BENDIX ATR-2 ANTILOCK TRACTION VALVE Service Data Sheet - Page 4

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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

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

TRAILER SUPPLY

INLET EXHAUST

PROPORTIONING

PISTON

REAR AXLE

RESERVOIR

FIGURE 6 - TRACTOR - TRAILER SERVICE BRAKE RELEASING

™

valve's service port

CONTROLLER

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.

TRACTION

SOLENOID

BRAKE

VALVE

CHECK VALVE

RELAY PISTON

INLET EXHAUST

MODULATOR

SERVICE BRAKE