How do air brakes work?

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Viper007Bond

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Dark_Templer_102 wanted to know in another thread:

Also how does "air brakes" work...like the ones on 18-wheelers
AFAIK, when no air pressure is applied, the brakes either are on or off (depending on the type of brake) due to a strong spring. Then, when air pressure is applied, the brake goes to the opposite position.

Trailers with air brakes are set to be turned on (braking) wehn there's no air pressure so that they stay stationary when not attached to the truck and so that they will apply the brakes if air pressure is lost.

I believe the main brakes stay off on semi-trucks except for the parking brake, but I'm really not sure about that.
 

Jostyrostelli

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This is an aribrake, big trucks have them:


It is a big drum brake,
You can see the air chamber, this is the compartment where the air is stored (duh?!) When you brake, the air is "compressed" and pushes the rod (blue) out, which moves the green slack adjuster, the slack adjuster is connected to the rotating cam. This one forces the 'shoe' onto the drumb.

The reason why they mostly are aplied on big heavy trucks:
Trucks are heavy, when a truck cruises at 50mph the engine of that truck may have had to work for miles to get to that speed, the brakes have to do the opposite thing, make the car stop as fast as possible..
If your car accelerates from 0-60 over 1300ft the brakes have to stop the car in about 130feet. So the brakes are about 10 times as powerful as your engine! And therefore it creates LOADS of heat. Some brakes may even rise in temperature of 300 degrees Celcius (600 degrees Fahrenheit). IF the brakes are worn out, it will even be higher!

When things heat up they expand, so do brakes. Don't know the exact numbers how much it mm it expands, but that's whats causing the Brake-fading, when expanding, the drum moves away from the 'shoe' and you will not be able to get the same braking power as before.
 

MXM

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I saw some trucks racing, and because of amount of heat brakes generate in tight corner, they actually spray water (or something) on them. Huge white clouds mixing with dark diesel exhaust on acceleration :)
 
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