Troubleshooting guide

28
For more information, visit www.bendix.com or www.foundationbrakes.com • 1-800-AIR-BRAKE (1-800-247-2725)
Spring Brake Valves
The Bendix
®
SR-1
™
spring brake valve is used in
dual circuit brake systems and serves two functions:
fi rst, during normal operation, it limits hold-off pressure
to the spring brakes to 90 or 95 psi. Next, if a loss of
pressure occurs in the rear brake service supply, the
valve will provide a modulated spring brake application
proportional to the driver’s service braking pressure
delivered to the front axle.
The Bendix
®
SR-7
®
spring brake modulating valve
is used in dual air brake systems with spring brake
actuators. It provides a rapid application of the spring
brake actuator when parking; modulates the spring brake
actuator application (using the dual brake valve, should
a primary failure occur in the service brake system); and
prevents compounding of service and spring forces.
Simple Spring Brake Anti-Compound System
Brake compounding can occur in a spring brake parking
system due to the direct in-line arrangement of both
the spring chamber and brake chamber. It occurs in
unprotected systems when parking AND service brake
applications are made at the same time.
An example of this situation occurs when a vehicle is
parked on a steep incline; the driver holds the service
brakes applied (preventing the vehicle from rolling
backwards), then actuates the park control which “sets”
or applies the spring brakes. For a brief time, the air
applied service brakes and the mechanical spring
brakes both exert a braking force on the slack adjusters
and foundation brakes. The forces of the spring and air
applications are additive and can cause damage to the
foundation brake components (cam shaft splines, shoes,
drum, etc.) and/or slack adjuster.
An anti-compounding system is especially important in
protecting the adjusting mechanism of automatic slack
adjusters from damage caused by over-torque that
occurs during a compounded application of the brakes.
Anti-compounding prevents the simultaneous application
of both the air and spring brakes by directing application
air to the spring brakes when both are applied at once.
In the simple anti-compounding schematic shown on this
page, the double check valve allows service application
air to apply the service brakes AND move into the spring
cavity if they are also applied (no air pressure and springs
are also applying brakes).
Bendix
®
SR-7
®
Spring Brake Valve
Bendix
®
SR-1
™
Spring Brake Valve
The anti-compounding function of the double check valve
is built into several air brake devices such as the Bendix
®
SR-7
®
, R-7
™
, R-14
®
and QR-1C
®
valves. When these
devices are used in the system, a separate double check
valve for anti-compounding is not needed.
It is recommended that the service connection to the
anti-compounding device (double check valve) come
from a point between the service brake chamber and
the fi rst “upstream” service device (in this case a quick
release valve).
The Control System: Spring Brake Valves
Park Control
Valve
Spring Brake
Parking Quick
Release Valve
Service
Brake Valve
Service Quick
Release Valve
Double
Check
Valve
Example of a Simple Spring Brake
Anti-Compound System