Brake Mechanical Linkage
topic describes the mechanical air brake parts which move when
the brakes are applied. Most of the line drawings in this topic
are the courtesy of AlliedSignal Bendix Truck Brake Systems, and
were taken from their Air Brake Handbook.
The air chambers
of the brake system contain a piston which pushes on a push rod.
The pounds per square inch (PSI) of air pressure felt on the piston
is multiplied by the number of square inches of the piston. The
drawing below shows a typical front wheel brake chamber and the
table shows the force generated by the air chamber push rod for
various air pressure values.
the air chamber pressure, we can control the braking force applied
to the brakes. The truck brake pedal (sometimes referred to as
a brake tredle) operates a dual valve which outputs air pressure
which is proportional to the angle of motion of the pedal.
pedal valve has separate valving for the front brakes and the
rear brakes. As you step harder on the pedal, the air pressure
outputs from this valve increases. As you let up on the brake
pedal, the air pressure outputs decrease accordingly.
pedal angle determines the amount of air pressure delivered to
the brake circuit. For simplicity sake, we will assume that both
the front brake circuit and the rear brake circuit consists of
the air chambers only. If you refer to our tractor
brake system topic, you will see that it is much more complicated
in real truck systems. The
trailer control (hand control valve) works in the same way as
the foot brake valve. The angle of trailer control valve motion
determines the air pressure from the trailer control valve to
the service brake chambers on the trailer.
to the right has the air chamber behind the brake drum, and you
can see the air chamber push rod connected to the slack adjuster.
As air pressure is applied to the air chamber, the slack adjuster
is moved by the push rod.
adjuster rotates the brake s-cam, and the scam spreads apart the
cam rollers which spread the brake shoes apart and forces them
to rub against the brake drum. This collection of parts is called
the brake foundation. When the air pressure to the air chamber
is removed, the air chamber push rod withdraws backwards, moving
the slack adjuster backwards, which moves the scam, which allows
the brake shoes to be pulled back to the release position by the
As the foundation
brake shoe linings wear, the brake shoes require further travel
to effect the same braking forces. The slack adjusters allow for
taking up this slack as the linings wear. If the slack was not
adjusted, then the air chamber would have to travel further to
effect the same braking force as the brake linings wear. This
would cause an additional time delay before the air chambers could
fill with enough air to make the brake shoes start rubbing the
brake drum. There
are many different brands of slack adjuster, but only two types
of slack adjuster. Manual slack adjusters require periodic manual
adjustments, and the automatic slack adjusters maintain proper
adjustment as the brakes are used. The picture above is a new
manual slack adjuster. A cutaway view of this slack adjuster is
shown to the left.
ring around the adjustment screw in the picture. This is called
the lock sleeve in the cutaway view. To move the adjusting screw,
you first push in the lock sleeve, and then use a wrench to adjust
the slack adjusting screw. The worm shaft rotation changes the
relationship between the spline and the slack adjuster arm.
As the brake
shoe lining wears, the angle between the slack adjuster arm and
the spline has to be adjusted. When you have finished with the
adjustment, always make sure that the spring loaded lock sleeve
pops back out to keep the adjusting screw from turning.
grease fitting in the picture above, and the grease hole in the
cutaway view above. Frequent and responsible greasing of the slack
adjuster keeps moisture out of the worm gear. This allows the
slack adjuster to be adjusted as the brake linings wear down.
When you obtained your CDL license, you probably had to demonstrate
knowledge about how to determine when the slack adjusters need
slack adjusters reduce the amount of attention which is needed
with regard to slack adjuster slack. But even automatic slack
adjusters can fail to adjust the slack properly, so they still
need to be checked on occasion. Automatic slack adjusters need
to be greased on a regular basis to keep moisture out, and to
lubricate the auto-adjust mechanism so that it keeps working properly.
the drawing below for the following explaination. As the brakes
are applied, the motion of the slack adjuster causes a lifting
force on the link which rotates the pinion gear clockwise. This
rotation tightens the slack adjuster until the force of brake
shoe contact increases the resistance on the worm gear. This increased
resistance of the worm gear makes the clutch spring slip, so that
the slack tightening ceases at this point. When the brakes are
released, the anti-reverse spring prevents the worm shaft from
turning backwards, so the clutch spring slips as the slack adjuster
rotates back to the rest
position. The clutch spring and the anti-reverse spring are wound
in opposite directions. This is why the two springs grip and slip
in opposing directions. Both springs can be overpowered by the
manual adjuster hex.
automatic slack adjusters can over adjust, which results in brake
shoes which are to tight, which causes excessive dragging of the
brake shoes, which causes heat damage to the brake foundation
parts. Most of the automatic slack adjusters have a manual override
capability similar to the one shown above. Consult with the manufacturer
for the procedures for manual adjustment if the slack adjuster
sets the brake shoes to tight.
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