Troubleshooting guide

22
For more information, visit www.bendix.com or www.foundationbrakes.com • 1-800-AIR-BRAKE (1-800-247-2725)
The Control System: Foundation Drum Brakes
Brake
Block
Brake Lining
Foundation Drum Brake Linings
When cam-actuated drum brakes are applied, the friction
material contacts the brake drum. The friction between
the lining and drum effectively converts the kinetic energy
of the vehicle into thermal energy, heating up the brake
drum and lining.
The friction material used in the brake must have the
capability of withstanding the heat until dissipated. It
takes the right combination of ingredients to provide all
the desirable characteristics: including a torque output
suffi cient to stop the vehicle, long lining life, long drum life
and noise reduction.
All friction material is identifi ed by stenciled codes on its
edge. These codes consist of the name or brand of the
manufacturer, the formula identifi cation, the specifi c part
number and usually two letters designating the lining’s
general friction level (examples: EF, FG). It should be
noted that these stenciled letters are not considered to
be accurate enough for choosing replacement linings.
To ensure proper braking, always use either genuine
original equipment linings, or an approved material with
direct performance comparison.
In an S-Cam type foundation drum brake, the pneumatic
system is linked by an air line to the air service and/or
spring brake chamber, which is then connected to the
arm of a slack adjuster by a push rod and clevis. The
slack adjuster is installed on the spline of the forged brake
cam shaft. The slack adjuster acts as a lever, converting
the linear force of the chamber push rod into a torsional
force needed to apply the brakes.
When torque is applied to the cam shaft, the “S” shape on
one end of the cam spreads the brake shoes, forcing the
brake lining into contact with the brake drum, therefore
retarding and/or stopping the vehicle.
Foundation drum brakes (aka S-Cam) are offered in
a variety of drum diameters to meet vehicle braking
requirements, with the most common being the 16½
inch version. The cam brake has a “leading-trailing”
shoe design with one or more fi xed anchor points for the
shoes, opposite the cam end of the shoe.
The structure of the S-Cam-actuated brakes generally
consists of stamped steel brake shoes and spiders for
standard duty applications, and cast shoes and spiders
for heavy-duty applications.
Foundation Drum Brakes
See the section on page 21 of this handbook for
more information about selecting friction materials
that meet the requirements of the Reduced Stopping
Distance (RSD) mandates.
S-Cam Foundation Drum Brake and
Examples of Friction Material