets of driving wheels, these being coupled together by coupling rods.