Lesson 2/Learning Event 1
The hand control valve assembly has an inlet and an exhaust valve
mounted on one stem. The valve is normally held in the exhaust
position by the intake and exhaust valve spring. One type of hand
control valve has a movable exhaust tube that is pushed upward by an
exhaust tube spring and downward by a cam plate spring. A hand lever
on the control valve is used to rotate a cam changing the tension on
the cam plate spring.
With the hand lever in the released position, the exhaust tube spring
holds the exhaust tube up. Any air pressure in the trailer brake
control line is exhausted through the exhaust tube and outlet on the
control valve. Pulling the hand lever to the applied position
increases the cam plate spring tension on top of the exhaust tube.
This moves the exhaust tube down, first contacting the exhaust valve
and blocking the air exhaust passage through the tube. Then it moves
the air exhaust and inlet valve assembly down, pushing the air inlet
valve off its seat. Air supply pressure flows past the open inlet
valve into the control line to apply the trailer brakes. The control
air pressure also pushes upward on the exhaust tube tending to lift
it and close the inlet valve. When the pressures above and below the
tube are equal, the inlet valve closes. By changing the position of
the hand lever, the amount of spring tension pushing the exhaust tube
down will change, and the driver can regulate the amount of control
line pressure.
TRAILER COUPLING HOSES AND CONNECTORS
Two air outlets are provided at the rear of the truck for connecting
its brake system to the trailer brakes. One outlet contains a tag
with the word "EMERGENCY" printed on it; the second outlet has a tag
with the label "SERVICE." Two air line connections on the trailer
also have emergency and service tags. When connecting the two brake
systems together, the emergency line on the truck must be connected
to the emergency line on the trailer. Likewise, the service lines
must be connected together. The service line connects the air
control line of the truck to the control line of the trailer to
control the normal application of the brakes. The emergency line
connects the air supply of the truck to the emergency relay valve of
the trailer. If the emergency line should break or be disconnected,
the trailer brakes automatically apply.
An air shutoff cock is located at each trailer connection outlet on
the truck. The cocks must be turned off to prevent the loss of air
when the truck brake system is not connected to a trailer brake
system.
45
Previous Page
