IMPROVEMENTS IN STEAM ENGINES
Year: 1855

page_text: LETTERS PATENT to Alexander Dalgety, of the firm of Dalgety and
Ledger, of Deptford, in the County of Kent, Engineers, for the Invention
of "IMPROVEMENTS IN STEAM ENGINES."
Sealed the 18th September 1855, and dated the 29th March, 1855.
PROVISIONAL SPECIFICATION left by the said Alexander Dalgety at
the Office of the Commissioners of Patents, with his Petition, on the
29th March 1855.
I, ALEXANDER DALGETY, of the firm of Dalgety & Ledger, of Deptford, in the
County of Kent, Engineers, do hereby declare the nature of the said Invention
for "IMPROVEMENTS IN STEAM ENGINES" to be as follows:-
This Invention relates to the general construction & arrangement of the
several working parts of steam engines, whether of the condensing or high
pressure principle. According to one modification, the engine will consist of
one long cylinder, the ends of which must be circular and bored true, whilst
in the centre is cast a chamber of any convenient form, made steam-tight by
side covers, and containing the crank. A pair of semi-trunk or tubular pistons
work inside the circular or bored pistons of the cylinder. These pistons are
in connection by means of suitable connecting rods with a double throw crank

page_text: working inside the central or crank chamber, and are thus made to approach
to and recede from each other alternately. On each end of the cylinder and
concentric with its bore is bolted or otherwise secured a smaller cylinder of
rather greater length than the piston tube or trunk, and the cylinder may be
either opened or closed at the end. A guide is thus formed in which the
trunk or tubular end of each piston works. The connecting rods are
jointed to the extreme end of the trunks or piston tubes farthest from the
pistons, thus avoiding the upward or downward thrust during the revolutions
of the crank. By this arrangement it will be obvious that as the ends of the
trunks are closed, the area of the two inner sides of the pistons next the
cranks are greater than the outer sides next the trunks. Steam being admitted
by a slide valve on the smaller area of the pistons, they are caused to approach
each other. When the stroke of the pistons is nearly completed, a number
of passages communicating from the outer sides of the pistons to the central
steam-tight crank chamber are opened, by the piston passing a short distance
beyond them, whereupon the steam which has already done its work is
allowed to expand into the central chamber. The communication is then
closed by the receding of the pistons, and the steam in the end cylinders is
allowed to exhaust, the valves being set for that purpose by suitable valve gear
& eccentrics placed on the main shaft. The steam contained in the central
chamber now acts by expansion upon the large area of each piston simul-
taneously, thereby causing them to recede from each other and complete the
double stroke of the engine. On the completion of the stroke, the steam is
allowed to escape from the central chamber through an exhaust aperture,
which is opened or closed at the proper moment by suitable valve gear. The
crank shaft is supported in steam-tight bearings, cast or otherwise secured to
the sides of the central or crank chamber. When used as a marine engine,
two cylinders are placed side by side with the cranks at right angles.
By another mode of working, the steam may be allowed to enter only one end
cylinder, such steam, expanding as before described, into the central chamber,
& acting upon both pistons simultaneously, as in the former case. Thus, the
pistons will be brought together or caused to approach each other by steam
pressure on one of the pistons only, and will be caused to recede from each other
by a diminished pressure on the larger area of both the pistons simultaneously.
The slide valves for admitting the steam into the cylinder are worked by side
rods and cross heads, such rods being connected to bell cranks which are attached
to vertical slides worked by eccentrics on the crank shaft. The escape valve
is worked by rods from the cross heads of the steam slides, which rods are
jointed to links also jointed to the spindle-of the valve, but so disposed as to

page_text: be at an angle with the rods from the cross heads. By this means, when
these rods approach to or recede from each other, a knee-joint action will
occur at the links, and the valve will be opened or closed accordingly.
Another modification of the same principal, that is, the crank being placed
in the centre of the cylinder or between the two working pistons, consists in
having the pistons either cast in one piece, or connected by rods or other
suitable contrivance; in this case a single throw crank only is required,
connected to either end of the double piston. The crank may either be open
to the atmosphere, or may be enclosed in a steam-tight casing, as in the first-
described arrangement. The steam is admitted by suitable valve gearing at
the opposite ends of the cylinder alternately. In some cases a trunk may be
fitted to or cast on to one or both ends of the double piston,, working in a
guide or guides, similar to the first-described arrangement. The connection
with the connecting rod will then be, as herein-before described, placed in
the extreme end of one of the trunks, thus removing from the piston the
upward or downward thrust of the connecting rod. In place of a connecting
rod, the crank pin may slide in a slot formed in a rib, either cast with or
secured to the piston.