i iS

MODERN STEAM PRACTICE.

from B to E, then cut the crank path at F, the dotted line A F is theline of cut-off by crank when the piston has travelled five-eighths ofits stroke. This angle is always the same, no matter whether thestroke or length of the crank is longer or shorter; that is to say, ifthe connecting rod bears the same proportion to the crank, namely,five to one.

THE CRANK AND ECCENTRIC PATHS.

The crank and eccentric paths are identical; the path of the oneis exactly the path of the other. Each revolves around the samecentre, namely, that of the main shaft of the engine. The crank andeccentric centres each describe an arc, the length of the chordsvarying with the circles described. The chord A B described by thecrank centre being greater than that delineated by the eccentric

Flg. 64.—The Crank and Eccentric Paths.

a b, Chord of the arc of supply on crank path. c d, Chord of the arc of supply on eccentric path.e f, Versed sineof the chord of the arc of supply on crank path. g h, Versed sine of the chordof the arc of supply on eccentric path. 1, Centre of engine shaft.

centre on C D, consequently their versed sines must likewise vary.Thus, in the example, the large circle denotes the path of the crank,and the small circle that of the eccentric; A is the commencing ofthe IN stroke, and B the point of cut-off determined on. It is veryevident that the point A, or crank pin, has travelled from A to B,while that of the eccentric has travelled from C to D; draw the linesA B and C D, bisect ABatF, and draw the lines E F and G H throughthe centre I. The line E F is the versed sine of the chord for the crankpath, and GH is the versed sine of the chord for the eccentric path.In all cases the versed sine of the chord for the crank path must, in