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By THOMAS HAWLEY.
RIDING CUT-OFF VALVES--PECULIARITIES AND MERITS OF THE DIFFERENT
STYLES.
In considering the slide valve in its simple form with or without lap,
we find there are certain limitations to its use as a valve that would
give the best results. The limitation of most importance is that its
construction will not allow of the proper cut off to obtain all the
benefits of expansion without hindering the perfect action of the
valve in other particulars. At this economical cut off the opening of
the steam port is very little and very narrow, and although this is
attempted to be overcome by exceedingly wide ports, sixteen inches in
width in many cases in locomotive work, this great width adds largely
to the unbalanced area of the valve. The exhausting functions of the
valve are materially changed at the short cut off, and when much lap
is added to overcome this defect, there usually takes place a choking
of the exhaust port. You might inquire, why not make the port wider,
but this would increase the minimum amount of load on the valve, and
this must not be overlooked. Then the cut off is a fixed one, and we
can govern only by throttling the pressure we have raised in the
boiler or by using a cut off governor and the consequent wastes of an
enormous clearance space.
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