Page:Valve-gears for steam-engines (IA valvegearsforste00peab).pdf/100

CHAPTER IV.

RADIAL VALVE-GEARS. The name radial valve-gear has been applied to a number of reversing-gears that differ widely in detail and in general appearance, but agree in that they derive the mid-gear motion of the valve from some source that is equivalent to an eccentric with 90° angular advance, and they combine with this motion another that is equivalent to that of an eccentric with no angular advance. The general conception of this form of valve gear is most easily obtained from an example. Walschaert Gear. This gear is chosen as the first example of the type because the elements are easily distinguished. In Fig.I,Plate XXII, H is the engine cross-head, and a is the head of the valve-spindle. The valve is moved through a radius-rod, one end of which carries a block that may be set at any position in a slotted link dF, and the other takes hold of a combining-lever af^ that receives motion from the engine cross-head. The slotted link swings on a fixed trunnion at G and is moved by an eccentric OE, which has no angular advance. In Fig. 2 the diagram in thin lines shows the gear at a dead-point, and the diagram in heavy lines shows the gear when the crank has moved through the angle

COC=Θ

If the motion of the engine cross-head can be considered to be harmonic, then it is clear that the motion that it gives to the valve could be derived from an eccentric with 90° angular advance;this motion is made equal to the lap plus the lead. 88