THE LOCOMOTIVE BOILER.
QUESTION 82. _How does the quantity of steam generated in locomotive
boilers in a given time compare with that generated in the boilers of
stationary and marine engines?_
_Answer_. Locomotive engine boilers must produce much more steam in
a given time, in proportion to their size, than is required of the
boilers of any other class of engines, (excepting perhaps those of
steam fire-engines,) because the space which locomotive boilers can
occupy and also their weight is limited.
QUESTION 83. _How is their steam-generating capacity increased above
that of marine and stationary boilers?_
_Answer_. By creating a very strong draft of air through the fire and
then passing the smoke and heated air through a great many small tubes,
which are surrounded by water. By this means the smoke and hot air are
divided into many small streams or currents which are exposed to the
inside surface of the tubes to which and to the surrounding water their
heat is imparted.
QUESTION 84. _How is the action of the exhaust steam in producing a
draft in the chimney explained?_
_Answer_. The exhaust steam escapes from the cylinders through one or
two contracted openings or exhaust-nozzles (_f_, Plate II, also shown
in fig. 40[21]), which point directly up the centre of the chimney or
smoke-stack. The exhaust steam escapes from this orifice with great
velocity, and expands as it rises, so that it fills the pipe _p_ and
the smoke-stack _R R_. It thus acts somewhat like a plunger or piston
forced violently up the chimney, and pushes up the air above it, and,
owing to the friction of the particles of air, carries that which
surrounds it along up the stack, from which it all escapes finally into
the open air, thus leaving a partial vacuum behind in the smoke-box.
The external pressure of the atmosphere then forces in air through any
and every opening in the smoke-box, to take the place of that already
drawn out or exhausted from it. As the only inlet is through the tubes,
to which the gases of combustion have free access from the fire-box,
and as the external air can only pass through the fire-grate, and
through the burning fuel, to reach the fire-box, there is a constant
draft of air through the grate as long as the waste steam escapes from
the blast-pipe and up the chimney. It is thus that, within certain
limits, the more the steam that is required, the more the steam that is
produced; for all the steam used in the engine draws in the air in its
final escape, to excite the fire to generate more steam.[22] Sometimes
one blast-orifice is used for each cylinder, as shown in plates II and
III and fig. 40; in other cases the exhaust steam from each cylinder
escapes through the same orifice.
[21] The term _blast-orifice_ is also often used to designate these
parts of locomotives.
[22] Colburn’s Locomotive Engineering.
QUESTION 85. _How much water is it necessary to evaporate in order
to furnish the steam required to run an ordinary train at its usual
speed?_
_Answer._ For an ordinary “American” locomotive,[23] weighing 60,000
lbs. and with cylinders of 16 inches diameter and 24 inches stroke,
from 6,000 to 12,000 lbs. of water must be evaporated per hour.
[23] In speaking of “American” locomotives, we mean locomotives like
that shown in Plate I, with four driving-wheels and a four-wheeled
truck, and shall so use the term hereafter.
QUESTION 86. _How much water will a pound of coal evaporate in ordinary
practice?_
_Answer._ The quantity of water which is converted into steam by a
pound of coal varies very materially with the quality of the coal, and
the construction and condition of the boiler; but from 6 to 8 lbs. of
water per pound of coal is about the average performance of ordinary
locomotives. It is, therefore, necessary to burn from 500 to 2,000 lbs.
of coal per hour in order to generate the quantity of steam required by
ordinary engines.
QUESTION 87. _How large a grate is needed to burn this quantity of
coal?_
_Answer._ The maximum rate of combustion may be taken at about 125 lbs.
of coal on each square foot of grate surface per hour, so that to burn
2,000 lbs. we need a grate with about 16 square feet of surface.
QUESTION 88. _How much heating surface is needed for a given size of
grate?_
_Answer._ In common practice about 50 square feet of heating surface
are given for each square foot of grate. There are, however, no reasons
for the proportions of either grate or heating surface which are given,
excepting that it has been found that they work well in practice. It
is, however, quite certain that the larger a boiler is, and the greater
its heating surface in proportion to the steam it must generate, other
things being equal, the more economical will it be in its consumption
of fuel, or, in other words, the more water will it evaporate per pound
of coal.
QUESTION 89. _Why is it necessary to use small tubes or flues in order
to have the required amount of heating surface?_
_Answer_. Because there is a great deal more surface in a small tube
of a given length, in proportion to the space it occupies, than in a
large one. Thus a tube _two_ inches in diameter and eleven feet long
has 829 square inches of surface, and one _four_ inches in diameter has
1,658 square inches, or just double the quantity. But the four-inch
tube occupies _four times_ as much space as the other, as it is twice
as high and twice as wide. Therefore, in proportion to the space it
occupies, the tube which is two inches in diameter has twice as much
surface as the larger one. If we compare a two-inch with an eight-inch
tube, we will find that the former has _four_ times as much surface,
in proportion to its size, as the eight-inch tube. As the size and
weight of locomotive boilers are limited, it is therefore necessary, in
order to get the requisite heating surface in the space to which we are
confined, to use tubes of small diameter.
Small tubes also have the advantage that they may be made of thinner
material, and yet have the same strength to resist a bursting pressure
from within, or a collapsing pressure from without, as larger tubes
made of thicker metal. The advantage of thin tubes is, that the heat
inside of them is conducted to the water outside more rapidly than it
would be through thicker metal, which is important when combustion is
as rapid as it is in locomotive boilers.
The reason tubes of smaller diameter than two inches are not ordinarily
used is because they are then liable to become stopped up with cinders
and pieces of unconsumed fuel.
QUESTION 90. _How is the fire-box of a locomotive constructed?_
_Answer._ It usually consists of a rectangular box (_G_, figs. 41 and
42) about three feet wide[24] and, for the size of engine we have
selected as an example, about five or five and a half feet long inside.
This box is composed of metal plates, either iron, steel or copper,
which, excepting on the front side, are from ⁵⁄₁₆ to ³⁄₈ of an inch
thick. This box is called the _inside shell_ of the fire-box, and is
surrounded by another shell, _A B C D E F_, fig. 42, of either iron
or steel plates, of about the same thickness as those composing the
inside. This is called the _outside shell_ of the fire-box and, as
already explained, is so much larger than the inside that there is a
space, called the _water-space_, from 2¹⁄₂ to 4¹⁄₂ inches wide, on all
the sides of the fire-box between the inner and outer plates.
[24] The width is dependent upon the distance between the rails, or
_gauge_ of the road, as it is called. The above size is for a 4 feet
8¹⁄₂ in. gauge.
The top _g_, _g_, of the inside shell, which is called the
_crown-sheet_ or _crown-plate_, is flat, whereas the outside shell is
arched, as shown in fig. 42. To the front plate of the inside shell the
tubes _a a′_, _a a′_ are attached. For this reason its thickness is
usually made greater than that of the other plates, and is usually from
³⁄₈ to ³⁄₄ of an inch. The edges of one of the plates at each corner
of the fire-box, where they are united together, as shown in figs. 41
and 42, are bent at right angles, and the other is fastened to it with
rivets from ⁵⁄₈ to ³⁄₄ of an inch in diameter.
The inside and the outside shells of the fire-box are united to each
other by a wrought-iron bar or _ring_ (_A F_, figs. 41 and 42) which
completely surrounds the inner shell and closes the water-space between
the two shells. This bar is bent and welded to the proper form to
extend around the bottom of the inside fire-box, and it is riveted to
both shells. The water in the water-space is in free communication
with the rest of the water in the boiler; and thus the flat sides of
the respective shells of the fire-box are exposed to the full pressure
of the steam, which tends to burst the outside shell and collapse the
inside one. These flat sides, by themselves, would be unable to resist
the strain upon them, but as the strain upon the respective fire-boxes
is in opposite directions, and necessarily equal for equal areas of
surface, tie-bolts, _n_, _n_, _n_, _n_, (figs. 41 and 42,) or, as they
are called _stay-bolts_, which are from ³⁄₄ to 1 inch in diameter,
are screwed through the plates at frequent intervals, usually from
3¹⁄₂ to 4¹⁄₂ in. apart, so as to connect the two fire-boxes securely
together, the ends of the stay-bolts being also riveted or spread out
by hammering so as still further to increase their holding power. These
bolts, owing to the expansion and contraction of the boiler and other
strains to which they are subjected, very frequently break, and if they
are made of solid bars of metal there is no way of discovering with
certainty whether they are in good condition or not without taking the
boiler to pieces. They should therefore be made of the best quality of
wrought iron, brass or copper and should also be made tubular, that is
they should have a hole through the centre, so that when they break the
water will escape at the fracture into the hole and the leak will thus
indicate the defect and danger. The latter is much greater from this
cause than is usually supposed, and it is not unusual to find on taking
a boiler to pieces that a large number of the stay-bolts are broken.
[Illustration: _Fig. 41._
Scale ¹⁄₄ in. = 1 foot.]
[Illustration: _Fig. 42._
Scale ¹⁄₄ in. = 1 foot.]
QUESTION 91. _How can the strain on the flat surface of a boiler
between the stay-bolts be calculated?_
_Answer_. By MULTIPLYING THE AREA IN INCHES BETWEEN ADJACENT
STAY-BOLTS BY THE PRESSURE. The reason for this is, that each stay-bolt
must sustain the pressure on a part of the plate to which it is
attached. Thus in fig. 43 it is plain that the bolt _s_ must sustain
the pressure on one-half of that part of the plate between it and the
bolts _v_, _t_, _w_, _u_, around it, or the pressure on the square _a
b d c_, whose sides are equal to the distance (4 inches) between the
centres of the bolts. With a pressure of 100 pounds per square inch,
the calculation would therefore be: 4 × 4 × 100 = 1,600 lbs. on each
bolt.
[Illustration: Fig. 43. Scale 1¹⁄₂ in. = 1 foot.]
Stay-bolts should never be subjected to a strain of more than
one-eighth or one-tenth of their breaking strength.
QUESTION 92. _How do stay-bolts often fail without breaking?_
_Answer._ By tearing or _stripping_ the thread of the bolt, or that
in the plate, but oftener perhaps by the stretching of the plates
between the holes. With a heavy pressure, the tendency of the plates
between the holes, especially if they are heated very hot, is to
“bulge” outward and thus stretch the hole in every direction until
it is so large that the bolt is drawn out without much injury to the
screw-thread.
QUESTION 93. _How is the flat-top or crown-sheet strengthened?_
_Answer._ It is sometimes strengthened with stay-bolts similar to those
used for the sides, which pass through the inner and outer shells;[25]
but usually the crown-sheet is strengthened by a series of iron bars,
(_f_, _f_, fig. 41 and 42) called _crown-bars_, placed on edge, and of
considerable depth, which are firmly fastened to it by T-head rivets
or bolts. The crown-sheet can therefore only be crushed downwards by
bending these bars, which are of great strength. They usually extend
crosswise of the length of the fire-box, but are sometimes placed
lengthwise. These bars bear on the fire-box only at each end, as shown
in fig. 42, and are usually made with a projection, _k_, _k_, which
rests on the edge of the side plates. Iron rings or washers from ³⁄₄ to
1¹⁄₂ inches thick are interposed between the plate and the bars at the
points where the bolts or rivets which secure the rivets pass through.
This permits the water to circulate under the bars, and prevents the
crown-sheet from being burnt or overheated, as it would be if the
water were excluded from the whole under surface of the crown-bars.
[26]The crown-bars are also attached to the outer shell and the dome by
_braces_, _e_, _e_, _l_, _l_.
[25] This method of staying crown-sheets has been extensively used on
the Baltimore & Ohio and Reading railroads, and is now very generally
used in Europe.
[26] Colburn’s Locomotive Engineering.
The opening _c_, fig. 41, at the back end is for the door through which
fuel is supplied to the grate.
QUESTION 94. _How are the grates constructed?_
_Answer._ They are generally made of cast-iron bars, and for burning
coal are usually arranged so that the fire can be shaken by moving
the bars. For burning anthracite coal, the grates are sometimes made
of wrought-iron tubes, through which a current of water circulates to
prevent them from being overheated.
QUESTION 95. _How are cinders and burning coals prevented from falling
through the grate upon the road?_
_Answer._ By attaching a sheet-iron receptacle or ash-pan (_d′ d′_,
fig. 41) as it is called, under the grate, which it completely encloses
from the outside air. This then serves two purposes, as it is often
important when the engine is standing still to prevent any access of
air to the fire-box, and therefore the ash-pan is made to fit tightly
to the fire-box. Suitable doors, or _dampers_ as they are called, are
placed in front and behind, and sometimes on the sides, which can be
opened or closed to admit or exclude air as may be needed.
QUESTION 96. _How are the tubes or flues of a locomotive arranged?_
_Answer._ They are fastened into accurately drilled holes in the tube
sheet (_a_, _a_, figs. 41 and 42) which forms the front of the fire-box
and in similar holes in a plate (_a′_, _a′_, fig. 41,) which forms the
front end of the cylindrical part of the boiler. They thus connect
the fire-box with the smoke-box. The tubes are arranged so that each
tube will have a space of from ⁵⁄₈ to ⁷⁄₈ of an inch between it and
those adjoining. The position of the holes for the tubes in relation
to each other is determined by describing from the centre of one tube
(_o_, fig. 44) a circle with a radius, _o k_, equal to the sum of
the diameter of a tube and the distance which they are intended to
be apart, and then subdividing this circle with the radius into six
parts, _k_, _r_, _s_, _l_, _g_ and _p_. Each point of subdivision and
also the centre, _o_, of the circle will be the centre of a tube. By
drawing them from these centres it will be found that the distances
_a b_, _c d_ between adjoining tubes will be the same between all of
them. By describing circles from the centres of the outside tubes and
subdividing the circles as before the position of other tubes will
be determined around those first laid down. This can, of course, be
carried out indefinitely. A difference in the arrangement of the tubes
will be observed if, when we subdivide the first circle shown in fig.
44, instead of commencing from the intersection of a vertical line we
begin from a horizontal line, _h i_, as shown in fig. 45. In the former
case the tubes are said to be in _vertical rows_, and in the latter in
_horizontal rows_. It is apparent from the figures and as shown by the
arrows that the water can circulate in ascending currents more freely
when tubes are arranged in vertical rows than when they are arranged
horizontally.
[Illustration: Fig. 44. Scale ¹⁄₄.]
[Illustration: Fig. 45. Scale ¹⁄₄.]
QUESTION 97. _How are the tubes fastened and made water-tight in the
tube-sheets?_
[Illustration: Fig. 46. Scale ¹⁄₄.]
[Illustration: Fig. 47.]
_Answer._ They are inserted into the holes drilled to receive them, and
the ends are allowed to project about a quarter of an inch beyond the
tube-sheets. A tapered plug, fig. 46, is then driven into the tube,
to expand it so that it will fit the hole. A tool is used called a
_tube-expander_, fig. 47, which is what might be called an expanding
plug, consisting of a number of sections, _a_, _b_, _c_, _d_, _e_, _f_,
_g_, _h_, held together by a spring clasp _s_, which embraces them, as
shown in the engraving. This plug when the sections are drawn together
is inserted into the mouth of the tube, and the tapered plug _p p_,
is then driven into the opening left in the center of the cluster of
sections, which are thus expanded. By this means, the ridge _a b c
d_ expands the tube at the inner edge of the tube-sheet, forming a
ridge or corrugation, as shown at _c c_, fig. 48. At the same time
the shoulder _j k l_ on the tool expands the outer edge of the tube
somewhat as is shown at _f f_, fig. 48. By repeating this process,
and slightly turning the expander each time, the tubes can be made
perfectly water-tight. There are other forms of tube-expanders, but the
one described, known as Prosser’s expander, is more generally used than
any other. In many cases, after the tubes are expanded with the tool
described, the outer edge is turned over still more with what is called
a _thumb-tool_, fig. 49, probably from its resemblance in form to a
man’s thumb. By placing the curved shoulder _a_ on the end _f_, fig.
48, of the tube it is turned over, somewhat in the form shown in the
engraving, by repeated blows of a hammer on the end of the tool. Copper
ferrules, represented by the black shading, _a a_, are also much used
now on the outside of locomotive tubes, and it is said that with them
the joints can be kept tight much easier than without. By turning over
the outside edge of the tube as shown in fig. 48, it not only protects
the copper ferrule, but, as the tubes must act as braces to sustain
the pressure of steam in the flat tube-sheets, it gives the joints the
requisite strength for resisting such strains.
[Illustration: Fig. 48. Scale ¹⁄₄.]
[Illustration: Fig. 49.]
QUESTION 98. _How can the strain on the cylindrical part of a boiler be
calculated?_
_Answer._ BY MULTIPLYING THE DIAMETER IN INCHES BY THE LENGTH IN INCHES
AND THE PRODUCT BY THE STEAM-PRESSURE PER SQUARE INCH. Thus for a
boiler 48 inches in diameter and 10 feet long with 100 pounds pressure
the calculation would be 48 × 120 × 100 = 576,000 lbs.
QUESTION 99. _Why do we multiply the diameter, instead of the
circumference, by the length, to get the strain on the cylindrical
part?_
_Answer._ The reason for multiplying by the diameter instead of by the
circumference is because only a portion of the pressure on the inside
surface of the boiler exerts a force to burst the shell at any one
point. Thus, supposing the following diagram, fig. 50, to represent
a section of a boiler, if we have a force acting on the shell in the
direction of the line _a b_, at the point _b_, where it is exerted
against the shell of the boiler, it would be composed of two forces,
one acting in the direction _b e_, and tending to tear the boiler
apart on the line _c d_, and the other acting in the direction _f b_,
to tear it apart on the line _h g_. It is so with all pressure inside
the boiler, excepting that, say _a h_, which acts exactly at right
angles to the line of rupture _c d_, it is all composed of two forces,
only one of which tends to tear the boiler apart at one point. It is
therefore only a part of the pressure on the circumference which tends
to burst the boiler at a given place, and that part is equivalent to
the pressure on a surface whose width is equal to the diameter and not
the circumference.
[Illustration: Fig. 50.]
[Illustration: Fig. 51.]
This we know is a little difficult for those to understand who are
not familiar with the principles of what is called the “resolution of
forces,” and we will therefore try to make it clear in another way.
To do this we will suppose that we have a boiler, _a b_, fig. 51,
made in two halves and bolted together at _a_ and _b_ by flanges. It
is evident that if we brought a pressure against the inside of the
flanges in the direction of the darts _c_ and _d_, such a pressure
would not have a tendency to tear apart the bolts _a_ and _b_. Some
distortion of the boiler might in fact take place, if, for example,
we put a jack-screw inside and forced out the flanges as indicated,
without subjecting the bolts to a tensile strain. We see therefore that
the forces acting in the direction _c_ and _d_ have no tendency to tear
apart the bolts at _a_ and _b_, but it is only the forces such as _e_,
_f_ and _g_, which act at right angles to _a b_, that exert a strain on
the flanges.
That this force is equivalent to a pressure on a surface with a width
equal to the diameter of the boiler is apparent if we suppose that we
have a boiler, _a b_, fig. 52, and that each half, _c_ and _d_, is
nearly filled with some substance, say wood or cement, which is fitted
so tight that no steam can get between it and the shell of the boiler.
It is apparent now that if we admit steam into the space _f_, the force
exerted on the bolts _a_ and _b_ is that due to the pressure on the
surface of the wood or cement exposed to the steam whose width is
equal to the diameter of the boiler. It might be said though that if
this substance were elastic, like india-rubber, the effect of the steam
would be different. If it were elastic, and a pressure on the surfaces
_f_ caused it to spread in the direction _g_ and _h_ so as to produce
a pressure in those directions, it would, as has already been shown,
not exert a force on the bolts _a_ and _b_ to tear them apart, but have
a tendency to rupture the boiler at right angles to _a b_. The sides
of the boiler must therefore have a strength sufficient to resist this
force which tends to tear them asunder. If the boiler is made of iron
³⁄₈ inch thick there would be a sectional area of 45 square inches on
each side, or a total of 90 square inches, to resist this strain, so
that each square inch must bear 6,400 lbs. of strain. The correctness
of this rule can be demonstrated by the use of mathematics, which would
be out of place here. Its practical truth has however been proved by
experiment.
[Illustration: Fig. 52.]
QUESTION 100. _How much strain per square inch is good boiler plate
capable of resisting, and how much is it safe to subject it to?_
_Answer._ There is great variation in the tensile strength[27] of
rolled iron boiler plate, but that of good plate will average about
50,000 pounds per square inch, if the strain is applied in the
direction of the “grain” or the fibres of the iron[28], and about ten
per cent. less if the strain is applied crosswise of the grain. It
has, however, been found by experiment that when a tensile strain is
applied to a bar of iron or other material, it is stretched a certain
amount in proportion to the length of the bar and to the degree of
strain to which it is subjected. It is found that if this strain does
not exceed about one-fifth of that which would break the bar, it will
recover its original length, or will contract after being stretched,
when the strain is removed. The greatest strain which any material
will bear without being permanently stretched is called its _limit
of elasticity_, and so long as this is not exceeded no appreciable
permanent elongation or “set” will be given to iron by any number
of applications of such strains or loads. If, however, the limit of
elasticity is exceeded, the metal will be permanently elongated, and
this elongation will be increased by repeated applications of the
strain until finally the bar will break. At the same time the character
of the metal will be altered by the repeated application of strains
greater than its elastic limit, and it will become brittle and less
able to resist a sudden strain, and will ultimately break short off.
It is therefore unsafe to subject iron, or in fact any other material,
to strains greater than its elastic limit. This limit for iron boiler
plates may be taken at about one-fifth its breaking, or, as it is
called, _ultimate_ strength. It should be remembered, however, in
this connection, that it often happens that the steam pressure is not
the greatest force the boiler must withstand, as sudden or unequal
expansion and contraction are probably more destructive, to locomotive
boilers especially, than the pressure of the steam.
[27] A force exerted to pull any material apart is called a _tensile
strain_, and if exerted to compress it is called a _compressive
strain_.
[28] It should be explained that in the process of manufacturing iron
by rolling, the iron is stretched out into fibres in the direction in
which it passes between the rolls.
QUESTION 101. _How are the plates of boilers fastened together?_
_Answer._ With rivets, which are made with a head at one end, and are
inserted while they are red-hot into holes drilled or punched in the
edges of the plates. After they are in the holes a head is formed on
the other end, either with blows from hand hammers, or by a machine
constructed for the purpose. In these machines the rivet after it is
in the holes is brought between a fixed and a movable die, the head
which is made with the rivet being placed against the fixed die, and
the movable die is then pressed, either by steam or hydraulic pressure
with great force against the other end of the rivet, thus forcing the
end of the rivet into the form of the die, which is made of the proper
shape and size for the rivet head. The powerful pressure which is thus
brought on the rivet causes it to be pressed into all parts of the two
holes, thus completely filling them both; whereas with hand riveting,
the holes are not nearly so completely filled, as it is impossible with
blows of a hammer to subject the rivets to so powerful or uniform a
pressure as the machine brings upon them.
[Illustration: _Fig. 53._]
QUESTION 102. _What is the strength of riveted seams compared with that
of the solid plate?_
_Answer._ The strength of a riveted seam depends very much upon the
arrangement and proportion of the rivets, but with the best design
and construction, the seams are always weaker than the solid plates,
as it is always necessary to cut away a part of the plate for the
rivet holes, which weakens the plate in three ways: 1. By lessening
the amount of material to resist the strains. 2. By weakening that
left between the holes. 3. By disturbing the uniformity of the
distribution of the strains. The first cause of weakness is obvious
from an inspection of an ordinary seam, riveted with a single row of
rivets, fig. 53. In this we have two plates 7¹⁄₂ inches wide and ³⁄₈
thick fastened with four rivets ¹¹⁄₁₆ inches in diameter and 1⁷⁄₈
inches from centre to centre. The section of the plate calculated with
decimals[29] would therefore be .375 × 7.5 = 2.81 square inches. A
piece ¹¹⁄₁₆ inch wide and ³⁄₈ inch thick would be removed to form each
hole, or a sectional area for the whole plate of .375 × .6875 × 4 =
1.03 square inches, so that the section of the plate would be reduced
through the holes 2.81 - 1.03 = 1.78 square inches. In other words,
on the dotted line _a b_ it will have only about 63 per cent. of the
sectional area of the solid plate.
[29] In the following calculations all the dimensions have for
convenience been reduced to decimals.
The second cause of the reduction of strength is owing to the injury
sustained by the plates during the process of drilling and punching.
The knowledge existing regarding this subject is not very accurate,
although numerous experiments have been made to determine the exact
amount of weakening caused by punching plates. It is, however, certain
that in many cases the strength of the metal _left_ between the holes
of boiler plates is reduced from 10 to 30 per cent. by the process of
punching. It is probable, however, that soft ductile metal is injured
less than that which is harder and more brittle. Some kinds of steel
plates are especially liable to injury from punching. It is also
probable that the condition of the punch, and the proportions of the
die used with it, have much to do with its effect upon the metal
The third cause of weakness is owing to the fact that if one or more
holes are made in a plate of any material, and it is then subjected
to a tensile strain, the strain, instead of being equally distributed
through the section left between the holes, will be greatest in that
part of the metal nearest them. This can be illustrated by taking
a band of india-rubber, fig. 54, and cutting a round hole in it to
represent a rivet hole. If we draw two parallel lines, _A B_, across
the band and then stretch it, the lines, instead of remaining parallel
when the band is stretched, will separate most next to the hole, as
shown in fig. 55, indicating that the fibres of the rubber nearest the
hole are strained most. A similar effect takes place when a plate of
iron is stretched, so that a fracture is liable to begin next to the
hole, after which the plate will be broken as it were in detail.
[Illustration: _Fig. 54._]
[Illustration: _Fig. 55._]
QUESTION 103. _How may a boiler seam like that shown in fig. 53 break?_
_Answer._ It may break in three different ways:
1. By the plate tearing between the rivet holes on the line _a b_.
2. By the rivets shearing off.
3. By the plate in front of the rivets crushing, as shown in fig. 56.
[Illustration: _Fig. 56._]
QUESTION 104. _How can the strength of a boiler seam be calculated at
each of these three points?_
_Answer._ The strength through the rivet holes is calculated by TAKING
THE AREA IN SQUARE INCHES OF THE METAL WHICH IS LEFT BETWEEN THE RIVET
HOLES, AND MULTIPLYING IT BY THE ULTIMATE STRENGTH OF THE METAL AFTER
THE HOLES ARE MADE. Thus, in fig. 53, the area of each of the plates
between the rivet holes is 1.78 square inches. As already stated, good
boiler plate will break at a strain of about 50,000 pounds in the
direction of its fibres,[30] but the strength of the metal left between
punched holes is probably 20 per cent. and that between drilled holes
10 per cent. less than that of the solid plate. We must, therefore, in
calculating the strength of a punched seam, take the ultimate strength
of the metal between the holes at only 40,000 pounds per square inch.
The calculation for the strength through the holes would therefore be:
1.78 × 40,000 = 71,200 pounds.
[30] Boiler plates should always be so arranged that the greatest
strain will come on them in the direction of their greatest strength,
which is parallel with the fibres of the metal.
It has also been found by experiment that the strength of rivets to
resist shearing is about the same as that of good boiler plate to
resist tearing apart, or 50,000 lbs. per square inch. The strength of
the rivets, therefore, is calculated by MULTIPLYING THE AREA IN SQUARE
INCHES OF ONE RIVET BY THE NUMBER OF RIVETS, AND THE PRODUCT BY THE
STRENGTH OF THE METAL TO RESIST SHEARING. The calculation for fig. 53
would therefore be:
Area of ¹¹⁄₁₆ rivet = .3712 × 4 × 50,000 = 74,240.
or a little more than the strength of the plates through the holes.
The resistance offered by a plate to the crushing strain of a rivet
has been found also by experiment to be about 90,000 pounds per square
inch. It can be proved that the area which resists the crushing strain
of a rivet in a plate, fig. 53, IS MEASURED BY MULTIPLYING THE DIAMETER
OF THE RIVET BY THE THICKNESS OF THE PLATE. The calculation for the
strength of this part of the seam will therefore be: diameter of hole =
.6875 × .375 × 4 × 90,000 = 92,812.
The strength of the solid plate would be EQUAL TO ITS SECTIONAL AREA
MULTIPLIED BY 50,000 POUNDS, or 7.5 × .375 × 50,000 = 140,625 pounds.
The ultimate strength of our seam would then be as follows:
Plates through rivet holes (tearing) = 71,200 lbs.
Rivets (shearing) = 72,240 lbs.
Plates in front of rivets (crushing) = 92,812 lbs.
Solid plate (tearing) = 140,625 lbs.
It will thus be seen that the strength of the weakest part of the above
seam, fastened with a single row of rivets in punched holes, is very
little more than half (50.6 per cent.) of that of the plates. It will
be noticed that the weakest part of the seam is the plates between the
holes.
[Illustration: _Fig. 57._]
[Illustration: _Fig. 58._]
[Illustration: _Fig. 59._]
[Illustration: _Fig. 60._]
QUESTION 105. _How can the strength of such a single-riveted boiler
seam be increased?_
_Answer._ The most obvious way of increasing the strength of such a
seam is to place, or, as it is called, _space_, the rivets further
apart, which would leave more metal between the holes, and thus
strengthen the seam at its weakest part. But if this is done, it is
said that there is difficulty in keeping the seam water-tight, as the
plates are then liable to spring apart between the rivets. Another way
of increasing its strength is to drill the rivet holes. As already
stated, the difference in the strength of the metal left between
drilled and punched holes has been shown to be from 10 to 20 per cent.
There is also another advantage in drilling the holes for rivets. In
punching them, it is necessary to punch each plate separately, and
even with the utmost care and skill it is impossible to get the holes
to match perfectly. Some of them will overlap each other, as shown in
fig. 57, so that when the rivet is set, it will assume somewhat the
form shown in fig. 58. There is then danger that those rivets which
fill the holes that match each other will be subjected to an undue
strain. If, for example, we have five rivet holes, as shown in fig. 59,
and only the centre ones correspond with each other, then the rivets
in all the other holes will assume somewhat the form shown in fig. 58,
and therefore the centre rivet _c_, in fig. 59, which fits the holes
accurately, must take the strain of the other four until they draw
up “to a bearing.” Under such circumstances, which are not unusual,
there will be great danger either of shearing off the rivet _c_, or
of starting a fracture in the plates, as indicated by the irregular
line _a b_, between the adjoining rivets. It is also obvious that a
rivet like the one in fig. 58 will not hold the plates together so
well as one which fits more perfectly, as shown in section in fig.
53, and therefore there is more danger of leakage between the plates
from badly fitted rivets than from those which fill the holes more
perfectly; consequently rivets which fit imperfectly must be placed
nearer together than those which are well fitted. It is true that
rivets which are set with a riveting machine fill any inaccuracies of
the holes more perfectly than those which are set by hand. But even if
they are made to fill the holes as shown in fig. 60 they are still not
so strong to resist shearing nor so efficient in holding the plates
together as they would be if the holes conformed more perfectly to each
other. In drilling the holes, the second plate can be drilled from the
holes in the first, so that the holes in each will correspond with
each other perfectly. The rivets will therefore fit more accurately,
and consequently can be spaced further apart, and still keep the
plates tight, and thus have more material between the holes, which is
the weakest part of the seam. It has been shown that a rivet ¹¹⁄₁₆
inch in diameter has a resistance to shearing of 18,560 pounds. There
is therefore no advantage in spacing such rivets further apart than
1¹³⁄₁₆ from centre to centre, because the metal left between drilled
holes that distance apart would be slightly stronger than the rivets.
If therefore the rivets are placed further apart, their diameter must
be increased. There is, however, a limit beyond which the diameters
of rivets cannot be increased with advantage, because if we increase
their diameters, their sectional area to resist shearing is increased
in proportion _to the square of the diameter_, whereas the section of
metal in the plate to resist crushing is increased only _in proportion
to the diameter_. This will be apparent if we compare a rivet ¹⁄₂ inch
with one 1 inch in diameter. The first has a sectional area of .1963
inch, the other .7854 inch, or _four_ times that of the first one. Now
the area which resists the crushing strain of the rivets is increased
only in proportion to their diameters, or is _twice_ as much for the
one as for the other. If, therefore, we increase the diameters of the
rivets, we very soon reach a point at which the plate has less strength
to resist crushing than the rivet has to resist shearing. The diameter
of rivet which will give just the same resistance to both strains
varies with the thickness of the plates; with ³⁄₈ inch plates a ⁷⁄₈
rivet will have a resistance to shearing of 30,065 pounds and the plate
in front of it a resistance to crushing of 29,530 pounds. A ⁷⁄₈ rivet
is, therefore, the largest size which can be used to advantage in ³⁄₈
plates. If now we were to space such rivets so far apart that the metal
left between the holes would have a strength just equal to that of the
rivets, we would have the strongest possible seam that can be made with
a single row of rivets. This distance would be 1³⁄₄ inches between the
edges of the rivets, or 2⁵⁄₈ from center to center, as shown in fig.
61. The following table will show the strength of such a seam composed
of four rivets, and two plates 10¹⁄₂ inches wide,[31] with drilled
holes:
Plates through rivet holes (tearing) 118,125 lbs.
Rivets (shearing) 120,260 lbs.
Plates in front of rivets (crushing) 118,125 lbs.
Solid plates (tearing) 196,875 lbs.
[31] It has been necessary to take for an illustration, plates of a
different width from the preceding example, in order to get an even
number of spaces between the rivets in each case.
[Illustration: _Fig. 61._]
From this it is seen that the strength of the seam with drilled plates
is 60 per cent. of that of the solid plates, or it is about 18¹⁄₂ per
cent. stronger than that made with plates having punched holes and the
rivets nearer together. It should be noted that a great part of the
superiority of the seams made with drilled holes is due to the superior
accuracy of the work done in that way, which makes it possible to
use larger rivets spaced further apart. It is probable that with the
use of some recently designed machines, intended to produce greater
accuracy in punching rivet holes, part of the above advantage may be
realized with that kind of work. The greatest distance that rivets
may be spaced apart without incurring danger of leakage between the
plates must, however, be determined more by practical than theoretical
considerations. It is certain, however, that rivets may be spaced much
further apart than they are in ordinary practice, and the seams still
be kept tight, if the work is done with sufficient accuracy and care.
QUESTION 106. _What other methods are there of making boiler seams
which are stronger than those which have been described?_
_Answer._ In this country two rows of rivets are used and also what is
called a “_welt_,” or _covering-strip_, the latter with both single and
double-riveted seams.
[Illustration: _Fig. 62._]
QUESTION 107. _How are the rivets arranged when two rows are used?_
_Answer._ They are often placed just behind each other as shown in fig.
62, which is called _chain-riveting_. Such an arrangement of rivets
obviously adds nothing to the strength of the seam, because its weakest
part, as has already been shown, is the section of the plates through
the rivet-holes, and this is not at all strengthened by adding another
row of rivets, because the plates are just as liable to break through
the rivet-holes with two rows of rivets as they would be with one.
[Illustration: _Fig. 63._]
A much better arrangement is to place them alternately in the two
rows, as shown in fig. 63. Rivets arranged in that way are said to be
_staggered_, or placed _zigzag_. The method of laying off the holes
and proportioning such a seam in order to get the most strength is,
first, to determine the greatest distance which can be allowed between
the rivet-holes, and yet keep the seams water-tight. Supposing this is
1⁵⁄₁₆ inches,[32] with plates ³⁄₈ inch thick, the diameter of rivet
whose sectional area will give an equal amount of strength must then be
calculated. From the preceding data it will be found that for drilled
plates the resistance of the portion left between such holes will be
22,148 pounds, and a rivet ³⁄₄ inch in diameter will have a resistance
of 22,085 pounds. On the line _a b_, fig. 63, which is to be the first
row of rivets, we will draw one hole, _c_. From the center of this hole
we will describe the arc of a circle, _d e_, with a radius, _c d_,
equal to the sum of the distance between the holes, 1³⁄₈ inches, and
one-half the diameter of the hole, or ³⁄₈ inch, making the radius 1³⁄₄
inches. From _d_, the intersection of this arc with the line of rivets,
_a b_, with the same radius, we will step off the distance _d f_ will
then be the centre of the second rivet-hole on the line _a_, _b_. The
rivet can therefore be drawn, and from its centre, with the same radius
employed before, another arc, _d g_, should be drawn. If now we draw
a rivet-hole (_h_) between the two arcs and touching each of them, we
will have all three of the holes so arranged that the metal between
the holes _c_, _f_, will be just equal to that between them and the
hole _h_. In other words, the strength of the plates on the line _c f_
is just the same as on the line _c h f_. Therefore the strength of the
plates on the straight line _a b_ is just the same as on the zigzag
line _c h f_, etc. The strength of this seam would therefore be as
follows:
Plates through rivet-holes (tearing) 110,742 lbs.
Rivets (shearing) 110,425 lbs.
Plates in front of rivets (crushing) 126,562 lbs.
Solid plates (tearing) 158,205 lbs.
[32] That and even greater distances between the edges of the holes
are now used successfully with ³⁄₈-inch plates.
The weakest portion of this seam, it will be seen, has a strength
of 70 per cent. of the plate, or is 38 per cent. stronger than a
single-riveted seam with punched holes. If we were to use ⁷⁄₈ inch
rivets and make the spaces between them 1³⁄₄ inches, the strength of a
similar seam would be as follows:
Plates through rivet-holes (tearing) 147,656 lbs.
Rivets (shearing) 150,325 lbs.
Plates in front of rivets (crushing) 147,656 lbs.
Solid plates (tearing) 205,078 lbs.
This seam would then have a strength of 72 per cent. of the solid
plates, or be 42¹⁄₄ per cent. stronger than the ordinary riveted seam
with punched holes. It is important to observe that an increase of
strength results from the use of larger rivets spaced farther apart
than is usual in ordinary practice, and that this is possible only with
the best and most accurate workmanship.
[Illustration: _Fig. 64._]
QUESTION 108. _What is the form of construction of boiler seams made
with a welt or covering-strip?_
_Answer._ The plates (_a_, _b_, fig. 64) are lapped over each other as
for an ordinary seam. Another plate, _c_, about nine inches wide, is
then placed on the inside of the seam and bent so as to conform to the
lap of the two plates. The rivets _r_, whether a double or single row,
pass through all three plates, and two more rows of rivets are put next
to the edges of the covering plate, _c_. It is plain that the strength
of the seam, _r_, is increased up to a certain point by an amount just
equal to that of the rivets in the edges of the covering plate. If,
however, these are placed too close together, the plates _a_ and _b_
will be weaker through the outside rows of rivets than the seam is
through either of the outside ones and the middle one taken together.
If, for example, we take a single-riveted seam, like that shown in fig.
53, whose strength is only a little more than half that of the solid
plate, and should add to it a covering plate, as shown in fig. 64,
and then space the rivets in the edges of the covering plate the same
distance apart as in the middle seam, then obviously the plates would
be just as liable to break through the outer rows of holes as through
the center row before the covering plate was added. If, however, the
holes in the two outside plates are spaced at say twice the distance
apart, or 3³⁄₄ inches, then the only way the seam can break through
the outer rows of holes is by shearing the rivets, because the plates
between the holes are then stronger than the rivets. But before these
rivets can be sheared, the centre seam must give way. Thus the strength
of such a seam is equal to THE SUM OF THE STRENGTH AT THE WEAKEST
POINTS OF THE MIDDLE AND THE OUTSIDE SEAMS. The strength of the plates
between the holes of the outside rows of rivets must, however, be as
great as the sum referred to, otherwise the seam will be the weakest
at that point, and the failure will occur there. The rivets in the
outside rows should be spaced at least twice as far apart as those in
the middle seam. The number of rivets to resist shearing will then be
50 per cent. greater, so that the strength of a seam like that shown in
fig. 53, with a covering plate added, will be as follows:
Plates through outside rows of rivet-holes (tearing) 91,880 lbs.
Rivets in one outside and middle row (shearing) 111,360 lbs.
Plates in front of rivets (crushing) 139,218 lbs.
Solid plates (tearing) 140,625 lbs.
An ordinary single-riveted seam with punched holes, with a welt or
covering plate added, would thus have a strength equal to 65.3 per
cent. of the solid plate, or be 29 per cent. stronger than the seam
without the covering plate. It is probable, however, that the injury
to the plate from punching the outside rows of holes which are further
apart is not so great as it is when they are punched nearer together
and to the edge, so that the strength is somewhat greater than our
estimate.
The relative strength of the different forms of seams described in
percentage of the strength of the solid plate is then as follows:
Percentage
of strength
compared with
solid plate.
Single-riveted seam, punched holes ¹¹⁄₁₆ rivets 50.6
Single-riveted seam, drilled holes ⁷⁄₈ rivets 60.
Double-riveted seam, drilled zigzag holes ³⁄₄ rivets 70.
Double-riveted seam, drilled zigzag holes ⁷⁄₈ rivets 72.
Single riveted seam, punched holes with covering plate 65.3
QUESTION 109. _Are there any other forms of boiler seams used?_
_Answer._ In Europe what are called _butt-joints_ are used to some
extent. In these the ends of the two plates abut against each other,
with a covering strip on one or both sides. This form of joint is,
however, not used in this country, and therefore its peculiarities will
not be described.
QUESTION 110. _How are the seams of boilers made water-tight?_
_Answer._ By what is called _caulking_. That is, by the use of a blunt
instrument somewhat resembling a chisel, the end of which is placed
against one or both of the edges of the plates _c_, _d_, fig. 53, which
are then riveted down by blows of a hammer, somewhat as the joints of a
ship are made tight. Before the edges, which are called the _caulking
edges_, of the plates are made tight in this way, they are cut or
trimmed off with a chisel. In this process the plate under the edge is
often injured seriously by the carelessness of workmen, who sometimes
allow the chisel to cut a groove in the plate under the edge, thus
weakening it at a point where the greatest strength is needed. There is
also danger of forcing the plates apart in the manner shown in fig. 65,
if it is done carelessly and with a heavy hammer.
[Illustration: _Fig. 65._]
QUESTION 111. _How are the flat ends of the boiler strengthened?_
_Answer._ By braces, _u_, _u_, _r_, _r_, fig. 41, which are fastened to
the end plates and to the outer shell of the boiler. They are fastened
at one end to ~⟂~ shaped pieces called _crow-feet_, which are riveted
to the end plates of the boiler. At the other end they are made with a
broad foot, which is riveted to the outer shell of the boiler. Especial
attention should be given to the form, proportion and arrangement
of these braces when a boiler is constructed, and they should be
frequently examined while the engine is in use, as they are liable to
be neglected or carelessly constructed and to become weakened or broken
by corrosion, or the constant strain to which they are subjected.
Great ignorance is often displayed in the design and proportions of
these braces, especially in their attachments to the shell of the
boiler.
QUESTION 112. _How should the braces for strengthening the ends of a
boiler be proportioned?_
_Answer._ Every part should be made equally strong. If, for example,
the brace itself is made of a bar of round iron one inch in diameter,
its sectional area would be .7854 square inch. The iron used for these
braces should be of such strength that a force of not less than 50,000
lbs. per square inch of transverse section should be required to tear
it apart lengthwise. A bar of the size referred to would therefore
require not less than 39,270 pounds to pull it apart. All the other
parts should be capable of resisting an equal strain. It is, for
example, not unusual to find a brace of the size we have described,
and even of larger diameter, fastened to a crow-foot which is attached
to the boiler plate with two rivets ⁵⁄₈ inches in diameter. A similar
fastening for the other end of the brace is also often used. The
sectional area of these two rivets is considerably less than that of
the brace, and at the same time the strain is brought upon them at such
an angle as to have a tendency to “snap” them off. For this reason,
and also because a rivet is apt to be deteriorated in strength by the
hammering, the rivets should always have a sectional area very nearly
or quite double that of the bar which forms the brace. The metal around
the eyes through which the pins are inserted should also be carefully
proportioned, and the transverse section at any one point should always
be at least 1¹⁄₂ times greater than that of the bar. The area of the
pins used for attaching the bars to the crow-feet should always be a
little more than half that of the bar. That is, an inch bar should have
a pin not less than ³⁄₄ inch in diameter. When flat braces are used,
it is not unusual to find a bar 3 inches wide with a hole an inch in
diameter punched or drilled so near the end that a pin is sure either
to pull out the end of the bar or else to break it crosswise at the
hole with a much less strain than the brace itself would resist. The
ends of braces should always be enlarged enough to give them sufficient
strength to resist as much strain as the bar itself. Although these
precautions may appear unimportant, and unfortunately are often so
regarded, yet it is upon just such details as these that the lives and
the safety of every locomotive runner, fireman and others near them are
constantly dependent.
QUESTION 113. _How much water is usually carried in a locomotive
boiler?_
_Answer._ There must always be enough water in the boiler to cover
completely all the parts which are exposed to the fire, otherwise they
will be heated to so high a temperature as to be very much weakened or
permanently injured. In order to be sure that all the heating-surface
will at all times be covered with water, it is usually carried so that
its surface will be from 4 to 8 inches above the crown-sheet.
QUESTION 114. _How much space should there be over the water for steam?_
_Answer._ No exact rule can be given to determine this. It may,
however, generally be assumed that the more steam space the better. In
order to increase the steam room, locomotive boilers are very generally
made in this country with what is called a _wagon-top, C_, fig. 41,
that is, the outside shell of the boiler over the fire-box is elevated
from 4 to 12 or even 18 inches above the cylindrical part.
QUESTION 115. _What is a steam-dome and for what purpose is it
intended?_
_Answer._ A _steam-dome_, _X_, fig. 41, is a cylindrical chamber made
of boiler-plate and attached to the top of the boiler. Its object is to
increase the steam room and to furnish a reservoir which is elevated
considerably above the surface of the water, from which the supply of
steam to be used in the cylinders can be drawn. The reason for drawing
the steam from a point considerably above the water is that during
ebullition more or less spray or particles of water are thrown up
and mixed with the steam. When this is the case, steam is said to be
_wet_, and when there is little or no unevaporated water mixed with it
it is said to be _dry_. It is found by experience that wet steam is
much less efficient than that which is dry. There is also danger that
the cylinders, pistons or other parts of the machinery may be injured
if much water is carried over from the boiler with the steam, because
water will be discharged so slowly from the cylinders that there is not
time for it to escape before the piston must complete its stroke, so
that the cylinder-heads will be “_knocked out_,” or the cylinder itself
or the piston will be broken. The reason for drawing or “_taking_”
steam from a point considerably above the water is because there is
less spray there than there is near the surface, and the hottest steam,
which is also the dryest, ascends to the highest part of the steam
space.
QUESTION 116. _Where is the dome usually placed?_
_Answer._ In this country it is usually placed over the fire-box, but
in Europe it is placed further forward, either about the centre of the
boiler or near the front end of the tubes. Sometimes two domes are used
on engines, in this country, one over the fire-box and another near the
front end.
QUESTION 117. _How is the steam conducted from the dome to the
cylinders?_
_Answer._ By a pipe _I m m_, fig. 41, called the _dry-pipe_, which
extends from the top of the dome to the front tube-plate. On the front
side of the tube-plate and inside the smoke-box two curved pipes, _O_,
fig. 41, (shown also in fig. 40,) called _steam-pipes_, are attached
to the dry-pipe at one end, and to the cylinders at the other. The
vertical portion of the dry-pipe in the dome, sometimes called the
_throttle-pipe_, is usually made of cast iron, the horizontal part of
wrought iron, and the steam pipes of cast iron.
QUESTION 118. _How is the loss of heat from locomotive boilers by
radiation and convection prevented?_
_Answer._ By covering the boiler and dome with wood, called _lagging_,
about ⁷⁄₈ inch thick, which is a poor conductor of heat, and then
covering the outside of the wood with Russia iron, the smooth, polished
surface of which is a poor radiator of heat.
QUESTION 119. _What is the smoke-box for?_
_Answer._ The smoke-box _Q_ is simply a convenient receptacle for the
smoke before it escapes into the chimney or smoke-stack, which is
attached to the top of the smoke-box. It also affords a convenient
place for the steam and exhaust pipes, where they are surrounded with
hot air and smoke, and not exposed to loss of heat by radiation. The
front end of the smoke-box is usually made of cast iron, with a large
door in the centre which affords access to the inside.
QUESTION 120. _How are the chimneys or smoke-stacks of locomotives
constructed?_
_Answer._ The forms of smoke-stacks which have been used are almost
numberless. For burning bituminous coal and wood they are generally
made with a central pipe, _R_, fig. 41, and a conical-shaped cast-iron
plate, _S_, called the _cone_ or _spark deflector_, which, as the
latter name implies, is intended to deflect the motion of the sparks
and cinders which are carried up with the ascending current of smoke
and air in the pipe _R_, so as to prevent them from escaping into the
open air while they are incandescent, or “alive.” A wire netting, _t
t_, is also provided, which is intended as a sort of sieve to enclose
the sparks and cinders, and at the same time allow the smoke to escape.
The receptacle _h h_ is intended as a chamber in which the burning
cinders will be extinguished before they escape. For burning anthracite
coal, a simple straight pipe, without a deflector or wire netting, is
ordinarily used.
QUESTION 121. _What are the proportions and materials usually employed
in the construction of smoke-stacks?_
_Answer._ The inside pipe _R_, fig. 41, is usually made of the same
diameter as the cylinders, or an inch or two smaller. For the other
dimensions there are no established rules, excepting for the height
of the top of the chimney above the rail, which is usually from 14 to
15 feet. The outsides of smoke-stacks are made of sheet iron, but the
upper part is now sometimes made of cast iron, so as to withstand the
abrasion of the sparks and cinders longer than sheet iron will. For
very warm and damp climates, the outsides of smoke-stacks are sometimes
made of copper to resist corrosion, which is very destructive to all
iron structures in those countries. The wire netting is made of iron or
steel wire from ¹⁄₁₀ to ¹⁄₃₀ of an inch in diameter, and with from 3 to
4 meshes to the inch.
QUESTION 122. _What is the pipe N, fig. 41, intended for?_
_Answer._ It is intended to conduct the exhaust steam and a portion of
the smoke from the bottom of the smoke-box, where the steam escapes,
to the base of the smoke-stack. In nearly all European engines, the
exhaust steam escapes at the top of the smoke-box just below the
aperture of the smoke-stack. There is, however, often difficulty in
equalizing the draft in the tubes, that is, to get an equal amount of
smoke to pass through all of them. By the use of the pipe _P_, called
the _inside pipe_ or _petticoat pipe_, it is thought that the draft
in the tubes can be equalized much better, as a part of the smoke is
drawn out of the smoke-box at the top and part at the bottom of the
smoke-box. The pipe is usually made in two parts, which slide into each
other like a telescope. The distance of the upper end from the top
of the smoke-box and that of the lower end from the bottom can thus
be increased or diminished, and if the draft is greater through the
upper tubes than through the lower ones, or _vice versa_, it can be
regulated or equalized by simply raising or lowering the top or bottom
of the petticoat pipe. Sometimes this pipe is made with openings and a
kind of deflectors over them between the two ends. It is then called a
_flounced petticoat pipe_, for obvious reasons.
No exact theory can be stated regarding the proportions of these pipes,
or the results effected by them, which can be determined only by
practical experience. Some more accurate knowledge concerning them is,
however, much needed.