Bendix Commercial Vehicle Systems AIR PRESSURE BALANCE PRESSURE TESTS - Manual (Page 4)

EXAMPLE:
Foundation Brake Threshold Pressure Test
7. When testing is complete, uncage the parking
brakes.
RECOMMENDATIONS:
Bendix recommends that all brake threshold pressures
for all axles (average calculated value from the test
results) be within a 2 psi range. This is to enable the
brake force distribution to be within acceptable limits,
especially at low brake application pressures. Industry
studies have indicated that 80% of all brake applications
made in actual service are below 20 psi.
For pressure balance, on increasing pressures we
recommend that the pressure recorded at each axle be
within 2 psi of one another for trailer control line pressures
of 10 to 40 psi. The decreasing pressures recorded
should also be within 2 psi of one another in this pressure
range. For axles fitted with special valves which enable
the threshold pressure recommendation to be achieved
or which proportion or limit the delivery to the chambers
this recommendation is not valid. Under these conditions
the specific application must be analyzed in more detail
to understand the brake distribution.
There are many factors that affect vehicle brake balance.
Balancing the brake threshold pressures and the
pressure at the chambers of all axles are important
factors in achieving brake balance. However, this does
not necessarily ensure even lining wear or brake force
distribution. Consideration must also be given such
factors as tire size, brake size, brake type, friction
material and others.
NOTE: The difference recorded in these tests can be
attributed to specific vehicle design considerations.
Before any brake system modification are made the test
data should be reviewed by both the vehicle OEM and
our Engineering Departments.
FOUNDATION BRAKE THRESHOLD
PRESSURE TESTING
Objective: To determine the air pressure required to
make and release contact between the lining
and drum at each foundation brake on the
vehicle.
Equipment: Test gauge (use a gauge with a 0-60 psi
range.)
Important: A low range (0-60 psi) pressure gauge is
required for accurate data for the threshold
pressure test. Using a higher range (0-100,
or 0-150 psi) gauge can result in bad data.
Testing:
1. Perform the vehicle preparation and place the vehicle
dash controls in the normal over-the-road position.
2. If the vehicle being tested is a single unit, such as a
straight truck or bus, install the test gauge directly in
the #1 or primary circuit delivery of the brake valve.
For tractors or combination vehicles, install the test
gauge in the trailer service (control) line (between
tractor and trailer).
3. Raise the axle until tires (wheel ends) are no longer
in contact with the floor.
4. While manually rotating a wheel, gradually make a
service brake application as registered on the gauge
until braking torque (drag) occurs. Note the gauge air
pressure and record it in the “increasing” (INC) column
for the appropriate brake in the chart.
5. Increase service air pressure to 40 psi. While
gradually decreasing the air pressure at the gauge,
attempt to rotate the wheel. Note the gauge air
pressure at which the wheel can be freely rotated and
record that pressure in the “decreasing” (DEC) column
for the appropriate brake in the chart.
6. Add the pressure recorded in the “increasing” (INC)
and “decreasing” (DEC) columns together then divide
by two. Record that value in the “average” (AVG)
column for the appropriate brake. This average is the
threshold pressure for that brake.
For example: If the number in the “increasing” column
was 9 and the number in the “decreasing” column
was 3, then 9 +3 = 12, 12÷2 = 6, and 6 is recorded in
the average column.
BW1555 © Bendix Commercial Vehicle Systems LLC 11/2002 Printed in U.S.A.
BR.# INC DEC AVG BR.# INC DEC AVG
2
11 1
12
9 3 6
8 3.5 6.5