If one port is more
exposed than the other, adjust by means of the eccentric screws till a
balance is obtained. Should the ports still not be fully uncovered, the
throw of the eccentric is too small, and you must either make a new
eccentric or reduce the width of the valve. (The second course has the
disadvantage of reducing the expansive working of the steam.) Excess
movement, on the other hand, implies too great an eccentric throw.
Setting the Eccentric.--Turn the crank full forward, so that a line
through the crank pin and shaft centres is parallel to the bed. Holding it
in this position, revolve the eccentric (the screw of which should be
slackened off sufficiently to allow the eccentric to move stiffly) round
the shaft in a clockwise direction, until it is in that position below the
shaft at which the front steam port just begins to show. Then tighten up
the eccentric lock screw.[1]
[Footnote 1: The reader is referred to an excellent little treatise,
entitled "The Slide Valve" (Messrs. Percival Marshall and Co., 26 Poppin's
Court, Fleet Street, E.C. Price 6d.), for a full explanation of the
scientific principles of the slide valve.
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