NOTE ON VERY EARLY CUT-OFF, AND ON REVERSING GEARS IN GENERAL.
_Very early Cut-off._—Theoretically there is no limit to the range of
cut-off which may be obtained from an ordinary slide valve, but a
moment’s consideration will show that if we require a valve to cut off
steam very early in the stroke of the piston so as to cause it to expand
throughout an increased fraction of that stroke, the _lap_ must be
considerably increased beyond that which is usual, if the valve-face is
to be enabled to keep the steam confined in the cylinder for a
sufficient period after cut-off is effected.
As a consequence of increased lap, the _travel_ of the valve, made up of
_lap plus the required port-opening_, will need to be increased
correspondingly; the _linear advance_ (the sum of lead plus an
_increased_ lap) will be increased also, and the valve will therefore
come back earlier into its central position (where compression begins)
than would the normal valve, so making compression begin earlier and
last longer than before. In an unbalanced valve, this increased travel,
together with the abnormal compression, cannot be tolerated, so that for
obtaining an early cut-off without too much compression, with a sliding
valve, gear of the “separate cut-off slide,” or Meyer type, is employed.
This is characterised by a cut-off plate or plates on the back of a main
slide valve, which may have the usual exhaust cavity, and in addition
possesses supply steam-conduits leading through the valve from the back
to the “lap-edges” (fig. 38). Exhaust and compression are timed by the
edges of the central exhaust cavity; lead is given by the “lap-edges,”
but cut-off is controlled by movement of the cut-off plates on the back
of the moving main valve, so that though a “lap-edge” may not at any
time have closed one of the steam-ports proper, a cut-off plate worked
by a separate eccentric can close the appropriate steam-conduit above
referred to, and so stop the current of steam flowing past the lap-edge
to the still open port. Moreover, by a suitable setting of the
eccentrics of the main and cut-off valves, it is possible to arrange
that, at the instant of cut-off, the two valves shall be moving in
opposite directions, and so shall effect a sharp cut-off, whereby
wire-drawing is reduced.
[Illustration: FIG. 38.—Slide Valve, with Cut-off Plate on back.]
With the model referred to in the earlier chapters, the action of a
Meyer or similar gear can be readily investigated if an extra
eccentric-arm be added to the crank-pin disc and an extra travel-scale
marked upon the baseplate.
In fig. 39 is shown the “Buckeye” valve used in America on the Buckeye
engine; _a_ is the main valve, which determines the admission, release
and compression of the steam; and _b_ is the small auxiliary valve which
rides on the top of the main valve _a_, and which effects the cut-off.
This valve is constructed so as to have all the advantages of a
perfectly balanced valve as well as those of the Meyer valve. There are
two faces between which the main valve slides steam tight, and the small
valve works on a face inside of the main valve, as shown in the figure.
Steam is exhausted past the outer edges of the main valve. In order that
steam may enter the cylinder, the port in the main valve must be in
communication with the steam port and at the same time it must not be
covered by the auxiliary valve _b_. In the figure the valves are shown
in such a position as to admit steam to the left-hand end of the
cylinder and to allow it to exhaust from the right-hand end, as
indicated by the arrows.
[Illustration: FIG. 39.—The Buckeye Valve.]
Each valve is moved by its own eccentric, quite independently of the
other. By changing the advance or eccentricity, or both, of the
auxiliary valve, the cut-off may be changed as desired, without in any
way affecting the motion of the main valve. And as the main valve
determines the admission, release, and compression of the steam, it is
possible, with this valve, to change the cut-off as desired without
affecting these at all.
_The Trick or Allen Valve._—In connection with the subject of early
cut-offs the Trick or Allen valve may be mentioned. It is made with a
passage-way through the back of the valve, as shown in fig. 40, adapted
to deliver an extra supply of steam into the port by rendering available
an additional proportion of the area thereof, when the narrow
port-opening given by the lap-edge alone, during linked-up working, is
found to wire-draw the steam. Whilst the lap-edge _a_, for instance,
uncovers the edge _b_ of the right-hand port, the left-hand end of the
Trick passage becomes uncovered by moving beyond the edge _c_ of the
port-face, so that a current of steam flows from left to right through
that passage to join the stream passing the edge _a_. The total area of
the two supply-streams flowing into the right-hand port being
practically twice that which would be available were the Trick passage
absent.
[Illustration: FIG. 40.]
The occurrence and duration of the operations of steam-distribution are
exactly the same with this valve as with an ordinary slide-valve of
corresponding proportions, the distinctive feature of the valve having
no effect upon them so far as their timing is concerned.
_Reversal._—It may be of interest to consider the question—“Why do we
need _reversing_ gears?” The gears in common use are nearly all variable
expansion gears, as well as being _reversing_ gears, but it is desired
to consider now only the question of reversing. Are reversing gears
really needed? Suppose that one could have a transparent cylinder and
valve-chest, so that movement of the piston and valve could be clearly
seen. Suppose also that the connecting-rod, crank, valve gear, and
fly-wheel could _not_ be seen. We look into the cylinder and see valve
and piston reciprocating regularly, and in harmony with one another. We
turn away, and before we look again, the engine, let us assume, has been
reversed without our knowledge; yet when we turn again to the engine
there are the piston and valve reciprocating just as they were at first
(_although they must have “changed step,” as we shall presently find_),
and, look as intently as we may, with nothing else visible we shall be
unable to tell that the engine has been reversed. The valve is doing now
exactly what it did before, performing its operations in the same order.
Why have we had to employ reversing gear? Solely because—when the engine
ceases to run _ahead_ for instance, stopping at half-stroke the piston
and valve will be in a certain position, and will have been moving in
certain directions relatively to one another. And to reverse the
_piston’s_ motion, we want the valve
_Firstly_—To take up a new position on the port-face, so as, for
example, to give steam where it had previously given exhaust, and thus
we get reversal of the _engine_; and
_Secondly_—To move away from that new position, after the engine has
started astern, in the same direction as that in which it was moving
when the engine was running ahead. And seeing that the valve gear
derives motion from the crank-shaft directly or indirectly, we cannot
give the valve motion in this _same_ direction from an engine now moving
reversely, unless we also reverse the _gear_. And thus we may conclude
that a reversing gear is needed simply for the purpose of regulating the
direction of the first motion of the valve after stopping (_i.e._, for
“changing the step”), for after it next reaches the end of its travel
its motion will be in all respects just as it was when the engine ran
ahead,—so that, if we had a single-cylinder engine and never stopped it
except on the dead centres, we could drive its valve _by a separate
valve-engine always running one way_, and our main engine would, if
pushed off by hand, run equally well either ahead or astern, whilst
served by a valve having lap, lead, and travel as is customary. It is
hoped that this little speculation may not appear unprofitable, for it
has a bearing on the action of the reversing slides in radial valve
gears such as those of Hackworth, Walschaert, and Joy.
[Illustration: FIG. 41.]
FIG. 41 SHOWS ENLARGED KEY-VIEW OF CARDBOARD CRANK-DISC, WHICH SHOULD
TURN WITHIN THE CIRCLE DRAWN IN THE CENTRE OF FIG. 1.
The arm marked C is the crank-arm, and serves as an index finger in the
recording and reading of results.
The mark 1 is the index-mark for the forward eccentric to operate a
valve without lap.[5]
The mark 2 is the index for a forward eccentric set for lead, using a
valve without lap.[5]
The mark 3 is for a forward eccentric for a valve with lap and lead run
in “full gear.”[6]
The mark 4 is for a forward eccentric for a valve with lap and lead run
with a link motion partially “linked up.”[6]
The mark 5 is for a valve with lap and lead run with the link motion in
mid-gear.[5]
The mark 6 is for a backward eccentric for a valve with lap and lead,
run with a link motion partially linked-up.[6]
The mark 7 is a backward eccentric for a valve with lap and lead run in
“full gear.”[6]
The mark 8 is for a backward eccentric set for lead, using a valve
without any lap.[5]
The mark 9 is for a backward eccentric for a valve without lap.[5]
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Footnote 5:
Used with No. 1 travel-scale, fig. 1.
Footnote 6:
Used with No. 2 travel-scale, fig. 1.
The radial lines between the centre of the disc and the small circles
represent the actual throw assumed for the various eccentrics and their
angular position in relation to the crank; where the actual throw is not
sufficient to enable the arms to reach the edge of the disc, the
throw-lines are produced to the edge.
------------------------------------------------------------------------
=Transcriber’s Notes=
• Missing or obscured punctuation was silently corrected.
• Typographical errors were silently corrected.
• The “0” in fig. 1 was transcribed as the number zero to represent the
center of the scale, and the “O” in fig. 2 was transcribed as the
letter O to represent the outlet.
• Text in italics is enclosed by underscores (_italics_).
• Superscripts are written using a caret (^) followed by the text
enclosed in braces (^{superscript}).
• Subscripts are written using an underscore (_) followed by the text
enclosed in braces (_{subscript}).