The result is that a change will bring about a leaky valve,
something that every engineer abhors.
The construction of the Buckeye engine, which is also of this type, is
such that the travel of the valve on the back of the main valve is
always the same, no matter what the cut off may be. Then this engine
makes use of our second proposition as a means of effecting the cut
off, viz., by advancing the eccentric. You will readily observe that
anything that will cause the cut off valve to reach a certain point
earlier in the stroke will bring about an earlier cut off as it
hastens everything all around. This is the plan pursued in the
Buckeye, in which the governor, of the shaft type, turns the eccentric
forward or back according as the load demands. Then, in addition, the
valve is balanced partially, the attempt not being made to produce an
absolutely balanced valve, on the ground that there should be friction
enough to keep the surfaces bright and to prevent leakage. The most
perfect valve will, of course, be entirely balanced under all
conditions of pressure so as to move with perfect ease. With the
riding cut off valve in connection with the plain slide valve, this is
not accomplished, and it does not matter whether it is partially
unbalanced to prevent leakage or not, the fact that it is not entirely
balanced prevents it reaching the ideal valve.
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