small independent compressor on each car, but is equipped with a large
storage tank, in which air is carried under high pressure—250 to
300 pounds per square inch. This storage tank is filled at regular
intervals when the car passes some point on its route at which a
compressor is located. In this case the car is obliged to stop long
enough to make connection to the tank of the compressor plant, and
to allow the car storage tank to be filled. This operation, however,
does not take long. The advantages of the system are a saving of the
weight and a saving in the maintenance of a small compressor on each
car. From the main storage tank under the car, air is led through a
reducing valve to an auxiliary storage tank. This reducing valve allows
enough air to pass through to maintain a pressure of about 50 pounds
per square inch in the auxiliary storage tank. The auxiliary storage
tank corresponds to the regular storage tank on a system employing
compressors on each car. The method of operation after the air has
entered the auxiliary storage tank is the same as with any air-brake
system.
[Illustration: Fig. 54. Westinghouse Air Compressor.]
Fig. 55 shows the arrangement of the apparatus under the cars of the
St. Louis Transit Company. The two storage tanks are each 6 feet long
by 18 inches in diameter and are mounted one on each side of the car.
Their combined capacity is equivalent to about 100 cubic feet at 45
lbs. pressure. The tanks are charged through an outlet near one side of
the car. This outlet contains a check valve and cock to prevent leakage.
The service or low pressure reservoir has a capacity of about 2½ cubic
feet. The position of the reducing valve between the high and low
pressure valves may be noted in the illustration.
=Momentum Brakes.= Momentum or friction brakes have been used to some
extent both on single-truck and on double-truck cars, but particularly
on single-truck cars. They derive the power to operate the brakes from
the momentum of the car by means of a friction clutch on the car axle.
The difference in various kinds of momentum brakes lies chiefly in the
design of the clutch mechanism. The clutch must evidently be arranged
to act very smoothly, and must be under very accurate control, as the
force with which the brakes are applied depends directly upon the pull
exerted by the clutch.
[Illustration: Fig. 55. Arrangement of Storage Air Brake Apparatus.]
In the Price momentum brake a flat disc is cast on the car wheel, which
is turned off to a smooth surface. Against this disc a friction clutch
acts, which has a leather face. The clutch is operated by a motorman’s
lever through a set of levers. A small movement in the motorman’s lever
forces the clutch against the disc on the car axle. The clutch winds up
the brake chain, and thus supplies power to apply the brakes.
Other momentum or friction clutch brakes have been devised, most of
which also use an application of leather on iron for the clutch, as
this has been found to be most reliable, and to be least affected
by the grease and dirt that is liable to work in between the clutch
surfaces.
=G. E. Electric Brake.= The General Electric Company’s electric brake
makes use of current generated by the motors acting as dynamos, to stop
the car. In order to accomplish this, a brake controller is provided
which reverses the armature connections of the motors, and so connects
them to operate as dynamos sending current through a resistance in the
circuit; the amount of current flowing and the braking effect depending
on the car speed and the resistance. In some forms of brake controller,
the two controllers are combined in one cylinder, so that the motorman,
to apply the electric brake, simply continues the movement of the
handle past the “off” position. In others, the brake-controller drum is
separate, but is interlocked with the main controller so that it can be
used only when the main controller is off.
However the controller may be arranged, the principle involved is
that when the motors are revolving by the motion of the car, and the
armature connections are reversed as they would be to reverse the
direction of motion of the car, the motors begin to generate current as
series-wound dynamos. The amount of current generated and the retarding
effect will depend on two things—namely, the speed of the car, with
the consequent electromotive force in the motors, and the amount of
resistance in the circuit. The amount of resistance is regulated by the
motorman by means of his electric brake controller. The function of
the electric brake controller is to reverse the motors and to insert
enough resistance in the circuit to make a comfortable stop. This
current in the motors acting as dynamos, in itself acts as a powerful
brake to retard the motion of the car. In the General Electric type of
electric brake, the current generated in the motors, in addition to
having this retarding effect in the motors themselves, is conducted to
brake discs that act as magnetic clutches against one of the car wheels
on each axle. The car wheel has a disc cast upon it, and against this
the magnetic disc acts. The magnetic disc contains a coil which is in
series in the brake circuit.
In applying an electric brake of this kind the motorman first puts
the controller on a point that inserts considerable resistance in the
circuit. When the motors have slowed down, the electromotive force, of
course, drops, so that to maintain the same braking current there must
be a reduction of the amount of resistance, until, when the car is
almost at a standstill, the resistance is nearly all cut out. It might
seem at first that the current would die down before the car came to
a stop, but it is found that there is enough induction in the motor
fields to cause current to flow for a short time after the car has
stopped. The residual magnetism in the steel in the fields of the motor
is sufficient to cause the motors to begin to generate current when the
electric-brake controller is first turned on.
The greatest advantage of an electric brake using motors as generators
is in the fact that the braking current instantly falls in value as
soon as the wheels begin to slide, and releases the brake until the
wheels again revolve. In fact, it is almost impossible to skid the
wheels as they are sometimes skidded by being locked by brake shoes.
This not only prevents flat wheels but insures a quick stop, because
when the wheels are locked and sliding, the braking or retarding power
is only about one-third what it was before the wheels began to slide.
The electric brake requires extra large motors because of the heating
caused by the current generated while braking.
[Illustration: Fig. 56. Magnetic Brake Shoe.]
=Westinghouse Electromagnetic Brake.= The Westinghouse magnetic brake
is in principle similar to the General Electric brake as far as the use
of motors as generators is concerned; but, instead of assisting the
motors by means of a magnetic brake disc acting against the car wheel,
a magnetic brake shoe is used (see Fig. 56), which acts against both
car wheel and track. This not only retards through the medium of the
wheels but acts directly on the track. It is not dependent upon the
coefficient of friction between the wheels and track; and it should,
therefore, be possible to stop much more quickly than with any form of
brake depending upon the coefficient of friction between the wheels and
track.
[Illustration: MAGNETIC BRAKE SHOWING METHOD OF ATTACHING TO CAR FRAME
AND TRUCKS.
Westinghouse Air Brake Co.]
=Track Brakes.= Track brakes have been used to some extent on very
hilly electric roads. These have a shoe fastened to the truck frame,
which acts directly on the track.
=Motors as Emergency Brakes.= The motors can of course be used to brake
the car by simply reversing them if current is applied to them from the
line. But in case the trolley flies off or if the circuit breaker or
the fuse opens the circuit, or the supply of current is interrupted for
any other reason, they may be used as brakes by throwing the reverse
lever and moving the controller handle to the multiple position of
a two-motor equipment or by simply throwing the reverse lever of a
four-motor equipment. These movements throw the motors in multiple and
connect the fields and armatures of the motor in such relation that
they can generate current. One of the motors then acts similarly to a
generator in a power house, deriving its power from the momentum of the
moving car instead of from an engine, and sends current through the
other motor of the pair which acts like any auxiliary motor trying to
revolve its wheels in the opposite direction from that in which they
are revolving. The motors of a four-motor equipment are permanently
connected in two multiple groups as long as the reverse is not in the
central position. In the two motor equipment such connections are not
made until the controller handle is turned to the multiple position.
As the external resistance is beyond the junction of the two motor
circuits, the braking effect is not increased by cutting out the
resistance.
The difference in the residual magnetism of the fields or in the
magnetic qualities of the fields of the two motors is primarily the
cause of the generation of the current. The motors at first act in
opposition, but one of them generates the higher voltage and forces
a current through the other. This current overcomes the residual
magnetism of the second motor, thereby changing its polarity and
both motors then act in series to send the current through the low
resistance path afforded by the windings. Any current passing increases
the strength of the fields and consequently the voltages, so that
abnormal currents are generated and the braking action is consequently
severe.
This generating action does not take place before the reverse lever is
thrown because the connections of the armatures and fields are such
that any current generated by reason of the residual magnetism of the
fields, flows in such a direction through these that this magnetism is
destroyed. The current then ceases to flow. This explains why current
is not generated in No. 2 motor with a K type of controller during the
change-over period when it is short-circuited, or in equipments when
the trolley flies off and the controller is turned on.
[Illustration: Fig. 57. Pneumatic Sander.]
=Brake Shoes.= The subject of brake shoes is of very little importance
on the smaller cars traveling at slow speeds and controlled alone by
hand brakes. On the larger high speed interurban cars, the brake shoe
question becomes an important item because of the rapidity with which
they are worn away. On such cars shoes sometimes last but about one
week. This means eight shoes per week per car or an expense of about
$4.00 per car per week.
Brake shoes are usually of soft gray cast iron with inserts of steel,
although some companies use very hard iron. They are usually fastened
by means of a key to a brake shoe head permanently attached to the
brake rigging. The brake levers are so adjusted that the shoes clear
the wheels about ³⁄₁₆-inch when the brakes are released. This distance
increases as the shoes wear, so that the brakes must be adjusted
frequently to take up the slack and prevent waste of air.
=Track Sanders.= A sprinkling of sand on the rail increases wonderfully
the adhesion of the rail and wheel. There is usually on cars some
provision made for scattering sand on the rails immediately in front
of the leading wheels. From sand boxes placed under the seats in the
smaller cars, or on the truck of the larger ones, flexible hose or
pipes drop within an inch or two of the rail in front of the leading
wheels. A valve under the control of the motorman regulates the flow of
sand to the rail. Sometimes air pressure is used to blow the sand out
of the sand box into the hose. In this case air pressure is obtained
from the air brake system, and an air valve leading to the sand box is
placed in the motorman’s cab. A section through a pneumatic sander of
this kind is shown in Fig. 57.
[Illustration: Fig. 58. Curves of Braking Tests.]
=Coefficient of Friction.= It has been found by experiment that the
coefficient of friction between the car wheel and rail is about 25
per cent of the weight on the wheel when the rails are dry; that is,
a car wheel having a weight of 2,000 pounds upon it would not be able
to exert either an accelerating or a retarding force exceeding 25 per
cent of this, or 500 pounds. This is when the wheel is rolling. There
is apparently a kind of locking or inter-meshing of the rough surfaces
of wheel and rail when the wheel is rolling, because it is found that
when a wheel begins to skid or slide, the coefficient of friction falls
off about two-thirds. The maximum braking or retarding force that can
be obtained, therefore, in a dry rail, amounts to 25 per cent of the
weight of the car. If the rail is slippery this is much reduced; or
if the wheels are allowed to slide it is also much reduced. If more
retarding force than can be obtained through the medium of a wheel
rolling on the rail is desired, it must be obtained either by the track
brakes or by magnetism.
[Illustration: Fig. 59. Automatic Coupler.]
=Rate of Retardation in Braking.= The rate of retardation of cars in
braking is usually 1 to 2 miles per hour per second. In other words a
car going at a speed of 40 miles an hour will usually be stopped in 40
to 20 seconds.
The plotted results of some braking tests (Fig. 58) show a higher rate
of acceleration. These tests were made on an interurban car weighing
about 63,000 pounds, equipped with straight air brakes. Of the six
curves shown, that giving the highest rate of retardation is No. 4.
This shows a stop from a speed of 38 miles per hour in 9½ seconds or a
rate of retardation of about 4 miles per hour per second. All of the
curves shown are for emergency stops. They show about the highest rate
of retardation that could be made with the equipment.
=Drawbars and Couplers.= For small surface cars a crude drawbar is
usually provided consisting simply of a straight iron bar pivoted under
the car and provided with a cast-iron pocket near the end. A coupling
pin passing through the pocket of one coupler and through a hole in the
end of the bar of the other, holds the two cars together.
The requirements of a coupler for heavier cars such as those used on
interurban and elevated roads are more exacting. The ends of the bars
are usually pivoted under the car about five feet back from the bumper.
A spring cushion intervenes between the pivot point and the drawbar
head. The illustrations, Figs. 59 and 60, show the action of the Van
Dorn Automatic coupler, which is the one used by all the elevated lines
in the United States. The link is placed in one of the drawbar heads
and the pin in the other. As the cars come together the wedge-shaped
end of the link forces its way between the pin and a spring. When the
faces of the drawbar heads meet, the spring forces the link to engage
the pin. The mechanism is designed especially to prevent lost motion
between coupler heads because, unlike steam railroad drawbars, electric
car drawbars must swivel to round curves and a great amount of play at
the point of coupling with swiveling drawbars would allow the couplers
to bend under a pushing strain.
[Illustration: Fig. 60.]
CAR CONSTRUCTION.
=Car Bodies.= In cities there are three general types in common use;
namely, box cars, suited for winter use only; open cars, suited for
summer use only; and semi-convertible cars, which can be adapted to
either summer or winter use. The open and box cars are the older types.
The semi-convertible car is usually provided with a center aisle, and
cross seats on each side of this aisle. [Illustration: Fig. 61. Side
Elevation and Plan of Car.] The windows are large, so that they can be
lowered or raised in summer to make something approaching the character
of an open car. The car bottom, which forms the basis for the entire
car structure, is constructed with longitudinal sills either of steel
or of wood combined with steel. One form of construction employs as
the main supports two steel channel bars extending the full length
of the car. Steel I-beams are sometimes used. Where wood is used in
combination with steel for longitudinal sills, the steel is usually in
the form of flat steel plates between the timbers. Most cars seat about
one passenger per foot of length over all.
[Illustration: Fig. 62. Cross-Section of Car Body.]
Many more difficulties are met in the construction of passenger
cars for electric railways than in steam coach construction. The
electric car must have low steps and platforms and turn short curves.
The difficulties are largely in the floor framing of the car. The
platforms at each end are usually eight to ten inches lower than the
floor of the interior. As the car must frequently be designed to pass
around curves of small radius, often of only thirty or forty feet,
sufficient clearance must be provided for the swing of the trucks.
This necessitates that the trucks of a double truck car be set far
enough back towards the center of the car to clear the dropped platform
timbers, shown in Fig. 63. In the illustration shown, Fig. 61, the
truck centers are but 21 feet 8 inches apart, while the ends overhang
the truck centers 11 feet 4½ inches. It is difficult to support this
overhanging weight properly. The difficulty is increased by the fact
that the rear platform is often crowded with passengers having an
aggregate weight of one ton or more. Trusses manifestly cannot be
employed to give rigidity to the long platform. This is usually given
in cars of wood construction by reinforcing the platform timbers
with steel plates as shown in the figure. In order that the dropping
tendency of the platform shall not bow up the body of the car between
the trucks this portion must be braced rigidly. The space below the
windows and above the side sill is utilized for this purpose. The side
sill is moreover strengthened by having steel plates bolted to it.
[Illustration: Fig. 63. Reinforcing Plates.]
The longitudinal members of the body framing are termed sills. These
are usually of long leaf yellow pine. Various combinations of wood and
steel are employed for sills, an example of which is seen in Figs. 61
and 62. The sills are kept the proper distance apart by “bridgings”
or cross sills mortised into them at intervals and by “end sills.”
The whole framing is tied together by the rods running parallel to
the bridging. These tie rods are often provided with turn buckles for
tightening when occasion may require. The outer sills are termed side
sills; those nearest the center of the car, the center sills or draft
timbers; while those between are called intermediate timbers.
The remaining portion of the car is constructed much after the manner
of a steam coach. The posts between the windows are mortised into
the side sill at the bottom and into a top sill at their upper end.
They are laterally braced by a belt rail immediately under the window
opening, both the belt rail and the posts being gained out so that the
rail fits flush with the posts. A wide letter board gained into the
post just below the side plate adds to the bracing of the side of the
car, as does also an iron truss usually one-fourth to one-half inch
thick and two to three inches wide which is gained into the posts on
the inside running just under the windows between the truck centers,
and then descends to pass through the side sills and fasten by a bolt
underneath.
The roof consists of the upper and lower decks. That portion over the
platform or vestibule is termed the hood. Rigidity is given to the
whole upper portion of the car by the end plates resting on the corner
posts and extending between the side plates at either end of the car
body proper, and by steel carlins which conform to the peculiar shape
of the roof and extend between the side plates. The steel carlins
are usually placed over alternate side posts. Bolted on either side
of them and placed at intervals of about twelve inches between are
wood carlins. The wood carlins of the lower deck extend from the side
plate, to which they are fastened by screws, to the top sill, which is
immediately below the windows of the upper deck. Above these windows is
the top plate, supporting the carlins of the upper deck, which extend
between and a few inches beyond the two top plates. Poplar sheathing
three-eighths or one-half inch is nailed over carlins and on this heavy
canvas usually of six or eight ounce duck is stretched tightly. Several
coats of heavy paint on the canvas and a trolley board for supporting
the trolley stand complete the roof. On the underside of the carlins
the headlining, usually of birch or birdseye maple, is secured. This
forms the interior finish of the ceiling of the car.
=Steel Car Framing.= As a result of the demands of the officials of
the New York Subway for cars of greater strength and less subject to
danger from fire, much progress has been made in the last few years
in the construction of cars with steel framing. Steel construction is
much more expensive than that in which the framing is of wood and is
considerably heavier. The advantages lie partly in the fact that it is
more durable, but the great reason for the interest with which the new
style of construction has been received is that the danger of collapse
and consequent injury to passengers, in case of accident, is greatly
diminished.
=Car Weights.= The total weight of a street car with a body 16 feet
long over corner posts mounted on a single truck with two motors is
approximately 14,000 pounds. Of this the body weighs about 4,500
pounds, the truck 4,400 pounds, and the motors and the electrical
equipment the remaining 5,100 pounds. The weights of the separate
parts of a certain interurban car measuring 52 feet 6 inches over the
bumpers mounted on double trucks, one of which carried two motors, is
body 34,065, motor truck 9,565, trail truck 6,670, electrical equipment
12,800; total 63,100.
An interurban car of about the same size as the one just mentioned
but equipped with four motors gave the following weights: Body with
controller and resistance grids 39,000 pounds, trucks 19,130 pounds,
motors 15,420 pounds; total 73,550 pounds.
=Car Painting.= A great deal of attention is given to the proper
painting of cars. A car painted with care and proper materials always
presents an attractive appearance, while one carelessly painted is
readily noticeable. New cars go through an elaborate painting process.
The time required is from two to three weeks. The following scheme may
be regarded as an example of a good process:
A coat of primer is given the car the first day. On the third
day all irregularities are puttied up smooth. On the fourth and
fifth days a heavy primer is applied, one coat on each day. A
coat of filler is given on the sixth day and allowed to harden
the following day. The next paint applied is termed a guide coat.
This is of a color different from the preceding ones and serves
as a guide for the rubbers, who on the following day go over the
car with mineral wool, fine sandpaper, or pumice stone, and rub it
until the guide coat is worn away. This assures an even and smooth
surface. On the tenth day the car is allowed to stand. A coat of
the color desired is applied, one on each of the following three
days. On the fourteenth and fifteenth days the car is striped
with the desired ornaments and lettered. This is usually done in
aluminum or gold leaf. The car is then given three coats of varnish
on alternate days, and the work is completed. The best practice
brings the cars in once each year to be revarnished.
[Illustration: TYPICAL HIGH GRADE TRACK CONSTRUCTION
J. G. White & Co.]
ELECTRIC RAILWAYS.
PART II.
OVERHEAD CONSTRUCTION.
[Illustration: Fig. 64.]