THE SYSTEM AND APPARATUS OF THE BATCHELLER PNEUMATIC TUBE COMPANY.
=General Arrangement and Adaptability of the System.=—The experience
gained in the construction and operation of the Philadelphia
post-office tubes has naturally suggested improvements that can be
made in future construction, and, furthermore, it has taught us what
the requirements will be of an extensive system of tubes laid in the
streets of our cities, both for the transmission of mail and for
a general commercial business. Since the Philadelphia post-office
tube was completed, we have been busily engaged in working out all
the details of a system of many stations so connected together that
carriers can be despatched in the most direct manner possible from
any station to any other. It is the purpose of the present chapter to
describe this system.
While the Pneumatic Transit Company has ample field in the State of
Pennsylvania to carry out the work which it has mapped out, a field
broad enough to yield a good profit for the capital invested, there
is no reason why the system should be limited to one State. So, in
order to obtain a broader charter, covering all places where pneumatic
service may be needed, a new corporation was formed and styled the
BATCHELLER PNEUMATIC TUBE COMPANY.
It is impossible to lay down a rigid system equally well adapted to
all places and purposes. We must accommodate ourselves somewhat to
circumstances. For example, the post-office department may require
one size of tube, arranged to operate in a particular way, while the
requirements of a parcel delivery business would be utterly different.
The geographical location of the stations will have much to do with the
general arrangement; also the condition of the streets. Some of the
streets of our large cities are so filled with water- and gas-pipes,
electrical conduits, sewers, steam-pipes, etc., etc., that it is almost
impossible to find space for pneumatic tubes, especially of large
diameter. Railway or water facilities have much to do with the location
of a central pumping station, on account of the coal supply. All of
these and many other things have to be taken into consideration in
planning a system for any locality.
We have an example of a peculiar location and conditions in a proposed
line of tubes over the New York and Brooklyn bridge connecting the main
post-offices of those cities. This would be in many respects a unique
plant. Two air-compressors would be used, one at each office.
In order to give a general idea how a large number of stations can be
connected into one system, the diagram Fig. 21 has been drawn.
We have already referred to the attempts of Clay and Lieb to devise
means whereby several stations could be located along a main line and
carriers be sent from any station to any other through the main line.
Their method was to use branch tubes leading off from the main line
with switches at the junctions. They deflected the air-current into
the branch by placing an automatic closed valve in the main line just
beyond the junction, returning the air from the branch to the main line
just beyond the valve. The carriers were to open and close this valve
automatically as they passed.
The branch and switch system has many attractions for the inventor,
and upon first thought it would seem the most feasible solution of
the problem. It has been the dream of more than one inventor, as the
records of the patent-office show, but no one has succeeded in working
it out. The current of air cannot be divided; carriers passing from the
branch into the main line must not collide with other carriers running
in the main line; a certain minimum distance must always be maintained
between the carriers in the same tube; when a carrier is despatched
it must go directly to the station for which it is intended without
further attention from the sender and it must not interfere with other
carriers; expense of manufacture prohibits the use of any but round
smooth tubes up to eight inches in diameter, hence projections cannot
be placed upon the carrier to give it an individuality and cause it to
operate a switch at any particular point along the line; the carrier is
free to rotate in a round tube about its longitudinal axis, therefore,
its individuality must be indicated by some symmetrical marking about
this axis, if it is to be automatic in its operation; the speed of
the carrier is so high that electrical contacts placed in distinctive
positions on the carriers cannot be used while it is in motion, for
mechanism having inertia could not be moved during the short time that
the electric circuit would be closed; only the simplest attachments
can be made to the carrier, for constructional reasons and because of
the rough usage that they receive. These and numerous other reasons
make the problem most difficult. We have not attempted to solve it by
the use of branch tubes and electrically operated switches, but have
adopted the simpler and equally effective method of carrying the main
line through each of the stations that it unites. In our system each
carrier has an individuality determining the station at which it will
be discharged from the tube. By a simple attachment to, the front end
of the carrier, consisting of a circular metal disk, the sender so
marks the carrier that it will pass all stations until it arrives at
the station for which it was destined and will there pass out of the
tube. In addition to this a method has been devised whereby carriers
can be inserted into the tube without the possibility of collision with
carriers already running in the tube.
[Illustration: FIG. 21.
A DIAGRAM SHOWING VARIOUS METHODS OF CONNECTING THE STATIONS OF A LARGE
SYSTEM WITH PNEUMATIC TUBES.]
Referring now to the diagram, Fig. 21, we have here an imaginary system
which we will suppose to be located in some large city. The two large
squares I and II indicate central pumping stations, and the small
squares A, B, C, D, etc., indicate receiving and sending stations. Some
of the stations, such as A, B, C, D, E, F, and Y, which do a large
amount of business and may be supposed to be large retail stores, are
connected directly with the central station by double tubes, one for
sending and the other for receiving carriers. Two smaller stores, such
as G and H, may be located on the same line. At I, J, K, and L we have
four stations, all connected by the same double line of tubes. These
stations we will imagine to be located in the residence section of
the city. Carriers containing parcels of merchandise or other matter
destined for private residences would be sent from the stores A, B, C,
etc., to the central station I, where they would be transferred to the
line 2 and be adjusted to stop at the station nearest the residence to
which the parcels were addressed. From this station the parcels would
be delivered by messengers to the residences. If a carrier is to be
sent from the central station I to station K, it will be so adjusted
before it is put into the tube that it will pass stations I and J, but
be discharged automatically from the tube when it arrives at station
K. In a similar manner carriers can be despatched from station L to
station I or from station J to station L. In passing through the
central station the carriers are manually transferred from one line to
another.
In another part of the city we may have another central pumping
station, II; and the two central stations may be connected by a double
trunk line, 3. Again, we have lines radiating from this central
station, as shown by station Y. There will be some localities where it
will be an advantage to arrange the stations upon a loop, as shown in
circuit 4, where stations S, T, U, V, W, and X are connected together
in this way. Or we can combine the two arrangements of loop and direct
line, as shown in circuit 5. Stations O and R are on the double line,
but from O a loop is formed including stations N, M, P, and Q. Here it
is supposed that the stations O and R do a much larger business with
the central station II than the stations N, M, P, and Q, this being
the principal reason for placing them on the double line. All carriers
must be returned to the station from which they were sent, or others
to replace them, otherwise there will be an accumulation of carriers
at some of the stations. It is like a railway: there must be as many
trains despatched in one direction as the other, each day. Station O
can receive a carrier from the central station and return it directly,
but when station N receives one it must be returned via M, P, Q, O,
and R, a much longer route than that by which it was received. This
disadvantage is compensated, when stations N, M, P, and Q do only a
small amount of business, by the less cost of laying a single line.
If a carrier is to be sent from M to N, it must go via P, Q, and O,
being manually transferred at O from the “down” to the “up” line. P
can send directly to Q, but Q must send to P via O, N, and M. R can
send directly to O and O to R. Similarly in circuit 4 the carriers must
all travel around the loop in the same direction, shown by the arrows.
Station S can receive carriers directly from the central station, but
they must return via U, W, X, V, and T.
Again, we may have a double-loop line, as indicated in the diagram
by circuit 6. Here five stations, _a_, _b_, _c_, _d_, and _e_, are
connected by a loop consisting of two lines of tube, in which the air
circulates in one direction in one line and in the opposite direction
in the other. Here _b_ can send directly to _c_, _c_ directly to
_b_, and _e_ to _b_ via _d_ and _c_, or via central and _a_. This
is an arrangement that would be used where there is a large amount
of business between the stations on the loop. As stated before, the
best arrangement for any particular locality depends entirely upon
circumstances.
=Size of Tubes.=—The pneumatic-tube system that we are describing
is not limited to any particular size of tube. The size is usually
determined by the number and size of packages to be transported.
A small tube, two or three inches in diameter, is best suited for
telegrams and messages; mail, parcels, etc., require a six- or
eight-inch tube, while mail pouches and bulky material, a thirty-six
inch or possibly larger tube. We divide tubes into three classes,
according to their size, naming them small, large, and very large
tubes. By small tubes we mean those not larger than three or possibly
four inches in diameter. Large tubes are those having a diameter more
than four inches and not more than eight inches. Very large tubes
include all that are more than eight inches. This classification is
for convenience, but it has a deeper significance. For example, in
the transportation of mail, it must either be handled in bulk, that
is, in pouches, or in broken-bulk, that is, loose or tied up in small
packages. There are many advantages in transporting it in broken-bulk,
in fact, there are very few places where it could be handled in any
other way. For this service six- or eight-inch tubes—not larger—are
best suited. The carriers are light enough to be easily handled; they
are not so large in capacity as to make it necessary to wait for an
accumulation of mail to fill them; they can be delivered from the tube
on to tables at any point in the building where the mail is wanted,
for cancelling, distribution, or pouching, thus rendering a very rapid
service; the mail is kept moving in an almost constant stream, keeping
the postal employees more uniformly employed; special carriers can be
despatched with “special delivery” letters. In other words, the most
rapid service can be rendered by this size of tube.
If a larger than eight-inch tube is to be used for mail service,
it should be not less than thirty-six inches. Carriers larger than
eight inches cannot be handled: they are too heavy. They are also too
heavy to slide through the tube, hence, must be mounted upon wheels.
It is not practical to make a carrier on wheels less than eighteen
or twenty-four inches, and the carrier must be at least twenty-four
inches to contain a large mail-pouch. Now, if we are going to despatch
mail-pouches through a pneumatic tube we must send more than one in a
carrier, otherwise the service will be too slow. Such large carriers
could not be despatched oftener than once or at most twice in a minute.
Suppose we were to transport the mail from a railway station to a main
post-office. A train arrives with, say, sixty pouches. If only one
pouch could be put into a carrier and the carriers could be despatched
at half-minute intervals, it would take thirty minutes to despatch
all the pouches. Now, suppose we make the tube thirty-six inches.
The carriers will be eight feet long and will contain from twelve to
fifteen pouches. Five carriers would contain the entire train-load of
mail, and they could be despatched in four or five minutes.
[Illustration: FIG. 22.
CROSS-SECTION OF A 36-INCH TUBE.]
[Illustration: FIG. 23.
CARRIER FOR A 36-INCH TUBE.]
=System of Very Large Tubes.=—The cross-section of a thirty-six-inch
tube is shown in Fig. 22. It is built flat on the bottom and sides,
with an arched top. The floor is of concrete containing creosoted
ties; the side walls and top are of brick, plastered with cement upon
the interior. The two tubes may be built one above the other or
side by side, depending upon the condition of the streets, but one
common separating wall will serve for both. The carriers, one of which
is shown in Fig. 23, run on two rails laid close to the sides of the
tube. At curves a guard-rail is placed upon the side wall, making it
impossible for a carrier to leave the track. The carriers are made
of hard wood with an iron frame, and are as light as consistent with
the service required of them. They are open on top. Their outside
dimensions are thirty-four inches by thirty-four inches by eight feet.
The sending and receiving apparatus for these very large tubes have to
be specially designed for each particular station, so no attempt will
be made here to describe them. The air-pressure required depends upon
the length of the line. If it were not more than six or eight ounces
a fan would be used to maintain the air-current, but for pressures
above this, up to a pound or two per square inch, some form of positive
blower would be used.
At the stations considerable floor space or “yard room” would be
required for side tracks, switches, etc. Usually the basement of a
building would have to be utilized for the termination of such a tube.
There are but few places in our large cities where the streets are so
free from pipes, sewers, conduits, etc., that it would be practicable
to build a thirty-six-inch pneumatic tube. When the service can be
rendered by an eight-inch tube, the cost of installation favors its
adoption. Steep grades cannot be ascended by these very large tubes,
while the eight-inch tubes can be placed vertically. We do not say that
there is no use for eighteen- and twenty-four-inch tubes, but the
demand for them would be in special cases and we will not discuss them
here. For ordinary mail and parcel service we recommend the use of
six- and eight-inch tubes. An eight-inch carrier is twenty-four inches
long, about seven inches inside diameter, and will contain five hundred
ordinary letters. It weighs about thirteen pounds empty, and one can
be despatched every six to ten seconds. We estimate that eighty per
cent. of all the parcels delivered from a large retail department store
could be wrapped up to go into these carriers. The minimum radius of
curvature of an eight-inch tube is eight feet.
=General Arrangement of Apparatus in the Stations. Two-Station,
Two-Compressor Line.=—We will now proceed to a description of our
system in detail. Figs. 24, 25, and 26 are diagrams showing how the
tubes, air-compressor, tanks, sending and receiving apparatus are
connected together at the stations. These diagrams are drawn to
represent an eight-inch tube, but essentially the same arrangement
would be used for smaller tubes.
Fig. 24 represents a line of two stations with an air-compressor at
each station. Such an arrangement is proposed for the line of postal
tubes over the New York and Brooklyn bridge, or for any two stations
located a very long distance apart, say six or eight miles.
[Illustration: FIG. 24.
DIAGRAM OF A TWO-STATION, TWO-COMPRESSOR LINE.]
Referring to the diagram, we have at station A an air-compressor, _c_,
which draws its supply of air from the tank _e_, and delivers it,
compressed to the necessary pressure, into the tank _d_. From the tank
_d_ the air flows to the sending apparatus, _a_, and thence through
the tube _f_ to the station B. Upon arrival at B it flows through the
receiving apparatus _m_, and then by the pipe _l_ to the tank _j_. A
second air-compressor, _o_, is located at station B, and it draws its
supply of air from the tank _j_. The tank _j_ has an opening to the
atmosphere, _i_, through which air can enter when the air-compressor
draws more than is supplied from the pipe _l_. The opening _i_ in the
tank _j_ serves as an escape for air when the air-compressor at station
A is started before that at station B. Stations A and B are similar in
their arrangements. At B the air-compressor _o_ delivers its compressed
air to the tank _k_, from which it flows to the sending apparatus _n_,
and thence through the tube _g_ back to station A. Upon its arrival at
A it passes through the receiving apparatus and enters the tank _e_,
which is open to the atmosphere at _h_. The tanks _d_ and _k_ serve
as separators to remove from the air any dirt and oil coming from the
compressors, and they form a cushion, deadening, to some extent, the
pulsations of the compressors and making the current of air in the
tubes more steady and uniform. The tanks _e_ and _j_ form traps to
catch any moisture, oil, or dirt coming out of the tubes.
Carriers are placed in the tubes and despatched by means of the sending
apparatus _a_ and _n_. They are received from the tubes and delivered
on to tables by means of the receiving apparatus _b_ and _m_. It will
be seen that the arrangement is such that the air flows through one
tube and returns through the other, the same air being used over and
over again. Any air that escapes at the sending and receiving apparatus
is replaced by an equal amount entering the tanks _e_ and _j_ from the
atmosphere. By thus keeping the same air circulating in the tubes we
prevent an accumulation of moisture in the tubes.
The air is at its maximum pressure in the tanks _d_ and _k_. The
pressure falls gradually as it flows along the tubes and is down to
atmospheric when it enters the tanks _e_ and _j_. The pressure at the
receivers, _b_ and _m_, is just sufficient to push the carriers out on
to the tables. The construction of the sending and receiving apparatus
will be described in another place.
[Illustration: FIG. 25.
DIAGRAM OF A TWO-STATION, ONE-COMPRESSOR LINE.]
=Two-Station, One-Compressor Line.=—Fig. 25 is a diagram showing two
stations, A and B, connected by a double line of tubes, both operated
by one air-compressor located at station A. This is the arrangement
used in the Philadelphia post-office line, and is the arrangement that
will ordinarily be used for all two-station lines except where unusual
conditions require something different. Station A is arranged precisely
like station A in Fig. 24, so it need not be described again. The
air flows from the sending apparatus _a_ through the tube _f_ to the
receiving apparatus _p_ at station B. From the receiver _p_ it flows
through the pipe _l_ to the sending apparatus _n_ and thence through
the tube _g_ back to station A. The receiver _p_ at station B is what
we will call a closed receiver,—_i.e._, it delivers the carrier from
the tube on to the table without opening the tube to the atmosphere.
The use of this form of receiver is made necessary by the fact that
the air-pressure in the tube at this station is considerably above
atmospheric. The air-pressure is at a maximum in the tank _d_. It falls
gradually along the tube _f_, and when the air arrives at the receiver
_p_, at station B, the pressure has fallen nearly to one-half its
maximum amount in the tank _d_. On its return journey through the tube
_g_ the pressure continues falling until it reaches the atmospheric
pressure when the air enters the tank _e_ at station A.
The entire line of tube, going and returning, is operated by air at a
pressure above the atmospheric. There is no exhausting in the return
tube. It is distinctly a _pressure_ system.
[Illustration: FIG. 26.
DIAGRAM OF A THREE- TO EIGHT-STATION LINE.]
=Three- to Eight-Station Line.=—Thus far we have described only
two-station lines. In Fig. 26 we have a diagram of three stations
connected together by a double line of tubes, and the arrangement
would be similar if it were extended to four, five, six, seven, or
eight stations. The stations are called A, B, and H. Station A is
arranged exactly the same as stations A in Figs. 24 and 25, therefore,
needs no description. Station B, being an intermediate station, is
quite differently arranged from any of the preceding. From station
A the air flows through the tube _f_ to station B, where it enters
the automatic receiving and transferring apparatus, _s_. From this
it flows through the tube _f_{´}_ to the sending apparatus _r_, and
thence through the tube _f_{´´}_ to the next station, which may be
another intermediate station, C, or the terminal station H. Station H
is arranged like station B, Fig. 25. The air from the tube _f_{´´}_
enters the receiver _p_, and is then returned, through the pipe _l_, to
the sending apparatus _n_. From the sending apparatus _n_ it continues
on its return journey through the tube _g_ to the intermediate station
B, where it enters the receiver and transfer apparatus _t_, then passes
to the sending apparatus _q_, and through the tube _g_{´´}_ back to
the receiver _b_ at station A. Thus we have followed the air-current
out through one tube and back through the other. The current is kept
circulating by the compressor located at station A. The pressure is
at a maximum in the tank _d_, and falls gradually as the air flows
along the tube until it returns to the tank _e_, when the pressure has
fallen to atmospheric. A carrier is despatched from station A, and
after passing through the tube _f_ arrives at station B, where it stops
momentarily in the automatic receiver and transfer apparatus _s_. If
the carrier is intended for station B, and was properly adjusted when
it was despatched at A, it will be discharged from the apparatus _s_
on to the table _u_. But if it were intended for some other station
and were so adjusted, after the delay of two or three seconds in
the apparatus _s_, it will be automatically transferred to the tube
_f_{´},_ pass through the sending apparatus _r_, and go on its journey
through tube _f_{´´}_ to the next station. If it is not discharged from
the tube at any of the intermediate stations, it will finally arrive
at the terminal station H and there stop. Just how the carriers are
adjusted and the details of the receiving and transfer apparatus will
be described hereafter. Carriers arrive at station B from H, or other
stations on the line, through tube _g_, in the apparatus _t_, which
either discharges them on to the table _u_ or sends them on through
the tube _g_{´}_ and _g_{´´}_ to station A. Carriers are despatched
from station B to station A by means of the sending apparatus _q_, and
from station B to other stations along the line, C, D, E, F, G, and
H, by means of the sending apparatus _r_. Thus, from B carriers can
be sent and received in either direction. In order to prevent the
possibility of a collision of carriers by attempting to despatch one
at station B at the instant another is passing through the sending
apparatus, an automatic lock is attached to each sending apparatus.
Just outside the station B, say three hundred feet on each side, are
located manholes, and in these manholes boxes are attached to the tube
containing an electric circuit-closing apparatus, so arranged that
when a carrier passes it will close an electric circuit leading to the
sending apparatus in the station. These manholes and circuit-closers
are shown and located on the diagram at _v_ and _w_. Wires _x_ and _y_
lead from them to the sending apparatus _r_ and _q_. When a carrier
from station A passes the box _v_, it closes the electric circuit _x_,
which sets a time-lock on the sending apparatus _r_, holding this
apparatus locked, so that it is impossible to despatch a carrier for,
say, twelve seconds, a sufficient time for the carrier coming from the
station A to pass station B and get three hundred feet beyond it. After
the twelve seconds have elapsed the sending apparatus is unlocked and
a carrier can then be despatched. In a similar manner a carrier coming
from station H, in passing the box _w_, closes the electric circuit _y_
and locks the sending apparatus _q_ for a sufficient length of time to
let the carrier pass the station. This resembles, in some respects,
the “block system” as used on railroads. A “block” about six hundred
feet in length, depending upon the speed of the carriers, is made at
each intermediate station with the station in the centre of the block.
Whenever a carrier enters this “block” the sending apparatus at the
station is locked, and a carrier cannot be inserted into the tube to
collide with the one which is passing. It will be noted that a carrier
in passing out of the “block” does not unlock the sending apparatus;
this is done automatically at a definite time after the carrier entered
the block. The unlocking is entirely independent of the carrier after
it has entered the block, and the reason it is so arranged is this:
suppose that a second carrier enters the “block” before the first one
leaves it; if the first carrier unlocked the apparatus when it left the
“block,” then it would be unlocked with the second carrier in the block
and a collision might occur, but by arranging it as we have done, if a
second carrier enters the “block” before the first has passed out, the
sending apparatus remains locked for a period of time beginning with
the arrival of the first carrier in the “block” and ending, say, twelve
seconds after the arrival of the last carrier, which is sufficient time
for the last carrier to pass out of the block. Of course, if a carrier
becomes wedged in the tube a collision may occur, but this very seldom
if ever happens. The details of the locking apparatus will be described
in another place.
If stations A, B, ... and H were arranged on a loop, as shown in
circuit 6, Fig. 21, then station H, Fig. 26, would be at the central,
or station A. If it were a single loop, like circuit 4, Fig. 21,
then there would be only one sending apparatus and one receiving and
transferring apparatus at the intermediate stations.
A telephone circuit will include all stations, in order to give orders
to the station attendants and to signal to the central station in case
of an accident, when it might be necessary to stop the air-compressor.
The telephone wires, in the form of a lead-covered cable, are laid in
the same trench with the tubes and fastened to them.
[Illustration: FIG. 27.
SENDING APPARATUS.]
[Illustration: FIG. 28.
SENDING APPARATUS.—LONGITUDINAL SECTION.]
=The Sending Apparatus.=—We have, in the preceding pages, frequently
spoken of the sending apparatus, and have described it as mechanism by
which carriers are inserted into the tube. In the Philadelphia postal
line this apparatus consisted of a large valve, operated by hand. For
an eight-inch tube such a valve would be too large and heavy to be
manually operated. Furthermore, that type of apparatus is not suited
to an intermediate station, where carriers have to pass through it. To
meet all of these requirements we have designed an apparatus, of which
Fig. 27 is a side elevation, Fig. 28 a longitudinal section, and Fig.
29 a cross-section. Referring to the longitudinal section, Fig. 28,
the sending apparatus is shown inserted into the line of a pneumatic
tube, A, A. We have a movable section of tube, B, that can be swung
about the large bolt, G, at the top, into and out of line with the main
tube, A, A. When the section of tube B is being swung to one side, the
air-current has a by-pass through the slots E and F and the U-shaped
pipe D. The joints at the ends of the movable section B are packed with
specially-formed leathers. Referring to the cross-section, Fig. 29,
when the movable section of tube B is swung out of line with the main
tube, another and similar tube, C, takes its place. The two movable
tubes, B and C, are made in one piece, so that they must always move
together. They are connected together at each end by plates, M, that
serve not only as connecting-plates, but covers for the ends of the
main-line tube while the tubes B and C are being moved. The tubes B
and C swing between four plates or wings, L, that extend out on each
side of the apparatus. They serve as guards, and, at certain positions
of the swinging tubes, prevent the air from escaping.
We will, for convenience, call the system of swinging tubes B and C,
with their supports, etc., the swing-frame or simply the frame. This
frame is moved or swung from one position to the other by means of a
cylinder and piston, H, placed in an inclined position under it. A lug,
N, is cast on the tube B, to which the connecting-rod, O, is attached.
The cross-head, P, slides upon an inclined guide, Q. On top of the
cylinder is placed a controlling valve, made in the form of a piston
slide-valve. The piston in the cylinder H is moved by the pressure of
the air taken from the main tube through the pipe I. The apparatus is
operated by a hand-lever, K. When this lever is pulled, it moves the
sliding-head R, and this, through the spring S, moves the controlling
valve, if the valve is not locked. If it is locked, pulling the lever
simply compresses the spring S. When the controlling valve is moved to
the right the air in the cylinder H escapes through the passage V and
the port J to the atmosphere, and compressed air from the main tube
flows through the pipe I, the passages T and U, to the cylinder H,
under the piston, causing the piston to move up the inclined cylinder
and swing the frame until the tube C is in line with the main tube.
Carriers are despatched by placing them in the tube C, then pulling
the lever K, and swinging the frame until the tube C is in line with
the main tube. The carrier is then taken up and carried along by the
current of air in the main tube.
[Illustration: FIG. 29.
SENDING APPARATUS.—CROSS-SECTION.]
Replacing the hand-lever K in its original position returns the frame
to its normal position.
[Illustration: FIG. 30.
SENDING TIME-LOCK.]
=Sending Time-Lock.=—In any system of large pneumatic tubes a short
time should elapse between the despatching of carriers, in order that
they may not collide in the tube, and to give the receiving apparatus
at the stations time to act. To insure the impossibility of having
carriers despatched too rapidly, we place on the sending apparatus a
time-lock that will automatically lock it for a determined length of
time after each carrier is despatched, the time-lock being adjustable
for any desired time. The time-lock, W, is shown attached to the
sending apparatus in Fig. 27. When the swing-frame is swung to despatch
a carrier, it pulls up the rod X by means of a link and bell-crank, Y,
thereby locking the controlling valve of the cylinder H and starting
the time-lock W, which will unlock the controlling valve after the
required time has elapsed. The details of this time-lock are shown
in Fig. 30. It consists of a long vertical cylinder, A, containing a
piston, B, and a spiral spring, C, that tends to force the piston to
the bottom of the cylinder. The cylinder is filled with oil, and holes,
D, in the piston allow the oil to pass freely through it when it is
moved upward in the cylinder. When the piston moves downward an annular
collar, E, forming a valve, closes the holes in the piston and prevents
the oil from passing through. Extending from one side of the piston
to the other is a by-pass, F, in the wall of the cylinder. When the
piston moves downward the displaced oil is forced to flow through this
by-pass. A small cock, G, is arranged in the by-pass to throttle the
stream of oil flowing through it. The opening in this cock, or the
amount of throttling, is indicated on the outside by an index and dial,
Z (see Fig. 27). When the piston B is raised and allowed to descend by
the force of the spring C, it forces the oil through the by-pass F and
the cock G. If the latter is wide open the piston will descend quickly,
but if it is nearly closed the piston will descend very slowly. In
other words, the time of descent can be regulated by opening and
closing the cock G. The reading on the dial Z can be made seconds of
time that elapse while the piston is descending.
Above the cylinder is a cross-head, H, that moves up and down between
vertical guides. This cross-head is moved by the rod X, also shown
in Fig. 27, that receives its motion from the swinging frame of the
sending apparatus. A piston rod, I, attached to the piston in the
cylinder, extends up through the travelling cross-head but is not
attached to it. On the piston-rod are two enlargements, J and K, one
made a solid part of it, the other formed by two nuts. The travelling
cross-head H carries a pawl, L, that engages under the shoulder formed
by the nuts K. This pawl is kept against the piston-rod by the spring
M. The enlargement, J, on the piston-rod forms a shoulder that bears
against the bell-crank, N, that connects with the bolt, O, which locks
the controlling valve. In the present down position of the piston and
piston-rod, the enlargement J, by pressing against the bell-crank N,
holds the bolt O in an unlocked position. When a carrier is despatched
the cross-head H is lifted by the rod X, and carries with it the piston
and piston-rod, compressing the spring C. This upward movement of the
piston-rod allows the bolt O to be thrown by a spring, not shown in the
figure, and so lock the controlling valve of the sending apparatus.
As the cross-head continues its upward movement, the pawl L comes in
contact with the end of the screw P and disengages the piston-rod. This
allows the piston to descend as rapidly as the oil can pass through the
by-pass and cock G. When the piston has reached nearly to the bottom of
the cylinder, the shoulder J, on the piston-rod, engages the bell-crank
N and withdraws the bolt O, thereby unlocking the controlling valve.
The time that the sending apparatus is locked depends upon the time
required for the piston to descend. While the sending apparatus is
locked against the sending of another carrier, it is not so locked
that the swing-frame cannot be returned to its normal position and
another carrier inserted ready to be sent as soon as the necessary time
expires. This time is usually not more than ten seconds. Not only may
the second carrier be placed in the tube C, Fig. 29, ready to be sent,
but the handle K may be pulled and fastened in the notch _a_, thereby
compressing the spring S, which, as soon as the controlling valve is
unlocked, will move the valve and automatically despatch the carrier.
The controlling valve is locked by the passage of a bolt through the
hole _b_, in a block carried on the end of the valve stem, when it
returns to the normal position shown in the figure. Usually little or
no time will be lost in thus locking the sending apparatus, for the
small amount of time that the apparatus is locked will be needed in
handling the carriers.
=Intermediate Station Time-Lock.=—We have another time-lock attached to
the sending apparatus that has been already referred to in describing
the “block system” used at intermediate stations; a time-lock to
prevent carriers being inserted into the tube at intermediate stations
while another carrier is passing that station. This time-lock is shown
in Fig. 27 at W´, and is shown in detail by a sectional drawing, Fig.
31.
When a carrier closes an electric circuit in passing one of the boxes
located in a manhole about three hundred feet from an intermediate
station, it indicates its approach to the station by exciting the
electro-magnet A, Fig. 31. This magnet pulls down its armature B and
raises the small piston valve C, which admits compressed air to a small
chamber, D. The air is supplied to this chamber from the main tube
through the pipe E. In one end of this chamber is fitted a piston, F,
held to one end of its stroke by a spring, G. When compressed air is
admitted to the chamber D, this piston is moved to the left, and by
such movement throws the controlling valve of the sending apparatus
into its normal position (shown in Fig. 29) and holds it there. This
forms a positive lock, and, no matter in what position the sending
apparatus may be, it puts the tube B, Fig. 29, into line with the main
tube so that the approaching carrier can pass through the apparatus.
The piston-rod H, Fig. 31, is connected to the finger _d_, Fig. 29,
and by rocking this finger moves the controlling valve, or prevents it
being moved by the handle K.
[Illustration: FIG. 31.
INTERMEDIATE STATION TIME-LOCK.]
Returning now to Fig. 31, we have on the top of the chamber D, in
addition to the electro-magnet A and its armature B, a differential
cylinder and piston, K, L, whose function is to close the valve C
when the chamber D is filled with air. The piston K is smaller than
the piston L, and sustains a constant air-pressure, supplied through
the small pipe M M, from the pipe E, which leads to the main tube.
When the chamber D becomes filled from the pipe E through the valve
C, the pressure in the chamber moves the piston L upward against the
pressure on the piston K, because of the greater area of the piston
L. This movement of the differential piston raises the lever I, which
passes through a slot in the stem of the differential piston, and thus
closes the valve C. The air in the chamber now gradually escapes to
the atmosphere through a small orifice Q; in fact it has been escaping
here all the time while the chamber was being filled, but the opening
through the valve C is so many times larger than the orifice Q that the
escape of air was not sufficient to prevent the chamber from filling.
Now, however, that all supply to the chamber is shut off, the air in
the chamber is gradually being discharged through the orifice. When
nearly all the air has escaped, the piston F will return to its normal
position, shown in the figure, and unlock the controlling valve.
The time required for the air to escape from the chamber, D, is the
time that the sending apparatus will be locked, and this time can be
regulated by varying the size of the orifice Q. The opening of the
orifice, or the time that the sending apparatus is locked, is indicated
by an index and dial, P.
This locking mechanism is secured to a bracket on the side of the
large cylinder H, Fig. 27, in a position where it can be easily
inspected. The moving parts of the electro-magnetic valve—for such is
the valve C, with the magnet A, Fig. 31—are made very light, in order
that they may respond easily and quickly to the closing of the electric
circuit.
It is a disadvantage to have stations too numerous upon the same line,
especially if they do a large amount of business, for each station
will delay the sending of carriers from the others more or less, and
the interference will be greatest during the busiest hours of the
day. This condition is inherent in any system of large tubes where
carriers have to be run a certain minimum distance apart, and cannot be
overcome by any mechanism. But the disadvantage is greatly overshadowed
by the advantage of being able to connect several stations by one
line, instead of having to run independent lines from each station to
the central, especially when the business of the individual stations
is not sufficient to occupy a separate tube all the time. It makes
it possible to have stations where otherwise the business would not
warrant the cost of installation and expense of operation. We recommend
the establishing of not more than eight stations on a line, and usually
a smaller number than this, depending, of course, upon the amount of
business to be done at each station.
[Illustration: FIG. 32.
ELECTRO-PNEUMATIC CIRCUIT-CLOSER.]
=The Electro-Pneumatic Circuit-Closer.=—There is one piece of
mechanism used in connection with the sending apparatus that we have
yet to describe, and that is the circuit-closing device located in
the manholes in the street. Since the carriers travel at a high
rate of speed, they should not be made to operate any mechanism by
impact with fingers or levers protruding into the tube when it can be
avoided, even though the work to be done is so slight as the closing
of an electric circuit, for the repeated impacts cannot fail to work
injury to the carriers and the mechanism to be operated, no matter how
carefully they are designed. To avoid such impacts, we have designed
the electro-pneumatic circuit-closer, shown by the drawing in Fig. 32.
It is operated by a passing carrier, but pneumatically rather than
mechanically. In the figure we have a pneumatic tube, A, A, in which a
carrier, B, is moving in the direction indicated by the arrow. At two
points, about twenty or thirty feet apart, two small holes are tapped
into the tube and pipes, C and D, are screwed in. These pipes lead to
two chambers in a cast-iron box, F, separated by a diaphragm, E. This
diaphragm is insulated electrically from the box supporting it, and
is connected with the wire G. Just out of contact with the diaphragm
is an insulated screw, H, connected with the wire I. These wires lead
to the time-lock, already described, on the sending apparatus at the
station. When no carrier is passing, the air-pressure is the same on
both sides of the diaphragm, but when a carrier enters that part of the
tube between the two points where the pipes C and D are connected, the
equality of pressure on opposite sides of the diaphragm is destroyed.
There is always a slightly greater pressure in rear of the carrier than
in front of it, equal to the frictional resistance of the carrier in
the tube. It is this difference of pressure in front and in rear of
the carrier that moves it through the tube. When the carrier is in
the position shown in the figure, the same difference of pressure will
exist on opposite sides of the diaphragm, and it will be deflected into
contact with the screw H, thereby closing the electric circuit. When
the carrier has passed, equality of pressure on opposite sides of the
diaphragm is established and the diaphragm takes its normal position,
out of contact with the screw H. This apparatus is easily attached to
the tube, and it contains no mechanism to get out of order.
[Illustration: FIG. 33.
OPEN RECEIVER.]
=The Open Receiver.=—Wherever the pressure in the tube is down nearly
to atmospheric, we can use an open receiver to discharge the carriers
from the tube. This is a receiver that opens the tube to the atmosphere
and allows the carrier to come out. Such a receiver is used at the
main post-office in the Philadelphia postal-line, and was described in
the last chapter. The present receiver is similar in operation, but
contains some improvements in details. Fig. 33 is a side elevation of
the apparatus, Fig. 34 is a longitudinal section, and Fig. 35 is a
cross-section through the cylinder and valve, showing the sluice-gate.
[Illustration: FIG. 34.
OPEN RECEIVER.—LONGITUDINAL SECTION.]
[Illustration: FIG. 35.
OPEN RECEIVER.—SLUICE-GATE MECHANISM.]
Referring to the longitudinal section, the apparatus is attached to
the end of a pneumatic tube, A. The current of air from the tube
A flows through the slots B into a pipe, C, that conducts it to a
tank near the air-compressor. About the centre of the apparatus is a
sluice-gate, E, that is raised and lowered by a piston in a vertical
cylinder, F, located just above the sluice-gate. This piston is moved
by air-pressure taken from some part of the system. When a carrier
arrives from the tube A, it passes over the slots B and runs into the
air-cushion D, where it comes gradually to rest. Checking the
momentum of the carrier compresses the air in front of it considerably,
and this excess of pressure is utilized to move a small slide-valve
that controls the movement of the piston in the cylinder F, so that as
soon as the carrier has come to rest the sluice-gate rises and allows
the carrier to be pushed out with a low velocity on to a table. The
small pipe G conducts a small portion of the air compressed in front
of the retarded carrier to the controlling valve, H, seen in Figs. 33
and 35. Referring now to the section of the valve and cylinder, Fig.
35, the pipe G enters the top of a small valve-cylinder containing a
hemispherical piston, I, that is held up by a spiral spring, J. This
spring has just sufficient tension to hold the piston I up against
the normal pressure of air in the tube. When a carrier arrives
and compresses the air in the air-cushion, the excess of pressure
forces the piston I down against the spring J, and moves the piston
slide-valve K. This change of position of the slide-valve allows the
air in the cylinder F to escape to the atmosphere through the passage
L, passage P, and pipe M, while compressed air from some part of the
main tube enters through the port N and passage O to the under side of
the piston in the cylinder F. This moves the piston up, carrying with
it the sluice-gate E.
There is just sufficient pressure in the tube in rear of the carrier
to push the carrier past the gate and on to the table. As the carrier
moves out it raises a finger, Q, Fig. 34, that projects into its
path. Raising this finger extends the spring R, Fig. 33, and rotates
the lever S, bringing the pawl T under the end of the controlling
valve-stem. When the carrier has passed out and the finger Q is free
to descend, the spring R rotates the lever S back to its original
position, and thereby raises the controlling slide-valve, which causes
the sluice-gate to close. By having the upward motion of the finger Q
simply extend the spring R, and the downward motion, by the force of
the spring, move the valve, we are enabled to have several carriers
pass out of the tube together without having the sluice-gate close
until the last carrier has passed out. If raising the finger Q moved
the valve, then when the first carrier passed out, the gate would close
down upon the second. Attached to the receiving apparatus and extending
beyond it is a tube, U, cut away upon one side so that the carriers can
roll out of it on to a table, and having in the end a buffer to stop
the carriers if by any accident they come out of the tube with too much
speed. This buffer consists of a piston covered with several layers
of leather and having a stiff spring behind it. The whole apparatus
is supported from the floor upon suitable standards, and, for an
eight-inch tube, occupies a floor-space twelve feet long by two feet
wide, not including the table.
This is the simplest form of receiving apparatus. Owing to conditions
of pressure already explained, its use is confined principally to the
pumping stations. The only care that it requires is an occasional
cleaning and oiling.
[Illustration: FIG. 36.
CLOSED RECEIVER.]
[Illustration: FIG. 37.
CLOSED RECEIVER.—LONGITUDINAL SECTION.]
=The Closed Receiver.=—Next we will turn our attention to the closed
receiving apparatus used at all terminal stations where the pressure in
the tube is considerably above the pressure of the atmosphere, so much
so that the tube cannot be opened to allow the carrier to pass out
without an annoying blast of air and a high velocity of the carrier.
This apparatus is similar to the receiver used in the sub-post-office
of the Philadelphia postal line, but contains several modifications
and improvements tending towards simplification. Fig. 36 shows it in
elevation, and Fig. 37 in longitudinal section. As in the open receiver
just described, the air from the tube A is deflected through slots B
into a branch pipe, C, that conducts it from the receiving apparatus
to the sending apparatus and return tube. The carriers arrive from the
tube A, pass over the slots B, where the air makes its exit, and run
into an air-cushion, D. This air-cushion is a tube about twice the
length of the carrier, closed at one end, and supported upon trunnions.
When the carrier has been brought to rest, this closed section of tube
is tilted by the movement of a piston in a cylinder to an angle that
allows the carrier to slide out; the tube then returns to its original
position. If the end of the air-cushion was closed perfectly tight the
carrier, after coming to rest, would rebound and might be caught in the
joint between the stationary and movable parts of the apparatus, when
the air-cushion tube tilted. To prevent the rebounding of the carrier a
relief-valve, E, has been placed in the head of the air-cushion tube.
It is held closed against the normal pressure in the tube by a spiral
spring, but the excessive pressure created by checking the momentum of
the carrier opens the valve and allows a little air to escape through
the passage F and pipe G, down the pedestal H, to the atmosphere. When
the air-cushion or receiving tube D is tilted to discharge a carrier,
the circular plate I covers the end of the main tube. In order to
prevent carriers sticking in the receiving tube when it is tilted,
and to insure their prompt discharge, the pipe J is provided. In the
tilted position of the receiving tube, the end of this pipe coincides
with the end of the main tube, from which it receives air to hasten
the discharge of the carrier. A check-valve, K, prevents the air from
flowing backward in this pipe when a carrier is being received in the
air-cushion chamber. The opening of this check-valve can be adjusted by
a screw, thereby regulating the speed of ejection of the carrier.
[Illustration: FIG. 38.
INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.]
[Illustration: FIG. 39.
INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.—VERTICAL
SECTION.]
The carrier is discharged down a chute, L, which has a buffer at the
bottom, and from the chute it rolls off on to a table. The buffer is
made similar to the buffer in the open receiver already described.
The cylinder and piston M, that operate to tilt the receiving tube D,
are supported upon the base of the apparatus under the closed end of
the receiving tube. The cross-head of the piston- and connecting-rods
travels between guides that are made a part of the upper cylinder
head. The movement of the piston in the cylinder M is controlled by
a piston slide-valve exactly similar to the one shown in Fig. 35.
The slide-valve is moved, in the same manner, by the air compressed
ahead of the carrier when it is brought to rest in the air-cushion
D. The air is conducted from the air-cushion to the controlling
slide-valve through a small pipe, N, Fig. 36. This pipe leads to one
of the trunnions, where it has a joint to allow for the tilting of
the receiving tube. When the carrier is discharged from the receiving
tube, it raises a finger, O, Fig. 37, located just outside the tube.
Raising this finger pulls the rod P, Fig. 36, extends the spring
Q, turns the lever R, and catches the pawl S, under the end of the
controlling valve stem. When the carrier has passed down the chute and
allowed the finger O to drop down, the spring Q turns the lever R back
to its original position and moves the controlling valve. This causes
the receiving tube to return to a horizontal position, where it is
ready to receive the next carrier.
At first this apparatus may seem a little cumbersome, but nothing could
work better. It is certain in its action and almost noiseless. Carriers
are received, discharged, and the receiving tube returned to its normal
position in four seconds, and it can be done in less time if necessary.
=The Intermediate Station Receiving and Transfer Apparatus.=—One other
form of receiving apparatus remains to be described, and this is the
apparatus used at intermediate stations to intercept all carriers
intended for that station and to send the others on through the tube
to the next station. A side elevation of the apparatus is shown in
Fig. 38 and a vertical section in Fig. 39. The tubes are led into an
intermediate station, carried upward, and then, with a bend of one
hundred and eighty degrees, are connected to the top of the receiving
and transfer apparatus, as shown in the diagram, Fig. 26. The object of
this arrangement will be seen as we describe the apparatus. Referring
to the sectional drawing, Fig. 39, the connection of the tube A is seen
at the top. As in the other receivers, the current of air arriving
from the tube A is deflected through slots, B, into a passage, C, made
in the frame of the apparatus. From this passage it enters the tube D
through the slots E. The tube D leads to the sending apparatus and
on to the next station, as seen in Fig. 26. The carriers are received
in a closed section of tube F, which forms an air-cushion, similar to
the closed receiver last described. This receiving tube F is made a
part of what we might term a wheel. This wheel fits accurately into a
circular casing and is supported by two trunnions or axles, upon which
it revolves. The wheel has a broad flat rim, G, that covers the end of
the tube at H when the wheel is revolved, and, in the normal position
in which it is shown in the figure, covers the interior openings I, J,
K, and L, in the casing. Leather packing is provided around each of the
openings to prevent the escape of air between the face of the wheel
and the interior face of the casing. From the bottom of the receiving
tube F a passage, M, leads past a check-valve, N, to the tube D. When
a carrier arrives from the tube A, it descends into the receiving tube
F, compressing the air in front of it. This compressed air begins to
escape through the passage M, but the high velocity of it closes the
check-valve N as much as possible. A stop on the stem of the valve
prevents it being closed entirely. The small opening past the valve
allows some of the air to pass, thereby preventing the carrier from
rebounding on the air-cushion. As soon as the carrier has come to rest,
the check-valve N, by its own weight, opens wide, and the carrier, by
its weight, settles gradually down to the bottom of the receiving tube.
The wheel containing the receiving tube and the carrier will then be
revolved by the cylinder and piston O, which is operated by compressed
air taken from the tube through the pipe P. If the carrier is for this
station, the wheel will rotate through an angle of forty-five degrees
and discharge the carrier through the opening J, down the chute Q, from
which it will roll on to a table arranged to receive it. If, however,
the carrier is intended for some other station, the wheel will rotate
through an angle of ninety degrees and discharge the carrier through
the opening K into the tube D, and it will go on its way to the next
station. This selection of carriers is brought about in a comparatively
simple manner. At the bottom of the receiving tube F there are two
vertical needles, R and S, shown upon a larger scale in Fig. 40. The
needles R and S are contained in tubes having an insulating lining
which keeps them out of electrical contact with the frame of the
apparatus. Wires _a_ and _b_ make connection with the needles through
metal plugs that form a guide for the needles, and through the springs
U and V. Directly below the needle R is an insulated spring clip, W,
held by two bolts and connected to the wire _e_.
[Illustration: FIG. 40.
A DETAIL OF THE INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS.]
The end of a carrier is represented at T. As the carrier settles down
to the bottom of the receiving tube, it comes in contact with the
ends of the needles and presses them down, they being supported by
two springs U and V. As the needle R is moved down, it makes contact
with the spring clip W, located just below it, and closes an electric
circuit that includes the electro-magnet X, Figs. 38 and 39, on the
valve of the rotating cylinder O. When this electro-magnet is excited
it attracts its armature and moves the piston slide-valve Y, that
admits air to the top of the piston in the cylinder O, and allows the
air under the piston to escape to the atmosphere. The piston moves
downward and revolves the wheel by means of a connecting rod.
Upon the end of the carrier T is placed a thin circular metal disk,
_f_, which may be copper, brass, tin-plate or any metal that is not
easily oxidized. The diameter of this disk of metal determines the
station at which the carrier will be discharged from the tube. Disks of
various diameters, that may be attached to the carrier, are represented
by dashed lines, _g_, in Fig. 40. When the carrier comes in contact
with the two needles R and S, if the circular metal disk on the front
end of the carrier has a diameter sufficient to span the space between
the two needles, in the position in which it is held, then an electric
circuit, made by the wires _a_ and _b_, will be closed through the
needles and the metal disk on the carrier. The metal disk makes a
short-circuit from one needle to the other. If the metal disk is not
large enough to span the distance between the two needles, then the
electric circuit remains broken.
Returning again to Fig. 39, we have the opening J, where the carriers
are discharged, closed by a sluice-gate. This gate is opened and
closed by a piston moving in a cylinder, _h_, shown in Fig. 38. A
piston slide-valve, _i_, similar in all respects to the valve on the
cylinder O, controls the movement of the piston in this cylinder and
the sluice-gate to which it is attached. The slide-valve is moved in
one direction, that opens the sluice-gate, by an electro-magnet in the
circuit of the wires _a_ and _b_, Fig. 40.
When the electric circuit made by these wires is closed by a disk
on the front end of a carrier, short-circuiting the two needles,
the valve is moved by the electro-magnet in the circuit, and the
sluice-gate is opened. As the wheel, including the receiving tube and
carrier, revolves, a lug, _j_, Fig. 38, on the outside of the wheel
comes in contact with the open sluice-gate and the wheel can rotate
no farther. A blast of air through the valve L, Fig. 39, assisted by
gravity, pushes the carrier out of the receiving tube, through the
opening J and down the chute Q, on to the receiving table.
Had the disk on the front end of the carrier been too small to span
the distance between the two needles, the circuit would not have been
closed, the sluice-gate would not have been opened, no obstruction
would have been placed in the path of the lug _j_, on the wheel, and
the wheel would have continued its rotation through ninety degrees
until the receiving tube F came in line with the tube D. During the
latter part of the rotation, a pin on the wheel engages a lever, _k_,
Fig. 38, and turns a valve, _l_, Fig. 39, stopping the flow of air
through the passage C, compelling it to take another route through
the passage _m_, and the receiving tube F, taking with it the carrier
into the tube D. When the carrier leaves the receiving tube and passes
through either of the openings J or K, it engages one of the fingers,
_n_ or _o_, that lie in its path. These fingers are connected by rods
and levers to the valves on the rotating and sluice-gate cylinders.
The ejected carrier pushes these fingers to one side, and after it has
passed the fingers return, by the force of a spring, to their former
position and move the valves, causing the sluice-gate to close and the
wheel to rotate backward into its normal position ready to receive
the next carrier. The connection between the fingers and the valves is
similar to the mechanism on the open and closed receivers, so need not
be described in detail here.
The speed with which the carriers are ejected from the receiving tube
through the opening J and down the chute Q is regulated by the valve
L, which can be opened or closed by a hand-wheel, _p_. Before the
wheel and receiving tube can be rotated, the needles must be withdrawn
from the receiving tube, and this is accomplished by a small cylinder
and piston, _q_, shown in Fig. 40. The needles and their encasement
are attached to a cross-head, _r_, on the end of a hollow piston-rod,
_s_. When air is admitted to the top of the piston in the rotating
cylinder O, Fig. 39, it is also admitted through the pipe _t_, Fig.
38, to the cylinder and upper side of the piston _q_, Fig. 40. This
moves the piston _q_ down against the force of a spring, _u_, and
withdraws the needles from the receiving tube. This takes place after
the needles have served their purpose and before the wheel is rotated.
The piston _q_ has much less inertia than the wheel, therefore it
moves much quicker. When the wheel begins to rotate it closes a valve,
_v_, in the pipe _t_, Fig. 38, confining the air in the cylinder _q_,
and preventing the needles from being raised by the spring _u_ before
the wheel returns to its normal position. If by any accident the
needles should be raised, no serious harm would result, for their ends
would simply bear against the face of the wheel. If this took place
constantly, grooves might be worn in the face of the wheel; for this
reason the valve _v_ is provided.
In order to facilitate the inspection of the needles and electric
contact springs W, they are contained in a cylindrical brass case,
_w_, that is held in place beneath the receiving tube by two bolts. By
removing the nuts from these bolts the entire mechanism can be removed,
examined, and cleaned. It also gives easy access to the receiving tube.
The receiving tube is long enough to receive two carriers, if it should
ever happen that two arrive at the same time.
[Illustration: FIG. 41.
DIAGRAM OF CONTACT-DISKS AND NEEDLES.]
To show how the apparatus at the various stations is arranged to
correspond with the disks of various sizes attached to the front of the
carriers, a diagram, Fig. 41, has been made, in which the needles at
the bottom of the receiving tubes of the apparatus at six intermediate
stations are represented at A, B, C, D, E, and F. Six disks of
different sizes are represented at _a_, _b_, _c_, _d_, _e_, and _f_.
The needles are placed farthest apart at station A and nearer together
at each succeeding station until we arrive at station F, where they
are nearest together. If we wish to send a carrier to station A from
the central, we place the largest disk, _a_, upon the front end of it.
When it arrives at station A, it closes the electric circuit between
the needles and is discharged from the tube. Should we wish to send a
carrier to station D, then we place the disk _d_ upon the front end of
it. When the carrier arrives at the station A, the disk is not large
enough to span the needles; therefore the sluice-gate is not opened
and the carrier is sent on in the tube. When it arrives at stations
B and C, the same thing occurs again, but when it reaches station D,
the needles are sufficiently close together so that the disk makes an
electric circuit between them, and the carrier is discharged from the
tube, as was intended when despatched. Since the carriers always travel
in the same direction in a tube, the first station at which they arrive
where the needles are near enough together to have both touch the disk,
will be the station at which the carrier was intended to stop. Carriers
can be despatched from any station, but if we wish to send from say D
to A, they must either travel around a loop or be sent through a return
tube in which the needles are arranged in the reverse order. If no disk
is placed on the carrier, it will go to the last station on the line.
There are other attachments that might be made to the front end of
the carriers in order to have them stop at any desired station along
a line. We have worked out two other systems which are entirely
mechanical in their operation, not using electric circuits and
electro-magnets to move the valves. While such a mechanical system has
some advantages over the present combined mechanical and electrical
system, yet there is one great advantage in the latter, and that is the
simplicity of the attachment made to the carrier. A round flat disk of
tin-plate is attached to the front end; it is something that is not
in the way; it does not prevent standing the carriers on end in racks
to fill them; it is not easily injured, and only those who have had
experience can realize the rough usage that the carriers receive; it
is quickly and easily attached to the carrier, and it is so cheap that
when bent it can be thrown away.
=Carriers.=—The carriers are similar in all respects to those used in
the Philadelphia postal-line, that have been described in the preceding
chapter and illustrated in Figs. 18, 19, and 20. When there are
intermediate stations upon the lines, means are provided for attaching
disks to the front end of the carriers. The disks have a central stem
that secures them to the bolt in the centre of the head, and are so
arranged that they can be quickly attached or removed.
Many experiments have been made to find the best material for
bearing-rings, but thus far nothing better than a specially-prepared
woven fabric has been found. These rings will run about a thousand
miles, when they become so reduced in diameter that they have to be
replaced by new ones.
The most essential elements of a carrier are strength, lightness, and
security of the contents. Aluminum has frequently been proposed as a
suitable material for the bodies of carriers, but for the same weight
steel is much stronger, especially in thin rolled sheets, and for this
reason it has been used.
One of the most perplexing problems that presented itself in working
out the details of the system was to design a secure and reliable lock
for the lids of the carriers. We believe that the one which has been
adopted fulfils all requirements in a satisfactory manner.
Some experiments have been made with carriers that open on the side,
but structurally they are weak and unsuited to stand the blows that
carriers frequently receive. They are not so easily and quickly filled
and emptied as those that open on the end. These remarks apply to
carriers for large tubes. In small tubes for the transportation of cash
in retail stores, carriers with side openings are found convenient.
When United States mail is sent through tubes not used exclusively for
postal service, carriers with special locks can be used, so that they
can be opened only by post-office employees.
=Air Supply.=—This completes the description of the special apparatus
used in this system, but we have yet to say something regarding the
machines that supply the air. In Paris the water from the city mains
has been used to compress or exhaust the air used in small tubes,
but to operate large tubes in most of our cities steam is the only
available power. Except in isolated cases, an independent steam plant
will be erected to supply the air for a system of tubes. This plant
should be designed with a view to obtaining the maximum economy in coal
consumption, labor, water, cartage, and incidental expenses. We might
say that the same general rules of economy which govern the design and
construction of electric-lighting plants should be applied to the plans
and construction of air-compressing plants.
Three types of blowing machines are used,—viz., centrifugal fans,
positive blowers, and air-compressors.
=Fans.=—Very large tubes of moderate length can be operated by ordinary
centrifugal fans. These fans are capable of supplying air under a
pressure not exceeding ten or twelve ounces per square inch with very
good efficiency. They are the simplest and most inexpensive of all
blowing-machines.
=Blowers.=—When tubes have a length and diameter that require a
pressure from one to four pounds per square inch, some form of positive
blower of the Root type can be used with economy. Their construction
is familiar to nearly every one at all interested in machinery, so we
need give no space to their description here.
[Illustration: FIG. 42.
THE STURTEVANT STEEL PRESSURE BLOWER.]
[Illustration: FIG. 43.
ROOT’S POSITIVE PRESSURE BLOWER.]
[Illustration: FIG. 44.
SECTION OF ROOT’S TRUE CIRCLE BLOWER.]
[Illustration: FIG. 45.
THE GREEN BLOWER.]
[Illustration: FIG. 46.
SECTION OF THE GREEN BLOWER.]
=Air-Compressors.=—By far the greater number of our tubes require an
air-pressure of more than five pounds per square inch. For such air
supply we recommend some form of air-compressor, and usually this is
driven by a steam-engine, which forms a part of the compressor. In
making our selection we should bear in mind the conditions under which
the compressor will run. Usually it must be kept in constant operation
at least ten hours per day, and frequently for a much longer period.
This makes it important that the compressor be substantially built
and supported upon a solid and firm foundation. The bearings should
be broad, of good wearing material that has a low coefficient of
friction, and provided at all times with ample lubrication. If poppet
valves are used in the air-cylinders, and they are most common, the
speed in revolutions per minute should not be high. Duplex are better
than single cylinder compressors, because they deliver the air in a
more steady stream,—the pulsations are less. For constant running,
economy of steam is an important item; therefore some good type of
cut-off valve should be provided. The air-cylinders should not be
water-jacketed unless the pressure is above twenty-five pounds per
square inch. It is better to use the air as warm as possible, for it
will soon be cooled after entering the tube. A speed-governor should
be provided with compressors which are to run at constant speed, but
usually they will be run to maintain a constant pressure in the tank,
and to this end a good and reliable form of pressure-governor should
be provided, together with some reliable safety device to stop the
engine when the speed exceeds a safe limit. But most important of all
is to have the valves of the air-cylinders large in area; otherwise
the efficiency of the machine will be very low. With machines working
under eighty pounds pressure, a difference in pressure of one pound
on opposite sides of the valves has but little effect, but when
the machine is only compressing to five or ten pounds, one pound
is a very large proportion of the total pressure and reduces the
efficiency. Besides these few suggestions, only the requirements of
good engineering need be demanded. In Figs. 42, 43, 44, 45, 46, and
47 we show a fan, two blowers, and an air-compressor suited to the
requirements of pneumatic-tube service that can be found in the market,
and that are built by responsible concerns. We believe they are all
good of their kind, but do not recommend any particular make.
=The Tube, Line Construction, etc.=—Up to the present time we have
found no material better suited for the straight parts of pneumatic
tubes than cast iron, machined upon the interior. It gives a smooth and
accurate tube. It can be made in most convenient lengths. It is strong
and not easily deformed. The bell-joint, calked with lead and oakum,
having the tubes fitted together male and female at the bottom of the
bell, is the best joint yet devised for pneumatic tubes. It is slightly
yielding, accommodating itself to slight changes of length of tube
due to changes of temperature, and it allows slight bends to be made
at each joint. The joints are very accurate, presenting no shoulders
to obstruct the passage of carriers. The joints can be made by men
accustomed to laying water- and gas-pipe. The cast iron is so stiff
that it is not distorted in calking, as may be done with wrought-iron
tube. The principal objections to its use are the expense of boring and
the readiness with which it corrodes upon the interior.
[Illustration: FIG. 47.
RAND COMPOUND COMPRESSOR OF MODERATE SIZE.]
We are always hoping that wrought-iron or steel tubes will be so much
improved in uniformity of dimensions and smoothness of interior that we
can use them, but our experiments thus far have been discouraging. It
may be that some of the new processes of making tubes will give us
what we want, but we have not yet found it.
Small tubes and the short bends of large tubes are made of brass, it
being the most suitable material. It would be very difficult to bend
iron tubes without involving great expense. The thickness of the bent
portion of an eight-inch tube is usually three-sixteenths of an inch
and never less than one-eighth of an inch.
Where the ground is firm, no other support is needed for the tubes
than to tamp the earth solidly about them. In order to economize space
in the streets, it is customary to lay the tubes one above the other;
and it is very convenient, although not necessary, to separate them by
cast-iron saddle brackets. Such an arrangement has to be frequently
departed from in order to overcome obstructions in the streets and to
get through narrow passages. At all low points in a tube line, traps
are provided to catch any moisture that may accumulate. These traps
are made accessible for frequent inspection by means of man-holes
or otherwise. The tube is usually laid about three feet below the
pavement. This distance has frequently to be varied, but it never
becomes so small as to render the tubes liable to injury from heavy
trucks passing over the pavement.