The Live Steamer, July-August 1950
"Dead Centers" and "Valve-Terms", having been briefly touched on in the first two articles in The Live Steamer, Combination levers perhaps should be next. They are used with both Baker and Walschaert valve gears.
The same method of figuring out their sizes applies to either valve gear. The only difference in the combination level layout is that which is due to inside or outside admission valves.
The Combination lever gives a movement to the valve of one lap plus one lead, times two. This motion is constant, as it derives its movement from the crosshead, whose travel is always the same. With the reverse lever in mid-gear, the valve will receive no movement from the valve gear, but will get the lap and lead motion from the crosshead.
When laying out the Combination lever, it is desireable to keep the distance "B" as great as possible, say 5/16 inch to 9/32 inch. If made much less than this, there will not be much room for the pins, they would have to be small and would war out rather fast. The other dimensions can be applied in the formula in their proper places and the whole is easily worked out by arithmetic.