IMPROVEMENTS IN EXCENTRICS FOR ACTUATING THE VALVES OF STEAM' ENGINES, AND PRODUCING RECIPROCATING MOTION
Year: 1880

page_text: PROVISIONAL SPECIFICATION left by Edmund Edwards at the Office. of
the Commissioners of Patents on the 6th July 1880. A communication from
abroad by José Lopes, of Fayal, in the Kingdom of Portugal.
EDMUND EDWARDS,' of the Firm' of Edwards and Co., Engineers, and Patent
Agents, of 40, Southampton Buildings, Chancery Lane, in the County of Middlesex..
"IMPROVEMENTS IN EXCENTRICS FOR ACTUATING THE VALVES OF STEAM' ENGINES,
AND PRODUCING RECIPROCATING MOTION." [A communication from abroad' by
José Lopes, of Fayal, in the Kingdom of Portugal.]
This Invention has for its object a novel arrangements of apparatus to be
especially as a substitute for the excentric as ordinarily constructed, and used for
actuating the valves of steam engines, but it may be used in other circumstances
where it is desired to convert a circular movement into a reciprocating rectilinear
movement. In the accompanying Drawing, Figure 1 represents the novel excentric,
and Figures 2, 3, and 4 represent it in other positions, whilst it is describing half
a revolution during the stroke of the piston from one end of its cylinder to the .
other.
In Figure 1 the position of the excentric corresponds with the half stroke of the
slide valve, the piston being at the dead point at one or other end of the cylinder,
and the valve closed. Figure 2 shows the full opening of the port for the
admission of steam. Figure 3 represents the point when the valve commences
to close the steam port; and in Figure 4 the valve is closed, the piston being at
the opposite end of its stroke, and the excentric having described half a revolution.
Between the position shewn in Figures 1 and 2, the main driving shaft moves
through an angle of 43° 30', opening wide the admission port at one end of the
cylinder. From Figures 2 to 3, the shaft moves through an angle of 93°, whilst the
port remains wide open.
From Figure 3 to Figure 4, the shaft moves through an angle of 43° 30' whilst
the valve is closing, the shaft having then revolved through a complete semi-
circumference (43° 30' + 93° + 43° 30′ : 180°) the diameter B, B2 (Figure 1).
During the other semicircumference the same process is repeated.
The necessary data for the construction of the improved apparatus are the
following: First. The diameter of the driving shaft to which the excentric is to be
fitted. Second. The thickness, B, C, Figure 1, of the excentric, which should be
proportioned to the work to be done, and may be more or less. Third. The travel
of the valve C², D, Figure 1. When these data are known, the proportions and shape
of the novel excentric are determined as follows:-First. The point A is taken

page_text: equidistant between B and B³, and representing the centre of the driving shaft,
from which the following circles are described, viz.-First, with the radius A, B, &
circle, which represents the diameter of the driving shaft; second, with the radius
A, C, or A, C2, a segment which represents the thickness of the inner part of the
excentric; and finally, with the radius A, D, a segment which represents the outer
boundary of the excentric. From the point D a line is drawn, of equal length in
both directions, tangentially to the segment at D, and equal in length to C, D, and
the points J, K, are thus fixed. From these points two parallel lines are drawn,
which cut at 4 points the circle described by the outer segment F, G, of the
excentric, and the dead points of the engine are determined when the arc F, G,
touches two of these points at the same time.
The parrallelogram is completed by the addition of a fourth line J, K, parallel
to that passing through D, and in the same relative position on the other side of
the driving shaft.
In actual work, the two opposite segments of the excentric are the most
important, since they operate together and uniformly, one I, H, on the line K, K²
and the other F, G, on the line J, J, during the part of the stroke in which the
admission port to the cylinder is fully open.
The entire excentric is completed by describing from the point F the segment
G, H, with a radius equal to C, D, cutting the point G, and joining the arc H, I;
and from the point G, the segment F, I, with the same radius, cutting the point F
and joining the arc I, H.
Round the excentric or cam thus formed is fitted the parallelogram or frame
J, J, K, K2, which is made of iron or other material sufficiently strong for the
work which it has to do. To this frame is connected the rod by which the valve
is driven, or other mechanism to which. it is desired to communicate a reciprocating
rectilinear movement.
This frame works between two parallel guides P, p, P, p; fixed firmly in the
proper position, and adjusted accurately to the outer faces of the frame, as shewn.
The proportions and details of construction of the novel apparatus described may be
modified to suit varying circumstances.