Valve gear on a steam locomotive is used to control the admission of steam into the cylinders of a locomotive. Specifically, the valve gear controls the timing or duration of time boiler pressure steam is allowed into the cylinders. For efficiency reasons, you would only want steam to be admitted into the cylinders during the short "intake" portion of the stroke which would allow the steam to expand throughout the "expansion" portion of the stroke. However, for maximum power when starting a steam locomotive from rest, boiler pressure steam would need to be admitted almost throughout the entire expansion portion of the stroke.
Because of these changing conditions, the engineer needs to be able to control or adjust the valve gear which changes the timing of steam admission. This was done using a large lever (often called "The Johnson Bar") in the cab. Because of the weights of all of the linkage and the forces transmitted through them, some of the forces were transmitted back to the Johnson bar. Therefore, when adjusting this lever, the engineer would have to brace himself and hold onto it quite tightly.
When using the Johnson bar to reduce the time during which steam is admitted into the cylinders, the admission of steam is essentially being cut off earlier in the stroke. Therefore, it is often said that the Johnson bar controls the "cut off". This is also called "hooking up" a steam locomotive.
Here is an account of proper use of the Johnson bar while starting a steam locomotive. The article was written by a college of mine at the Minnesota Transportation Museum describing the exact locomotive that I used to run. Reading it brings back a lot of good memories.
The following types or configurations of valve gear were used on locomotives in North America:
Stephenson valve gear was invented by employees of the Stephenson's Locomotive Works during the 1840's. The design used eccentrics on the driving axle of the locomotive to control valve motion. In the USA it quickly became the most popular valve gear through the 1800's. However, because the valve gear mechanism was located between the wheels, it was difficult to maintain.
Walschaerts valve gear was invented around the same time as Stephenson valve gear but did not become popular until the very late 1800's. After that it quickly became the most popular valve gear on steam locomotives. It's main advantage was that it was located completely outside the wheels and therefore was much easier to maintain than Stephenson valve gear.
Baker valve gear was similar to Walschaerts valve gear except that it replaced the slider and expansion link with a pin-joint mechanism. In most linkage mechanisms, a sliding joint will be the source of most problems. The Baker valve gear did not have any slider/block joints. It was made up completely of pin joints, thus requiring less maintenance.
Baker valve gear became the main competitor to Walschaerts valve gear. Curiously, one of its criticisms was that it had too many pin joints! It was popular on the following railroads:
Note: I believe there is an error in the animation shown above. The error is in how the "gear connecting rod" connects to the "reverse yoke" and the "bell crank". Compare the animation to this picture. Unfortunatly, the site from which these animations were obtained is no longer active.
Southern valve gear was designed by engineers from the Southern Railway (thus the name). It was similar to Walschaerts except that it did away with the combination lever and union links connected to the crosshead. It had a curved expansion link (like on Walschaerts) except that it was mainly horizontal instead of vertical like on Walschaerts.
It was used primarily on the following:
Young valve gear was invented by an employee of the C&NW named O. W. Young. This valve gear was first applied to a steam locomotive on the Grand Trunk in 1915. Young valve gear eliminated the need for the eccentric crank. It took advantage of the quartering of the drivers by using the piston rod motion on one side of the locomotive to control the steam valves on the other side of the locomotive. As a result, it was said to have put less dynamic loads on the main driver. It was also purported to produce better valve timing events which resulted in more power.
Young valve gear was used on Union Pacific 4-8-2s and 2-10-2s.