AIR SPRINGS AND CUSHIONS
UNLIKE water, air is a highly compressible, elastic fluid, and as such
furnishes an excellent medium for the storage of energy. It acts just
like a clock spring into which energy may be introduced and stored
by winding or compressing the spring. The energy remains locked up
in the spring and when the spring is released, it gives back just
as much energy as was put into it, except for slight frictional and
heat losses. Air is a much better spring than steel or any other
metal because it never loses its elasticity from fatigue and because
it has an enormous capacity for the storage of energy. It possesses
one serious drawback, however. Much of the energy that is expended
in compressing it is converted into heat. If the air were to be used
immediately and without transmitting it to a distance, there would be
no advantage in extracting the heat, but heat cannot be stored in air
for long and it would gradually escape from the storage reservoir or
air receiver and from the pipes leading the air to the machines that it
was to operate. As the heat escaped it would lower the pressure of the
air and hence much of the energy would be lost.
HEAT OF COMPRESSION
In Chapter V we described a hydraulic system of compressing air and
noted that one of the advantages of this system is that it delivers
air cooled to the temperature of the water. This, of course, does not
mean that there is no loss due to heat. The air bubbles as they are
compressed are cooled by the water that compresses them; in other
words, the heat of compression passes off into the water, making the
water warmer than it would otherwise be. Heat is not produced without
expenditure of energy and a certain proportion of the water power is
thus wasted.
In most cases air is compressed by steam or electrically driven
compressors, and in such machines the heat due to compression is a
serious matter. The air cylinders are water-jacketed to carry off the
heat. But air is a poor conductor; it acquires heat faster than it can
give it off to the water surrounding the cylinders. In compound air
compressors the air compressed in one cylinder is cooled before being
passed on to the next cylinder, where it is further compressed. The
heat loss in compressing air in a single stage up to 100 pounds gauge
pressure is about 30 per cent. Air that enters a compressor at the
normal pressure of the atmosphere and with an initial temperature of
60 degrees F., if not cooled will become heated to 415 pounds gauge
pressure. The higher the initial heat of the air, the greater the rise
of temperature. If a volume of air be subjected to 294 pounds gauge
pressure, it will occupy about one-tenth of its former volume. If the
air was introduced into the compressor at zero, it would acquire a
temperature of about 650 degrees; if introduced at 60 degrees, it would
show an increase of about 800 degrees; and, if started at 100 degrees,
it would show an increase of 900 degrees in passing through the
compressor. This shows the advantage of compressing the air in stages
and cooling the air between stages.
Sometimes the heat developed is sufficient to produce a disastrous
explosion. Air is noncombustible, but the oils used to lubricate the
compressor are vaporized by the heat and when mixed with air form a
powerful explosive. Care has to be taken that none but high-grade
oil with a high flash point be used in the compressor and that the
temperature of compression does not rise to near the flash point lest
the vaporized oil be ignited.
That fire can be produced by sudden compression of air has long been
known in the Philippine Islands. The natives use a small air tube with
a close-fitting plunger. Combustible matter is placed in the bottom of
the tube, and on striking the plunger a sharp blow this is ignited.
When compressed air is used in an air motor it expands and in so doing
absorbs heat. The more rapidly it expands, the more heat it absorbs.
This heat it extracts from the motor and from the atmosphere into which
it escapes, and it is a common occurrence to find a thick coating of
frost around the exhaust port. This is due to condensation of moisture
in the atmosphere or in the compressed air itself, which, because
of the rapid extraction of heat, is converted into snow. On cold
days enough frost may be produced in the exhaust pipe to clog it and
interrupt the operation of the motor, and frost sometimes clogs the air
lines leading to the motor.
[Illustration: WHARF ON THE PACIFIC COAST PROTECTED BY A PNEUMATIC
BREAKWATER
Air issuing from the submerged perforated pipes breaks up the waves]
[Illustration: GENERAL VIEW OF A SUN POWER PLANT IN EGYPT]
[Illustration: CLOSE-UP OF ONE OF THE REFLECTORS SHOWING THE BOILER]
The fact that compressed air on expanding is cooled, makes it an ideal
power for use in mining machinery. Steam is inconvenient because of the
difficulty of transmitting it to the machines without loss of heat,
because it would heat the machines so that they cannot be handled
readily and because the exhaust steam would fill the mine with an
impenetrable fog; electricity is dangerous in mines that are apt to
contain explosive gases, because the wires are liable to be broken and
cause sparks by short circuiting, and because sparks are likely to form
between the brushes and commutators of the motors; but compressed air
has none of these objections, and, furthermore, the discharge from the
machines furnishes the operators with ample supplies of fresh cool air
which drives out disagreeable and dangerous gases.
AIR-DRIVEN HAMMERS
The ordinary rock drill is really a pneumatic hammer. The tool, which
is chisel-shaped, is used as a hammer to pound a hole in the rock. The
tool is driven up and down or in and out by compressed air bearing
alternately on opposite sides of a piston. The elasticity of the
compressed air acts as a cushion to relieve the machine from shock. The
drill is mounted upon a tripod in such a way that it may be operated in
any direction.
The pneumatic riveter is another form of compressed-air hammer.
The machine is held in the hands, and the tool, which is rapidly
reciprocated by air pressure, pounds the red-hot shank of a rivet.
While this is being done an assistant holds a hammer against the head
of the rivet. However, there is a type of riveter which has a U-shaped
frame. One arm of the frame carries the reciprocating tool while the
other reaches around and bears against the head of the rivet.
All sorts of tools have been built in which a small air-driven motor
furnishes the motive power. There are hand drills in which the tool is
revolved by a set of pistons, shears for shearing sheep which operate
somewhat on the principle of hair clippers, and pneumatic chisels used
for chipping stone. Pneumatic motors are widely used in operating
cranes and air hoists. They consist of a simple cylinder and piston,
and are used for short, direct lifts of all sorts.
STOPPING TRAINS WITH AIR
Air has played a most important part in transportation. When the air
brake was first introduced its purpose was to prevent collisions and
provide greater safety of operation. It was not generally realized that
efficient air brakes are not only a safety precaution, but a means of
increasing schedule speeds. The more quickly a train can stop, the
better speed it may make, particularly on a schedule that calls for
frequent stops.
In the first air brake invented by George Westinghouse, the locomotive
was provided with a reservoir in which air was stored and compressed
by means of a steam-operated air pump. The cars were each provided
with a cylinder and piston connected to the air reservoir through a
valve conveniently located in the engineer’s cab. The piston of the
air cylinder was connected with the air brakes so that whenever the
engineer wished to stop his train he merely turned on the compressed
air and all the brakes in the train were operated. This was a great
saving over the previous system of providing brakemen to operate hand
brakes.
Unfortunately, the problem of stopping a train was not as simple as
all this. On a long train the cylinders near the engine were the first
to receive pressure sufficient to operate the brakes, and as a result,
the forward part of the train was retarded more than the rear part at
the start of the brake operation, and the train came to a stop with
a series of jolts. Another serious disadvantage was the fact that
an enormous reservoir was required to furnish all the air necessary
for a long train. But the most serious drawback was the fact that
occasionally a train broke apart and then the engineer was powerless
to control the brakes of the detached portion of the train. If the
accident occurred on a down grade, there would result a collision
between the cars running by their own momentum and gravity and those
attached to the engine. This led to the present system of reversing the
process which consists in keeping air pressure constantly in the train
pipe and setting the brakes by relieving the pressure in the train pipe.
Each car is provided with its own cylinder which furnishes the
necessary air for the operation of its brakes. There is, of course, a
main cylinder on the locomotive in which air is pumped at high pressure
by a steam-operated pump. When the pressure of the main reservoir
falls below a predetermined amount, the air pump starts operating
automatically and continues until the requisite pressure is restored.
Air from the reservoir is fed to the train pipe at a certain pressure
and feeds the local reservoirs on the cars of the train. At each car
it passes through a very ingenious triple valve, which consists of a
cylinder with a double piston, one operated by pressure in the train
pipe, and the other by pressure in the local cylinder. The piston
operated by the pressure of the train pipe is larger than the other and
consequently the valve piston is normally pressed back, uncovering a
small port through which air from the train pipe feeds into the local
reservoir. In this way the air in each local reservoir is maintained
at the same pressure as that of the train pipe. Whenever air is let
out of the train pipe, the piston is pushed out by the excess pressure
on the local reservoir side and uncovers another port which permits
air to flow from the local reservoir to the brake cylinder operating
the brakes. The advantage of this system is that each car is always
supplied with sufficient power to act on the brakes immediately without
having to draw its supply of air from the main reservoir. In case the
train breaks apart, the air in the train pipe escapes and the brakes
are set automatically.
There have been a number of improvements in air brakes aimed to make
the operation of the brakes more uniform and to insure immediate
action, even on a very long train. Modern express trains weighing 920
tons and traveling at a velocity of 60 miles per hour, can be brought
to a standstill in 860 feet, or practically their own length. Automatic
arrangements are provided to insure the application of the brakes
at graduated rate, except, of course, when the emergency brakes are
applied. As the speed of the train slows down the pressure is gradually
relieved, otherwise the train would stop with a severe jolt. The rate
of applying the brakes so as to provide a smooth retardation depends in
large measure upon the weight of the train.
On the New York subways, it is interesting to note, every passenger
who boards a train has his weight recorded by the brake mechanism, and
allowance is automatically made for the inertia that his mass adds to
the train when it is in motion. The weight operates through a system of
levers to control the amount of pressure that is applied to the brakes
so that when the engineer operates the brake lever, he does not have to
consider whether the cars are light or jammed full of passengers; the
brake mechanism itself takes care of this.
PROPELLING CARS WITH AIR
If air can be used to stop a train, why cannot air be used to propel
it? This question occurred to many inventors and they answered it by
building air-propelled cars and locomotives. Pneumatic cars were tried
out on street railways, but they have had to give way to electric
cars which do not need to carry their power around with them, but can
draw it from a central power plant through a trolley wire. In only a
few situations, such as in mines where electric sparking is feared,
are air-propelled cars and locomotives still used to any considerable
extent.
However, air plays another and highly important part in transportation
and here it is employed not to deliver energy stored in it, but to
serve as a cushion. Without the soft, flexible grip of the air-filled
rubber tubes with which automobile wheels are shod, high-speed
motoring would be practically impossible. There is nothing that can
compare with air for absorbing shocks and unevennesses in the road.
Whenever an obstruction is encountered, not only is the shock absorbed
by elasticity of the air, but the impact is immediately distributed
uniformly over the whole tire, so that the strain on the tire is not
localized and the life of the tire is correspondingly increased. There
have been many attempts to introduce substitutes for pneumatic tires,
such as combinations of metallic springs and straps, but these have
failed, chiefly for the reason that they cannot distribute the shock
as the pneumatic tire does. Consequently they are not only liable
to damage, but they do not absorb the obstructions as readily as the
pneumatic tire does and the vehicle which they carry is subjected to
heavy stresses and strains.
Air as a cushion is used to prevent the rebound of the springs of an
automobile and is also widely employed in machinery to absorb the
momentum of moving parts or to slow the action of a spring. In a door
check, for instance, a powerful spring is provided which would slam the
door shut were it not for the cushioning action of air. A plunger is
connected to the door and slides in a cylinder on the door frame. The
air compressed by the plunger can escape only very slowly through a
small port.
AIR CUSHIONS FOR ELEVATORS
Air cushions are used in elevators to prevent too rapid a fall of
the car in case of accident. The lower part of the elevator shaft is
completely inclosed and fitted with steel doors to form an air pocket.
The car fits the shaft closely enough to compress the air under it when
it is moving downward. At normal speeds, this air escapes quite readily
around the sides of the car and through cracks in the doors, but when
the speed is excessive the air cannot escape fast enough and sufficient
pressure is built up to retard the car so that it will strike the
bottom with a moderate impact not at all dangerous to the passengers.
In the Woolworth Building, New York, the highest elevator shafts are
680 feet high and the air pockets are 137 feet deep--i. e., they reach
up to the tenth story. If the car broke away from its supports at the
top of the shaft it would be traveling at the rate of 132 miles an hour
when it struck the air pocket, but before it reached the bottom the
air would bring it practically to a stop. Imagine a heavy automobile
traveling at 132 miles per hour and brought down to a standstill in
half a city block! The passengers would be hurled out of their seats,
but in an elevator the passengers are standing and can brace themselves
against the pressure produced by their own momentum. The velocity
acquired in falling 543 feet to the air pocket must be overcome in 137
feet or ¼ of the distance. This means that the weight of each passenger
is multiplied by four. A man who weighed 150 pounds would find that
his pressure on the floor of the car had mounted to 600 pounds. One of
the inventors of the pneumatic cushion for elevators was killed when
testing out a car, not because of the impact of the car, but because
he was foolish enough to sit in a chair. The chair gave way under his
suddenly acquired weight and the poor man was fatally stabbed by one of
the splinters.
Air jets are used for a variety of purposes ranging from tamping
railway tracks to painting buildings. When a track is depressed under
the tamping action of trains passing over it the ties must be raised
and new ballast introduced under them so as to bring them up to level.
In order to save hand labor and insure the perfect filling of all
cavities a pneumatic tamper is sometimes used. The ballast, consisting
of sand or gravel up to ¾ inch in diameter, is fed out of a hopper and
meets a blast of air that hurls it into place like shot from an air
rifle. This furnishes a very solid and compact road bed.
It is in much the same way that weather-stained stone walls are
cleansed by means of a sand blast. The sand particles projected by the
jet of air act like myriads of tiny bullets which chip away the face
of the stone.
LAYING CEMENT WITH AN AIRGUN
It is after this same fashion that the cement gun is used to project
cement against wire reenforcement to form walls of buildings. Carl
E. Akeley was led to the invention of the cement gun by his efforts
to find an expeditious and economical method of mounting specimens
of large animals for the Field Museum in Chicago. He constructed a
pneumatic device for spraying cement and water upon a canvas-covered
framework, thus building up a body upon which the skin of the animal
could be mounted. This machine was improved and tried out successfully,
on a large scale, in constructing buildings.
The machine as now constructed consists of a hopper into which a
proper mix of sand and cement is introduced. Compressed air blows this
mixture out of a nozzle. Here it meets a jet of water also propelled
by compressed air. The water, sand, and cement combine and strike the
wall or surface to be coated with such an impact as to make a compact
fine-grained coating known to the trade as “gunite.”
Large surfaces can be painted much more readily and more evenly with
compressed air than with a brush. A widely spreading air jet is used
which draws the paint out of a receptacle, breaking it up into minute
droplets that are projected as a mist against the surface, covering it
with a uniform coating.
AIR AS A TENSION SPRING
So far we have been considering air as a compression spring. It may
also be employed in a manner somewhat analogous to a tension spring.
When air is exhausted from a receptacle we have a partial vacuum which
will produce work, but in the opposite direction from that produced by
compressed air. Of course there is no such thing as suction, and it is
the pressure of the atmosphere which is employed to do the work.
We are apt to overlook the fact that a vacuum cleaner depends upon
atmospheric pressure to drag the dust and dirt out of a carpet.
A vacuum cleaner would not work on the moon, where there is no
atmospheric pressure. The apparatus consists of an air pump, usually
driven by an electric motor, which exhausts, or partially exhausts,
the air from a foot piece that is dragged along the carpet. As the air
under atmospheric pressure rushes in through the carpet to take the
place of the air that has been pumped out, it carries with it all loose
particles of dirt. The dirt is filtered out of the air or a mechanical
trap is provided to catch and retain the dirt while the air is allowed
to escape. Large-scale vacuum cleaners are used for cleaning city
streets. The dirt is loosened by revolving brushes and is then drawn up
by atmospheric pressure into a tank.
Another use of atmospheric pressure is to be found in the pneumatic
tubes which convey mail from one post office to another. Air is
exhausted from the tube and when a carrier is inserted which fits the
tube closely the atmospheric pressure behind it forces the carrier
along to its destination. The same system is used to carry money to and
from the cashier’s office in department stores.
In later developments of the pneumatic mail carrier compressed air
was used back of the carrier, but the principle of operation was not
altered. In either case propulsion is effected by the difference of
air pressure on the opposite sides of the car.
In 1869 Alfred E. Beach built an experimental subway line under
Broadway, New York, near Warren Street. The line was only 200 feet
long. A cylindrical car large enough to hold eighteen people, fitted
the bore snugly and was propelled in one direction by compressing the
air in the tunnel and in the other direction by exhausting the air.