Walschaerts Valve Gear

The valve gear controls the opening and closing of the valves in the cylinders, and also
provide the means of reversing the direction of rotation of the wheels. There are two main
valve gear systems in use on locomotives. The Walschaerts Valve Gear for used on outside cylinders and the Stephensons Link gear for used between
the frames.
Both of these systems basically operate from a second Crank or Excentric system that
provides a movement 90° advanced on the main piston rod. As we saw above the efficiency
of a steam engine can be improved by allowing the steam to Expand in the cylinder before it
is exhausted. Unfortunately this adjustment depends on the speed. When starting the
pistons only move slowly and a brute force 'Blow' system is the most effective. However at
speed a dramatic improvement in efficiency can be obtained by increasing the amount of
expansion allowed.