Bendix ATR-2 User Manual

®

Bendix® ATR-2™ AntiLock Traction Relay Valve

SD-13-4812

CONTROLLER OR COVER

PLATE MOUNTING HOLES:

SEE NOTE

MOUNTING

BRACKET

BODY

SUPPLY PORT

NOTE: AN OPTIONAL COVER PLATE IS INSTALLED OVER THE TOP WHEN THE

ATR-2™ VALVE IS MOUNTED REMOTE FROM THE ANTILOCK TRACTION CONTROLLER

FIGURE 1 - ATR-2™ ANTILOCK TRACTION RELAY VALVE

BOBTAIL

PROPORTIONING

VALVING

SERVICE PORT

CONTROL PORT

DELIVERY PORT (4)

DESCRIPTION

The A TR-2™ antilock traction relay valve is a specialized air
brake valve developed for use on Bendix antilock/traction
equipped tractors only .

It is essentially three separate valves working in combination
in a single housing. An R-14™ style service relay valve is
the base valve and is fitted with a modified cover containing
a bobtail tractor brake proportioning valve (reference Bendix
Service Data Sheet SD-03-1067) and a traction control
solenoid. The A TR-2™ valve contains both air and electric
components and provides both the service braking and
traction control (differential braking) functions. A Bendix
antilock traction controller can be mounted to the ATR-2
valve or a cover plate can be installed and the antilock
controller mounted elsewhere on the vehicle.

The A TR-2™ valve replaces the standard relay valve used to
control the rear axle service brakes and performs the
standard relay function during tractor-trailer operation. During
tractor bobtail operation, the ATR-2™ valve reduces brake
application pressure to the lighter rear axle(s) brakes to
improve vehicle controllability and minimize rear axle(s)
wheel skid before antilock is required. The ATR-2™ valve
provides added vehicle braking control and reduces the
number of times antilock is actually needed.

Like the standard relay valve it replaces, the A TR-2™ valve
and its attached antilock controller is normally mounted
near the service brakes it serves. A mounting bracket,
furnished with the valve, permits either frame or cross
member mounting. All air connections on the A TR-2™ valve
are identified with cast, embossed letters for ease of
identification and installation. The letter identification and
air line connections are shown below for reference.

ATR-2™ VALVE
AIR CONNECTION EMBOSSED IDENT .

Supply (to reservoir) SUP

™

Delivery (to brake chamber) DEL
Service (to brake valve rear delivery) SER
Control (to delivery of trailer supply valve) CON

The A TR-2™ valve is part of the R-12™ family of relay valves
which includes the R-12™, R-14™, BP-R1™, AR-1™ valves.
The internal components of the relay portion of all of these
valves are interchangeable with the R-12™ valve and therefore
the same basic components are used to service all of them.

The A TR-2™ valve is available with various crack pressures
to accommodate specific applications, however the standard
is 4 psi.

TRACTION CONTROL

SOLENOID

COVER

SUPPLY

PORT

2 PIN SOLENOID

CONNECTOR

CONTROLLER

ATR-2™ ANTILOCK
TRACTION RELAY

FIGURE 2 - ANTILOCK TRACTION ASSEMBLY

INITIAL CHARGING

During the initial build up of tractor system air pressure,
reservoir air flows into the supply port and through internal
body and cover passages to the blend back piston and
from there to the supply of the normally closed (NC) traction
control solenoid. When sufficient air pressure is applied to
the blend back piston, it moves against the resistance of
its spring until it comes to rest against the service piston.

Activating the trailer supply valve (dash control with red
octagonal button), simultaneously charges the trailer and
the A TR-2™ valve’s control port. Air entering the ATR-2
valve’s control causes the proportioning piston to move
toward its stop in the cover . The integral inlet and exhaust
valve is carried along with the piston until it seats and seals
the exhaust passage. Continued movement of the
proportioning piston opens the inlet seat. With the tractor
and trailer air system fully charged the vehicle may be
operated.

™

OPERATION
GENERAL

Because the ATR-2™ valve is actually a relay valve, the
following description of operation refers to its function in the
vehicle’s air brake system and does not address all of the
separate antilock components and their operation. For a
description of antilock operation refer to the appropriate
Service Data Sheet covering the electronic controller used
with the A TR-2™ valve.

SOLENOID

CONNECTOR

PROPORTIONING

PISTON

CONTROL

INLET-EXHAUST

VALVE

BLEND

BACK

PISTON

TRACTION CONTROL

SOLENOID

SERVICE

DOUBLE

CHECK

VALVE

RELAY PISTON

INLET-EXHAUST

VALVE

SERVICE BRAKES APPLYING - TRACTOR/
TRAILER COMBINATION (FIGURE 4)

Brake application air enters the A TR-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. Air flows through the service piston then through
the center of both the blend back and proportioning pistons
to the inlet and exhaust valve. Flowing around the inlet
valve, application air moves through a passage in the cover
to the top of the service relay piston. In response to air
pressure, the relay piston moves into contact with the
exhaust portion of its inlet and exhaust valve. With the
exhaust passage sealed, continued movement of the piston
unseats the inlet portion of the inlet and exhaust valve,
allowing supply air from the reservoir to flow out the A TR-2
valve’s delivery ports to the brake chambers.

SERVICE BRAKES HOLDING - TRACTOR/
TRAILER COMBINATION (FIGURE 5)

The air pressure being delivered to the brake chambers is
also present beneath the relay piston. When the air pressure
above and below relay piston is equal, the piston moves
slightly allowing the inlet valve to return to its seat. The
exhaust valve remains closed. With both the inlet and
exhaust valves closed, air pressure in the brake chambers
is held stable and neither increases nor decreases.

™

SUPPLY

EXHAUST

FIGURE 3 - SECTIONAL A TR-2™ ANTILOCK TRACTION ASSY.

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DELIVERY

SERVICE BRAKES RELEASING - TRACTOR/
TRAILER COMBINATION (FIGURE 6)

When the brake application is released, air from above the
relay piston, flows back through the proportioning, blend
back and service pistons to the foot brake valve and is
exhausted. As air pressure is reduced above the relay
piston, pressure beneath it lifts the piston away from the
exhaust valve and opens the exhaust passage. Air from
the service brake chambers returns to the ATR-2™ valve
and flows out the open exhaust.

TRAILER

SUPPLY

CONTROLLER

TRACTION SOLENOID

BRAKE

VALVE

PROPORTIONING

PISTON

CHECK VALVE

INLET EXHAUST

RELAY PISTON

INLET EXHAUST

MODULATOR

REAR AXLE

RESERVOIR

SERVICE BRAKE

FIGURE 4 - SERVICE BRAKE APPLICATION

CONTROLLER

TRAILER SUPPLY

INLET EXHAUST

PROPORTIONING

PISTON

TRACTION SOLENOID

BRAKE

VALVE

CHECK VALVE

RELAY PISTON

INLET EXHAUST

MODULATOR

REAR AXLE

RESERVOIR

FIGURE 5 - SERVICE BRAKES HOLDING

SERVICE BRAKE

3

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 A TR-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 p si 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 A TR-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 A TR-2™ valve and flows out the open exhaust.

TRAILER SUPPLY

INLET EXHAUST

PROPORTIONING

PISTON

REAR AXLE

RESERVOIR

CONTROLLER

TRACTION
SOLENOID

BRAKE

VALVE

CHECK VALVE

RELAY PISTON

INLET EXHAUST

MODULATOR

SERVICE BRAKE

FIGURE 6 - TRACTOR - TRAILER SERVICE BRAKE RELEASING

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