A. The Walschaert gear has an eccentric crank attached to the end of the
main pin on each side of the locomotive, with an eccentric rod from this
pin to the connection at the bottom end of the link. This eccentric is
located so it serves for both forward and back motion. The link swings
on a center trunnion and cannot be moved up and down as the Stephenson
link, but the link block can be moved from one end of the link to the
other to reverse the engine; or part way toward the center of the link
to change the cut-off. A radius rod connects the link block to the valve
stem. There are two motions given to the valve stem, one from the link
block which regulates the travel of the valve for the cut-off and
reversing; the other motion is from a connection with the cross-head
which gives the valve a positive motion to take care of the lap and
lead. To give this motion there is used a combination lever or a lap and
lead lever connected to a cross-head arm by the union link.