edit: Solved. Thanks guys
In: Engineering
Air brakes are common on larger vehicles for various reasons, but mainly because the air is constantly recycled, bleeding off system heat as they go, unlike hydraulic systems which just use the same fluid which can get hot and even boil to failure.
Also, the way that airbrakes work is to “fail on” rather than “fail off” like hydraulic brakes do. Here’s what that means.
In a hydraulic system, system failure means you got no brakes. That’s bad.
Airbrakes, however, have a failsafe built in called a spring brake, which, upon system failure, applies the brakes, hard. There’s a very stiff spring that pushes the brakes on, until air pressure pushes against the spring and holds it open. If you lose air pressure, the brakes turn on, not off. The vehicle stops, hard, upon system failure. I think the benefits of that are obvious.
The compressor that provides the compressed air for the system is engine driven, so whenever the engine is running, it’s making compressed air.
What this means is that the PSSHT sound comes from three sources:
1.) The operator releases the air from the system to set the spring brake, which also functions as the parking brake of the vehicle. This will happen whenever the operator pulls a yellow, diamond shaped button on the dash out. It sets the spring/parking brakes and makes the vehicle immobile. Buses do this for safety whenever they stop at a bus stop to ensure the vehicle doesn’t move as people get on and off. PSSHT
2.) If the operator applies the brake pedal at all, when they let off the pedal, the air that was fed into the system to apply the brakes is vented so that the brakes unapply. So, anytime you hit the brake pedal and then lift off, PSSHT.
3.) Because the compressor is engine driven, it is constantly making compressed air. Eventually, it makes more than the system needs, and a pressure actuated relief valve will blow some un-needed air out of the system to maintain the proper pressures and prevent overpressure. PSHHT.
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