A Catechism of the Steam Engine
by
John Bourne
Part 6 out of 8
_A._--If the screw be properly proportioned to the resistance that the
vessel has to overcome, the slip will not be more than 10 per cent., but in
some cases it amounts to 30 per cent., or even more than this. In other
cases, however, the slip is nothing at all, and even less than nothing; or,
in other words the vessel passes through the water with a greater velocity
than if the screw were working in a solid nut.
568. _Q._--Then it must be by the aid of the wind or some other extraneous
force?
_A._--No; by the action of the screw alone.
569. _Q._--But how is such a result possible?
_A._--It appears to be mainly owing to the centrifugal action of the screw,
which interposes a film or wedge of water between the screw itself and the
water on which the screw reacts. This negative slip, as it is called,
chiefly occurs when the pitch of the screw is less than its diameter, and
when, consequently, the velocity of rotation is greater than if a coarser
pitch had been employed. There is, moreover, in all vessels passing through
the water with any considerable velocity, a current of water following the
vessel, in which current, in the case of a screw vessel, the screw will
revolve; and in certain cases the phenomenon of negative slip may be
imputable in part to the existence of this current.
570. _Q._--Is the screw propeller as effectual an instrument of propulsion
as the radial or feathering paddle?
_A._--In all cases of deep immersion it appears to be quite as effectual as
the radial paddle, indeed, more so; but it is scarcely as effectual as the
feathering paddle, with any amount of immersion, and scarcely as effectual
as the common paddle in the case of light immersions.
COMPARATIVE ADVANTAGES OF PADDLE AND SCREW VESSELS.
571. _Q._--Whether do you consider paddle or screw vessels to be on the
whole the most advantageous?
_A._--That is a large question, and can only receive a qualified answer. In
some cases the use of paddles is indispensable, as, for example, in the
case of river vessels of a limited draught of water, where it would not be
possible to get sufficient depth below the water surface to enable a screw
of a proper diameter to be got in.
572. _Q._--But how does the matter stand in the case of ocean vessels?
_A._--In the case of ocean vessels, it is found that paddle vessels fitted
with the ordinary radial wheels, and screw vessels fitted with the ordinary
screw, are about equally efficient in calms and in fair or beam winds with
light and medium immersions. If the vessels are loaded deeply, however, as
vessels starting on a long voyage and carrying much coal must almost
necessarily be, then the screw has an advantage, since the screw acts in
its best manner when deeply immersed, and the paddles in their worst. When
a screw and paddle vessel, however, of the same model and power are set to
encounter head winds, the paddle vessel it is found has in all cases an
advantage, not in speed, but in economy of fuel. For whereas in a paddle
vessel, when her progress is resisted, the speed of the engine diminishes
nearly in the proportion of the diminished speed of ship, it happens that
in a screw vessel this is not so,--at least to an equal extent,--but the
engines work with nearly the same rate of speed as if no increase of
resistance had been encountered by the ship. It follows from this
circumstance, that whereas in paddle vessels the consumption of steam, and
therefore of fuel, per hour is materially diminished when head winds occur,
in screw vessels a similar diminution in the consumption of steam and fuel
does not take place.
573. _Q._--But perhaps under such circumstances the speed of the screw
vessel will be the greater of the two?
_A._--No; the speed of the two vessels will be the same, unless the
strength of the head wind be so great as to bring the vessels nearly to a
state of rest, and on that supposition the screw vessel will have the
advantage. Such cases occur very rarely in practice; and in the case of the
ordinary resistances imposed by head winds, the speed of the screw and
paddle vessel will be the same, but the screw vessel will consume most
coals.
574. _Q._--What is the cause of this peculiarity?
_A._--The cause is, that when the screw is so proportioned in its length as
to be most suitable for propelling vessels in calms, it is too short to be
suitable for propelling vessels which encounter a very heavy resistance. It
follows, therefore, that if it is prevented from pursuing its spiral course
in the water, it will displace the water to a certain extent laterally, in
the manner it does if the engine be set on when the vessel is at anchor;
and a part of the engine power is thus wasted in producing a useless
disturbance of the water, which in paddle vessels is not expended at all.
575. _Q._--If a screw and paddle vessel of the same mould and power be tied
stern to stern, will not the screw vessel preponderate and tow the paddle
vessel astern against the whole force of her engines?
_A._--Yes, that will be so.
576. _Q._--And seeing that the vessels are of the same mould and power, so
that neither can derive an advantage from a variation in that condition,
does not the preponderance of the screw vessel show that the screw must be
the most powerful propeller?
_A._---No, it does not.
577. _Q._--Seeing that the vessels are the same in all respects except as
regards the propellers, and that one of them exhibits a superiority, does
not this circumstance show that one propeller must be more powerful than
the other?
_A._--That does not follow necessarily, nor is it the fact in this
particular case. All steam vessels when set into motion, will force
themselves forward with an amount of thrust which, setting aside the loss
from friction and from other causes, will just balance the pressure on the
pistons. In a paddle vessel, as has already been explained, it is easy to
tell the tractive force exerted at the centre of pressure of the paddle
wheels, when the pressure urging the pistons, the dimensions of the wheels
and the speed of the vessel are known; and that force, whatever be its
amount, must always continue the same with any constant pressure on the
pistons. In a screw vessel the same law applies, so that with any given
pressure on the pistons and discarding the consideration of friction, it
will follow that whatever be the thrust exerted by a paddle or a screw
vessel, it must remain uniform whether the vessel is in motion or at rest,
and whether moving at a high or a low velocity through the water. Now to
achieve an equal speed during calms in two vessels of the same model, there
must be the same amount of propelling thrust in each; and this thrust,
whatever be its amount, cannot afterward vary if a uniform pressure of
steam be maintained. The thrusts, therefore, caused by their respective
propelling instruments, when a screw and paddle vessel are tied stern to
stern, must be the same as at other times; and as at other times those
thrusts are equal, so must they be when the vessels are set in the
antagonism supposed.
578. _Q._--How comes it then that the screw vessel preponderates?
_A._--Not by virtue of a larger thrust exerted by the screw in pressing
forward the shaft and with it the vessel, but by the gravitation against
the stern of the wave of water which the screw raises by its rapid
rotation. This wave will only be raised very high when the progress of the
vessel through the water is nearly arrested, at which time the centrifugal
action of the screw is very great; and the vessel under such circumstances
is forced forward partly by the thrust of the screw, and partly by the
hydrostatic pressure of the protuberance of water which the centrifugal
action of the screw raises up at the stern.
579. _Q._--Can you state any facts in corroboration of this view?
_A._--The screw vessel will not preponderate if a screw and paddle vessel
be tied bow to bow and the engines of each be then reversed. In, some screw
vessels the amount of thrust actually exerted by the screw under all its
varying circumstances, has been ascertained by the application of a
dynamometer to the end of the shaft. By this instrument--which is formed by
a combination of levers like a weighing machine for carts--a thrust or
pressure of several tons can be measured by the application of a small
weight; and it has been found, by repeated experiment with the dynamometer,
that the thrust of the screw in a screw vessel when towing a paddle vessel
against the whole force of her engines, is just the same as it is when the
two vessels are maintaining an equal speed in calms. The preponderance of
the screw vessel must, therefore, be imputable to some other agency than to
a superior thrust of the screw, which is found by experiment not to exist.
580. _Q._--Has the dynamometer been applied to paddle vessels?
_A._--It has not been applied to the vessels themselves, as in the case of
screw vessels, but it has been employed on shore to ascertain the amount of
tractive force that a paddle vessel can exert on a rope.
581. _Q._--Have any experiments been made to determine the comparative
performances of screw and paddle vessels at sea?
_A._--Yes, numerous experiments; of which the best known are probably those
made on the screw steamer Rattler and the paddle steamer Alecto, each
vessel of the same model, size, and power,--each vessel being of about 800
tons burden and 200 horses power. Subsequently another set of experiments
with the same object was made with the Niger screw steamer and the Basilisk
paddle steamer, both vessels being of about 1000 tons burden and 400 horses
power. The general results which were obtained in the course of these
experiments are those which have been already recited.
582. _Q._--Will you recapitulate some of the main incidents of these
trials?
_A._--I may first state some of the chief dimensions of the vessels. The
Rattler is 176 feet 6 inches long, 32 feet 8-1/2 inches broad, 888 tons
burden, 200 horses power, and has an area of immersed midship section of
380 square feet at a draught of water of 11 feet 5-1/2 inches. The Alecto
is of the same dimensions in every respect, except that she is only of 800
tons burden, the difference in this particular being wholly owing to the
Rattler having been drawn out about 15 feet at the stern, to leave abundant
room for the application of the screw. The Rattler was fitted with a
dynamometer, which enabled the actual propelling thrust of the screw shaft
to be measured; and the amount of this thrust, multiplied by the distance
through which the vessel passed in a given time, would determine the amount
of power actually utilized in propelling the ship. Both vessels were fitted
with indicators applied to the cylinders, so as to determine the amount of
power exerted by the engines.
583. _Q._--How many trials of the vessels were made on this occasion?
_A._--Twelve trials in all; but I need not refer to those in which similar
or identical results were only repeated. The first trial was made under
steam only, the weather was calm and the water smooth. At 54 minutes past 4
in the morning both vessels left the Nore, and at 30-1/2 minutes past 2 the
Rattler stopped her engines in Yarmouth Roads, where in 20-1/2 minutes
afterward she was joined by the Alecto. The mean speed achieved by the
Rattler during this trial was 9.2 knots per hour; the mean speed of the
Alecto was 8.8 knots per hour. The slip of the screw was 10.2 per cent. The
actual power exerted by the engines, as shown by the indicator, was in the
case of the Rattler 334.6 horses, and in the case of the Alecto 281.2
horses; being a difference of 53.4 horses in favor of the Rattler. The
forward thrust upon the screw shaft was 3 tons, 17 cwt., 3 qrs., and 14
lbs. The horse power of the shaft--or power actually utilized--ascertained
by multiplying the thrust in pounds by the space passed through by the
vessel in feet per minute, and dividing by 33,000, was 247.8 horses power.
This makes the ratio of the shaft to the engine power as 1 to 1.3, or, in
other words, it shows that the amount of engine power utilized in
propulsion was 77 per cent. In a subsequent trial made with the vessels
running before the wind, but with no sails set and the masts struck, the
speed realized by the Rattler was 10 knots per hour. The slip of the screw
was 11.2 per cent. The actual power exerted by the engines of the Rattler
was 368.8 horses. The actual power exerted by the engines of the Alecto was
291.7 horses. The thrust of the shaft was equal to a weight of 4 tons, 4
cwt., 1 qr., 1 lb. The horse power of the shaft was 290.2 horses, and the
ratio of the shaft to the engine power was 1 to 1.2. Here, therefore, the
amount of the engine power utilized was 84 per cent.
584. _Q._--If in any screw vessel the power of the engine be diminished by
shutting off the steam or otherwise, you will then have a larger screw
relatively with the power of the engine than before?
_A._--Yes.
585. _Q._--Was any experiment made to ascertain the effect of this
modification?
_A._--There was; but the result was not found to be better than before. The
experiment was made by shutting off the steam from the engines of the
Rattler until the number of strokes was reduced to 17 in the minute. The
actual power was then 126.7 horses; thrust upon the shaft 2 tons, 2 cwt., 3
qrs., 14 lbs; horse power of shaft 88.4 horses; ratio of shaft to engine
power 1 to 1.4; slip of the screw 18.7 per cent. In this experiment the
power utilized was 71 per cent.
586. _Q._--Was any experiment made to determine the relative performances
in head winds?
_A._--The trial in which this relation was best determined lasted for seven
hours, and was made against a strong head wind and heavy head sea. The
speed of the Rattler by patent log was 4.2 knots; and at the conclusion of
the trial the Alecto had the advantage by about half a mile. Owing to an
accidental injury to the indicator, the power exerted by the engines of the
Rattler in this trial could not be ascertained; but judging from the power
exerted in other experiments with the same number of revolutions, it
appears probable that the power actually exerted by the Rattler was about
300 horses. The number of strokes per minute made by the engines of the
Rattler was 22, whereas in the Alecto the number of strokes per minute was
only 12; so that while the engines of the Alecto were reduced, by the
resistance occasioned by a strong head wind, to nearly half their usual
speed, the engines of the Rattler were only lessened about one twelfth of
their usual speed. The mean thrust upon the screw shaft during this
experiment, was 4 tons, 7 cwt., 0 qr., 16 lbs. The horse power of the shaft
was 125.9 horses, and the slip of the screw was 56 per cent. Taking the
power actually exerted by the Rattler at 300 horses, the power utilized in
this experiment is only 42 per cent.
587. _Q._--What are the dimensions of the screw in the Rattler?
_A._--Diameter 10 feet, length 1 foot 3 inches, pitch 11 feet. The
foregoing experiments show that with a larger screw a better average
performance would be obtained. The best result arrived at, was when the
vessel was somewhat assisted by the wind, which is equivalent to a
reduction of the resistance of the hull, or to a smaller hull, which is
only another expression for a larger proportionate screw.
588. _Q._--When you speak of a larger screw, what increase of dimension do
you mean to express?
_A._--An increase of the diameter. The amount of reacting power of the
screw upon the water is hot measured by the number of square feet of
surface of the arms, but by the area of the disc or circle in which the
screw revolves. The diameter of the screw of the Rattler being 10 feet, the
area of its disc is 78.5 square feet; and with the amount of thrust already
mentioned as existing in the first experiment, viz. 8722 lbs., the reacting
pressure on each square foot of the screw's disc will be 108-1/2 lbs. The
immersed midship section being 380 square feet, this is equivalent to 23
lbs. per square foot of immersed midship section at a speed of 9.2 knots
per hour.
589. _Q._--In smaller vessels of similar form, will the resistance per
square foot of midship section be more than this?
_A._--It will be considerably more. In the Pelican, a vessel of 109-3/4
square feet of midship section, I estimate the resistance per square foot
of midship section at 30 lbs., when the speed of the vessel is 9.7 knots
per hour. In the Minx with an immersed midship section of 82 square feet,
the resistance per square foot of immersed midship section was found by the
dynamometer to be 41 lbs. at a speed of 8-1/2 knots; and in the Dwarf, a
vessel with 60 square feet of midship section, I estimate the resistance
per square foot of midship section at 46 lbs. at a speed of 9 knots per
hour, which is just double the resistance per square foot of the Rattler.
The diameter of the screw of the Minx is 4-1/2 feet, so that the area of
its disc is 15.9 square feet, and the area of immersed midship section is
about 5 times greater than that of the screw's disc. The diameter of the
screw of the Dwarf is 5 feet 8 inches, so that the area of its disc is
25.22 square feet, and the area of immersed midship section is 2.4 times
greater than that of the screw's disc. The pressure per square foot of the
screw's disc is 214 lbs. in the case of the Minx, and 109-1/2 lbs. in the
case of the Dwarf.
590. _Q._--From the greater proportionate resistance of small vessels, will
not they require larger proportionate screws than large vessels?
_A._--They will.
591. _Q._--Is there any ready means of predicting what the amount of thrust
of a screw will be?
_A._--When we know the amount of pressure on the pistons, and the velocity
of their motion relatively with the velocity of advance made by the screw,
supposing it to work in a solid nut, it is easy to tell what the thrust of
the screw would be if it were cleared of the effects of friction and other
irregular sources of disturbance. The thrust, in fact, would be at once
found by the principle of virtual velocities; and if we take this
theoretical thrust and diminish it by one fourth to compensate for friction
and lateral slip, we shall have a near approximation to the amount of
thrust that will be actually exerted.[1]
[1] See Treatise on the Screw Propeller, by J. Bourne, C. E.
COMPARATIVE ADVANTAGES OF DIFFERENT SCREWS.
592. _Q._--What species of screw do you consider the best?
_A._--In cases in which a large diameter of screw can be employed, the
ordinary screw or helix with two blades seems to be as effective as any
other, and it is the most easily constructed. If, however, the screw is
restricted in diameter, or if the vessel is required to tow, or will have
to encounter habitually strong head winds, it will be preferable to employ
a screw with an increasing pitch, and also of such other configuration that
it will recover from the water some portion of the power that has been
expended in slip.
593. _Q._--How can this be done?
_A._--There are screws which are intended to accomplish, this object
already in actual use. When there is much slip a centrifugal velocity is
given to the water, and the screw, indeed, if the engine be set on when the
vessel is at rest, acts very much as a centrifugal fan would do if placed
in the same situation. The water projected outward by the centrifugal force
escapes in the line of least resistance, which is to the surface; and if
there be a high column of water over the screw, or, in other words, if the
screw is deeply immersed, then the centrifugal action is resisted to a
greater extent, and there will be less slip produced. The easiest
expedient, therefore, for obviating loss by slip is to sink the screw
deeply in the water; but as there are obvious limits to the application of
this remedy, the next best device is to recover and render available for
propulsion some part of the power which has been expended in giving motion
to the water. One device for doing this consists in placing the screw well
forward in the dead wood, so that it shall be overhung by the stern of the
ship. The water forced upward by the centrifugal action of the screw will,
by impinging on the overhanging stern, press the vessel forward in the
water, just in the same way as is done by the wind when acting on an
oblique sail. I believe, the two revolving vanes without any twist or
obliquity on them at all, would propel a vessel if set well forward in the
dead wood or beneath the bottom, merely by the ascent of the water up the
inclined plane of the vessel's run; and, at all events, a screw so placed
would, in my judgment, aid materially in propelling the vessel when her
progress was resisted by head winds.
594. _Q._--But you said there are some kinds of screws which profess to
accomplish this?
[Illustration: Fig. 49. THE EARL OF DUNDONALD'S PROPELLER.]
_A._--There are screws which profess to counteract the centrifugal velocity
given to the water by imparting to it an equal centripetal force, the
consequence of which will be, that the water projected backward by the
screw, instead of taking the form of the frustum of a cone, with its small
end next the screw, will take the form of a cylinder. One of these forms of
screw is that patented by the Earl of Dundonald in 1843, and which is
represented in fig. 49. Another is the form of screw already represented in
fig. 48, and which was patented by Mr. Hodgson in 1844. Mr. Hodgson bends
the arms of his propellers backward, not into the form of a triangle, but
into the form of a parabola, to the end that the impact of the screw on the
particles of the water may cause them to converge to a focus, as the rays
of light would do in a parabolic reflector. But this particular
configuration is not important, seeing that the same convergence which is
given to the particles of the water, with a screw of uniform pitch bent
back into the form of a parabola, will be given with a screw bent back into
the form of a triangle, if the pitch be suitably varied between the centre
and the circumference.
595. _Q._--Then the pitch may be varied in two ways?
_A._--Yes: a screw may have a pitch increasing in the direction of the
length, as would happen in the case of a spiral stair, if every successive
step in the ascent was thicker than the one below it; or it may increase
from the centre to the circumference, as would happen in the case of a
spiral stair, if every step were thinner at the centre of the lower than at
its outer wall. When the pitch of a screw increases in the direction of its
length, the leading edge of the screw enters the water without shock or
impact, as the advance of the leading edge per revolution will not be
greater than the advance of the vessel. When the pitch of a screw increases
in the direction of its diameter, the central part of the screw will
advance with only the same velocity as the water, so that it cannot
communicate any centrifugal velocity to the water; and the whole slip, as
well as the whole propelling pressure, will occur at the outer part of the
screw blades.
596. _Q._--Is there any advantage derived from these forms of screws?
_A._--There is a slight advantage, but it is so slight as hardly to balance
the increased trouble of manufacture, and, consequently, they are not
generally or widely adopted.
597. _Q._--What other kinds of screw are there proposing to themselves the
same or similar objects?
_A._--There is the corrugated screw, the arms of which are corrugated, so
as it were to gear with the water during its revolution, and thereby
prevent it from acquiring a centrifugal velocity. Then there is Griffith's
screw, which has a large ball at its centre, which, by the suction it
creates at its hinder part, in passing through the water, produces a
converging force, which partly counteracts the divergent action of the
arms. Finally, there is Holm's screw, which has now been applied to a good
number of vessels with success.
598. _Q._--Will you describe the configuration and action of Holm's screw?
_A._--First, then, the screw increases in the direction of its length, and
this increase is very rapid at the following edge, so that, in fact, the
following edge stands in the plane of the shaft, or in the vertical
longitudinal plane of the vessel. Then the ends of the arms are bent over
into a curved flange, the edge of which points astern, and the point where
this curved flange joins the following edge of the screw is formed, not
into an angle, but into a portion of a sphere, so that this corner
resembles the bowl of a spoon. When the screw is put into revolution, the
water is encountered by the leading edge of the screw without shock, as its
advance is only equal to the advance of the vessel, and before the screw
leaves the water it is projected directly astern. At the same time, the
curved flange at the rim of the screw prevents the dispersion of the water
in a radial direction, and it consequently assumes the form of a column or
cylinder of water, projected backward from the ship.
599. _Q._--What is the nature of Beattle's screw?
_A._--Beattie's screw is an arrangement of the screw propeller whereby it
is projected beyond the rudder, and the main object of the arrangement is
to take away the vibratory motion at the stern,--an intention which it
accomplishes in practice. There is an oval eye in the rudder, to permit the
screw shaft to pass through it.
600. _Q._--When the diameter of the cylinder of water projected backward by
a screw, and the force urging it into motion are known, may not the
velocity it will acquire be approximately determined?
_A._--That will not be very difficult; and I will take for illustration the
case of the Minx, already referred to, which will show how such a
computation is to be conducted. The speed of this vessel, in one of the
experiments made with her, was 8.445 knots; the number of revolutions of
the screw per minute, 231.32; and the pressure on each square foot of area
of the screw's disc, 214 lbs. If a knot be taken to be 6075.6 feet, then
the distance advanced by the vessel, when the speed is 8.445 knots, will be
3.7 feet per revolution, and this advance will be made in about .26 of a
second of time. Now the distance which a body will fall by gravity, in .26
of a second, is 1.087 feet; and a weight of 214 lbs. put into motion by
gravity, or by a pressure of 214 lbs., would, therefore, acquire a velocity
of 1.087 feet during the time one revolution of the screw is being
performed. The weight to be moved, however, is 3.7 cubic feet of water,
that being the new water seized by the screw each revolution for every
square foot of surface in the screw's disc; and 3.7 cubic feet of water
weigh 231.5 lbs., so that the urging force of 214 lbs. is somewhat less
than the force of gravity, and the velocity of motion communicated to the
water will be somewhat under 1.087 feet per revolution, or we may say it
will be in round numbers 1 foot per revolution. This, added to the progress
of the vessel, will make the distance advanced by the screw through the
water 4.7 feet per revolution, leaving the difference between this and the
pitch, namely 1.13 feet, to be accounted for on the supposition that the
screw blades had broken laterally through the water to that extent. It
would be proper to apply some correction to this computation, which would
represent the increased resistance due to the immersion of the screw in the
water; for a column of water cannot be moved in the direction of its axis
beneath the surface, without giving motion to the superincumbent water, and
the inertia of this superincumbent water must, therefore, be taken into the
account. In the experiment upon the Minx, the depth of this superincumbent
column was but small. The total amount of the slip was 36.53 per cent.; and
there will not be much error in setting down about one half of this as due
to the recession of the water in the direction of the vessel's track, and
the other half as due to the lateral penetration of the screw blades.
601. _Q._--Is it not important to make the stern of screw vessels very
fine, with the view of diminishing the slip, and increasing the speed?
_A._--It is most important. The Rifleman, a vessel of 486 tons, had
originally engines of 200 horses power, which propelled her at a speed of 8
knots an hour. The Teazer, a vessel of 296 tons, had originally engines of
100 horses power, which propelled her at a speed of 6-1/2 knots an hour.
The engines of the Teazer were subsequently transferred to the Rifleman,
and new engines of 40 horse power were put into the Teazer. Both vessels
were simultaneously sharpened at the stern, and the result was, that the
100 horse engines drove the Rifleman, when sharpened, as fast as she had
previously been driven by the 200 horse engines; and the 40 horse engines
drove the Teazer, when sharpened, a knot an hour faster than she had
previously been driven by the 100 horse engines. The immersion of both
vessels was kept unchanged in each case; and the 100 horse engines of the
Teazer, when transferred to the Rifleman, drove that vessel, after she had
been sharpened, 2 knots an hour faster than they had previously driven a
vessel not much more than half the size. These are important facts for
every one to be acquainted with who is interested in the success of screw
vessels, and who seeks to obtain the maximum of efficiency with the minimum
of expense.[1]
[1] See Treatise on the Screw Propeller, by John Bourne, C. E.
PROPORTIONS OF SCREWS.
602. _Q._--In fixing upon the proportions of a screw proper to propel any
given vessel, how would you proceed?
_A._--I would first compute the probable resistance of the vessel, and I
would be able to find the relative resistances of the screw and hull, and
in every case it is advisable to make the screw as large in diameter as
possible. The larger the screw is, the greater will be the efficiency of
the engine in propelling the vessel; the larger will be the ratio of the
pitch to the diameter, which produces a maximum effect; and the smaller
will be the length of the screw or the fraction of a convolution to produce
a maximum effect.
603. _Q._--Will you illustrate this doctrine by a practical example?
_A._--The French screw steamer Pelican was fitted successively with two
screws of four blades, but the diameter of the first screw was 98.42
inches, and the diameter of the second 54 inches. If the efficiency of the
first screw by represented by 1, that of the second screw will be
represented by .823, or, in other words, if the first screw would give a
speed of 10 knots, the second would give little more than 8. The most
advantageous ratio of pitch to diameter was found to be 2.2 in the case of
the large screw, and 1.384 in the case of the small. The fraction of a
convolution which was found to be most advantageous was .281 in the case of
the large screw, and .450 in the case of the small screw.
604. _Q_--Were screws of four blades found to be more efficient than screws
with two?
_A_--They were found to have less slip, but not to be more
efficient, the increased slip in those of two blades being balanced by the
increased friction in those of four. Screws of two blades, to secure a
maximum efficiency, must have a finer pitch than screws of four.
605. _Q._--Are the proportions found to be most suitable in the case of the
Pelican applicable to the screws of other vessels?
_A._--Only to those which have the same relative resistance of screw and
hull. Taking the relative resistance to be the area of immersed midship
section, divided by the square of the screw's diameter, it will in the case
of the Rattler be 380/100 or 3.8. From the experiments made by MM. Bourgois
and Moll on the screw steamer Pelican, they have deduced the proportions of
screws proper for all other classes of vessels, whether the screws are of
two, four, or six blades.
606. _Q._--Will you specify the nature of their deductions?
_A._--I will first enumerate those which bear upon screws with two blades.
When the relative resistance is 5.5 the ratio of pitch to diameter should
be 1.006, and the fraction of the pitch or proportion of one entire
convolution should be 0.454. When the relative resistance is 5, the ratio
of pitch to diameter should be 1.069, and fraction of pitch 0.428; relative
resistance 4.5, pitch 1.135, fraction 0.402; relative resistance 4, pitch
1.205, fraction 0.378; relative resistance 3.5, pitch 1.279, fraction
0.355; relative resistance 3, pitch 1.357, fraction 0.334; relative
resistance 2.5, pitch 1.450, fraction 0.313; relative resistance 2, pitch
1.560, fraction 0.294; relative resistance 1.5, pitch 1.682, fraction
0.275. The relative resistance of 4 is that which is usual in an auxiliary
line of battle ship, 3.5 in an auxiliary frigate, 3 in a high speed line of
battle ship, 2.5 in a high speed frigate, 2 in a high speed corvette, and
1.5 in a high speed despatch boat.
607. _Q._--What are the corresponding proportions of screws of four blades?
_A._--The ratios of the pitches to the diameter being for each of the
relative resistances enumerated above, 1.342, 1.425, 1.513, 1.607, 1.705,
1.810, 1.933, 2.080, and 2.243, the respective fractions of pitch or
fractions of a whole convolution will be 0.455, 0.428, 0.402, 0.378, 0.355,
0.334, 0.313, 0.294, and 0.275.
608. _Q._--And what are the corresponding proportions proper for screws of
six blades?
_A._--Beginning with the relative resistance of 5.5 as before, the proper
ratio of pitch to diameter for that and each of the successive resistances
in the case of screws with six blades, will be 1.677, 1.771, 1.891, 1.2009,
2.131, 2.262, 2.416, 2.600, 2.804; and the respective fractions of pitch
will be 0.794, 0.749, 0.703, 0.661, 0.621, 0.585, 0.548, 0.515, and 0.481.
These are the proportions which will give a maximum performance in every
case.[1]
[1] In my Treatise on the Screw Propeller I have gone into these various
questions more fully than would consort with the limits of this
publication.
SCREW VESSELS WITH FULL AND AUXILIARY POWER.
609. _Q._--Do you consider that the screw propeller is best adapted for
vessels of full power, or for vessels with auxiliary power?
_A._--It is, in my opinion, best adapted for vessels with auxiliary power,
and it is a worse propeller than paddle wheels for vessels which have
habitually to encounter strong head winds. Screw vessels are but ill
calculated--at least as constructed heretofore--to encounter head winds,
and the legitimate sphere of the screw is in propelling vessels with
auxiliary power.
610. _Q._--Does the screw act well in conjunction with sails?
_A._--I cannot say it acts better than paddles, except in so far as it is
less in the way and is less affected by the listing or heeling over of the
ship. A small steam power, however, acts very advantageously in aid of
sails, for not only does the operation of the sails in reducing the
resistance of the hull virtually increase the screw's diameter, but the
screw, by reducing the resistance which has to be overcome by the sails and
by increasing the speed of the vessel, enables the sails to act with
greater efficiency, as the wind will not rebound from them with as great a
velocity as it would otherwise do, and a larger proportion of the power of
the wind will also be used up. In the case of beam winds, moreover, the
action of the screw, by the larger advance it gives to the vessel will
enable the sails to intercept a larger column of wind in a given time. It
appears, therefore, that the sails add to the efficiency of the screw, and
that the screw also adds to the efficiency of the sails.
611. _Q._--What is the comparative cost of transporting merchandise in
paddle steamers of full power, in screw steamers of auxiliary power, and in
sailing ships?
_A._--That will depend very much upon the locality where the comparison is
made. In the case of vessels performing distant ocean voyages, in which
they may reckon upon the aid of uniform and constant winds, such as the
trade winds or the monsoon, sailing ships of large size will be able to
carry more cheaply than any other species of vessel. But where the winds
are irregular and there is not much sea room, or for such circumstances as
exist in the Channel or Mediterranean trades, screw vessels with auxiliary
power will constitute the cheapest instrument of conveyance.
612. _Q._--Are there any facts recorded illustrative of the accuracy of
this conclusion?
_A._--A full paddle vessel of 1000 tons burden and 350 horses power, will
carry about 400 tons of cargo, besides coal for a voyage of 500 miles, and
the expense of such a voyage, including wear and tear, depreciation, &c.,
will be about 190_l_. The duration of the voyage will be about 45-1/2
hours. A screw vessel of 400 tons burden and 100 horses power, will carry
the same amount of cargo, besides her coals, on the same voyage, and the
expense of the voyage, including wear and tear, depreciation, &c., will be
not much more than 60_l_. An auxiliary screw vessel, therefore, can carry
merchandise at one third of the cost of a full-powered paddle vessel. By
similar comparisons made between the expense of conveying merchandise in
auxiliary screw steamers and sailing ships on coasting voyages, it appears
that the cost in screw steamers is about one third less than in the sailing
ships; the greater expedition of the screw steamers much more than
compensating for the expense which the maintenance of the machinery
involves.
SCREW AND PADDLES COMBINED.
613. _Q._--Would not a screw combined with paddles act in a similarly
advantageous way as a screw or paddles when aided by the wind?
_A._--If in any given paddle vessel a supplementary screw be added to
increase her power and speed, the screw will act in a more beneficial
manner than if it had the whole vessel to propel itself, and for a like
reason the paddles will act in a more beneficial manner. There will be less
slip both upon the paddles and upon the screw than if either had been
employed alone; but the same object would be attained by giving the vessel
larger paddles or a larger screw.
614. _Q._--Have any vessels been constructed with combined screw and
paddles?
_A._--Not any that I know of, except the great vessel built under the
direction of Mr. Brunel. The Bee many years since was fitted with both
screw and paddles, but this was for the purpose of ascertaining the
relative efficiency of the two modes of propulsion, and not for the purpose
of using both together.
615. _Q._--What would be the best means of accelerating the speed of a
paddle vessel by the introduction of a supplementary screw?
_A._--If the vessel requires new boilers, the best course of procedure
would be to work a single engine giving motion to the screw with high
pressure steam, and to let the waste steam from the high pressure engine
work the paddle engines. In this way the power might be doubled without any
increased expenditure of fuel per hour, and there would be a diminished
expenditure per voyage in the proportion of the increased speed.
616. _Q._--What would the increased speed be by doubling the power?
_A._--The increase would be in the proportion of the cube root of 1 to the
cube root of 2, or it would be 1.25 times greater. If, therefore, the
existing speed were 10 miles, it would be increased to 12-1/2 miles by
doubling the power, and the vessel would ply with about a fourth less coals
by increasing the power in the manner suggested.
617. _Q._--Is not high pressure steam dangerous in steam vessels?
_A._--Not necessarily so, and it has now been introduced into a good number
of steam vessels with satisfactory results. In the case of locomotive
engines, where it is used so widely, very few accidents have occurred; and
in steam vessels the only additional source of danger is the salting of the
boiler. This may be prevented either by the use of fresh water in the
boiler, or by practising a larger amount of blowing off, to insure which it
should be impossible to diminish the amount of water sent into the boiler
by the feed pump, and the excess should be discharged overboard through a
valve near the water level of the boiler, which valve is governed by a
float that will rise or fall with the fluctuating level of the water. If
the float be a copper ball, a little water should be introduced into it
before it is soldered or brazed up, which will insure an equality of
pressure within and without the ball, and a leakage of water into it will
then be less likely to take place. A stone float, however, is cheaper, and
if properly balanced will be equally effective. All steam vessels should
have a large excess of boiling feed water constantly flowing into the
boiler, and a large quantity of water constantly blowing off through the
surface valves, which being governed by floats will open and let the
superfluous water escape whenever the water level rises too high. In this
way the boiler will be kept from salting, and priming will be much less
likely to occur. The great problem of steam navigation is the economy of
fuel, since the quantity of fuel consumed by a vessel will very much
determine whether she is profitable or otherwise. Notwithstanding the
momentous nature of this condition, however, the consumption of fuel in
steam vessels is a point to which very little attention has been paid, and
no efficient means have yet been adopted in steam vessels to insure that
measure of economy which is known to be attainable, and which has been
attained already in other departments of engineering in which the benefits
of such economy are of less weighty import. It needs nothing more than the
establishment of an efficient system of registration in steam vessels, to
insure a large and rapid economy in the consumption of fuel, as this
quality would then become the test of an engineer's proficiency, and would
determine the measure of his fame. In the case of the Cornish engines, a
saving of more than half the fuel was speedily effected by the introduction
of the simple expedient of registration. In agricultural engines a like
economy has speedily followed from a like arrangement; yet in both of these
cases the benefits of a large saving are less eminent than they would be in
the case of steam navigation; and it is to be hoped that this expedient of
improvement will now be speedily adopted.
CHAPTER X.
EXAMPLES OF ENGINES.
* * * * *
OSCILLATING PADDLE ENGINES.
618. _Q._--Will you describe the structure of an oscillating engine as made
by Messrs. Penn?
_A._--To do this it will be expedient to take an engine of a given power,
and then the sizes may be given as well as an account of the configuration
of the parts: we may take for an example a pair of engines of 21-1/2 inches
diameter of cylinder, and 22 inches stroke, rated by Messrs. Penn at 12
horses power each. The cylinders of this oscillating engine are placed
beneath the cranks, and, as in all Messrs. Penn's smaller engines, the
piston rod is connected to the crank pin by means of a brass cap, provided
with a socket, by means of which it is cuttered to the piston rod. There is
but one air pump, which is situated within the condenser between the
cylinders, and it is wrought by means of a crank in the intermediate
shaft--this crank being cut out of a solid piece of metal as in the
formation of the cranked axles of locomotive engines. The steam enters the
cylinder through the outer trunnions, or the trunnions adjacent to the
ship's sides, and enters the condenser through the two midship trunnions--a
short three ported valve being placed on the front of the cylinder to
regulate the flow of steam to and from the cylinder in the proper manner.
The weight of this valve on one side of the cylinder is balanced by a
weight hung upon the other side of the cylinder; but in the most recent
engines this weight is discarded, and two valves are used, which balance
one another. The framing consists of an upper and lower frame of cast iron,
bound together by eight malleable iron columns: upon the lower frame the
pillow blocks rest which carry the cylinder trunnions, and the condenser
and the bottom frame are cast in the same piece. The upper frame supports
the paddle shaft pillow blocks; and pieces are bolted on in continuation of
the upper frame to carry the paddle wheels, which are overhung from the
journal.
619. _Q._--What are the dimensions and arrangement of the framing?
_A._--The web, or base plate of the lower frame is 3/4 of an Inch thick,
and a cooming is earned all round the cylinder, leaving an opening of
sufficient size to permit the necessary oscillation. The cross section of
the upper frame is that of a hollow beam 6 inches deep, and about 3-1/2
inches wide, with holes at the sides to take out the core; and the
thickness of the metal is 13/16ths of an inch. Both the upper and the lower
frame is cast in a single piece, with the exception of the continuations of
the upper frame, which support the paddle wheels. An oval ring 3 inches
wide is formed in the upper frame, of sufficient size to permit the working
of the air pump crank; and from this ring feathers run to the ends of the
cross portions of the frame which supports the intermediate shaft journals.
The columns are 1-1/2 inches in diameter; they are provided with collars at
the lower ends, which rest upon bosses in the lower frame, and with collars
at the upper ends for supporting the upper frame; but the upper collars of
two of the corner columns are screwed on, so as to enable the columns to be
drawn up when it is required to get the cylinders out. The cross section of
the bottom frame is also of the form of a hollow beam, 7 inches deep,
except in the region of the condenser, where it is, of course, of a
different form. The depth of the boss for the reception of the columns is a
little more than 7 inches deep on the lower frame, and a little more than 6
inches deep on the upper frame; and the holes through them are so cored
out, that the columns only bear at the upper and lower edges of the hole,
instead of all through it--a formation by which the fitting of the columns
is facilitated.
620. _Q._--What are the dimensions of the condenser?
_A._--The condenser, which is cast upon the lower frame, consists of an
oval vessel 22-1/2 inches wide, by 2 feet 4-1/4 inches long, and 1 foot
10-1/2 inches deep; it stands 9 inches above the upper face of the bottom
frame, the rest projecting beneath it; and it is enlarged at the sides by
being carried beneath the trunnions.
621. _Q._--What are the dimensions of the air pump?
_A._--The air pump, which is set in the centre of the condenser, is 15-1/4
inches in diameter, and has a stroke of 11 inches. The foot valve is
situated in the bottom of the air pump, and its seat consists of a disc of
brass, in which there is a rectangular flap valve, opening upwards, but
rounded on one side to the circle of the pump, and so balanced as to enable
the valve to open with facility. The balance weight, which is formed of
brass cast in the same piece as the valve itself, operates as a stop, by
coming into contact with the disc which constitutes the bottom of the pump;
the disc being recessed opposite to the stop to enable the valve to open
sufficiently. This disc is bolted to the barrel of the pump by means of an
internal flange, and before it can be removed the pump must be lifted out
of its place. The air pump barrel is of brass to which is bolted a cast
iron mouth piece, with a port for carrying the water to the hot well;
within the hot well the delivery valve, which consists of a common flap
valve, is situated. The mouth piece and the air pump barrel are made tight
to the condenser, and to one another, by means of metallic joints carefully
scraped to a true surface, so that a little white or red lead interposed
makes an air tight joint. The air pump bucket is of brass, and the valve of
the bucket is of the common pot lid or spindle kind. The injection water
enters through a single cock in front of the condenser--the jet striking
against the barrel of the air pump. The air pump rod is maintained in its
vertical position by means of guides, the lower ends of which are bolted to
the mouth of the pump, and the upper to the oval in the top frame, within
which the air pump crank works; and the motion is communicated from this
crank to the pump rod by means of a short connected rod. The lower frame is
not set immediately below the top frame, but 2-1/2 inches behind it, and
the air pump and condenser are 2-1/2 inches nearer one edge of the lower
frame than the other.
622. _Q._--What are the dimensions of the cylinder?
_A._--The thickness of the metal of the cylinder is 9/16ths of an inch; the
depth of the belt of the cylinder is 9-1/2 inches, and its greatest
projection from the cylinder is 2-1/2 inches. The distance from the lower
edge of the belt to the bottom of the cylinder is 11-1/2 inches, and from
the upper edge of the belt to the top flange of the cylinder is 9 inches.
The trunnions are 7-1/4 inches diameter in the bearings, and 3-1/2 inches
in width; and the flanges to which the glands are attached for screwing in
the trunnion packings are 1-1/2 inch thick, and have 7/8ths of an inch of
projection. The width of the packing space round the trunnions is 5/8ths of
an inch, and the diameter of the pipe passing through the trunnion
4-5/8ths, which leaves 11/16ths for the thickness of the metal of the
bearing. Above and below each trunnion a feather runs from the edge of the
belt or bracket between 3 and 4 inches along the cylinder, for the sake of
additional support; and in large engines the feather is continued through
the interior of the belt, and cruciform feathers are added for the sake of
greater stiffness. The projection of the outer face of the trunnion flange
from the side of the cylinder is 6-1/2 inches; the thickness of the flange
round the mouth of the cylinder is 3/4 of an inch, and its projection 1-3/8
inch; the height of the cylinder stuffing box above the cylinder cover is
4-1/8 inches, and its external diameter 4-3/8 inches--the diameter of the
piston rod being 2-1/8 inches. The thickness of the stuffing box flange is
1-1/8 inch.
623. _Q._--Will you describe the nature of the communication between the
cylinder and condenser?
_A._--The pipe leading to the condenser from the cylinder is made somewhat
bell mouthed where it joins the condenser, and the gland for compressing
the packing is made of a larger internal diameter in every part except at
the point at which the pipe emerges from it, where it accurately fits the
pipe so as to enable the gland to squeeze the packing. By this construction
the gland may be drawn back without being jammed upon the enlarged part of
the pipe; and the enlargement of the pipe toward the condenser prevents the
air pump barrel from offering any impediment to the free egress of the
steam. The gland is made altogether in four pieces: the ring which presses
the packing is made distinct from the flange to which the bolts are
attached which force the gland against the packing, and both ring and
flange are made in two pieces, to enable them to be got over the pipe. The
ring is half checked in the direction of its depth, and is introduced
without any other support to keep the halves together, than what is
afforded by the interior of the stuffing box; and the flange is half
checked in the direction of its thickness, so that the bolts which press
down the ring by passing through this half-checked part, also keep the
segments of the flange together. The bottom of the trunnion packing space
is contracted to the diameter of the eduction pipe, so as to prevent the
packing from being squeezed into the jacket; but the eduction pipe does not
fit quite tight into this contracted part, but, while in close contact on
the lower side, has about 1/32nd of an inch of space between the top of the
pipe and the cylinder, so as to permit the trunnions to wear to that extent
without throwing a strain upon the pipe. The eduction pipe is attached to
the condenser by a flange joint, and the bolt holes are all made somewhat
oblong in the perpendicular direction, so as to permit the pipe to be
slightly lowered, should such an operation be rendered necessary by the
wear of the trunnion bearings; but in practice the wear of the trunnion
bearings is found to be so small as to be almost inappreciable.
624. _Q._--Will you describe the valve and valve casing?
_A._--The length of the valve casing is 16-1/2 inches, and its projection
from the cylinder is 3-1/2 inches at the top, 4-1/4 inches at the centre,
and 2-1/2 inches at the bottom, so that the back of the valve casing is not
made flat, but is formed in a curve. The width of the valve casing is 9
inches, but there is a portion of the depth of the belt 1-1/2 inch wider,
to permit the steam to enter from the belt into the casing. The valve
casing is attached to the cylinder by a metallic joint; the width of the
flange of this joint is 1-1/4 inch, the thickness of the flange on the
casing 1/2 inch, and the thickness of the flange on the cylinder 5/8ths of
an inch. The projection from the cylinder of the passage for carrying the
steam upwards, and downwards, from the valve to the top and bottom of the
cylinder, is 2-1/4 inches, and its width externally 8-5/8 inches. The valve
is of the ordinary three ported description, and both cylinder and valve
faces are of cast iron.
625. _Q._--What description of piston is used?
_A._--The piston is packed with hemp, but the junk ring is made of
malleable iron, as cast iron junk rings have been found liable to break:
there are four plugs screwed into the cylinder cover, which, when removed,
permit a box key to be introduced, to screw down the piston packing. The
screws in the junk ring are each provided with a small ratchet, cut in a
washer fixed upon the head, to prevent the screw from turning back; and the
number of clicks given by these ratchets, in tightening up the bolts,
enables the engineer to know when they have all been tightened equally. In
more recent engines, and especially in those of large size, Messrs. Penn
employ for the piston packing a single metallic ring with tongue piece and
indented plate behind the joint; and this ring is packed behind with hemp
squeezed by the junk ring as in ordinary hemp-packed pistons.
626. _Q._--Will you describe the construction of the cap for connecting the
piston rod with the crank pin?
_A._--The cap for attaching the piston rod to the crank pin, is formed
altogether of brass, which brass serves to form the bearing of the crank
pin. The external diameter of the socket by which this cap is attached to
the piston rod is 3-5/16 inches. The diameter of the crank pin is 3 inches,
and the length of the crank pin bearing 3-7/8 inches. The thickness of the
brass around the crank pin bearing is 1 inch, and the upper portion of the
brass is secured to the lower portion, by means of lugs, which are of such
a depth that the perpendicular section through the centre of the bearing
has a square outline measuring 7 inches in the horizontal direction, 3-7/8
inches from the centre of the pin to the level of the top of the lugs, and
2-1/2 inches from the centre of the pin to the level of the bottom of the
lugs. The width of the lugs is 2 inches, and the bolts passing through them
are 1-1/4 inch in diameter. The bolts are tapped into the lower portion of
the cap, and are fitted very accurately by scraping where they pass through
the upper portion, so as to act as steady pins in preventing the cover of
the crank pin bearing from being worked sideways by the alternate thrust on
each side. The distance between the centres of the bolts is 5 inches, and
in the centre of the cover, where the lugs, continued in the form of a web,
meet one another, an oil cup 1-5/8 inch in diameter, 1-1/8 inch high, and
provided with an internal pipe, is cast upon the cover, to contain oil for
the lubrication of the crank pin bearing. The depth of the cutter for
attaching the cap to the piston rod is 1-1/4 inch and its thickness is
3/8ths of an inch.
627. _Q._--Will you describe the means by which the air pump rod is
connected with the crank which works the air pump?
[Illustration: Fig. 50. AIR PUMP CONNECTING ROD AND CROSS HEAD. Messrs.
Penn.]
_A._--A similar cap to that of the piston rod attaches the air pump crank
to the connecting rod by which the air pump rod is moved, but in this
instance the diameter of the bearing is 5 inches, and the length of the
bearing is about 3 inches. The air pump connecting rod and cross head are
shown in perspective in fig. 50. The thickness of the brass encircling the
bearing of the shaft is three fourths of an inch upon the edge, and 1-1/8
inch in the centre, the back being slightly rounded; the width of the lugs
is 1-5/8 inch, and the depth of the lugs is 2 inches upon the upper brass,
and 2 inches upon the lower brass, making a total depth of 4 inches. The
diameter of the bolts passing through the lugs is 1 inch, and the bolts are
tapped into the lower brass, and accurately fitted into the upper one, so
as to act as steady pins, as in the previous instance. The lower eye of the
connecting rod is forked, so as to admit the eye of the air pump rod; and
the pin which connects the two together is prolonged into a cross head, as
shown in fig. 50. The ends of this cross head move in guides. The forked
end of the connecting rod is fixed upon the cross head by means of a
feather, so that the cross head partakes of the motion of the connecting
rod, and a cap, similar to that attached to the piston rod, is attached to
the air pump rod, for connecting it with the cross head. The diameter of
the air pump rod is 1-1/2 inch, the external diameter of the socket
encircling the rod is 2-1/8 inches, and the depth of the socket 4-1/2
inches from the centre of the cross head. The depth of the cutter for
attaching the socket to the rod is 1 inch, and its thickness 5/16 inch. The
breadth of the lugs is 1-3/8 inch, the depth 1-1/4 inch, making a total
depth of 2-1/2 inches; and the diameter of the bolts seven eighths of an
inch. The diameter of the cross head at the centre is 2 inches, the
thickness of each jaw around the bearing 1 inch, and the breadth of each
9/16 inch.
628. _Q._--What are the dimensions of the crank shaft and cranks?
_A._--The diameter of the intermediate shaft journal is 4-3/16 inches, and
of the paddle shaft journal 4-3/8 inches; the length of the journal in each
case is 5 inches. The diameter of the large eye of the crank is 7 inches,
and the diameter of the hole through it is 4-3/8 inches; the diameter of
the small eye of the crank is 5-1/4 inches, the diameter of the hole
through it being 3 inches. The depth of the large eye is 4-1/4 inches, and
of the small eye 3-3/4 inches; the breadth of the web is 4 inches at the
shaft end, and 3 inches at the pin end, and the thickness of the web is
2-5/8 inches. The width of the notch forming the crank in the intermediate
shaft for working the air pump is 3-1/2 inches, and the width of each of
the arms of this crank is 3-15/16 inches. Both the outer and inner corners
of the crank are chamfered away, until the square part of the crank meets
the round of the shaft. The method of securing the cranks pins into the
crank eyes of the intermediate shaft consists in the application of a nut
to the end of each pin, where it passes through the eye, the projecting end
of the pin being formed with a thread upon which the nut is screwed.
629. _Q._--Will you describe the eccentric and eccentric rod?
[Illustration: Fig. 51. ECCENTRIC AND ROD. Messrs. Penn.]
_A._--The eccentric and eccentric rod are shown in fig. 51. The eccentric
is put on the crank shaft in two halves, joined in the diameter of largest
eccentricity by means of a single bolt passing through lugs on the central
eye, and the back balance is made in a separate piece five eighths of an
inch thick, and is attached by means of two bolts, which also help to bind
the halves of the eccentric together. The eccentric strap is half an inch
thick, and 1-1/4 inch broad, and the flanges of the eccentric, within which
the strap works, are each three eighths of an inch thick. The eccentric rod
is attached to the eccentric hoop by means of two bolts passing through
lugs upon the rod, and tapped into a square boss upon the hoop; and pieces
of iron, of a greater or less thickness, are interposed between the
surfaces in setting the valve, to make the eccentric rod of the right
length. The eccentric rod is kept in gear by the push of a small horizontal
rod, attached to a vertical blade spring, and it is thrown out of gear by
means of the ordinary disengaging apparatus, which acts in opposition to
the spring, as, in cases where the eccentric rod is not vertical, it acts
in opposition to the gravity of the rod.
630. _Q._--Will you explain in detail the construction of the valve
gearing, or such parts of it as are peculiar to the oscillating engine?
_A._--The eccentric rod is attached by a pin, 1 inch in diameter, to an
open curved link or sector with a tail projecting upward and passing
through an eye to guide the link in a vertical motion. The link is formed
of iron case-hardened, and is 2-3/4 inches deep at the middle, and 2-3/8
inches deep at the ends, and 1 inch broad. The opening in the link, which
extends nearly its entire length, is 1-5/16 inch broad; and into this
opening a brass block 2 inches long is truly fitted, there being a hole
through the block 3/4 inch diameter, for the reception of the pin of the
valve shaft lever. The valve shaft is 1-3/4 inch diameter at the end next
the link or segment, and diminishes regularly to the other end, but its
cross section assumes the form of an octagon in its passage round the
cylinder, measuring mid-way 1-1/4 inch deep, by about 3/4 inch thick, and
the greatest depth of the finger for moving the valve is about 1 inch. The
depth of the lever for moving the valve shaft is 2 inches at the broad, and
1-1/4 inch at the narrow end. The internal breadth of the mortice in which
the valve finger moves is 5/16 inch, and its external depth is 1-3/4 inch,
which leaves three eighths of an inch as the thickness of metal round the
hole; and the breadth, measuring in the direction of the hole, is 1-1/2
inch. The valve rod is three fourths of an inch in diameter, and the
mortice is connected to the valve rod by a socket 1 inch long, and 1-1/8
inch diameter, through which a small cutter passes. A continuation of the
rod, eleven sixteenths of an inch diameter, passes upward from the mortice,
and works through an eye, which serves the purpose of a guide. In addition
to the guide afforded to the segment by the ascending tail, it is guided at
the ends upon the columns of the framing by means of thin semicircular
brasses, 4 inches deep, passing round the columns, and attached to the
segment by two 3/8 inch bolts at each end, passing through projecting
feathers upon the brasses and segment, three eighths of an inch in
thickness. The curvature of the segment is such as to correspond with the
arc swept from the centre of the trunnion to the centre of the valve lever
pin when the valve is at half stroke as a radius; and the operation of the
segment is to prevent the valve from being affected by the oscillation of
the cylinder, but the same action, would be obtained by the employment of a
smaller eccentric with more lead. In some engines the segment is not formed
in a single piece, but of two curved blades, with blocks interposed at the
ends, which may be filed down a little, to enable the sides of the slot to
be brought nearer, as the metal wears away.
631. _Q._--What kind of plummer blocks are used for the paddle shaft
bearings?
_A._--The paddle shaft plummer blocks are altogether of brass, and are
formed in much the same manner as the cap of the piston rod, only that the
sole is flat, as in ordinary plummer blocks, and is fitted between
projecting lugs of the framing, to prevent side motion. In the bearings
fitted on this plan, however, the upper brass will generally acquire a good
deal of play after some amount of wear. The bolts are worked slack in the
holes, though accurately fitted at first; and it appears expedient,
therefore, either to make the bolts very large, and the sockets through
which they pass very deep, or to let one brass fit into the other.
632. _Q._--How are the trunnion plummer blocks made?
_A._--The trunnion plummer blocks are formed in the same manner as the
crank shaft plummer blocks; the nuts are kept from turning back by means of
a pinching screw passing through a stationary washer. It is not expedient
to cast the trunnion plummer blocks upon the lower frame, as is sometimes
done; for the cylinders, being pressed from the steam trunnions by the
steam, and drawn in the direction of the condenser by the vacuum, have a
continual tendency to approach one another; and as they wear slightly
toward midships, there would be no power of readjustment unless the plummer
blocks were movable. The flanges of the trunnions should always fit tight
against the plummer block sides, but there should be a little play sideways
at the necks of the trunnions, so that the cylinder may be enabled to
expand when heated, without throwing an undue strain upon the trunnion
supports.
633. _Q._--What kind of paddle wheel is supplied with these oscillating
engines?
_A._--The wheels are of the feathering kind, 9 feet 8 inches in diameter,
measuring to the edges of the floats; and there are 10 floats upon each
wheel, measuring 4 feet 6 inches long each, and 18-1/2 inches broad. There
are two sets of arms to the wheel, which converge to a cast iron centre,
formed like a short pipe with large flanges, to which the arms are affixed.
The diameter of the shaft, where the centre is put on, is 4-1/2 inches, the
external diameter of the pipe is 8 inches, and the diameter of the flanges
is 20 inches, and their thickness 1-1/4 inch. The flanges are 12 inches
asunder at the outer edge, and they partake of the converging direction of
the arms. The arms are 2-1/4 inches broad and half an inch thick; the heads
are made conical, and each is secured into a recess upon the side of the
flange by means of three bolts. The ring which connects together the arms,
runs round at a distance of 3 feet 6 inches from the centre, and the
projecting ends of the arms are bent backward the length of the lever which
moves the floats, and are made very wide and strong at the point where they
cross the ring, to which they are attached by four rivets. The feathering
action of the floats is accomplished by means of a pin fixed to the
interior of the paddle box, set 3 inches in advance of the centre of the
shaft, and in the same horizontal line. This pin is encircled by a cast
iron collar, to which rods are attached 1-3/8 inch diameter in the centre,
proceeding to the levers, 7 inches long, fixed on the back of the floats in
the line of the outer arms. One of these rods, however, is formed of nearly
the same dimensions as one of the arms of the wheel, and is called the
driving arm, as it causes the cast iron collar to turn round with the
revolution of the wheel, and this collar, by means of its attachments to
the floats, accomplishes the feathering action. The eccentricity in this
wheel is not sufficient to keep the floats in the vertical position, but in
the position between the vertical and the radial. The diameter of the pins
upon which the floats turn is 1-3/8 inch, and between the pins and paddle
ring two stud rods are set between each of the projecting ends of the arms,
so as to prevent the two sets of arms from being forced nearer or further
apart; and thus prevent the ends of the arms from hindering the action of
the floats, by being accidentally jammed upon the sides of the joints.
Stays, crossing one another, proceed from the inner flange of the centre to
the outer ring of the wheel, and from the outer flange of the centre to the
inner ring of the wheel, with the view of obtaining greater stiffness. The
floats are formed of plate iron, and the whole of the joints and joint pins
are steeled, or formed of steel. For sea-going vessels the most approved
practice is to make the joint pins of brass, and also to bush the eyes of
the joints with brass; and the surface should be large to diminish wear.
634. _Q._--Can you give the dimensions of any other oscillating engines?
_A._--In Messrs. Penn's 50 horse power oscillating engine, the diameter of
the cylinder is 3 feet 4 inches, and the length of the stroke 3 feet. The
thickness of the metal of the cylinder is 1 inch, and the thickness of the
cylinder bottom is 1-3/4 inch, crossed with feathers, to give it additional
stiffness. The diameter of the trunnion bearings is 1 foot 2 inches, and
the breadth of the trunnion bearings 5-1/2 inches. Messrs. Penn, in their
larger engines, generally make the area of the steam trunnion less than
that of the eduction trunnion, in the proportion of 32 to 37; and the
diameter of the eduction trunnion is regulated by the internal diameter of
the eduction pipe, which is about 1/5th of the diameter of the cylinder.
But a somewhat larger proportion than this appears to be expedient: Messrs.
Rennie make the area of their eduction pipes, in oscillating engines, 1/22d
of the area of the cylinder. In the oscillating engines of the Oberon, by
Messrs. Rennie, the cylinder is 61 inches diameter, and 1-1/2 inch thick
above and below the belt, but in the wake of the belt it is 1-1/4 inch
thick, which is also the thickness of metal of the belt itself. The
internal depth of the belt is 2 feet 6 inches, and its internal breadth is
4 inches. The piston rod is 6-3/4 inches in diameter, and the total depth
of the cylinder stuffing box is 2 feet 4 inches, of which 18 inches
consists of a brass bush--this depth of bearing being employed to prevent
the stuffing box or cylinder from wearing oval.
635. _Q._--Can you give any other examples?
_A._--The diameter of cylinder of the oscillating engines of the steamers
Pottinger, Ripon, and Indus, by Miller & Ravenhill, is 76 inches, and the
length of the stroke 7 feet. The thickness of the metal of the cylinder is
1-11/16 inch; diameter of the piston rod 8-3/4 inches; total depth of
cylinder stuffing box 3 feet; depth of bush in stuffing box 4 inches; the
rest of the depth, with the exception of the space for packing, being
occupied with a very deep gland, bushed with brass. The internal diameter
of the steam pipe is 13 inches; diameter of steam trunnion journal 25
inches; diameter of eduction trunnion journal 25 inches; thickness of metal
of trunnions 2-1/4 inches; length of trunnion bearings 11 inches;
projection of cylinder jacket, 8 inches; depth of packing space in
trunnions, 10 inches; width of packing space in trunnions, or space round
the pipes, 1-1/2 inch; diameter of crank pin 10-1/4 inches; length of
bearing of crank pin 15-1/2, inches. There are six boilers on the tubular
plan in each of these vessels; the length of each boiler is 10 feet 6
inches, and the breadth 8 feet; and each boiler contains 62 tubes 3 inches
in diameter, and 6 feet 6 inches long, and two furnaces 6 feet 4-1/2 inches
long, and 3 feet 1-1/2 inch broad.
636. _Q._--Is it the invariable practice to make the piston rod cap of
brass in the way you have described?
_A._--In all oscillating engines of any considerable size, the cover of the
connecting brass, which attaches the crank pin to the connecting rod, is
formed of malleable iron; and the socket also, which is cuttered to the end
of the piston rod, is of malleable iron, and is formed with a T head,
through which bolts pass up through the brass, to keep the cover of the
brass in its place.
637. _Q._--Is the piston of an oscillating engine made deeper than in
common engines?
_A._--It is expedient, in oscillating engines, to form the piston with a
projecting rim round the edge above and below, and a corresponding recess
in the cylinder cover and cylinder bottom, whereby the breadth of bearing
of the solid part of the metal will be increased, and in many engines this
is now done.
638. _Q._--Would any difficulty be experienced in keeping the trunnions
tight in a high pressure oscillating engine?
_A._--It is very doubtful whether the steam trunnions of a high pressure
oscillating engine will continue long tight if the packing consists of
hemp; and it appears preferable to introduce a brass ring, to embrace the
pipe, cut spirally, with an overlap piece to cover the cut, and packed
behind with hemp.
639. _Q._--How is the packing of the trunnions usually effected?
_A._--The packing of the trunnions, after being plaited as hard as
possible, and cut to the length to form one turn round the pipe, is dipped
into boiling tallow, and is then compressed in a mould, consisting of two
concentric cylinders, with a gland forced down into the annular space by
three to six screws in the case of large diameters, and one central screw
in the case of small diameters. Unless the trunnion packings be well
compressed, they will be likely to leak air, and it is, therefore,
necessary to pay particular attention to this condition. It is also very
important that the trunnions be accurately fitted into their brasses by
scraping, so that there may not be the smallest amount of play left upon
them; for if any upward motion is permitted, it will be impossible to
prevent the trunnion packings from leaking.
DIRECT ACTING SCREW ENGINE.
640. _Q._--Will you describe the configuration and construction of a direct
acting screw engine?
_A._--I will take as an example of this species of engine, the engine
constructed by Messrs. John Bourne & Co., for the screw steamer Alma, a
vessel of 500 tons burden. This engine is a single steeple engine laid on
its side, and in its general features it resembles the engines of the
Amphion already described, only that there is one cylinder instead of two.
The cylinder is of 42 inches diameter and 42 inches stroke, and the vessel
has been propelled by this single engine at the rate of fourteen miles an
hour.
641. _Q._--Is not a single engine liable to stick upon the centre so that
it cannot be started or reversed with facility?
_A._--A single engine is no doubt more liable to stick upon the centre than
two engines, the cranks of which are set at right angles with one another;
but numerous paddle vessels are plying successfully that are propelled by a
single engine, and the screw offers still greater facility than paddles for
such a mode of construction. In the screw engine referred to, as the
cylinder is laid upon its side, there is no unbalanced weight to be lifted
up every stroke, and the crank, whereby the screw shaft is turned round,
consists of two discs with a heavy side intended to balance the momentum of
the piston and its connections; but these counter-weights by their
gravitation also prevent the connecting rod and crank from continuing in
the same line when the engine is stopped, and in fact they place the crank
in the most advantageous position for starting again when it has to be set
on.
642. _Q._--Will you explain the general arrangement of the parts of this
engine?
_A._--The cylinder lies on its side near one side of the vessel, and from
the end of the cylinder two piston rods extend to a cross head sliding
athwartships, in guides, near the other side of the vessel. To this cross
head the connecting rod is attached, and one end of it partakes of the
motion of the cross head or piston, while the other end is free to follow
the revolution of the crank on the screw shaft.
643. _Q._--What is the advantage of two discs entering into the composition
of the crank instead of one?
_A._--A double crank, such as two discs form with the crank pin, is a much
steadier combination than would result if only one disc were employed with
an over-hung pin. Then the friction on the neck of the shaft is made one
half less by being divided between the two bearings, and the short
prolongation of the shaft beyond the journal is convenient for the
attachment of the eccentrics to work the valves.
644. _Q._--Will you enumerate some of the principal dimensions of this
engine?
_A._--The bottom frame, on which also the condenser is cast, forms the base
of the engine: on one end of it the cylinder is set; on the other end are
the guides for the cross head, and in the middle are the bearings for the
crank shaft. The part where the cylinder stands is two feet high above the
engine platform, and the elevation to the centre of the guides or the
centre of the shaft is 10 inches higher than this. The metal both of the
side frames and bottom flange is 1-1/4 inch thick. The cylinder has flanges
cast on its sides, upon which it rests on the bottom frame, and it is sunk
between the sides of the frame so as to bring the centre of the cylinder in
the same plane as the centre of the screw shaft. The opening left at the
guides for the reception of the guide blocks is 6 inches deep, and the
breadth of the bearing surface is 11 inches. The cover of the guides is 8
inches deep at the middle, and about half the depth at the ends, and holes
are cored through the central web for two oil cups on each guide. The brass
for each of the crank shaft bearings is cut into four pieces so that it may
be tightened in the up and down direction by the bolts, which secure the
plummer block cap, and tightened in the athwartship direction, which is the
direction of the strain, by screwing up a wedge-formed plate against the
side of the brass, a parallel plate being applied to the other side of the
brass, which may be withdrawn to get out the wedge piece when the shaft
requires to be lifted out of its place. The air pump is bolted to one side
of the bottom frame, and a passage is cast on it conducting from the
condenser to the air pump. In this passage the inlet and outlet valves at
each end of the air pump are situated, and appropriate doors are formed
above them to make them easily accessible. The outlet passage leading from
the air pump communicates with the waste water pipe, through which the
water expelled by the air pump is discharged overboard.
645. _Q._--Is the cylinder of the usual strength and configuration?
_A._--The cylinder is formed of cast iron in the usual way, and is 1-1/8
inch thick in the barrel. The ends are of the same thickness, but are each
stiffened with six strong feathers. The piston is cast open. The bottom of
it is 5/8ths of an inch thick, and it is stiffened by six feathers 3/4 of
an inch thick; but the feather connecting the piston rod eyes is 1-1/4 inch
thick, and the metal round the eyes is 2 inches thick. The piston is closed
by a disc or cover 5/8ths of an inch thick, secured by 15 bolts, and this
cover answers also the purpose of a junk ring. The piston packing consists
of a single cast iron ring 3-1/2 inches broad, and 1/2 inch thick, packed
behind with hemp. This ring is formed with a tongue piece, with an indented
plate behind the cut; and the cut is oblique to prevent a ridge forming in
the cylinder. The total thickness of the piston is 5-1/2 inches. The piston
rods are formed with conical ends for fitting into the piston, but are
coned the reverse way as in locomotives, and are secured in the piston by
nuts on the ends of the rods, these nuts being provided with ratchets to
prevent them from unscrewing accidentally.
646. _Q._--What species of slide valve is employed?
_A._--The ordinary three ported valve, and it is set on the top of the
cylinder. The cylinder ports are 4-1/2 inches broad by 24 inches long; and
to relieve the valve from the great friction due to the pressure on so
large a surface, a balance piston is placed over the back of the valve, to
which it is connected by a strong link; and the upward pressure on this
piston being nearly the same as the downward pressure on the valve, it
follows that the friction is extinguished, and the valve can be moved with
great case with one hand. The balance piston is 21 inches in diameter. In
the original construction of this balance piston two faults were committed.
The passage communicating between the condenser and the top of the balance
piston was too small, and the pins at the ends of the link connecting the
valve and balance piston were formed with an inadequate amount of bearing
surface. It followed from this misproportion that the balance piston, being
adjusted to take off nearly the whole of the pressure, lifted the valve off
the face at the beginning of each stroke. For the escape of the steam into
the eduction passage momentarily impaired the vacuum subsisting there, and
owing to the smallness of the passage leading to the space above the
balance piston, the vacuum subsisting in that space could not be impaired
with equal rapidity. The balance piston, therefore, rose by the upward
pressure upon it momentarily predominating over the downward pressure on
the valve; but this fault was corrected by enlarging the communicating
passage between the top of the balance piston and the eduction pipe. The
smallness of the pins at the ends of the link connecting the valve and
balance piston, caused the surfaces to cut into one another, and to wear
very rapidly, and the pins and eyes in this situation should be large in
diameter, and as long as they can be got, as they are not so easily
lubricated as the other bearings about the engine, and are moreover kept at
a high temperature by the steam. The balance piston is packed in the same
way as the main piston of the engine. Its cylinder, which is only a few
inches in length, is set on the top of the valve casing, and a trunk
projects upwards from its centre to enable the connecting link to rise up
in it to attain the necessary length.
[Illustration: Fig 52. CONNECTING ROD. Messrs. Bourne & Co.]
647. _Q._--What is the diameter of the piston rods and connecting rod?
_A._--The piston rods, which are two in number, are 3 inches diameter, and
12 feet 10 inches long over all. They were, however, found to be rather
small, and have since been made half an inch thicker. The connecting rod
consists of two rods, which are prolongations of the bolts that connect the
sides of the brass bushes which encircle the crank pin and cross head. The
connecting rod is shown in perspective in fig. 52. The rods composing it
are each 2-3/4 inches in diameter.
648. _Q._--Will you describe the configuration of the cross head.
_A._--The cross head, exhibited in fig. 53, is a round piece of iron like a
short shaft, with two unequal arms keyed upon it, the longer of which _b_
works the air pump, and the shorter _c_ works the feed pump. The piston
rods enter these arms at _a A._ The cross head is 8 inches diameter where
it is embraced by the connecting rod at _e_, and 7 inches diameter where
the air pump and feed pump arms are fixed on. The ends of the cross head _d
d_, for a length of 12 inches, are reduced to 3 inches diameter where they
fit into round holes in the centre of the guide blocks. Those blocks are of
cast iron 6 inches deep, 11 inches wide, and 14 inches long, and they are
formed with flanges 1 inch thick on the inner sides of the blocks. The
projection of the air pump lever from the centre of the cross head is 1
foot 9 inches, and it is bent 5-3/4 inches to one side to enable it to
engage the air pump rod. The eye of this arm is 6 inches broad and about 2
inches thick. At the part where one of the piston rods passes through it,
the arm is 8 inches deep and 6 inches wide; but the width thereafter
narrows to 3 inches, and finally to 2 inches; and the depth of the web of
the arm reduces from 8 inches at the piston rod, to 4 inches at the eye,
which receives the end of the air pump rod. The feed pump arm is only 3
inches thick, and has 9 inches of projection from the centre of the cross
head; but the eye attached to it on the opposite side of the cross head for
the reception of the other piston rod is of the same length as that part of
the air pump arm which one of the piston rods passes through. The piston
rods have strong nuts on each side of each of these arms to attach them to
the arms, and also to enable the length of the piston rods to be suitably
adjusted, to leave equal clearance between the piston and each end of the
cylinder at the termination of the stroke.
[Illustration: Fig. 53.
CROSS HEAD AND PUMP ARMS. Messrs. Bourne & Co.]
649. _Q._--Will you recapitulate the main particulars of the air pump?
_A._--The air pump is made of brass 12-1/2 inches diameter and 42 inches
stroke, and the metal of the barrel is 9/16ths of an inch thick. The air
pump bucket is a solid piston of brass, 6-1/2 inches deep at the edge, and
7 inches deep at the eye; and in the edge three grooves are turned to hold
water which answers the purpose of packing. The inlet and outlet valves of
the air pump consist of brass plates 1/2 inch with strong feathers across
them, and in each plate there are six grated perforations covered by india
rubber discs 7 inches in diameter. These six perforations afford
collectively an area for the passage of the water equal to the area of the
pump. The air pump rod is of brass, 2-1/2 inches diameter.
650. _Q._--What are the constructive peculiarities of the discs and crank
pin?
_A._--The discs, which are 64 inches diameter, are formed of cast iron, and
are 2-1/2 inches thick in the body, and 5 inches broad at the rim. The
crank shaft is 8-1/2 inches diameter, and the central boss of the disc
which receives the shaft measures 10 inches through the eye, and the metal
of the eye is 3 inches thick. In the part of the disc opposite to the crank
pin, the web is thickened to 10 inches for nearly the whole semicircle,
with the view of making that side of the disc heavier than the other side;
and when the engine is stopped, the gravitation of this heavy side raises
the crank pin to the highest point it can attain, whereby it is placed in
mid stroke, and cannot rest with the piston rods and connecting rod in a
horizontal line. The crank pin is 8-1/2 inches diameter, and the length of
the bearing or rubbing part of it is 16 inches. It is secured at the ends
to the discs by flanges 18 inches diameter, and 2 inches thick. These
flanges are indented into thickened parts of the discs, and are each
attached to its corresponding disc by six bolts 2 inches diameter,
countersunk in the back of the disc, and tapped into the malleable iron
flange. Besides this attachment, each end of the pin, reduced to 4-1/2
inches diameter, passes through a hole in its corresponding disc, and the
ends of the pin are then riveted over. The crank pin is perforated through
the centre by a small hole about 3/4 of an inch in diameter, and three
perforations proceed from this central hole to the surface of the pin. Each
crank shaft bearing is similarly perforated, and pipes are cast in the
discs connecting these perforations together. The result of this
arrangement is, that a large part of the oil or water fed into the bearings
of the shaft is driven by the centrifugal action of the discs to the
surface of the crank pin, and in this way the crank pin may be oiled or
cooled with water in a very effectual manner. To intercept the water or oil
which the discs thus drive out by their centrifugal action, a light paddle
box or splash board of thin sheet brass is made to cover the upper part of
each of the discs, and an oil cup with depending wick is supported by the
tops of these paddle boxes, which wick is touched at each revolution of the
crank by a bridge standing in the middle of an oil cup attached to the
crank pin. The oil is wiped from the wick by the projecting bridge at each
revolution, and subsides into the cup from whence it proceeds to lubricate
the crank pin bearing. This is the expedient commonly employed to oil the
crank pins of direct acting engines; but in the engine now described, there
are over and above this expedient, the communicating passages from the
shaft bearings to the surface of the pin, by which means any amount of
cooling or lubrication can be administered to the crank pin bearing,
without the necessity of stopping or slowing the engine.
[Illustration: Fig. 54. DOUBLE DISC CRANK. Messrs. Bourne & Co.]
651. _Q._--What is the diameter of the screw shaft?
_A._--The screw shaft is 7-1/2 inches diameter, but the bearings on each
side of the disc are 8-1/2 inches diameter, and 16 inches long. Between the
side of the disc and the side of the contiguous bearings there is a short
neck extending 4-3/4 inches in the length of the shaft, and hollowed out
somewhat to permit the passage of the piston rod; for one piston rod passes
immediately above the shaft on the one side of the discs, and the other
piston rod passes immediately below the shaft on the other side of the
discs. A short piece of one piston rod is shown in fig. 54.
[Illustration: Fig. 55. THRUST BEARING. Messers. Bourne & Co.]
[Illustration: Fig. 56. COUPLING CRANKS. Messers. Bourne & Co.]
652. _Q._--How is the thrust of the screw shaft received?
_A._--The thrust of the screw shaft is received upon 7 collars, each 1 inch
thick, and with 1 inch of projection above the shaft. The plummer block for
receiving the thrust of the shaft is shown in fig. 55, and the coupling to
enable the screw propeller to be disconnected from the engine, so that it
may revolve freely when the vessel is under sail, is shown in fig. 56. When
it is required to disengage the propeller from the engine, the pins passing
through the opposite eyes shown fig. 56, are withdrawn by means of screws
provided for that purpose, and the propeller and the engine are thenceforth
independent of one another.
[Illustration: Fig. 57. LINK MOTION. Messrs. Bourne & Co.]
653. _Q._--Will you describe the arrangement of the valve gearing?
_A._--The end of the screw shaft, after emerging from the bearing beside
the disc, is reduced to a diameter of 4 inches, and is prolonged for 4-1/2
inches to give attachment to the cam or curved plate which gives motion to
the expansion valve. This plate is 3-1/2 inches thick, and a stud 3-1/2
inches diameter is fixed in the plate at a distance of 5 inches from the
centre of the shaft. To this stud an arm is attached which extends to a
distance of 2 inches from the centre of the shaft in the opposite
direction, and the end of this arm carries a pin of 2-1/2 inches diameter.
From the pin most remote from the centre of the shaft, a rod 2-1/2 inches
broad and 1 inch thick extends to the upper end of the link of the link
motion; and from the pin least remote from the centre of the shaft, a
similar rod extends to the lower end of the link of the link motion. This
link, which is represented in fig. 57, is 2-1/4 inches broad, 1 inch thick,
and is capable of being raised or lowered 25 inches in all. In the open
part of the link is a brass block, which, by raising or lowering the link,
takes either the position in which it is represented at the centre of the
link, or a position at either end of it. Through the hole in the brass
block a pin passes to attach the brass to the end of a lever fixed on the
valve shaft; so that whatever motion is imparted to the brass block is
communicated to the valve through the medium of this lever. If the brass
block be set in the middle of the link, no motion is communicated to it,
and the valve being consequently kept stationary and covering both ports,
the engine stops. If the link be lowered until the brass block comes to the
upper end of the link, the valve receives the motion of the eccentric for
going ahead, and the engine moves ahead; whereas if the link be raised
until the brass block comes to the lower end of the link, the valve
receives the motion of the backing eccentric, and the engine moves astern.
Instead of eccentrics, however, pins at the end of the shaft are employed
in this engine, the arrangement partaking of the nature of a double crank;
but the backing pin has less throw than the going ahead pin, whereby the
efficient length of the link for going ahead is increased; and the
operation of backing, which does not require to be performed at the highest
rate of speed, is sufficiently accommodated by about half the throw being
given to the valve that is given in going ahead. A valve shaft extends
across the end of the cylinder with two levers standing up, which engage
horizontal side rods extending from a small cross head on the end of the
valve rod. A lever extends downwards from the end of the valve shaft, which
is connected by a pin to the brass block within the link; and the link is
moved up or down by the starting handle, which, by means of a spring bolt
shooting into a quadrant, holds the starting handle at any position in
which it may be set.
654. _Q._--What is the diameter and pitch of the screw propeller?
_A._--The diameter is 7 feet and the pitch 14 feet. The propeller is Holm's
conchoidal propeller. Its diameter is smaller than is advisable, being
limited by the draught of water of the vessel; and the vessel was required
to have a small draught of water to go over a bar. This engine makes, under
favorable circumstances, 100 strokes per minute. The speed of piston with
this number of strokes is 700 feet per minute, and the engine works
steadily at this speed, the shock and tremor arising from the arrested
momentum of the moving parts being taken away by the counterbalance applied
at the discs.
LOCOMOTIVE ENGINE.
655. _Q._--Will you describe the principal features of a modern locomotive
engine?
_A._--I will take for this purpose the locomotive Snake, constructed by
John V. Gooch for the London and South Western Railway, as an example of a
modern locomotive of good construction, adapted for the narrow gauge. The
length of the wheel base of this engine is 12 feet 8-1/2 inches. There are
two cylinders, each 14-1/4 inches diameter and 21 inches stroke. The total
weight of the engine is 19 tons; and this weight is so distributed on the
wheels as to throw 8 tons on the leading wheels, 6 tons on the driving
wheels, and 5 tons on the hind wheels. The engine is made with outside
cylinders, and the cylinders are raised somewhat out of the horizontal line
to enable them better to clear the leading wheels.
656. _Q._--What are the dimensions of the boiler?
_A._--The interior of the fire box is 3 feet 7-1/4 inches wide by 3 feet
5-1/2 inches long, measuring in the direction of the rails. The area of the
fire grate is consequently 12.4 square feet. The bars are somewhat lower on
the side next the fire door than at the side next the tubes, and the mean
height of the crown of the fire box above the bars is 3 feet 10 inches. The
top edge of the fire door is about 7 inches lower than the crown of the
fire box. The fire box is divided transversely by a corrugated feather or
bridge of plate iron, containing water, about 3-1/2 inches wide, and of
about one-third of the height of the fire box in the centre of the feather,
and about two-thirds the height of the fire box at the sides where it joins
the sides of the fire box. The internal shell of the fire box tapers
somewhat upwards to facilitate the disengagement of the steam. It is about
2 inches narrower and shorter at the top than at the bottom; the water
space between the external and internal shell of the fire box being 2
inches at the bottom and 3 inches at the top.
657. _Q._--Of what material is the fire box composed?
_A._--The external shell of the fire box is formed of iron plates 3/8ths of
an inch thick, and the internal shell is formed of copper plates 1/4 inch
thick, but the tube plate is 3/4 inch thick. The fire grate is rectangular,
and the internal and external shells are tied together by iron stay bolts
3/4 inch diameter, and pitched about 4 inches apart. The roof of the fire
box is stiffened by six strong bars extending from side to side of the fire
box like beams, and the top of the fire box is secured to these bars, so
that it cannot be forced down without breaking or bending them.
658. _Q._--What are the dimensions of the barrel of the boiler?
_A._--The barrel of the boiler is 3 feet 7-1/2 inches in diameter, and 10
feet long. It is formed of iron plates 3/8ths of an inch thick, riveted
together. It is furnished with 181 brass tubes 1-7/8 inch diameter and 10
feet long, secured at the ends by ferules. The tube plate at the smoke box
end is 5/8ths of an inch thick, and the tube plates above the tubes are
tied together by eight iron rods 7/8ths of an inch thick, extending from
end to end of the boiler. The metal of the tubes is somewhat thicker at the
end next the fire, being 13 wire gauge at fire box end, and 14 wire gauge
at smoke box end. The rivets of the boiler are 3/4 inch diameter and 1-1/2
inch pitch. The plating of the ash pan is 5/16ths of an inch thick, and the
plating of the smoke box is 3/16ths of an inch thick.
659. _Q._--Will you describe the structure of the framework on which the
boiler and its attachments rest, and in which the wheels are set?
_A._--The framework or framing consists of a rectangular structure of plate
iron circumscribing the boiler, with projecting lugs or arms for the
reception of the axles of the wheels. In this engine the sides of the
rectangle are double, or, as far as regards the sides, there are virtually
two framings, one for the reception of the driving axles, and the other for
the reception of the axles not connected with the engine. The whole of the
parts of the outer and inner framings are connected together by knees at
the corners, and the double sides are elsewhere connected by intervening
brackets and stays, so as to constitute the whole into one rigid structure.
The whole of the plating of the inside frame is 3/4 inch thick and 9 inches
deep. The plating of the outside frame is of the same thickness and depth
at the fore part, until it reaches abaft the position of the cylinders and
guides, where it reduces to 1/2 inch thick. The axle guard of the leading
wheels is formed of 3/4 plate bolted to the frame with angle iron guides.
The axle guards of the trailing wheels are formed of two 1/2 inch plates,
with cast iron blocks between them to serve as guides. The ends of the
rectangular frame are formed of plates 3/4 thick, and at the front end
there is a buffer beam of oak 4-1/2 inches thick and 15 inches deep. The
draw bolt is 2 inches diameter. There are two strong stays on each side,
joining the barrel of the boiler to the inside framing, and one angle iron
on each side joining the bottom of the smoke box to the inside framing.
660. _Q._--Of what construction are the wheels?
_A._--The wheels and axles are of wrought iron, and the tires of the wheels
are of steel. The driving wheels are 6 feet 6-1/2 inches in diameter, and
the diameter of crank pin is 3-1/2 inches. The diameter of the smaller
wheels is 48-1/2 inches. The axle boxes are of cast iron with bushes of
Fenton's metal, and the leading axle has four bearings. The springs are
formed of steel plates, 3 feet long, 4 inches broad, and 1\2 inch thick.
The axle of the driving wheel has two eccentrics, forged solid upon it, for
working the pumps.
661. _Q._--Will you specify the dimensions of the principal parts of the
engine?
_A._--Each of the cylinders which is 14-1/4 inches diameter, has the valve
casing cast upon it. The steam ports are 13 inches long and 1-5/8 inches
broad, and the exhaust port is 2-1/2 inches broad. The travel of the valve
is 4-1/8 inches, the lap 1 inch, and the lead 1/4 inch. The piston is 4
inches thick: its body is formed of brass with a cover of cast iron, and
between the body and the cover two flanges, forged on the piston rod, are
introduced to communicate the push and pull of the piston to the rod. The
piston rod is of iron, 2-1/2 inches diameter. The guide bars for guiding
the top of the piston rod are of steel, 4 inches broad, fixed to rib iron
bearers, with hard wood 1/4 of an inch thick, interposed. The connecting
rod is 6 feet long between the centres, and is fitted with bushes of white
metal. The eccentrics are formed of wrought iron, and have 4-1/8 inches of
throw. The link of the link motion is formed of wrought iron. It is hung by
a link from a pin attached to the framing; and instead of being susceptible
of upward and downward motion, as in the case of the link represented in
fig. 57 a rod connecting the valve rod with the movable block in the link,
is susceptible of this motion, whereby the same result is arrived at as if
the link were moved and the block was stationary. One or the other
expedient is preferable, according to the general nature of the
arrangements adopted. The slide valve is of brass, and the regulator
consists of two brass slide valves worked over ports in a chest in the
steam pipe, set in the smoke box. The steam pipe is of brass, No. 14. wire
gauge, perforated within the boiler barrel with holes 1/12th of an inch in
diameter along its upper side. The blast pipe, which is of copper, has an
orifice of 4-1/4 inches diameter. There is a damper, formed like a Venetian
blind, with the plates running athwartships at the end of the tubes.
[Illustration: Fig. 58. SAFETY VALVE. Gooch.]
662. _Q._--Of what construction is the safety valve?
_A._--There are two safety valves, consisting of pistons 1-3/16 inch in
diameter, and which are kept down by spiral springs placed immediately over
them. A section of this valve is given in fig. 58.
663. _Q._--What are the dimensions of the feed pumps?
_A._--The feed pumps are of brass, with plungers 4 inches diameter and
3-1/4 inches stroke. The feed pipe is of copper, 2 inches diameter. A good
deal of trouble has been experienced in locomotives from the defective
action of the feed pump, partly caused by the leakage of steam into the
pumps, which prevented the water from entering them, and partly from the
return of a large part of the water through the valves at the return stroke
of the pump, in consequence of the valve lifting too high. The pet cock--a
small cock communicating with the interior of the pump--will allow any
steam to escape which gains admission, and the air which enters by the cock
cools down the barrel of the pump, so that in a short time it will be in a
condition to draw. The most ordinary species of valve in the feed pumps of
locomotives, is the ball valve.
Notwithstanding the excellent performance of the best examples of
locomotive engines, it is quite certain that there is still much room for
improvement; and indeed various sources of economy are at present visible,
which, if properly developed, would materially reduce the expense of the
locomotive power. In all engines the great source of expense is the fuel;
and although the consumption of fuel has been greatly reduced within the
last ten or fifteen years, it is capable of being still further reduced by
certain easy expedients of improvement, which therefore it is important
should be universally applied. One of these expedients consists in heating
the feed water by the waste steam; and the feed water should in every case
be sent into the boiler _boiling hot_, instead of being quite cold, as is
at present generally the case. The ports of the cylinders should be as
large as possible; the expansion of the steam should be carried to a
greater extent; and in the case of engines with outside cylinders, the
waste steam should circulate entirely round the cylinders before escaping
by the blast pipe. The escape of heat from the boiler should be more
carefully prevented; and the engine should be balanced by weights on the
wheels to obviate a waste of power by yawing on the rails. The most
important expedient of all, however, lies in the establishment of a system
of registering the performance of all new engines, in order that
competition may stimulate the different constructors to the attainment of
the utmost possible economy; and under the stimulus of comparison and
notoriety, a large measure of improvement would speedily ensue. The
benefits consequent on public competition are abundantly illustrated by the
rapid diminution of the consumption of fuel in the case of agricultural
engines, when this stimulus was presented.
CHAPTER XI
OF VARIOUS FORMS, APPLICATIONS, AND APPLIANCES OF THE STEAM ENGINE.
In the English edition of this work, the first part of this chapter is
devoted to examples of Portable and fixed Agricultural engines, of
different makers and styles of workmanship, but not in sufficient detail,
nor illustrated on large enough scale to be of practical value as models,
forming rather in fact an illustrated catalogue of the manufacturer, than a
study for the mechanic. On this account, they have been entirely omitted,
and their place supplied by a few illustrations from American workmanship,
not only of Steam Engines, of various forms and applications, but also of
various machines, or appliances, connected with the working of engines, as
for the determination, or regulation of pressure, of the boilers; for the
supply or feed of the boilers, the regulation of the speed of the engine,
and the like.
The Gauges used in this country to show the pressures of steam in boilers
are of various constructions, but perhaps the most common is the Bourdon,
or, as it is known here, the Ashcroft gauge, from the party introducing it,
and holding the patent. Fig. 59 represents its interior construction. It
consists of a thin metallic tube, _a_, bent into nearly a complete circle
closed at one end, the steam being introduced at the other, at _b_. The
effect of the pressure of the steam on the interior of the tube is to
expand the circle, more or less according to the pressure, the elasticity
of the metal returning the circle to its original position, when the
pressure is removed. The free or closed end of the tube is connected by a
link _c_ with a lever _d_, at the opposite end of which is segmental gear,
in gear with a pinion, on which is a hand, which marks the pressure on a
dial. The dial and hand are not shown on the cut, but are on the exterior
case removed to show the construction.
[Illustration: Fig. 59.]
[Illustration: Fig. 60.]
Fig. 60 is an elevation of a boiler with Clark's Patent Steam and Fire
Regulator attached, for the control of the draft of the chimney by the
pressure of steam in the boiler. It consists of a chamber, _a_, with a
flexible diaphragm or cover on top, in communication with the boiler. On
this diaphragm rests a plunger or piston, which is held down like a safety
valve, by a lever and weight, _b_. The end of the lever is connected with a
balanced damper, _c_, in the chimney. The weight, _b_, is placed at any
required position on the lever, and when the pressure of steam in the
boiler, exerted on the diaphragm, becomes sufficient to raise the weight,
the lever rises, and the damper begins to close, and to check the draft in
the chimney. When properly adjusted, the machine works on a variation of
from, one to two pounds between the extremes of motion. When the dampers
are very large, say 3 feet or over, they should be set on rollers, like
common grindstone rollers; the regulator should be attached directly to the
damper, the length of the pipe connecting the regulator with the boiler
being of no account.
[Illustration: Fig. 61.]
Porter's Patent Governor, fig. 61, is a modification of the ordinary
centrifugal governor. Very small balls are employed, from 2-1/4 to 2-5/8
inches in diameter. These swing from a single joint at the axis of the
spindle, which is the most sensitive arrangement, and make from 300 to 350
revolutions per minute, at which speed their centrifugal force lifts the
counterpoise. The lower arms are jointed to the upper ones at the centres
of the balls, and connect with the slide by joints about two inches apart.
The counterpoise may be attached to the slide in any manner; for the sake
of elegance, it is put in the form of a vase rising between the arms, its
stem forming the slide. The vase is hollow and filled with lead, and weighs
from 60 lbs. to 175 lbs. It moves freely on the spindle, through nearly
twice the vertical distances traversed by the balls, and is capable of
rising from 2-1/2 to 3 inches, before its rim will touch the arms. It is
represented in the figure as lifted through about one half of its range of
action.
The standard is bored out of the solid, forming a long and perfect bearing
for the spindle; the arms and balls are of gun metal, the joint pins of
steel; every part of the governor is finished bright, except the bracket
carrying the lever, and the square base of the standard, which are painted.
The pulley is from 3 to 10 inches in diameter, and makes in the larger
sizes about 125 revolutions, and in the smaller 230 revolutions per minute;
the higher speed of the governor being got up by gearing.
Mr. Porter warrants the following action in this governor, operating any
regulating valve or cut-off which is in reasonably good order. The engine
should be run with the stop-valve wide open, and, except the usual oiling,
will require no attention from the engineer, under any circumstances, after
it is started, until it is to be stopped. No increase in the pressure of
steam will affect its motion perceptibly. The extreme possible variation in
the speed, between that at which the regulating valve will be held wide
open, and that at which it will be closed, is from 3 to 5 per cent., being
least in the largest governors. This is less than 1/6 of the variation
required by the average of ordinary governors, and is with difficulty
detected by the senses. The entire load which the engine is capable of
driving may be thrown on or off at once, and one watching the revolutions
cannot tell when it is done. The governor will be sensibly affected by a
variation in the motion of the engine of 1 revolution in 800.
Notwithstanding this extreme sensitiveness, or rather by reason of it, it
will not oscillate, but when the load is uniform will stand quite, or
nearly, motionless.
For the supply of the water to the boiler, in many positions, it is very
convenient to have a pump unconnected with the engine. On this account it
is very usual in this country to have what are called donkey pumps or
engines independent of the main engines, which can be used to feed the
boilers, or for supplying water for many other purposes.
Fig. 62 is a longitudinal section of the Worthington Steam Pump, the first
of its kind, and for many years in successful operation.
The general arrangement is that of a Steam Cylinder, the piston rod of
which, carried through into the water cylinder and attached directly to the
water plunger, works back and forth without rotary motion, and of course
without using either crank or fly wheel.
[Illustration: Fig. 62.]
In the figures, _a_ is the Steam Cylinder--_b_, the Steam Chest--_d_, a
handle for regulating the steam valve--_f_, the starting bar _g, g_,
tappets attached to the valve rod, which is moved by the contact of the arm
_e_, on the piston rod with said tappets--_h_, the double-acting water
plunger working through a packing ring--_o, o_, force valves--_o', o'_,
suction valves. The pump piston is represented as moving from right to
left, the arrows indicating the course of the water through the passages.
The suction valves _o'_, on the right side, and the force valves _o_, on
the left side, are show open; _x_, is an air chamber made of copper; _s_,
the suction pipe terminating in a vacuum chamber; made by prolonging the
suction pipe, and closing it perfectly tight at the top, the connection
being made to the pump by a branch as shown; _m, m_, are hand-hole plates,
affording easy access to the water valves; _n, n_, small holes through the
plunger, which relieve the pressure near the end of the stroke, to give
momentum to throw the valves when working at slow speed.
[Illustration: Fig. 63.]
Fig. 63 is a perspective view of H.R. Worthington's Duplex Steam Pump. The
prominent peculiarity of this pump is its valve motion. As seen in the cut,
two steam pumps are placed side by side (or end to end, if desired). Each
pump, by a rock shaft connected with its piston rod, gives a constant and
easy motion to the steam valve of the other. Each pump therefore gives
steam to and starts its neighbor, and then finishes its own stroke, pausing
an instant till its own steam valve, being opened by the other pump, allows
it to make the return stroke.
This combined action produces a perfectly positive valve motion without
dead points, great regularity and ease of motion, and entire absence of
noise or shock of any kind. Both kinds of pumps are made by Mr.
Worthington, of various size according to the requirements, the duplex
being used for boiler feed and for the supply of cities with water.
Fig. 64 is a side elevation of the Woodward Steam Pump. The pump is direct
acting. The steam and water piston being on the same rod, but momentum is
obtained to throw the valves by means of a fly wheel, placed beyond the
pump, and connected with the piston rod by a cross head and a yoke. The
machine is simple in its construction and action, and is extensively used.
Giffard's Injector, both in Europe and this country, is quite extensively
used to supply the place of a pump, as independent feed for all classes of
boilers. It is represented in elevation and section, figs. 65 and 66.
[Illustration: Fig. 64.]
[Illustration: Fig. 65.]
[Illustration: Fig. 66.]
_A_, steam pipe leading from the boiler. _B_, a perforated tube or
cylinder, through which the steam passes into the space _b_. _C_ screwed
rod for regulating the passage of steam through the annular conical space
_c_, and worked by the handle _d/_. _E_, suction pipe, leading from the
tank or hot well to small chamber _m_. _F_, annular conical opening or
discharge pipe, the size of which is regulated by the movement of the tube
or cylinder _B_. _G_, hand wheel for actuating the cylinder _B_. _H_,
opening, in connection with the atmosphere, intervening between discharge
pipe _F_ and the receiving pipe through which the water is forced. _I_,
tube through which the water passes to the boiler. _K_, valve for
preventing the return of the water from the boiler when the injector is not
working. _L_, waste or overflow pipe. _M_, nut to tighten the packing rings
_g_ and upper packing _i_ in cylinder _B_. _N_, lock nut to hold _M_.
The pipe _A_ is connected with the steam space of the boiler at its highest
part, to obtain as dry steam as possible. The passage of the steam into _A_
is controlled by a cock, as is also the feed pipe to the boiler. In
working, both are opened, the steam passes through _A_ into the space _b_,
and issuing through the nozzle _c_ with the pressure due to its head, and a
partial vacuum by its contact with the feed water, it drives this water in
connection with the jet through the pipe _F_ into the pipe _I_ in
connection with the water space of the boiler.
_Method of Working._--Turn the wheel so as to permit a small quantity of
water to flow to the instrument. Open the steam cock connecting the
apparatus with the boiler. Turn slightly the handle, which will admit a
small quantity of steam to the apparatus; a partial vacuum is thus
produced, causing the water to enter through the supply pipe. As soon as
this happens, which can be observed at the overflow pipe, the supply of
steam or water may be increased as required, up to the capacity of the
instrument, regulating either by means of the wheel and handle, so as to
prevent any overflow. The quantity of water delivered into the boiler, may
be varied by means of the stop cocks on the steam and water pipes, without
altering the handles on the injector; a graduated cock on the water supply
pipe is very convenient for this purpose.
The machines are manufactured by Wm. Sellers & Co. Philadelphia.
As an example of Portable Steam Engines, of which there are large numbers
in this country of different manufacturers, we give the representation
(fig. 67) of one made by J.C. Hoadley, of Lawrence, Mass.
[Illustration: Fig. 67.]
In these machines, the rules and proportions of the locomotive engine are
adapted to the requirements of stationary power, for all purposes under
forty horse power. The leading ideas are: high velocity, high pressure,
good valve motion, large fire-box, numerous and short flues, and steam
blast. The characteristic features are: great strength of boiler, fully
adequate to bear with safety 200. lbs. pressure per sq. in., great
compactness and simplicity, large and adjustable wearing surfaces, and the
entire absence of all finish, or polish, for mere show.
The cylinder is placed over the centre of the boiler, at the fire-box end,
so that the strain due to the engine is central to the boiler (which serves
as bed plate); the starting valve is under the hand of the engineer when at
the fire door; and both ends of the crank shaft are available for driving
pulleys.
For the sake of compactness, the cylinders are set low, by means of a
depression in the boiler between the stands of the crank shaft, to admit of
the play of the crank and connecting rod. All the parts are attached to the
boiler, which is made of sufficient strength to bear all extra strain due
to the working of the engine.
They have feed water heater, force pumps, Jackson's governor and valve,
belt for governor, belt pulley, turned on the face, steam gauge;
everything, in short, necessary to the convenient working of a steam
engine. All engines are fired up and tried before they leave the shop, and
they are warranted tight, safe, and complete.
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