A Catechism of the Steam Engine
by
John Bourne
Part 5 out of 8
_A._--Yes; and the general arrangement of the valves of double acting air
pumps, such as are usual in direct acting screw engines, is that
represented in the figure of Penn's trunk engine already described in
Chapter I. Each inlet and outlet valve consists of a number of india rubber
discs set over a perforated brass plate, and each disc is bound down by a
bolt in the middle, which bolt also secures a brass guard set above the
disc to prevent it from rising too high. The usual configuration of those
valves is that represented in figs. 42, 43, and 44; figs. 42 and 43 being a
section and ground plan of the species of valve used by Messrs. Penn, and
fig. 44 being a section of that used by Messrs. Maudslay. It is important
in these valves to have the india rubber thick,--say about an inch thick
for valves eight inches in diameter. It is also advisable to make the
central bolts with a nut above and a nut below, and to form the bolt with a
counter sunk neck, so that it will not fall down when the top nut is
removed. The lower point of the bolt should be riveted over on the nut to
prevent it from unscrewing, and the top end should have a split pin through
the point for the same purpose. The hole through which the bolt passes
should be tapped, though the bolt is not screwed into it, so that if a bolt
breaks, a temporary stud may be screwed into the hole without the necessity
of taking out the whole plate. The guard should be large, else the disc may
stretch in the central hole until it comes over it; but the guard should
not permit too much lift of the valve, else a good deal of the water and
air will return into the pump at the return stroke before the valve shuts.
Penn's guard is rather small, and Maudslay's permits too much lift.
459. _Q._--What is the proper area through the valve gratings?
_A._--The collective area should be at least equal to the area of the pump
piston, and the lower edges of the perforations should be rounded off to
afford more free ingress or egress to the water.
460. _Q._--Is there much strain thrown on the plates in which the valves
are set?
_A._--A good deal of strain; and in the earlier direct acting screw engines
these plates were nearly in every case made too light. They should be made
thick, have strong feathers upon them, and be very securely bolted down
with split pins at the points of the bolts, to prevent them from
unscrewing. The plate will be very apt to be broken should some of the
bolts become loose. Of course all the bolts and split pins, as well as the
plates and guards, must be of brass.
461. _Q._--How are the plates to be taken out should that become necessary?
_A._--They are usually taken out through a door in the top of the hot well
provided for that purpose, which door should be as large as the plates
themselves; and it is a good precaution to cast upon this door--which will
be of cast iron--six or eight stout projecting feet which will press upon
the top of the outlet or delivery valve plate when the door is screwed
down. The upper or delivery valve plate and the lower or foot valve plate
should have similar feet. A large part of the strain will thus be
transferred from the plates to the door, which can easily be made strong
enough to sustain it. It is advisable that the plates should lie at an
angle so that the shock of the water may not come upon the whole surface at
once.
462. _Q._--Does the double acting air pump usual in direct acting screw
engines, produce as good a vacuum as the single acting air pump usual in
paddle engines?
_A._--It will do so if properly constructed; but I do not know of any case
of a double acting air pump, with india rubber valves, which has been
properly constructed.
463. _Q._--What is the fault of such pumps?
_A._--The pump frequently works by starts, as if at times it did not draw
at all, and then again on a sudden gorged itself with water, so as to throw
a great strain upon the working parts. The vacuum, moreover, is by no means
so good as it should be, and it is a universal vice of direct acting screw
engines that the vacuum is defective. I have been at some pains to
investigate the causes of this imperfection; and in a sugar house engine
fitted with pumps like those of a direct acting screw engine to maintain a
vacuum in the pans, I found that a better vacuum was produced when the
engine was going slowly than when it was going fast; which is quite the
reverse of what was to have been expected, as the hot water which had to be
removed by the condensation of the steam proceeding from the pan, was a
constant quantity. In this engine, too, which was a high pressure one, the
irregularities of the engine consequent upon the fitful catching of the
water by the pump, was more conspicuous, as the working of this vacuum pump
was the only work that the engine had to perform.
464. _Q._--And were you able to discover the cause of these irregularities?
_A._--The main cause of them I found to be the largeness of the space left
between the valve plates in this class of pumps, and out of which there is
nothing to press the air or water which may be lying there. It consequently
happens, that if there be the slightest leakage of air into the pump, this
air is merely compressed, and not expelled, by the advance of the air pump
piston. It expands again to its former bulk on the return of the pump
piston, and prevents the water from entering until there is such an
accumulation of pressure in the condenser as forces the water into the
pump, when the air being expelled by the water, causes a good vacuum to be
momentarily formed in the pump when it gorges itself by taking a sudden
gulp of water. So soon, however, as the pressure falls in the condenser and
some more air leaks into the pump, the former imperfect action recurs and
is again redressed in the same violent manner.
465. _Q._--Is this irregular action of the pump the cause of the imperfect
vacuum?
_A._--It is one cause. Sometimes one end of the pump will alone draw and
the other end will be inoperative, although it is equally open to the
condenser, and this will chiefly take place at the stuffing box end, where
a leakage of air is more likely to occur. I find, however, that even when
both ends of the pump are acting equally and there is no leakage of air at
all, the vacuum maintained by a double acting horizontal pump with india
rubber valves, is not so good as that maintained by a single acting pump of
the kind usual in old engines.
466. _Q._--Will you specify more precisely what were the results you
obtained?
_A._--When the vacuum pan was exhausted by the pumps without any boiling
being carried on in the pan, but only a little cold water being let into
it, and also into the pumps to enable them to act in their best manner, it
was found that whereas with the old pump a vacuum of 114 on the sugar
boiler's gauge could be readily obtained, equal to about 29-1/2 inches of
mercury, the lowest that could possibly be got with the new horizontal pump
was 122 degrees of the sugar boiler's gauge, or 29 inches of mercury, and
to get that the engine must not go faster than 10 or 12 strokes per minute.
The proper speed of the engine was 75 strokes per minute, but if allowed to
go at that speed the vacuum fell to 130 of the sugar maker's gauge, or
28-1/2 inches of mercury. When the steam was let into the worms of the pan
so as to boil the water in it, the vacuum was 134 at 75 revolutions of the
engine, and went down to 132 at 40 revolutions, but rose again to 135,
equal to about 28-1/4 inches of mercury, at 20 revolutions.
467. _Q._--To what do you attribute the circumstance of a better vacuum
being got at low speeds than at high speeds?
_A._--It is difficult to assign the precise reason, but it appears to be a
consequence of the largeness of the vacant space between the valve plates.
When the piston of the air pump is drawn back, the air contained in this
large collection of water will cause it to boil up like soda water; and
when the piston of the pump is forced forward, this air, instead of being
expelled, will be again driven into the water. There will consequently be a
quantity of air in the pump which cannot be got rid of at all, and which
will impair the vacuum as a matter of course.
468. _Q._--What expedient did you adopt to improve the vacuum in the engine
to which you have referred?
_A._--I put blocks of wood on the air pump piston, which at the end of its
stroke projected between the valve plates and forced the water out. I also
introduced a cock of water at each end of the pump between the valve
plates, to insure the presence of water at each end of the pump to force
the air out. With these ameliorations the pump worked steadily, and the
vacuum obtained became as good as in the old pump. I had previously
introduced an injection cock into each end of the air pump in steam
vessels, from which I had obtained advantageous results; and in all
horizontal air pumps I would recommend the piston and valve plates to be so
constructed that the whole of the water will be expressed by the piston. I
would also recommend an injection cock to be introduced at each end of the
pump.
PUMPS, COCKS, AND PIPES.
469. _Q._--Will you explain the arrangement of the feed pump?
_A._--In steam vessels, the feed pump plunger is generally of brass, and
the barrel of the pump is sometimes of brass, but generally of cast iron.
There should be a considerable clearance between the bottom of the plunger
and the bottom of the barrel, as otherwise the bottom of the barrel may be
knocked out, should coal dust or any other foreign substance gain
admission, as it probably would do if the injection water were drawn at any
time from the bilge of the vessel, as is usually done if the vessel springs
a leak. The valves of the feed pump in marine engines are generally of the
spindle kind, and are most conveniently arranged in a chest, which may be
attached in any accessible position to the side of the hot well. There are
two nozzles upon this chest, of which the lower one leads to the pump, and
the upper one to the boiler. The pipe leading to the pump is a suction pipe
when the plunger ascends, and a forcing pipe when the plunger descends. The
plunger in ascending draws the water out of the hot well through the lowest
of the valves, and in descending forces it through the centre valve into
the space above it, which communicates with the feed pipe. Should the feed
cock be shut so as to prevent any feed water from passing through it, the
water will raise the topmost valve, which is loaded to a pressure
considerably above the pressure of the steam, and escape into the hot well.
This arrangement is neater and less expensive than that of having a
separate loaded valve on the feed pipe with an overflow through the ship's
side, as is the more usual practice.
470. _Q._--Will you describe what precautions are to be observed in the
construction of the cocks used in engines?
_A._--All the cocks about an engine should be provided with bottoms and
stuffing boxes, and reliance should never be placed upon a single bolt
passing through a bottom washer for keeping the plug in its place, in the
case of any cock communicating with the boiler; for a great strain is
thrown upon that bolt if the pressure of the steam be high, and if the plug
be made with much taper; and should the bolt break, or the threads strip,
the plug will fly out, and persons standing near may be scalded to death.
In large cocks, it appears the preferable plan to cast the bottoms in; and
the metal of which all the cocks about a marine engine are made, should be
of the same quality as that used in the composition of the brasses, and
should be without lead, or other deteriorating material. In some cases the
bottoms of cocks are burnt in with hard solder, but this method cannot be
depended upon, as the solder is softened and wasted away by the hot salt
water, and in time the bottom leaks, or is forced out. The stuffing box of
cocks should be made of adequate depth, and the gland should be secured by
means of four strong copper bolts. The taper of blow-off cocks is an
important element in their construction; as, if the taper be too great, the
plugs will have a continual tendency to rise, which, if the packing be
slack, will enable grit to get between the faces, while, if the taper be
too little, the plug will be liable to jam, and a few times grinding will
sink it so far through the shell that the waterways will no longer
correspond. One eighth of an inch deviation from the perpendicular for
every inch in height, is a common angle for the side of the cock, which
corresponds with one quarter of an inch difference of diameter in an inch
of height; but perhaps a somewhat greater taper than this, or one third of
an inch difference in diameter for every inch of height, is a preferable
proportion. The bottom of the plug must be always kept a small distance
above the bottom of the shell, and an adequate surface must be left above
and below the waterway to prevent leakage. Cocks formed according to these
directions will be found to operate satisfactorily in practice, while they
will occasion perpetual trouble if there be any malformation.
471. _Q._--What is the best arrangement and configuration of the blow-off
cocks?
_A._--The blow-off cocks of a boiler are generally placed some distance
from the boiler; but it appears preferable that they should be placed quite
close to it, as there are no means of shutting off the water from the pipe
between the blow-off cock and the boiler, should fracture or leakage there
arise. Every boiler must be furnished with a blow-off cock of its own,
independently of the main blow-off cocks on the ship's sides, so that the
boilers may be blown off separately, and may be shut off from one another.
The preferable arrangement appears to be, to cast upon each blow-off cock a
bend for attaching the cock to the bottom of the boiler, and the plug
should stand about an inch in advance of the front of the boiler, so that
it may be removed, or re-ground, with facility. The general arrangement of
the blow-off pipes is to run a main blow-off pipe beneath the floor plates,
across the ship, at the end of the engines, and into this pipe to lead a
separate pipe, furnished with a cock, from each boiler. The main blow-off
pipe, where it penetrates the ship's side, is furnished with a cock: and in
modern steam vessels Kingston's valves are also used, which consist of a
spindle or plate valve, fitted to the exterior of the ship, so that if the
internal pipe or cock breaks, the external valve will still be operative.
Some expedient of this kind is almost necessary, as the blow-off cocks
require occasional regrinding, and the sea cocks cannot be re-ground
without putting the vessel into dock, except by the use of Kingston's
valves, or some equivalent expedient.
472. Q.--What is the proper construction and situation of the injection
cocks, and waste water valves?
A.--The sea injection cocks are usually made in the same fashion as the sea
blow-off cocks, and of about the same size, or rather larger. The injection
water is generally admitted to the condenser by means of a slide valve, but
a cock appears to be preferable, as it is more easily opened, and has not
any disposition to shut of its own accord. In paddle vessels the sea
injection pipes should be put through the ship's sides in advance of the
paddles, so that the water drawn in may not be injuriously charged with
air. The waste water pipe passing from the hot well through the vessel's
side is provided with a stop valve, called the discharge valve, which is
usually made of the spindle kind, so as to open when the water coming from
the air pump presses against it. In some cases this valve is a sluice
valve, but the hot well is then almost sure to be split, if the engine be
set on without the valve having been opened. The opening of the waste water
pipe should always be above the load water line, as it will otherwise be
difficult to prevent leakage through the engine into the ship when the
vessel is lying in harbor.
473. Q.--What is the best arrangement of gauge cocks and glass gauges?
A.--Gauge cocks are generally very inartificially made, and occasion
needless annoyance. They are rarely made with bottoms, or with stuffing
boxes, and are consequently, for the most part, adorned with stalactites of
salt after a short period of service. The water discharged from them, too,
from the want of a proper conduit, disfigures the front of the boiler, and
adds to the corrosion in the ash pits. It would be preferable to combine
the gauge cocks appertaining to each boiler into a single upright tube,
connected suitably with the boiler, and the water flowing from them could
be directed downward into a funnel tube communicating with the bilge. The
cocks of the glass tubes, as well as of the gauge cocks, should be
furnished with stuffing boxes and with bottoms, unless the water enters
through the bottom of the plug, which in gauge cocks is sometimes the case.
The glass gauge tubes should always be fitted with a cock at each neck
communicating with the boiler, so that the water and steam may be shut off
if the tube breaks; and the cocks should be so made as to admit of the
tubes being blown through with steam to clear them, as in muddy water they
will become so soiled that the water cannot be seen. The gauge cocks
frequently have pipes running up within the boiler, to the end that a high
water level may be made consistent with an easily accessible position of
the gauge cocks themselves. With the glass tubes, however, this species of
arrangement is not possible, and the glass tubes must always be placed in
the position of the water level.
474. Q.--What is the proper material of the pipes in steam vessels?
A.--Most of the pipes of marine engines should be made of copper. The steam
pipes may be of cast iron, if made very strong, but the waste water pipes
should be of copper. Cast iron blow-off pipes have in some cases been
employed, but they are liable to fracture, and are dangerous. The blow-off
and feed pipes should be of copper, but the waste steam pipe may be of
galvanized iron. Every pipe passing through the ship's side, and every pipe
fixed at both ends, and liable to be heated and cooled, should be furnished
with a faucet or expansive joint; and in the case of the cast iron pipes,
the part of the pipe fitting into the faucet should be turned. In the
distribution of the faucets of the pipes exposed to pressure, care must be
taken that they be so placed that the parts of the pipe cannot be forced
asunder, or turned round by the strain, as serious accidents have occurred
from the neglect of this precaution.
475. _Q._--What is the best mode of making pipes tight where they penetrate
the ship's side?
_A._--In wooden vessels the pipes where they pierce the ship's side, should
be made tight, as follows:--the hole being cut, a short piece of lead pipe,
with a broad flange at one end, should be fitted into it, the place having
been previously smeared with white lead, and the pipe should then be beaten
on the inside, until it comes into close contact all around with the wood.
A loose flange should next be slipped over the projecting end of the lead
pipe, to which it should be soldered, and the flanges should both be nailed
to the timber with scupper nails, white lead having been previously spread
underneath. This method of procedure, it is clear, prevents the possibility
of leakage down through the timbers; and all, therefore, that has to be
guarded against after this precaution, is to prevent leakage into the ship.
To accomplish this object, let the pipe which it is desired to attach be
put through the leaden hause, and let the space between the pipe and the
lead be packed with gasket and white lead, to which a little olive oil has
been added. The pipe must have a flange upon it to close the hole in the
ship's side; the packing must then be driven in from the outside, and be
kept in by means of a gland secured with bolts passing through the ship's
side. If the pipe is below the water line the gland must be of brass, but
for the waste water pipe a cast iron gland will answer. This method of
securing pipes penetrating the side, however, though the best for wooden
vessels, will, it is clear, fail to apply to iron ones. In the case of iron
vessels, it appears to be the best practice to attach a short iron nozzle,
projecting inward from the skin, for the attachment of every pipe below the
water line, as the copper or brass would waste the iron of the skin if the
attachment were made in the usual way.
DETAILS OF THE SCREW AND SCREW SHAFT.
476. _Q._--What is the best method of fixing the screw upon the shaft?
_A._--The best way is to cut two large grooves in the shaft coming up to a
square end, and two corresponding grooves or key seats in the screw boss
opposite the arms. Fit into the grooves on the shaft keys with heads, the
length of which is equal to half the depth of the boss, and with the ends
of the keys bearing against the ends of the grooves in the shaft. Then ship
on the propeller, and drive other keys of an equal length from the other
side of the boss, so that the points of the keys will nearly meet in the
middle; next burr up the edge of the grooves upon the heads of the keys, to
prevent them from working back; and finally tap a bolt into the side of the
boss to penetrate the shaft. Propellers so fitted will never get slack.
477. _Q._--What is the best way of fitting in the screw pipe at the stern?
_A._--It should have projecting rings, which should be turned; and cast
iron pieces with holes in them, bored out to the sizes of these rings,
should be secured to the stern frames, and the pipe be then shipped through
all. Before this is done, however, the stern post must be bored out by a
template to fit the pipe, and the pipe is to be secured at the end to the
stern post either by a great external nut of cast iron, or by bolts passing
through the stern post and through lugs on the pipe. The pipe should be
bored throughout its entire length, and the shaft should be turned so as to
afford a very long bearing which will prevent rapid wear.
478. _Q._--How is the hole formed in the deadwood of the ship in which the
screw works?
_A._--A great frame of malleable iron, the size of the hole, is first set
up, and the plating of the ship is brought to the edge of this hole, and is
riveted through the frame. It is important to secure this frame very firmly
to the rest of the ship, with which view it is advisable to form a great
palm, like the palm of a vice, on its inner superior corner, which,
projecting into the ship, may be secured by breast-hook plates to the
sides, whereby the strain which the screw causes will be distributed over
the stern, instead of being concentrated on the rivets of the frame.
479. _Q._--Are there several lengths of screw shaft?
_A._--There are.
480. _Q._--How then are these secured to one another?
_A._--The best mode of securing the several lengths of shaft together is by
forging the shafts with flanges at the ends, which are connected together
by bolts, say six strong bolts in each, accurately fitted to the holes.
[Illustration: Fig 44. End of the Screw Shaft of Correo, showing the mode
of receiving the Thrust. A, discs; B, tightening wedge.]
481. _Q._--How is the thrust of the shaft usually received?
_A._--In some cases it is received on a number of metal discs set in a box
containing oil; and should one of these discs stick fast from friction, the
others will be free to revolve. This arrangement, which is represented in
fig. 44, is used pretty extensively and answers the purpose perfectly. It
is of course necessary that the box in which the discs A are set, shall be
strong enough to withstand the thrust which the screw occasions. Another
arrangement still more generally used, is that represented in figs. 55 and
56, p. 331. It is a good practice to make the thrust plummer block with a
very long sole in the direction of the shaft, so as to obviate any risk of
canting or springing forward when the strain is applied, as such a
circumstance, if occurring even to a slight extent, would be very likely to
cause the bearing to heat.
482. _Q._--Are there not arrangements existing in some vessels for enabling
the screw to be lifted out of the water while the vessel is at sea?
_A._--There are; but such arrangements are not usual in merchant vessels.
In one form of apparatus the screw is set on a short shaft in the middle of
a sliding frame, which can be raised or lowered in grooves like a window
and the screw shaft within the ship can be protruded or withdrawn by
appropriate mechanism, so as to engage or leave free this short shaft as
may be required. When the screw has to be lifted, the screw shaft is drawn
into the vessel, leaving the short shaft free to be raised up by the
sliding frame, and the frame is raised by long screws turned round by a
winch purchase on deck. A chain or rope, however, is better for the purpose
of raising this frame, than long screws; but the frame should in such case
be provided with pall catches like those of a windlass, which, if the rope
should break, will prevent the screw from falling.
DETAILS OF THE PADDLES AND PADDLE SHAFT.
483. _Q._--What are the most important details of the construction of
paddle wheels?
_A._--The structure of the feathering wheel will be hereafter described in
connection with an account of the oscillating engine; and it will be
expedient now to restrict any account of the details to the common radial
paddle, as applied to ocean steamers. The best plan of making the paddle
centres is with square eyes, and each centre should be secured in its place
by means of eight thick keys. The shaft should be burred up against the
head of these keys with a chisel, so as to prevent the keys from coming
back of their own accord. If the keys are wanted to be driven back, this
burr must be cut off, and if made thick, and of the right taper, they may
then be started without difficulty. The shaft must of course be forged with
square projections on it, so as to be suitable for the application of
centres with square eyes. Messrs. Maudslay & Co. bore out their paddle
centres, and turn a seat for them on the shaft, afterward fixing them on
the shaft with a single key. This plan is objectionable for the two
reasons, that it is insecure when new, and when old is irremovable. The
general practice among the London engineers is to fix the paddle arms at
the centre to a plate by means of bolts, a projection being placed upon the
plates on each side of the arm, to prevent lateral motion; but this method
is inferior in durability to that adopted in the Clyde, in which each arm
is fitted into a socket by means of a cutter--a small hole being left
opposite to the end of each arm, whereby the arm may be forced back by a
drift.
484. _Q._--How are the arms attached to the outside rings?
_A._--Some engineers join the paddle arms to the outer ring by means of
bolts; but unless very carefully fitted, those bolts after a time become
slack sideways, and a constant working of the parts of the wheel goes on in
consequence. Sometimes the part of the other ring opposite the arm is
formed into a mortise, and the arms are wedged tight in these holes by
wedges driven in on each side; but the plan is an expensive one, and not
satisfactory, as the wedges work loose even though riveted over at the
point. The best mode of making a secure attachment of the arms to the ring,
consists in making the arms with long T heads, and riveting the cross piece
to the outer ring with a number of rivets, not of the largest size, which
would weaken the outer ring too much. The best way of securing the inner
rings to the arms is by means of lugs welded on the arms, and to which the
rings are riveted.
485. _Q._--What are the scantlings of the paddle floats?
_A._--The paddle floats are usually made either of elm or pine; if of the
former, the common thickness for large sea-going vessels is about 2-1/2
inches; if of the latter, 3 inches. The floats should have plates on both
sides, else the paddle arms will be very liable to cut into the wood, and
the iron of the arms will be very rapidly wasted. When the floats have been
fresh put on they must be screwed up several times before they come to a
bearing. If this be not done, the bolts will be sure to get slack at sea,
and all the floats on the weather side may be washed off. The bolts for
holding on the paddle floats are made extra strong, on account of the
corrosion to which they are subject; and the nuts should be made large, and
should be square, so that they may be effectually tightened up, even though
their corners be worn away by corrosion. It is a good plan to give the
thread of the paddle bolts a nick with a chisel, after the nut has been
screwed up, which will prevent the nut from turning back. Paddle floats,
when consisting of more than one board, should be bolted together edgeways,
by means of bolts running through their whole breadth. The floats should
not be notched to allow of their projection beyond the outer ring, as, if
the sides of the notch be in contact with the outer ring, the ring is soon
eaten away in that part, and the projecting part of the float, being
unsupported, is liable to be broken off.
486. _Q._--Do not the wheels jolt sideways when the vessel rolls?
_A._--It is usual to put a steel plate at each end of the paddle shafts
tightened with a key, to prevent end play when the vessel rolls, but the
arrangement is precarious and insufficient. Messrs. Maudslay make their
paddle shaft bearings with very large fillets in the corner, with the view
of diminishing the evil; but it would be preferable to make the bearings of
the crank shafts spheroidal; and, indeed, it would probably be an
improvement if most of the bearings about the engine were to be made in the
same fashion. The loose end of the crank pin should be made not spheroidal,
but consisting of a portion of a sphere; and a brass bush might then be
fitted into the crank eye, that would completely encase the ball of the
pin, and yet permit the outer end of the paddle shaft to fall without
straining the pin, the bush being at the same time susceptible of a slight
end motion. The paddle shaft, where it passes through the vessel's side, is
usually surrounded by a lead stuffing box, which will yield if the end of
the shaft falls; this stuffing box prevents leakage into the ship from the
paddle wheels: but it is expedient, as a further precaution, to have a
small tank on the ship's side immediately beneath the stuffing box, with a
pipe leading down to the bilge to catch and conduct away any water that may
enter around the shaft.
487. _Q._--How is the outer bearing of the paddle wheels supplied with
tallow?
_A._--The bearing at the outer end of the paddle shaft is sometimes
supplied with tallow, forced into a hole in the plummer block cover, as in
the case of water wheels; but for vessels intended to perform long voyages,
it is preferable to have a pipe leading down to the oil cup above the
journal from the top of the paddle box, through which pipe oil may at any
time be supplied.
488. _Q._--Will you explain the method of putting engines into a steam
vessel?
_A._--As an illustration of this operation it may be advisable to take the
case of a side lever engine, and the method of proceeding is as follows:--
First measure across from the inside of paddle bearers to the centre of the
ship, to make sure that the central line, running in a fore and aft
direction on the deck or beams, usually drawn by the carpenter, is really
in the centre. Stretch a line across between the paddle bearers in the
direction of the shaft; to this line, in the centre of the ship where the
fore and aft mark has been made, apply a square with arms six or eight feet
long, and bring a line stretched perpendicularly from the deck to the
keelson, accurately to the edge of the square: the lower point of the line
where it touches the keelson will be immediately beneath the marks made
upon the deck. If this point does not come in the centre of the keelson, it
will be better to shift it a little, so as to bring it to the centre,
altering the mark upon the deck correspondingly, provided either paddle
shaft will admit of this being done--one of the paddle brackets being
packed behind with wood, to give it an additional projection from the side
of the paddle bearer. Continue the line fore and aft upon the keelson as
nearly as can be judged in the centre of the ship; stretch another line
fore and aft through the mark upon the deck, and look it out of winding
with the line upon the keelson. Fix upon any two points equally distant
from the centre, in the line stretched transversely in the direction of the
shaft; and from those points, as centres, and with any convenient radius,
sweep across the fore and aft line to see that the two are at right angles;
and, if not, shift the transverse line a little to make them so. From the
transverse line next let fall a line upon each outside keelson, bringing
the edge of the square to the line, the other edge resting on the keelson.
A point will thus be got on each outside keelson, perpendicularly beneath
the transverse line running in the direction of the shaft, and a line drawn
between those two points will be directly below the shaft. To this line the
line of the shaft marked on the sole plate has to be brought, care being
taken, at the same time, that the right distance is preserved between the
fore and aft line upon the sole plate, and the fore and aft line upon the
central keelson.
489. _Q._--Of course the keelsons have first to be properly prepared?
_A._--In a wooden vessel, before any part of the machinery is put in, the
keelsons should be dubbed fair and straight, and be looked out of winding
by means of two straight edges. The art of placing engines in a ship is
more a piece of plain common sense than any other feat in engineering, and
every man of intelligence may easily settle a method of procedure for
himself. Plumb lines and spirit levels, it is obvious, cannot be employed
on board a vessel, and the problem consists in so placing the sole plates,
without these aids, that the paddle shaft will not stand awry across the
vessel, nor be carried forward beyond its place by the framing shouldering
up more than was expected. As a plumb line cannot be used, recourse must be
had to a square; and it will signify nothing at what angle with the deck
the keelsons run, so long as the line of the shaft across the keelsons is
square down from the shaft centre. The sole plates being fixed, there is no
difficulty in setting the other parts of the engine in their proper places
upon them. The paddle wheels must be hung from the top of the paddle box to
enable the shaft to be rove through them, and the cross stays between the
engines should be fixed in when the vessel is afloat. To try whether the
shafts are in a line, turn the paddle wheels, and try if the distance
between the cranks is the same at the upper and under, and the two
horizontal centres; if not, move the end of the paddle shaft up or down,
backward or forward, until the distance between the cranks at all the four
centres is the same.
490. _Q._--In what manner are the engines of a steam vessel secured to the
hull?
_A._--The engines of a steamer are secured to the hull by means of bolts
called holding down bolts, and in wooden vessels a good deal of trouble is
caused by these bolts, which are generally made of iron. Sometimes they go
through the bottom of the ship, and at other times they merely go through
the keelson,--a recess being made in the floor or timbers to admit of the
introduction of a nut. The iron, however, wears rapidly away in both cases,
even though the bolts are tinned; and it has been found the preferable
method to make such of the bolts as pass through the bottom, or enter the
bilge, of Muntz's metal, or of copper. In a side lever engine, four Muntz's
metal bolts may be put through the bottom at the crank end of the framing
of each engine, four more at the main centre, and four more at the
cylinder, making twelve through bolts to each engine; and it is more
convenient to make these bolts with a nut at each end, as in that case the
bolts may be dropped down from the inside, and the necessity is obviated of
putting the vessel on very high blocks in the dock, in order to give room
to put the bolts up from the bottom. The remainder of the holding down
bolts may be of iron, and may, by means of a square neck, be screwed into
the timber of the keelsons as wood screws--the upper part being furnished
with a nut which may be screwed down upon the sole plate, so soon as the
wood screw portion is in its place. If the cylinder be a fixed one it
should be bolted down to the sole plate by as many bolts as are employed to
attach the cylinder cover, and they should be of copper or brass, in any
situation that is not easily accessible.
491. _Q._--If the engines become loose, how do you refix them?
_A._--It is difficult to fix engines effectually which have once begun to
work in the ship, for in time the surface of the keelsons on which the
engines bear becomes worn uneven, and the engines necessarily rock upon it.
As a general rule, the bolts attaching the engines to the keelsons are too
few and of too large a diameter: it would be preferable to have smaller
bolts, and a greater number of them. In addition to the bolts going through
the keelsons or the vessel's bottom, there should be a large number of wood
screws securing the sole plate to the keelson, and a large number of bolts
securing the various parts of the engine to the sole plate. In iron
vessels, holding down bolts passing through the bottom are not expedient;
and there the engine has merely to be secured to the iron plate of the
keelsons, which are made hollow to admit of a more effectual attachment.
492. _Q._--What are the proper proportions of bolts?
_A._--In well formed bolts, the spiral groove penetrates about one twelfth
of the diameter of the cylinder round which it winds, so that the diameter
of the solid cylinder which remains is five sixths of the diameter over the
thread. If the strain to which iron may be safely subjected in machinery is
one fifteenth of its utmost strength, or 4,000 lbs. on the square inch,
then 2,180 lbs. may be sustained by a screw an inch in diameter, at the
outside of the threads. The strength of the holding down bolts may easily
be computed, when the elevating force of the piston or main centre is
known; but it is expedient very much to exceed this strength in practice,
on account of the elasticity of the keelsons, the liability to corrosion,
and other causes.
THE LOCOMOTIVE ENGINE.
493. _Q._--What is the amount of tractive force requisite to draw carriages
on railways?
_A._--Upon well formed railways with carriages of good construction, the
average tractive force required for low speeds is about 7-1/2 lbs. per ton,
or 1/300th of the load, though in some experimental cases, where particular
care was taken to obtain a favorable result, the tractive force has been
reduced as low as 1/500th of the load. At low speeds the whole of the
tractive force is expended in overcoming the friction, which is made up
partly of the friction of attrition in the axles, and partly of the rolling
friction, or the obstruction to the rolling of the wheels upon the rail.
The rolling friction is very small when the surfaces are smooth, and in the
case of railway carriages does not exceed 1/1000th. of the load; whereas
the draught on common roads of good construction, which is chiefly made up
of the rolling friction, is as much as 1/36th of the load.
494._Q._--In reference to friction you have already stated that the
friction of iron sliding upon brass, which has been oiled and then wiped
dry, so that no film of oil is interposed, is about 1/11th of the pressure,
but that in machines in actual operation, where there is a film of oil
between the rubbing surfaces, the friction is only about one third of this
amount, or 1/33d of the weight. How then can the tractive resistance of
locomotives at low speeds, which you say is entirely made up of friction,
be so little as 1/500th. of the weight?
_A._--I did not state that the resistance to traction was 1/500th of the
weight upon an average--to which condition the answer given to a previous
question must be understood to apply--but I stated that the average
traction was about 1/300th of the load, which nearly agrees with my former
statement. If the total friction be 1/300th of the load, and the rolling
friction be 1/1000th of the load, then the friction of attrition must be
1/429th of the load; and if the diameter of the wheels be 36 in., and the
diameter of the axles be 3 in., which are common proportions, the friction
of attrition must be increased in the proportion of 36 to 3, or 12 times,
to represent the friction of the rubbing surface when moving with the
velocity of the carriage, 12/429ths are about 1/35th of the load, which
does not differ much from the proportion of 1/33d as previously determined.
495. _Q._--What is the amount of adhesion of the wheels upon the rails?
_A._--The adhesion of the wheels upon the rails is about 1/5th of the
weight when the rails are clean, or either perfectly wet or perfectly dry;
but when the rails are half wet or greasy, the adhesion is not more than
1/10th or 1/12th of the weight or pressure upon the wheels. The weight of a
locomotive of modern construction varies from 20 to 25 tons.
496. _Q._--And what is its cost and average performance?
_A._--The cost of a common narrow gauge locomotive, of average power,
varies from L1,900 to L2,200; it will run on an average 130 miles per day,
at a cost for repairs of 2-1/2d. per mile; and the cost of locomotive
power, including repairs, wages, oil, and coke, does not much exceed 6d.
per mile run, on economically managed railways. This does not include a
sinking fund for the renewal of the engines when worn out, which may be
taken as equivalent to 10 per cent. on their original cost.
497. _Q._--Does the expense of traction increase much with an increased
speed?
_A._--Yes; it increases very rapidly, partly from the undulation of the
earth when a heavy train passes over it at a high velocity, but chiefly
from the resistance of the atmosphere and blast pipe, which constitute the
greatest of the impediments to motion at high speeds. At a speed of 30
miles an hour, the atmospheric resistance has been found in some cases to
amount to about 12 lbs. a ton; and in side winds the resistance even
exceeds this amount, partly in consequence of the additional friction
caused from the flanges of the wheels being forced against the rails, and
partly because the wind catches to a certain extent the front of every
carriage, whereby the efficient breadth of each carriage, in giving motion
to the air in the direction of the train, is very much increased. At a
speed of 30 miles an hour, an engine evaporating 200 cubic feet of water in
the hour, and therefore exerting about 200 horses power, will draw a load
of 110 tons. Taking the friction of the train at 7-1/2 lbs. per ton, or 825
lbs. operating at the circumference of the driving wheel--which, with 5 ft.
6 in. wheels, and 18 in. stroke, is equivalent to 4,757 lbs. upon the
piston--and taking the resistance of the blast pipe at 6 lbs. per square
inch of the pistons, and the friction of the engine unloaded at 1 lb. per
square inch, which, with pistons 12 in. in diameter, amount together to
1,582 lbs., and reckoning the increased friction of the engine due to the
load at 1/7th of the load, as in some cases it has been found
experimentally to be, though a much less proportion than this would
probably be a nearer average, we have 7018.4 lbs. for the total load upon
the pistons. At 30 miles an hour the speed of the pistons will be 457.8
feet per minute, and 7018.4 lbs. multiplied by 457.8 ft. per minute, are
equal to 3213023.5 lbs. raised one foot high in the minute, which, divided
by 33,000, gives 97.3 horses power as the power which would draw 110 tons
upon a railway at a speed of 30 miles an hour, if there were no atmospheric
resistance. The atmospheric resistance is at the rate of 12 lbs. a ton,
with a load of 110 tons, equal to 1,320 lbs., moving at a speed of 30 miles
an hour, which, when reduced, becomes 105.8 horses power, and this, added
to 97.3, makes 203.1, instead of 200 horses power, as ascertained by a
reference to the evaporative power of the boiler. This amount of
atmospheric resistance, however, exceeds the average, and in some of the
experiments for ascertaining the atmospheric resistance, a part of the
resistance due to the curves and irregularities of the line has been
counted as part of the atmospheric resistance.
498. _Q._--Is the resistance per ton of the engine the same as the
resistance per ton of the train?
_A._--No; it is more, since the engine has not merely the resistance of the
atmosphere and of the wheels to encounter, but the resistance of the
machinery besides. According to Mr. Gooch's experiments upon a train
weighing 100 tons, the resistance of the engine and tender at 13.1 miles
per hour was found by the indicator to be 12.38 lbs.; the resistance per
ton of the train, as ascertained by the dynamometer, was at the same speed
7.58 lbs., and the average resistance of locomotive and train was 9.04 lbs.
At 20.2 miles per hour these resistances respectively became 19.0, 8.19,
and 12.2 lbs. At 441 miles per hour the resistances became 34.0, 21.10, and
25.5 lbs., and at 57.4 miles an hour they became 35.5, 17.81, and 23.8 lbs.
499. _Q._--Is it not maintained that the resistance of the atmosphere to
the progress of railway trains increases as the square of the velocity?
_A._--The atmospheric resistance, no doubt, increases as the square of the
velocity, and the power, therefore, necessary to overcome it will increase
as the cube of the velocity, since in doubling the speed four times, the
power must be expended in overcoming the atmospheric resistance in half the
time. At low speeds, the resistance does not increase very rapidly; but at
high speeds, as the rapid increase in the atmospheric resistance causes the
main resistance to be that arising from the atmosphere, the total
resistance will vary nearly as the square of the velocity. Thus the
resistance of a train, including locomotive and tender, will, at 15 miles
an hour, be about 9.3 lbs. per ton; at 30 miles an hour it will be 13.2
lbs. per ton; and at 60 miles an hour, 29 lbs. per ton. If we suppose the
same law of progression to continue up to 120 miles an hour, the resistance
at that speed will be 92.2 lbs. per ton, and at 240 miles an hour the
resistance will be 344.8 lbs. per ton. Thus, in doubling the speed from 60
to 120 miles per hour, the resistance does not fall much short of being
increased fourfold, and the same remark applies to the increase of the
speed from 120 to 240 miles an hour. These deductions and other deductions
from Mr. Gooch's experiments on the resistance of railway trains, are fully
discussed by Mr. Clark, in his Treatise on railway machinery, who gives the
following rule for ascertaining the resistance of a train, supposing the
line to be in good order, and free from curves:--To find the total
resistance of the engine, tender, and train in pounds per ton, at any given
speed. Square the speed in miles per hour; divide it by 171, and add 8 to
the quotient. The result is the total resistance at the rails in lbs. per
ton.
500._Q._--How comes it, that the resistance of fluids increases as the
square of the velocity, instead of the velocity simply?
_A._--Because the height necessary to generate the velocity with which the
moving object strikes the fluid, or the fluid strikes the object, increases
as the _square_ of the velocity, and the resistance or the weight of a
column of any fluid varies as the height. A falling body, as has been
already explained, to have acquired twice the velocity, must have fallen
through four times the height; the velocity generated by a column of any
fluid is equal to that acquired by a body falling through the height of the
column; and it is therefore clear, that the pressure due to any given
velocity must be as the square of that velocity, the pressure being in
every case as twice the altitude of the column. The work done, however, by
a stream of air or other fluid in a given time, will vary as the cube of
the velocity; for if the velocity of a stream of air be doubled, there will
not only be four times the pressure exerted per square foot, but twice the
quantity of air will be employed; and in windmills, accordingly, it is
found, that the work done varies nearly as the cube of the velocity of the
wind. If, however, the work done by _a given quantity_ of air moving at
different speeds be considered, it will vary as the squares of the speeds.
501. _Q._--But in a case where there is no work done, and the resistance
varies as the square of the speed, should not the power requisite to
overcome that resistance vary as the square of the speed?
_A._--It should if you consider the resistance over a given distance, and
not the resistance during a given time. Supposing the resistance of a
railway train to increase as the square of the speed, it would take four
times the power, so far as atmospheric resistance is concerned, to
accomplish a mile at the rate of 60 miles an hour, that it would take to
accomplish a mile at 30 miles an hour; but in the former case there would
be twice the number of miles accomplished in the same time, so that when
the velocity of the train was doubled, we should require an engine that was
capable of overcoming four times the resistance at twice the speed, or in
other words, that was capable of exerting eight times the power, so far as
regards the element of atmospheric resistance. We know by experience,
however, that it is easier to attain high speeds on railways than in steam
vessels, where the resistance does increase nearly as the square of the
speed.
502. _Q._--Will you describe generally the arrangement of a locomotive
engine?
_A._--The boiler and engine are hung upon a framework set on wheels, and,
together with this frame or carriage, constitute what is commonly called
the locomotive. Behind the locomotive runs another carriage, called the
tender, for holding coke and water. A common mode of connecting the engine
and tender is by means of a rigid bar, with an eye at each end through
which pins are passed. Between the engine and tender, however, buffers
should always be interposed, as their pressure contributes greatly to
prevent oscillation and other irregular motions of the engine.
503. _Q._--How is the framing of a locomotive usually constructed?
_A._--All locomotives are now made with the framing which supports the
machinery situated within the wheels; but for some years a vehement
controversy was maintained respecting the relative merits of outside and
inside framing, which has terminated, however, in the universal adoption of
the inside framing. It is difficult, in engines intended for the narrow
gauge, to get cylinders within the framing of sufficient diameter to meet
the exigencies of railway locomotion; by casting both cylinders in a piece,
however, a considerable amount of room may be made available to increase
their diameters. It is very desirable that the cylinders of locomotives
should be as large as possible, so that expansion may be adopted to a large
extent; and with any given speed of piston, the power of an engine either
to draw heavy loads, or achieve high velocities, will be increased with
every increase of the dimensions of the cylinder. The framing of
locomotives, to which the boiler and machinery are attached, and which
rests upon the springs situated above the axles, is formed generally of
malleable iron, but in some engines the side frames consist of oak with
iron plates riveted on each side. The guard plates are in these cases
generally of equal length, the frames being curved upward to pass over the
driving axle. Hard cast iron blocks are riveted between the guard plates to
serve as guides for the axle bushes. The side frames are connected across
the ends, and cross stays are introduced beneath the boiler to stiffen the
frame sideways, and prevent the ends of the connecting or eccentric rods
from falling down if they should be broken.
504. _Q._--What is the nature and arrangement of the springs of
locomotives?
_A._--The springs are of the ordinary carriage kind, with plates connected
at the centre, and allowed to slide on each other at their ends. The upper
plate terminates in two eyes, through each of which passes a pin, which
also passes through the jaws of the bridle, connected by a double threaded
screw to another bridle, which is jointed to the framing; the centre of the
spring rests upon the axle box. Sometimes the springs are placed between
the guard plates, and below the framing which rests upon their extremities.
One species of springs which has gained a considerable introduction,
consists of a number of flat steel plates with a piece of metal or other
substance interposed between them at the centre, leaving the ends standing
apart. It would be preferable, perhaps, to make the plates of a common
spring with different curves, so that the leaves, though in contact at the
centre, would not be in contact with the ends with light loads, but would
be brought into contact gradually, as the strain conies on: a spring would
thus be obtained that was suitable for all loads.
505. _Q._--What is the difference between inside and outside cylinder
engines?
_A._--Outside cylinders are so designated when placed upon the outside of
the framing, with their connecting rods operating upon pins in the driving
wheels; while the inside cylinders are situated within the framing, and the
connecting rods attach themselves to cranks in the driving axle.
506. _Q._--Whether are inside or outside cylinder engines to be preferred?
_A._--A diversity of opinion obtains as to the relative merits of outside
and inside cylinders. The chief objection to outside cylinders is, that
they occasion a sinuous motion in the engine which is apt to send the train
off the rails; but this action may be made less perceptible or be remedied
altogether, by placing a weight upon one side of the wheels, the momentum
of which will just balance the momentum of the piston and its connections.
The sinuous or rocking motion of locomotives is traceable to the arrested
momentum of the piston and its attachments at every stroke of the engine,
and the effect of the pressure thus created will be more operative in
inducing oscillation the farther it is exerted from the central line of the
engine. If both cylinders were set at right angles in the centre of the
carriage, and the pistons were both attached to a central crank, there
would be no oscillation produced; or the same effect would be realized by
placing one cylinder in the centre of the carriage, and two at the sides--
the pistons of the side cylinders moving simultaneously: but it is
impossible to couple the piston of an upright cylinder direct to the axle
of a locomotive, without causing the springs to work up and down with every
stroke of the engine: and the use of three cylinders, though adopted in
some of Stephenson's engines, involves too much complication to be a
beneficial innovation.
507. _Q._--Whether are four-wheeled or six-wheeled engines preferable?
_A._--Much controversial ingenuity has been expended upon the question of
the relative merits of the four and six-wheeled engines; one party
maintaining that four-wheeled engines are most unsafe, and the other that
six-wheeled engines are unmechanical, and are more likely to occasion
accidents. The four-wheeled engines, however, appear to have been charged
with faults that do not really attach to them when properly constructed;
for it by no means follows that if the axle of a four-wheeled engine
breaks, or even altogether comes away, that the engine must fall down or
run off the line; inasmuch as, if the engine be properly coupled with the
tender, it has the tender to sustain it. It is obvious enough, that such a
connection may be made between the tender and the engine, that either the
fore or hind axle of the engine may be taken away, and yet the engine will
not fall down, but will be kept up by the support which the tender affords;
and the arguments hitherto paraded against the four-wheeled engines are, so
far as regards the question of safety, nothing more than arguments against
the existence of the suggested connection. It is no doubt the fact, that
locomotive engines are now becoming too heavy to be capable of being borne
on four wheels at high speeds without injury to the rails; but the
objection of damage to the rails applies with at least equal force to most
of the six-wheeled engines hitherto constructed, as in those engines the
engineer has the power of putting nearly all the weight upon the driving
wheels; and if the rail be wet or greasy, there is a great temptation to
increase the bite of those wheels by screwing them down more firmly upon
the rails. A greater strain is thus thrown upon the rail than can exist in
the case of any equally heavy four-wheeled engine; and the engine is made
very unsafe, as a pitching motion will inevitably be induced at high
speeds, when an engine is thus poised upon the central driving wheels, and
there will also be more of the rocking or sinuous motion. Locomotives,
however, intended to achieve high speeds or to draw heavy loads, are now
generally made with eight wheels, and in some cases the driving wheels are
placed at the end of the engine instead of in the middle.
508. _Q._--As the question of the locomotive boiler has been already
disposed of in discussing the question of boilers in general, it now only
remains to inquire into the subject of the engine, and we may commence with
the cylinders. Will you state the arrangement and construction of the
cylinders of a locomotive and their connections?
_A._--The cylinders are placed in the same horizontal plane as the axle of
the driving wheels, and the connecting rod which is attached to the piston
rod engages either a crank in the driving axle or a pin in the driving
wheel, according as the cylinders are inside or outside of the framework.
The cylinders are generally made an inch longer than the stroke, or there
is half an inch of clearance at each end of the cylinder, to permit the
springs of the vehicle to act without causing the piston to strike the top
or bottom of the cylinder. The thickness of metal of the cylinder ends is
usually about a third more than the thickness of the cylinder itself, and
both ends are generally made removable. The priming of the boiler, when it
occurs, is very injurious to the cylinders and valves of locomotives,
especially if the water be sandy, as the grit carried over by the steam
wears the rubbing surfaces rapidly away. The face of the cylinder on which
the valve works is raised a little above the metal around it, both to
facilitate the operation of forming the face and with the view of enabling
any foreign substance deposited on the face to be pushed aside by the valve
into the less elevated part, where it may lie without occasioning any
further disturbance. The valve casing is sometimes cast upon the cylinder,
and it is generally covered with a door which may be removed to permit the
inspection of the faces. In some valve casings the top as well as the back
is removable, which admits of the valve and valve bridle being removed with
greater facility. A cock is placed at each end of locomotive cylinders, to
allow the water to be discharged which accumulates in the cylinder from
priming or condensation; and the four cocks of the two cylinders are
usually connected together, so that by turning a handle the whole are
opened at once. In Stephenson's engines, however, with variable expansion,
there is but one cock provided for this purpose, which is on the bottom of
the valve chest.
509. _Q._--What kind of piston is used in locomotives?
_A._--The variety of pistons employed in locomotives is very great, and
sometimes even the more complicated kinds are found to work very
satisfactorily; but, in general, those pistons which consist of a single
ring and tongue piece, or of two single rings set one above the other, so
as to break joint, are preferable to those which consist of many pieces. In
Stephenson's pistons the screws were at one time liable to work slack, and
the springs to break.
510. _Q._--Will you explain the connection of the piston rod with the
connecting rod?
_A._--The piston rods of all engines are now generally either case hardened
very deeply, or are made of steel; and in locomotive engines the diameter
of the piston rod is about one seventh of the diameter of the cylinder, and
it is formed of tilted steel. The cone of the piston rod, by which it is
attached to the piston, is turned the reverse way to that which is adopted
in common engines, with the view of making the cutter more accessible from
the bottom of the cylinder, which is made to come off like a door. The top
of the piston rod is secured with a cutter into a socket with jaws, through
the holes of which a cross head passes, which is embraced between the jaws
by the small end of the connecting rod, while the ends of the cross head
move in guides. Between the piston rod clutch and the guide blocks, the
feed pump rod joins the cross head in some engines.
511. _Q._--What kind of guides is employed for the end of the piston rod?
_A._--The guides are formed of steel plates attached to the framing,
between which work the guide blocks, fixed on the ends of the cross head,
which have flanges bearing against the inner edges of the guides. Steel or
brass guides are better than iron ones: Stephenson and Hawthorn attach
their guides at one end to a cross stay, at the other to lugs on the
cylinder cover; and they are made stronger in the middle than at the ends.
Stout guide rods of steel, encircled by stuffing boxes on the ends of the
cross head, would probably be found superior to any other arrangement. The
stuffing boxes might contain conical bushes, cut spirally, in addition to
the packing, and a ring, cut spirally, might be sprung upon the rod and
fixed in advance of the stuffing box, with lateral play to wipe the rod
before entering the stuffing box, to prevent it from being scratched by the
adhesion of dust.
512. _Q._--Is any provision made for keeping the connecting rod always of
the same length?
_A._--In every kind of locomotive it is very desirable that the length of
the connecting rod should remain invariable, in spite of the wear of the
brasses, for there is a danger of the piston striking against the cover of
the cylinder if it be shortened, as the clearance is left as small as
possible in order to economize steam. In some engines the strap encircling
the crank pin is fixed immovably to the connecting rod by dovetailed keys,
and a bolt passes through the keys, rod, and strap, to prevent the
dovetailed keys from working out. The brass is tightened by a gib and
cutter, which is kept from working loose by three pinching screws and a
cross pin or cutter through the point. The effect of this arrangement is to
lengthen the rod, but at the cross head end of the rod the elongation is
neutralized by making the strap loose, so that in tightening the brass the
rod is shortened by an amount equal to its elongation at the crank pin end.
The tightening here is also effected by a gib and cutter, which is kept
from working loose by two pinching screws pressing on the side of the
cutter. Both journals of the connecting rod are furnished with oil cups,
having a small tube in the centre with siphon wicks. The connecting rod is
a thick flat bar, with its edges rounded.
513. _Q._--How is the cranked axle of locomotives constructed?
_A._--The cranked axle of locomotives is always made of wrought iron, with
two cranks forged upon it toward the middle of its length, at a distance
from each other answerable to the distance between the cylinders. Bosses
are made on the axle for the wheels to be keyed upon, and bearings for the
support of the framing. The axle is usually forged in two pieces, which are
afterward welded together. Sometimes the pieces for the cranks are put on
separately, but the cranks so made are liable to give way. In engines with
outside cylinders the axles are made straight-the crank pins being inserted
in the naves of the wheels. The bearings to which the connecting rods are
attached are made with very large fillets in the corners, so as to
strengthen the axle in that part, and to obviate side play in the
connecting rod. In engines which, have been in use for some time, however,
there is generally a good deal of end play in the bearings of the axles
themselves, and this slackness contributes to make the oscillation of the
engine more violent; but this evil may be remedied by making the bearings
spheroidal, whereby end play becomes impossible.
514. _Q._--How are the bearings of the axles arranged?
_A._--The axles bear only against the top of the axle boxes, which are
generally of brass; but a plate extends underneath the bearing, to prevent
sand from being thrown upon it. The upper part of the box in most engines
has a reservoir of oil, which is supplied to the journal by tubes with
siphon wicks. Stephenson uses cast iron axle boxes with brasses, and grease
instead of oil; and the grease is fed upon the journal by the heat of the
bearing melting it, whereby it is made to flow down through a hole in the
brass. Any engines constructed with outside bearings have inside bearings
also, which are supported by longitudinal bars, which serve also in some
cases to support the piston guides; these bearings are sometimes made so as
not to touch the shafts unless they break.
515. _Q._--How are the eccentrics of a locomotive constructed?
_A._--In locomotives the body of the eccentric is of cast iron, in inside
cylinder engines the eccentrics are set on the axle between the cranks, and
they are put on in two pieces held together by bolts; but in straight axle
engines the eccentrics are cast in a piece, and are secured on the shaft by
means of a key. The eccentric, when in two pieces, is retained at its
proper angle on the shaft by a pinching screw, which is provided with a jam
nut to prevent it from working loose. A piece is left out of the eccentric
in casting it to allow of the screw being inserted, and the void is
afterward filled by inserting a dovetailed piece of metal. Stephenson and
Hawthorn leave holes in their eccentrics on each side of the central arm,
and they apply pinching screws in each of these holes. The method of fixing
the eccentric to the shaft by a pinching screw is scarcely sufficiently
substantial; and cases are perpetually occurring, when this method of
attachment is adopted, of eccentrics shifting from their place. In the
modern engines the eccentrics are forged on the axles.
516. _Q._--How are the eccentric straps constructed?
_A._--The eccentric hoops are generally of wrought iron, as brass hoops are
found liable to break. When formed of malleable iron, one half of the strap
is forged with the rod, the other half being secured to it by bolts, nuts,
and jam nuts. Pieces of brass are, in some cases, pinned within the
malleable iron hoop; but it appears to be preferable to put brasses within
the hoop to encircle the eccentric, as in the case of any other bearing.
When the brass straps are used, the lugs have generally nuts on both sides,
so that the length of the eccentric rod may be adjusted by their means to
the proper length; but it is better for the lugs of the hoops to abut
against the necks of the screws, and, if any adjustment be necessary from
the wear of the straps, washers can be interposed. In some engines the
adjustment is effected by screwing the valve rod, and the cross head
through which it passes has a nut on either side of it, by which its
position upon the valve rod is determined.
517. _Q._--Will you describe the eccentric rod and valve levers?
_A._--In the engines in use before the introduction of the link motion, the
forks of the eccentric rod were of steel, and the length of the eccentric
rod was the distance between the centre of the crank axle and the centre of
the valve shaft; but in modern engines the use of the link motion is
universal. The valve lever in locomotives is usually longer than the
eccentric lever, to increase the travel of the valve, if levers are
employed; but it is better to connect the valve rod to the link of the link
motion without the intervention of levers. The pins of the eccentric lever
in the old engines used to wear quickly; Stephenson used to put a ferule of
brass on these pins, which being loose, and acting like a roller,
facilitated the throwing in and out of gear, and when worn could easily be
replaced, so that there was no material derangement of the motion of the
valve from play in this situation.
518. _Q._--What is the arrangement of a starting lever?
_A._--The starting lever travels between two iron segments, and can be
fixed in any desired position. This is done by a small catch or bell crank,
jointed to the bottom of the handle at the end of the lever, and coming up
by the side of the handle, but pressed out from it by a spring. The smaller
arm of this bell crank is jointed to a bolt, which shoots into notches,
made in one of the segments between which the lever moves. By pressing the
bell crank against the handle of the lever the bolt is withdrawn, and the
lever may be shifted to any other point, when, the spring being released,
the bolt flies into the nearest notch.
519. _Q._--In what way does the starting handle act on the machinery of the
engine to set it in motion?
_A._--Its whole action lies in raising or depressing the link of the link
motion relatively with the valve rod. If the valve rod be attached to the
middle of the link, the valve will derive no motion from, it at all, and
the engine will stop. If the attachment be slipped to one end of the link
the engine will go ahead, and if slipped to the other end it will go
astern. The starting handle merely achieves this change of position.
520. _Q._--Will you explain the operation of setting the valve of a
locomotive?
_A._--In setting the valves of locomotives, place the crank in the position
answerable to the end of the stroke of the piston, and draw a straight
line, representing the centre line of the cylinder, through the centres of
the crank shaft and crank pin. From the centre of the shaft describe a
circle with the diameter equal to the throw of the valve; another circle to
represent the crank shaft; and a third circle to represent the path of the
crank pin. From the centre of the crank shaft, draw a line perpendicular to
the centre line of the cylinder and crank shaft, and draw another
perpendicular at a distance from the first equal to the amount of the lap
and the lead of the valve: the points in which this line intersects the
circle of the eccentric are the points in which the centre of the eccentric
should be placed for the forward and reverse motions. When the eccentric
rod is attached directly to the valve, the radius of the eccentric, which
precedes the crank in its revolution, forms with the crank an obtuse angle;
but when, by the intervention of levers, the valve has a motion, opposed to
that of the eccentric rod, the angle contained by the crank and the radius
of the eccentric must be acute, and the eccentric must follow the crank: in
other words, with a direct attachment to the valve the eccentric is set
_more_ than one fourth of a revolution in advance of the crank, and with an
indirect attachment the eccentric is set _less_ than one fourth of a circle
behind the crank. If the valve were without lead or lap the eccentric would
be exactly one fourth of a circle in advance of the crank or behind the
crank, according to the nature of the valve connection; but as the valve
would thus cover the port by the amount of the lap and lead, the eccentric
must be set forward so as to open the port to the extent of the lap and
lead, and this is effected by the plan just described.
521. _Q._--In the event of the eccentrics slipping round upon the shaft,
which you stated sometimes happens, is it necessary to perform the
operation of setting the valve as you have just described it?
_A._--If the eccentrics shift upon the shaft, they may be easily refixed by
setting the valve open the amount of the lead, setting the crank at the end
of the stroke, and bringing round the eccentric upon the shaft till the
eccentric rod gears with the valve. It would often be troublesome in
practice to get access to the valve for the purpose of setting it, and this
may be dispensed with if the amount of lap on the valve and the length of
the eccentric rod be known. To this end draw upon a board two straight
lines at right angles to one another, and from their point of intersection
as a centre describe two circles, one representing the circle of the
eccentric, the other the crank shaft; draw a straight line parallel to one
of the diameters, and distant from it the amount of the lap and the lead:
the points in which his parallel intersects the circle of the eccentric are
the positions of the forward and backward eccentrics. Through these points
draw straight lines from the centre of the circle, and mark the
intersection of these lines with the circle of the crank shaft; measure
with a pair of compasses the chord of the arc intercepted between either of
these points, and the diameter which is at right angles with the crank, and
the diameters being first marked on the shaft itself, then by transferring
with the compasses the distance found in the diagram, and marking the
point, the eccentric may at any time be adjusted without difficulty.
[Illustration: Fig. 45.]
522. _Q._--Will you describe the structure and arrangement of the feed
pumps of locomotive engines?
_A._--The feed pumps of locomotives are generally made of brass, but the
plungers are sometimes made of iron, and are generally attached to the
piston, cross head, though in Stephenson's engines they are worked by rods
attached to eyes on the eccentric hoops. There is a ball valve, fig. 45,
between the pump and the tender, and two usually in the pipe leading from
the pump to the boiler, besides a cock close to the boiler, by which the
pump may be shut off from the boiler in case of any accident to the valves.
The ball valves are guided by four branches, which rise vertically, and
join together at the top in a hemispherical form. The shocks of the ball
against this cap have in some cases broken it after one week's work, from
the top of the cage having been flat, and the branches not having had their
junction at the top properly filleted. These valve guards are attached in
different ways to the pipes; when one occurs at the junction of two pieces
of pipe it has a flange, which along with the flanges of the pipes and that
of the valve seat are held together by a union joint. It is sometimes
formed with a thread at the under end, and screwed into the pipe. The balls
are cast hollow to lessen the shock, as well as to save the metal. In some
cases where the feed pump plunger has been attached to the cross head, the
piston rod has been bent by the strain; and that must in all cases occur,
if the communication between the pump and boiler be closed when the engine
is started, and there be no escape valve for the water.
523. _Q._--Are none but ball valves used in the feed pump?
_A._--Spindle valves have in some cases been used instead of ball valves,
but they are more subject to derangement; but piston valves, so contrived
as to shut a portion of water in the cage when about to close, might be
adopted with a great diminution of the shock. Slide valves might be
applied, and would probably be found preferable to any of the expedients at
present in use. In all spindle valves opened and shut rapidly, it is
advisable to have the lower surface conical, to take off the shock of the
water; and a large lift of the valve should be prevented, else much of the
water during the return stroke of the pump will flow out before the valve
shuts.
524. _Q._--At what part of the boiler is the feed water admitted?
_A._--The feed pipe of most locomotive engines enters the boiler near the
bottom and about the middle of its length. In Stephenson's engine the water
is let in at the smoke box end of the boiler, a little below the water
level; by this means the heat is more fully extracted from the escaping
smoke, but the arrangement is of questionable applicability to engines of
which the steam dome and steam pipe are at the smoke box end, as in that
case the entering cold water would condense the steam.
525. _Q._--How are the pipes connecting the tender and locomotive
constructed, so as to allow of play between the engine and tender without
leakage?
_A._--The pipes connecting the tender with the pumps should allow access to
the valves and free motion to the engine and tender. This end is attained
by the use of ball and socket joints; and, to allow some end play, one
piece of the pipe slides into the other like a telescope, and is kept tight
by means of a stuffing box. Any pipe joint between the engine and tender
must be made in this fashion.
526. _Q._--Have you any suggestion to make respecting the arrangement of
the feed pump?
_A._--It would be a material improvement if a feed pump was to be set in
the tender and worked by means of a small engine, such as that now used in
steam vessels for feeding the boilers. The present action of the feed pumps
of locomotives is precarious, as, if the valves leak in the slightest
degree, the steam or boiling water from the boiler will prevent the pumps
from drawing. It appears expedient, therefore, that at least one pump
should be far from the boiler and should be set among the feed water, so
that it will only have to force. If a pump was arranged in the manner
suggested, the boiler could still be fed regularly, though the locomotive
was standing still; but it would be prudent to have the existing pumps
still wrought in the usual way by the engine, in case of derangement of the
other, or in case the pump in the tender might freeze.
527. _Q._--Will you explain the construction of locomotive wheels?
_A._--The wheels of a locomotive are always made of malleable iron. The
driving wheels are made larger to increase the speed; the bearing wheels
also are easier on the road when large. In the goods engines the driving
wheels are smaller than in the passenger engines, and are generally coupled
together. Wheels are made with much variety in their constructive details:
sometimes they are made with cast iron naves, with the spokes and rim of
wrought iron; but in the best modern wheels the nave is formed of the ends
of the spokes welded together at the centre. When cast iron naves are
adopted, the spokes are forged out of flat bars with T-formed heads, and
are arranged radially in the founder's mould, the cast iron, when fluid,
being poured among them. The ends of the T heads are then welded together
to constitute the periphery of the wheel or inner tire; and little
wedge-form pieces are inserted where there is any deficiency of iron. In
some cases the arms are hollow, though of wrought iron; the tire of wrought
iron, and the nave of cast iron; and the spokes are turned where they are
fitted into the nave, and are secured in their sockets by means of cutters.
Hawthorn makes his wheels with cast iron naves and wrought iron rims and
arms; but instead of welding the arms together, he makes palms on their
outer end, which are attached by rivets to the rim. These rivets, however,
unless very carefully formed, are apt to work loose; and it would probably
be found an improvement if the palms were to be slightly indented into the
rim, in cases in which the palms do not meet each other at the ends. When
the rim is turned it is ready for the tire, which is now made of steel.
528. _Q._--How do you find the length of bar necessary for forming a tire?
_A._--To find the proper length of bar requisite for the formation of a
hoop of any given diameter, add the thickness of the bar to the required
diameter, and the corresponding circumference in the table of
circumferences of circles is the length of the bar. If the iron be bent
edgewise the breadth of the bar must be added to the diameter, for it is
the thickness of the bar measured radially that is to be taken into
consideration. In the tires of railway wheels, which have a flange on one
edge, it is necessary to add not only the thickness of the tire, but also
two thirds of the depth of the flange; generally, however, the tire bars
are sent from the forge so curved that the plain edge of the tire is
concave, and the flange edge convex, while the side which is afterward to
be bent into contact with the cylindrical surface of the wheel is a plane.
In this case the addition of the diameter of two thirds of the depth of the
flange is unnecessary, for the curving of the flange edge has the effect of
increasing the real length of the bar. When the tire is thus curved, it is
only necessary to add the thickness of the hoop to the diameter, and then
to find the circumference from a table; or the same result will be obtained
by multiplying the diameter thus increased by the thickness of the hoop by
3.1416.
529. _Q._--How are the tires attached to the wheels?
_A._--The materials for wheel tires are first swaged separately, and then
welded together under the heavy hammer at the steel works; after which they
are bent to the circle, welded, and turned to certain gauges. The tire is
now heated to redness in a circular furnace; during the time it is getting
hot, the iron wheel, turned to the right diameter, is bolted down upon a
face plate or surface; the tire expands with the heat, and when at a cherry
red, it is dropped over the wheel, for which it was previously too small,
and it is also hastily bolted down to the surface plate; the whole mass is
then quickly immersed by a swing crane in a tank of water five feet deep,
and hauled up and down till nearly cold; the tires are not afterward
tempered. The tire is attached to the rim with rivets having countersunk
heads, and the wheel is then fixed on its axle.
530. _Q._--Is it necessary to have the whole tire of steel?
_A._--It is not indispensable that the whole tire should be of steel; but a
dovetail groove, turned out of the tire at the place where it bears most on
the rail, and fitted with a band of steel, will suffice. This band may be
put in in pieces, and the expedient appears to be the best way of repairing
a worn tire; but particular care must be taken to attach these pieces very
securely to the tire by rivets, else in the rapid revolution of the wheel
the steel may be thrown out by the centrifugal force. In aid of such
attachment the steel, after being introduced, is well hammered, which
expands it sideways until it fills the dovetail groove.
531. _Q._--Is any arrangement adopted to facilitate the passage of the
locomotive round curves?
_A._--The tire is turned somewhat conical, to facilitate the passage of the
engine round curves--the diameter of the outer wheel being virtually
increased by the centrifugal force of the engine, and that of the inner
wheel being correspondingly diminished, whereby the curve is passed without
the resistance which would otherwise arise from the inequality of the
spaces passed over by wheels of the same diameter fixed upon the same axle.
The rails, moreover, are not set quite upright, but are slightly inclined
inward, in consequence of which the wheels must be either conical or
slightly dished, to bear fairly upon the rails. One benefit of inclining
the rails in this way, and coning the tires, is that the flange of the
wheels is less liable to bear against the sides of the rail, and with the
same view the flanges of all the wheels are made with large fillets in the
corners. Wheels have been placed loose upon the axle, but they have less
stability, and are not now much used. Nevertheless this plan appears to be
a good one if properly worked out.
532. _Q._--Are any precautions taken to prevent engines from being thrown
off the rails by obstructions left upon the line?
_A_.--In most engines a bar is strongly attached to the front of the
carriage on each side, and projects perpendicularly downward to within a
short distance of the rail, to clear away stones or other obstructions that
might occasion accidents if the engine ran over them.
CHAPTER IX.
STEAM NAVIGATION.
* * * * *
RESISTANCE OF VESSELS IN WATER.
533. _Q._--How do you determine the resistance encountered by a vessel
moving in water?
_A._--The resistance experienced by vessels moving in water varies as the
square of the velocity of their motion, or nearly so; and the power
necessary to impart an increased velocity varies nearly as the cube of such
increased velocity. To double the velocity of a steam vessel, therefore,
will require four times the amount of tractive force, and as that
quadrupled force must act through twice the distance in the same time, an
engine capable of exerting eight times the original power will be
required.[1]
534. _Q._--In the case of a board moving in water in the manner of a paddle
float, or in the case of moving water impinging on a stationary board, what
will be the pressure produced by the impact?
_A._--The pressure produced upon a flat board, by striking water at right
angles to the surface of the board, will be equal to the weight of a column
of water having the surface struck as a base, and for its altitude twice
the height due to the velocity with which the board moves through the
water. If the board strike the water obliquely, the resistance will be
less, but no very reliable law has yet been discovered to determine its
amount.
535. _Q._--Will not the resistance of a vessel in moving through the water
be much less than that of a flat board of the area of the cross section?
_A._--It will be very much less, as is manifest from the comparatively
small area of paddle board, and the small area of the circle described by
the screw, relatively with the area of the immersed midship section of the
vessel. The absolute speed of a vessel, with any given amount of power,
will depend very much upon her shape.
536. _Q._--In what way is it that the shape of a vessel influences her
speed, since the vessels of the same sectional area must manifestly put in
motion a column of water of the same magnitude, and with the same velocity?
_A._--A vessel will not strike the water with the same velocity when the
bow lines are sharp as when they are otherwise; for a very sharp bow has
the effect of enabling the vessel to move through a great distance, while
the particles of water are moved aside but a small distance, or in other
words, it causes the velocity with which the water is moved to be very
small relatively with the velocity of the vessel; and as the resistance
increases as the square of the velocity with which the water is moved, it
is conceivable enough in what way a sharp bow may diminish the resistance.
537. _Q._--Is the whole power expended in the propulsion of a vessel
consumed in moving aside the water to enable the vessel to pass?
_A._--By no means; only a portion, and in well-formed vessels only a small
portion, of the power is thus consumed. In the majority of cases, the
greater part of the power is expended in overcoming the friction of the
water upon the bottom of the vessel; and the problem chiefly claiming
consideration is, in what way we may diminish the friction.
538. _Q._--Does the resistance produced by this friction increase with the
velocity?
_A._--It increases nearly as the square of the velocity. At two nautical
miles per hour, the thrust necessary to overcome the friction varies as the
1.823 power of the velocity; and at eight nautical miles per hour, the
thrust necessary to overcome the friction varies as the 1.713 power of the
velocity. It is hardly proper, perhaps, to call this resistance by the name
of friction; it is partly, perhaps mainly, due to the viscidity or adhesion
of the water.
539. _Q._--Perhaps at high velocities this resistance may become less?
_A_.--That appears very probable. It may happen that at high velocities the
adhesion is overcome, so that the water is dragged off the vessel, and the
friction thereafter follows the law which obtains in the case of solid
bodies. But any such conclusion is mere speculation, since no experiments
illustrative of this question have yet been made.
540. _Q._--Will a vessel experience more resistance in moving in salt water
than in moving in fresh?
_A._--If the immersion be the same in both cases a vessel will experience
more resistance in moving in salt water than in moving in fresh, on account
of the greater density of salt water; but as the notation is proportionably
greater in the salt water the resistance will be the same with the same
weight carried.
541. _Q._--Discarding for the present the subject of friction, and looking
merely to the question of bow and stern resistance, in what manner should
the hull of a vessel be formed so as to make these resistances a minimum?
_A._--The hull should be so formed that the water, instead of being away
driven forcibly from the bow, is opened gradually, so that every particle
of water may be moved aside slowly at first, and then faster, like the ball
of a pendulum, until it reaches the position of the midship frame, at which
point it will have come to a state of rest, and then again, like a
returning pendulum, vibrate back in the same way, until it comes to rest at
the stern. It is not difficult to describe mechanically the line which the
water should pursue. If an endless web of paper be put into uniform motion,
and a pendulum carrying a pencil or brush be hung in front of it, then such
pendulum will trace on the paper the proper water line of the ship, or the
line which the water should pursue in order that no power may be lost
except that which is lost in friction. It is found, however, in practice,
that vessels formed with water lines on this principle are not much
superior to ordinary vessels in the facility with which they pass through
the water: and this points to the conclusion that in ordinary vessels of
good form, the amount of power consumed in overcoming the resistance due to
the wave at the bow and the partial vacuity at the stern is not so great as
has heretofore been supposed, and that, in fact, the main resistance is
that due to the friction.
[1] This statement supposes that there is no difference of level between
the water at the bow and the water at the stern. In the experiments on the
steamer Pelican, the resistance was found to vary, as the 2.28th power of
the velocity, but the deviation from the recognized law was imputed to a
difference in the level of the water at the bow and stern.
EXPERIMENTS ON THE RESISTANCE OF VESSELS.
542. _Q._--Have experiments been made to determine the resistance which
steam vessels experience in moving through the waters?
_A._--Experiments have been made both to determine the relative resistance
of different classes of vessels, and also the absolute resistance in pounds
or tons. The first experiments made upon this subject were conducted by
Messrs. Boulton and Watt, and they have been numerous, long continued, and
carefully performed. These experiments were made upon paddle vessels.
543. _Q._--Will you recount the chief results of these experiments?
_A._--The purpose of the experiments was to establish a coefficient of
performance, which with any given class of vessel would enable the speed,
which would be obtained with any given power, to be readily predicted. This
coefficient was obtained by multiplying the cube of the velocity of the
vessels experimented upon, in miles per hour, by the sectional area of the
immersed midship section in square feet, and dividing by the numbers of
nominal horses power, and this coefficient will be large in the proportion
of the goodness of the shape of the vessel.
544. _Q._--How many experiments were made altogether?
_A._--There were five different sets of experiments on five different
classes of vessels. The first set of experiments was made in 1828, upon the
vessels Caledonia, Diana, Eclipse, Kingshead, Moordyke, and Eagle-vessels
of a similar form and all with square bilges and flat floors; and the
result was to establish the number 925 as the coefficient of performance of
such vessels. The second set of experiments was made upon the superior
vessels Venus, Swiftsure, Dasher, Arrow, Spitfire, Fury, Albion, Queen,
Dart, Hawk, Margaret, and Hero-all vessels having flat floors and round
bilges, where the coefficient became 1160. The third set of experiments was
made upon the vessels Lightning, Meteor, James Watt, Cinderella, Navy
Meteor, Crocodile, Watersprite, Thetis, Dolphin, Wizard, Escape, and
Dragon-all vessels with rising floors and round bilges, and the coefficient
of performance was found to be 1430. The fourth set of experiments was made
in 1834, upon the vessels Magnet, Dart, Eclipse, Flamer, Firefly, Ferret,
and Monarch, when the coefficient of performance was found to be 1580. The
fifth set of experiments was made upon the Red Rover, City of Canterbury,
Herne, Queen, and Prince of Wales, and in the case of those vessels the
coefficient rose to 2550. The velocity of any of these vessels, with any
power or sectional area, may be ascertained by multiplying the coefficient
of its class by the nominal horse power, dividing by the sectional area in
square feet, and extracting the cube root of the quotient, which will be
the velocity in miles per hour; or the number of nominal horse power
requisite for the accomplishment of any required speed may be ascertained
by multiplying the cube of the required velocity in miles per hour, by the
sectional area in square feet, and dividing by the coefficient: the
quotient is the number of nominal horse power requisite to realize the
speed.
545. _Q._--Seeing, however, that the nominal power does not represent an
invariable amount of dynamical efficiency, would it not be better to make
the comparison with reference to the actual power?
_A._--In the whole of the experiments recited, except in the case of one or
two of the last, the pressure of steam in the boiler varied between 2-3/4
lbs. and 4 lbs. per square inch, and the effective pressure on the piston
varied between 11 lbs. and 13 lbs. per square inch, so that the average
ratio of the nominal to the actual power may be easily computed; but it
will be preferable to state the nominal power of some of the vessels, and
their actual power as ascertained by experiment.
546. _Q._--Then state this.
_A._--Of the Eclipse, the nominal power was 76, and the actual power 144.4
horses; of the Arrow, the nominal power was 60, and the actual 119.5;
Spitfire, nominal 40, actual 64; Fury, nominal 40, actual 65.6; Albion,
nominal 80, actual 135.4; Dart, nominal 100, actual 152.4; Hawk, nominal
40, actual 73; Hero, nominal 100, actual 171.4; Meteor, nominal 100, actual
160; James Watt, nominal 120, actual 204; Watersprite, nominal 76, actual
157.6; Dolphin, nominal 140, actual 238; Dragon, nominal 80, actual 131;
Magnet, nominal 140, actual 238; Dart, nominal 120, actual 237; Flamer,
nominal 120, actual 234; Firefly, nominal 52, actual 86.6; Ferret, nominal
52, actual 88; Monarch, nominal 200, actual 378. In the case of swift
vessels of modern construction, such as the Red Rover, Herne, Queen, and
Prince of Wales, the coefficient appears to be about 2550; but in these
vessels there is a still greater excess of the actual over the nominal
power than in the case of the vessels previously enumerated, and the
increase in the coefficient is consequent upon the increased pressure of
the steam in the boiler, as well as the superior form of the ship. The
nominal power of the Red Rover, Herne, and City of Canterbury is, in each
case, 120 horses, but the actual power of the Red Rover is 294, of the
Herne 354, and of the City of Canterbury 306, and in some vessels the
excess is still greater; so that with such variations it becomes necessary
to adopt a coefficient derived from the introduction of the actual instead
of the nominal power.
547. _Q._--What will be the average difference between the nominal and
actual powers in the several classes of vessels you have mentioned and the
respective coefficients when corrected for the actual power?
_A._--In the first class of vessels experimented upon, the actual power
was about 1.6 times greater
than the nominal power; in the second class, 1.67 times greater; in the
third class, 1.7 times
greater; and in the fourth, 1.96 times greater; while in such vessels as
the Red Rover and City of
Canterbury, it is 2.65 times greater; so that if we adopt the actual
instead of the nominal power in
fixing the coefficients, we shall have 554 as the first coefficient, 694
as the second, 832 for the
third, and 806 for the fourth, instead of 925, 1160, 1430, and 1580 as
previously specified; while
for such vessels as the Red Rover, Herne, Queen, and Prince of Wales, we
shall have 962 instead of
2550. These smaller coefficients, then, express the relative merits of
the different vessels without
reference to any difference of efficacy in the engines, and it appears
preferable, with such a
variable excess of the actual over the nominal power, to employ them
instead of those first referred
to. From the circumstance of the third of the new coefficients being
greater than the fourth, it
appears that the superior result in the fourth set of experiments arose
altogether from a greater
excess of the actual over the nominal power.
548. _Q._--These experiments, you have already stated, were all made on
paddle vessels. Have similar coefficients of performance been obtained in
the case of screw vessels?
_A._--The coefficients of a greater number of screw vessels have been
obtained and recorded, but it would occupy too much time to enumerate them
here. The coefficient of performance of the Fairy is 464.8; of the Rattler
676.8; and of the Frankfort 792.3. This coefficient, however, refers to
nautical and not to statute miles. If reduced to statute miles for the
purpose of comparison with the previous experiments, the coefficients will
respectively become 703, 1033, and 1212; which indicate that the
performance of screw vessels is equal to the performance of paddle vessels,
but some of the superiority of the result may be imputed to the superior
size of the screw vessels.
INFLUENCE OF THE SIZE OF VESSELS UPON THEIR SPEED.
549. _Q._--Will large vessels attain a greater speed than small, supposing
each to be furnished with the same proportionate power?
_A._--It is well known that large vessels furnished with the same
proportionate power, will attain a greater speed than small vessels, as
appears from the rule usual in yacht races of allowing a certain part of
the distance to be run to vessels which are of inferior size. The velocity
attained by a large vessel will be greater than the velocity attained by a
small vessel of the same mould and the same proportionate power, in the
proportion of the square roots of the linear dimensions of the vessels. A
vessel therefore with four times the sectional area and four times the
power of a smaller symmetrical vessel, and consequently of twice the
length, will have its speed increased in the proportion of the square root
of 1 to the square root of 2, or 1.4 times.
550. _Q._--Will you further illustrate this doctrine by an example?
_A._--The screw steamer Fairy, if enlarged to three times the size while
retaining the same form, would have twenty-seven times the capacity, nine
times the sectional area, and nine times the power. The length of such a
vessel would be 434 feet; her breadth 63 feet 4-1/2 inches; her draught of
water 16-1/2 feet; her area of immersed section 729 square feet; and her
nominal power 1080 horses. Now as the lengths of the Fairy and of the new
vessel are in the proportion of 1 to 3, the speeds will be in the
proportion of the square root of 1 to the square root of 3; or, in other
words, the speed of the large vessel will be 1.73 times greater than the
speed of the small vessel. If therefore the speed of the Fairy be 13 knots,
the speed of the new vessel will be 22.49 knots, although the proportion of
power to sectional area, which is supposed to be the measure of the
resistance, is in both cases precisely the same. If the speed of the Fairy
herself had to be increased to 22.29 knots, the power would have to be
increased in the proportion of the cube of 13 to the cube of 22.49, or 5.2
times, which makes the power necessary to propel the Fairy at that speed
equal to 624 nominal horses power.
STRUCTURE AND OPERATION OF PADDLE WHEELS.
551. _Q._--Will you describe the configuration and mode of action of the
paddle wheels in general use?
_A._--There are two kinds of paddle wheels in extensive use, the one being
the ordinary radial wheel, in which the floats are fixed on arms radiating
from the centre; and the other the feathering wheel, in which each float is
hung upon a centre, and is so governed by suitable mechanism as to be
always kept in nearly the vertical position. In the radial wheel there is
some loss of power from oblique action, whereas in the feathering wheel
there is little or no loss from this cause; but in every kind of paddle
there is a loss of power from the recession of the water from the float
boards, or the _slip_ as it is commonly called; and this loss is the
necessary condition of the resistance for the propulsion of the vessel
being created in a fluid. The slip is expressed by the difference between
the speed of the wheel and the speed of the vessel, and the larger this
difference is the greater the loss of power from slip must be--the
consumption of steam in the engine being proportionate to the velocity of
the wheel, and the useful effect being proportionate to the speed of the
ship.
552. _Q._--The resistance necessary for propulsion will not be situated at
the circumference of the wheel?
_A._--In the feathering wheel, where every part of any one immerged float
moves forward with the same horizontal velocity, the pressure or resistance
may be supposed to be concentrated in the centre of the float; whereas, in
the common radial wheel this cannot be the case, for as the outer edge of
the float moves more rapidly than the edge nearest the centre of the wheel,
the outer part of the float is the most effectual in propulsion. The point
at which the outer and inner portions of the float just balance one another
in propelling effect, is called the _centre of pressure_; and if all the
resistances were concentrated in this point, they would have the same
effect as before in resisting the rotation of the wheel. The resistance
upon any one moving float board totally immersed in the water will, when
the vessel is at rest, obviously vary as the square of its distance from
the centre of motion--the resistance of a fluid varying with the square of
the velocity; but, except when the wheel is sunk to the axle or altogether
immersed in the water, it is impossible, under ordinary circumstances, for
one float to be totally immersed without others being immersed partially,
whereby the arc described by the extremity of the paddle arm will become
greater than the arc described by the inner edge of the float; and
consequently the resistance upon any part of the float will increase in a
higher ratio than the square of its distance from the centre of motion--the
position of the centre of pressure being at the same time correspondingly
affected. In the feathering wheel the position of the centre of pressure of
the entering and emerging floats is continually changing from the lower
edge of the float--where it is when the float is entering or leaving the
water--to the centre of the float, which is its position when the float is
wholly immerged; but in the radial wheel the centre of pressure can never
rise so high as the centre of the float.
553. _Q._--All this relates to the action of the paddle when the vessel is
at rest: will you explain its action when the vessel is in motion?
_A._--When the wheel of a coach rolls along the ground, any point of its
periphery describes in the air a curve which is termed a cycloid; any point
within the periphery traces a prolate or protracted cycloid, and any point
exterior to the periphery traces a curtate or contracted cycloid--the
prolate cycloid partaking more of the nature of a straight line, and the
curtate cycloid more of the nature of a circle. The action of a paddle
wheel in the water resembles in this respect that of the wheel of a
carriage running along the ground: that point in the radius of the paddle
of which the rotative speed is just equal to the velocity of the vessel
will describe a cycloid; points nearer the centre, prolate cycloids, and
points further from the centre, curtate cycloids. The circle described by
the point whose velocity equals the velocity of the ship, is called the
_rolling circle_, and the resistance due to the difference of velocity of
the rolling circle and centre of pressure is that which operates in the
propulsion of the vessel. The resistance upon any part of the float,
therefore, will vary as the square of its distance from the rolling circle,
supposing the float to be totally immerged; but, taking into account the
greater length of time during which the extremity of the paddle acts,
whereby the resistance will be made greater, we shall not err far in
estimating the resistance upon any point at the third power of its distance
from the rolling circle in the case of light immersions, and the 2.5 power
in the case of deep immersions.
554. _Q._--How is the position of the centre of pressure to be determined?
_A._--With the foregoing assumption, which accords sufficiently with
experiment to justify its acceptation, the position of the centre of
pressure may be found by the following rule:--from the radius of the wheel
substract the radius of the rolling circle; to the remainder add the depth
of the paddle board, and divide the fourth power of the sum by four times
the depth; from the cube root of the quotient subtract the difference
between the radii of the wheel and rolling circle, and the remainder will
be the distance of the centre of pressure from the upper edge of the
paddle.
555. _Q._--How do you find the diameter of the rolling circle?
_A._--The diameter of the rolling circle is very easily found, for we have
only to divide 5,280 times the number of miles per hour, by 60 times the
number of strokes per minute, to get an expression for the circumference of
the rolling circle, or the following rule may be adopted:--divide 88 times
the speed of the vessel in statute miles per hour, by 3.1416 times the
number of strokes per minute; the quotient will be the diameter in feet of
the rolling circle. The diameter of the circle in which the centre of
pressure moves or the effective diameter of the wheel being known, and also
the diameter of the rolling circle, we at once find the excess of the
velocity of the wheel over the vessel.
556. _Q._--Will you illustrate these rules by an example?
_A._--A steam vessel of moderately good shape, and with engines of 200
horses power, realises, with 22 strokes per minute, a speed of 10.62 miles
per hour. To find the diameter of the rolling circle, we have 88 times
10.62, equal to 934.66, and 22 times 3.1416, equal to 69.1152; then 934.66
divided by 69.1152 is equal to 13.52 feet, which is the diameter of the
rolling circle. The diameter of the wheel is 19 ft. 4 in., so that the
diameter of the rolling circle is about 2/3ds of the diameter of the wheel,
and this is a frequent proportion. The depth of the paddle board is 2 feet,
and the difference between the diameters of the wheel and rolling circle
will be 5.8133, which will make the difference of their radii 2.9067; and
adding to this the depth of the paddle board, we have 4.9067, the fourth
power of which is 579.64, which, divided by four times the depth of the
paddle board, gives us 72.455, the cube root of which is 4.1689, which,
diminished by the difference of the radii of the wheel and rolling circle,
leaves 1.2622 feet for the distance of the centre of pressure from the
upper edge of the paddle board in the case of light immersions. The radius
of the wheel being 9.6667, the distance from the centre of the wheel to the
upper edge of the float is 7.6667, and adding to this 1.2622, we get 8.9299
feet as the radius, or 17.8598 feet as the diameter of the circle in which
the centre of pressure revolves. With 22 strokes per minute, the velocity
of the centre of pressure will be 20.573 feet per second, and with 10.62
miles per hour for the speed of the vessel, the velocity of the rolling
circle will be 15.576 feet per second. The effective velocity will be the
difference between these quantities, or 4.997 feet per second. Now the
height from which a body must fall by gravity, to acquire a velocity of
4.997 feet per second, is about .62 feet; and twice this height, or 1.24
feet, multiplied by 62-1/2, which is the number of Lbs. weight in a cubic
foot of water, gives 77-1/2 Lbs. as the pressure on each square foot of the
vertical paddle boards. As each board is of 20 square feet of area, and
there is a vertical board on each side of the ship, the total pressure on
the vertical paddle boards will be 2900 Lbs.
557. _Q._--What pressure is this equivalent to on each square inch of the
pistons?
_A._--A vessel of 200 horses power will have two cylinders, each 50 inches
diameter, and 5 feet stroke, or thereabout. The area of a piston of 50
inches diameter is 1963.5 square inches, so that the area of the two
pistons is 3927 square inches, and the piston will move through 10 feet
every revolution; and with 22 strokes per minute this will be 220 feet per
minute, or 3.66 feet per second. Now, if the effective velocity of the
centre of pressure and the velocity of the pistons had been the same, then
a pressure of 2900 Lbs. upon the vertical paddles would have been balanced
by an equal pressure on the pistons, which would have been in this case
about .75 Lbs. per square inch; but as the effective velocity of the centre
of pressure is 4.997 feet per second, while that of the pistons is only
3.66 feet per second, the pressure must be increased in the proportion of
4.997 to 3.66 to establish an equilibrium of pressure, or, in other words,
it must be 1.02 Lbs. per square inch. It follows from this investigation,
that, in radial wheels, the greater part of the engine power is distributed
among the oblique floats.
558. _Q._--How comes this to be the case?
_A._--To understand how it happens that more power is expended upon the
oblique than upon the vertical floats, it is necessary to remember that the
only resistance upon the vertical paddle is that due to the difference of
velocity of the wheel and the ship; but if the wheel be supposed to be
immersed to its axle, so that the entering float strikes the water
horizontally, it is clear that the resistance on such float is that due to
the whole velocity of rotation; and that the resistance to the entering
float will be the same whether the vessel is in motion or not. The
resistance opposed to the rotation of any float increases from the position
of the vertical float-where the resistance is that due to the difference of
velocity of the wheel and vessel--until it reaches the plane of the axis,
supposing the wheel to be immersed so far, where the resistance is that due
to the whole velocity of rotation; and although in any oblique float the
total resistance cannot be considered operative in a horizontal direction,
yet the total resistance increases so rapidly on each side of the vertical
float, that the portion of it which is operative in the horizontal
direction, is in all ordinary cases of immersion very considerable. In the
feathering wheel, where there is little of this oblique action, the
resistance will be in the proportion of the square of the horizontal
velocities of the several floats, which may be represented by the
horizontal distances between them; and in the feathering wheel, the
vertical float having the greatest horizontal velocity will have the
greatest propelling effect.
559. _Q._--Should the floats in feathering wheels enter and leave the water
vertically?
_A._--The floats should be so governed by the central crank or eccentric,
that the entering and emerging floats have a direction intermediate between
a radius and a vertical line.
560. _Q._--Can you give any practical rules for proportioning paddle
wheels?
_A._--A common rule for the pitch of the floats is to allow one float for
every foot of diameter of the wheel; but in the case of fast vessels a
pitch of 2-1/2 feet, or even less, appears preferable, as a close pitch
occasions less vibration. If the floats be put too close, however, the
water will not escape freely from between them, and if set too far apart
the stroke of the entering paddle will occasion an inconvenient amount of
vibratory motion, and there will also be some loss of power. To find the
proper area of a single float:--divide the number of actual horses power of
both engines by the diameter of the wheel in feet; the quotient is the area
of one paddle board in square feet proper for sea going vessels, and the
area multiplied by 0.6 will give the length of the float in feet. In very
sharp vessels, which offer less resistance in passing through the water,
the area of paddle board is usually one-fourth less than the above
proportion, and the proper length of the float may in such case be found by
multiplying the area by 0.7. In sea going vessels about four floats are
usually immersed, and in river steamers only one or two floats. There is
more slip in the latter case, but there is also more engine power exerted
in the propulsion of the ship, from the greater speed of engine thus
rendered possible.
561. _Q._--Then is it beneficial to use small floats?
_A._--Quite the contrary. If to permit a greater speed of the engine the
floats be diminished in area instead of being raised out of the water, no
appreciable accession to the speed of the vessel will be obtained; whereas
there will be an increased speed of vessel if the accelerated speed of the
engine be caused by diminishing the diameter of the wheels. In vessels
intended to be fast, therefore, it is expedient to make the wheels small,
so as to enable the engine to work with a high velocity; and it is
expedient to make such wheels of the feathering kind, to obviate loss of
power from oblique action. In no wheel must the rolling circle fall below
the water line, else the entering and emerging floats will carry masses of
water before them. The slip is usually equal to about one-fourth of the
velocity of the centre of pressure in well proportioned wheels; but it is
desirable to have the slip as small as is possible consistently with the
observance of other necessary conditions. The speed of the engine and also
the speed of the vessel being fixed, the diameter of the rolling circle
becomes at once ascertainable, and adding to this the slip, we have the
diameter of the wheel.
CONFIGURATION AND ACTION OF THE SCREW.
562. _Q._--Will you describe more in detail than you have yet done, the
configuration and mode of action of the screw propeller?
_A._--The ordinary form of screw propeller is represented in figs. 46 and
47; fig. 46 being a perspective view, and fig. 47 an end view, or view such
as is seen when looking upon the end of the shaft. The screw here
represented is one with two arms or blades. Some screws have three arms,
some four and some six; but the screw with two arms is the most usual, and
screws with more than three arms are not now much employed in this country.
The screw on being put into revolution by the engine, preserves a spiral
path in the water, in which it draws itself forward in the same way as a
screw nail does when turned round in a piece of wood, whereas the paddle
wheel more resembles the action of a cog wheel working in a rack.
[Illustration: Fig. 46. Fig. 47. ORDINARY FORM OF SCREW PROPELLER.]
563. _Q._--But the screw of a steam vessel has no resemblance to a screw
nail?
_A._--It has in fact a very close resemblance if you suppose only a very
short piece of the screw nail to be employed, and if you suppose, moreover,
the thread of the screw to be cut nearly into the centre to prevent the
wood from stripping. The original screw propellers were made with several
convolutions of screw, but it was found advantageous to shorten them, until
they are now only made one-sixth of a convolution in length.
564. _Q._--And the pitch you have already explained to be the distance in
the line of the shaft from one convolution to the next, supposing the screw
to consist of two or more convolutions?
_A._--Yes, that is what is meant by the pitch. If a thread be wound upon a
cylinder with an equal distance between the convolutions, it will trace a
screw of a uniform pitch; and if the thread be wound upon the cylinder with
an increasing distance between each convolution, it will trace a screw of
an increasing pitch. But two or more threads may be wound upon the cylinder
at the same time, instead of a single thread. If two threads be wound upon
it they will trace a double-threaded screw; if three threads be wound upon
it they will trace a treble-threaded screw; and so of any other number. Now
if the thread be supposed to be raised up into a very deep and thin spiral
feather, and the cylinder be supposed to become very small, like the newel
of a spiral stair, then a screw will be obtained of the kind proper for
propelling vessels, except that only a very short piece of such screw must
be employed. Whatever be the number of threads wound upon a cylinder, if
the cylinder be cut across all the threads will be cut. A slice cut out of
the cylinder will therefore contain a piece of each thread. But the
threads, in the case of a screw propeller, answer to the arms, so that in
every screw propeller the number of threads entering into the composition
of the screw will be the same as the number of arms. An ordinary screw with
two blades is a short piece of a screw of two threads.
565. _Q._--In what part of the ship is the screw usually placed?
[Illustration: Fig. 48]
_A._--In that part of the run of the ship called the dead wood, which is a
thin and unused part of the vessel just in advance of the rudder. The usual
arrangement is shown in fig. 48, which represents the application to a
vessel of a species of screw which has the arms bent backwards, to
counteract the centrifugal motion given to the water when there is a
considerable amount of slip.
566. _Q._--How is the slip in a screw vessel determined?
_A._--By comparing the actual speed of the vessel with the speed due to the
pitch and number of revolutions of the screw, or, what is the same thing,
the speed which the vessel would attain if the screw worked in a solid nut.
The difference between the actual speed and this hypothetical speed, is the
slip.
567. _Q._--In well formed screw propellers what is the amount of slip found
to be?
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