MARINE ENGINES.

393

is a stuffing box and gland, and at the other end the copper pipeis bolted to a plain flange; while the part in the gland is quiteloose, expanding and contracting with the varying pressure.

Eccentric and link motion. —Various descriptions of valve gearare in use, but the one generally adopted is the double eccentrics andlink motion. This is only introduced as a sure means of handlingthe engines, and is but rarely used for working expansively, as in thelocomotive, for the obvious reason that were the slide valve travel-

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Fig. 275.—Eccentric Sheaves.a, Eccentric. bb, Keys. C, Bolt foreccentric.

Fig. 276.—Eccentric Strap and Rod.

A, Eccentric strap. b, Rod. c, Lubricator cup. d, Jawand pin for link.

ling only a small portion of its stroke, and kept running for weeksin this reduced grade, a groove would be formed on the cylinderface, which would pass steam into the condenser when the valve orlink was put in full gear, and so impair the vacuum; and it thusbecomes imperative to have a separate expansion valve. In theexample in Fig. 275 the eccentrics are cast in two halves andsecured with bolts and nuts, in other examples they are cast solid;in both cases they are firmly secured to the shaft by keys, as shown.