AIR BRAKES - GENERAL INFORMATION
The air brake system on a tractor-trailer combination
must provide balanced braking at all wheels and axles of
the combination for optimum brake performance and life.
This means that the braking effort produced at each
wheel of the combination must be capable of doing its
share of the work in controlling the speed of the two
There are several factors which can affect brake
balance. These are:
Loads must be properly distributed between
tractor and trailer. A balanced condition should
not be expected with the combination vehicle if
the load is not properly distributed. The brakes
on each axle are designed to meet specific
performance criteria based on the axles Gross
Axle Weight Rating (GAWR). For example,
15400 kg (34,000 lb.) GAWR tandem axles used
on both the tractor and trailer in combination are
deigned to meet similar performance standards.
Overloading or under loading either the tractor or
the trailer tandem axle can cause unbalanced
braking in the combination vehicle. A typical
symptom would be early lockup of the brakes on
an axle loaded to less than the rated capacity.
Tractor and trailer brakes must be compatible.
The foundation brakes used on the tractor and
trailer should be of the same type. Operating
sensitivity, are different for wedge, cam and disc-
operating conditions, the intermixing of brake
types can cause or exaggerate an imbalance
condition between tractor and trailer(s). Extreme
combination equipped with brakes of different
Tractor and trailer brakes must be maintained in
proper adjustment. The force output of an air
brake chamber push rod Increases linearly for
approximately 75% of its maximum travel.
Beyond 75%, the force is reduced. Therefore, if
the brake adjustment is not maintained within
specification and the push rod travel is allowed
to increase beyond 75% of maximum travel, the
brake torque output at the wheel will be reduced.
For example, when air pressure of 689 kPa (100
psi) is applied to a Type 30 service air brake
chamber, approximately 3,000 lbs. of force is
produced with a push rod travel of 38mm (1-1/2
in.) or less. When the push rod travel is
increased to 57mm (2-1/4 in.) at the same air
pressure, the force produced decreases to
approximately 11,121N (2,500 lbs.), a reduction
of about 17%. A reduction in force at the push
rod causes a similar reduction in the brake
torque output at the wheel. A reduction in the
brake torque output capability at any one or
several wheels of a combination vehicle places
added work on the brakes at other wheels. The
result can be uneven lining and drum or rotor
wear (between brake assemblies), trailer surging
and increased brake fade.
If automatic adjusters are used, they should be
used on both tractor and trailer to insure
compatibility between the two units. While
automatic adjusters do maintain the brakes in
reasonable adjustment, it is important that the
adjusters be inspected periodically to be sure
that they are functioning properly.
Tractor and trailer brake assemblies and air sys-
maintained. Corroded anchor pins, frozen
camshafts, weak or broken return springs, or
deformed shoes, etc., can reduce the efficiency
of the brake assembly at any wheel(s). It is
important to inspect the air system - reservoirs,
damage. This includes draining the reservoirs
Whenever complaints of touchy brakes, inadequate
braking, short service life of brake linings, drums or
rotors on either the tractor or the trailer (but not both) are
encountered, there may be an imbalance between the
tractor and trailer air brake systems.
It is suggested that a brake analysis be performed. This
analysis includes testing for 1) pressure balance, 2)
pressure build-up rate (timing), and 3) torque balance.
However, before conducting the tests, the following
questions should be considered:
Are all or a majority of the drivers operating the
combination units obtaining the same results or
registering the same complaints?
Are the same results or complaints being
registered on similar combination units?
Are the same results or complaints being
registered under similar operating conditions?
combination unit or did complaints originate after
some brake service was performed, or vehicle
operating conditions changed?
The answers to these questions may help in identifying
the cause(s). Such problems could be the result
of incorrect or malfunctioning brake components,
or simply the conditions under which the
combination vehicle is operated.
If the response to these questions seems to indicate that
a brake Imbalance condition may be present in
the air brake system, the preliminary checks and
test pro-cedures in the following text can be
used to isolate the cause.
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