How it Works Part 12

THE WESTINGHOUSE AIR-BRAKE.

This system is somewhat more complicated than the vacuum, though equally reliable and powerful. Owing to the complexity of certain parts, such as the steam air-pump and the triple-valve, it is impossible to explain the system in detail; we therefore have recourse to simple diagrammatic sketches, which will help to make clear the general principles employed.

The air-brake, as first evolved by Mr. George Westinghouse, was a very simple affair--an air-pump and reservoir on the engine; a long pipe running along the train; and a cylinder under every vehicle to work the brakes. To stop the train, the high-pressure air collected in the reservoir was turned into the train pipe to force out the pistons in the coach cylinders, connected to it by short branch pipes. One defect of this "straight" system was that the brakes at the rear of a long train did not come into action until a considerable time after the driver turned on the air; and since, when danger is imminent, a very few seconds are of great importance, this slowness of operation was a serious fault. Also, it was found that the brakes on coaches near the engine went on long before those more distant, so that during a quick stop there was a danger of the forward coaches being b.u.mped by those behind. It goes without saying that any coaches which might break loose were uncontrollable. Mr. Westinghouse therefore patented his _automatic_ brake, now so largely used all over the world. The brake ensures practically instantaneous and simultaneous action on all the vehicles of _a train of any length_.

[Ill.u.s.tration: FIG. 88.--Diagrammatic sketch of the details of the Westinghouse air-brake. Brake "off."]

The principle of the brake will be gathered from Figs. 88 and 89. P is a steam-driven air-pump on the engine, which compresses air into a reservoir, A, situated below the engine or tender, and maintains a pressure of from 80 to 90 lbs. per square inch. A three-way c.o.c.k, C, puts the train pipe into communication with A or the open air at the wish of the driver. Under each coach is a triple-valve, T, an auxiliary reservoir, B, and a brake cylinder, D. The triple-valve is the most noteworthy feature of the whole system. The reader must remember that the valve shown in the section is _only diagrammatic_.

Now for the operation of the brake. When the engine is coupled to the train, the compressed air in the main reservoir is turned into the train pipe, from which it pa.s.ses through the triple-valve into the auxiliary reservoir, and fills it till it has a pressure of, say, 80 lbs. per square inch. Until the brakes are required, the pressure in the train pipe must be maintained. If accidentally, or purposely (by turning the c.o.c.k C to the position shown in Fig. 89), the train-pipe pressure is reduced, the triple-valve at once s.h.i.+fts, putting B in connection with the brake cylinder D, and cutting off the connection between D and the air, and the brakes go on. To get them off, the pressure in the train pipe must be made equal to that in B, when the valve will a.s.sume its original position, allowing the air in D to escape.

The force with which the brake is applied depends upon the reduction of pressure in the train pipe. A slight reduction would admit air very slowly from B to D, whereas a full escape from the train pipe would open the valve to its utmost. We have not represented the means whereby the valve is rendered sensitive to these changes, for the reason given above.

[Ill.u.s.tration: FIG. 89.--Brake "on."]

The latest form of triple-valve includes a device which, when air is rapidly discharged from the train pipe, as in an emergency application of the brake, opens a port through which compressed air is also admitted from the train pipe _directly_ into D. It will easily be understood that a double advantage is hereby gained--first, in utilizing a considerable portion of the air in the train pipe to increase the available brake force in cases of emergency; and, secondly, in producing a quick reduction of pressure in the whole length of the pipe, which accelerates the action of the brakes with extraordinary rapidity.

It may be added that this secondary communication is kept open only until the pressure in D is equal to that in the train pipe. Then it is cut off, to prevent a return of air from B to the pipe.

An interesting detail of the system is the automatic regulation of air-pressure in the main reservoir by the air-pump governor (Fig. 90).

The governor is attached to the steam-pipe leading from the locomotive boiler to the air-pump. Steam from the boiler, entering at F, flows through valve 14 and pa.s.ses by D into the pump, which is thus brought into operation, and continues to work until the pressure in the main reservoir, acting on the under side of the diaphragm 9, exceeds the tension to which the regulating spring 7 is set. Any excess of pressure forces the diaphragm upwards, lifting valve 11, and allowing compressed air from the main reservoir to flow into the chamber C. The air-pressure forces piston 12 downwards and closes steam-valve 14, thus cutting off the supply of steam to the pump. As soon as the pressure in the reservoir is reduced (by leakage or use) below the normal, spring 7 returns diaphragm 9 to the position shown in Fig. 90, and pin-valve 11 closes. The compressed air previously admitted to the chamber C escapes through the small port _a_ to the atmosphere. The steam, acting on the lower surface of valve 14, lifts it and its piston to the position shown, and again flows to the pump, which works until the required air-pressure is again obtained in the reservoir.

[Ill.u.s.tration: FIG. 90.--Air-pump of Westinghouse brake.]

[21] This resembles the upper part of the rudimentary water injector shown in Fig. 15. The reader need only imagine pipe B to be connected with the train pipe. A rush of steam through pipe A creates a partial vacuum in the cone E, causing air from the train pipe to rush into it and be expelled by the steam blast.

Chapter XI.

RAILWAY SIGNALLING.

The block system--Position of signals--Interlocking the signals--Locking gear--Points--Points and signals in combination--Working the block system--Series of signalling operations--Single line signals--The train staff--Train staff and ticket--Electric train staff system--Interlocking--Signalling operations--Power signalling--Pneumatic signalling--Automatic signalling.

Under certain conditions--namely, at sharp curves or in darkness--the most powerful brakes might not avail to prevent a train running into the rear of another, if trains were allowed to follow each other closely over the line. It is therefore necessary to introduce an effective system of keeping trains running in the same direction a sufficient distance apart, and this is done by giving visible and easily understood orders to the driver while a train is in motion.

In the early days of the railway it was customary to allow a time interval between the pa.s.sings of trains, a train not being permitted to leave a station until at least five minutes after the start of a preceding train. This method did not, of course, prevent collisions, as the first train sometimes broke down soon after leaving the station; and in the absence of effective brakes, its successor ran into it. The advent of the electric telegraph, which put stations in rapid communication with one another, proved of the utmost value to the safe working of railways.

THE BLOCK SYSTEM.

Time limits were abolished and distance limits subst.i.tuted. A line was divided into _blocks_, or lengths, and two trains going in the same direction were never allowed on any one block at the same time.

The signal-posts carrying the movable arms, or semaph.o.r.es, by means of which the signalman communicates with the engine-driver, are well known to us. They are usually placed on the left-hand side of the line of rails to which they apply, with their arms pointing away from the rails.

The side of the arms which faces the direction from which a train approaches has a white stripe painted on a red background, the other side has a black stripe on a white background.

The distant and other signal arms vary slightly in shape (Fig. 91). A distant signal has a forked end and a V-shaped stripe; the home and starting signals are square-ended, with straight stripes. When the arm stands horizontally, the signal is "on," or at "danger"; when dropped, it is "off," and indicates "All right; proceed." At the end nearest the post it carries a spectacle frame glazed with panes of red and green gla.s.s. When the arm is at danger, the red pane is opposite a lamp attached to the signal post; when the arm drops, the green pane rises to that position--so that a driver is kept as fully informed at night as during the day, provided the lamp remains alight.

[Ill.u.s.tration: FIG. 91.--Distant and home signals.]

POSITION OF SIGNALS.

On double lines each set of rails has its own separate signals, and drivers travelling on the "up" line take no notice of signals meant for the "down" line. Each signal-box usually controls three signals on each set of rails--the distant, the home, and the starting. Their respective positions will be gathered from Fig. 92, which shows a station on a double line. Between the distant and the home an interval is allowed of 800 yards on the level, 1,000 yards on a falling gradient, and 600 yards on a rising gradient. The home stands near the approach end of the station, and the starting at the departure end of the platform. The last is sometimes reinforced by an "advance starting" signal some distance farther on.

It should be noted that the distant is only a _caution_ signal, whereas both home and starting are _stop_ signals. This means that when the driver sees the distant "on," he does not stop his train, but slackens speed, and prepares to stop at the home signal. He must, however, on no account pa.s.s either home or starting if they are at danger. In short, the distant merely warns the driver of what he may expect at the home.

To prevent damage if a driver should overrun the home, it has been laid down that no train shall be allowed to pa.s.s the starting signal of one box unless the line is clear to a point at least a quarter of a mile beyond the home of the next box. That point is called the _standard clearing point_.

Technically described, a _block_ is a length of line between the last stop signal worked from one signal-box and the first stop signal worked from the next signal-box in advance.

[Ill.u.s.tration: FIG. 92.--Showing position of signals. Those at the top are "off."]

INTERLOCKING SIGNALS.

A signalman cannot lower or restore his signals to their normal positions in any order he likes. He is compelled to lower them as follows:--Starting and home; _then_ distant. And restore them--distant; _then_ starting and home. If a signalman were quite independent, he might, after the pa.s.sage of a train, restore the home or starting, but forget all about the distant, so that the next train, which he wants to stop, would dash past the distant without warning and have to pull up suddenly when the home came in sight. But by a mechanical arrangement he is prevented from restoring the home or starting until the distant is at danger; and, _vice versa_, he cannot lower the last until the other two are off. This mechanism is called _locking gear_.

LOOKING GEAR.

There are many different types of locking gear in use. It is impossible to describe them all, or even to give particulars of an elaborate locking-frame of any one type. But if we confine ourselves to the simplest combination of a stud-locking apparatus, such as is used in small boxes on the Great Western Railway, the reader will get an insight into the general principles of these safety devices, as the same principles underlie them all.

[Ill.u.s.tration: FIG. 93.--A signal lever and its connections. To move the lever, C is pressed towards B raising the catch-rod from its nick in the rack, G G G, guides; R R, anti-friction rollers; S, sockets for catch-rod to work in.]

The levers in the particular type of locking gear which we are considering have each a tailpiece or "tappet arm" attached to it, which moves backwards and forwards with the lever (Fig. 93). Running at right angles to this tappet, and close to it, either under or above, are the lock bars, or stud bars. Refer now to Fig. 94, which shows the ends of the three tappet arms, D, H, and S, crossed by a bar, B, from which project these studs. The levers are all forward and the signals all "on." If the signalman tried to pull the lever attached to D down the page, as it were, he would fail to move it on account of the stud _a_, which engages with a notch in D. Before this stud can be got free of the notch the tappets H and S must be pulled over, so as to bring their notches in line with studs _b_ and _c_ (Fig. 95). The signalman can now move D, since the notch easily pushes the stud _a_ to the left (Fig.

96). The signals must be restored to danger. As H and S are back-locked by D--that is, prevented by D from being put back into their normal positions--D must be moved first. The interlocking of the three signals described is merely repeated in the interlocking of a large number of signals.

[Ill.u.s.tration: FIG. 94.]

[Ill.u.s.tration: FIG. 95.]

On entering a signal-box a visitor will notice that the levers have different colours:--_Green_, signifying distant signals; _red_, signifying home and starting signals; _blue_, signifying facing points; _black_, signifying trailing points; _white_, signifying spare levers.

These different colours help the signalman to pick out the right levers easily.

To the front of each lever is attached a small bra.s.s tablet bearing certain numbers; one in large figures on the top, then a line, and other numbers in small figures beneath. The large number is that of the lever itself; the others, called _leads_, refer to levers which must be pulled before that particular lever can be released.

[Ill.u.s.tration: FIG. 96.]

[Ill.u.s.tration: FIG. 97.--Model signal equipment in a signalling school.

(By permission of the "G.W.R. Magazine").]

POINTS.

Mention was made, in connection with the lever, of _points_. Before going further we will glance at the action of these devices for enabling a train to run from one set of rails to another. Figs. 98 and 99 show the points at a simple junction. It will be noticed that the rails of the line to the left of the points are continued as the outer rails of the main and branch lines. The inner rails come to a sharp V-point, and to the left of this are the two short rails which, by means of s.h.i.+fting portions, decide the direction of a train's travel. In Fig. 98 the main line is open; in Fig. 99, the branch. The s.h.i.+fting parts are kept properly s.p.a.ced by cross bars (or tie-rods), A A.