284

DETAILS OF SCREW ENGINES.

wi]l be noticed, however, that the steam cannotbe admitted at both ends of the cylindersimultaneously. This, however, is no greatfault, which practice has proved.

SLIDE VALVES.

The steam engine becomes, indeed, a pooraffair when the slide valve is out of order,and it is from this cause that many productionsare introduced to accomplish the best effect.The slide valve of the present day has growninto large proportions; it is a common prac-tice to make the length five feet, and thewidth six feet, for marine purposes. Now, withthese dimensions, obviously a large surface isexposed to the action of the steam. Thearrangement and formation of the ports,passages, and bars, are simple matters com-pared with the production of a perfect andconstant working contact for the respectivesurfaces. To accomplish this, the action ofthe steam must be duly considered, and themechanical means for adjustment.

The valve mostly used in the present day isthe type known as the double-ported equili-brium. An example of this kind is shown byFig. 134, as arranged by Messrs. Penn. Thevalve is depicted at half stroke, so that theports are covered. When the valve is at thefull stroke, the width of the opening causedequals half of the supply port. The steamadmitted to the inner ports passes through thevalves, and thus the pressure is relieved fromthe surfaces in contact. To prevent the steamacting on the back of the valve, a packingring is located thereon—shown in the sectionand in the half plan. The valve rod passesthrough the body, and is secured at the

extremity by a nut. The casing is the ordinarydesign, and secured by studs and nuts to the

Fig. 134.

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MESSRS. PENN’S SLIDE VALVE.

cylinder. For an engine 250 horse powernominal, the length of the slide valve is 5 feet4f inches, the width being 4 feet 2 inches,and the diameter of the rod 3 inches. Thisexample is in connection with the cylinderdepicted by Fig. 115 in page 274.

Messrs. Napier’s practice for the presentdetails is represented by Fig. 135, both in

Fig. 135.

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messes, napier’s slide valve.

: longitudinal and transverse sections.” This