While waiting for the bus departure time at the rest facility, the one that suddenly appeared as we ducked into the distance. hissing sound it can be scary sometimes. Let’s take a look at the reason for this oft-heard disturbing noise:

The reason for this situation seen in various heavy vehicles: It’s an air brake system. Frequently used in both heavy industry and road transport, these vehicles usually weigh more than 10 tons. It is not possible for buses and trucks so heavy to stop with normal truck braking system.

When heavy vehicles have to slow down and stop on the road, the driver’s effort must be minimal.

If it was necessary to stop a bus that can stop with a compressed air braking system with a normal brake, 30-40 times more had to exert pressure. It is impossible to transfer the force exerted on the brake pedal to the tires of heavy vehicles. Therefore, the air goes to the vehicle wheel and creates mechanical movement with pneumatic force before acting on the drum or brake disc.

Buses are equipped with sensitive air bellows suspensions (shock absorbers) for comfort.

In order to maintain the pressure value of the air accumulated here after a certain pressure level, must be evacuated. 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, distribution valve, hand brake lever, trailer control valve, relay valve, bellows and emergency bellows, load adjustment consists of devices such as valves.

With evolving technology, the air produced once inside the vehicle is used in mechanisms other than the brakes.

Thanks to this system, severe servo conditions in clutch systems are alleviated with air servo valves. For example, in addition to the hydraulic part of the coupling, a pneumatic part was added and the air coupling system was created.

The air produced in the vehicle is also used in suspension systems. Similar “air suspension systems” It has replaced mechanical scissor-type 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-filled shock absorber system used in passenger cars.

ABS, EBS, TEBS High-end systems such as these are now used in heavy-duty vehicles.

Notice pressure changes thanks to sensors All the devices listed above send error reports to the driver display. In addition, with the help of brake diagnosis tools, all faults in the brake system can be traced with one tool.

Sources: 1, 2