Steam motor

A steam motor is a form of steam engine used for light locomotives. They represented one of the final developments of the steam locomotive, in the final decades of the widespread use of steam power.

The principle of the steam motor is to use the developments of the high-speed steam engine, and to apply them to light locomotives. Rather than a large conventional locomotive having only two cylinders, rotating at the speed of the driving wheels, the steam motor uses several small cylinders geared to run at high speeds. With all other factors remaining the same, doubling the speed of a piston engine doubles its power. The steam motor allowed small, light engines to be used. As many of the engine's performance losses remain constant, or are also related to the engine size, these small engines could also be more efficient overall.

All steam motors had the following characteristics:

Motors were of a standard size, according to the manufacturer's product line. Where greater power was required, multiple motors were used, one per axle or bogie.

The crankshaft and often the valve gear, was enclosed within a crankcase that contained an oil sump. This provided a generous supply of lubrication and also excluded dirt.

This allows a high crankshaft speed, encouraging efficiency. Although not all geared steam locomotives made use of this, particularly the US designs such as the Shay and the Climax, it was an essential part of the steam motor concept.

The final drive of early Sentinel locomotives was by chain. Later designs, particularly those by Abner Doble, preferred spur gears. Gear drives required the steam motor to be mounted low-down, alongside the axle.

Other beneficial characteristics were commonly found, but were not essential.

This was usually poppet valves driven by camshafts. In the steam motors built by Sentinel, the motor was derived from their already advanced steam wagon design.

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