Brake Chambers Explained
air brake chambers are what convert the air pressure force into
mechanical push rod force which engages the brake shoes of the
foundation brake system. This topic will not address rotary brake
chambers because they are not used on highway rigs. Anytime that
you work with air brake chambers you had better show some respect.
These push rods can generate thousands of pounds of force. If
one of your body parts gets in the way, .... to bad, .... you
Brake Air Chamber
the right is a typical front brake air chamber installed on the
front axle. Below, is a diagram of a front brake air chamber,
courtesy of GMC Truck Division. It has a single diaphragm which
pushes the push rod to the right when air enters into the chamber
from the air inlet. All of these brake chambers have sizing information.
The larger the chamber size, the more force the chamber can generate.
Chamber sizing is an important factor in designing brake systems
which will help to keep the truck in control during panic stops.
Always make sure that replacement air chambers are the correct
size for that installation. Consult the manufacturer's specifications
when replacing air chambers.
the second air inlet at the top, which has a plug in it. This
second inlet permits hose physical arrangement options. The push
rod return spring helps to expel the air and return the rod to
it's withdrawn position when the chamber air pressure is released.
that the push rod yoke is threaded onto the push rod, and a locking
nut locks the yoke into place. Whenever you need to adjust the
air chamber rod length, just loosen the locking nut, and rotate
the yoke for proper adjustment while holding the push rod from
have also included an exploded view of the front brake chamber
here on the left. Front brake chambers are fairly straight forward,
and understanding the front brake chamber will aid in understanding
the spring brake chambers which are described next.
Spring Brake Air Chamber
rear brake air chamber is more complicated because it serves two
purposes. One part of this dual chamber is called the spring brake
chamber and the other part is called the service brake chamber.
The service brake chamber is what activates when you step on the
tractor brake pedal. The spring brake chamber is what is charged
to release the spring brakes. These diagrams below are courtesy
of Neway Anchorlok International.
brakes were designed so there would always be a fail-safe method
of stopping an air brake vehicle, if for some reason, all air
supply pressure were suddenly lost while the vehicle is in motion.
In the diagram to the right, the heavy duty spring in the right
hand compartment (spring brake chamber) forces the push rod to
the left which engages the slack adjuster when there is no air
pressure in the spring brake chamber.
This forms the emergency brake action to stop a vehicle in motion
should the air supply fail. Release of air pressure in the spring
brake chamber is what takes place when you set your air parking
brakes. The parking brake valve on the cab dash, releases the
air pressure in the spring brake chambers, and the powerful spring
applies hundreds of pounds of brake force on the push rod, thereby
rotating the slack adjuster and setting the brakes.
the diagram to the left, the spring brake chamber has been pressurized,
and the spring brake diaphragm has compressed the heavy duty spring,
which allows the push rod to release the pressure on the slack
adjuster arm, which releases the parking brake.
When you release the parking brake, you actually pressurize the
spring brake chamber. Now the service brake chamber can move the
push rod to the left when the brake pedal is pressed. Trailers
also have spring brakes which function just like the tractor rear
wheel spring brakes. The trailer service brake chambers can be
charged when the truck foot brake is applied, or when the trailer
hand brake is applied.
diagram on the right shows the service brake chamber when full
service braking action is applied. Notice how the left hand chamber
air on its right side has pushed the chamber diaphragm to the
left which forces the push rod to the left also.
What is not shown in this diagram is that when the service brake
chamber moved the push rod to the left, the push rod moved away
from the spring brake chamber diaphragm because it is not fastened
to the spring brake diaphragm. The spring brake diaphragm only
pushes on the push rod (much like you would push the end of a
broom handle with the palm of your hand), so the push rod is free
to move to the left with the service brake chamber when service
brake action is required.
can be emergency situations where one or more service brake chambers
can not be pressurized for what ever reason. When this situation
arises, most spring brake chambers have a release bolt mounted
on the outside of the spring chamber housing.
You can override the spring brake feature by installing this service
bolt, and using a wrench to draw the spring brake diaphragm to
the right, which compresses the heavy duty spring and releases
the spring brake force on the push rod. Usually, you insert the
bolt and twist it 1/4 turn to lock it into place, then you tighten
the nut, In this configuration, that spring brake is disabled,
but the service brake can still work. This is only to be used
for emergency situations, because it disables the protective spring
but not least, never, ever, attempt to disassemble a spring brake
air chamber unless you are fully qualified. The spring forces
inside the spring brake chamber can be deadly when that heavy
duty spring is suddenly released by disassembly of the spring
brake chamber. Remember the old joke, where opening the peanut
can would release a spring which would scare the unwary? Well
the spring brake chamber has a similar surprise which could kill
you or others if you don't know what you are doing. This spring
is capable of hundreds (if not thousands) of pounds of brute force
when it is released.
to top of page , Driver
Knowledge Page ,
Pressure components page , HomePage.