Air Brakes

(Air brake assembly)


Heavy trucks use air brakes exclusively. Most of these are drum type units like the one shown above. Air enters the chamber when the brakes are applied, the push rod moves out turning the slack adjuster which rotates the "S" cam and forces the shoes into the drum.


Brakes are a heat engine, they convert kinetic energy (motion) into thermal energy or heat. It interesting to figure out just how powerful your brakes must be to do the job of stopping your rig. Suppose you've got a 400 horsepower engine and are about grossed out at 80,000 lbs. your engine can probably get you to about 40 mph in 1/4 mile, that's 1320 feet. Your brakes should be able to stop you in about 133 feet with this speed and weight, making them 10 times as powerful as your engine!

A full stop from 60 mph might raise your drum temperatures to 600 degrees F. This is about the limit for safe operation. If the brakes aren't right, or the load is not distributed properly, then some drums might go to 800 - 1000 degrees F. This is definitely dangerous. The drum will increase in diameter about 0.01" per 100 degree temperature rise. So that even at 600 degrees your drums will be 0.055" larger than at 50 degrees. This will increase the pushrod stroke about 0.40", call it half an inch. The question is: Do you have that much additional stroke available? See the pages below for more information.

More Air Brake Topics:

Brake fade

Adjusting brakes

Brakes and jackknifing

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