[Rear brake OPERATION]
With the general drum brake type, when the
brake pedal is depressed, two pistons in the
wheel cylinder force the brake shoes outward,
restraining the turn of the drum.
The more the brake shoes get worn, the longer
distance the pistons must move. As a result, the
brake pedal travel (pedal-to-wall clearance)
increases. Then the shoe clearance must be
adjusted by the shoe adjusting screws. Thus
periodical adjustment is required for the drum
brake type in general.
This rear brake is provided with a self-adjusting
system which automatically adjusts the shoe-to-
drum clearance (pedal-to-wall clearance) caused
by such brake shoe wear.
Clearance correction
In each rear wheel cylinder, pistons, piston cups,
and a piston spring (1) are installed. When the
brake pedal is depressed, fluid pressure is applied
to the inside of the chamber on the piston (2),
(3).
Being actuated by this pressure, the piston (2)
moves to the left (piston (3) moves to the right)
in the following figure and presses the brake
shoe against the brake drum, thus producing
brake force.
(2)
(3)
Fig. 19-9
At this time, the distance the brake shoe moves
is “B”,
that is, the distance that “A” (the end of
the long hole made in the brake shoes web)
moves till it contacts the lever (1) which is fitted
in the long hole.
Fig. 19-10
When the brake pedal is depressed, the piston
and brake shoe move toward the brake drum
side by the aforementioned distance “B” and
“A” of the brake shoe web contacts the lever
(1). As the brake shoe gets worn and the brake
shoe clearance becomes larger, the force applied
to the lever (1) at the time of such a contact
becomes larger. When it exceeds 10
-
12 kg
(22
-
26 Ibs), the “A” of the brake shoe web
moves the lever (1) as much as the amount of
the brake shoe lining wear toward the direction
as shown with an arrow in the figure. Thus the
shoe is forced against the drum and the brake
force is produced.
19-9
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