Brake

Brake, a contrivance for controlling or diminishing the speed of a carriage, train, revolving cylinder, etc., by means of friction. Ordinarily we have a block of iron or hard wood pressing against the wheel tyres with force more or less regulated. The magnitude of the friction produced is very nearly proportional to the pressure applied, and levers are generally adopted to increase the applied force sufficiently. This force may be produced by hand, by atmospheric pressure as in vacuum-brakes, or by steam pressure as in the Westinghouse-brake. The chief applications of the brake are on trains, whose motion requires most careful control. A train brake must be automatic, or self-acting, i.e. if the train or part of it suddenly tends to increase its speed unduly, the necessary check should be applied mechanically, without requiring a man to apply it. Also it should be continuous, durable, simple in construction, and powerful. In the chain-brake the brake-blocks are kept apart from the carriage-wheels by a long continuous chain kept stretched by means of a drum on the brake van. If the chain is slackened by breaking, or by turning the drum, compressed springs force the blocks against the wheel tyres and the brake is in action. In the vacuum-brake, a continuous pipe extends along the length of the train. By means of an air-pump on the locomotive a vacuum is maintained in this pipe and in a series of brake-cylinders connected with each carriage. Each brake-cylinder contains a piston which, with vacuum-pressure on each side, will not move. When air is let in on one side by fracture of the pipe, or by giving it convenient entry, the piston moves and actuates the brake-blocks. The Westinghouse-brake, which is the best example of the pressure-brake type, is noticed separately.