SLIDING, ROLLING, WALKING, AND CREEPING
IN ALL forms of transportation friction plays a most important part. If
there were no such thing as friction, it would be impossible for us to
set an object in motion by the means that are now commonly in use, and
once the object were moving it would be impossible to stop it except by
bringing it up against a wall set squarely across its path.
In transportation on land friction is a much more serious bar to motion
than it is in marine or aerial transportation. There are two kinds
of friction that we have to contend with; sliding friction when two
contacting surfaces are dragged by each other; and rolling friction
when one surface rolls upon another. It is impossible to obtain two
surfaces that are absolutely smooth; there are minute elevations in
each that sink into minute depressions of the other like intermeshing
teeth and a grinding action takes place as these microscopic
inequalities are broken away. Oil reduces the friction by filling up
these inequalities, but the oil itself offers a certain amount of
friction just as water does along the sides of a moving ship.
ROLLING FRICTION
Rolling friction is of a different kind. The intermeshing inequalities
or microscopic teeth are lifted out of contact with, one another
just as the teeth of gear wheels are carried out of mesh. But there
is another cause of friction due to the fact that no objects are so
microscopic that they do not sink into each other to some extent. A
wheel is always rolling out the surface it is turning on just as a
rolling pin rolls out dough. If the surface is of elastic material
such as a steel rail, it springs back into place immediately after
the passage of the wheel, but the wheel must constantly travel in
the trough of a wave which accompanies it along the rail. There is a
similar wave in the wheel itself and this ironing-out action produces
heat in the wheel and the rail. It is particularly noticeable in the
flexible tires of automobiles which, after a run on even a smooth road,
become too hot to be grasped with the bare hand merely because of the
waves of compression and decompression to which they are subjected.
Both rolling and sliding friction are increased by pressure because
the depression is greater and because inequalities are brought into
more intimate engagement with one another. The degree of friction also
depends upon the nature of the substances in contact, but theoretically
the area of contact does not make any difference. It is just as hard to
push a block along a smooth surface on its edge as on its side.
PUTTING ROLLERS BETWEEN LOAD AND ROAD
As intimated in Chapter I the forerunner of the wheel was probably the
roller. It was much easier to move a heavy object on rollers because
rolling friction was substituted for sliding friction, but the rollers
would not stay under the object; they traveled only half as fast as
the load they carried. To make them keep up with the load they had to
be mounted on axles which were fastened either directly to the load or
to a cart body on which the load was supported. Thus the wheel came to
be invented, but except for the fact that it stays by its load and does
not roll out from under it a common wheel is not to be compared with a
roller for efficiency. To be sure, it substitutes rolling friction for
sliding friction where it contacts with the road, but the friction at
the axle is sliding rather than rolling. However, drawing an object on
wheels is a decided improvement over sliding it along the road, for two
reasons: the sliding friction at the axle is reduced to a minimum by
choosing materials that will slide upon each other with comparatively
little resistance, by polishing them smooth and by lubricating them.
But even if these precautions were not taken there would be a distinct
advantage in the use of wheels because of the relatively shorter travel
at the axle than at the rim of the wheel. If a wheel is thirty inches
in diameter and turns on an axle one inch in diameter, it will travel
thirty times as far at the rim as it does at the axle; hence the
sliding friction at the axle is far less than it would be at the point
of contact with the ground, were the wheel locked so that it could not
turn. But it is not necessary to have any sliding friction at the axle
if we revert to the old roller system that prevailed before the day of
the wheel. The axle may be considered the load and the axle bearing
the road. We can then put rollers between load and road. Because the
road is a circular one that travels with the load we can line it with
rollers throughout its length, and the load will never lack for rollers
to roll upon. Thus we have the roller bearing which is so widely
used in modern vehicles. Ball bearings operate on the same principle
except that the balls furnish less contacting surface and are not so
suitable for supporting heavy loads, as are rollers. Sliding friction
is almost completely eliminated and unless heavily loaded a wheel on
ball bearings will not heat even when the bearings are not lubricated.
REDUCING ROAD FRICTION
While there is little friction between a wheel and the roadway upon
which it travels, the roughness of the road is a very important factor.
Every time a wheel goes over a stone the entire vehicle must be lifted;
this represents so much wasted energy. The advantage of the pneumatic
tire lies in the fact that it absorbs small inequalities without making
it necessary for the entire vehicle to rise over them. This means less
load lifted and hence less work done. Large wheels are better than
small ones because they do not sink so deeply into depressions and
because they surmount small obstacles more readily. On a steel track
the size of the wheel does not make so much difference as it does on a
road because the track surface is smooth and is but little depressed
by the wheel. It has been estimated that a horse can pull ten times as
great a load on rails as on an ordinary macadamed road. Some years ago
broad steel tracks were laid in some of our city streets for the use of
horse-drawn trucks. They served very well as far as the vehicles were
concerned. The road friction was reduced considerably, but the fact
was overlooked that the horses needed a good friction surface under
their feet. Two horses could not pull a truck along the track without
walking on the track and the smooth steel made such slippery footing
that the tracks had to be torn out and replaced with common paving.
INCREASING TRACK FRICTION
The difference between sliding and rolling friction is well illustrated
in a locomotive. The driving wheels are turned around by steam power.
They must either roll or slide on the track and the load they will
pull without slipping is a measure of the excess of sliding friction
over rolling friction. To increase the traction or the adhesion of the
locomotive to the track it is provided with a number of driving wheels.
In some of our largest locomotives driving wheels are placed under the
tender so as to obtain a maximum of traction.
ANCIENT LINEAGE OF THE AUTOMOBILE
When it was first proposed to substitute steam propulsion for the horse
it was not realized that a rail would furnish enough traction to permit
of hauling heavy loads, and some of the early locomotives that ran on
rails were provided with toothed wheels that engaged in racks alongside
the rails. In fact, the earliest locomotives were not built to run on
rails but on ordinary roads; in other words, they were automobiles.
The motor car can therefore boast of a more ancient lineage than the
railroad engine. Joseph Cugnot of France is said to have built a
three-wheeled steam carriage in 1769 which was so top-heavy that it
upset when making a sharp turn at three miles per hour. Several steam
carriages were built in England in the eighteenth century, but they
were not successful. The real father of steam traction was Richard
Trevithic, of Camborne, Cornwall, whose first steam carriage, built
in 1801, carried eight passengers. His third machine, built in 1804,
was the first to run on rails. This was strictly a locomotive intended
to haul cars. It ran with its load at the astonishing speed of five
miles an hour. Trevithic was the first to exhaust the steam from the
cylinders into the smoke-stack and thereby increase the draft through
the furnace and generated steam at higher pressure.
All the early locomotives used toothed gears to turn the driving wheels
until George Stephenson introduced connecting rods to drive the driving
wheels direct from the pistons. George Stephenson’s “Rocket,” built in
1829, won a prize of 500 pounds in a speed contest when it established
a record of 24 1-6 miles per hour. It also established the doubtful
honor of being the first mechanical speed monster to exact the toll of
human life. On its prize run it ran over a man and killed him.
STEPHENSON’S LINK MOTION
[Illustration: FIG. 61.--STEPHENSON’S LINK MOTION]
Stephenson’s son Robert is commonly credited with the invention of
an ingenious link motion, although the invention is also claimed for
W. T. James of New York. We must pause a moment for a description of
this ingenious link motion, because it became a standard in locomotive
construction that is still in service. As explained in Chapter IX steam
should be used expansively in order to obtain a maximum of efficiency.
After a certain amount of steam has been admitted into the cylinder
it is cut off from the boiler and it pushes the piston by its own
expansion. The speed of the locomotive must be varied by varying the
point of cut-off, and the direction of motion of the locomotive is
reversed by reversing the motion of the valve. In the Stephenson link
motion this is all effected by the operation of a single lever. Fig.
61 shows the arrangement. The valve is moved back and forth by the
rod _A_ which is connected to the arm _B_ suspended from the frame of
the locomotive. The drive shaft is shown at _C_ and it carries two
eccentrics projecting on opposite sides of the shaft which move the
rods _D_ and _E_ back and forth. The outer ends of these rods are
connected to opposite ends of a curved link _F_ and a pin _G_ on the
arm _B_ engages a slot in the link. The link is held up by a rod _H_.
As the drive shaft rotates the slotted link is oscillated back and
forth on its own center. When the link is lowered so that the pin _G_
is at the center of the slot there is no motion of the valve rod _A_,
but when the pin is at the top or at the bottom of the slot the valve
rod has its greatest motion. As the top of the link is moving in one
direction while the bottom is moving in the other direction, it will
be plain that the action of the valve when _G_ is at the top of the
link will be the reverse of the action when _G_ is at the bottom. The
link is raised and lowered by means of a lever _I_ which is connected
to one arm of a T-shaped lever _J_ which on one side of its fulcrum is
connected to the rod _H_ and on the other to the counterweight _K_.
There are certain defects in the Stephenson link motion that we cannot
discuss here. These became serious as engines grew larger and more
powerful so that to-day it has been largely superseded by other valve
gears. The most important of these is the Walschaerts gear, invented by
a Belgian engineer. This gear, unlike Stephenson’s, is conspicuously
placed outside the drivers and is particularly noticeable because
of its peculiar grasshopper motion. The action of the gear is so
complicated that a description of it would be out of place in this book.
MODERN LOCOMOTIVES
We cannot enter minutely into the development of the locomotive from
the crude machine of Trevithic’s time to monsters of to-day. There has
been a progressive growth of locomotives in power and in speed. Our
biggest freight engines are so powerful that they cannot be used for
pulling alone because they can pull a greater load than the draw-bars
of the cars can stand. If placed at the head of a long train they would
yank the forward cars loose from the rest. Hence they are placed at the
rear of the train to act as pushers or in the middle of the train where
half their energy is expended in pushing the cars ahead of them and the
other half in pulling the rest of the train.
The most powerful steam locomotive of to-day (1921) weighs 342 tons and
its tender 107 making a total of 449 tons. Its length is 105 feet and
its boiler 8 feet 7⅛ inches in diameter. Its low pressure cylinders (4
feet in diameter) are larger than the locomotive boilers of 50 years
ago. Its high pressure cylinders are 30 inches in diameter and the
stroke is 32 inches. It may be operated either compound or simple, _i.
e._, the smaller cylinders may exhaust into the larger ones or they
may take steam direct from the boiler. The tractive effort compound
is 147,200 pounds and simple 176,600 pounds and the total horsepower
developed is 5,040. Each cylinder drives five coupled drivers, in other
words there are twenty power-driven wheels with a pair of trailers and
a pair of pilot wheels. Six and a half tons of coal are consumed per
hour.
In the matter of speed a mile per minute has become common and regular
scheduled runs over long distances at an average well above sixty miles
per hour have been maintained, but the present tendency is to reduce
speed somewhat in favor of safety.
There are two inventions that have made possible the high speeds
of modern railroad travel: the air brake, which has already been
described; and the block signal system. The latter, of which there are
a number of different types, being electrical, does not properly belong
in this book.
Stoking a large freight engine or a high speed passenger locomotive is
strenuous work. Three tons of coal per hour is not an uncommon rate of
consumption. The fireman on a fast express train gets little rest. To
relieve him of this exhaustive work automatic stokers are now being
used. These convey the coal from the tender to the fire box and feed
the fuel at a uniform rate. In place of solid fuel, oil is extensively
used in regions where it may readily be obtained. This simplifies
the task of firing the locomotives. There has also been some use of
powdered coal which is blown into the furnace in much the same way as
oil is.
To-day steam locomotives have found a serious competitor in the
electric locomotive, which is steadily increasing in favor.
Where traffic is heavy, where long tunnels make the smoke and gases
of a steam locomotive dangerous, where electric power is plentiful,
the steam locomotive must give place to electricity. As the cost of
coal mounts, the electrification of railroads will spread and it will
be only a matter of time before the electric locomotive, which is far
more economical in its use of power, will completely supplant the steam
locomotive.
FIRST AUTOMOBILE ACCIDENT
As we have already noted, the automobile antedated the railway
locomotive. It was an accident that took the primitive steam carriage
off the high roads and put it on rails. In 1802 Richard Trevithic,
while speeding along a road at the frightful speed of ten miles per
hour, lost control of his machine, ran into a fence and ripped off
a number of palings. That accident spelled the doom of the early
automobile. So dangerous a machine was not allowed to run at large.
Special tolls and restrictions were placed on power-driven road
vehicles. As late as 1896 when the automobile had become a practical
machine in France and was being rapidly developed in this country,
England still had a law prohibiting any power-propelled vehicle to
travel over the highways at a higher speed than four miles per hour
and required further that the vehicle be preceded by a man bearing a
red flag.
The most important early American road car was that built by Richard
Dudgeon in 1855, which made a record speed of forty miles per hour.
In 1889 Serpollet, in France, invented the flash boiler and interest
in steam-driven automobiles was revived. In a flash boiler water is
turned into steam as it is used by injecting small quantities at a time
in a red-hot tube. In the meantime, however, the internal combustion
engine was being developed. Lenoir, in France, was the first to build
a motor car driven by an internal combustion engine. He obtained a
patent on such a vehicle in 1860. However, the real father of the
modern automobile was Gottlieb Daimler of Germany, who patented a motor
vehicle in 1884 driven by an internal combustion engine. The next year
Karl Benz, another German, built an automobile.
AMERICAN PIONEERS OF THE MOTOR CAR
Pioneer work in this country began with Charles E. Duryea in 1891.
Five years later he took one of his machines to England, where it
entered a fifty-two-mile race between London and Brighton. There were
many entrants from France, Germany, and other European countries.
The American car won the race by nearly an hour over its nearest
competitor. Commercial building of automobiles began in America in
1894. However, it was not until the opening of the twentieth century
that America took hold of the motor car in real earnest. Since then the
rise of the automobile industry has been phenomenal. American methods
of manufacture were applied and cars were turned out in quantity.
In 1916 the annual production exceeded a million and to-day the
production is about two million passenger cars and over three hundred
thousand trucks. The motor car industry is largely responsible for the
wonderful progress in American machine tools that has been made in the
past two decades.
On the race track the motor car has established a record speed of 131
miles per hour, but of greater utility has been the motor truck which
now competes with railroads in the transportation of freight. Owing to
New York’s inadequate terminal facilities, it takes less time to haul
a load from New York to Philadelphia by motor truck than to take the
same load from the warehouse, transport it across the Hudson River, and
load it on a freight car. It has made the country a part of the city. A
short run brings the farmer’s produce to the markets and his passenger
car keeps him in close touch with city life. The motor tractor has
lightened his work on the farm and has enabled him to conduct farming
operations over vast areas. Animal power is gradually giving way to
mechanical power. This, however, is a special branch of automotive
engineering which we must look into.
WHEELS VS. LEGS
The success of the automobile depends upon the character of the roads
it has to traverse. Wheels provide the best form of transportation
over a smooth, hard road, but if the road is soft, the wheel will sink
into the surface and will be greatly impeded. In traveling over mud or
sand runners are preferable to wheels because they have a broader and
longer bearing surface. In the snowy regions of the north and the sandy
regions of Africa and Asia Minor, sledges are used in place of wheeled
vehicles. Sliding friction is less than rolling friction under such
conditions. In fact, only where man has constructed special roads is
it possible to use wheeled locomotion. It is because in nature we must
contend with all sorts of surface conditions, soft and hard and seldom
smooth, that the rolling principle of locomotion is not to be found
in any species of animal. The legs of an animal are levers, just as a
wheel is a system of levers, but in the former case the levers can be
folded or extended to adapt themselves to all the unevennesses of the
ground. No animal can begin to run as fast as an automobile on a good
road, but on the other hand we have yet to build a machine which will
run as fast as a horse on soft and uneven ground.
WALKING MACHINES
Farm operations must be conducted on loose and broken surfaces for
which the ordinary wheeled vehicle is unfitted. In some cases broad
wheels are used to keep them from sinking into the soil and they are
furnished with cleats so as to give them a good grip on the ground,
but long ago it was realized that if the horse was to be completely
displaced on the farm a more adaptable form of locomotion than that of
wheels must be furnished. Inventors sought to make machines that would
walk. One interesting machine of this class which met with a certain
degree of success attempted to combine walking and rolling. Large
wheels were used which were provided with a series of feet or tread
plates connected by knuckle joints to the wheel rims. As the wheels
revolved, these treads came successively into contact with the ground
and the machine actually walked on its feet. The feet were broad
enough to prevent the machine from sinking into the ground and they
adapted themselves to inequalities in the surface. Instead of having to
roll over a rock that lay in its path the machine would plant its feet
on the rock and lift itself over.
A very curious machine of the walking type has been developed for
excavating machines that operate in soft swampy ground. The machine
actually walks over surfaces that a light carriage could not negotiate.
It consists of a large central turntable flanked on each side by a pair
of broad and long tread plates which serve as feet. When the dredge is
to be moved, it plants the feet on the ground, lifts up the turntable,
moves it forward, and sets it down again; then the feet are lifted,
moved forward, and planted on the ground again while the turntable is
moved forward again. When it is desired to make a turn the machine is
swiveled around to the desired direction while sitting on the turntable
with its feet raised clear of the ground. Of course the speed of such a
machine is very low, but transportation is of secondary consideration.
The main purpose of the machine is to excavate ditches in soft ground
and only occasionally does it have to move its position. The turntable
provides a broad base that distributes the load over a comparatively
large area which prevents the machine from sinking into the mud, and if
it should sink into a soft spot it lifts itself out vertically instead
of having to roll out.
“CATERPILLAR” TRACTION
Long ago it occurred to inventors that a machine could travel over
trackless wastes if they carried their own tracks with them. The idea
dates back more than a century, but only in comparatively recent times
has it been developed to a practical stage. The track-laying mechanism
takes different forms, but in one prominent type it consists of a
series of plates linked together to form a chain or belt that passes
around a series of wheels. As the machine progresses the plates are
successively laid down in front of the wheels and picked up behind
them. The wheels consist of a large number of rollers whose axles
are spring-supported so that they are capable of a certain amount of
vertical movement and as the belt they roll upon is made up of separate
plates they can adjust themselves to irregularities of the ground and
creep over an uneven surface. Its close resemblance to the creeping
locomotion of a caterpillar has led to the adoption of the trade name
“caterpillar” by one of the large tractor manufacturers. The tractor
belt is driven by spur wheels at each end and the rollers serve merely
to distribute the weight of the machine along the belt. The traction is
exceedingly great because the belt is broad and long, giving it a large
gripping surface. For the same reason it will not sink into plowed
ground. Because of its flexible tread surface it will creep over rocks
and stumps, waddle down into a ditch and climb up the opposite bank. So
powerful is it that it will crash through underbrush with ease and even
small trees yield before it. It will run over soft mud, deep sand, and
even snow with equal facility. The tread belts on either side of the
tractor are separately driven and by making one belt run faster than
the other it is possible to steer the machine. In fact it can be made
to turn around in its own length by stopping the belt on one side and
driving the belt on the other.
Such a machine is ideal for plowing and other are hauled with ease over
the soft plowed ground. For ordinary farming purposes speed is not
essential, but the possibility of rapid travel with tread-belt traction
was demonstrated in the World War when small “tanks” were built which
could run at the rate of twelve miles per hour.