4
SERVICE APPLICATION - BOBTAIL TRACTOR
(FIGURE 7)
When the trailer supply valve (dash control with red octagonal
button) is activated to disconnect the trailer, air in the
ATR-2
™
valve’s control port and trailer supply line is
exhausted to atmosphere. During bobtail tractor operation,
service application air enters the ATR-2
™
valve’s service port
and is conducted to the single check valve. The check
valve diaphragm flexes in response to application pressure
and seals the passage to the open exhaust of the traction
solenoid. Application air passes through the blend back
piston and exerts a force on the full effective diameter of the
piston. The blend back piston remains stationary at
application pressures below approximately 80 psi because
of opposing reservoir air acting on the large diameter.
Application air flowing through the blend back piston also
exerts a force on the small diameter of the proportioning
piston while simultaneously flowing through the center of it
on its way to the inlet and exhaust valve. Once past the
inlet and exhaust valve, service air pressure exerts a force
on the larger diameter end of the proportioning piston, which
opposes the air pressure and spring force exerted on the
other end. The proportioning piston inlet valve remains open
until a preset, initial application pressure has reached the
relay piston assuring that the foundation brake shoes are
brought into contact with the drum. As service pressure
begins to exceed the preset initial application, the
proportioning piston will have moved sufficiently to close its
inlet valve without opening the exhaust.
The inlet remains closed, preventing additional air delivery
to the relay piston and a subsequent increase in brake
chamber pressure, until service application pressure
exceeds approximately 30 psi. Service applications above
this pressure will result in a proportioned increase of the
preset initial brake application to the tractor’s rear brakes.
Proportioning occurs due to the difference in effective area
on each end of the proportioning piston. Tractor rear axle
brake proportioning will occur for all service applications
between approximately 30 and 80 psi.
Service applications of more than 80 psi cause the blend
back piston to begin to move. Service air pressure acting
on the full diameter of the blend back piston over comes
the resistance of reservoir pressure acting on the large
diameter of the other side. Above 80 psi the ratio between
control and delivered air pressure is reduced and “blends
back” from a proportioned delivery to a full 1 to 1 delivery.
Complete “blend back” to a 1 for 1 delivery is achieved when
a full brake application is made.
SERVICE BRAKES RELEASING - BOBTAIL
TRACTOR (FIGURE 8)
When the brake application is released, all air pressure
from between the closed proportioning piston inlet valve and
the ATR-2
™
valve’s service port returns to the brake valve
and is exhausted. Air from above the relay piston, flows
back to the proportioning piston causing it to move. As it
moves, the proportioning piston unseats the exhaust valve
allowing air from above the relay piston to escape to
atmosphere. Reducing the air pressure above the relay
piston, causes pressure beneath it to lift the piston away
from the exhaust valve. Air from the service brake chambers
returns to the ATR-2
™
valve and flows out the open exhaust.
CONTROLLER
BRAKE
VALVE
CHECK VALVE
RELAY PISTON
INLET EXHAUST
REAR AXLE
RESERVOIR
TRAILER SUPPLY
PROPORTIONING
PISTON
TRACTION
SOLENOID
INLET EXHAUST
MODULATOR
FIGURE 6 - TRACTOR - TRAILER SERVICE BRAKE RELEASING
SERVICE BRAKE
