Lesson 3/Learning Event 2
Learning Event 2:
OPERATION OF STRAIGHT AIR-BRAKE SYSTEM
So far, we have discussed the components contained in airbrake
systems and what they do. Knowledge of how the components function
is important so that a mechanic can diagnose troubles without making
As mentioned before, the components of the compressed air system are
about the same as those used in 2 1/2 and 5ton trucks. The
operation of air compressors, governors, reservoirs, highpressure
relief valves, and drain cocks of airbrake systems is the same as
those discussed in earlier lessons for airhydraulic systems. Again,
in this system, the maximum pressure is about 105 PSI.
The brake valve (or brake application valve) is the device that an
operator uses to control pressure to the brakes. It is mounted under
the floor of the cab and is controlled by brake pedal movement. It
is made so that the driver can vary the air pressure admitted to the
brake chambers. As we will see later, the more air pressure there is
in the brake chamber, the more the brake shoes will be forced against
the brake drum. There are several types of brake valves, but they
all do about the same job. The main difference between the types is
that some are operated by the foot pedal only, while others are
operated by a foot pedal but have a handoperated limit control.
Standard brake valves are fitted with a lever that is connected and
operated by a foot pedal.
As the lever is moved toward its fully applied position, mechanical
force is applied to the top of the diaphragm in the brake valve.
This is done by the action of the plunger and pressure regulating
spring assembly. As the diaphragm moves downward, a force is applied
to the middle of the rocker arm and onto the inlet and exhaust valve.
Because the exhaust valve spring is weaker than the inlet valve
spring, the exhaust valve is forced down onto its seat before the
inlet valve is forced down to open.