Page:Model Steam Locomotives by Greenly Henry.pdf/202

Motion Details rod fixing in the manner shown in Fig. 234, which provides a differential screw form of valve adjustment at the same time.

In small engines eccentric sheaves are often grooved in the centre and a rib formed on the bearing surface of the strap to freely fit this groove. This reduces the overall size of the sheave, which is important where they are cut solid with the axle.

In large engines, and where this consideration may not apply, the groove is best arranged in the sheave. The double-flanged sheave, shown in Fig. 230, is not often used. By adopting the central rib or groove method just described, the dimensions of overall width can be cut down to the minimum.

The two designs for “indirect” link motion, i.e. a valve gear in which the motion is transmitted through a rocking shaft to valves on top of the cylinders, illustrated in Fig.235, are interesting arrangements suitable for 3i-in. gauge (and larger) models. The upper one is the better of the two, as it eliminates the careful fitting necessary in making and assembling the intermediate valve spindle, rocker and die block used in the other scheme. All wearing surfaces are either round pins or drilled holes. 193