THE CYLINDERS, PISTONS, GUIDE-RODS AND CONNECTING-RODS.
QUESTION 159. _How are the steam cylinders constructed?_
_Answer._ They are made of hard cast iron, and have the steam and
exhaust ports and valve-seats cast with them. The harder the iron the
better will the cylinders withstand the wear of the pistons and valves,
but they must at the same time be made soft enough, so that after they
are cast the inside can be bored out perfectly cylindrical, the ends
turned off, the bolt-holes drilled, and the valve-seats planed smooth.
Fig. 92 represents a longitudinal section through the centre of the
cylinder and steam-chest. Fig. 94 is a plan of the same parts with the
cover of the steam-chest and the valve removed. The left-hand side
of fig. 95 shows a transverse section through the centre _c d_, fig.
92, of the cylinders, and the right side is a section through the
steam-pipe _G h_, fig. 94. The same letters indicate like parts in the
three different views.
The cylinders of locomotives in this country are now universally placed
on the outside of the wheels, as has already been described. In order
to fasten them securely together and to the boiler, they are attached
to what is called a _bed-plate_ or _bed-casting_, _D D_, figs. 94 and
95, which is placed between them. Some builders make this bed-casting
in a separate piece, and the cylinders are then bolted to it on the
outside, about at the dotted lines, _l_, _m_, fig. 95. Others cast
one-half of it with each cylinder, as shown in our engravings, and then
bolt them together at the line _i_, _j_, which is the centre of the
engine. The bed-casting is also bolted to the smoke-box by the flanges
_E_, _E_. The cylinders are bolted to the frame _F_ with bolts, _m_ and
_k_, fig. 95.
After the cylinders are bored out, and the ends turned off, _heads_,
_A_ and _B_, figs. 92 and 94, are fitted with steam-tight joints to
each end. These heads are fastened with bolts and nuts, _a_, _a_, _a_,
to flanges, _C_, _C_.
QUESTION 160. _How is the steam conducted to and from the cylinders?_
_Answer._ Two pipes or passages are cast in each cylinder, the one, _G
G′_, fig. 95, for admitting steam into the steam-chest, and the other,
_H H′_, for exhausting it from the cylinders. The one _G G′_ is called
the _steam-passage_, and the other, _H H′_, the _exhaust-passage_. The
steam-passage terminates at one end with a round opening, _G_, figs.
94 and 95, to which the steam-pipe _o_, figs. 40 and 95, is attached
inside of the smoke-box. At the other end it divides into two branches,
_G′_, _G′_, fig. 94, each of which terminates in an opening, _g_, _g′_,
inside of the steam-chest. The steam is thus delivered at both ends of
the chest, and can pass freely into each of the steam-ports. By making
the cylinders in this way, they are exactly alike for each side of the
engine, or, to use a shop phrase, there are “_no rights and lefts_,”
so that a cylinder casting can be used for either side of the engine.
This method of making cylinders has been adopted by a number of the
principal builders in this country, but is by no means universal.
[Illustration: Fig. 92. Scale ³⁄₄ in. = 1 foot.]
[Illustration: _Fig. 93._]
[Illustration: Fig. 94. Scale ³⁄₄ in. = 1 foot.]
[Illustration: Fig. 95. Scale ³⁄₄ in. = 1 foot.]
QUESTION 161. _How is the steam-chest constructed?_
_Answer_. It usually consists of two castings, one of which, _J_, figs.
92 and 94, is a square cast-iron box made open at the top and bottom.
This rests on the top of the cylinder casting and is joined to the
latter with a steam-tight joint. On top of it is a cast-iron cover, _K
K_. The steam-chest and cover are held down by bolts, _p_, _p_, which
are screwed into the cylinder casting and have nuts on top.
QUESTION 162. _How are the slide-valves made to work steam-tight on the
valve-seats?_
_Answer_. They are first planed off smooth, and then filed and scraped
until the two touch each other over the whole of their surfaces in
contact. The valve-stem _v_, fig. 92, works steam-tight through a
stuffing-box on the steam-chest.
QUESTION 163. _How are the valves and pistons oiled?_
_Answer_. The oil is usually introduced into the steam-chest through a
cock, _c_, fig. 92, called an _oil-cock_. From this cock it flows down
upon the valve and is conducted by suitable holes and channels to the
valve-face and from there through the steam-ports to the cylinder and
piston. Sometimes, for greater convenience, oil-cocks _c_, _c_, fig.
71, are placed inside of the cab and communicate by pipes with the
steam-chests.
The valves are oiled by pouring oil or melted tallow into the oil-cocks
when the steam is shut off from the steam-chests and cylinders. When
the pistons are working in the cylinders without steam, they create
a partial vacuum, so that if oil is then poured into the oil-cocks it
will be sucked into the steam-chests, or, in other words, it will be
forced in by the pressure of the air above it. _Q_, fig. 71, is a shelf
attached to the boiler to receive an oil-can filled with oil or tallow,
which is thus melted or kept in a fluid condition by the heat of the
boiler.
QUESTION 164. _How are the cylinders and steam-chests protected so as
to prevent, as far as possible, the heat in the steam from being lost?_
_Answer._ The sides of the cylinders are covered with wood, _w_,
_w_, _w_, fig. 95, called the _cylinder lagging_, and the wood is
covered outside with Russia iron or brass, which is called the
_cylinder-casing_. The ends of the cylinders have light metal covers,
called _cylinder-head covers_, made of cast iron, brass or sheet metal.
The steam-chest is covered in a similar way so as to be surrounded
either with a covering of wood or of confined air. Sometimes coarse
felt is used for the purpose. The covering, excepting the cylinder
lagging, is not shown in the engravings.
QUESTION 165. _For what purpose are the cocks C, C, figs. 92 and 95, at
each end of the cylinder, used?_
_Answer._ They are used to exhaust the water which collects in the
cylinders. When the engine is not working the cylinders and steam-pipes
are all cooled off, so that when steam is first introduced into them
a great deal of it is condensed until they become warmed. Water is
also frequently carried over from the boiler with the steam. When
this occurs the boiler is said to _prime_, or to “_work water_.” This
water and that produced by the condensation of steam collect in the
bottom of the cylinder and will not escape through the exhaust-pipes
until the piston moves up so near to the end of the cylinder that the
water will fill the whole space between it and the cylinder-head.
As has already been stated, it will then escape so slowly that the
momentum of the piston and other machinery is liable to “knock out”
the cylinder-heads or even break the cylinder itself. The cocks _C_,
_C_, called _cylinder-cocks_, are therefore placed in the under side of
the cylinder, so that when they are open if there is any water in the
cylinder it will escape through the cocks. They are therefore always
opened when the engine is starting, or at any other time when there is
any indication that there is water in the cylinders.
QUESTION 166. _How are these cocks opened and closed?_
_Answer._ A shaft, _R R_, figs. 92 and 94, which extends across the
frames, has an arm, _R S_, fig. 92, at each end. These arms are
connected by rods, _S T_, with the handles of the cylinder-cocks.
The shaft also has a vertical arm, _R U_, the upper end of which is
connected by a rod with the cab. At the end of the rod is a suitable
handle, _f_, fig. 71, by which the cocks can be either opened or closed
at pleasure by the locomotive runner.
QUESTION 167. _How is the piston-rod fastened to the piston?_
_Answer._ It fits into a straight or tapered hole in the piston-head,
in which it is fastened either with a key, _k k_, as shown in figs. 96
and 97,[44] or by a nut on the front side of the piston.
[44] Fig. 96 is an end view of the piston with the follower-plate
removed. Fig. 97 is a section through the centre.
[Illustration: Fig. 96.]
[Illustration: Fig. 97.
Scale 1¹⁄₂ in. = 1 foot.]
QUESTION 168. _How is the piston constructed?_
_Answer._ It is made of two cast-iron pieces, _B_ and _C_, fig.
97, the one, _B_, called the _piston-head_ or _spider_, to which
the piston-rod _D_ is attached. The other part, _C_, called the
_follower-plate_, is bolted to the piston-head by the bolts _c_, _c_,
called _follower-bolts_. The piston-head has lugs or projections, _d_,
_d_, _d_, fig. 96, cast on the inside, to which the follower plate is
bolted. Hollow spaces are thus left between these lugs.
QUESTION 169. _How is the piston made to work steam-tight in the
cylinder?_
_Answer._ By means of two rings, _A_, _A_, figs. 96 and 97, called
_packing-rings_. These rings are turned of the same size or a little
larger in diameter than the cylinder. They are then cut open at one
point in their circumference so that they can be pressed apart or
expanded by the springs _a_, _a_, called _packing springs_, on the
inside of the rings. These springs are pressed out by the nuts and
bolts _b_, _b_, called _packing-bolts_ and _packing-nuts_, so that
when the rings wear they can be expanded so as to fill the cylinder
completely. The place where the one ring is cut is placed opposite that
of the opening in the other ring, or they are made to _break joints_,
as it is called. This is done to prevent the steam which leaks through
the opening where the one ring is cut from passing through to the other
side of the piston. These rings are usually made of brass and have
grooves, _c_, _c_, fig. 97, turned in them, which are filled with what
is called Babbitt’s metal. This metal is used because it is less liable
to scratch the cylinders than brass alone. Another ring, _l l_, made of
cast iron and as wide as the two brass rings, is placed inside of the
latter and is intended to furnish a bearing for the springs, and thus
distribute the pressure of the springs equally on the packing rings.
This iron ring is also cut open at one point.
QUESTION 170. _How is the piston-rod made to work steam-tight through
the cylinder-head?_
_Answer._ By what is called a _stuffing-box_. This consists of a
cylindrical chamber, _r r_, fig. 92, which is made about 1¹⁄₂ inches
larger in diameter than the piston-rod. This leaves a space ³⁄₄ of an
inch wide all around the rod. This space is filled with hemp or some
other fibrous material, called _packing_, saturated with oil or melted
tallow. This packing is compressed by a hollow cylinder, _s s_, called
a _gland_, the inside of which fits the piston-rod and the outside the
stuffing-box. This gland is forced into the stuffing-box by nuts, _t_,
_t_, which are screwed down on a flange, _u_, attached to the gland.
The packing is thus compressed in the stuffing-box and forced against
the piston-rod, which is made smooth and perfectly round and straight,
and against the side of the stuffing-box, so that no steam can escape
around the piston-rod. A brass ring or “_bushing_” is often put into
the cylinder-head and in the gland where it touches the piston-rod,[45]
because brass will bear the friction of the rod better than cast iron,
and when it is worn out it can be removed and a new one substituted in
its place.
[45] Locomotive piston-rods are now usually made of steel.
QUESTION 171. _Why is the end of the piston-rod made to work in guides?_
_Answer._ Because it must move in a straight line if it and the piston
work steam-tight in the cylinder. By referring back to fig. 2, it
is obvious that if a pressure be exerted against the piston _B_ and
communicated to the crank-pin _N_ by the connecting-rod _E_, the
latter, excepting at the dead-points, will exert a pressure either
upward or downward, according to the direction the piston is moving.
This pressure would bend the piston-rod if no provision were made to
prevent it. For this reason, therefore, the end of the piston-rod
is attached to what is called a _cross-head_, _L_, figs. 92 and 94,
which works in guides, _M_, _M_. The cross-head is made of cast iron
and has slides, _N_, _N_, figs. 93 and 94, one on each side, each of
which works between a pair of guide-bars or rods, _M_, _M_, shown
in section in fig. 93.[46] These guide-rods, or _guides_ as they
are called, are planed and finished with great accuracy so as to be
straight and smooth, and are attached to the cylinder-head at one end,
and to a support, _O_, called the _guide-yoke_, which is fastened to
the frame at _F_, fig. 94, and also usually attached to the boiler.
The guides are set with great care, so as to be exactly parallel with
the axis or centre line of the cylinder, so that the cross-head will
slide in exactly the same path that the piston-rod will if it moves
in a straight line. If then the piston-rod and the connecting-rod are
attached to the cross-head, all the strain produced by the obliquity
of the connecting-rod will be borne by the guides, thus relieving the
piston-rod, and making it certain that it will move in a straight line.
[46] Fig. 93 is a transverse section through the guides at _n_, fig.
92.
QUESTION 172. _How are the piston and connecting-rods attached to the
cross-head?_
_Answer._ The end of the piston-rod fits into a tapered hole in
the cross-head and is held by a key, _w_, figs. 92 and 94. The
connecting-rod is attached to a pin, _Q_, called a _wrist-pin_, which
is cast with the cross-head.
QUESTION 173. _How is the wear of the slides lessened and compensated?_
_Answer._ Sometimes they are made with brass wearing pieces called
_gibs_, shown at _N_, _N_, fig. 93, which are placed between the
slides and the guides. These gibs can either be removed and new ones
substituted when they become very much worn, or by inserting thin
pieces of metal, called liners, between them and the cross-head, they
will be spread apart so as to fill the space between the slides.
The slides are now, however, oftener made without gibs, and have
recesses either cast or drilled in them, which are filled with either
Babbitt’s metal or glass bearings, which latter are said to wear very
well. The guides are bolted at each end to blocks, _x_, _x_, called
_guide-blocks_, which can be planed off so as to bring the guides
nearer together when they and the slides are worn. Sometimes liners are
placed between the blocks and the guides, which can be removed when it
is necessary to bring the guides nearer together.
QUESTION 174. _Are the top and bottom guides worn alike?_
_Answer._ No: the top guide in ordinary engines is worn the most,
because the pressure of the slides is always on the top guide when the
engine is running forward, and on the bottom guide when it is running
backward. This will be understood by referring back to the series
of figures from 11 to 24. It will be noticed that in the backward
stroke of the piston, represented by figs. 11 to 17, the strain on
the connecting-rod tends to _push_ the cross-head upward, and in the
forward part of the stroke, figs. 18 to 24, the connecting rod _pulls_
the cross-head in the same direction. If the crank turned the opposite
way, this action would be reversed and the cross-head would then be
alternately pushed and pulled downward, and the bottom guides would
then be worn the most. As nearly all locomotives run forward more than
backward, the top guides are usually worn the most.
QUESTION 175. _How are the slides oiled?_
_Answer._ Oil cups, _n_, _o_, figs. 92, 93 and 94, are placed about the
middle of the top guide. These cups usually have a reservoir to hold a
supply of oil, and are so constructed that it will be gradually fed on
the slides, which are thus constantly and regularly lubricated.
QUESTION 176. _How are the pumps worked from the piston-rods?_
_Answer._ The pump-plunger is attached to a projection, _W_, figs.
92 and 93, called the _pump-lug_, cast on one of the slides of the
cross-head. The plunger thus receives a reciprocating motion from the
piston.
QUESTION 177. _How are the connecting-rods made?_
_Answer._ They are made of flat bars of wrought iron. The rods
which connect the cross-heads with the driving-wheels are called
_main connecting-rods_, and the rods which connect or couple the
driving-wheels together are called _coupling-rods_.[47] Fig. 98
represents a side view and a plan of a main connecting-rod. In the side
view the end _B_, and in the plan both ends of the rod are shown in
section. It is attached to the wrist-pin at _A_ and to the crank-pin at
_B_. Fig. 99 represents similar views of a coupling-rod. To save room
in the engraving each of these rods is represented with a part of the
middle broken away. The main rods are usually made wider at _G_, next
the crank-pin, than at the other end, as it has been found that they
are most liable to break at that end. The coupling-rods are now made
either straight or somewhat wider in the centre.
[47] They are also often called _side_ or _parallel-rods_, but the
term _coupling-rods_ is considered the best.
[Illustration: _Fig. 98._]
[Illustration: _Fig. 99._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 178. _How are these rods prevented from getting loose on the
pins from the wear of the latter in the inside of the holes of the
rods?_
_Answer._ The ends of the rods are provided with what are called
_brass-bearings_,[48] or “_brasses_,” _c_, _d_, and _e_, _f_. These
brasses are made in pairs, so as to embrace the pins, from each side.
They are held by ~⊃~-shaped clamps, _s s s_, called _straps_, which are
bolted to the rods. When the brass-bearings become worn, they are
taken out of the straps, and a portion of their surfaces of contact
with each other is filed away, thus allowing them to come nearer
together, and thereby reducing the size of the hole which receives the
pin or journal. In order to prevent their being loose in the straps,
tapered or wedge-shaped _keys_, _k_, _k_, which bear against the
brasses, are fitted in the straps and rods. By driving down these keys
the brass bearings are forced together, thus reducing the size of the
hole for the journal, and making the rods fit tightly on the pins. A
hard steel plate, shown by dark shading in the engraving, is sometimes
interposed between the keys and the brasses to prevent the key from
indenting the surface of the soft brass. As the keys are very liable to
get loose and fall out, they are held either by screws and nuts, _x_,
_x_, as shown in the engraving, or by _set-screws_ on the side of the
rods. The whole arrangement of straps, keys and brasses is called a
_stub-end_.
[48] The portion of a shaft pin or spindle subjected to friction is
called a _journal_, and the surface which presses or “bears” against
it is called a _bearing_.
QUESTION 179. _How are the journals of the crank-pins oiled?_
_Answer._ By oil-cups attached to the straps, above the journals,
similar to the cups used on the _guide-rods_, but which are not shown
in the engravings of the connecting-rods. Sometimes _oil-cellars_, as
they are called, are attached to the under side of the straps. These
are metal boxes, which are filled with oil, which is agitated violently
by the rapid motion of the rods, and is thus applied to the journals
through holes drilled in the straps. In order to confine the oil and
prevent its leaking out around the journals of the coupling-rods, the
brasses at _n_, _n_, fig. 99, are usually made so as to enclose the
outside end of the crank-pin, which thus not only keeps the oil in, but
excludes the dust. The brasses are usually lined with Babbitt’s or some
other kind of soft metal, which is thought to be less liable to heat
from the friction of the journals.
QUESTION 180. _Are the coupling-rods always made with stub-ends?_
_Answer._ No; their ends are sometimes made in one piece--that is,
without straps or keys. The holes which receive the crank-pins then
have brass rings or _bushings_, as they are called, which fit tightly
and are driven into the holes, and form the bearings on the pins. When
these rings become worn they are driven out and new ones put in.
QUESTION 181. _What is meant by the term lost motion?_
_Answer._ It is used to designate the wear of machinery, which causes a
loss of motion in some of the parts. Thus if the bearings of the main
connecting-rods are worn, the piston must move a distance equal to the
wear at each end of the stroke before it moves the crank-pin. Lost
motion might therefore be called the looseness of the parts. When we
speak of _taking up_ the lost motion, we mean making parts which were
loose fit tightly.