By Prof. C.W. MacCord.
The accompanying engravings illustrate the arrangement of a propeller
engine of 20 inch bore and 22 inch stroke, whose cylinder and valve gear
were recently designed by the writer, and are in process of construction
by Messrs. Valk & Murdoch, of Charleston, S.C.
In the principal features of the engine, taken as a whole, as will be
perceived, there is no new departure. The main slide valve, following
nearly full stroke, is of the ordinary form, and reversed by a shifting
link actuated by two eccentrics, in the usual manner; and the expansion
valves are of the well known Meyer type, consisting of two plates on the
back of the main valve, driven by a third eccentric, and connected by a
right and left handed screw, the turning of which alters the distance
between the plates and the point of cutting off.
The details of this mechanism, however, present several novel features,
of which the following description will be understood by reference to
the detached cuts, which are drawn upon a larger scale than the general
plan shown in Figs. 1 and 2.
[Illustration: Figs. 1-2 IMPROVED STEAM ENGINE.--BY PROF MACCORD.]
The first of these relates to the arrangement of the right and left
handed screw, above mentioned, and of the device by which it is rotated.
Usually, the threads, both right handed and left handed, are cut upon
the cut-off valve stem itself, which must be so connected with the
eccentric rod as to admit of being turned; and in most cases the valve
stem extends through both ends of the steam chest, so that it must both
slide endwise and turn upon its axis in two stuffing boxes, necessarily
of comparatively large size.
All this involves considerable friction, and in the engine under
consideration an attempt has been made to reduce the amount of this
friction, and to make the whole of this part of the gear neater and more
compact, in the following manner:
Two small valve stems are used, which are connected at their lower ends
by a crosstail actuated directly by the eccentric rod, and at their
upper ends by a transverse yoke. This yoke, filling snugly between two
collars formed upon a sleeve which it embraces, imparts a longitudinal
motion to the latter, while at the same time leaving it free to rotate.
This sleeve has cut upon it the right and left handed screws for
adjusting the cut-off valves; and it slides freely upon a central
spindle which has no longitudinal motion, but, projecting through the
upper end of the valve chest, can be turned at pleasure by means of a
bevel wheel and pinion. The rotation of the spindle is communicated to
the sleeve by means of two steel keys fixed in the body of the latter
and projecting inwardly so as to slide in corresponding longitudinal
grooves in the spindle.
Thus the point of cutting off is varied at will while the engine is
running, by means of the hand wheel on the horizontal axis of the bevel
pinion, and a small worm on the same axis turns the index, which points
out upon the dial the distance followed. These details are shown in
Figs. 3, 4, and 5; in further explanation of which it may be added that
Fig. 3 is a front view of the valve chest and its contents, the cover,
and also the balance plate for relieving the pressure on the back of the
main valve (in the arrangement of which there is nothing new), being
removed in order to show the valve stems, transverse yoke, sleeve, and
spindle above described. Fig. 4 is a longitudinal section, and Fig. 5 is
a transverse section, the right hand side showing the cylinder cut by a
plane through the middle of the exhaust port, the left hand side being a
section by a plane above, for the purpose of exhibiting more clearly the
manner in which the steam is admitted to the valve chest; the latter
having no pipes for this service, the steam enters below the valve, at
each end of the chest, just as it escapes in the center.
The second noteworthy feature consists in this: that the cut-off
eccentric is not keyed fast, as is customary when valve gear of this
kind is employed, but is loose upon the shaft, the angular position in
relation to the crank being changed when the engine is reversed; two
strong lugs are bolted on the shaft, one driving the eccentric in one
direction, the other in the opposite, by acting against the reverse
faces of a projection from the side of The eccentric pulley.
The loose eccentric is of course a familiar arrangement in connection
with poppet valves, as well as for the purpose of reversing an engine
when driving a single slide valve. Its use in connection with the Meyer
cut-off valves, however, is believed to be new; and the reason for its
employment will be understood by the aid of Fig. 6.
For the purposes of this explanation we may neglect the angular
vibrations of the connecting rod and eccentric rod, considering them
both as of infinite length. Let O be the center of the shaft; let L O M
represent the face of the main valve seat, in which is shown the port
leading to the cylinder; and let A be the edge of the main valve, at the
beginning of a stroke of the piston. It will then be apparent that the
center of the eccentric must at that instant be at the point, C, if the
engine turn to the left, as shown by the arrow, and at G, if the
rotation be in the opposite direction; C and G then may be taken as the
centers of the "go-ahead" and the "backing" eccentrics respectively,
which operate the main valve through the intervention of the link.
Now, in each revolution of the engine, the cut-off eccentric in effect
revolves in the same direction about the center of the main eccentric.
Consequently, we may let R C S, parallel to L O M, represent the face of
the cut-off valve seat, or, in other words, the back of the main valve,
in which the port, C N, corresponds to one of those shown in Fig. 4; and
the motion of the cut-off valve over this seat will be precisely, the
same as though it were driven directly by an eccentric revolving around
the center, C.
In determining the position of this eccentric, we proceed upon the
assumption that the best results will be effected by such an arrangement
that when cutting off at the earliest point required, the cut-off valve
shall, at the instant of closing the port, be moving over it at its
highest speed. And this requires that the center of the eccentric shall
at the instant in question lie in the vertical line through C.
[Illustration: Figs. 3-12 IMPROVED STEAM ENGINE.--BY PROF MACCORD.]
Next, the least distance to be followed being assigned, the angle
through which the crank will turn while the piston is traveling that
distance is readily found; then, drawing an indefinite line C T, making
with the vertical line, G O, an angle, G C T. equal to the one thus
determined, any point upon that line may be assumed as the position of
the required center of the cut-off eccentric, at the beginning of the
stroke.
But again, in order that the cut-off may operate in the same manner when
backing as when going ahead, this eccentric must be symmetrically
situated with respect to both C and G; and since L O M bisects and is
perpendicular to G C, it follows that if the cut-off eccentric be fixed
on the shaft, its center must be located at H, the intersection of C T
with L M. This would require the edge of the cut-off valve at the given
instant to be at Q, perpendicularly over H; and the travel over the main
valve would be equal to twice C H, the virtual lever arm of the
eccentric, the actual traverse in the valve chest being twice O H, the
real eccentricity.
This being clearly excessive, let us next see what will occur if the
lever arm, CH, be reduced as in the diagram to CK. The edge of the
cut-off valve will then be at N; it instantly begins to close the port.
CN, but not so rapidly as the main valve opens the port, AB.
The former motion increases in rapidity, while the latter decreases;
therefore at some point they will become equal in velocity, and the
openings of the two ports will be the same; and the question is, Will
this maximum effective port area give a sufficient supply of steam?
This diagram is the same as the one actually used in the engine under
consideration, in which it was required to follow a minimum distance of
5 inches in the stroke of 22. Under these conditions it is found that
the actual port opening for that point of cutting off is three-fifths of
that allowed when following full stroke, whereas the speed of the piston
at the time when this maximum opening occurs is less than half its
greatest speed.
This, it would seem, is ample; but we now find the eccentric, K, no
longer in the right position for backing; when the engine is reversed it
ought to be at, P, the angle, POL, being equal to the angle, KOL. By
leaving it free, therefore, to move upon the shaft, by the means above
described, through the angle, KOP, the desired object is accomplished.
The real eccentricity is now reduced in the proportion of OK to OH,
while the lengths of the cut-off valves, and what is equally important,
their travel over the back of the main valve, are reduced in the
proportion of CK to CH, in this instance nearly one-half; a gain quite
sufficient to warrant the adoption of the expedient.
The third, and perhaps the most notable, peculiarity is the manner of
suspending and operating the main link. As before stated, this link is
used only for reversing, and is therefore always in "full gear" in one
direction or the other; and the striking feature of the arrangement here
used is that, whether going ahead or backing, there is _no slipping of
the link upon the link block_.
The link itself is of the simplest form, being merely a curved flat bar,
L, in which are two holes, A and B (Fig. 7), by which the link is hung
upon the pins, which project from the sides of the eccentric rods at
their upper ends.
This is most clearly shown in Fig. 8, which is a top view of the
reversing gear. The link block is a socket, open on the side next to the
eccentric rods, but closed on the side opposite, from which projects the
journal, J, as shown in Fig. 9, which is a vertical section by the
plane, XY. This journal turns freely in the outer end of a lever, M,
which transmits the reciprocating motion to the valve, through the
rock-shaft, O, and another lever, N. Connected with the lever, M, by the
bridge-piece, K, and facing it, is a slotted arm, G, as shown in the end
view, Fig. 10. The center line of this slot lies in the plane which
contains the axes of the journal, J, and of the shaft, O.
A block, E, is fitted to slide in the slotted arm, G; and in this block
is fixed a pin, P. A bridle-rod, R, connects P with the pin, A, of one
of the eccentric-rods, prolonged for that purpose as shown in Fig. 8;
and a suspension-rod, S, connects the same pin, P, with the upper end of
the reversing lever, T, which is operated by the worm and sector. The
distance, JO, in Fig. 10, or in other words the length of the lever, M,
is precisely equal to the distance, AB, in Fig. 7, measured in a right
line; and the rods, R and S, from center to center of the eyes, are also
each of precisely this same length. Further, the axis about which the
reversing lever, T, vibrates is so situated that when that lever, as in
Fig 11, is thrown full to the left, the pin in its upper end is exactly
in line with the rock-shaft, O.
When the parts are in this position, the suspension-rod, S, the arm, G,
and the lever, M, will be as one piece, and their motions will be
identical, consisting simply of vibration about the axis of the
rock-shaft, O. The motion of the lever, M, is then due solely to the
pin, B, which is in this case exactly in line with the journal, J, so
that the result is the same as though this eccentric rod were connected
directly to the lever; and the pin, P, being also in line with B and J,
and kept so by the suspension-rod, S, it will be seen that the
bridle-rod, R, will move with the link, L, as though the two were
rigidly fastened together.
When the reversing lever, T, is thrown full to the right, as in Fig. 12,
the pin, P, is drawn to the inner end of the slot in the arm, G, and is
thus exactly in line with the rock-shaft, O. The suspension-rod, S,
will, therefore, be at rest; but the pin, A, will have been drawn, by
the bridle-rod, R, into line with the journal, J, and the bridle-rod
itself will now vibrate with the lever, M, whose sole motion will be
derived from the pin, A.
There is, then, no block slip whatever when the link thus suspended and
operated is run in "full gear," either forward or backward.
If this arrangement be used in cases where the link is used as an
expansion device, there will be, of course, some block slip while
running in the intermediate gears. But even then, it is to be observed
that the motion of the pin, A, relatively to the rocker arm is one of
vibration about the moving center, J; and its motion relatively to the
sliding block, E, is one of vibration about the center, P, whose motion
relatively to E is a small amount of sliding in the direction of the
slot, due to the fact that the rocker arm itself, which virtually
carries the block, E, vibrates about O, while the suspension-rod, S,
vibrates about another fixed center. It will thus be seen that, finally,
the block slip will be determined by the difference in curvature of arcs
_which curve in the same direction_, whether the engine be running
forward or backward; whereas in the common modes of suspension the block
slip in one direction is substantially the half sum of the curvatures of
two arcs curving in opposite directions.
Consequently it would appear that the average action of the new
arrangement would be at least equal to that of the old in respect to
reducing the block slip when running in the intermediate gears, while in
the full gears it entirely obviates that objectionable feature.
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