A valve gear is a component that controls the intake and exhaust of steam in an engine. The mechanism is usually connected a steam engine's valves, and was often used in early locomotives. Such an engine is typically most efficient when the inlet is open for a short time, before it reaches the cutoff point when no more steam goes into the engine’s cylinder. There have been several designs over the years, each generally limited by requiring the same process to work the exhaust and inlet valves.
Steam locomotive nomenclature typically includes a range of parts which include the boiler, firebox, wheels, and a variety of pumps and valves. The most efficient valve gear is usually one that lets steam in during the intake so it can expand on the next phase of the cycle. Various types of mechanisms have been built to control the valve motion. To move a locomotive from rest, however, more power is generally required, so steam can be let in during the expansion phase as well. A device called a Johnson bar was often used for steam control; train engineers could pull on it to stop the flow.
One type of locomotive valve gear, invented in the 1840s, enabled motion control with mechanisms on the wheel axle of the train. This design was called the Stephenson valve gear. Another type created around the same time was typically located away from the wheel systems, and was mounted externally on the train car. The Walschaerts valve gear was used for many locomotive designs with outside cylinders. A similar design has pin joints, that don’t typically require as much maintenance, instead of sliding mechanisms.
The Southern valve gear features a horizontal design rather than a vertical one of many other variants. It was often used in the United States in the early 1900s, and had elements similar to other popular designs. The Joy valve gear, generally used in England, was often built into ships and locomotives in the late 1800s. This was smaller than other designs but some components often fractured under stress.
Able to make use of piston rod motion, the Young valve gear could control valves on the opposite side of a train. Power efficiency was typically better because the timing of opening and closing valves was more precise. Some historians believe that hundreds of types of valve gears were created, in efforts to make steam engines more efficient.