Mid gear. The reversing screw is in mid position so that the point G hardly moves as the driving wheel turns and the expansion link (shown in pale blue) rotates. Point G therefore acts as a fixed fulcrum in mid gear. As the drive rod moves, the point K on the crosshead, which moves along the slide bar, causes the point H to move the valve spindle through part of its available travel. When the main piston is at each end of its travel, point G is not displaced by rotation of the expansion link whatever the reverser position. The valve opening at this point is approx. ¼″ (the 'lap'). In mid gear the valve spindle moves through the central 50% of its available travel by an amount equal to the 'lead' plus the 'lap'. This amount corresponds to a fixed ratio, the distance GH/GJ, of the movemnent of the main piston. The buttons below will show an animated view as the driving wheel turns through 90°.
The valvegear is also illustrated at 25%, 50% and 100% of full forward gear and 25%, 50% and 100% of full reverse gear. For these views click on the relavant link.