Brakes. Any contrivance for controlling by friction the speed of carriages, wagons, trains, or revolving drums is called a brake. In its simplest form, as applied to road vehicles, wooden blocks are pressed by the driver through the medium of connecting-rods or levers against the tires of the wheels; the friction thus set up checks the speed, enabling the vehicle to go steadily down inclines, or to be rapidly pulled up.
Brakes, however, find their most important application on trains. Soon after the introduction of railways, improvements in the locomotive compelled engineers to devise means for controlling its speed. Stephenson arranged a brake, in which steam admitted into a small cylinder, pushed out its piston, and so through the action of levers pressed the blocks against the tires. It was a failure partly because it was applied only to wheels on one side of the engine, thus tending to cause it to leave the rails. Since then much labour has been spent on this question, chiefly with the object of making brakes continuous and automatic. The conditions a train-brake must fulfil are these: It must be continuous, automatic—i.e. in case of accident, such as the breaking apart of the train, it must instantaneously put itself on—be equally readily put on or off any vehicle, and be simple in construction and durable. Moreover, it should allow of vehicles being slipped or detached without putting it on, should be tell-tale to indicate if it is in working order, be constantly in use, and lastly, very powerful, capable of stopping a heavy train at a high speed in a few hundred yards.
The chief types now used are chain, vacuum, and pressure brakes; but the first great advance was in the application of the screw to regulate the pressure of the brake-blocks on the tires, a plan still used in hand-brakes, where compressed springs give great power; the blocks are now cast-iron. The chain-brake is simply a continuous chain extending under several vehicles, wound up by a drum on the brake-van so as to be in tension; this keeps the blocks off: but if the guard slacks the chain, or it breaks through the train parting, compressed springs come into play, forcing the blocks against the tires; it is thus partly automatic, and has been much used. Vacuum-brakes work on the principle of keeping up a vacuum in a continuous pipe or pipes extending under the train, and in brake-cylinders connected to them under each vehicle, the air being sucked out by ejectors or pumps on the locomotive. Each cylinder has a piston working in it which is, with a vacuum on both sides, in such a position that the blocks are off; but if air is let into pipes the vacuum on one side is destroyed and the piston moves over, putting on the brakes by the help of suitable levers. In other forms the cylinder is a flexible sack which is collapsed by the atmosphere, and puts on the brakes directly a vacuum in the pipes is produced.
The best known pressure-brake is the invention of Mr Westinghouse of Pittsburg, U.S.A., whose first brakes were non-automatic, but were superseded by his automatic brake. In the latter, compressed air, at from 80 to 100 lb. per square inch, is stored in a reservoir under each vehicle and in a pipe connecting them, also under each vehicle is a brake-cylinder with pistons, the rods of which operate the brake-levers, and lastly, a connecting valve called a 'triple valve.' As long as pressure is maintained in the pipe the brake is off; but if air escapes intentionally or otherwise from the pipe, the brakes are put on. The triple valve is an ingenious piece of mechanism. A piston with a sliding valve attached to its top works in a small case, with connections to the pipe, cylinder, and reservoir. If there is pressure in the pipe it acts on the under side of piston, keeping it with its valve in such a position that the brake-cylinder is in communication with the air (and so the brakes are off), while compressed air can feed to the reservoir; but if the pressure in the pipe falls, the excess pressure in the reservoir acts on the top of the piston, forces it and the valve down, shuts passage to the air, and allows compressed air to rush from the reservoir to the cylinder and put on the brakes. A little auxiliary valve in the slide valve gives great control over the pressure, and accidental leaks are provided against by grooves opening to the air which shut when the brakes are put into action. Generally each cylinder has two pistons kept close together by springs; the compressed air enters between them, forcing them apart, and so working the brake-levers. The hose couplings for pipes between the carriages are automatic—i.e. a valve opens in them when the couplings are made, and closes if they are disconnected (except when forcibly torn apart as in accidents). Captain Galton made a valuable set of experiments on the friction between blocks and tires on the Brighton line, proving conclusively amongst other things that the pressure should not be so great as to cause the wheels to skid. His results were communicated to the Institution of Mechanical Engineers. An elaborate series of tests of rival brakes was made at Newark in 1875, the general result proving the Westinghouse to be the most powerful and quickest in application—for instance, it stopped a train weighing 203 tons, running 56 miles an hour, in 1020 feet.
For revolving drums, the brake is usually some form of strap which can be tightened strongly against the drum, causing great friction, and so pulling up the drum.
Latterly the word brake has been used for any form of engine dynamometer which measures power given out by an engine by absorbing it in over-coming friction. The usual method is to hang a weight from a strap or series of blocks encircling a pulley or flywheel of the engine, then to tighten up these straps till the friction produced is great
Diagram of Westinghouse Automatic Brake, showing arrangement for one carriage:
A, reservoir; B, triple valve; C, slide valve; D, exhaust; E, brake-cylinder; F, release valve; G, spring; H, pistons; I, I, communication to brake-lever; K, main pipe extending all along the train; L, branch pipe; M, cock. enough to cause the flywheel to tend to carry the weights round, or, as it is said, to keep them 'floating.' The weights so floated measure the brake horse-power of the engine.