Walschaerts valve gear

Egide Walschaerts was awarded a gold medal, honorable mentions and a diploma at the 1878 Universal Exhibition in Paris, and also in Antwerp in 1883.

The first application in Britain was on a Single Fairlie 0-4-4T, exhibited in Paris in 1878 and purchased by the Swindon, Marlborough and Andover Railway in March 1882.

[3] According to Ernest Ahrons,[4] the locomotive saw very little service as nobody seems to have known how to set the valves and this led to enormous coal consumption.

Since the exhaust is also shut, during the rest of the stroke the steam that has entered the cylinder expands in isolation, and so its pressure decreases.

Economy also requires that the throttle be wide open and that the boiler pressure is at the maximum safe level to maximise thermal efficiency.

For economy, a steam engine is used of a size such that the most economical settings yield the right amount of power most of the time, such as when a train is running at steady speed on level track.

When greater power is necessary, e.g. when gaining speed when pulling out of a station and when ascending a gradient, the Walschaerts valve gear enables the engine driver to set the cutoff point near the end of the stroke, so that the full pressure of the boiler is exerted on the piston for almost the entire stroke.

In this position the secondary motion is eliminated and the piston valve travel is shortest, giving minimal injection and exhaust of steam.

Contrast this to when the die block is at the bottom of the expansion link (7), giving maximum steam injection and exhaust.

[6]) Once the locomotive has accelerated the driver can adjust the reverser toward the mid-gear position, decreasing cut-off to give a more economical use of steam.

The eccentric rod provides motion to the expansion link (7) which is pivoted in a central location back to the body of the locomotive.

In this way the secondary, out of phase, driver controlled component of motion is imparted to the top of the combination lever (12) by the radius bar (8).