This had the old
disadvantage that all the steam in the lower chest expanded with that
in the cylinder, at a consequent considerable loss. This was further
improved by causing the riding cut off to be upon the top of the main
valve, instead of its chest, and resulted in a considerable reduction
of the clearance space.
This is the simplest form, and is shown in Fig. 1. The steam is
supplied by a passage through the main valve which operates exactly as
an ordinary slide valve would. That is, the inside edges of the steam
passage are the same as the ordinary valve, the additional piece on
each end, if I may so term it, being merely to provide a passage for
the steam which can be closed, instead of allowing the steam to pass
the edge. The eccentric of the main valve is fastened to the shaft to
give the proper amount of lead, and the desired release and
compression, and the expansion valve is operated by a separate
eccentric fastened in line with or 180 deg. ahead of the crank. When the
piston, therefore, commences to move from the crank end to open the
port, D, the expansion valve is forced by its eccentric in the
opposite direction, and is closing the steam port and would have
closed it before the piston reached quarter stroke, thus allowing the
steam then in the cylinder to do work by expansion.
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