CERTAIN IMPROVEMENTS IN STEAM ENGINES
Year: 1868

page_text: PROVISIONAL SPECIFICATION left by George Holcroft and William
Nathan Dack at the Office of the Commissioners of Patents, with
their Petition, on the 2nd October 1868.
We, GEORGE HOLCROFT and WILLIAM NATHAN DACK, both of the
City of Manchester, Engineers, do hereby declare the nature of the
said Invention for "CERTAIN IMPROVEMENTS IN STEAM ENGINES," to be as.
follows:-
Our Invention relates to an improved mode of working the valves.
of steam engines described in the Specification of Letters Patent granted
to us on the Eleventh day of December, One thousand eight hundred
and sixty-seven, No. 3512.
In the said Patent we described our improved cut-off or expansion.
gear consisting of a variable stop (acted on and made variable by the
action of the governor) coming in contact with a fixed stop on the
valve spindle; now according to our present improvements we connect
the said differential stop to an eccentric rod or its equivalent, thereby
making it slide backwards and forwards with it, instead of being fixed
as described in the former Patent. This eccentric or its equivalent.
may be set opposite to the crank (or otherwise) and by placing a
plate or having a facing with ports between the slide valve and the

page_text: expansion valves the steam may be cut off at any part of the stroke
of the piston. The motion of the valve in this case will be positive
and not depending on the friction of one valve on the other.
Another part of our Invention consists in varying the point of cut-
off by means of a lever, one end being attached to or in connection
with the spindle of the cut-off valve or to the valve, and the other
end fixed. This lever passes between jaws on a sliding block and
alternately comes in contact with it; the position of this sliding block
is varied by the action of the governor, and the amount of steam
admitted to the cylinder is thereby varied according to the load. This
arrangement may be again worked by an eccentric as described above,
or the expansion valve may be carried on the back of the other as
in our former Patent. When worked by an eccentric it would give
motion to a second lever by means of the sliding block coming alternately
in contact with it; the sliding block or differential stop will have a
certain amount of play between it and the lever, so that when it is
near the fixed end it will allow it to traverse further than when near
to the end attached to the valve rod, thereby varying the amount of
expansion.
Another part of our Invention consists in the application of the
above-named plate or facing with ports situate between the slide and
expansion valves being cast to the valve box, or held to its place by
bolts or springs or their equivalents.
Another part of our Invention relates to the governor rod, which is
made in two parts, each with a head acted upon by springs contained
in a box, where the variable stop of expansion gears is in contact with
the stops connected to the valve spindle or eccentric rod; the governor
will be at liberty to act against one of the springs, and when the
stop is released the compressed spring will act on the variable stop
of the cut-off gear to increase or reduce the speed of the engine or
amount of steam admitted into the cylinder.