Lesson 3/Learning Event 2
When the trailer emergency line is connected to the emergency line of
the towing vehicle, air flows from the tractor reservoir, through the
emergency line, and into the relay emergency valve. This airflow
lifts the check valve and flows over the top of the emergency
diaphragm into the supply line to charge the trailer reservoir. At
the same time, air has also been flowing below the emergency
diaphragm and into the cavity above the pressure regulating
diaphragm. Air continues to flow in this manner until the air
pressure in the cavity above the pressure regulating diaphragm
reaches about 70 PSI.
Brake chambers are mounted at each wheel. Their purpose is to
convert the energy of compressed air into the mechanical force and
motion necessary to operate the brakes. One type of brake chamber
consists mainly of a housing, diaphragm and pushrod, and a spring.
As compressed air enters the brake chamber behind the diaphragm, the
diaphragm forces the adjuster lever which rotates a shaft and cam to
apply the brakes. The higher the air pressure admitted to the brake
chamber, the greater the force on the pushrod. When the air pressure
is released from the brake chamber, the spring returns the pushrod
and diaphragm to the released position.
Instead of the brake chamber discussed above, some vehicles use a
rotochamber. As air pressure enters the rotochamber behind the
diaphragm, it moves the diaphragm forward. The diaphragm moves along
the inside wall of the cylinder body with a rolling motion. This
motion of the diaphragm forces the pushrod forward. The higher the
air pressure admitted to the rotochamber, the greater the force on
the pushrod. If all the air pressure is released from the
rotochamber, the spring returns the diaphragm and pushrod to the
The air cylinder is another version of a brake chamber. When air
enters the compressed air opening and goes behind the piston, the
piston will move, driving the pushrod and compressing the piston.
When air pressure in back of the piston is released, the piston
spring returns the piston and pushrod to the released position.
A slack adjuster is used in airbrake systems to convert back and
forth motion to rotary motion and to provide a quick and easy way to
adjust the brakes to compensate for brake lining wear. All slack
adjusters consist mainly of a worm and gear contained in a body
having a lever arm. The gear is splined to fit the brake camshaft.
In normal braking, the entire slack adjuster moves as one solid unit.
It acts as a simple lever to transmit brake chamber forces to the
brake camshaft as the brakes are applied. All adjustments are made
by turning the worm shaft. This turns the worm gear and brake
camshaft and moves the brake shoes either closer to or farther from
the brake drum.