CONSTRUCTIVE DETAILS OF ENGINES.
PUMPING ENGINES.
422. _Q._--Will you explain the course of procedure in the erection of a
pumping engine, such as Boulton and Watt introduced into Cornwall?
_A._--The best instructions on this subject are those of Mr. Watt himself,
which are as follows:--Having fixed on the proper situation of the pump in
the pit, from its centre measure out the distance to the centre of the
cylinder, from which set off all the other dimensions of the house,
including the thickness of the walls, and dig out the whole of the included
ground to the depth of the bottom of the cellar, so that the bottom of the
cylinder may stand on a level with the natural ground of the place, or
lower, if convenient, for the less the height of the house above the
ground, the firmer it will be. The foundations of the walls must be laid at
least two feet lower than the bottom of the cellar, unless the foundation
be firm rock; and care must be taken to leave a small drain into the pit
quite through the lowest part of the foundation of the lever wall, to let
off any water that may be spilt in the engine house, or may naturally come
into the cellar. If the foundation at that depth does not prove good, you
must either go down to a better if in your reach, or make it good by a
platform of wood or piles, or both.
423. _Q._--These directions refer to the foundations?
_A._.--Yes; but I will now proceed to the other parts. Within the house,
low walls must be built to carry the cylinder beams, so as to leave
sufficient room to come at the holding down bolts, and the ends of these
beams must also be lodged in the wall The lever wall must be built in the
firmest manner, and run solid, course by course, with thin lime mortar,
care being taken that the lime has not been long slaked. If the house be
built of stone, let the stones be large and long, and let many headers be
laid through the wall: it should also be a rule, that every stone be laid
on the broadest bed it has, and never set on its edge. A course or two
above the lintel of the door that leads to the condenser, build into the
wall two parallel flat thin bars of iron equally distant from each other,
and from the outside and inside of the wall, and reaching the whole breadth
of the lever wall. About a foot higher in the wall, lay at every four feet
of the breadth of the front, other bars of the same kind at right angles to
the former course, and reaching quite through the thickness of the wall;
and at each front corner lay a long bar in the middle of the side walls,
and reaching quite through the front wall; if these bars are 10 feet or 12
feet long it will be sufficient. When the house is built up nearly to the
bottom of the opening under the great beam another double course of bars is
to be built in, as has been directed. At the level of the upper cylinder
beams, holes must be left in the walls for their ends, with room to move
them laterally, so that the cylinder may be got in; and smaller holes must
be left quite through the walls for the introduction of iron bars, which
being firmly fastened to the cylinder beams at one end, and screwed at the
other or outer end, will serve, by their going through both the front and
back walls, to bind the house more firmly together. The spring beams or
iron bars fastened to them must reach quite through the back wall, and be
keyed or screwed up tight; and they must be firmly fastened to the lever
wall on each side, either by iron bars, firm pieces of wood, or long strong
stones, reaching far back into the wall. They must also be bedded solidly,
and the residue of the opening must be built up in the firmest manner.
424. _Q._--If there be a deficiency of water for the purpose of
condensation, what course should be pursued?
_A._--If there be no water in the neighborhood that can be employed for the
purpose of condensation, it will be necessary to make a pond, dug in the
earth, for the reception of the water delivered by the air pump, to the end
that it may be cooled and used again for the engine. The pond may be three
or four feet deep, and lined with turf, puddled, or otherwise made water
tight. Throwing up the water into the air in the form of a jet to cool it,
has been found detrimental; as the water is then charged with air which
vitiates the vacuum.
425. _Q._--How is the piston of a pumping engine packed?
_A._--To pack the piston, take sixty common-sized white or untarred
rope-yarns, and with them plait a gasket or flat rope as close and firm as
possible, tapering for eighteen inches at each end, and long enough to go
round the piston, and overlapped for that length; coil this rope the thin
way as hard as possible, and beat it with a sledge hammer until its breadth
answers the place; put it in and beat it down with a wooden drift and a
hand mallet, pour some melted tallow all around, then pack in a layer of
white oakum half an inch thick, so that the whole packing may have the
depth of five to six inches, depending on the size of the engine; finally,
screw down the junk ring. The packing should be beat solid, but not too
hard, otherwise it will create so great a friction as to prevent the easy
going of the engine. Abundance of tallow should be allowed, especially at
first; the quantity required will be less as the cylinder grows smooth. In
some of the more modern pumping engines, the piston is provided with
metallic packing, consisting for the most part of a single ring with a
tongue piece to break the joint, and packed behind with hemp. The upper
edge of the metallic ring is sharpened away from the inside so as to permit
more conveniently the application of hemp packing behind it; and the junk
ring is made much the same as if no metallic packing were employed.
426. _Q._--Will you explain the mode of putting the engine into operation?
_A._--To set the engine going, the steam must be raised until the pressure
in the steam pipe is at least equal to three pounds on the square inch; and
when the cylinder jacket is fully warmed, and steam issues freely from the
jacket cock, open all the valves or regulators; the steam will then
forcibly blow out the air or water contained in the eduction pipe, and to
get rid of the air in the cylinder, shut the steam valve after having blown
through the engine for a few minutes. The cold water round the condenser
will condense some of the steam contained in the eduction pipe, and its
place will be supplied by some of the air from the cylinder. The steam
valve must again be opened to blow out that air, and the operation is to be
repeated until the air is all drawn out of the cylinder. When that is the
case shut all the valves, and observe if the vacuum gauge shows a vacuum in
the condenser; when there is a vacuum equivalent to three inches of
mercury, open the injection a very little, and shut it again immediately;
and if this produces any considerable vacuum, open the exhausting valve a
very little way, and the injection at the same time. If the engine does not
now commence its motion, it must be blown through again until it moves. If
the engine be lightly loaded, or if there be no water in the pumps, the
throttle valve must be kept nearly closed, and the top and exhaustion
regulators must be opened only a very little way, else the engine will make
its stroke with violence, and perhaps do mischief. If there is much
unbalanced weight on the pump end, the plug which opens the steam valve
must be so regulated, that the valve will only be opened very slightly; and
if after a few strokes it is found that the engine goes out too slowly, the
valve may be then so adjusted as to open wider. The engine should always be
made to work full stroke, that is, until the catch pins be made to come
within half an inch of the springs at each end, and the piston should stand
high enough in the cylinder when the engine is at rest, to spill over into
the perpendicular steam pipe any water which may be condensed above it; for
if water remain upon the piston, it will increase the consumption of steam.
When the engine is to be stopped, shut the injection valve and secure it,
and adjust the tappets so as to prevent the exhausting valve from opening
and to allow the steam valve to open and remain open, otherwise a partial
vacuum may arise in the cylinder, and it may be filled with water from the
injection or from leaks. A single acting engine, when it is in good order,
ought to be capable of going as slow as one stroke in ten minutes, and as
fast as ten strokes in one minute; and if it does not fulfil these
conditions, there is some fault which should be ascertained and remedied.
427. _Q._--Your explanation has reference to the pumping engine as
introduced into Cornwall by Watt: have any modifications been since made
upon it?
_A._--In the modern Cornish engines the steam is used very expansively, and
a high pressure of steam is employed. In some cases a double cylinder
engine is used, in which the steam, after having given motion to a small
piston on the principle of a high pressure engine, passes into a larger
cylinder, where it operates on the principle of a condensing engine; but
there is no superior effect gained by the use of two cylinders, and there
is greater complexity in the apparatus. Instead of the lever walls, cast
iron columns are now frequently used for supporting the main beam in
pumping engines, and the cylinder end of the main beam is generally made
longer than the pump end in engines made in Cornwall, so as to enable the
cylinder to have a long stroke, and the piston to move quickly, without
communicating such a velocity to the pump buckets as will make them work
with such a shock as to wear themselves out quickly. A high pressure of
steam, too, can be employed where the stroke is long, without involving the
necessity of making the working parts of such large dimensions as would
otherwise be necessary; for the strength of the parts of a single acting
engine will require to be much the same, whatever the length of the stroke
may be.
428. _Q._--What kind of pump is mostly used in draining deep mines?
_A._--The pump now universally preferred is the plunger pump, which admits
of being packed or tightened while the engine is at work; but the lowest
lift of a mine is generally supplied with a pump on the suction principle,
both with the view of enabling the lowest pipe to follow the water with
facility as the shaft is sunk deeper, and to obviate the inconvenience of
the valves of the pump being rendered inaccessible by any flooding in the
mine. The pump valves of deep mines are a perpetual source of expense and
trouble, as from the pressure of water upon them it is difficult to prevent
them from closing with violence; and many expedients have been contrived to
mitigate the evil, of which the valve known as Harvey and West's valve has
perhaps gained the widest acceptation.
429. _Q._--Will you describe Harvey and West's pump valve?
_A._--This valve is a compromise between the equilibrium valve, of the kind
employed for admitting the steam to and from the cylinder in single acting
engines, and the common spindle valve formerly used for that purpose; and
to comprehend its action, it is necessary that the action of the
equilibrium valve, which has been already represented fig. 34, should first
be understood. This valve consists substantially of a cylinder open at both
ends, and capable of sliding upon a stationary piston fixed upon a rod the
length of the cylinder, which proceeds from the centre of the orifice the
valve is intended to close. It is clear, that when the cylinder is pressed
down until its edge rests upon the bottom of the box containing it, the
orifice of the pipe must be closed, as the steam can neither escape past
the edge of the cylinder nor between the cylinder and the piston; and it is
equally clear, that as the pressure upon the cylinder is equal all around
it, and the whole of the downward pressure is maintained by the stationary
piston, the cylinder can be raised or lowered without any further exertion
of force than is necessary to overcome the friction of the piston and of
the rod by which the cylinder is raised. Instead of the rubbing surface of
a piston, however, a conical valve face between the cylinder and piston is
employed, which is tight only when the cylinder is in its lowest position;
and there is a similar face between, the edge of the cylinder and the
bottom of the box in which it is placed. The moving part of the valve, too,
instead of being a perfect cylinder, is bulged outward in the middle, so as
to permit the steam to escape past the stationary piston when the
cylindrical part of the valve is raised. It is clear, that if such a valve
were applied to a pump, no pressure of water within the pump would suffice
to open it, neither would any pressure of water above the valve cause it to
shut with violence; and if an equilibrium valve, therefore, be used as a
pump valve at all, it must be opened and shut by mechanical means. In
Harvey and West's valves, however, the equilibrium principle is only
partially adopted; the lower face is considerably larger in diameter than
the upper face, and the difference constitutes an annulus of pressure,
which will cause the valve to open or shut with the same force as a spindle
valve of the area of the annulus. To deaden the shock still more
effectually, the lower face of the valve is made to strike upon end wood
driven into an annular recess in the pump bucket; and valves thus
constructed work with very little noise or tremor; but it is found in
practice, that the use of Harvey and West's valve, or any contrivance of a
similar kind, adds materially to the load upon the pump, especially in low
lifts where the addition of a load, to the valve makes a material addition
to the total resistance which the engine has to overcome. Instead of end
wood driven into a recess for the valve to strike upon, a mixture of tin
and lead cast in a recess is now frequently used, and is found to be
preferable to the wood.
430. _Q._--Is there any other kind of pump valve which is free from the
shocks incidental to the working of common valves?
_A._--In some cases canvass valves are used for pumps, with the effect of
materially mitigating the shock; but they require frequent renewal, and are
of inferior eligibility in their action to the slide valve, which might in
many cases be applied to pumps without inconvenience.
431. _Q._--Could not a form of pump be devised capable of working without
valves at all?
_A._.--It appears probable, that by working a common reciprocating pump at
a high speed, a continuous flow of water might be maintained through the
pipes in such a way as to render the existence of any valves superfluous
after once the action was begun, the momentum of the moving water acting in
fact as valves. The centrifugal pump, however, threatens to supersede pumps
of every other kind; and if the centrifugal pump be employed there will be
no necessity for pump valves at all. There is less loss of effect by the
centrifugal pump than by the common pump.
432. _Q._--What is the best form of the centrifugal pump?
_A._--There are two forms in which the centrifugal pump may be applied to
mines;--that in which the arms diverge from the bottom, like the letter V;
and that in which revolving arms are set in a tight case near the bottom of
the mine, and are turned by a shaft from the surface. Such pumps both draw
and force; and either by arranging them in a succession of lifts in the
shaft of the mine, or otherwise, the water may be drawn without
inconvenience from any depth. The introduction of the centrifugal pump
would obviously extinguish the single acting engine, as rotative engines
working at a high speed would be the most appropriate form of engine where
the centrifugal pump was employed.
433. _Q._--This would not be a heavy deprivation?
_A._--The single acting engine is a remnant of engineering barbarism which
must now be superseded by more compendious contrivances. The Cornish
engines, though rudely manufactured, are very expensive in production, as a
large engine does but little work; whereas by employing a smaller engine,
moving with a high speed, the dimensions may be so far diminished that the
most refined machinery may be obtained at less than the present cost.
434. _Q._--Are not the Cornish engines more economical in fuel than other
engines?
_A._--It is a mistake to suppose that there is any peculiar virtue in the
existing form of Cornish engine to make it economical in fuel, or that a
less lethargic engine would necessarily be less efficient. The large duty
of the engines in Cornwall is traceable to the large employment of the
principle of expansion, and to a few other causes which may be made of
quite as decisive efficacy in smaller engines working with a quicker speed;
and there is therefore no argument in the performance of the present
engines against the proposed substitution.
VARIOUS FORMS OF MARINE ENGINES.
435. _Q._--What species of paddle engine do you consider to be the best?
_A._--The oscillating engine.
436. _Q._--Will you explain the grounds of that preference?
_A._--The engine occupies little space, consists of few parts, is easily
accessible for repairs, and may be both light and strong at the same time.
In the case of large engines the crank in the intermediate shaft is a
disadvantage, as it is difficult to obtain such a forging quite sound. But
by forging it in three cranked flat bars, which are then laid together and
welded into a square shaft, a sound forging will be more probable, and the
bars should be rounded a little on the sides which are welded to allow the
scoriae to escape during that operation. It is important in so large a
forging not to let the fire be too fierce, else the surface of the iron
will be burnt before the heart is brought to a welding heat. In some cases
in oscillating engines the air pump has been wrought by an eccentric, and
that may at any time be done where doubt of obtaining a sound intermediate
shaft is entertained; but the precaution must be taken to make the
eccentric very wide so as to distribute the pressure over a large surface,
else the eccentric will be apt to heat.
437. _Q._--Have not objections been brought against the oscillating engine?
_A._--In common with every other improvement, the oscillating engine, at
the time of its introduction, encountered much opposition. The cylinder, it
was said, would become oval, the trunnion bearings would be liable to heat
and the trunnion joints to leak, the strain upon the trunnions would be apt
to bend in or bend out the sides of the cylinder; and the circumstance of
the cylinder being fixed across its centre, while the shaft requires to
accommodate itself to the working of the ship, might, it was thought, be
the occasion of such a strain upon the trunnions as would either break them
or bend the piston rod. It is a sufficient reply to these objections to say
that they are all hypothetical, and that none of them in practice have been
found to exist--to such an extent at least as to occasion any
inconvenience; but it is not difficult to show that they are altogether
unsubstantial, even without a recourse to the disproofs afforded by
experience.
438. _Q._--Is there not a liability in the cylinder to become oval from the
strain thrown on it by the piston?
_A._--There is, no doubt, a tendency in oscillating engines for the
cylinder and the stuffing box to become oval, but after a number of years'
wear it is found that the amount of ellipticity is less than that which is
found to exist in the cylinders of side lever engines after a similar
trial. The resistance opposed by friction to the oscillation of the
cylinder is so small, that a man is capable of moving a large cylinder with
one hand; whereas in the side lever engine, if the parallel motion be in
the least untrue, which is, at some time or other, an almost inevitable
condition, the piston is pushed with great force against the side of the
cylinder, whereby a large amount of wear and friction is occasioned. The
trunnion bearings, instead of being liable to heat like other journals, are
kept down to the temperature of the steam by the flow of steam passing
through them; and the trunnion packings are not liable to leak when the
packings, before being introduced, are squeezed in a cylindrical mould.
439. _Q._--Might not the eduction trunnions be immersed in water?
_A._--In some cases a hollow, or lantern brass, about one third or one
fourth the length of the packing space, and supplied with steam or water by
a pipe, is introduced in the middle of the packing, so that if there be any
leakage through the trunnion, it will be a leakage of steam or water, which
will not vitiate the vacuum; but in ordinary cases this device will not be
necessary, and it is not commonly employed. It is clear that there can be
no buckling of the sides of the cylinder by the strain upon the trunnions,
if the cylinder be made strong enough, and in cylinders of the ordinary
thickness such an action has never been experienced; nor is it the fact,
that the intermediate shaft of steam vessels, to which part alone the
motion is communicated by the engine, requires to adapt itself to the
altering forms of the vessel, as the engine and intermediate shaft are
rigidly connected, although the paddle shaft requires to be capable of such
an adaptation. Even if this objection existed, however, it could easily be
met by making the crank pin of the ball and socket fashion, which would
permit the position of the intermediate shaft, relatively with that of the
cylinder, to be slightly changed, without throwing an undue strain upon any
of the working parts.
440. _Q._--Is the trunk engine inferior to the oscillating?
_A._--A very elegant and efficient arrangement of trunk engine suitable for
paddle vessels has latterly been employed by Messrs. Rennie, of which all
the parts resemble those of Penn's oscillating engine except that the
cylinders are stationary instead of being movable; and a round trunk or
pipe set upon the piston, and moving steam tight through the cylinder
cover, enables the connecting rod which is fixed to the piston to vibrate
within it to the requisite extent. But the vice of all trunk engines is
that they are necessarily more wasteful of steam, as the large mass of
metal entering into the composition of the trunk, moving as it does
alternately into the atmosphere and the steam, must cool and condense a
part of the steam. The radiation of heat from the interior of the trunk
will have the same operation, though in vertical trunk engines the loss
from this cause might probably be reduced by filling the trunk with oil, so
far as this could be done without the oil being spilt over the edge.
441. _Q._--What species of screw engine do you consider the best?
_A._--I am inclined to give the preference to a variety of the horizontal
steeple engine, such as was first used in H.M.S. Amphion. In this engine
the cylinders lie on their sides, and they are placed near the side of the
vessel with their mouths pointing to the keel. From each cylinder two long
piston rods proceed across the vessel to a cross head working in guides;
and from this cross head a connecting rod returns back to the centre of the
vessel and gives motion to the crank. The piston rods are so placed in the
piston that one of them passes above the crank shaft, and the other below
the crank shaft. The cross head lies in the same horizontal plane as the
centre of the cylinder, and a lug projects upwards from the cross head to
engage one piston rod, and downwards from the cross head to engage the
other piston rod. The air pump is double acting, and its piston or bucket
has the same stroke as the piston of the engine. The air pump bucket
derives its motion from an arm on the cross head, and a similar arm is
usually employed in engines of this class to work the feed and bilge pumps.
442. _Q._--Is not inconvenience experienced in direct acting screw engines
from the great velocity of their motion?
_A._--Not if they are properly constructed; but they require to be much
stronger, to be fitted with more care, and to have the bearing surfaces
much larger than is necessary in engines moving slowly. The momentum of the
reciprocating parts should also be balanced by a weight applied to the
crank or crank shaft, as is done in locomotives. A very convenient
arrangement for obtaining surface is to form the crank of each engine of
two cast iron discs cast with heavy sides, the excess of weight upon the
heavy sides being nearly equal to that of the piston and its connections.
When the piston is travelling in one direction the weights are travelling
in the opposite; and the momentum of the piston and its attachments, which
is arrested at each reciprocation, is just balanced by the equal and
opposite momentum of the weights. One advantage of the horizontal engine
is, that a single engine may be employed, whereby greater simplicity of the
machinery and greater economy of fuel will be obtained, since there will be
less radiating surface in one cylinder than in two.
CYLINDERS, PISTONS, AND VALVES,
443. _Q._--Is it a beneficial practice to make cylinders with steam
jackets?
_A._--In Cornwall, where great attention is paid to economy of fuel, all
the engines are made with steam jackets, and in some cases a flue winds
spirally round the cylinder, for keeping the steam hot. Mr. Watt, in his
early practice, discarded the steam jacket for a time, but resumed it
again, as he found its discontinuance occasioned a perceptible waste of
fuel; and in modern engines it has been found that where a jacket is used
less coal is consumed than where the use of a jacket is rejected. The cause
of this diminished effect is not of very easy perception, for the jacket
exposes a larger radiating surface for the escape of the heat than the
cylinder; nevertheless, the fact has been established beyond doubt by
repeated trials, that engines provided with a jacket are more economical
than engines without one. The exterior of the cylinder, or jacket, should
be covered with several plies of felt, and then be cased in timber, which
must be very narrow, the boards being first dried in a stove, and then
bound round the cylinder with hoops, like the staves of a cask. In many of
the Cornish engines the steam is let into casings formed in the cylinder
cover and cylinder bottom, for the further economisation of the heat, and
the cylinder stuffing box is made very deep, and a lantern or hollow brass
is introduced into the centre of the packing, into which brass the steam
gains admission by a pipe provided for the purpose; so that in the event of
the packing becoming leaky, it will be steam that will be leaked into the
cylinder instead of air, which, being incondensable, would impair the
efficiency of the engine. A lantern brass, of a similar kind, is sometimes
introduced into the stuffing boxes of oscillating engines, but its use
there is to receive the lateral pressure of the piston rod, and thus take
any strain off the packing.
444. _Q._--Will you explain the proper course to pursue in the production
of cylinders?
_A._--In all engines the valve casing, if made in a separate piece from the
cylinder, should be attached by means of a metallic joint, as such a
barbarism as a rust joint in such situations is no longer permissible. In
the case of large engines with valve casings suitable for long slides, an
expansion joint in the valve casing should invariably be inserted,
otherwise the steam, by gaining admission to the valve casing before it can
enter the cylinder, expands the casing while the cylinder remains unaltered
in its dimensions, and the joints are damaged, and in some cases the
cylinder is cracked by the great strain thus introduced. The chest of the
blow-through valve is very commonly cast upon the valve casing; and in
engines where the cylinders are stationary this is the most convenient
practice. All engines, where the valve is not of such a construction as to
leave the face when a pressure exceeding that of the steam is created in
the cylinder by priming or otherwise, should be provided with an escape
valve to let out the water, and such valve should be so constructed that
the water cannot fly out with violence over the attendants; but it should
be conducted away by a suitable pipe, to a place where its discharge can
occasion no inconvenience. The stuffing boxes of all engines which cannot
be stopped frequently to be repacked, should be made very deep; metallic
packing in the stuffing box has been used in some engines, consisting in
most instances of one or more rings, cut, sprung, and slipped upon the
piston rod before the cross head is put on, and packed with hemp behind.
This species of packing answers very well when the parallel motion is true,
and the piston rod free from scratches, and it accomplishes a material
saving of tallow. In some cases a piece of sheet brass, packed behind with
hemp, has been introduced with good effect, a flange being turned over on
the under edge of the brass to prevent it from slipping up or down with the
motion of the rod. The sheet brass speedily puts an excellent polish upon
the rod, and such a packing is more easily kept, and requires less tallow
than where hemp alone is employed. In side lever marine engines the
attachments of the cylinder to the diagonal stay are generally made of too
small an area, and the flanges are made too thick. A very thick flange cast
on any part of a cylinder endangers the soundness of the cylinder, by
inducing an unequal contraction of the metal; and it is a preferable course
to make the flange for the attachment or the framing thin, and the surface
large--the bolts being turned bolts and nicely fitted. If from malformation
in this part the framing works to an inconvenient extent, the best
expedient appears to be the introduction of a number of steel tapered
bolts, the holes having been previously bored out; and if the flanges be
thick enough, square keys may also be introduced, half into one flange and
half into the other, so as to receive the strain. If the jaw cracks or
breaks away, however, it will be best to apply a malleable iron hoop around
the cylinder to take the strain, and this will in all cases be the
preferable expedient, where from any peculiarities of structure there is a
difficulty in introducing bolts and keys of sufficient strength.
445. _Q._--Which is the most eligible species of piston?
_A._--For large engines, pistons with a metallic packing, consisting of a
single ring, with the ends morticed into one another, and a piece of metal
let in flush over the joint and riveted to one end of the ring, appears to
be the best species of piston; and if the cylinder be oscillating, it will
be expedient to chamfer off the upper edge of the ring on the inner side,
and to pack it at the back with hemp. If the cylinder be a stationary one,
springs may be substituted for the hemp packing, but in any case it will be
expedient to make the vertical joints of the ends of the ring run a little
obliquely, so as to prevent the joint forming a ridge in the cylinder. For
small pistons two rings may be employed, made somewhat eccentric internally
to give a greater thickness of metal in the centre of the ring; these rings
must be set one above the other in the cylinder, and the joints, which are
oblique, must be set at right angles with one another, so as to obviate any
disposition of the rings, in their expansion, to wear the cylinder oval.
The rings must first be turned a little larger than the diameter of the
cylinder, and a piece is then to be cut out, so that when the ends are
brought together the ring will just enter within the cylinder. The ring,
while retained in a state of compression, is then to be put in the lathe
and turned very truly, and finally it is to be hammered on the inside with
the small end of the hammer, to expand the metal, and thus increase the
elasticity.
446. _Q._--The rings should be carefully fitted to one another laterally?
_A._--The rings are to be fitted laterally to the piston, and to one
another, by scraping--a steady pin being fixed upon the flange of the
piston, and fitting into a corresponding hole in the lower ring, to keep
the lower ring from turning round; and a similar pin being fixed into the
top edge of the lower ring to prevent the upper ring from turning round;
but the holes into which these pins fit must be made oblong, to enable the
rings to press outward as the rubbing surfaces wear. In most cases it will
be expedient to press the packing rings out with springs where they are not
packed behind with hemp, and the springs should be made very strong, as the
prevailing fault of springs is their weakness. Sometimes short bent
springs, set round at regular intervals between the packing rings and body
of the piston, are employed, the centre of each spring being secured by a
steady pin or bolt screwed into the side of the piston; but it will not
signify much what kind of springs is used, provided they have sufficient
tension. When pistons are made of a single ring, or of a succession of
single rings, the strength of each ring should be tested previously to its
introduction into the piston, by means of a lever loaded by a heavy weight.
447. _Q._--What kind of piston is employed by Messrs. Penn?
_A._--Messrs. Penn's piston for oscillating engines has a single packing
ring, with a tongue piece, or mortice end, made in the manner already
described. The ring is packed behind with hemp packing, and the piece of
metal which covers the joint is a piece of thick sheet copper or brass, and
is indented into the iron of the ring, so as to offer no obstruction to the
application of the hemp. The ring is fitted to the piston only on the under
edge; the top edge is rounded to a point from the inside, and the junk ring
does not bear upon it, but the junk ring squeezes down the hemp packing
between the packing ring and the body of the piston.
448. _Q._--How should the piston rod be secured to the piston?
_A._--The piston rod, where it fits into the piston, should have a good
deal of taper; for if the taper be too small the rod will be drawn through
the hole, and the piston will be split asunder. Small grooves are sometimes
turned out of the piston rod above and below the cutter hole, and hemp is
introduced in order to make the piston eye tight. Most piston rods are
fixed to the piston by means of a gib and cutter, but in some cases the
upper portion of the rod within the eye is screwed, and it is fixed into
the piston by means of an indented nut. This nut is in some cases
hexagonal, and in other cases the exterior forms a portion of a cone which
completely fills a corresponding recess in the piston; but nuts made in
this way become rusted into their seat after some time, and cannot be
started again without much difficulty. Messrs. Miller, Ravenhill & Co. fix
in their piston rods by means of an indented hexagonal nut, which may be
started by means of an open box key. The thread of the screw is made flat
upon the one side and much slanted on the other, whereby a greater strength
is secured, without creating any disposition to split the nut. In side
lever engines it is a judicious practice to add a nut to the top of the
piston rod, in addition to the cutter for securing the piston rod to the
cross head. In a good example of an engine thus provided, the piston rod is
7 in. in diameter, and the screw 5 in.; the part of the rod which fits into
the cross head eye is 1 ft. 5-1/2 in. long, and tapers from 6-1/2 in. to
6-13/16 in. diameter. This proportion of taper is a good one; if the taper
be less, or if a portion of the piston rod within the cross head eye be
left untapered, as is sometimes the case, it is very difficult to detach
the parts from one another.
449. _Q._--Which is the most beneficial construction of slide valve?
_A._--The best construction of slide valve appears to be that adopted by
Messrs. Penn for their larger engines, and which consists of a three ported
valve, to the back of which a ring is applied of an area equal to that of
exhaustion port, and which, by bearing steam tight against the back of the
casing, so that a vacuum may be maintained within the ring, puts the valve
in equilibrium, so that it may be moved with an inconsiderable exercise of
force. The back of the valve casing is put on like a door, and its internal
surface is made very true by scraping. There is a hole through the valve so
as to conduct away any steam which may enter within the ring by leakage,
and the ring is kept tight against the back of the casing by means of a
ring situated beneath the bearing ring, provided with four lugs, through
which bolts pass tapped into bosses on the back of the valve; and, by
unscrewing these bolts,--which may be done by means of a box key which
passes through holes in the casing closed with screwed plugs,--the lower
ring is raised upwards, carrying the bearing ring before it. The rings must
obviously be fitted over a boss upon the back of the valve; and between the
rings, which are of brass, a gasket ring is interposed to compensate by its
compressibility for any irregularity of pressure, and each of the bolts is
provided with a ratchet collar to prevent it from turning back, so that the
engineer, in tightening these bolts, will have no difficulty in tightening
them equally, if he counts the number of clicks made by the ratchet. Where
this species of valve is used, it is indispensable that large escape valves
be applied to the cylinder, as a valve on this construction is unable to
leave the face. In locomotive engines, the valve universally employed is
the common three ported valve.
450. _Q._--Might not an equilibrium valve be so constructed by the
interposition of springs, as to enable it to leave the cylinder face when
an internal force is applied?
_A._--That can no doubt be done, and in some engines has been done. In the
screw steamer Azof, the valve is of the equilibrium construction, but the
plate which carries the packing on which the top ring rests, is an octagon,
and fits into an octagonal recess on the back of the valve. Below each side
of the octagon there is a bent flat spring, which lifts up the octagonal
plate, and with it the packing ring against the back of the valve casing;
and should water get into the cylinder, it escapes by lifting the valve,
which is rendered possible by the compressibility of the springs. An
equivalent arrangement is shown in figs. 39 and 40, where the ring is
lifted by spiral springs.
[Illustration: Fig. 39. EQUILIBRIUM GRIDIRON SLIDE VALVE. Longitudinal
Section. Scale 3/4 inch = 1 foot.]
451. _Q._--What species of valve is that shown in figs. 39 and 40?
[Illustration: Fig. 40. EQUILIBRIUM GRIDIRON SLIDE VALVE. Back View with
Ring removed. Scale 3/4 inch = 1 foot.]
_A._--It is an equilibrium gridiron valve; so called because it lets the
steam in and out by more than one port. A A are the ordinary steam
passages to the top and bottom of the cylinder; B B is the ring which rubs
against the back of the valve casing, and D is the eduction passage, S S S
S shows the limits of the steam space, for the steam penetrates to the
central chamber S S by the sides of the valve. When the valve is opened
upon the steam side, the cylinder receives steam through both ports at that
end of the cylinder, and both ports at the other end of the cylinder are at
the same time open to the eduction. The benefit of this species of valve
is, that it gives the same opening of the valve that is given in ordinary
engines, with half the amount of travel; or if three ports were made
instead of two, then it would give the same area of opening that is given
in common engines with one third the amount of travel. For direct acting
screw engines this species of valve is now extensively used.
452. _Q._--Will you describe the configuration and mode of attachment of
the eccentric by which the valve is moved?
_A._--In marine engines, whether paddle or screw, if moving at a slow rate
of speed, the eccentric is generally loose upon the shaft, for the purpose
of backing, and is furnished with a back balance and catches, so that it
may stand either in the position for going ahead, or in that for going
astern. The body of the eccentric is of cast iron, and it is put on the
shaft in two pieces. The halves are put together with rebated joints to
keep them from separating laterally, and they are prevented from sliding
out by round steel pins, each ground into both halves; square keys would
probably be preferable to round pins in this arrangement, as the pins tend
to wedge the jaws of the eccentric asunder. In some cases the halves of the
eccentric are bolted together by means of flanges, which is, perhaps, the
preferable practice. The eccentric hoop in marine and land engines is
generally of brass; it is expedient to cast an oil cup on the eccentric
hoop, and, where practicable, a pan should be placed beneath the eccentric
for the reception of the oil droppings. The notch of the eccentric rod for
the reception of the pin of the valve shaft is usually steeled, to prevent
inconvenient wear; for when the sides of the notch wear, the valve movement
is not only disturbed, but it is very difficult to throw the eccentric rod
out of gear. It is found to be preferable, however, to fit this notch with
a brass bush, for the wear is then less rapid, and it is an easy thing to
replace this bush with another when it becomes worn. The eccentric catches
of the kind usually employed in marine engines, sometimes break off at the
first bolt hole, and it is preferable to have a bolt in advance of the
catch face, or to have a hoop encircling the shaft with the catches welded
on it, the hoop itself being fixed by bolts or a key. This hoop may either
be put on before the cranks in one piece or afterwards in two pieces.
453. _Q._--Are such eccentrics used in direct acting screw engines?
_A._--No; direct acting screw engines are usually fitted with the link
motion and two fixed eccentrics.
AIR PUMP AND CONDENSER.
454. _Q._--What are the details of the air pump?
_A._--The air pump bucket and valves are all of brass in modern marine
engines, and the chamber of the pump is lined with copper, or made wholly
of brass, whereby a single boring suffices. When a copper lining is used,
the pump is first bored out, and a bent sheet of copper is introduced,
which is made accurately to fill the place, by hammering the copper on the
inside. Air pump rods of Muntz's metal or copper are much used. Iron rods
covered with brass are generally wasted away where the bottom cone fits
into the bucket eye, and if the casing be at all porous, the water will
insinuate itself between the casing and the rod and eat away the iron. If
iron rods covered with brass be used, the brass casing should come some
distance into the bucket eye; the cutter should be of brass, and a brass
washer should cover the under side of the eye, so as to defend the end of
the rod from the salt water. Rods of Muntz's metal are probably on the
whole to be preferred. It is a good practice to put a nut on the top of the
rod, to secure it more firmly in the cross head eye, where that plan can be
conveniently adopted. The part of the rod which fits into the cross head
eye should have more taper when made of copper or brass, than when made of
iron; as, if the taper be small, the rod may get staved into the eye,
whereby its detachment will be difficult.
455. _Q._--What species of packing is used in air pumps?
_A._--Metallic packing has in some instances been employed in air pump
buckets, but its success has not been such as to lead to its further
adoption. The packing commonly employed is hemp. A deep solid block of
metal, however, without any packing, is often employed with a satisfactory
result; but this block should have circular grooves cut round its edge to
hold water. Where ordinary packing is employed, the bucket should always be
made with a junk ring, whereby the packing may be easily screwed down at
any time with facility. In slow moving engines the bucket valve is
generally of the spindle or pot-lid kind, but butterfly valves are
sometimes used. The foot and delivery valves are for the most part of the
flap or hanging kind. These valves all make a considerable noise in
working, and are objectionable in many ways. Valves on Belidor's
construction, which is in effect that of a throttle valve hung off the
centre, were some years ago proposed for the delivery and foot valves; and
it appears probable that their operation would be more satisfactory than
that of the valves usually employed.
456. _Q._--Where is the delivery valve usually situated?
_A._--Some delivery valve seats are bolted into the mouth of the air pump,
whereby access to the pump bucket is rendered difficult: but more commonly
the delivery valve is a flap valve exterior to the pump. If delivery valve
seats be put in the mouth of the air pump at all, the best mode of fixing
them appears to be that adopted by Messrs. Maudslay. The top of the pump
barrel is made quite fair across, and upon this flat surface a plate
containing the delivery valve is set, there being a small ledge all round
to keep it steady. Between the bottom of the stuffing box of the pump cover
and the eye of the valve seat a short pipe extends encircling the pump rod,
its lower end checked into the eye of the valve seat, and its upper end
widening out to form the bottom of the stuffing box of the pump cover. Upon
the top of this pipe some screws press, which are accessible from the top
of the stuffing box gland, and the packing also aids in keeping down the
pipe, the function of which is to retain the valve seat in its place. When
the pump bucket has to be examined the valve seat may be slung with the
cover, so as to come up with the same purchase. For the bucket valves of
such pumps Messrs. Maudslay employ two or more concentric ring valves with
a small lift. These valves have given a good deal of trouble in some cases,
in consequence of the frequent fracture of the bolts which guide and
confine the rings; but this is only a fault of detail which is easily
remedied, and the principle appears to be superior to that of any of the
other metallic air pump valves at present in common use.
[Illustration: Fig. 41. TRUNK AIR PUMP. Scale 3/4 inch to 1 foot.]
457. _Q._--Are not air pump valves now very generally made of india rubber?
_A._--They are almost invariably so made if the engines are travelling
fast, as in the case of direct acting screw engines, and they are very
often made of large discs or rings of india rubber, even when the engines
travel slowly. A very usual and eligible arrangement for many purposes is
that shown in fig. 41, where both foot and delivery valves are situated in
the ends of the pump, and they, as well as the valve in the bucket are made
of india rubber rings closing on a grating. The trunk in the air pump
enables guide rods to be dispensed with.
[Illustration: Fig. 42. PENN'S DISK VALVE FOR AIR PUMP. Section.]
[Illustration: Fig. 43. PENN'S DISK VALVE FOR AIR PUMP. Ground Plan.]
[Illustration: Fig. 44. MAUDSLAY'S DISC VALVE FOR AIR PUMP. Section.]
458. _Q._--The air pump, when double acting, has of course inlet and outlet
valves at each end?
_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 £1,900 to £2,200; 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?