Chapter III (2)

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_.

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.

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.

THE STURTEVANT STEEL PRESSURE BLOWER.]

ROOT’S POSITIVE PRESSURE BLOWER.]

SECTION OF ROOT’S TRUE CIRCLE BLOWER.]

THE GREEN BLOWER.]

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.

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.