The simplest way of applying a brake is by muscular force, exerted through a hand or foot lever or through a screw, by which the brake block is pressed against the rim of the wheel or the band brake tightened on its drum. This method is sufficient in the case of most road vehicles, and is largely used on railway vehicles. But the power thus available is limited, and becomes inadequate for heavy vehicles moving at high speeds. Moreover, on a train consisting of a number of vehicles, the hand brakes on each of which are independent of all others, either a brakesman must be carried on each, or a number of the brakes must be left unused, with consequent loss of stopping power; while even if there is a brakesman on every vehicle it is impossible to secure that all the brakes throughout the train are applied with the promptness that is necessary in case of emergency.
Considerations of this sort led to the development of power brakes for railway trains. Of these there are five main classes:--
(1) Mechanical brakes, worked by springs, friction wheels on the axle, chains wound on drums, or other mechanical devices, or by the force produced when, by reason of a sudden checking of the speed of the locomotive, the momentum of the cars causes pressure on the draw-bars or buffing devices. (2) Hydraulic brakes, worked by means of water forced through pipes into proper mechanism for transmitting its force to the brake-shoes. (3) Electric brakes. (4) Air and vacuum brakes, worked by compressed air or by air at atmospheric pressure operating on a vacuum. (5) Brakes worked by steam or water from the boiler of the engine, operating by means of a cylinder; the use of these is generally limited to the locomotive. Of this kind is the counter-pressure or water brake of L. le Chatelier. If the valve gear of a locomotive in motion be reversed and the steam regulator be left open, the cylinders act as compressors, pumping air from the exhaust pipe into the boiler against the steam pressure. A retarding effect is thus exercised, but at the cost of certain inconveniences due to the passage of hot air and cinders from the smoke box through the cylinders. To remedy these, le Chatelier arranged that a jet of hot water from the boiler should be delivered into the exhaust pipe, so that steam and not the hot flue gases should be pumped back.