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
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
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.