IMPROVEMENTS IN VARIABLE EXPANSION VALVES FOR STEAM ENGINES
Year: 1879

page_text: LETTERS PATENT to Alexander Melville Clark, of 53, Chancery Lane, in the
County of Middlesex, Patent Agent, for the Invention of "IMPROVEMENTS IN
VARIABLE EXPANSION VALVES FOR STEAM ENGINES." A communication from
abroad by Alexis Bretonneau, Fils, of Paris, France.
Sealed the 18th November 1879, and dated the 10th June 1879.
PROVISIONAL SPECIFICATION left by the said Alexander Melville Clark at
the Office of the Commissioners of Patents on the 10th June 1879.
ALEXANDER MELVILLE CLARK, of 53, Chancery Lane, in the County of Middlesex,
Patent Agent.
"IMPROVEMENTS IN VARIABLE EXPANSION VALVES FOR STEAM
ENGINES."
[A communication from abroad by Alexis Bretonneau, Fils, of Paris,
France.]
This Invention relates to an improved system of variable expansion valve for
steam engines, and is illustrated in the accompanying Drawings. It consists of an
expansion slide valve A, Figures 1 and 2, on the back of the main slide valvo D,
and of two moveable cut off plates C, C¹, on the back of the expansion valve A, which
do not participate in the motion of the slide valve, but may be adjusted nearer or
farther apart from one another by means of a stud g on each engaging in a curved
slot in a plate B (shown separately in Figure 3), on turning which the slides C, C¹,
are moved towards or away from one another in order to cut off earlier or later, and
so vary the grade of expansion. A spring y presses lightly on the back of the
plates C to keep them close against the expansion valve A, and the latter against
the main slide D. The expansion valve A is actuated by a crank or eccentric
placed at K, Figure 4, diametrically opposite the driving crank of the engine, if the
latter runs both ways, or at K' a little in advance of this position if it runs one way
only, in order that the expansion valve A shall be moving at a certain velocity
when the cylinder ports are uncovered by the main valve, and shall not arrive at
the end of its travel until after the closing of the said ports, in order to avoid wire
drawing by the plates C at the beginning and end of the stroke. The two slide
valves D and A move independently, and the travel of the valve A depends on the
space available in each case; thus in the example shown, the valve chest being
short and narrow, the travel of the slide A is only 7/10ths of that of the main
valve D, and notwithstanding this there is no wiredrawing of the steam. The ports
of the main valve are also made wider at 1 and 2 in order that here also there shall

page_text: be no wiredrawing. The dotted lines show how the size of the port and the
length of the valve would be increased for a more extended travel of valve A.
The plate B is fixed on a steel spindle t of the smallest possible diameter, passing
through a stuffing box in the cover of the valve chest, and is operated by a governor
or hand lever.
The operation is as follows:-When the piston is at the commencement of its
stroke and the crank is at H, Figure 4, the crank or eccentric operating valve A is
at K¹, and the edge 4 of the port z is in line with the axis of plate B, so that if the
plates C, C¹, were brought quite close together the port z would not be uncovered
at all, so that steam would not be admitted to the main valve, and the cut off
would then be at the very beginning of the stroke. Supposing the plates C, C', to
be separated each for the distance y¹, Figure 4 (full size), or the travel of the
expansion valve A during the uncovering of the cylinder port by the main valve,
the expansion valve A would have to move this distance y' before the edge 4 of
port z would close the port, which would take place at the moment when the main
valve closes the cylinder port, and the cut off will then be at the end of the stroke
of the main valve, supposing it to be without lap; the intermediate positions of the
plates C, C, correspond to grades of expansion between these two extremes. The
apparatus is easily controlled directly by a governor; the power required to turn
plate B being very small, it works without shock, there being no stop, and is
consequently applicable to engines running at high speeds, as well as engines running
both ways. By placing the eccentric or crank which operates the expansion valve
A diametrically opposite to the driving crank this expansion valve is in position for
rotation either way of the crank shaft, and it is only necessary to operate the main
valve in order to reverse the engine, the expansion valve being left to the control of
the governor, if one be employed, whereby the elastic force of the steam is utilised
to the greatest possible extent. The governor holding the plates Cat their greatest
distance apart when the engine is stopped enables it to be started again in either
direction with full pressure, whereby irregularities and difficulties are avoided at
the moment of starting, and the engine can be worked as if it had an ordinary D
valve, which are very important advantages.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Alexander Melville Clark in the Great Seal Patent Office on the
10th December 1879.
ALEXANDER MELVILLE CLARK, of 53, Chancery Lane, in the County of Middlesex,
Patent Agent. "IMPROVEMENTS IN VARIABLE EXPANSION VALVES FOR STEAM
ENGINES." A communication from abroad by Alexis Bretonneau, Fils, of Paris,
France.
This Invention relates to an improved system of variable expansion valve for
steam engines, and is illustrated in the accompanying Drawings.
It consists of an expansion slide valve A, Figures 1 and 2, having two ports z, ≈¹,
and placed on the back of the main slide valve D, and of two adjustable cut off
plates C, C, on the back of the expansion valve A which do not participate in the
motion of the slide valve, but may be adjusted nearer or further apart from one
another by means of a steel study screwed into each, engaging in a curved cam slot
in a plate B (shown separately in Figure 3), on turning which the slides C, C¹, are
moved towards or away from one another in order to cut off carlier or later, and so
vary the grade of expansion. A spring y presses lightly on the back of the plates
C to keep them close against the expansion valve A, and the latter against the main
slide D. The expansion valve A is actuated by a crank or eccentric placed at K,
Figure 4, diametrically opposite the driving crank of the engine, if the latter runs
both ways, or at K', a little in advance of this position, if it runs one way only, in
order that the expansion valve A shall be moving at a certain velocity when the
cylinder ports are uncovered by the main valve, and shall not arrive at the end of
its travel until after the closing of the said ports, in order to prevent the steam
being wiredrawn by the plates Ceither at the admission or at the cut off. The two
slide valves D and A move independently, and the travel of the valve A depends on
the space available in each case; thus in the example shown, the valve chest being
short and narrow, the travel of the slide A is only 7/10ths of that of the main valve
D, and notwithstanding this the steam is not wiredrawn. The ports of the main
valve are also made wider at 1 and 2 in order to avoid wiredrawing the steam at
this point. The dotted lines a show how the size of the port and the length of the
valve would be increased for a more extended travel of valve A. The plate B is
fixed on a steel spindle t of the smallest possible diameter passing through a stuffing
box in the cover of the valve chest, and is operated by a governor or hand lever.
In order to set out the slots in plate B for the studs g I mark off on the line a, à¹,
Figure 3, beginning at a point 1 a little below line m, m³, the distances 1, 2, 3, 4, 5,
6, 7, 8, 9, 10, which correspond to the different positions the plate C must occupy
in order to cut off at 1/10th, 10ths, 3/10ths, &c. (or other period) of the piston stroke,
and from these points 1, 2, 3, &c., with o as a centre, I describe arcs whose intersec-
tion with the radii 01, 02, 03, &c., give the centre line of the curve for the slot which
is to operate the plate C; for the other slot I proceed in the same way for the
divisions of the line n, n, but instead of the radii I draw parallels 1-1, 2-2, 3-3, &c.,
to lines which, like a, a¹, pass through points in the centre line of the first curve,
and are tangents to a circle o, o', described with the radius o, a¹. This gives two
curves differing in accordance with the influence of the length of the connecting rod
and of the eccentric of valve A.
The operation is as follows:-When the piston is at the beginning of its stroke
and the crank is at H, Figure 4, the crank or eccentric operating valve A is at K',
and the edge 4 of the port z is in line with the axis of plate B. Now, supposing
the plates C, C¹, were brought into close contact, the port z would not be uncovered
at all, steam would not be admitted to the main valve, and the cut off would be at
the very beginning of the stroke; on the other hand, supposing the plates C, C¹, to
be separated each for the distance y', Figure 4 (full size), which corresponds to the

page_text: travel of the expansion valve A during the uncovering of the cylinder port by the
main valve, the expansion valve A would have to move this distance y¹ before
the edge 4 of port z would close the port, which it would do at the moment when
the main valve closes the cylinder port, and the cut off would then be at the end
of the travel of main valve, supposing it to be without lap. The intermediate positions
of the plates C, C¹, between these two extremes correspond to various grades of
expansion.
The apparatus is easily controlled directly by a governor, the power required to
turn plate B being very small; it works without shock, there being no stop, and is
consequently applicable to engines running at high speeds as well as to engines
running both ways by placing the eccentric or crank which operates the expansion
valve A diametrically opposite to the driving crank; this expansion valve is in
position for rotation either way of the crank shaft, and it is only necessary to shift
the reversing gear of the main valve in order to reverse the engine, the expansion
valve being left to the control of the governor, if one be employed, whereby the
elastic force of the steam is utilized to the greatest possible extent. As the governor
separates the plates C, and keeps them at their greatest distance apart when the
engine is stopped, the latter may be started again in either direction with full
admission, whereby irregularities and difficulties are avoided at the moment of
starting, and the engine can be worked as if it had an ordinary D valve, which are
very important advantages.
Having described the nature of the said Invention, and the manner of performing
the same, I declare that what I claim as the Invention to be protected by the
herein before in part recited Letters Patent is, the improved system of expansion
valve, consisting of the adjustable plates C, C¹, in combination with and adjusted
by the plate B, having cams of peculiar form as shown, and a spring y, the whole
applied in connection with an expansion slide valve A substantially as shown and
described.
In witness whereof, I, the said Alexander Melville Clark, have bereunto set my
hand and seal, this Ninth day of December, in the year of our Lord One
thousand eight hundred and seventy nine.