Lesson 2/Learning Event 2
Now let's discuss the complete compressed air system of the 5ton,
6x6 truck. The truck has service and emergency trailer couplings at
the front as well as at the rear. If a truck must be towed, the
trailer couplings at the front can be connected to the rear trailer
couplings of the towing truck. With the two brake systems connected
in this manner, the brakes of both trucks can be controlled from the
towing truck. Three doublecheck valves direct the flow of
controlled air pressure.
One doublecheck valve is located at the control line connection of
the airhydraulic cylinder. The center connection of the valve is
connected to the power cylinder. The control line from the air
hydraulic cylinder control valve is connected to an end connection of
the doublecheck valve. The control or service line from the front
of the truck is attached to the remaining end connection of the
When the control unit of the airhydraulic cylinder sends air
pressure to the doublecheck valve, the air pressure moves a sliding
piston in the valve closing the service line passage to the front of
the truck. The air can move freely from the control unit, through
the doublecheck valve, into the power cylinder to apply the brakes.
When the truck is being towed and the brakes are applied, the double
check valve prevents the escape of controlled air from the towing
truck. Brake air controlled pressure from the tow truck flows
through the service line to the doublecheck valve. The controlled
air moves the sliding piston closing the passage to the airhydraulic
control unit. Controlled air pressure now flows freely from the tow
truck, through the doublecheck valve, and into the power cylinder.
The two remaining doublecheck valves direct the controlled air
flowing to the trailer service coupling at the rear of the truck.
They are connected so that controlled pressure furnished from one
control unit cannot flow to another control unit. This must be done
to prevent the escape of controlled pressure. For instance, if
controlled pressure from the airhydraulic cylinder is allowed to
flow through the control lines to the hand control valve, the
pressure will be released through the openhand control exhaust valve.
The doublecheck valves used on the 5ton truck make it possible for
the brakes of a towed vehicle to be applied by using either the brake
pedal or the hand control of the towing truck. When the brake pedal
is pushed down, the brakes of both the towing and the towed vehicle
are applied. When the hand control valve is applied, the double
check valves direct the controlled air so only the brakes of the
towed vehicle are applied.