Part 11 (2/2)

[18] Only the ”drum” type of armature is treated here

[19] This refers to continuous-current dynamos only

[20] The Robertson

Chapter X

RAILWAY BRAKES

The Vacuuhouse air-brake

In the early days of the railway, the pulling up of a train necessitated the shutting off of steareat distance away The train gradually lost its velocity, the process being hastened to a coine and guard's van The goods train of to-day inobsolete in passenger traffic

An advance was th of the train, was arranged so as to pull on subsidiary chains branching off under each carriage and operating levers connected with brake blocks pressing on every pair of wheels The guard strained the ear in his van This system was, however, radically defective, since, if any one branch chain was shorter than the rest, it alone would get the strain Further of the ely, brakes operated by steae was placed a cylinder, in connection with a ineer wished to apply the brakes, he turned high-pressure stea into the brake cylinders, drove out in each a piston operating the brake gear

Unfortunately, the stea the pipe, was condensed, and in cold weather failed to reach the rear carriages Water formed in the pipes, and this was liable to freeze If the train parted accidentally, the apparatus of course broke down

Hydraulic brakes have been tried; but these are open to several objections; and railway engineers now make use of air-pressure as the most suitable form of power Whatever air system be adopted, experience has shown that three features are essential:--(1) The brakes must be kept ”off” artificially (2) In case of the train parting accidentally, the brakesall the vehicles of the train to a standstill (3) It reater or less force, according to the needs of the case

At the present day one or other of two systems is used on practically all automatically-braked cars and coaches These are known as--(1) The _vacuu the pressure of the atmosphere on a piston from the other side of which air has been house auto compressed air The action of these brakes will now be explained as simply as possible

THE VACUUM AUTOMATIC BRAKE

Under each carriage is a vacuu on trunnions, E E, so that ita little when the brakes are applied Inside the chaht by a rubber ring rolling between it and the cylinder walls The piston rod works through an air-tight stuffing-box in the botto, and when it rises operates the brake rods It is obvious that if air is exhausted from both sides of the piston at once, the piston will sink by reason of its oeight and that of its attachments If air is now admitted below the piston, the latter will be pushed upwards with a maximum pressure of 15 lbs to the square inch The ball-valve ensures that while air can be sucked from _both_ sides of the piston, it can be admitted to the lower side only

[Illustration: FIG 85--Vacuum brake ”off”]

[Illustration: FIG 86--Vacuuine that a train has been standing in a siding, and that air has gradually filled the vacuuine is coupled on, and the driver at once turns on the steam ejector,[21]

which sucks all the air out of the pipes and chahout the train The air is sucked directly froh pipe D; and froh the channel C, lifting the ball, which, as soon as exhaustion is complete, or when the pressure on both sides of the piston is equal, falls back on its seat On air being adh D and into the space B (Fig 86) below the piston, but is unable to pass the ball, so that a strong upward pressure is exerted on the piston, and the brakes go on To throw them off, the space below the piston must be exhausted This is to be noted: If there is a leak, as in the case of the train parting, _the brakes go on at once_, since the vacuum below the piston is automatically broken

[Illustration: FIG 87--Guard's valve for applying the Vacuum brake]

For ordinary stops the vacuum is only partially broken--that is, an air-pressure of but froency stops full atmospheric pressure is used In this case it is advisable that air should enter at _both_ ends of the train; so in the guard's van there is installed an ingenious autouard pressing down a lever, but which opens of itself when the train-pipe vacuu 87 shows this device in section Seated on the top of an upright pipe is a valve, _A_, connected by a bolt, B, to an elastic diaphrag the bottom of the chah it from end to end When the vacuum is broken slowly, the pressure falls in D as fast as in the pipe; but a sudden inrush of air causes the valve A to be pulled off its seat by the diaphragm C, as the vacuum in D has not been broken to any appreciable extent Air then rushes into the train pipe through the valve It is thus evident that the driver controls this valve as effectively as if it were on the engine These ”eency” valves are sometie is slipped, taps on each side of the coupling joint of the train pipe are turned off by the guard in the ”slip;” and when he wishes to stop hethe valve Under the van is an auxiliary vacuum chamber, frouard, after the slip has parted from the train, finds that he has applied his brakes too hard, he can put this chamber into communication with the brake cylinder, and restore the vacuuain

When a train has come to rest, the brakes must be sucked off by the ejector Until this has been done the train cannot be moved, so that it is impossible for it to leave the station unprepared to make a sudden stop if necessary