Bendix AR-2 User Manual

®

Bendix® AR-2™ AntiLock Relay Valve

ANTILOCK CONTROLLER

MOUNTING HOLES(4)

MOUNTING

BRACKET

SD-13-4796

SERVICE

SUPPLY

DELIVERY

FIGURE 1 - AR-2™ ANTILOCK RELAY VAL VE ASSEMBLY

DESCRIPTION

The AR-2™ antilock relay is a specialized air brake valve
developed for use on antilock equipped tractors only. It is
essentially a BP-R1™ “bobtail” brake proportioning relay valve
(Bendix Service Data Sheet SD-03-1067) with a modified
cover that permits direct attachment of a Bendix® antilock
controller such as the EC-15™. When combined with an
antilock controller the resulting assembly is referred to as a
antilock controller relay assembly. For example when the
AR-2™ valve is combined with the EC-15™ electronic
controller, the resulting assembly is referred to as a CR-16
controller relay assembly .

While it is intended for use on antilock equipped tractors,
the AR-2™ valve functions solely as an air brake relay and
contains no electronics of its own.

DELIVERY

The AR-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 AR-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 AR-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 AR-2™ valve

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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 AR-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.

CONTROL

1

BLEND BACK

PISTON

SPRING

CONTROL PORT

EXHAUST

INLET/EXHAUST VALVE

PROPORTIONING

PISTON

SUPPLY PORT

EXHAUST

FIGURE 2 - SECTIONAL VIEW AR-2™ ANTILOCK RELAY VAL VE

SERVICE PORT

SERVICE PISTON

RELAY PISTON

SPRING (NOT IN ALL

MODELS)

INLET/EXHAUST VALVE

DELIVERY PORT

AR-2™ VALVE
AIR CONNECTION EMBOSSED IDENT .

Supply (to reservoir) SUP
Delivery (to brake chamber) DEL
Service (to brake valve rear service delivery) SE R
Control (to delivery of trailer supply valve) CO N

The AR-2™ valve is part of the R-12™ family of relay valves
which includes the R-12™, R-14™, BP-R1™, and 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 AR-2™ valve incorporates various crack pressures to
accommodate specific applications, the standard is 4 psi.

OPERATION

GENERAL

Because the AR-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 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 AR-2

valve (ie; SD-13-4785 covering the EC-15™ electronic
controller). Other useful information regarding antilock
system components used in conjunction with the AR-2
valve is contained in publications SD-13-4793 covering the
M-21™ antilock modulator and SD-13-4754 covering the
WS-20™ wheel speed sensor.

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. 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 AR-2™ valve control port. Air entering the AR-2™ valve
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.

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™

2

TRAILER

SUPPLY

VALVE

INLET-OPEN

EXHAUST-CLOSED

REAR AXLE

SERVICE

CHAMBER

ANTILOCK

CONTROLLER

SERVICE

BRAKE

VALVE

RELAY PISTON

INLET-OPEN

EXHAUST-CLOSED

™

M-21

MODULATOR

FIGURE 3 - TRACTOR SERVICE BRAKES APPL YING - TRACTOR TRAILER COMBINATION

SERVICE BRAKES APPLYING - TRACTOR/TRAILER
COMBINA TION

Brake application air enters the AR-2™ valve service port
and is conducted 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 AR-2™ valve delivery ports
to the brake chambers.

SERVICE BRAKES RELEASING - TRACTOR/TRAILER
COMBINA TION

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 AR-2™ valve and flows
out the open exhaust.

SERVICE APPLICATION - BOBT AIL TRACTOR

When the trailer supply valve (dash control with red octagonal
button) is activated to disconnect the trailer, air in the AR-2
valve control port and trailer supply line is exhausted to

SERVICE BRAKES HOLDING - TRACTOR/TRAILER
COMBINA TION

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

atmosphere. During bobtail tractor operation, service
application air enters the AR-2™ valve service port and as it
passes through the blend back piston air 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

decreases.

™

3

ANTILOCK

CONTROLLER

TRAILER

SUPPLY

VALVE

INLET-OPEN

EXHAUST-CLOSED

REAR AXLE

SERVICE

CHAMBER

FIGURE 4 - TRACTOR SERVICE BRAKES HOLDING - TRACTOR TRAILER COMBINATION

SERVICE

BRAKE

VALVE

RELAY PISTON

INLET-CLOSED

EXHAUST-CLOSED

™

MODULATOR

M-21

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.

4

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 - BOBT AIL TRACT OR

When the brake application is released, all air pressure from
between the closed proportioning piston inlet valve and the
AR-2™ valve 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
AR-2™ valve and flows out the open exhaust.