THE RUNNING GEAR.
QUESTION 246. _What is meant by the running gear of a locomotive?_
_Answer._ It means those parts, such as the wheels, axles and frames,
which carry the other parts of the engine. As the Germans express it,
it is the “_wagon_” of the locomotive.
QUESTION 247. _How may the wheels be classified?_
_Answer._ As _driving_ and _carrying_ or _truck wheels_.
QUESTION 248. _What service must the driving wheels perform?_
_Answer._ The driving wheels, as indicated by their name, “drive”
or move the locomotive on the track, as was explained in answer to
Questions 64, 65 and 66. As their adhesion depends upon the pressure
with which they bear upon the rails, they must carry either a part or
the whole of the weight of the engine.
QUESTION 249. _What proportion of the weight of ordinary locomotives is
usually carried on the driving wheels?_
_Answer._ Eight-wheeled “American” locomotives, which are most commonly
used in this country, have about two-thirds of their weight on the
driving wheels.
QUESTION 250. _What is meant by the “truck” of a locomotive?_
_Answer._ It means one or more pairs of wheels which are attached to a
separate frame and to the locomotive by a flexible connection, so that
the axles are not held rigidly at right angles to the main frame, but
can assume positions which approximate to that of radii of the curves
of the track. In plates I, II and III, _E E_ are the truck wheels, _b′
b′_ the truck frame, and _y_, plate II, and fig. 40, the centre-pin,
around which the truck frame turns.
QUESTION 251. _What service does the truck perform?_
_Answer._ It carries the weight of the front end of the locomotive, and
also guides it into and around curves and _switches_.[67]
[67] A switch is a movable pair of rails, by which a locomotive is
enabled to run from one track to another.
QUESTION 252. _How does it perform the latter service?_
_Answer._ It does it very much in the same way as the front wheels of
an ordinary wagon enable it to turn around corners; that is, the truck
wheels being attached to a separate frame, which is connected to the
locomotive by a centre-pin, just as the front axle of an ordinary wagon
is connected by the king-bolt, can turn.
[Illustration: _Fig. 149._
_Fig. 150._
Scale, ³⁄₈ inch = 1 foot.]
[Illustration:
_Fig. 151._]
QUESTION 253. _Why are two pairs of wheels used on a locomotive instead
of one, as on an ordinary wagon?_
_Answer._ Because it is necessary to have one pair of wheels guide
the other. In an ordinary wagon the front axle is guided by the pole
or shafts. Nearly every one knows the difficulty of moving a wagon
when the pole or shafts are removed, especially if it be pushed from
behind. The movement of the front axle is then uncontrolled, and it
is impossible to direct the motion of the vehicle. The same thing
would occur with a locomotive if a single pair of wheels were used,
and attached in the same way as the front axle of a wagon. Thus if a
single pair of wheels were connected to a locomotive by a centre-pin,
_a_, fig. 149, so that the axle would be free to move around this
pin, then if one of the wheels should strike an obstruction, say a
stone, _s_, fig. 150, there would be nothing to prevent the axle from
being thrown into the position shown in fig. 150, and the wheels would
be quite sure to leave the track. When two pairs of wheels are used
and both axles attached to the same frame, which is connected to the
engine by a centre-pin, _s_, fig. 151, between the two axles, then
the wheels in moving round the centre-pin must move around the centre
_s_ in arcs of circles, _m n_, _m n_, described from the centre _s_.
These arcs, it will be observed, cross the rails. Now if the wheels
should move in that direction, the flange of one of them would come in
contact with the rail and prevent it from moving any farther. It is
therefore evident that wheels arranged in that way can only move about
the centre-pin as far as the curvature of the track will permit. Trucks
are sometimes used with only one pair of wheels, but the centre-pin
is then placed some distance behind the centre of the axle, or in the
same relation to it that the centre _s_ is to the axle _a a′_ in fig.
151. It is evident that if the frame for such a truck turns around the
centre-pin, the wheels must move across the track in the same way
as represented by the arcs _m n_, in fig. 151. The construction and
operation of trucks with a single pair of wheels will be more fully
explained hereafter.
[Illustration:
_Fig. 152._
_Fig. 153._
_Scale
³⁄₁₆ Inch = 1 Foot._
_Fig. 154._
_Fig. 155._
_Fig. 156._]
QUESTION 254. _Why will a locomotive run around curves easier if the
front axles are attached to a truck frame which is connected to the
locomotive by a flexible connection?_
_Answer._ Because the truck axles can then assume positions which
conform very nearly to the radii of the curves of the track, and it is
well known that if two or more axles, each with a pair of wheels on
it, are attached to a frame with their centre lines parallel with each
other, as shown in fig. 152, they will roll in a straight line, but if
the centre lines of the axles are inclined to each other, as shown in
fig. 153, the tendency will be to roll in a curve, the radius of which
will depend upon the degree of inclination of the axles to each other.
In order to make the wheels in fig. 152 roll on the curves _a b_ and _c
d_, it will be necessary to slide them laterally a distance equal to
that between the curves and the straight lines _m o_ and _p r_, and as
the length of the outside curve is greater than the inside one, if the
wheels are fastened to the axles so they cannot turn on them and roll
on the curves, either the wheels on the inside or those on the outside
must slip a distance equal to the difference in the length of the two
curves. Considerable force will therefore be required to overcome the
resistance due to the combined lateral and circumferential sliding of
the wheels, so that more power will be needed to make them roll in
a curve than is necessary to make them roll in a straight line. If,
however, the axles are inclined to each other, then the wheels will
naturally roll on a curved path, and it will not be necessary to slide
them sideways to make them conform to such a path. But if the wheels
are all attached to the axles so that those on the same axle cannot
turn independently of each other and are all of the same diameter, then
either the inside or the outside ones must slip, because the path in
which the outside ones roll is longer than the inside curve, so that
even if the axles are inclined to each other more power will be needed
to roll the truck in a curved path than to roll the wheels shown in
fig. 152 in a straight line. It is, however, a fact that a cone or
a portion of a cone like that shown in fig. 154 will of itself roll
on a curve. It will do the same thing if the middle is cut away, as
indicated by the dotted lines in fig. 154 and as shown in fig. 155. If
now the wheels are made so that their peripheries[68] form portions
of a cone and the axles are inclined to each other as shown in fig.
156, then there will be no slipping on the track, because the outside
wheel, being larger in diameter than the inside one, advances further
in one revolution than the latter does, and thus rolls on the longest
path in the same time that the inside or smaller wheel does on the
shorter one. When this is the case, such wheels will roll in a curve
as easily as those in fig. 152 will in a straight line. The degree of
inclination of the axles and of the sides of the cone must, however,
vary with the radius of the curve. But if the axles are parallel to
each other, and the wheels conical, as represented in fig. 157, they
will not roll either in a straight line or in a curve without great
difficulty, because if they roll in a straight line, the wheels on
one side being larger in diameter than those on the other, either the
larger or the smaller ones must slip on the path in which they roll.
If they roll on a curve, then each pair of wheels has a tendency to
roll in a curve independent of the other, and therefore the wheels must
slip laterally if both pairs roll on the same track. Thus, suppose
two pairs of wheels, _a_, _a′_, and _b_, _b′_, fig. 157, to be made
conical and attached to a frame so that their axles are parallel to
each other. Now each pair of such wheels will have a tendency to roll
in circular paths, _a′ i_, _a h_, and _b′ k_, _b j_, the centres of
which are at _m_ and _n_, or at the apices of the cones of which their
peripheries form a part. If they are made to roll in circular paths, _c
d_, _e f_, described from a point _g_, then each pair of wheels must
slip laterally over the space between the paths _a′ i_, _a h_, in which
they would naturally roll and that in which they are made to roll. Thus
the wheel _a_ would slide laterally the distance between the curve _a
h_ and _a f_, and _a′_ that between _a′ i_ and _c d_; _b_ would slide
from _b j_ to _b f_ and _b′_ from _b′ k_ to _b′ d_. It will thus be
seen that in order that two pairs of wheels may roll with equal ease
in a straight line and in curves, the wheels in the one case must be
of equal diameters and the axles parallel, and in the other case the
wheels must be of unequal diameters and their axles be _radial_[69] to
the curve. This is equally true of any number of pairs of wheels. If
we have three, four, or any number of axles, with wheels all attached
to the same frame, if their axles are parallel, and the wheels of the
same diameter, they will roll in a straight line; but if their wheels
are conical and their axles radial, they will roll in a curve.
[68] The periphery is the outside surface on which the wheel rolls.
This part of a wheel is usually called the “_tread_.”
[69] That is, that their centre lines incline towards each other, and
if extended far enough would meet at the centre of the curve.
[Illustration:
_Fig. 157._
_Scale_
_³⁄₁₆ In. = 1 Foot._]
For the preceding reasons it is therefore sufficiently obvious that
if a locomotive is to run on both straight and curved tracks, on
the former the wheels should be of the same diameter and the axles
parallel, and on the latter the wheels should be conical and the axles
radial.
[Illustration: _Fig. 158._
_Fig. 159._
Scale ³⁄₈ in. = 1 foot.]
QUESTION 255. _How are the wheels made so that in curves they will act
as though they were of the conical form described and on a straight
track all be of the same diameters?_
_Answer._ The periphery or tread of each wheel is made conical, but of
the same size as the other, and with the small diameter of the cone
outside, as shown in fig. 158. On a straight track if the position of
the wheels on the rails is such that their two flanges are equally
distant from the rails, as shown, then obviously at the points of
contact with the rails the wheels are of the same diameter. That
is, _a_ is equal to _b_. But in running on a curved track, if the
wheels are of the same diameter, as has been shown, they will roll in
a straight line and consequently towards the outside of the curve.
The flange _c_--supposing it to be at the outside of the curve--will
therefore roll towards the rail, and consequently the outside wheel
will rest on the rail at a point nearer the flange, as shown in fig.
159, where the diameter _a_ is larger, and the inside one further from
the flange where the diameter _b_ is smaller than at _a_ and _b_ in
fig. 158; and consequently the action of the wheels is the same as
though their peripheries were made of the form shown in fig. 157.
QUESTION 256. _How are the axles of locomotives made to assume a
position radial to the curves in the track?_
_Answer._ This is only done approximately, as the mechanical
difficulties in the way of doing it perfectly are so great as to
render it impracticable. By attaching the truck to the locomotive by
a flexible connection or centre-pin, _s_, as shown in fig. 160 (which
represents a plan of the wheels of an ordinary locomotive), it is plain
that the truck axles _e f_ and _g h_, instead of remaining parallel
to the driving-axles _a b_ and _c d_, will, by turning around the
centre-pin, _s_, adjust themselves to the curve so as to approximate as
closely to radii as is possible for two axles which are held parallel
to each other. Of course the further apart they are the greater will be
their divergence from the position of radii, and whether the tread
of the wheels be cylindrical or conical the further apart their axles
are the greater will be the divergence of the paths in which they would
naturally roll from that of any curve on which they must roll. Thus, if
the axles were twice as far apart as they are represented in fig. 157,
and in the position shown in the dotted lines _l l′_ and _o o′_, the
wheels, if they are conical, would then naturally roll in curves drawn
from the centres _p_ and _q_. If the wheels are cylindrical, they would
roll in straight lines. In either case the divergence of their paths
_l s_ and _l r_ from the curve of the track is greater than _a h_ and
_a′ i_, the paths in which they would roll if their axles were nearer
together. This divergence increases with the distance between the
axles, and therefore the lateral slip of the wheels must be in the same
proportion.
[Illustration:
_Fig. 160._
_Scale ³⁄₁₆ Inch -- 1 Foot._]
QUESTION 257. _Is the resistance to rolling diminished by placing the
truck axles nearer together?_
_Answer._ It is, within certain limits. The nearer each other they are
placed, the closer will the centre-pin of the truck be to the centre of
the axles. The closer it is to the centre of the axle, the greater is
the tendency of the wheels to become “slewed,” or to assume a diagonal
position to the rails as represented in fig. 150, and thus increase
the resistance and also the danger of running off the track. The
increase of resistance from this cause, after the axles reach a certain
distance from each other, is greater than the decrease from a closer
approximation to the position of radii. In ordinary locomotives it is
necessary to place the truck wheels from 5 ft. 6 in. to 6 ft. apart, in
order to get the cylinders between them in a horizontal position. This
distance apart works very well in ordinary practice.
QUESTION 258. _What is meant by flange friction?_
_Answer._ It is the friction of the flanges of the wheels against the
head of the rails. Thus if two pairs of wheels, _a a′_, _b b′_, fig.
151, be placed on a curve and rolled in the direction of the dart, the
wheel _a_ will roll towards the outside of the curve until the flange
comes in contact with the rail. As already explained, if two axles
are parallel to each other, no matter whether the wheels are conical
or cylindrical, they must slip laterally in order to roll in a curved
path. As the flange must follow the curve of the rail, it forces the
wheel laterally and thus compels it to roll in the curved path into
which the rail is bent. As the wheel offers considerable resistance
to sliding there is a corresponding pressure of the flange against
the rail, and consequently the revolutions of the wheel produce an
abrasive action between the two. This action is obviously increased
with the distance between the axles, because, as has been shown, the
lateral slip of the wheels is then greater than when they are nearer
together. It is also obvious that if the wheels are parallel with the
rails there will be no abrasive action of the flanges, but that the
greater the angle at which the wheels stand to the rails the harder
will the flanges rub against the rails, and the greater will be the
flange friction. With the aid of geometry it can very easily be proved
that the farther apart two parallel axles are, the greater will be the
angle of the wheels to the rails on a curved track, and, therefore, the
greater will be their flange friction. It must, however, be remembered
that if the wheels are so close together that they are liable to
become “slewed,” or assume a diagonal position across the rails, as
shown in fig. 150, the angle at which the wheels would stand to the
rails would thus be very much increased. It has therefore come to be a
very generally recognized rule that the centres of axles should never
be placed nearer together than the distance between the rails.
QUESTION 259. _Is the flange friction of all the wheels of a truck the
same on any given curve?_
_Answer._ No; of the front wheels obviously only the flange of the one
on the outside of the curve comes in contact with the rail. As the
centrifugal force of the engine presses the back pair of wheels towards
the outside of the curve, the flange of the outside wheel alone comes
in contact with the rail. But as this wheel is constantly rolling
_away_ from the rail, as shown by the dotted lines _h g_, fig. 151,
obviously the friction of its flange is less than that of the front
outside wheel, which always rolls _towards_ the rail. The flange of the
back inside wheel is carried outwards by the centrifugal force and also
by the tendency of the wheels to roll on their largest diameters on a
curve, so that its flange will not touch the rail.
QUESTION 260. _Can the axles of driving wheels assume positions radial
to the track?_
_Answer._ In ordinary engines they cannot. Various plans have been
devised for the purpose of enabling them to do so, but it is only
recently that they have met with any success. Some of these plans
will be described hereafter. It is, however, of less importance that
the driving axles, when they are behind the centre of the locomotive,
should assume positions radial to a curved track than that the front
wheels should. This is illustrated by a common road wagon, as all know
the ease with which such a vehicle can turn a corner if we run it with
the front axle ahead, and the difficulty of doing so when the back axle
is in front. In the case of a locomotive the reason for it is very much
the same as that which makes the flange friction of the back wheels
of a truck less than that of the front ones. From fig. 160 it will be
seen that the outside driving-wheels, when the engine is running with
the truck in front, are rolling _from_ the rail and not against it.
As stated before, the centrifugal force of the engine when in motion
has a tendency to throw the wheels towards the outside of the curve.
It will also be noticed that the front driving axle is near the centre
of that portion of the curve which lies between the centre s of the
truck and the centre _k_ of the back axle. If it were in the middle
between them, it would be exactly radial to the curve; being near the
middle, it approximates closely to that position, and therefore the
flange friction of its wheels is very slight. It will be noticed that
if the flange of the back or trailing-wheel on the inside of the curve
were not kept away from the rail it would roll toward and impinge
against that rail. But it will be noticed that the flange of the front
driving-wheel will come in contact with the inside rail before that
on the back wheel can touch it. For this reason, and also on account
of the effect of the centrifugal force exerted on the engine and the
tendency of the wheels to roll on their largest diameters, the flange
of the inside back wheel is kept out of contact with the rail, and as
the back wheel on the outside of the curve rolls away from the rail
there is very little friction of the flanges of the back driving-wheels.
It will also be noticed from fig. 160, that if the radius of the
curve is very short, the bend of the rails between the back pair
of driving-wheels and the centre of the truck is so great that the
inside rail will press hard against the flange of the front or main
driving-wheel next that rail. This of course produces a great deal of
friction, and if the curve is excessively short the flange will mount
on top of the rail and the tread of the opposite wheel will fall off
from its rail. For this reason the centre-pin of the truck is sometimes
arranged so that it can move laterally, that is cross-wise of the
track. In fig. 160 the centre-pin is represented as having moved some
distance from the actual centre of the truck, which is represented with
dotted lines. The front wheels of locomotives are also sometimes made
with wide “flat” tires, that is, tires without flanges, so that there
will be no friction against the one rail and no danger of falling off
the other.
Another action also takes place which facilitates the motion of the
driving-wheels of ordinary engines around curves. Every one knows how
easy the direction in which the front wheels of a common wagon can be
controlled by taking hold of the end of the tongue or pole. With the
leverage which it gives the wheels and axle can easily be directed
wherever it is desired. A similar action takes place in an ordinary
locomotive. The front driving-axles are guided by the truck, which is
attached to the frame ten or twelve feet in front of the driving-axle,
and thus the track exerts a leverage to guide the movement of the
driving-axles, just as a common wagon can be guided by the pole.
If the locomotive is run backward, then none of these advantages exist,
and the flange friction of the back driving-wheels is excessive.
Engines such as construction locomotives, which run backward as much
as forward, wear out the flanges of the back wheels very rapidly on
crooked roads.
QUESTION 261. _What is meant by the “spread” of the wheels or axles?_
_Answer._ It is the distance between the centres of two axles.
QUESTION 262. _What is the “wheel-base” of a locomotive?_
_Answer._ It is the distance between the centres of the front and back
or trailing-wheels. On ordinary engines, such as that illustrated in
plate I, it is the distance from the centre of the front truck to the
centre of the back driving-wheels.
QUESTION 263. _Is the “coning” of the tread of the wheels of much
practical importance?_
_Answer._ There is great difference of opinion regarding it, but even
if its action is very beneficial, the advantage is very soon lost,
owing to the wear of the wheels. It is, therefore, believed that the
advantage is more apparent in theory than in practice.
QUESTION 264. _How are the driving-wheels of locomotives constructed?_
_Answer._ They are made of cast iron with wrought-iron or steel tires
around the outside. Fig. 161 represents a perspective view of a pair of
locomotive wheels and axle. The central portion of the wheel, that is
the hub, spokes and rim, are cast in one piece. Usually the hub and
the rim, and sometimes the spokes, are cast hollow. The central portion
of the wheel, that is the part which is made of cast iron, is called
the _wheel-centre_.
[Illustration: Fig. 161.]
QUESTION 265. _How are the tires fastened on the wheel-centres?_
_Answer._ The insides of the tires are usually turned out somewhat
smaller than the outside of the wheel-centre. The tire is then heated
so that it will expand enough to go on the centre. It is then cooled
off, and the contraction of the metal binds it firmly around the cast
iron part of the wheel. As an additional security bolts or set-screws,
_a_, _a_, fig. 161, are screwed through the rim and into the tire to
prevent it from slipping off in case it becomes loose. In some cases
the wheel-centre and the inside of the tire are turned conical, and the
tires are then put on cold and held on with hook-headed bolts, _C_, as
shown in fig. 162, which is a section of the tire and the rim of the
wheel. The wheel-centre is made largest on the inside. As the strain
against the flange of the tire is inward, the cone of the wheel-centre
resists this strain. If it was curved or tapered the reverse way, the
strain would come on the bolts, and it would also be impossible to
remove the tires without first taking the wheels off the axles. This
method of putting on tires has the advantage that they can be removed
quickly and without heating the tires.[70]
[70] It is exclusively used on the Baltimore and Ohio Railroad.
[Illustration: Fig. 162. Scale 3 in. = 1 foot.]
QUESTION 266. _Are there any standard sizes for the inside diameters of
tires?_
_Answer._ Yes. To avoid the great inconvenience arising from the
diversity in the _inside diameters_ of tires, the American Railway
Master Mechanics’ Association has recommended that the inside diameter
of tires should be made 36, 40, 44, 50, 56 and 62 inches. The thickness
for the first three sizes to be 3 in. and the last three 2¹⁄₂ in.
QUESTION 267. _How are the driving-wheels fastened on the axles?_
_Answer._ The hubs are accurately bored out to receive the axles, and
the latter are turned off so as to fit the hole bored in the wheel. The
axles are then forced into the wheel by a powerful pressure produced
either with a hydraulic or screw press, made for the purpose. In order
to prevent the strain upon the crank-pins from turning the wheels upon
the axle, they are keyed fast with square keys driven into grooves cut
in the axle and in the wheel to receive them. The ends of these keys
are shown at _b_, fig. 161.
QUESTION 268. _How are the crank-pins made?_
_Answer._ They are made of wrought iron or steel and accurately turned
to the size required for the journals for the connecting-rods. Fig.
164 represents one of the main crank-pins, and fig. 163 a back pin for
an American engine. The main pin has two journals, one, _B_, to which
the main connecting-rod is attached, and the other, _A_, receiving
the coupling-rod. The back pin has only one journal, _A_, for the
coupling-rod.
QUESTION 269. _How are the crank-pins fastened to the wheels?_
_Answer._ They are turned so as to fit accurately holes which are bored
in the wheels, and are usually “straight” or cylindrical. The pins are
then either driven in with blows from a heavy weight swung from the
end of a rope, or else pressed in with a screw or hydraulic press.
Sometimes the holes are bored tapered or conical and the pins turned to
the same form. They are then ground in with emery and oil, so as to fit
perfectly, and are secured by a large nut and key on the inside of the
wheel.
[Illustration: _Fig. 163._
_Fig. 164._
Scale 1¹⁄₂ in. = 1 foot.]
QUESTION 270. _What are the pieces A, A, fig. 161, between the spokes
of the wheel for?_
_Answer._ They are called _counterbalance weights_, and are put in the
wheels to balance the weight of the crank-pins, connecting-rods and
pistons. The principle of their action will be explained hereafter.
QUESTION 271. _On what part of the axle does the weight of the engine
rest?_
_Answer._ It rests on the driving-axle boxes _L_, fig. 161, which are
placed just inside and close to the wheel.
[Illustration: Fig. 165.]
QUESTION 272. _What are the driving-axle boxes for and how are they
made?_
_Answer._ They are cast iron blocks, _L_, fig. 161, which embrace and
rest on the axle. The part of the axle on which the box bears is called
the _journal_. Each box has a brass bearing, _c_, fig. 165, which bears
on top of the journal and which is consequently exposed to the friction
and wear. Fig. 165 is a perspective view of a driving-box, which shows
what is called the _oil-cellar_, _d_. This is a receptacle underneath
the axle which is filled with wool or cotton waste and saturated with
oil for the purpose of lubricating the journal. The oil-cellar is held
in its position by two bolts, _f_, _f_, which pass through it and the
driving-box casting. By removing the bolts the oil-cellar can easily be
removed and the box can then be taken off the axle.
[Illustration: Fig. 166.
Fig. 167.
Scale ¹⁄₂ in. = 1 foot.]
QUESTION 273. _How are the truck wheels made?_
_Answer._ They are made of cast iron, usually in one piece. Figs. 166
and 167 represent sections of two forms of wheels used for cars. Those
used for locomotive trucks are similar to these, excepting that they
are usually a little smaller in diameter. They are made with a disc or
plate which unites the tire to the hub, and in some cases they have
ribs cast in the inside, as shown in the two figures. Some are made
with single and others with double plates, as shown in the engravings,
and still others with spokes similar to the driving-wheels. The tread
of the wheel is hardened by a process called _chilling_. This is done
by pouring the melted cast iron into a mould of the form of the tread
of the wheel. The mould is also made of cast iron, but being cold cools
the melted iron very suddenly, and thus hardens it somewhat as steel is
hardened when it is heated and plunged into cold water.[71]
[71] It should be mentioned here that it is only certain kinds of
cast iron which will be hardened in this way, or will “_chill_,” as
it is called. The cause to which this chilling property is due is not
known.
QUESTION 274. _How are the boxes, journals and journal-bearings of the
truck-wheels made?_
_Answer._ They are very similar to those for the driving-wheels, their
chief difference being that those for the truck-wheels are smaller than
those for the driving-wheels.
QUESTION 275. _How are the frames for locomotives constructed?_
_Answer._ The frames, _H H H_, plates I, II and III, are made of bars
of wrought iron from 3 to 4 inches thick and about the same in width.
They are usually made in two parts, the one at the back part of the
engine, to which the driving-boxes and axles are attached, and the
other at the front end, to which the cylinders are bolted. The back
part, or main frame, as it is called, is represented in figs. 168 and
169, and consists of a top bar, _H H_, to which pieces, _a_, _a′_,
_b_, _b′_, called _frame-legs_, are welded. Two of these form what is
called a _jaw_, which receives the axle-box, as shown in fig. 169. To
the bottom of each jaw a _clamp_, _c_, is bolted to hold the two legs
together. The two legs, _a_ and _b′_, are united by a brace, _d d_,
welded to the bottom of the legs. A brace, _m_, unites the back end of
the frame with the leg _b_, and is welded to each.
The front part of each frame consists of a single bar, _e_, which is
bolted to the back end, as represented in figs. 168 and 169, which show
the construction clearer than any description would. These front bars
extend forward to the front end of the engine, and a heavy timber,
called a _bumper-timber_, _E′ E′_, plates I, II and III, extends across
from one to the other and is bolted to each of them. This timber is
intended to receive the shock or blow when the locomotive runs
against any object, such as a car. The _cow-catcher_ or _pilot_, _S_,
is fastened to this timber.
[Illustration: _Fig. 168._
_Fig. 169._
Scale ³⁄₈ in. = 1 foot.]
The front bar of the frames also has usually two lugs or projections
forged on it, between which the cylinders are attached. The latter are
securely held in their position by wedges, which are driven in between
the lugs and the cylinder castings.
The frames, as already stated, are made of wrought iron and are
accurately planed off over their whole surface.
QUESTION 276. _How are the frames fastened to the boiler?_
_Answer._ As already stated, they are fastened to the cylinders with
wedges and bolts, and as the cylinders are bolted to the smoke-box the
frames are thus rigidly attached to the front end of the boiler. In
order to strengthen those portions of the frames which extend beyond
the front of the smoke-box and to which the bumper-timber is attached,
diagonal braces, _r′ r′_, plates I, II and III are bolted both to the
timber and to each of the frames at their lower ends. The upper ends
are bolted to the smoke-box. Other braces, _d′_, plate II, are also
fastened to the frames and to the barrel of the boiler. The frames
are fastened to the fire-box by clamps, _I_, _I_, plate I, called
_expansion clamps_. These clamps embrace the frames so that the latter
can slide through the former longitudinally. There are also usually
two diagonal braces not shown in plate II, the upper ends of which are
fastened to the back end of the shell of the fire-box at about the
level of the crown-sheet, and the lower ends to the back ends of the
frames. There are also usually transverse braces attached to the lower
part of the frames, thus uniting the two together. The guide-yokes, _j
j_, plate I, are also usually bolted to the frames and to the boiler.
In many cases one only is used, which extends across from one frame to
the other and is fastened to the boiler.
QUESTION 277. _Why are the frames attached to the shell of the fire-box
so as to slide longitudinally through the fastenings?_
_Answer._ Because when the boiler becomes heated it expands, and if it
could not move independent of the frames its expansion would create
a great strain on both itself and the frames. The fastenings to the
fire-box are therefore made so that the frames can move freely through
them lengthwise, but in no other direction.
QUESTION 278. _How much more will the boiler expand than the frames in
getting up steam?_
_Answer._ From ¹⁄₄ to ⁵⁄₁₆ of an inch.
QUESTION 279. _Why is it necessary to support the engine on springs?_
_Answer._ [72]Because, however well a road may be kept up, there will
always be shocks in running over it; these occur at the rail joints and
especially when the ballasting of the ties is not quite perfect. These
shocks affect the wheels first, and by them are transferred through
the axle-boxes to the frame, the engine and the boiler. The faster the
locomotive runs, the more powerful do they become, and therefore the
more destructive to the engine and road, and consequently the faster a
locomotive has to run the more perfect should be the arrangement of the
springs.
[72] The above answer and much of the material referring to springs
has been translated from “Die Schule des Locomotivführers,” by
Messrs. J. Brosius and R. Koch.
If we strike repeatedly with a hammer on a rail, the latter is soon
destroyed, while it can bear without damage a much greater weight than
the hammer lying quietly on it. The axles, axle-boxes and wheels strike
like a hammer on the rails at each shock, while the shock of the rest
of the parts of the engine first reaches and bends the springs, but on
the rails has only the effect of a load greater than usual resting on
them. Another comparison will make still plainer the lessening by the
springs of the injurious effect which the weight of the boiler, etc.,
exercises on the rails.
A light blow with a hammer on a pane of glass is sufficient to shatter
it. If, however, on the pane of glass is laid some elastic substance,
such as india-rubber, and we strike on that, the force of the blow
or the weight of the hammer must be considerably increased before
producing the above-named effect. If the locomotive boiler is put in
place of the hammer, the springs in place of the india-rubber, and the
rails in place of the glass, the comparison will agree with the case
above. From this consideration it will be seen how important it is
to make the weights of the axles, axle-boxes and wheels as light as
possible.
QUESTION 280. _How are the driving-axle boxes arranged so that the
weight of the engine will rest on springs?_
_Answer._ They are arranged so as to slide up and down in the jaws.
Springs, _B_, _B′_, fig. 169, are then placed over the axle-boxes and
above the frames. These springs rest on ~∩~-shaped saddles, _G′_, _G′_,
which bear on the top of the axle-boxes. The frames are suspended to
the ends of the springs by rods or bars, _g_, _g′_, _g_, _g′_, called
_spring-hangers_. As the boiler and most of the other parts of the
engine are fastened to the frames, their weight is suspended on the
ends of the springs, which, being flexible, yield to the weight which
they bear.
QUESTION 281. _How are the frames protected from the wear of the
axle-boxes which results from their sliding up and down in the jaws?_
_Answer._ The insides of the legs, _a_, _a′_, _b_, _b′_, are protected
with _shoes_ or _wedges_, _h_, _h_, which are held stationary, and the
box slides against the faces of the shoes, thus wearing the shoe or
wedge but not the frame.
QUESTION 282. _Why are the shoes usually made wedge-shaped?_
_Answer._ They are made in that way so that when they become worn, by
moving one or both of them up in the jaws, the space between them is
narrowed and the lost motion is taken up. They are moved by the screws
_i_, _i_. If the boxes should become loose from wear, it would cause
the engine to thump at each revolution of the wheels or stroke of the
piston.
QUESTION 283. _How are the springs for the driving-wheels made?_
_Answer._ They are made of steel plates which are placed one on top
of the other. These plates are of different lengths, as shown at _B_,
_B′_, in fig. 169, and are from 3 to 4 in. wide and ⁵⁄₁₆ to ⁷⁄₁₆ thick.
The length of the springs measured from the centre of one hanger to the
centre of the other is usually about three feet.
[Illustration: _Fig. 170._]
[Illustration: _Fig. 171._]
QUESTION 284. _What determines the amount which a spring will bend
under a given load?_
_Answer._ The number of plates, their thickness, length and breadth,
and of course the material of which they are made. This can be
explained if we suppose we have a spring plate of a uniform thickness,
_h_, and a triangular form, of which fig. 170 is a side view and 171
a plan, and that it is clamped fast at its base, _b_. It is a well
known mechanical law that any material of this form and under these
conditions will have a uniform strength through its whole length to
support any load, _P_, suspended at its end, and also that it will bend
or deflect in the form of an arc of a circle.
[Illustration: _Fig. 172._
_Fig. 173._
_Fig. 174._
_Fig. 175._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 285. _How are locomotive springs usually made?_
_Answer._ In locomotives the arrangement of springs is always such that
they are either supported in the middle and moved at the two ends, or
such that they are supported at the two ends and loaded in the middle;
for our consideration it is indifferent which of the two kinds of
springs is taken for the present illustration. That shown in plan and
elevation in figs. 172 and 173, which is formed of a wide plate placed
diagonally, and which in reality consists of two such triangular pieces
as were represented in fig. 171 united at their bases _m m_, and loaded
at two opposite corners, _e_ and _f_, would answer the requirements
mentioned if the great breadth, _m m_, were not an obstacle. This
breadth is obviated by cutting the spring into several strips, _a a_,
_b b_, _c c_, _d d_, ... _i_, fig. 173, of equal width, and placing
these not side by side, but one over the other, as shown in figs. 174
and 175.
In order that the separate strips and layers of the spring so made,
figs. 174 and 175, may not slip out of place, the strips _a a_, _b b_,
etc., are made in one piece, and all the plates are inclosed with a
strap, _F_, figs. 176 and 177. The plates, instead of being cut from
a piece like that represented in fig. 173, are, however, made out of
steel of the proper width, and the ends, instead of being cut off
pointed as represented, are sometimes drawn out thinner on the ends,
like the point of a chisel, or oftener still cut off straight, as shown
in fig. 177.
[Illustration: _Fig. 176._
_Fig. 177._
_Fig. 178._
Scale ³⁄₄ in. = 1 foot.]
In order to hold the plates together a band, _F_, is put around the
middle. This is put on hot, and becomes tight by contracting as it
cools. The centre of the spring has a hole drilled through it with
a pin, _s_, fig. 178 (which shows a cross section of a spring), to
prevent the plates from sliding endwise. The plates at each end usually
have a depression, _a_, fig. 179 (which is a cross section of a plate
on a larger scale than the preceding figure), made in them on one side,
and a corresponding elevation, _b_, on the other. The elevation on one
plate fits into the depression on the other, and thus prevents the
plates from slipping sideways.
[Illustration: _Fig. 179._
Scale 6 in. = 1 foot.]
[Illustration: _Fig. 180._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 286. _How should springs be curved?_
_Answer._ Springs should be curved so that when they bear the greatest
load which they must carry they will be straight. If they are curved
too much they are subjected not only to a strain which bends the
plates, but to one which has a tendency to compress them endwise.
Thus if a spring like that represented in fig. 180 is bent into a
half-circle, it is obvious that the strain at the ends has no tendency
at all to bend the plates, but only to compress them endwise. Near the
middle the strain will of course bend the spring. In the one direction
the spring is flexible and elastic, and in the other it is not; and
as the strain of compression depends on the amount of curvature, the
greater the latter is, the less flexibility and elasticity the spring
will have.
Springs are often given a double curve, as shown in fig. 181. This is
not to be recommended, because when a spring bends the plates must
slide on each other. If they have but a single curve, they will do so
and remain in contact through their whole length, but if they have two
curves they will separate and therefore “gape,” as it is called.
[Illustration: _Fig. 181._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 287. _What is meant by the elasticity of a spring?_
_Answer._ It is the amount which a spring will deflect or bend under a
given load without having its form permanently changed. If the bending
is so great that the spring does not recover its original form when
the load is removed, then the strain to which it is subjected is said
to exceed the _limits of elasticity_, and if repeated often it will
ultimately break the spring.
QUESTION 288. _What is meant by the elastic strength and the ultimate
strength of a spring?_
_Answer._ The _elastic strength_ is the strain it will bear without
being strained beyond the limits of elasticity, and the _ultimate
strength_ is the strain which will break it.
QUESTION 289. _What determines the strength of a spring?_
_Answer._ It depends of course (1) upon the material of which the
spring is made; (2) its strength increases in proportion to the number
of plates, and (3) to their width, and (4) in proportion to the square
of their thickness, and (5) as the length diminishes.
Thus, if we wanted to double the strength of a spring like that shown
in figs. 170 and 171, it could be done in either of the following ways:
(1) by making it of material twice as strong; (2) by putting another
plate just like it on top; (3) by doubling the width of the base _b_,
which would make the strength of the whole plate twice what it was
before; (4) by making the whole plate about four-tenths thicker, which
would increase its strength, as already stated, in proportion to the
square of the thickness as 1.4 × 1.4 = 2 nearly; (5) by reducing the
length to one-half what it is in fig. 170.
QUESTION 290. _What determines the elasticity of a spring?_
_Answer._ (1) The material of which it is made; with the same material
the elasticity increases (2) as the number, and (3) as the width of
the plates diminishes, and (4) with the cube of the length, and (5)
decreases with the cube of the thickness of plate.
Thus, supposing the plate in figs. 170 and 171 to be ³⁄₈ in. thick and
the deflection _d_ 2¹⁄₂ in.; the latter would be only half as much or
1¹⁄₄ in. (1) if it were made of material twice as stiff, or (2) with
two such plates, or (3) with one twice as wide at the base. If (4) the
length were doubled, the deflection would be equal to 2 × 2 × 2 = 8
times what it was before, or in proportion to the cube of the length.
If (5) the thickness were doubled the deflection would be _reduced_ in
the same proportion, and would be only one-eighth of 2¹⁄₂ in., or ⁵⁄₁₆
in.
[Illustration: _Fig. 182._
_Fig. 183._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 291. _What should be the proportion of the plates of a spring
in relation to each other?_
_Answer._ The lower plates should diminish regularly in their lengths.
The reason for this will be apparent from the fact which has already
been stated, that if a triangular plate of uniform thickness is clamped
fast at its base, it will, if loaded at the end, be of uniform strength
throughout its whole length. It is immaterial what the length of the
base of such a triangle is, if the two sides are of equal length and
the thickness of the plate is uniform, not only its strength but the
amount of deflection or bending from any load will be equal all through
its length. If, therefore, we make a spring by cutting a plate formed
of two such triangular pieces united at their bases into strips, as
has already been explained, evidently the spring made of them will
have a uniform strength throughout its whole length. As the strips thus
made diminish in length regularly, it is evident that if the spring
plates are made of steel rolled of the requisite width, their length
should be the same as that of those cut from the plate referred to
above. When this is the case the lower outline, _a b b a_, fig. 183,
of the spring will, when the spring is not bent, be straight lines.
Sometimes the lower outline of springs is made curved, as shown in fig.
182. This gives too much strength near the strap _F_, and too little
near the ends. In drawing springs, therefore, it is best to lay them
out with the plates straight, as shown in figs. 182 and 183, and after
determining the thickness, drawing a straight line from the strap to
the end of the longest plate will give the best form of the spring
and the length of each of the plates. It is necessary, however, to
put a sufficient number of long plates in each spring to give it the
required strength next to the attachment of the hanger. Sometimes one
or more of these long plates are made heavier than the rest. The evil
of this method of construction will be apparent if it is remembered
that the greatest permissible deflection up to the breaking of the
spring decreases with the _cube_ of the thickness of the plate and
its strength increases with the _square_ of the thickness. Now if we
have a spring with say ten plates ³⁄₈ in. thick and one on top ³⁄₄ in.
thick, the thick plate will have a strength _four_ times that of the
thin plates, but its elasticity will be only one-eighth that of the
thin plates, and therefore it will require eight times as much load to
bend it any given distance as is needed to bend the thinner plates the
same distance. But its strength is only four times that of the thin
plates, so that for any given amount of elasticity the thick plate must
bear twice as much load as it has strength to carry. This shows what a
great mistake is committed if some of the plates are made thicker than
others, a conclusion which is supported by practical experience, as
it is found that if the top plates are made thicker than others, the
thick ones break most frequently, which is the necessary result of the
supposed strengthening by increasing the thickness of the top plates.
QUESTION 292. [73]_How can we find by calculation the elasticity or
deflection of a given steel spring?_
[73] The following rules for calculating the proportion and strength
of steel springs are from Clark’s Railway Machinery.
_Answer._ BY MULTIPLYING THE BREADTH OF THE PLATES IN INCHES BY THE
CUBE OF THE THICKNESS IN SIXTEENTHS, AND BY THE NUMBER OF PLATES:
DIVIDE THE CUBE OF THE SPAN[74] IN INCHES BY THE PRODUCT SO FOUND, AND
MULTIPLY BY 1.66. THE RESULT IS THE ELASTICITY IN SIXTEENTHS OF AN INCH
PER TON OF LOAD.
[74] The span is the distance between the centres of the
spring-hangers when the spring is loaded.
QUESTION 293. _How can we find the span due to a given elasticity and
number and size of plates?_
_Answer._ BY MULTIPLYING THE ELASTICITY IN SIXTEENTHS PER TON BY
THE BREADTH OF PLATE IN INCHES, AND BY THE CUBE OF THE THICKNESS IN
SIXTEENTHS, AND BY THE NUMBER OF PLATES: DIVIDE BY 1.66, AND FIND THE
CUBE ROOT OF THE QUOTIENT. THE RESULT IS THE SPAN IN INCHES.
QUESTION 294. _How can we find the number of plates due to a given
elasticity, span, and size of plate?_
_Answer._ BY MULTIPLYING THE CUBE OF THE SPAN IN INCHES BY 1.66; THEN
MULTIPLYING THE ELASTICITY IN SIXTEENTHS BY THE BREADTH OF PLATE IN
INCHES, AND BY THE CUBE OF THE THICKNESS IN SIXTEENTHS: DIVIDE THE
FORMER PRODUCT BY THE LATTER. THE QUOTIENT IS THE NUMBER OF PLATES.
QUESTION 295. _How can we find the working strength, that is the
greatest weight it should bear in practice, of a given steel-plate
spring?_
_Answer_. BY MULTIPLYING THE BREADTH OF PLATES IN INCHES BY THE SQUARE
OF THE THICKNESS IN SIXTEENTHS, AND BY THE NUMBER OF PLATES; MULTIPLY,
ALSO, THE WORKING SPAN IN INCHES BY 11.3: DIVIDE THE FORMER PRODUCT
BY THE LATTER. THE RESULT IS THE WORKING STRENGTH IN TONS (OF 2,240
POUNDS) BURDEN.
QUESTION 296. _How can we find the span due to a given strength, and
number and size of plate?_
_Answer_. BY MULTIPLYING THE BREADTH OF PLATE IN INCHES BY THE SQUARE
OF THE THICKNESS IN SIXTEENTHS, AND BY THE NUMBER OF PLATES; MULTIPLY,
ALSO, THE STRENGTH IN TONS BY 11.3: DIVIDE THE FORMER PRODUCT BY THE
LATTER. THE RESULT IS THE WORKING SPAN IN INCHES.
QUESTION 297. _How can we find the number of plates due to a given
strength, span and size of plates?_
_Answer_. BY MULTIPLYING THE STRENGTH IN TONS BY THE SPAN IN INCHES,
AND BY 11.3; MULTIPLY ALSO, THE BREADTH OF PLATE IN INCHES BY THE
SQUARE OF THE THICKNESS IN SIXTEENTHS: DIVIDE THE FORMER PRODUCT BY THE
LATTER. THE RESULT IS THE NUMBER OF PLATES.
QUESTION 298. _How can we find the required amount of curvature or set
of the spring before it is loaded?_
_Answer._ BY MULTIPLYING THE ELASTICITY, PER TON, IN INCHES, BY THE
WORKING STRENGTH IN TONS; ADD THE PRODUCT TO THE DESIRED WORKING
COMPASS. THE SUM IS THE WHOLE ORIGINAL SET, TO WHICH AN ALLOWANCE OF
¹⁄₈ TO ³⁄₈ IN. SHOULD BE ADDED TO THE PERMANENT SETTING OF THE SPRING.
QUESTION 299. _How are the spring-hangers attached to the ends of the
springs?_
_Answer._ A great variety of methods have been used. The most common
ones are those shown in fig. 169. There the hanger embraces the spring
at the ends, _g_, _g_, (shown on an enlarged scale at _a_, in figs.
176 and 177.) The end of the spring has two projections forged on its
end to receive the upper end of the hanger, which is made to fit the
groove thus formed between the two projections. The other end, _b_,
of the spring, figs. 176 and 177, has an eye cut in it which receives
the hanger _b_. The latter is made of a single bar, and also has an
eye, _c_, to receive a key which sustains the weight suspended on the
hanger _b_. The back end of the front springs and the front end of the
back springs are made in this way because they come on the side of the
fire-box, and if their width was increased by the thickness of the
hanger, as shown at _a_ in fig. 177, it would rub against and wear the
outer shell of the fire-box.
QUESTION 300. _How are the lower ends of the hangers attached?_
_Answer._ The front hanger, _g_, fig. 169, of the front spring, and the
back hanger, _g_, of the back spring have eyes and pins in their lower
ends, _k_, as shown in the engraving. The pins are supported by rubber
springs, _l_, _l_, which are held between two concave castings, _n_,
_k_, one of each of which rests against the frames. The object of the
rubber springs is to relieve the spring-hangers from sudden shocks and
strains. The benefit derived from their use is believed to be purely
imaginary, as the spring itself, if sufficiently elastic, should absorb
the sudden shocks which the wheels and axles will convey to the hangers.
QUESTION 301. _Why are the ends g′, g′ of the springs attached to the
lever_[75] _A A?_
[75] This lever is called an _equalizing lever_ or _beam_, or, more
briefly, an _equalizer_.
_Answer._ Because if there is a spring for every axle and the hangers
are fastened to the frame, then evidently the locomotive has as many
points of support as it has axle-boxes. Every shock from the rails is
transferred through the wheel and the axle to the nearest axle-box
and the spring belonging to it, and the latter must be made strong
enough to receive and dispose of the whole of it. If the adjacent
hangers, _g′_, _g′_, fig. 169, of the adjoining springs, _B_ and _B′_,
are connected by an equalizing lever, _A A_, which turns on the fixed
point _C_, then the shock which affects one wheel will be transferred
first to the corresponding spring. From this spring a part of the shock
will be transferred to the frame by the hanger _g_, and a part by the
hanger _g′_ to the equalizer, which will transfer the pressure to the
adjoining spring _B′_. If by some unevenness of the road or a powerful
oscillation of the locomotive, a spring is momentarily burdened, the
equalizer thus causes the next wheel to receive part of this load.
The advantages of this arrangement are evident: since the springs have
to receive only a part of the shocks, they can be made less strong and
therefore more flexible. The danger of running off the track and that
of breaking axles, springs and hangers, is therefore reduced by the use
of equalizing levers.
QUESTION 302. _How are the equalizing levers constructed?_
_Answer_. They are made of wrought iron and are supported in the centre
by a fulcrum, _C_, which is fastened to the frame or boiler or both.
The spring-hangers _g′, g′_ are usually attached to the lever by eyes
and keys. Sometimes eyes are made in the lever, as shown in fig. 169,
and the hanger is inserted into the eye and held either with a key or
else with projections which are forged on the hanger below the lever.
In other cases the hangers are made with an eye which embraces the end
of the lever.
QUESTION 303. _How is the distribution of weight of the engine affected
by the equalizing levers?_
_Answer_. The weight is equally distributed on all the driving-wheels.
This is apparent if it is observed that the weight suspended from each
of the spring-hangers of each spring in fig. 169 must be the same; for
if the weights in the two hangers, _g′_ and _g′_, were unequal, then
the end of the spring which supports the heaviest weight would be drawn
down until the pressure was equalized. If the weights suspended from
the two hangers, _g′_ and _g′_, attached to the equalizing lever were
unequal, then the one supporting the greatest load would draw up its
end of the equalizer until the weights were again in equilibrium.
[Illustration: _Fig. 184._
_Fig. 185._
Scale ³⁄₈ in. = 1 foot.]
Another effect of the equalizing levers is that each side of the
locomotive is supported in such a way that the action is the same as
it would be if it was supported on one point. If, for example, we
have a heavy beam, say a piece of timber like that shown by _A B_,
fig. 184, suspended at one point, _C_, in its centre, to the middle,
_a_, of a long spring, _D E_, the ends of which rest on two supports,
_F_ and _G_, it is evident that if the point of suspension is at the
middle, _C_, of the timber and _a_ of the spring, the weight of the
timber will rest equally on the two supports, _F_ and _G_, and that
the ends of the timber can move up or down or vibrate about the point
of suspension, _C_, without affecting the distribution of weight on
the supports, _F_ and _G_. If, now, the timber is suspended from three
points, _A_, _C_ and _B_, fig. 185, that is, its middle and two ends,
as shown in fig. 185, the ends, _A_ and _B_, being attached to the ends
of the springs _b c_ and _d e_, the latter resting on the supports _F_
and _G_, and connected at their opposite ends to an equalizer, _f g_,
whose fulcrum is at _a_, it is evident that each of the end hangers
must support one-half of that part of the weight of the timber between
it and the middle, and that the centre hanger must support one-half
the weight between the middle and the two ends. Thus the hanger _A b_
must support one-half the weight of the timber between _A_ and _C_,
and _B e_ must support one-half of that between _B_ and _C_; in other
words, the end hangers would each sustain one-fourth of the weight of
the timber and the middle one-half of its weight. If the weight of
the timber is 1,000 pounds, the end hangers would each sustain 250
and the middle one 500 pounds. The weight of the middle of the timber
is hung on the equalizer, and one-half, or 250 pounds of it is thus
transferred to each of its ends _f_ and _g_, and thence to the hangers
_f c_ and _g d_, and thus to the springs, so that the ends, _c_ and
_d_, of the springs, sustain a weight of 250 pounds, therefore, as the
opposite ends also sustain the same weight, it is evident that each
of the springs bears a total load of 500 pounds, or one-half of the
weight of the timber, which is the same load they sustained in fig.
164. If the ends of a timber supported as shown in fig. 185 are moved
up or down about the centre point of suspension, it is evident that
the distribution of weight would not be affected any more than it was
in fig. 164 by a similar movement, because if the ends of the timber
move as shown by the dotted lines around the centre point of suspension
_C_, the end _A_ will ascend as much as _B_ descends. The same thing
is true of the ends _b_ and _e_ of the springs and of their opposite
ends _c_ and _d_, and also of the ends of the equalizer, so that when
the timber, springs and equalizer are in the position shown by the
dotted lines, it is in equilibrium, just as it was when the timber
was horizontal; and therefore the weight on the supports is the same
in both cases, thus showing that the load _A B_ can move about the
centre of suspension when supported as shown in fig. 185 as freely as
it can if arranged as shown in fig. 184. It therefore follows that in
the distribution of the weight of each side of the locomotive on the
wheels and on the track, it may be regarded the same as though it was
supported at one point, which is the fulcrum of the equalizing-lever.
QUESTION 304. _What advantage results from supporting the weight of the
back part of the locomotive on two points?_
_Answer_. If the back part of the locomotive rests on only two points
and the front end on the centre of the truck, then the whole weight
of the engine will be sustained on three points. Now it is a well
known fact that any tripod, like that on which an engineer’s level is
mounted, or a three-legged stool, will adjust itself to any surface,
however uneven, and stand firmly in any position; whereas if there are
more than three points of support, if they are all of the same length
the surface on which they rest must be a plane, otherwise some of them
will not touch. All railroad tracks have inequalities of surface, and
therefore it is of the utmost importance that a locomotive should be
able to adjust itself on its points of support to any unevenness of the
track on which it must run. This is possible only when the weight rests
on three points of support.
[Illustration: _Fig. 186._
_Fig. 187._
_Fig. 188._
_Scale, ³⁄₈ inch = 1 foot._]
QUESTION 305. _How is the truck constructed?_
_Answer_. It consists, as has already been stated, of two pairs of
wheels.[76] These are attached to a frame, _b′ b′_, plates I, II and
III. The axles have boxes called _truck-boxes_, and brass bearings
similar to those used on the driving-axles. These boxes work in jaws,
also similar to those on the main engine frame, excepting that they
have no attachment to prevent them from being worn by the motion of
the boxes up and down in the jaws. Fig. 186 is a horizontal section,
fig. 187 a plan, and fig. 188 a transverse section[77] of a truck. The
frame _C D E F_, fig. 187, shown also at _h′ h′_, figs. 186 and 188, is
of rectangular form and is forged in one piece. The legs _f f_ which
form the jaws for the boxes, are bolted to the frame as shown in fig.
186. To the lower end of these legs a brace, _g g g_, is bolted, which
thus unites them together. On each side one spring, _S F S_, is placed
under the frame and in the reverse or inverted position to that of
the driving-springs. A pair of equalizing levers, _G G G_, is placed
on each side of the truck, one of them on the inside of the frame and
the other on the outside, as shown in the plan. The ends of these
equalizers rest on the top of the truck-boxes, and the springs are
attached to the levers at _i i_ by the hangers, _j j_. The truck-frame
rests on the top of the spring-strap, _F_, which is made of the form
of an arc of a circle, or “_rounded_,” as it is termed by workmen, so
that it can move freely about the point of support. It is evident that
this arrangement of spring and equalizer operates in the same way as
that employed for the driving-wheels in distributing the weight on each
of the wheels, and that the truck-frame is supported on two points,
_k_, _k_, figs. 186 and 187. The weight of the front end of the engine
rests on a cast iron _centre-plate_, _H H_. This centre-plate rests
on four bars, _l l_, _l l_, and _m m, m m_, two of which are bolted
to the frame transversely and the other two longitudinally, as shown
in the plan. These bars are elevated in the centre as shown in figs.
186 and 188. The transverse bars are trussed with two corresponding
bars, _n n_, fig. 188, below. These _truss-bars_ as they are called,
are bolted to the upper bars with bolts, _o_, _o_, but are separated
from the top-bars by distance pieces, _P_, _P_, figs. 186 and 188.
The centre-plate _H H_, called the _lower centre-plate_, has an
annular groove in it, which receives a corresponding projection on
the casting _K K_, called the _upper centre-plate_, which is bolted
to the bed-plate of the cylinders, as shown in plate II. The upper
centre-plate has a pin, _y_, called a _centre-pin_, fig. 186 and plate
II, attached to it, which passes through the lower centre-plate, and
has a key underneath the latter plate. This key is intended to prevent
the engine from “jumping” off of the truck on a rough track or in case
of accident. The annular groove and the projection which fits into it
are intended to receive the strain which otherwise would bear against
the centre-pin and would be liable to break or bend it.
[76] In some rare cases three pairs of wheels are employed for
locomotive trucks. Six-wheeled trucks are very commonly used under
passenger cars.
[77] The right half is a section through the centre of the axle, or
of _G g_, of fig. 186, and the left half a section through the centre
of the truck, or on _g g_, of fig. 186.
From this description it will be seen that while the truck-frame rests
on two points, _k_ and _k_, the weight of the engine is supported by
the centre-plate of the truck. As the back part substantially rests
on the centres of the two equalizers, it will be seen that this
distribution of the weight fulfills the conditions of the tripod, or as
it has been called, the “_three-legged principle_.”
[Illustration: _Fig. 189._
_Fig. 190._
_Fig. 191._
Scale ³⁄₈ in. = 1 foot.]
QUESTION 306. _How are trucks arranged so as to give them lateral
motion?_
_Answer._ When this is done, the lower centre-plate is usually
suspended in some way from the truck-frame on links or hangers, so that
it can swing laterally. One method of doing this is shown in figs. 189,
190 and 191. Fig. 190 is a front view, fig. 191 a plan, and fig. 189 a
transverse section of such an arrangement. The centre-plate _H H_ has
cast with it an extension, _B B_, the ends of which are suspended on
links, _L_, _L_, called _suspension-links_, the upper ends of which are
attached to bars, _m_, _m_, which are set edgeways and extend across
the truck-frames. It is evident that with this arrangement the lower
centre casting can swing crosswise of the track on the links _L_, _L_,
and that the front end of the engine will thus have a lateral motion
independent of the truck.