A manual of marine engineering: comprising the designing, construction, and working of marine machinery by Seaton Albert Edward 1848-1930

Author:Seaton, Albert Edward, 1848-1930
Language: eng
Format: epub, pdf
Tags: Marine engineering
Publisher: London, Griffin
Published: 1896-03-25T05:00:00+00:00

Fig. 84. —Proportions of a Common Valve.

Let x be the outside lap of the valve at the front end, and y that at the back end. Then,

H

and K

• + y-

Let * be the inside lap at the front, and w that at the back. Then,

F

VALVE GEAR : LINK MOTION.

28S

Fig. 85. —Proportions of a Trick Valve. Let x, y, z, and w be the laps as before. Then,

G

Also,

G and K = ~

"R 1 and 0 = — - w.

0

+ | inch ; and D = -- + j inch.

The openings through the valve laps or covers must be as large as possible, but need not exceed the ordinary opening of the valve to steam at the outer edge ; then

G+P=K+N;

and G + Q

Valve Gear: Link Motion.—The common form of motion employed to work the valves of a screw engine consists essentially of two eccentrics keyed on the crank-shaft in such a position relative to the crank that when one is operating on the valve, the engine will propel the ship a-head and is said to be in head-gear, and when the other, the engine will propel the ship stern first and is said to be in stern-gear, their rods being connected by a bar or bars on which is a block to which the valve spindle is attached. This bar connection is called the link, and is of such a form that by sliding it through the block, the head or stern eccentric may at pleasure be brought to operate on the valve.

In designing a link gear, the most important objects are to give the valve such motions as shall cause it to open to steam slightly before the piston is at the end of its stroke, the amount by which it

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