Bendix Commercial Vehicle Systems A2LS User Manual

®

Bendix® A2LS™ Hydraulic Drum Brakes w/Parking

DIRT SHIELD

SD-20-6608

CROSS-OVER PIPE

RETAINING SPRING

WHEEL CYLINDER END

SHOE RETURN SPRING

EXPANDER END SHOE

RETURN SPRING

WHEEL CYLINDER

EXPANDER

FIGURE 1 - A2LS™ AUT OMA TIC ADJUSTING TWO LEADING SHOE DRUM BRAKE

DESCRIPTION

The A2LS™ drum brake is a hydraulically operated duo-duplex
brake with an automatic adjustment and parking brake
feature. The A2LS™ drum brake (automatic adjust, two
leading shoes) provides two leading shoes in either direction
of drum rotation and an automatic adjustment of the brake
shoes to compensate for lining wear.

GASKET

SHOE HOLD-DOWN CLIP (NOT

FITTED TO EARLY BRAKES)

The single web brake shoes are located by slots in the wheel
cylinder tappet head, the expander input tappet/piston and
abutment tappet, and are retained by the shoe return springs.
As with all Bendix automatic adjusting brakes, provision is
made for manual override of the automatic adjuster. This
should only be needed for initial adjustment of the shoes to
a specific drum clearance, or to back off shoes to enable a
lipped drum to be removed.

CROSS-OVER

PIPE

CAP SCREWS

GROMMETS

Both the wheel cylinder unit and expander unit have cast
integral brake mounting lugs, each taking the brake torque
load. The wheel cylinder unit contains two single acting
hydraulic cylinders with automatic adjusters. The expander
unit contains a double acting hydraulic cylinder with a park
brake wedge expander . Wheel cylinder and expander units
have either a 1-5/8 inch (41,3mm) or 1-3/4 inch (44,5mm)
diameter bore depending upon vehicle brake requirements.
The units are hydraulically connected together with a bridge
pipe or crossover tube.

The brake assembly incorporates an integral backing plate
(dirtshield) to protect the brake from excessive water and
dirt intrusion. The backing plate (dirtshield) contains a steady
post for each brake shoe to insure that lining material is
kept parallel to the drum braking surface. Both brake shoes
are free to slide on their abutment faces against the backing
plate (dirtshield) and are therefore able to centralize
themselves to the drum. Lining material is 3/4 inch (19,1 1
mm) thick in the center of the shoe tapering to approximately
5/8 inch (15,9 mm) at each end. Lining inspection holes and

1

manual override access holes are covered by rubber
grommets.

INPUT/

TAPPET

WHEEL

CYLINDER

PISTONS

PISTON

(ARROW INDICATES DIRECTION OF DRUM ROTATION)

TORQUE

LOAD

TORQUE

LOAD

WHEEL

CYLINDER

PISTONS

FIGURE 2 - SERVICE BRAKE APPLICA TION

ABUTMENT

ROLLER/TAPPET

PISTON

TAPPET

OPERATION

On brake pedal application, hydraulic fluid pressure is
delivered to the wheel cylinder and expander unit. This
hydraulic pressure forces the wheel cylinder pistons
outwards, Figure 2, moving the brake shoes against the
drum. At the same time hydraulic pressure also forces the
input tappet piston and abutment tappet in the expander
unit outward, moving the opposite end of the brake shoes
toward the drum. The expander input tappet/piston pushes
against one shoe, and the roller tappet/piston pushes on
the expander abutment tappet and through this to the other
shoe.

(ARROW INDICATES DIRECTION OF DRUM ROTATION)

FIGURE 4 - REVERSE DRUM ROTA TION

When the brake shoe lining contacts the drum the shoes
are forced around, sliding on the abutments, in the direction
Of drum rotation. In the forward direction, Figure 3, the torque
load from one shoe is taken by the wheel cylinder piston
bottoming in its bore in the wheel cylinder, and from the
opposite shoe by the expander abutment tappet resting
against the expander unit body. In the reverse direction,
Figure 4, the torque load from one shoe is taken by the
other wheel cylinder piston bottoming in its bore in the wheel
cylinder, and from the opposite shoe by the expander input
tappet/piston head resting against the expander body. In
this way, regardless of forward or reverse drum rotation,
the brake will always be a two leading shoe brake.

P ARKING BRAKE APPLICA TION

On application of the parking brake, the spring actuator
screwed into the back of the expander unit housing pushes
the wedge assembly forward. As the wedge assembly moves

TORQUE

LOAD

FIGURE 3 - FORWARD DRUM ROT A TION

2

TORQUE

LOAD

WEDGE

ASSEMBLY

FIGURE 5 - P ARKING BRAKE APPLICA TION

PISTON

DRIVE RING

SEAL

SPRING
ADJUSTER

ADJUSTER

SHAFT HELIX

DRIVE RING

FIGURE 6 - WHEEL CYLINDER ASSEMBL Y CUT -AW A Y

forward, the wedge rollers force the expander abutment tappet
and roller tappet/piston apart, Figure 5, and directly, or
through the expander input tappet/piston the brake shoes
are forced against the drum. In parking brake operation the
brake assembly is a one leading and one trailing shoe brake,
in either forward or reverse drum rotation.

AUTOMATIC ADJUSTMENT OF BRAKE SHOES

In the A2LS™ drum brakes, automatic adjustment is
controlled by the wheel cylinder units. Adjuster assemblies
within the wheel cylinder units control the clearance between
the drum and brake shoe by the back-lash built into each
adjuster mechanism. During braking application, when no
adjustment is required, brake shoe travel takes place within
the back-lash available. As the linings wear and the dist ance
between the lining and drum exceeds the predetermined
clearance, travel of the piston exceeds the back-lash available
and adjustment of the brake shoe is automatically made.
Each wheel cylinder piston has an adjuster mechanism
screwed into it (Figure 6). The rotation of this adjuster relative
to the piston adjusts the brake shoes according to the
amount of lining wear. The adjuster shaft has a fast helix
thread; onto this is mounted a drive ring having a
corresponding thread with an axial clearance (back-lash)
built in.

MANUAL OVERRIDE

WHEEL

The adjuster drive ring is held in light contact with its seat in
the cylinder body by the drive ring spring. The brake shoe
tip engaging in the piston tappet head, and a spring between
the adjuster shaft assembly and the face of the adjuster
bore, insure that a back-lash is maintained on the mating
threads of the adjuster shaft and piston.

When the brake pedal is applied, hydraulic fluid pressure
causes the piston to move outwards, lifting the adjuster shaft

AT REST

SHOE RETURN

SPRING LOAD

DRIVE

RING

SEATED

DRIVE

RING

ADJUSTER

SCREW

PISTON AND ADJUSTER

(SCREW MOVE OUT)

ADJUSTER

SCREW
SEATED

FIGURE 7 - ADJUSTER OPERA TION

NORMAL APPLICATION

(NO ADJUSTMENT REQUIRED)

SPRING LOAD

DRIVE

RING

SEATED

ADJUSTER

SCREW

DRIVE

RING

3

BRAKE APPLICATION

(ADJUSTMENT REQUIRED)

DRIVE RING-LIFTED OFF

SEATING-ROTATES RIDES

DOWN HELIX AND RE-SEATS

FIGURE 8 - ADJUSTER OPERA TION

BRAKES HELD ON (DRIVE RING

ALREADY SETTLED IN NEW

POSITION)

DRIVE RING

RE-SEATED

off its seat in the cylinder body. If the movement of the
adjuster shaft helix through the adjuster drive ring is less
than the axial clearance (back-lash), no rotation of the
adjuster shaft takes place. (Figure 7)

When the movement of the adjuster shaft helix through the
adjuster drive ring is greater than the axial clearance, rotation
of the drive ring relative to the adjuster shaft will occur . (Figure

8) As the adjuster shaf t helix passes through the drive ring,
the drive ring, lightly loaded by the drive ring spring, will
rotate in its seat in the cylinder body . As the brake pedal is
released, the brake shoe return springs cause the brake
shoe to push the piston and adjuster shaft assembly back
through the cylinder.

As the axial tolerance between the adjuster shaft helix and
adjuster drive ring is taken up, the load on the drive ring
helix forces it onto its seat. The resultant friction keeps it
from further rotation. As the piston and adjuster continue
through the cylinder, the adjuster shaft helix is forced to
rotate through the drive ring. (Figure 9) This rotation of the
adjuster shaft, relative to the piston, unscrews the adjuster
shaft from the piston, increasing the overall length of the
adjuster and piston assembly . This in turn reduces the brake
shoe lining to drum clearance.

The automatic adjustment rate is variable; each application
of the brake takes up only a percentage of the excess shoe
to drum clearance. This avoids over adjustment when the
drums get hot and expand.

Note: Due to manufacturing tolerances, it is unlikely that all

brake shoes will run at the same clearances. No
detrimental effect will result and no attempt should be
made to standardize these clearances.

MANUAL OVERRIDE MECHANISM

The manual override mechanism is used to override the
automatic adjuster to set the initial shoe to drum clearance.
This initial adjustment is made after new brake shoes have
been installed, or other maintenance work has been
performed requiring the removal of the brake drum. This
adjustment ‘is necessary to avoid excessive movement of
the wheel cylinder pistons and to achieve a correct and
stabilized running clearance.

The manual override is also used to un-adjust the brakes
when removing the drum when a lipped drum condition
exists.

IMPORT ANT! PLEASE READ:

BRAKES HELD ON (DRIVE RING

ALREADY SETTLED IN NEW POSITION)

SHOE RETURN

SPRING LOAD

DRIVE RING

RE-SEATED

FIGURE 9 - ADJUSTER OPERA TION

4

DRIVE RING

SEATED

RETURN STROKE

ADJUSTMENT COMPLETED

The manual override should not be used as an
alternative to the auto-adjuster. It is essential that the
auto-adjust mechanism be kept in good operating
condition to insure safe and reliable brake operation.

PREVENTIVE MAINTENANCE

The A2LS™ drum brakes normally do not require any service
except when new linings are being installed. The automatic
adjusters eliminate the need for any periodic adjustment of
the brakes. To insure safe operation of the brakes, the
following service checks should be performed.

The following is a recommended time table; the actual times
may vary depending upon the use of the vehicle and the
type of operation and condition the vehicle is exposed to.

LINE HAUL VEHICLES

Every 50,000 miles (80,000 km), six months, or 900
operating hours, whichever occurs first, perform the service
checks outlined in this manual.

CITY DELIVERY

Every 15,000 miles (24,000 km), two months, or 300
operating hours, whichever occurs first, perform the service
checks outlined in this manual.

SERVICE CHECKS

With the vehicle on a level surface, block the wheels to
prevent movement of the vehicle.

1. Remove one or more of the lining inspection plugs.
Check the thickness of the brake linings.

2. Activate the brake pedal. Observe the brake shoes
through the inspection holes and make sure that the
brake shoes move against the drums smoothly and
promptly. Release the brake pedal and make sure the
brake shoes retract promptly and smoothly .

3. Apply the parking brake. Observe the brake shoes
through the inspection hole and make sure the brake
shoes engage the drum.

4. Check all hydraulic fittings and the surface of the
backing plate (dirtshield) for any evidence of hydraulic
fluid leakage.

5. Refer to the vehicle manual for service checks of other
brake related items.

WARNING!

When working on or around brake systems and
components, the following precautions must be
observed:

WARNING! PLEASE READ AND FOLLOW
THESE INSTRUCTIONS TO AVOID
PERSONAL INJURY OR DEATH:

When working on or around brake systems and
components, the following precautions must be
observed at all times:

1. Park the vehicle on a level surface, apply the

parking brakes, and always block the wheels.
When working around or under the vehicle, stop
the engine and remove the key from the ignition.
Always keep hands away from chambers as they
may apply as system pressure drops. Always wear
safety glasses.

2. When working in the engine compartment, the

engine should be shut off and the ignition key
should be removed. Where circumstances require
that the engine be in operation, extreme caution
should be used to prevent personal injury resulting
from contact with moving, rotating, leaking, heated
or electrically charged components.

3. Do not attempt to install, remove, disassemble or

assemble a component until you have read and
thoroughly understand the recommended
procedures. Use only the proper tools and observe
all precautions pertaining to the use of those tools.

4. Following the vehicle manufacturer’s
recommended procedures, deactivate the electrical
system in a manner that safely removes all
electrical power from the vehicle.

5. If the vehicle is equipped with an air over hydraulic
brake system or any auxiliary pressurized air
system, make certain to drain the air pressure from
all reservoirs before beginning ANY work on the
vehicle. If the vehicle is equipped with an AD-IS
air dryer system or a dryer reservoir module, be
sure to drain the purge reservoir.

6. Never connect or disconnect a hose or line
containing pressure; it may whip. Never remove a
component or pipe plug unless you are certain all
system pressure has been depleted.

7. Never exceed manufacturer’s recommended
pressure.

8. Never attempt to disassemble a component until
you have read and understand all recommended
procedures. Some components contain powerful
springs and injury can result if not properly
disassembled. Use only proper tools and observe
all precautions pertaining to use of those tools.

9. Use only genuine Bendix® replacement parts,
components and kits.

A. Use only components, devices and mounting

and attaching hardware specifically designed
for use in hydraulic brake systems.

B. All replacement hardware, tubing, hose,

fittings, etc. must be of equivalent size, type
and strength as the original equipment.

10. Components with stripped threads or damaged
parts should be replaced rather than repaired. Do
not attempt repairs requiring machining or welding
unless specifically stated and approved by the
vehicle and component manufacturer.

11. Prior to returning the vehicle to service, make
certain all components and systems are restored to
their proper operating condition.

ADJUSTMENT

OPENING

FIGURE 10 - REAR VIEW OF THE BACKING PLA TE
(DIRTSHIELD)

™

5