While waiting for the departure time of the bus at the resting facility, that sudden hissing sound as we dive into the distance can sometimes be frightening. Let’s find out what is the reason for this frequently heard disturbing sound:
The reason for this situation seen in various heavy vehicles is the air brake system. These vehicles, which are frequently used both in heavy industry and in road transportation, are generally over 10 tons in weight. It is not possible for buses and trucks that are that heavy to stop with the normal braking system of trucks.
When heavy vehicles need to slow down and stop on the road, the driver’s force should be at a minimum.
If it was necessary to stop a bus that can stop with a compressed air brake system with normal brakes, it would be necessary to apply 30-40 times more pressure to the brake. It is impossible to transmit the force applied to the brake pedal to the tires of heavy vehicles. Therefore, the air goes up to the vehicle wheel and creates mechanical movement with pneumatic power before it acts on the drum or brake disc.
Buses are equipped with sensitive air bellows suspensions (shock absorbers) for comfort.
The air accumulated here must be evacuated after a certain pressure level in order to maintain the pressure value. During this process, that famous hissing sound is produced.
This system, which is the cause of the hissing sound; It is a system consisting of compressed air supply unit, continuous brake system, foot brake system, hand brake unit and compressor, brake pedal valve, relief valve, distributor valve, hand brake lever, trailer control valve, relay valve, bellows and emergency bellows, load adjustment. consists of apparatus such as valves.
With the developing technology, the air produced once inside the vehicle has started to be used in other mechanisms besides the brakes.
Thanks to this system, heavy servo conditions in clutch systems have been alleviated with air servo valves. Thus, a pneumatic part was added next to the hydraulic part of the clutch and the air clutch system emerged.
The air produced in the vehicle is also used in suspension systems. Thus, “air suspension systems” have replaced mechanical spring suspension systems. The basic logic of this system is to provide suspension by placing air-filled balloons between the chassis and the load. It should not be confused with the air-charged shock absorber system used in passenger cars.
High-end systems such as ABS, EBS, TEBS are now used in heavy vehicles.
Thanks to the sensors that note pressure changes, all apparatuses mentioned above transmit error reports to the driver display. In addition, with the help of brake diagnostic tools, all malfunctions in the brake system can be revealed with a single tool.
Sources: 1, 2
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