Troubleshooting guide
20
For more information, visit www.bendix.com or www.foundationbrakes.com • 1-800-AIR-BRAKE (1-800-247-2725)
The Control System: Actuators
Actuators
During service braking, actuators convert the air pressure
being produced by the driver pushing on the brake pedal
into a mechanical push rod force acting on the foundation
brakes. Air enters the actuator and pressurizes a chamber
containing a rubber diaphragm. The air pushes against
the diaphragm, pushing against the service return spring
and moving the push-plate (and push rod) forward. See
the Section 8 for information about the physics behind
how actuators work.
Brake chambers are available in many sizes, providing
a wide range of output forces and stroke lengths.
Different size brake chambers are identifi ed by numbers
which specify the effective area of the diaphragm. For
example, a “type 30” brake chamber has 30 square
inches of effective area.
Spring brake actuators are composed of separate air
and mechanical actuators in a single housing. Mounted
at the wheel of the axle it serves, they function as service,
parking and emergency brakes.
Service Brake
Chamber
(No Spring Brake)
Parking/ Emergency
Brake Housing
Non-serviceable
Power Spring
Service
Push Rod
Service Return
Spring
Service
Chamber
Release Tool T-Bolt
(Used during brake
service to restrain the
power spring.)
T-Bolt
Storage
Pocket
Non-Pressure
Housing
Yoke
Service
Diaphragm
Clamp
Band
Non-Serviceable
Parking Diaphragm
Non-Serviceable
Section Permanently
Sealed
Welded Clevis
Threaded Push Rod
Dust Cap installed
Dust Cap
removed
Release
Tool
(Spring brake components are not serviceable, and are shown here for informational purposes only.)
The service chamber portion of a spring brake actuator
functions the same as described at the start of this
section.
The parking/emergency brake housing side of the spring
brake actuator contains a powerful spring that — when
engaged — holds a parked vehicle in position. When the
driver prepares to move away and releases the parking
brake, the spring force is countered by the introduction of
air pressure into a chamber within the spring brake portion
of the actuator.
A spring brake, therefore, contains two actuators which
use air pressure in opposite ways. The service actuator
requires air pressure to apply the brakes, while the park
(or emergency actuator) uses air pressure to release the
brakes.
In an emergency, the air pressure restraining the
powerful spring can be released and the spring brakes
can be used to help stop the vehicle. A feature — called
anti-compounding — helps to prevent the simultaneous
application of both the spring and service brakes.
Bendix
®
NG-3
™
Piston-Style
Spring Brake Chamber
Air Disc Brake Actuator
(uses ADB-style
push rod)
The No Touch
™
Power Spring shown
here — designed to
prevent spring corrosion
— is used in Bendix
®
EverSure
®
Spring Brakes.
Diaphragm