Of the many varieties of locomotive valve gears, two principal type played important role. The Stephenson gear located between the wheels, this gear was successful up to the time when modern developments in locomotive practice crowded the space between wheels. When this condition become serious the Stephenson gear was abandoned in favor of the Walschaert type shown in this animation, which, in various modifications, has been adopted later on practically all modern locomotives.
The principal advantage of the Walschaert gear lieas in accessibility of its parts, which are placed entirely outside the driving wheels. This facilitates inspection, oiling, and cleaning.
The various parts of the Walschaert gear are pin-connected and are easily lubricated; hence troubles due to overheating are reduced to the minimum. The Walschaert gear also transmits the moving force of the valve in practically a straight line; consequently there are less springing and yielding of the parts than in the Stephenson gear. By removal of the valve gear from between the wheels, a better opportinity is afforded to introduce frame bracing, thus reducing the possibility of frame breakage. The Welschaert type of valve gear also possesses advantages of steam distribution and operation.
The general adoption of the Walschaert valve gear has been accompanied by a number of variations which, do not eliminate its esential elements. The Baker, the Young, and the Southern valve gears, other well-known locomotive valve gears, are simply variations of the Walschaert gear.