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). Inside and outside admission valves The Walschaerts gear can be linked to inside or outside admission valves. This article has only considered inside-admission piston valves until now, but outside-admission valves (slide valves and some piston valves) can use Walschaerts valve gear. If the valves have outside admission the radius bar connects to the combination lever below the valve stem rather than above. Layout To lay out Walschaerts gear, the proportions of the combination lever must be chosen. A displacement of the union link end by half the piston travel must cause a valve rod displacement of the valve lead plus the valve lap. The ratio of distance from union link end to pivot with radius rod to the distance between the valve rod end to the pivot with the radius rod should be in the same proportion as half piston travel to valve lap plus lead. When the piston is at either dead centre movement of the radius rod should not move the valve rod. Therefore the expansion link die slot should be an arc of a circle of radius equal to the length of the radius rod. The throw of the return crank must give the required oscillation of the expansion link. Variants There have been many variants of Walschaerts valve gear, including: Baker valve gear Baguley valve gear Bagnall–Price valve gear J. T. Marshall valve gear See also Johnson Bar (locomotive) Steam locomotive nomenclature Notes References Bibliography See The Encyclopedia of Railroads, O. S. Nock, 1977 The name is Walschaert, changed by the family in 1830 when Belgium became independent of the Netherlands. It was patented under the name Walschaert because that