THE SLIDE-VALVE.
QUESTION 41. _What are the essential conditions which a slide-valve
must fulfill in governing the admission and exhaust of steam to and
from the cylinder of an ordinary engine?_
_Answer._ 1. It must admit steam to one end only of the cylinders at
one time. 2. It must allow the steam to escape from one end at least
as soon as it is admitted to the other end; and 3. it must cover
the steam-ports so as not to permit the steam to escape from the
steam-chest into the exhaust-port.
[Illustration:
_Fig. 10._
Scale ³⁄₁₆ in. = 1 inch.]
QUESTION 42. _What was the first form of slide-valve used?_
_Answer._ That represented in fig. 10. The smallest movement of this
valve either way opens one of the steam-ports for the admission of
steam and puts the other in communication with the exhaust-port. By
cutting a piece of ordinary writing paper to the form of the section
of the valve, and moving it on the line _f f_, the action of the valve
will be clearly shown.
[Illustration: _Plate I._
AMERICAN LOCOMOTIVE.
By The Grant Locomotive Works, Paterson, New Jersey.
_Scale, ³⁄₈ in. = 1 foot._]
QUESTION 43. _How was the admission and escape of the steam effected by
this valve?_
_Answer._ In order to explain this clearly, a series of diagrams
will be necessary. Before referring to them, however, it should be
explained first that the motion of an eccentric is exactly the same as
that of a small crank. It is in fact a crank with a crank-pin whose
diameter is very much enlarged. In the diagrams, figs. 11 to 25, the
eccentrics will therefore be represented as small cranks, and most
of the other parts by their centre-lines and points only, so as to
make the diagrams as simple as possible. The dimensions selected for
these illustrations are for the cylinder 16 in. diameter and 24 in.
stroke, and a connecting-rod 7 ft. long. The steam-ports are 1¹⁄₄ in.,
the exhaust-port 2¹⁄₂ in., and the metal or bars between them, which
are called _bridges_, are 1¹⁄₈ in. wide. The eccentric produces a
lateral movement of 3 in., which is called its _throw_. In fig. 11 the
piston is at the beginning of the backward stroke. The valve is then
in the centre of the valve-face, and the eccentric is consequently at
half-throw. The slightest movement of the crank in the direction of the
dart _N_ will move the eccentric enough to open the front steam-port
to the steam and the back one to the exhaust. In fig. 12 the piston
is represented as having moved 4 in. of its stroke; the valve has
then partly opened the front steam-port, and the other one is open to
the exhaust. In fig. 13 the piston has moved 8 in. of its stroke, and
the ports are now wide open, the front one to the steam and the back
one to the exhaust. In fig. 14 the piston has moved 12 in., or is at
half-stroke, and the valve has then moved as far as it will in that
direction. In fig. 15 the piston has moved 16 in. and the valve has
begun to return. In fig. 16 the piston has moved 20 in., and the valve
has nearly closed the front port to the steam and the other to the
exhaust. In fig. 17 the forward stroke is completed, and both ports
are closed by the valve. Figs. 18, 19, 20, 21, 22 and 23 represent
the piston and the valve on the return stroke in the positions
corresponding with those described for the backward stroke.
[Illustration:
_Fig. 11._
_Fig. 12._
_Fig. 13._
_Fig. 14._
_Fig. 15._
_Fig. 16._
_Fig. 17._
Scale ¹⁄₄ in. = 1 foot.
_Fig. 24._
_Fig. 23._
_Fig. 22._
_Fig. 21._
_Fig. 20._
_Fig. 19._
_Fig. 18._
Scale ¹⁄₄ in. = 1 foot.]
QUESTION 44. _Is there any other method by which the motion of a valve
can be represented by a drawing?_
_Answer._ Yes, by what are called _motion-curves_. It is, however,
difficult to explain these clearly, and as they are purely imaginary,
it is difficult to understand their nature and purpose. Close attention
will therefore be required to the following description:
We will suppose, in the first place, that the line _f f_, fig.
25, represents the valve-face, _c_ and _d_ the steam and _g_ the
exhaust-port, drawn to a larger scale than in the preceding figures.
We will now draw the valve _H_ in the position represented in fig. 11,
where the piston is at the beginning of the stroke. In order to show
the valve in the position represented in fig. 12, where the piston has
moved 4 in. of the stroke, we will draw a line 4-20, four inches below
and parallel to _f f_, and extend the lines, representing the edges of
the ports _c_, _d_ and _g_, downward. On the horizontal line 4-20 we
will now draw the edges _i_, _r_, _i′_, _r_, of the valve in the same
position in relation to the port _c_ that it has in fig. 12. We will
then draw another horizontal line, 8-16, eight inches below _f f_,
and parallel to it, and on this represent the valve in the position
shown in fig. 13. In the same way we will draw lines 12, 16, 20 and
24 in. below _f f_, and draw the valve on each one respectively in
the positions shown in figs. 14, 15, 16 and 17. The distance between
the lower line 24-0, and _f f_, will then represent the stroke of the
piston, or 24 in. If now we begin from the edge _h_ of the valve on
the line _f f_, and draw a curve, _h i j k l m n_, through the same
edge of the valve, represented on each of the parallel lines below,
the curve will indicate the position of the valve in relation to the
steam-port _c_ at each point of the stroke. To illustrate this, suppose
we draw lines 1-23, 2-22, and 3-21, one inch apart and parallel to _f
f_, and between it and 4-20. They will then represent the position of
the piston after it has moved 1, 2 and 3 in. from the beginning of the
stroke, and where they intersect the curved line will be the position
of the edge of the valve when the piston has moved 1, 2 and 3 in. of
the stroke. The curved line will in fact represent the position of the
valve at any point of the stroke between these lines. Other horizontal
lines, 5-19, 6-18, etc., can be drawn to represent every inch of the
rest of the stroke. The curve line _h i j k l m n_, or _motion-curve_
as it is called, will then show the exact position of the edge of the
valve and of the width of the opening of the steam-port during the
whole stroke. From it we see that the valve opens the port _c_ for the
admission of steam simultaneously with the movement of the piston, and
when the latter has made one inch of its stroke the port _c_ is half
open. At 4¹⁄₂ inches of the stroke the port is wide open,[12] and at
19¹⁄₂ inches it begins to be closed, but is not completely closed until
the end of the stroke.
[12] By cutting a paper section of the valve and placing it on the
diagram in each position named, it will probably help the reader
to understand the movement of the valve more and the nature of the
motion-curves.
[Illustration:
_Fig. 25._
Scale ³⁄₁₆ in. = 1 inch.]
Similar motion-curves, such as _h′ i′ j′ k′ l′ m′ n′_, (represented
in fig. 25) can be drawn to represent the position of the other edges
of the valve, and also for the return stroke. The latter are shown in
dotted lines. If we follow the curve _h′ i′ j′ k′ l′ m′ n′_, which
represents the position of the edge of the valve _h′_ which governs the
exhaust from the back end of the cylinder, we see that the port _d_ is
opened and closed to the exhaust simultaneously with the opening and
closing of the port _c_ for the admission of steam to the front end of
the cylinder, and that they both remain open until the completion of
the stroke.
The width that the ports are opened by the valve is thus ascertainable
from these diagrams, for any point of the stroke, and in fact can be
seen at a glance. By the aid of such motion-curves, the movement of
slide-valves can therefore be analyzed more perfectly than is possible
without them.
QUESTION 45. _What were some of the disadvantages of valves, like that
shown in fig. 10, and which are shown by the motion-curves in fig. 25._
_Answer._ The free admission of the steam until the completion of the
stroke by the piston was hurtful to the machinery, as it co-operated
with the momentum of the piston and its connections in producing undue
strains in the working parts. The steam then escaped from the cylinder
without expansion, so that much of its useful energy was lost. The
steam was not allowed to escape from one end of the cylinder until it
was admitted at the opposite end, and as the process of exhausting it
occupies some time, there was always more or less back pressure until
all the exhaust steam was expelled from the cylinder. In practice,
the imperfections of the valve-gear frequently delayed the opening of
the ports, both for admitting and exhausting steam, until after the
commencement of the stroke of the piston.[13]
[13] D. K. Clark’s “Railway Machinery.”
QUESTION 46. _How may some of these evils be overcome?_
_Answer._ By moving the eccentric forward on the axle so that the
motion of the valve is advanced to the same extent, and the admission
and exhaust of the steam will occur a little before the completion
of the stroke of the piston. In this way the steam is admitted into
the cylinder so as to act as a cushion to receive the momentum of the
piston, and some time is given to the exhaust steam to escape, before
the return stroke.
QUESTION 47. _What is meant by lead?_
_Answer._ By _lead_ is meant the width of the opening of the
steam-ports at the beginning of the stroke of the piston. On the steam
side of the valve it is called _outside lead_; on the exhaust side
_inside lead_. In fig. 26 the opening _h_ of the steam-port is the
outside lead and _h′_ the inside lead.
[Illustration:
_Fig. 26._
Scale ³⁄₁₆ in. = 1 inch.]
QUESTION 48. _What is meant by the travel of a valve?_
_Answer._ By the _travel_ we mean the motion of the valve back and
forth, or in other words its stroke. If the arms of the rocker are
of the same length, the travel of the valve is equal to the throw of
the eccentric. For the preceding illustrations we have selected an
eccentric with three inches throw, which is the travel of the valve.
[Illustration:
_Fig. 27._
Scale ³⁄₁₆ in. = 1 inch.]
QUESTION 49. _How is the steam made to work expansively with a
slide-valve?_
_Answer._ By giving the valve what is called _lap_. That is, by
allowing the edges of the valve when it is in the center of the
valve-seat to overlap the edges of the steam-ports, as shown in fig.
27. Where this overlap, _L L_, is on the outside of the valve, it is
called _outside lap_; when on the inside, _l l_, _inside lap_. When
a valve has lap, those portions of the _face_[14] _h_, _i_, and _h′_,
_q_, which cover the steam-ports, being wider than the ports, therefore
occupy some time in moving over them, during which time the steam is
enclosed in the end of the cylinder, as there is then no communication
either with the steam-chest or the exhaust-port. This action is shown
very clearly by the motion-curves in fig. 26. The valve in this case
has ¹⁄₄ inch lead. At 4¹⁄₂ inches of the stroke of the piston the valve
has moved as far as it will go in that direction, and the steam-port
has its maximum width of opening. From that point the valve will begin
to close the steam-port, and at 14¹⁄₂ inches of the stroke the port
will be entirely covered, and the steam therefore be _cut off_. The
port will remain closed until the piston has moved 21³⁄₄ inches, when
it will be observed from the motion-curve _r s t u v w x_, that the
port _c_ is opened to the exhaust and the steam escapes, or, as it is
technically called, the _release_ takes place. From the time the steam
is cut off to the time it is released, it works _expansively_ in the
cylinder.
[14] The valve-face is the surface of the valve in contact with the
valve-seat.
QUESTION 50. _What relation is there between the amount of lap_[15]
_and the degree of expansion?_
[15] In speaking simply of _lap_, outside lap is always meant.
_Answer._ The greater the lap with any given travel, the shorter will
be the period of admission of steam, and, consequently, the more time
and space for expansion.
QUESTION 51. _What is the effect of inside lap?_
_Answer._ It delays the release of the steam. Thus in fig. 26 the valve
has ¹⁄₈ in. inside lap. The motion-curve _r s t u v w x_ shows that
the release takes place during the back stroke at 21³⁄₄ in. If now
there was no inside lap, the dotted line _y_, _x_ would represent the
exhaust edges of the valve, and the release would then occur somewhat
earlier, or at 21 in. For this reason no inside lap is usually given to
valves for engines which run at high rates of speed, as it allows too
little time for the steam to escape. In fact, in some cases, what is
called _inside clearance_ is given to the valve; that is, the valve as
shown in fig. 29, when it is in the middle of the valve-face, does not
entirely cover the steam-ports. The effect of this is just the reverse
of that produced by inside lap; that is, it causes the release to occur
earlier in the stroke.
[Illustration:
_Fig. 29._
Scale ³⁄₁₆ in. = 1 inch.]
QUESTION 52. _With the same outside lap, what is the effect of changing
the travel of the valve?_
_Answer._ By increasing the travel, the period of admission is
increased and that for expansion lessened; and by reducing it, the
admission is lessened, and the degree of expansion is increased. This
is shown by the motion-curves in fig. 28, in which the same valve and
ports are represented as are shown in fig. 26, but the valve has a
travel of 5 instead of 3 inches. The valve also has the same lead. By
following the motion-curve _h i j k l m n_, it will be seen that the
steam is thus admitted up to 20¹⁄₂ inches of the stroke of the piston,
and the period of expansion, as compared with that in fig. 26, is
correspondingly lessened. It will also be seen by comparing fig. 26
with fig. 28 that with the short travel of the valve the ports are not
opened so wide as they are when the travel is increased. This evil is
practically obviated, however, by making the ports so long that with
a comparatively small opening they will still have area sufficient
to admit enough steam to fill the cylinders, and it is known that an
opening less than the whole area of the steam-ports is sufficient to
facilitate the passage of steam into the cylinder.
[Illustration:
_Fig. 28._
Scale ³⁄₁₆ in. = 1 inch.]
QUESTION 53. _How is the exhaust affected by lap and lead?_
_Answer._ The steam is released earlier in the stroke in proportion as
the amount of outside lap and lead is increased, but the steam-port is
also closed to the exhaust, or _compression_, as it is called, begins
earlier with lap and lead than without. Thus, in fig. 25, it will
be seen that at the beginning of the stroke both ports are entirely
closed; in fig. 26, however, in which the valve has both lap and lead,
the port _d_ is nearly wide open at the beginning of the stroke, and
by following the motion-curve _r s t u v w x_, which represents the
position of the exhaust edge of the valve, it will be seen that the
steam was released from the port _c_ before the piston had completed
its stroke, or when it had still nearly 3¹⁄₂ inches to move. In fig. 25
the port _c_ is not opened to the exhaust until the commencement of the
stroke, but it remains open to its completion, whereas in fig. 26 it is
closed, or compression begins, at 18 inches of the return stroke, as
shown by the dotted motion-curve.
QUESTION 54. _How does the action of the connecting-rod influence the
motion of the valve in relation to the piston?_
_Answer._ By delaying the movement of the crank in the backward
stroke of the piston, and accelerating it in the forward stroke.
This will be best explained by reference to fig. 14, in which the
piston is represented in the center of the cylinder, or the middle
of the backward stroke. If now we take a pair of dividers set to a
length equal to that of the connecting-rod, and from the center, _f_,
describe an arc of a circle, _a b_, from the center of the shaft, and
through the lower half of the circle which represents the path of the
crank-pin, we will find that the point of intersection, _a_, falls
short of the vertical line, _c d_, and that the crank-pin has not made
quite one-quarter of a revolution while the piston was moving through
the first half of the backward stroke. By referring to fig. 21, in
which the piston is again in the middle of its stroke, but is moving
forward, and by describing another arc of a circle, _b a_, from the
center of the shaft and intersecting the path of the crank-pin, it will
be seen that the latter has moved _more_ than a quarter revolution,
while the piston has made the first half of the forward stroke.
Owing to this _angularity_, as it is called, of the connecting-rod,
the crank-pin is behind the piston during its backward stroke and
ahead of it during the forward stroke. As the valve is moved by the
eccentric, and it in turn by the shaft and crank, any irregularities
of the latter are of course communicated to the valve. We therefore
find, by referring to fig. 26, that the point of cut-off occurs during
the backward stroke at 14¹⁄₂ inches, and during the forward stroke
at 12 inches. A similar inequality is observable in the points of
release for the front and back strokes. It is not, however, a matter
of very great practical importance with stationary engines which run
at comparatively slow speeds; but if it is thought desirable, the
period of admission and the point of release for both strokes can be
equalized, either by giving the valve more lead or lap at one end
than the other, or by making the one steam-port wider than the other.
The mechanism employed for moving locomotive slide-valves, however,
furnishes us with the means of modifying their motion in relation to
that of the piston, and of thus equalizing the periods of admission and
release for the front and back strokes. The methods of doing this will
be more fully explained hereafter.