IMPROVEMENTS IN STEAM ENGINES
Year: 1871

page_text: PROVISIONAL SPECIFICATION left by John Hopkinson at the Office
of the Commissioners of Patents, with his Petition, on the 20th
May 1871.
I, JOHN HOPKINSON, the younger, of Manchester, in the County
of Lancaster, Engineer, do hereby declare the nature of the said In-
vention for "IMPROVEMENTS IN STEAM ENGINES," to be as follows:-
The object of my improvements is to effect a variable expansion of
the steam regulated by the governor.
My Invention relates to that form of valve in which the steam is
admitted and the exhaust opened and closed by means of one or two
slide valves, and is cut off by means of an independent cut-off slide
moving at the back of the slide valve or valves.
The Invention consists also in the construction of the slide valves
and in the mode of setting the eccentrics, also in the manner of con-
trolling the setting of the cut-off eccentric by means of the governor.
The slide valve eccentric is set at about 90° in advance of the crank,
and the bars or solid faces of the slide valve are made of the same width
as the ports in the side of the cylinder instead of being wider or having

page_text: a "lap" as heretofore, they also correspond thereto, so that when the
engine is on either dead centre the bars of the slide valve just cover
each port, and as the engine passes the centre the exhaust is closed,
and the steam opened on the one side of the piston, whilst on the
other the exhaust is simultaneously opened. The expansion is varied
by altering the lead of the cut-off eccentric, the change of lead being
effected by the action of the governor. Any other means of obtaining
reciprocating motion may be used instead of eccentrics, provided only
the principle of changing the lead of the cut-off slide be applicable.
If eccentrics are used the slide valve eccentric is fixed on the crank shaft
but the cut-off eccentric is loose on the same. The two eccentrics are
connected together by a pin which is movable in a radial slot in one
eccentric, and is moved along a transverse slot in the other by the
action of a screw supported in bearings fixed therein. When the engine
goes too fast or too slow the governor operates on the screw, so moving
the pin along the transverse slot, and altering the setting of the loose
eccentric. Instead of connecting the two eccentrics it is sometimes
convenient to connect the loose eccentric to an independent disc fixed
to the crank shaft.
The mechanism I prefer for working the screw consists of a worm
wheel on the axis of the screw, and a worm to which a toothed wheel
is attached, the axis of the toothed wheel being parallel to the crank
shaft. As the toothed wheel revolves around the crank shaft it may be
acted on on either side by one of the prongs of a fork moved by the
governor. When the governor balls expand one prong of the fork acts
on the toothed wheel, so increasing the lead of the cut-off eccentric and
cutting off the steam earlier in the stroke, whereas when the balls
collapse the other prong of the fork is brought against the other side of
the toothed wheel, so causing the steam to be cut off at a later period.
The means by which the governor controls the lead of the cut-off valve
may
be modified.