=Walschaert and Baker-Pilliod valve gears=

1.  Q. Give a brief explanation of the Walschaert valve gear.

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.

2.  Q. Is the Walschaert gear direct or indirect?

A. It is direct when the link block is below the center of the link; it is indirect when the link block is above the center of the link.

3.  Q. What are the principal differences in the location of the Stephenson and Walschaert gears, and what advantages does this give the Walschaert?

A. The Stephenson gear is placed between the main frames and employs two eccentrics, with straps and rods on each side of the locomotive; one for forward and one for backward motion.  The Walschaert gear is placed outside the driving wheels and frame, has but one eccentric, which is a simple arm connected to the outside end of the crank pin for both forward and back motion.  The links are set above the wheels on a level with the steam chest, the combination lever next to the cross-head.  This gives it an advantage of a better chance to inspect all parts, the eccentric connections are much lighter and direct, which makes them less liable to wear or breakdown, and the valve has a constant lead.

4.  Q. How is the lead affected by movement of the reverse lever with the two gears?

A. With the Stephenson gear the lead increases as the reverse lever is hooked toward the center in both forward and back motion.  With the Walschaert gear the lead is the same in all positions of the lever, so that the lever is used to reverse the engine or adjust the cut-off.

5.  Q. In reversing, how do the two gears differ as to the movement of the link and link block?

A. With the Stephenson gear, when reversing, the link is raised and lowered, bringing the block which is not moved by the reverse lever under control of either the forward or back-up eccentric as is desired to move the engine the proper way.  With the Walschaert gear the link is not moved by the reverse lever, but the link block is raised and lowered in the link; the position of the block above or below the center of the link controlling the direction of motion.