Walschaerts valve gear

The Walschaerts valve gear is a mechanism that regulates the flow of steam to the pistons of a steam locomotive. The Belgian railway engineer Egide Walschaerts invented it in 1844, and in Germany and some neighbouring countries it is called the Heusinger valve gear after Edmund Heusinger von Waldegg, who devised the mechanism independently in 1849. It improved on the earlier Stephenson valve gear because the driver can vary the engine continuously between maximum economy and maximum power, changing the point at which steam is cut off without changing when admission begins. Because the whole gear can be mounted on the outside of the locomotive, the space between the frames stays clear and the parts are easy to reach for service and adjustment, which first won it adoption on some articulated locomotives. It was slow to gain popularity, and the first locomotive fitted with it was built in 1849 at the Belgian Tubize workshops. By the twentieth century it was the most commonly used type on larger locomotives, and in Europe its use was almost universal. The name is sometimes written without the final s because the gear was patented under that spelling.