This device was designed
specially as an inexpensive method of changing the common slide valve
into an automatic cut off. The cut off would not be as quick as in
other cases we have cited, depending here upon the movement of the
lower valve alone, and that, too, is in its slowest movement; whereas
in the other cases, the edges approaching each other, by the differing
movement of the valves the cut off is very rapid, provided the
distance to travel is not long. In this device considerable noise must
result by the cut off valve striking the cut off blocks, and a
considerable amount of leakage is likely to occur past this valve.
But there is one great objection in the valve gears thus far cited,
that the travel of the expansion valve upon the main valve is
variable. I have in mind the case of a Kendall & Roberts engine, which
had been run for a long time at no better economy than would be
obtained from a plain slide valve engine, and when it was attempted to
get an earlier cut off by separating the two cut off valves, they had
worn so much in their old place on the valve that shoulders were found
sufficient to cause a disagreeable noise and a leaky valve. This is
very apt to occur, not only where the valve is run for a long time on
one seat, but in cases of variation of the travel of the expansion
valve.
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