4
FIGURE 9 - ADJUSTER OPERATION
FIGURE 8 - ADJUSTER OPERATION
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 shaft 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.
IMPORTANT! PLEASE READ:
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.
DRIVE RING-LIFTED OFF
SEATING-ROTATES RIDES
DOWN HELIX AND RE-SEATS
BRAKES HELD ON (DRIVE RING
ALREADY SETTLED IN NEW
POSITION)
BRAKE APPLICATION
(ADJUSTMENT REQUIRED)
DRIVE RING
RE-SEATED
DRIVE RING
SEATED
DRIVE RING
RE-SEATED
SHOE RETURN
SPRING LOAD
RETURN STROKE
ADJUSTMENT COMPLETED
BRAKES HELD ON (DRIVE RING
ALREADY SETTLED IN NEW POSITION)
