jockey valve

plural jockey valves

1. A manually-controlled auxiliary valve on the regulator of a steam engine to control smaller amounts of steam than can be managed by the main valve.

   1 more example

   Jockey-valves of locomotive regulators are small auxiliary ports which can be opened by a suitable movement of the regulator-handle, when it is desired to pass no more than a limited amount of steam to the cylinders, as, for example, over level or slightly falling track with light loads. A jockey-valve enables such working to be conducted with more precision than is possible by a partial opening of the main regulator valve.

   1946 March and April, “The Why and The Wherefore: Jockey-Valves”, in Railway Magazine, page 129