Walschaerts Valve Gear - Technical Details

Technical Details

The primary lead motion is provided by the crosshead arm (9) and the union link (11). This pivoting bar gives the in phase component of motion to the bottom of the combination lever (12).

The secondary directional/amplitude motion is derived from a mechanical linkage made up of several components.

The flycrank (1) is rigidly attached to the con-rod pin connected to the main drive wheel. Note that this is the only suitable attachment point on any of the drive wheels that is not fouled by the passage of the coupling rod or the connecting rod. The flycrank is of a length such that the pin attachment to the fly crank rod (2) is 90 degrees out of phase with the lead motion.

The flycrank rod provides motion to the Expansion link (7) which is pivoted in a central location back to the body of the locomotive. The expansion link holds the Radius Bar (8), captive by a Die Block which is integral with the radius bar but is free to move vertically in a constrained curved path along the expansion link.

The vertical position of the radius bar is controlled in the cab by the driver adjusting the reverser which in turn controls the mechanical linkage; reach rod(3), lifting link (4), Lifting arm(5) and reverse arm and shaft(6).

In this way the secondary, out of phase, driver controlled component of motion is imparted to the top of the combination lever (12) by the radius bar (8).

The combination lever combines these two motions with the resultant acting upon the Valve Stem (13), suitably restrained by the Valve Stem Guide (10), which in turn acts upon the Piston Valve (14).