Lesson 2/Learning Event 1
AIR RESERVOIR
Two round steel tanks are used on each truck to hold a supply of
compressed air. The tanks are large enough to provide enough air
under pressure for several brake applications after the engine has
stopped running. The reservoirs also provide a place to trap
condensed oil and water vapors. The air, which is heated during
compression, is cooled in the tanks causing any vapors in the air to
condense. A drain cock is provided in the bottom of each reservoir
to drain trapped condensation.
SAFETY VALVE
A relief valve, known as the safety valve, is installed on one of the
reservoirs. It is used to prevent air pressure in the system from
building up above a safe operating pressure if the governor or
unloader mechanism should fail. The valve is held closed by spring
pressure and opened by air pressure. When the air pressure in the
reservoir exceeds 150 PSI, the valve opens exhausting the excess
pressure.
WARNING SIGNAL BUZZER
An electrically operated buzzer is located under the dash panel to
warn the vehicle operator if the air pressure falls below a safe
operating level. The buzzer sounds when an airoperated switch
closes to connect an electrical circuit between the buzzer and the
vehicle batteries. The switch is generally mounted under the dash
panel near the buzzer and is connected to an air line from the air
reservoir. Air pressure on a piston in the switch tends to open the
switch contacts, while spring pressure tends to close the switch
contacts. When the air pressure falls below 60 PSI, spring pressure
closes the switch and the buzzer sounds.
PRESSURE GAGE
An air pressure gage is located on the dash panel to show the amount
of pressure in the air system. The gage is made to read pressures
from 0 to 120 PSI.
HAND CONTROL VALVE
Some trucks have a hand control valve mounted on the steering column
for individual control of the brakes on towed vehicles. Two air
lines are connected to the valve: One is a supply line from the
truck's air reservoir, and the other is a control line that is
attached to the brakes of the trailer. A third threaded air opening
in the valve is an air exhaust outlet which is left open.
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