while the steam in front of it escaping to the condenser, exerts
but little pressure upon it. It is therefore always forcibly
pressed against the surfaces in contact with which it moves, and
is thereby maintained steam-tight. Indeed this pressure would
rapidly wear the rubbing surfaces, unless they were made
sufficiently extensive, and hardened so as to resist the effects
of the friction. Where fresh water is used, as in land boilers,
the slide may be made of hardened steel; and in the case of marine
boilers, it may be constructed of gun-metal. In this and all other
contrivances in which the apertures by which the steam is admitted
to and withdrawn from the piston are removed to any considerable
distance from the top and bottom of the cylinder, there is a waste
of steam, for the steam consumed at each stroke of the piston is
not only that which would fill the capacity of the cylinder, but
also the steam which fills the passage between the slide G and the
top or bottom of the cylinder. Any arrangement which would throw
the passages H and I on the other side of the slide G, that is,
between S and G, instead of being, as they are, between G and the
top and bottom of the cylinder, would remove this defect. This is
accomplished by a slide, which is usually called the D valve,
because, being semi-cylindrical in its form, and hollow, its cross
section resembles the letter D. This slide, which is that which at
present is in most general use, is represented in _figs._ 49, 50.;
E is the rod by which the slide is moved, passing [Pg231] through
a stuffing-box F; G G is the slide represented by a vertical
section, _a a_ being a passage in it extending from the top to the
bottom; S is the mouth of the great steam pipe coming from the
boiler; P is the pipe leading to the condenser; T H is a hollow
space formed in the slide always in communication with the steam
pipe S, and consequently always filled with steam from the boiler.
A transverse section of the slide and cylinder is represented in
_fig._ 51., where _a_ represents the top of the passage marked _a_
in _fig._ 49. In the position of the slide represented in _fig._
49., the steam filling the space T H has access to the top of the
cylinder, but is excluded from the bottom. The steam which was
below the piston, passing up the passage _a_, escapes through the
tube P to the condenser. When the piston has descended, the rod E
moves the slide downwards, so as to give it the position
represented in FIG. 50. The steam in T H has now access to the
bottom of the cylinder, while the steam above the piston passing
through P escapes to the condenser. In this way the operation of
the piston is continued and the steam consumed at each stroke only
exceeds the capacity of the cylinder by what is necessary to fill
the passages between the slide and the cylinder.
[Illustration: _Fig._ 49.]
[Illustration: _Fig._ 50.]
[Illustration: _Fig._ 51.]
In a slide constructed in this manner, the steam filling the space
T H has a tendency to press the slide back, so as to break the
contact of the rubbing surfaces, and thereby to cause the steam to
leak from the space T H to the back of the slide. This is
counteracted by the packing _x_, at the back of the slide.
In engines of very long stroke, the extent of the rubbing surfaces
of slides of this kind renders it difficult to keep [Pg232] them
in steam-tight contact and to insure their uniform wear. In such
cases, therefore, separate slides, upon the same principle, are
provided at the top and bottom of the cylinder, moved, however, by
a single rod of communication.
(135.) In slides, as we have here described them, the same motion
which admits steam to either end of the cylinder, withdraws it
from the other end. Such an arrangement is only compatible with
the operation of a cylinder which works without expansion; for in
such a cylinder the full flow of steam to the piston is only
interrupted for a moment during the change of position of the
slide. But if the steam act expansively, it would be necessary to
move the slide, so as to stop its flow to one end of the cylinder,
without at the same time obstructing the escape of steam from the
other end to the condenser. It would therefore be necessary that
the slide should close the passage leading to the cylinder at one
end, without at the same time obstructing the communication
between the passage from the cylinder to the condenser at the
other end. On the arrival of the piston, however, at the bottom of
the cylinder, it would be necessary immediately to put the lower
passage to the cylinder in communication with the steam pipe, and
the upper passage in communication with the condenser. This would
necessarily suppose two motions of the slide as well as some
modifications in its length. Let the length of the slide be such
that when the passage to the top of the cylinder is stopped, the
lower part of the slide shall not reach the passage to the lower
part of the cylinder; and let such a provision be made in the
mechanism by which the rod E governing the slide is driven that it
shall receive two motions during the descent of the piston, the
first to be imparted to it at the moment the steam is to be cut
off, and the second just before the termination of the stroke. Let
the position of the slide, at the commencement of the stroke, be
represented in _fig._ 52., and let it be required that the steam
shall be cut off at one half of the stroke. When the piston has
made half the stroke, the rod governing the slide is moved
downwards, so as to throw the slide into the position represented
in _fig._ 53. The passage between the steam pipe and the cylinder
is [Pg233] now stopped at both ends; but the passage from the
bottom of the cylinder to the condenser remains open. During the
remainder of the stroke, therefore, the steam in the cylinder
works expansively. As the piston approaches the bottom of the
cylinder, another motion is imparted to the rod governing the
slide, by which the latter is thrown into the position represented
in _fig._ 54. Steam now flows below the piston while the steam
above it passes to the condenser. In a similar manner, by two
motions successively imparted to the slide during the ascent of
the piston, the steam may be cut off at half stroke; and it is
evident that by regulating the time at which these motions are
given to the slide, the steam may be worked expansively, to any
required extent.
[Illustration: _Fig._ 52.]
[Illustration: _Fig._ 53.]
[Illustration: _Fig._ 54.]
It is easy to conceive various mechanical means by which, in the
same engine, the point at which the steam is cut off may be
regulated at pleasure.
In cases where the motion of the piston is very rapid, as in
locomotive engines, it is desirable that the passages to and from
the cylinder should be opened very suddenly. This is difficult to
be accomplished with any form of slide consisting of a single
aperture; but if, instead of admitting the steam to the cylinder
by a single aperture, the same magnitude of opening were divided
among several apertures, then a proportionally less extent of
motion in the slide would clear the passage for the steam, and
consequently greater suddenness of opening would be effected.
[Pg234]
The great advantages in the economy of fuel resulting from the
application of the expansive principle have, of late years forced
themselves on the attention of engineers, and considerable
improvements have been made in its application, especially in the
case of marine engines used for long voyages, in which the economy
of fuel has become an object of the last importance. The mechanism
by which expansive slides are moved, is made capable of adjustment,
so that the part of the stroke at which the steam is cut off, can be
altered at pleasure. The working power of the engine, therefore,
instead of being controlled by the throttle-valve, is regulated by
the greater or less extent to which the expansive principle is
applied. Steam of the same pressure is admitted to the cylinder in
all cases; but it is cut off at a greater or less portion of the
stroke, according to the power which the engine is required to
exert.
The last degree of perfection has been conferred on this principle
by connecting the governor with the mechanism by which the slide
is moved, so that the governor instead of acting on the
throttle-valve, is made to act upon the slide. By this means when,
by reason of any diminution of the resistance, the motion of the
engine is accelerated, the balls of the governor diverging shift
the cam or lever which governs the slide, so that the steam is cut
off after a shorter portion of the stroke, the expansive principle
is brought into greater play, and the quantity of steam admitted
to the cylinder at each stroke is diminished. If, on the other
hand, the resistance to the machine be increased, so as to
diminish the velocity of the engine, then the balls collapsing the
levers of the governor shift the cam which moves the slides, so as
to increase the portion of the stroke made by the piston before
the steam is cut off, and thereby to increase the amount of
mechanical power developed in the cylinder at each stroke. The
extent to which the expansive principle is capable of being
applied, more especially in marine engines, has been hitherto
limited by the necessity of using steam of very high pressure,
whenever the steam is cut off after the piston has performed only
a small part of the stroke. A method, however, is now (March,
1840) under experimental trial, by [Pg235] Messrs. Maudsley and
Field, by which the expansive principle may be applied to any
required extent without raising the steam in the boiler above the
usual pressure of from three to five pounds per square inch. This
method consists in the use of a piston of great magnitude. The
force urging the piston is thus obtained not by an excessive
pressure on a limited surface, but by a moderate pressure diffused
over a large surface. The entire moving force acting on the piston
before the steam is cut off, is considerably greater than the
resistance; but during the remainder of the stroke this force is
gradually enfeebled until the piston is brought to the extremity
of its play.
[Illustration: _Fig._ 55.]
(136.) Mr. Samuel Seaward, of the firm of Messrs. Seawards,
engineers, has contrived an improved system of slides, for which
he has obtained a patent. A section of Seaward's slides is
represented in _fig._ 55. The steam pipe proceeding from the
boiler to the cylinder is represented at A A, and it communicates
with passages S and S′ leading to the top and bottom of the
cylinder. These passages are formed in nozzles of iron or other
hard metal cast upon the side of the cylinder. These nozzles
present a smooth face outwards, upon which the slides B B′, also
formed with smooth faces, play. The slides B B′ are attached by
knuckle-joints to rods E E′, which move through stuffing-boxes,
and the [Pg236] connection of these rods with the slides is such
that the slides have play so as to detach their surfaces easily
from the smooth surfaces of the nozzles when not pressed against
these surfaces. The steam in the steam pipe A A will press against
the backs of the slides B B′, and keep their faces in steam-tight
contact with the smooth surfaces of the nozzles. These slides may
be opened or closed by proper mechanism at any point of the
stroke. When steam is to be admitted to the top of the cylinder,
the upper slide is raised and the passage S opened; and when it is
to be admitted to the bottom of the cylinder, the lower slide is
raised and the passage S′ opened; and its communication to the top
or bottom of the cylinder is stopped by the lowering of these
slides respectively. On the other side of the cylinder are
provided two passages C C′ leading to a pipe G, which is continued
to the condenser. On this pipe are cast nozzles of iron or other
metal presenting smooth faces towards the cylinder, and having
passages D D′ communicating between the top and bottom of the
cylinder respectively and the pipe G G leading to the condenser.
Two slides _b b'_, having smooth faces turned from the cylinder,
and pressing upon the faces of the nozzles D D′, are governed by
rods playing through stuffing-boxes, in the same manner as already
described. The faces of these slides being turned from the
cylinder, the steam in the cylinder having free communication with
them, has a tendency to keep them by its pressure in steam-tight
contact with the surfaces in which the apertures leading to the
condenser are formed. These two slides may be opened or closed
whenever it is necessary.
When the piston commences its descent, the upper steam slide is
raised, so as to open the passage S, and admit steam above the
piston; and the lower exhausting slide _b′_ is also raised, so as
to allow the steam below the piston to escape through G to the
condenser, the other two passages S′ and C being closed by their
respective slides. The slide which governs S is lowered at that
part of the stroke at which the steam is intended to be cut off,
the other slides remaining unchanged; and when the piston has
reached the bottom of the cylinder, the lower steam slide opens
the passage S′, and [Pg237] the upper exhausting slide opens the
passage C; and at the same time the lower exhausting slide closes
the passage C′. Steam being admitted below the piston through S′,
and at the same time the steam above it being drawn away to the
condenser through the open passage C and the tube G, the piston
ascends. When it has reached that point at which the steam is
intended to be cut off, the slide which governs S′ is lowered, the
other slides remaining unaltered, and the upward stroke is
completed in the same manner as the downward.
These four slides may be governed by a single lever, or they may
be moved by separate means. From the small spaces between the
several slides and the body of the cylinder, it will be evident
that the waste of steam by this contrivance will be very small.
In the slide valves commonly used, the packing of hemp at the back
of the slide, by which the pressure necessary to keep the slide in
steam-tight contact is obtained, requires constant attention from
the engine-man while the engine is at work. Any neglect of this
will produce a corresponding loss in the power of the engine; and
accordingly it is found that in many cases where engines work
inefficiently, the defect is owing either to ignorance or want of
attention on the part of the engine-man in the packing of the
slides. In Seaward's slides no hemp packing is used, nor is any
attention on the part of the engine-man required after the slides
are first adjusted. The slides receive the pressure necessary to
keep them in steam-tight contact with the surfaces of the nozzles
from the steam itself, which acts behind them.
The eduction and steam slides being independent of each other,
they may be adjusted so that the engine shall work expansively in
any required degree; and this may be accomplished either by
working the slides by separate mechanism, or by a single
eccentric.
One of the advantages claimed by the patentees for these slides
is, that the engines are secured from the accidents which arise
from the accumulation of water within the steam cylinder. If such
a circumstance should occur, the action of the piston will press
the water against the faces of the steam [Pg238] slides, and the
play allowed to them by their connection with the rods which move
them permits their faces to be raised from the surfaces of the
nozzles, so that the water collected in the cylinder shall be
driven into the steam pipe, and sent back from thence to the
boiler.
[Illustration: _Fig._ 56.]
[Illustration: _Fig._ 57.]
[Illustration: _Fig._ 58.]
(137.) Of the cocks or valves which are opened and closed by the
motion of an axis passing through their centre, the throttle-valve,
whether worked by hand or by the governor, is an example. But the
most common form for cocks is that of a cylindrical or slightly
conical plug (_fig._ 56.), inserted in an aperture of corresponding
magnitude passing across the pipe or passage which the cock is
intended to open or close. One or more holes are pierced
transversely in the cock, and when the cock is turned so that these
holes run in the direction of the tube, the passage through the tube
is opened; but when the passage through the cock is placed at right
angles to the tube, then the sides of the tube stop the ends of the
passage in the cock, and the passage through the tube is obstructed.
The simple cock is designed to open or close the passage through a
single tube. When the cock is turned, as in _fig._ 57., so that the
passage through the cock shall be at right angles to the length of
the tube, then the passage through the tube is stopped; but when the
cock is turned from that position through a quarter of a revolution,
as in _fig._ 58., then the passage through the cock takes the
direction of the passage through the tube, and the cock is opened,
and the passage through the tube unobstructed. In such a cock the
passage may be more or less _throttled_ by [Pg239] adjusting the
position of the cock, so that a part of the opening in it shall be
covered by the side of the tube.
(138.) It is sometimes required to put one tube or passage
alternately in communication with two others. This is accomplished
by a _two-way cock_. In this cock the passage is curved, opening
usually at points on the surface of the cock, at right angles to
each other. Such a cock has already been described, and its use
illustrated in the description of the Marquis of Worcester's
engine (17.); the two-way cock, as represented at K and R (_fig._
4.), being the means by which steam and water are alternately
supplied to the two forcing vessels.
[Illustration: _Fig._ 59.]
[Illustration: _Fig._ 60.]
(139.) When it is required to put four passages alternately in
communication by pairs, a _four-way cock_ is used. Such a cock has
two curved passages (_fig. 59._), each similar to the curved
passage in the two-way cock. Let S C B T be the four tubes which
it is required to throw alternately into communication by pairs.
When the cock is in the position _fig._ 59., the tube S
communicates with T, and the tube C with B. By turning the cock
through a quarter of a revolution, as in _fig._ 60., the tube S is
made to communicate with B, and the tube C with T; and if the cock
continue to be turned at intervals through a quarter of a
revolution, these changes of communication will continue to be
alternately made. It is evident that this may be accomplished by
turning the cock continually in the same direction.
The four-way cock is sometimes used as a substitute for the valves
or slides in a double-acting steam engine to conduct the steam to
and from the cylinder. If S represent a pipe conducting steam from
the boiler, C that which leads to the condenser, T the tube which
leads to the top of the cylinder, and B that which leads to the
bottom, then when the cock is in the position (_fig._ 59.), steam
would flow from the boiler to [Pg240] the top of the piston,
while the steam below it would be drawn off to the condenser; and
in the position (_fig._ 60.), steam would flow from the boiler to
the bottom of the piston, while the steam above it would be drawn
off to the condenser. Thus by turning the cock through a quarter
of a revolution towards the termination of each stroke, the
operation of the machine would be continued.
One of the disadvantages which is inseparable from the use of a
four-way cock for this purpose is the loss of the steam at each
stroke, which fills the tubes between the cock and the ends of the
cylinder. This disadvantage could only be avoided by the
substitution of two two-way cocks (138.) instead of a four-way
cock. A two-way cock at the top of the cylinder would open an
alternate communication between the cylinder and steam pipe, and
the cylinder and condenser, while a similar office would be
performed by another two-way cock at the other end.
The friction on cocks of this description is more than on other
valves; but this is in some degree compensated by the great
simplicity of the instrument. When the cock is truly ground into
its seat, being slightly conical in its form, the pressure of the
steam has a tendency to keep the surfaces in contact; but this
pressure also increases the friction, and has a tendency to wear
the seat of the cock into an elliptical shape. Consequently, such
cocks require to be occasionally ground and refitted.
(140.) The four-way cock, as above described, admits the steam to
one end of the piston at the same moment that it stops it at the
other end. It would therefore be inapplicable where steam is
worked expansively. A slight modification, however, analogous to
that already described in the slides, will adapt it to expansive
action. This will be accomplished by giving to one of the passages
through the cock one aperture larger than the other, and working
the cock so that this passage shall always be used to conduct
steam to the cylinder; also by enlarging both apertures of the
other passage, and using it always to conduct steam from the
cylinder. The effect of such an arrangement will be readily
understood.
[Illustration: _Fig._ 61.]
[Illustration: _Fig._ 62.]
[Illustration: _Fig._ 63.]
[Illustration: _Fig._ 64.]
Let the position of the cock at the commencement of the [Pg241]
descending stroke be represented in _fig._ 61. Steam flows from S
through T to the top of the cylinder, while it escapes from B
through C from the bottom of the cylinder. When the piston has
arrived at that point at which the steam is to be cut off, let the
cock be shifted to the position represented in _fig._ 62. The
passage of steam from the boiler is now stopped, but the escape of
steam from the bottom of the cylinder through C continues, and the
cock is maintained in this position until the piston approaches
the bottom of the cylinder, when it is further shifted to the
position represented in _fig._ 63. Steam now flows from S through
B to the bottom of the cylinder, while the steam from the top of
the cylinder escapes through C to the condenser. When the piston
has arrived at that point where the steam is to be cut off, the
cock is shifted to the position represented in _fig._ 64. The
communication between the steam and the bottom of the piston is
now stopped, while the communication between the top of the
cylinder and the condenser is still open. During the next double
stroke of the piston the position of the cock is similarly
changed, but in the contrary direction, and in the same way the
motion is continued. Under these circumstances the cock, instead
[Pg242] of being moved constantly in the same direction, as in the
case of the common four-way cock, will require to be moved
alternately in opposite directions.
PISTONS.
(141.) The office of a piston being to divide a cylinder into two
compartments by a movable partition which shall obstruct the passage
of any fluid from one compartment to the other, it is evident that
the two conditions which such an instrument ought to fulfil are,
_first_, that the contact of its sides with the surface of the
cylinder shall be so close and tight throughout its entire play that
no steam or other fluid can pass between them; _secondly_, that it
shall be so free from friction, notwithstanding this necessary
tightness, that it shall not absorb any injurious quantity of the
moving power.
Since, however accurately the surfaces of the piston and cylinder
may be constructed, there will always be in practice more or less
imperfection of form, it is evident that the contact of the
surface of the piston with the cylinder throughout the stroke can
only be maintained by giving to the circumference of the piston
sufficient elasticity to accommodate itself to such inequalities
of form. The substance, whatever it may be, used for this purpose,
and by which the piston is surrounded, is called _packing_.
In steam pistons the material used for packing must be such as is
capable of resisting the united effects of heat and moisture.
Hence leather and other animal substances are inapplicable.
The packing used for steam pistons is therefore of two kinds,
_vegetable packing_, usually hemp, or _metallic packing_.