3
FIGURE 7 - ADJUSTER OPERATION
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 distance
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.
DRIVE
RING
DRIVE
RING
ADJUSTER
SCREW
ADJUSTER
SCREW
DRIVE
RING
SEATED
ADJUSTER
SCREW
SEATED
DRIVE
RING
SEATED
SHOE RETURN
SPRING LOAD
PISTON AND ADJUSTER
(SCREW MOVE OUT)
SPRING LOAD
NORMAL APPLICATION
(NO ADJUSTMENT REQUIRED)
AT REST
FIGURE 6 - WHEEL CYLINDER ASSEMBLY CUT-AWAY
MANUAL OVERRIDE
WHEEL
PISTON
SEAL
ADJUSTER
SHAFT HELIX
ADJUSTER
DRIVE RING
DRIVE RING
SPRING
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
