2
FIGURE 2 - SERVICE BRAKE APPLICATION
INPUT/
TAPPET
PISTON
WHEEL
CYLINDER
PISTONS
ROLLER/TAPPET
PISTON
ABUTMENT
TAPPET
WHEEL
CYLINDER
PISTONS
FIGURE 5 - PARKING BRAKE APPLICATION
WEDGE
ASSEMBLY
FIGURE 4 - REVERSE DRUM ROTATION
manual override access holes are covered by rubber
grommets.
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.
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.
PARKING BRAKE APPLICATION
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
TORQUE
LOAD
(ARROW INDICATES DIRECTION OF DRUM ROTATION)
FIGURE 3 - FORWARD DRUM ROTATION
(ARROW INDICATES DIRECTION OF DRUM ROTATION)
TORQUE
LOAD
TORQUE
LOAD
