Compressed Air Brake
Before the introduction of the electrically operated braking system in the motor car there was some concern that one would lose the direct controlling influence. As far as trucks, with their compressed-air brakes are
concerned, this has been the case for quite some time now. The air-pressure developed by engine power can produce very high pressure through piston- or membrane cylinders. After decades a safety standard has
been achieved, whereby trucks no longer have a significantly higher brake damage than do motor cars.
Why then, is the compressed-air brake only found in mid-range- to heavy trucks? The reason is, that up to
twenty times more mass has to be braked than is the case with a motor car. This cannot be achieved by increasing the force on the brake pedal. The solution is that the total operating force must be produced by the
braking system, whereby, the compressed-air to the wheel-brakes is very sensitively regulated by the pedal-force from the driver. However, such an expensive and space consuming system is only possible in the truck-
and bus area.
The air pressure is generated, regulated and stored by the compressor (see figure), the air dryer, the pressure regulator and the
reservoir. The four-circuit protection valve is responsible for the safety in the event of a pressure drop. In contrast to the hydraulic brake, the pressure force is produced solely through compressed-air. The brake-pedal force from the driver presses
against a spring in the brake-valve and can be sensitively applied through an additional control system. The transmission of the pedal-force to the friction surface occurs through a membrane- or a piston cylinder.
For the hand-brake, strong springs are found in the rear brake cylinders. They can only be released after a certain amount of pressure has been reached. Thus, is driving without sufficient air-pressure not possible. On
the subject of the air-brakes, the following terms are important:
Single-circuit braking system:
All the brake-cylinders are supplied through one air pressure system.
Dual-circuit braking system:
The brake cylinders of the front axle are supplied by a different air pressure system than the cylinders of the rear axle. In the event of a breakdown of one system, the other remains
Single-conduit braking system:
In the case of the single conduit brake, the trailer is supplied and controlled, through a single conduit from the tractor unit. The more the pressure falls, the
stronger the braking in the trailer brake valve is. During braking, no air-pressure supply to the trailer is possible.
Dual-conduit braking system:
In the dual-conduit braking system, the trailer is supplied
from the tractor-unit through a red (reservoir) conduit and controlled through a yellow (brake) conduit. Also, fresh compressed-air flows to the trailer during a braking operation.
ApplicationPneumatic brake-systems are installed in mid-range-, heavy utility vehicles and in busses.