LOCOMOTIVES

_Locomotive engines_, or simply _locomotives_, are steam engines which,

mounted on a carriage, propel themselves by transmitting their motion to

wheels.

The parallel motion, the beam, and the fly wheel of the ordinary

stationary engine form no part of a locomotive. The principal parts are

the _framework_, the _fire box_, the _casing_ of the boiler, the _smoke

box_, the _steam cylinders_, with their valves, the _driving wheels_,

and the _feed pump_.

The framework rests on the axles of the wheels. The illustration on

another page shows clearly the arrangement and parts of a typical

locomotive. It will be observed that in the lower part of the _steam

dome_ is the _fire box_, from whence the flame and the products of

combustion pass into the smoke box, and then into the chimney after

having previously traversed the numerous brass _fire tubes_ which pass

through the boiler. The _boiler_, which connects the fire box with the

smoke box, is made of iron, and is cylindrical.

The steam passes from the boiler into two _cylinders_, placed on either

side of the smoke box. There, by means of a steam chest similar to that

already described, it acts alternately on the two faces of the piston,

the motion of which is transmitted to the axle of the large driving

wheels. After having acted on the pistons, the steam is forced through

the blast pipe into the chimney, thus increasing the draft.

The motion of the pistons is transmitted to the large driving wheels by

two connecting rods, which, by means of cranks, connect the piston rods

with the axles of the wheels. The alternating motion of the slide valve

is effected by means of eccentrics placed on the axles of the large

wheels.

WHEELS OF THE LOCOMOTIVE.--The wheels range ordinarily from forty-five

to eighty-five inches in diameter for drivers, thirty to forty-two

inches for truck wheels. They are made of castiron or steel body and

steel rim shrunk on. Spoked wheels are usual for drivers, solid wheels

for trucks. The tread is four to five inches wide, the flange (one to

one and one-quarter inch high) increasing this to five and one-half to

seven inches. A counterbalance weight is cast between the spokes

opposite the crank-pin seat. The axles, forged steel, are six to eight

inches in diameter (for drivers); the wheels are forced on their ends by

a powerful press. Cranked axles (for inside cylinders) are forged to

shape, rarely built up.

CONTROL.--The locomotive is controlled by the throttle-valve and the

reverse lever. Both are located in the cab, which is built at the rear

around the fire box, and serves also as firing platform.

AUXILIARIES.--The necessary auxiliaries of the locomotive are those

required for its operation as a power generator, and those necessary to

its service as a railroad vehicle or as a tractor. The _tender_ is the

most important in the first group. It is a separate vehicle attached

behind the locomotive, carrying a water tank of three to eight or nine

thousand gallons capacity, and a coal bin of two to ten tons capacity.

Eight-wheel (two-truck) tenders are usual. The coal space is at the

front of the tender, and the water tank occupies the rear half and

extends forward along the sides of the coal bin. The coal thus is

reached directly from the rear of the engine cab.

Water is supplied to the engine by pipes leading from the tank to

injectors on the engine. Feed pumps are rarely used for pumping the

water into the boiler, injectors in duplicate being depended on. The

safety valve, mounted on the top of the boiler, is of the spring poppet

type. A steam whistle is placed alongside, for use as warning and

train-movement signal; a bell operated from the cab by a cord is also

mounted on the boiler.

The air-brake equipment of the locomotive comprises the brake mechanism

for the engine itself, and an air pump with its governor, a main

reservoir, and the engineer’s valve, for supplying and manipulating the

brakes of the entire train. The air pump is a direct-coupled compressor

whose steam and air cylinders have a common piston-rod, attached in

vertical position to the side of the boiler in front of the cab. The

cylinder diameter is eight to ten inches. It pumps air into the main

reservoir, a cylindrical tank hung under the boiler. An automatic

pressure governor starts the compressor when the pressure falls and

stops it when the full reservoir pressure is restored.

The locomotive brake equipment consist of brake cylinder and lever

system connected to the wheel brake-shoes, but its valve control differs

somewhat from that of a car, so as to permit braking the engine alone if

desired. The engineer’s valve is a flat-seat rotary valve with positions

for supplying brakes, recharging the train-pipe, and closing all

connections. A separate valve is usually supplied to operate the engine

brake alone, working this as “straight-air” or non-automatic brake.

Reservoir and train-pipe gauges are mounted in the engine cab near the

brake handles. Steam brakes are no longer fitted on American

locomotives. The driving-wheels only of the locomotive are braked, but

the tender is fully braked.

The sand-box for increasing the driving-wheel friction on wet or greasy

rail is commonly set on top of the boiler with discharge pipes ending in

front of the drivers just above the rail. A compressed-air ejector is

now often used (pneumatic sander), in which case the sand-box may be

placed on the front sill or in other convenient position with equal

effectiveness.

THE CAB, with windows in front and sides, is built around the fire box,

providing a seat on either side which commands a view ahead over the

track. The reverse lever is placed on the right-hand, or engineer’s

side, from where also the throttle lever and brake-valve handles

are reached. Injector, whistle, sander, bell, drain cocks,

traction-increaser, and other appliances are controlled from here. The

headlight, set on top of the boiler in front of the stack, is usually an

oil lamp, with parabolic reflector. Acetylene and electric headlights

are extensively used in recent years, the latter supplied by a small

steam-turbine and dynamo combination.

CLASSES AND TYPES OF LOCOMOTIVES.--The wheel-arrangement of a locomotive

is, in conjunction with its total weight, the chief characteristic.

Freight locomotives, running at slow speeds, utilize a large adhesion,

and therefore have a large proportion of their weight carried on

drivers; they have less need for good guiding quality and steadiness at

great speeds. Passenger locomotives, working at high speeds, develop a

much lower tractive force, and therefore require less weight on drivers,

but need leading wheels for guiding quality and steadiness.

The number and arrangement of cylinders is another characteristic of

classification. Most locomotives have two cylinders, both simple.

Compounds are built with two, three or four cylinders.

RECENT DEVELOPMENTS.--The chief factor in the modern modification of

locomotive types and details is increase of size. The only limiting

factors are boiler capacity and weight on drivers.

Economy of operation has brought compounding into much favor even for

single-frame engines, and more recently has led to the wide adoption of

super-heating; these improvements also allow increased power to be

obtained from a boiler of given size.

WEIGHT AND POWER.--Locomotives weighing seventy-five to one hundred and

twenty-five tons (without tender) are common. In power, road locomotives

range from three hundred to one thousand five hundred horse power and

occasionally to two thousand horse power, the more modern ranging from

seven hundred to one thousand five hundred. High-speed passenger

locomotives are usually more powerful than heavier freights.

BOILER PERFORMANCE.--The distinguishing feature of the locomotive boiler

is its high evaporative capacity, and the very high rates of

fuel-burning. At full power one hundred pounds coal are burned per

square foot of grate surface per hour, by virtue of the strong draft

produced by the exhaust-steam blast. At moderate speeds twenty-five to

forty pounds are burned.

ELECTRIC LOCOMOTIVES.--The operation of heavy railroad service (_i.e._

trains of freight cars and long passenger trains) by electric power

requires the use of electric locomotives in place of the car-motor

arrangement of street railroads. Such locomotives have been built since

the middle of the nineties, and in considerable number since 1905. The

earlier ones had the motors geared to the axles, or directly mounted

thereon, but recent constructions have the motors mounted on the frame

or platform, above the wheels, so that their weight is carried by the

frame springs, and the motors drive the wheels through coupling-rods

either direct or by way of an intermediate jack-shaft. This form is

found to give smoother running and exert less destructive effect on the

track than the prior forms. In wheel arrangement these locomotives vary

greatly, but recent machines exhibit combinations of coupled drivers

with leading and trailing trucks not unlike the arrangement of steam

locomotives. Electric locomotives of two thousand to three thousand

horse power have been built, and are in regular use hauling trunk-line

trains.