121 Introduction to air powered brakes

As the size and weight of road vehicles increase there comes a time when not only are manual brakes inadequate, but there is no point in having power assistance because the amount of braking contributed by the driver's foot is insignificant relative to the principal source of power, be it vacuum or hydraulic energy, and therefore power operated brakes become essential. A further consideration is that the majority of heavy commercial or public service vehicles are propelled by diesel engines which do not have a natural source of vacuum and therefore require a vacuum pump (exhauster) driven from the engine to supply the vacuum energy. However, if a separate pump has to be incorporated to provide the necessary power transmitting media, a third energy source with definite advantages and few disadvantages can be used; that is compressed air.

Reciprocating compressors driven off the engine can operate efficiently and trouble-free at pressures in the region of 7-8 bar, whereas vacuum assisted brakes can only work at the most with depressions of 0.9 bar below atmospheric pressure. Consequently compressed air has a power factor advantage of between 7 and 8 times that for an equivalent vacuum source when used as a force transmitting media.

Conversely, hydraulic pumps are compelled to work at pressures of between 50 and 60 bar. The pressures generated in the pipe lines may reach values of 100 bar or even more. Consequently, because of these high pressures, small diameter servo cylinders and small bore pipes are utilized. This may appear to merit the use of hydraulic energy but, because of the very high working pressure in a hydraulic operated brake system, much more care has to be taken to avoid fluid leakage caused by wear or damage. Compressed air as a power transmitting media would operate at pressures of only one-tenth of an equivalent hydraulic source, but for large vehicles where there is more space, there is no real problem as much larger diameter cylinders can be used. In addition, if there is a leakage fault in a hydraulic layout it will eventually drain the supply fluid so that the brakes cannot continue to function, whereas small leakages of air in an air power operated braking system will not prevent the brakes operating even if this does take place at slightly reduced stopping efficiency.

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