Re: How Do Air Brakes Work?

John;

You asked how air brakes work.  Air brakes are used on heavy trucks, 
school busses, railroad trains, among others.  I assume you are interested 
in the more common applications, such as heavy trucks and school busses.  
Here, air brakes are used because muscular energy is not enough to safely 
and conveniently operate the brakes.  

There is a practical limit to how much stroke our legs can push a brake 
pedal.  The force our legs can exert is also limited.  The product of 
pedal force and stroke provides a measure of how much work the driver's 
leg can do.  Heavy vehicles require more work to brake them than lighter 
vehicles.  Brakes can be designed to require less work to operate them, 
but again, there is a practical limit. 

Human limitations can be by-passed with air brakes.  An air compressor is 
operated by the engine.  The compressed air is used to do the work of 
brake application.  The driver merely operates a valve that controls air 
pressure to the brakes.  This pressure is determined only by the pedal 
stroke.

Available air pressure is much lower than hydraulic pressure, so air 
brakes have large actuators that are called air chambers.  These come in 
different sizes to suit the brake and the axle loading.  A lightly loaded 
steering axle might use a small chamber, while a heavily loaded drive axle 
could use a much larger air chamber.  

Just as a human has a limiting leg stroke, an air chamber also has a 
limiting stoke.  For this reason, air brakes have to use the proper stroke 
air chamber for a particular brake.  The brake has to be adjusted as it 
wears, or the air chamber may not be able to produce enough actuation 
force.  Modern trucks and busses with air brakes use automatic slack 
adjusters.  Slack adjusters, as their name suggests, are used to adjust 
the amount of slack, or free play, in the brake.  This adjustment is 
critical in air brakes.  With too little slack, the brake may drag and 
overheat.  Too much slack and the brake may not generate enough braking 
effort to safely stop the vehicle. 

Air brake vehicles require parking brakes, and emergency braking--should 
the air system fail.  This is done with springs.  A spring chamber applies 
the brake by the spring force.  It is released by system air pressure.  A 
driver can apply the parking brake by a lever, similar to a conventional 
parking brake lever that releases the air pressure to the spring chamber.  
Should the air supply fail (for example, if an air line ruptures) the 
spring brakes provide emergency braking.

Modern trucks and busses have different types of air-operated brakes. 
There are air disc brakes and air drum brakes.  Air drum brakes can be 
operated by s-cams or wedges.  S-cam drum brakes are the most popular air 
brakes in use today.  With these, the rotation of the s-cam pushed rollers 
that are located at the ends of the brake shoes.  As the cam rotates, the 
brake shoes are pushed against the brake drum.  A very popular size of s-
cam drum air brake is 16.5 X 7.  This brake uses a drum that is 16.5 
inches in inside diameter.  The brake linings, called brake blocks are 7 
inches wide.  The brake drums are made of cast iron, and weigh over 100 
pounds each.

You probably have heard loud air release sounds coming from the brakes of 
heavy trucks and busses.  This sound comes from the release of the 
brakes.  When air brakes are applied, there is some air sound, but it is 
not easily heard.  If you have a chance, take a good look at a vehicle 
with air brakes.  Look for double air chambers on the driving axles.  One 
of these is used to operate the service brakes.  The second is the spring 
chamber, used for parking and emergency braking.  Note that the air 
chambers may be in front of, or behind the axle.  Also, note that the 
front brakes use much smaller air chambers.  This is because the front 
axles typically are more lightly loaded than the rear axles.

The following URL's provide more information on air brakes, and show what 
air brake systems look like.  
 http://www.ganet.org/dps/gcdl/5-1.htm
 http://www.e-z.net/~ts/ts/Brakpg.htm
 http://www.edccorp.com/products/brakedesign.html