made by the trammel may be seen easily. As the ends of the valve travel are approached, when the wheels are rotated, the trammel T is placed as shown in the pop in the steam chest cover (Fig. 43–I) and the vertical point is passed and repassed across the valve spindle as the latter moves until the latter reverses its direction of movement. The limiting position gives the line c₁ (Fig. 43–IV) marked on the spindle, and the distance a₁c₁ is the maximum port opening at the front end. Similarly a₂c₂ is the port opening at the back end. When valves are set to equal port openings instead of equal leads, the adjustments are such that a₁c₁ is made equal to a₂c₂. In this case there is a slight difference in the leads at the front and back ends.
It may be noted that as the reversing lever is “notched up” when running, in order to expand the steam in the cylinders, the lead increases with a Stephenson link motion having “open” eccentric rods.[1] For instance an engine, which has the valves set to equal leads of ⅛ in. when in full fore gear, may have a lead of ¼ in. when fully notched up, and moreover the link motion may be so suspended that the leads at both ends are not equal when the reversing lever is in this latter position. It is therefore usual for the valves to be set to equal leads, when the reversing gear is placed in the notch or position in which the
- ↑ For further particulars of Stephenson’s link motion with open and crossed rods, and the effect of the latter on the lead, see ''Steam Engine Valves and Valve Gears'' in this series of primers.
