Troubleshooting guide

59
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FIGURE 16 - BRAKING FORCES-EFFECT OF SLACK
ADJUSTER ARM LENGTH
Torque On Brake
Camshaft 4000 Inch
Pounds With Slack
Adjuster Arm 4
Inches Long.
Torque On Brake
Camshaft 6000 Inch
Pounds With Slack
Adjuster Arm 6
Inches Long.
Foundation Drum and Air Disc Brakes
Braking Forces – Effect of Air Pressure
This control of the braking force – by controlling the air
pressure in the chambers – is illustrated in Figure 14.
It shows the resulting forces (in pounds of various air
pressures) with a chamber having an effective diaphragm
area of 24 square inches.
Key Takeaway:
The important point is that the air pressure in a brake
chamber can be controlled so the brake chamber
will develop the required force.
Braking Forces – Effect of Brake Chamber
Size
Different sizes of vehicles and different axles of the
same vehicle may require different braking forces,
depending on the weight of the vehicle or the weight
distribution between axles. These variations in the
braking force are design variations, because the maximum
and minimum force required must be properly provided
before good performance can be obtained throughout
the entire braking range. (Note: Rotochambers are a
larger, heavier style of brake chamber typically used on
heavier, off-road equipment where their constant output
and longer push rod stroke is useful.)
Figure 15 illustrates the developed force (in pounds) of
each of several different sizes of brake chambers and
rotochambers when supplied with air pressure at 60
pounds per square inch. The effective area of the different
brake chambers generally varies from six square inches
to 36 square inches, and their developed force at 60
pounds air pressure generally varies from 360 pounds
to 2,160 pounds. This permits the choice of a chamber
size suitable for properly operating any size or type of
foundation brake.
Braking Forces – Effect of Slack Adjuster Arm
Length
Figure 16 illustrates how the principles of leverage apply
when a brake chamber and slack adjuster combination is
being selected to meet specifi c requirements.
With the same brake chamber force of 1,000 pounds,
the torque on the brake camshaft can be increased from
4,000 inch pounds to 6,000 inch pounds merely by using
a slack adjuster with a 6” arm instead of one with a 4”
arm.
In an S-Cam foundation brake, the full range of
braking forces for any vehicle is provided by the
use of different sizes of brake chambers and slack
adjusters.
A term used to express the relation of the brake chamber
size and slack adjuster arm length is “AL” factor. The
“AL” factor differs from torque – or turning moment –
because only the variable factors determining the force
are expressed. The reason for this is that an air pressure
of 60 pounds is generally used in calculating air braking
forces and, therefore, is considered constant.
FIGURE 17 - AIR DISC BRAKE - CUT-AWAY VIEW
Lever
Rotor
Eccentric
Bearing
Inner Brake
Pad
Outer
Brake Pad
Actuating
Beam
Actuator
Rod
Supply Port